Conformational Switching in Pyruvic Acid Isolated in Ar and N2Matrixes: Spectroscopic Analysis, Anharmonic Simulation, andTunnelingIgor Reva,† Claúdio M. Nunes,† Malgorzata Biczysko,‡,§ and Rui Fausto*,†

†CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal‡Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy§Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti OrganoMetallici (ICCOM-CNR), UOS di Pisa, Area dellaRicerca CNR, Via G. Moruzzi 1, I-56124 Pisa, Italy

ABSTRACT: Monomers of pyruvic acid (PA) isolated in cryogenicargon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided byfully anharmonic calculations of the fundamental modes, overtones,and combinations up to two quanta, including their infraredintensities. The initially dominating PA conformer (Tc) has a cisCCOH arrangement and is stabilized by a strong intramolecularH-bond. Selective near-infrared excitation of Tc at the first OHovertone (6630 cm−1 in Ar, 6643 cm−1 in N2) induced a large scaleconformational conversion to the higher-energy conformer (Tt) withtrans CCOH arrangement. Tt was then converted back to Tc byselective NIR irradiation at the first Tt OH overtone (6940 cm−1 inAr, 6894 cm−1 in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrixmolecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of thevibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc inthe dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speedconsiderably increased. The decay mechanisms are discussed.

1. INTRODUCTION

Pyruvic acid (PA) is well-known to participate in severalfundamental metabolic pathways in biological systems, being akey intersection intermediate in both aerobic and anaerobicenergy production processes in the cells.1 PA is an importantorganic acid widely used in the chemical, drug, and agro-chemical industries. Industrially, it is used mainly as a startingmaterial in the biosynthesis of pharmaceuticals, such as L-tryptophan, L-tyrosine, and alanine. It is also employed in theproduction of crop protection agents, polymers, cosmetics, andfood additives.2 Pyruvic acid, central to leaf carbon metabolism,is a precursor of many volatile organic compounds that impactair quality and climate.3 PA is one of the four (formic, acetic,pyruvic, and oxalic acids) most abundant organic acids in thegas and aerosol phases, which are believed to make animportant contribution to the formation of cloud condensationnuclei in such sources as vegetation emissions and biomass-burning.4

The atmospheric reactions and energy disposal in decom-position of PA is an important research topic.5,6 Thedecomposition of pyruvic acid was studied for thermal reactionsin the ground state7,8 as well as its photolysis.9 The photolysisof PA studied in aqueous solutions,10,11 in the gas phase,12

adsorbed on a metal surface in vacuo,13 and in the presence of

metal ions in condensed media14 indicate the existence ofdifferent reaction mechanisms and exit channels, dependent onthe environment. Some of these mechanisms involve protontunnelling.13 The most recent series of works on theatmospherically relevant chemistry of pyruvic acid has beenpublished by Vaida and co-workers.15−17 In particular, theyhave shown that photochemical reactions of pyruvic acid can beinitiated in the atmosphere by the vibrational OH overtonepumping by red sunlight, at energies well below the electronictransitions.18 Therefore, the experimental characterization ofthe near-infrared spectra in the volatile organic acids is relevantfor understanding of their atmospheric chemistry.18

The microwave spectra of gaseous pyruvic acid are well-known.19−21 The understanding of the main features of therotational spectrum of pyruvic acid is now fairly comprehensive.Spectroscopic constants for the lowest vibrational states havebeen determined with precision suitable for astrophysicalapplications.22 The vibrational spectrum of PA was reportedby Hollenstein et al. in 1978.23 However, all these studies

Special Issue: Markku Ras̈an̈en Festschrift

Received: September 22, 2014Revised: October 17, 2014Published: October 20, 2014

Article

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concern the most stable conformer of PA dominating in thethermal equilibrium at room temperature.The molecule of PA has two conformationally relevant

internal degrees of freedom (the rotations about theintercarbonyl CC and CO bonds), which may yieldthree different conformers of PA in its ground electronicstate.24 We shall follow here the nomenclature used for thepyruvic acid conformers by Ras̈an̈en et al.25 (Figure 1).

Conformer Tc, bearing a strongly stabilizing OH···Ointramolecular H-bond, corresponds to the lowest-energy formand has been extensively studied in the past bothexperimentally and theoretically.19−21,23−31 The highest-energyconformer, Ct, with a theoretically estimated relative energywithin the range 10−18 kJ mol−1,24,25,28−31 has never beenobserved experimentally. The existence of a nonidentifiedsecond, in terms of energy, conformer Tt (between 5 and 11 kJmol−1),24,25,28−31 of pyruvic acid in the gas phase had alreadybeen suggested.23,26 Indeed, several strongest infrared bands ofthis second conformer were unequivocally characterizedexperimentally in 2001 by Reva, Stepanian, Adamowicz, andFausto (hereafter abbreviated as RSAF),24 using matrixisolation infrared spectroscopy.In the RSAF study, the Tt conformer was populated

thermally in the gas phase. Its signature was identified bycomparing the infrared spectra obtained by trapping the vaporof the compound initially at different temperatures (296 and480 K) into a cryogenic (15 K) solid argon matrix with thetheoretically predicted infrared spectra for its different con-formers. Though the amount of the Tt conformer trapped fromthe PA vapor at 480 K was about 4 times that resulting fromdeposition of the PA vapor at 296 K, the relative population ofthis conformer in the matrix deposited from the vapor at 480 Kwas still below 10%.24 A further increase of the temperature ofthe gas led to thermal decomposition of the compound.If conformers of interest are not accessible thermally, they

may be generated in matrixes by using UV irradiation.32

However, besides conformational conversion, the UV irradi-ation may lead to partial decomposition of the studiedsystem.33 Recently, Gerbig and Schreiner (abbreviated as GS)reported on the UV-induced rotamerizations in a series ofα-ketocarboxylic acids, including pyruvic acid.34 In the quest ofa new conformer, we also tried UV irradiation of PA in thiswork. Only a limited amount (ca. 15−20% of the initiallydominating Tc form) could be converted to the minor Ttconformer by UV irradiations in the 370−260 nm range. We donot further elaborate on this topic here, because our resultswere similar to those obtained in the GS study.34

Application of tunable narrow-band NIR light sources topromote conformational conversions represents an alternativeapproach of studying higher-energy conformers, especially inthe cases when they are not accessible thermally. The ideaconsists of pumping a vibrational transition lying above the

torsional barrier for the conformational interconversion(usually an OH stretching overtone). Then, in the course ofvibrational relaxation to the torsional coordinate, a newconformer may be formed. This approach was pioneered in1997 by Pettersson, Lundell, Khriachtchev, and Ras̈an̈en(PLKR).35 By pumping the OH stretching overtone ofmatrix-isolated formic acid, they were able to generate anotherformic acid rotamer. During the past few years the PLKRapproach has developed as a very powerful and eleganttechnique35−43 and was successfully used for generation ofhigh-energy conformers, otherwise not accessible experimen-tally, for several simple carboxylic acids (e.g., formic,44,45

acetic,36 glycolic,46 oxalic,47 and propionic acids48) and aminoacids (e.g., glycine,49 alanine,41,50 β-aminoisobutyric acid42) aswell as the trans−trans conformer of hydroxyacetone.38Using narrow-band NIR radiation, it is possible to excite in

situ, in a very selective way, only molecules adopting a particularconformation. Should such irradiation lead to conversion intoanother structure, then it is possible to totally depopulate acertain conformer in favor of another one. The high selectivityof NIR vibrational excitation of matrix-isolated moleculesmakes this procedure a very powerful technique in the opticalcontrol of the relative populations in conformational mixtures.In the present study of PA, we report an alternative to

previous studies procedure for in situ generation of conformerTt from the matrix-isolated most stable conformer Tc. Theconformer-selective near-infrared (NIR) irradiation of thematrix-isolated PA resulted in a very efficient Tc → Ttconversion, on a very large scale. Moreover, it is also shownthat subsequent narrow-band selective in situ NIR excitation ofthe optically generated Tt conformer can also be used tosuccessfully convert this form back to the most stable Tcconformer. This is an illustrative example of a carboxylic acidbased molecular system allowing for a successful bidirectionalNIR-induced rotamerization. The selective narrow-band NIR-induced transformation of Tt into Tc is also compared with thespontaneous Tt → Tc conversion for the compound kept inthe dark or exposed to the broad-band mid-IR radiation of thespectrometer IR source.The observed large scale NIR-induced rotamerizations in PA,

combined with fully anharmonic calculations, accounting forboth mechanical and electrical anharmonic effects,51 permitteda detailed characterization of the vibrational spectrum of theminor Tt form, including the positions and IR intensities offundamental, overtone and combination bands.

2. EXPERIMENTAL METHODSA commercial sample of pyruvic acid (PA, Acros Organics, 98%purity) was used. The sample was placed in a glass tube andthen connected to the chamber of the cryostat through a needlevalve (NUPRO SS-4BMRG). The sample compartment (glasstube) was kept at 273 K (in melting water ice) to provide thecompound with adequate vapor pressure. The valve nozzle waskept at room temperature (298 K) to have the Tc conformerstrongly dominate in the thermal equilibrium.Prior to usage, the sample was purified by using the standard

freeze−pump−thaw method. To deposit the matrix, the vaporof PA was introduced into the cryostat through the needlevalve, together with a large excess of argon (N60) or nitrogen(N60) gas, both supplied by Air Liquide, coming from aseparate line. The used solute-to-matrix concentration ratioswere kept low enough to ensure the absence of aggregates ofPA in the matrixes. A CsI window, kept at 15 K during

Figure 1. Conformers of pyruvic acid. The uppercase letter (T, trans,= 180° and C, cis, = 0°) refers to the OCCO dihedral angleand the lowercase letter (t, trans, = 180° and c, cis, = 0°) refers to theCCOH dihedral angle.

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deposition and during the IR measurements, was used as theoptical substrate for the matrixes. Its temperature was measuredby a silicon diode sensor connected to a digital controller(Scientific Instruments, model 9650-1), which provides thestabilization accuracy of 0.1 K. In all experiments, an APDCryogenics closed cycle helium refrigeration system with a DE-202A expander was used.The mid-IR spectra were collected, with a resolution of 0.5

cm−1, using a Thermo Nicolet 6700 Fourier transform infraredspectrometer, equipped with a deuterated triglycine sulfate(DTGS) detector and a KBr beam splitter. The NIR spectrawere recorded, with a resolution of 1 cm−1, using the samespectrometer and a mercury cadmium telluride (MCT-B)detector with a CaF2 beam splitter. To avoid interference fromatmospheric H2O and CO2, a continuous flux of dry air wasused to purge the system. To protect matrixes from light withwavenumbers above ∼4200 cm−1 or above ∼2200 cm−1,standard Edmund Optics long-pass filters were used (trans-mission cutoff values of 2.40 or 4.50 μm, respectively).The matrixes were irradiated, through an outer quartz

window of the cryostat, using tunable narrow-band light (fwhm0.2 cm−1) provided by the idler beam of a Spectra PhysicsMOPO-SL optical parametric oscillator pumped by a pulsed(pulse energy 10 mJ, duration 10 ns, repetition rate 10 Hz)Quanta Ray Pro-Series Nd:YAG laser.

3. COMPUTATIONAL METHODSAll calculations were performed with the Gaussian 09 Rev. D01program package52 using the B3LYP53−55 functional and the6-311++G(d,p) basis set. Geometry optimizations wereperformed using TIGHT optimization criteria and werefollowed by harmonic frequency calculations, at the samelevel of theory, which also permitted us to characterize thenature of the stationary points. Anharmonic IR spectra weresubsequently computed by means of a fully automatedapproach,56−58 set within second-order vibrational perturbationmodel (VPT2),59 thus allowing for the evaluation ofanharmonic infrared intensities of not only fundamentals butalso overtones and combination bands.51,56,60 To computeanharmonic frequencies and IR intensities, the requiredsemidiagonal quartic potential energy and cubic electric dipolemoment surfaces were derived through numerical differ-entiations of the analytical second-derivatives of the energycomputed at geometries displaced from equilibrium along thenormal modes (with a 0.01 Å step). Fully anharmonic spectrawere evaluated at the GVPT2/DVPT2 level, allowing for aneffective treatment of anharmonic resonances (see ref 51 andreferences therein), applying default criteria for Fermi57,58 and1-156 resonances.51 The relative energetics were computedbeyond the harmonic approximation by means of simpleperturbation theory (SPT)61,62 combined with the hindered-rotor anharmonic oscillator (HRAO) model,62,63 usingresonance-free expression for the anharmonic zero pointvibrational energies (ZPVE)64 and vibrational wavenumbers.62

For a graphical comparison of theoretical spectra withexperiment, the calculated anharmonic frequencies, togetherwith the calculated infrared intensities, were used to convoluteeach peak with a Lorentzian function having a full width at half-maximum (fwhm) of 2 cm−1, so that the integral bandintensities correspond to the calculated infrared absoluteintensity.65 Note that the peak intensities (in units of “RelativeIntensity”) in such simulated spectra are reduced by a factor of0.3183 compared to the calculated intensity (in km mol−1).

4. RESULTS AND DISCUSSION

4.1. Potential Energy Surface and Matrix Isolation ofPyruvic Acid. The ground-state potential energy landscape ofpyruvic acid has been investigated previously at different levelsof theory.24,25,28−30 As found for other molecules bearing amethyl group adjacent to a carbonyl,66−68 in all minimumenergy structures of PA the methyl group assumes aconformation where one of the hydrogen atoms is syn-periplanar with respect to the carbonyl oxygen. This reducesthe conformationally relevant degrees of freedom of themolecule to only the internal rotations about the intercarbonylC−C and C−O bonds. A contour map representing thepotential energy surface (PES) of PA as a function of these twocoordinates is presented in Figure 2.

The contour map in Figure 2 shows the location of the threePA minima (Tc, Tt, and Ct) on the molecule PES and theirrelative energies. All conformers belong to the Cs symmetrypoint group. The lowest energy of the Tc conformer resultsfrom the presence in this form of a stabilizing intramolecularH-bond (CO···HO; d(O···O) ∼ 204 pm) inserted in afive-membered ring, which energetically compensates the lessfavorable cis arrangement of the CCOH frag-ment.36,69−71 In the second most stable form, Tt (ΔE ∼ 9 kJmol−1), the loss of the intramolecular hydrogen stabilizinginteraction is partially compensated by the more favorable transarrangement of the CCOH fragment.36,69−71 As shownbelow, these structural characteristics of the Tc and Ttconformers, in particular the inherently most stable arrange-ment of the CCOH fragment in Tt and the presence ofthe intramolecular H-bond in Tc stabilizing the intrinsically lessstable cis arrangement of the CCOH fragment in thisform, are fundamental to allow for the bidirectional optical

Figure 2. Relaxed potential energy surface map of pyruvic acidcalculated at the B3LYP/6-311++G(d,p) theory level, as a function ofthe OCCO and CCOH dihedral angles. The twodihedral angles were incremented in steps of 15°, and all the remainingcoordinates were optimized. Dots (•) indicate positions of Tc, Tt, andCt conformers and the Cc structure (transition state). The color baron the right designates the energy scale defined relatively to theelectronic energy of the lowest-energy form Tc (without the zero-point vibrational correction). The isoenergy lines are traced usingsteps of 2 and 4 kJ mol−1, below and above 20 kJ mol−1, respectively.

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control of the rotamerization reactions between these twoforms discussed in the next sections of this article.The higher-energy Ct form differs from the Tt form by a

180° rotation about the intercarbonyl C−C bond. Therepulsive interactions between the two carbonyl oxygenatoms in the Ct conformer are expected to be larger than therepulsive interactions involving the carbonyl and hydroxyloxygen atoms in the Tt conformer, thus justifying the higherenergy of Ct when compared with Tt. The Cc structurecorresponds to a first-order saddle point, with an estimatedrelative energy above 45 kJ mol−1.The energy barrier for conversion of the Tt form into the

most stable Tc conformer is estimated to be ca. ∼47 kJ mol−1(∼56 kJ mol−1 in the reverse direction). On the other hand, theenergy barrier associated with the Ct→ Tt conversion amountsto only ca. 2 kJ mol−1 (Figure 2). Such a low barrier is notenough to allow the survival of the Ct form during depositionof the compound in a cryogenic matrix at 15 K, due to thephenomenon known as conformational cooling.37,72−74 More-over, due to a high internal energy (ca. 14 kJ mol−1) the Ctconformer has a negligible population at the room temperatureand only the two most stable conformers Tc and Tt are thenexpected to be trapped in the matrixes. The spectrum of PAfreshly deposited in an argon matrix obtained in this work isessentially the same as in the previous RSAF matrix-isolationstudy.24 The amounts of the Tc and Tt forms in the cryogenicmatrix prepared from the room temperature PA vapor (Tc ∼95%; Tt ∼ 5%) are in a good agreement with their predictedpopulations (Table 1).4.2. Narrow-Band Selective NIR-Induced Rotameriza-

tion in Pyruvic Acid. In the present study, we used the NIRpumping to populate the Tt conformer of PA, through selectivevibrational excitation of the OH stretching overtone vibrationof the most stable Tc conformer. Such excitation introduces inthe molecule an energy well-above the predicted energy barrierfor the Tc → Tt conversion. Once generated, in this manner,the higher-energy Tt conformer could be subsequentlycharacterized in detail spectroscopically.

The experimental near-IR spectrum of PA isolated in anargon matrix was first collected to locate the spectral position ofthe νOH overtone of the Tc conformer. As shown in Figure 3a,the results indicated that 2νOH of Tc appears at ∼6630 cm−1,in good agreement with the B3LYP/6-311++G(d,p) calculatedanharmonic frequency for this vibration (6671 cm−1; Table 2and Figure 3b).The matrix-isolated PA monomers were then irradiated

several times at 6630 cm−1. The progress of changes wascontrolled by registration of infrared spectrum after eachirradiation. When the IR bands due to the Tc conformerdecreased considerably (by more than 70% of their initialintensity), this series of NIR irradiations was terminated.76 Theset of bands that intensify in the course of this irradiation hasbeen previously identified as belonging to the Tt form.24 In theprevious study, the maximum amount of Tt form could be

Figure 3. Fragments of spectra of pyruvic acid: (a) spectra in an Ar matrix at 15 K, immediately after deposition; (b) simulated spectra. See section 3for the details of simulation. Note change of the absorbance scale factor from the near-IR to mid-IR range. Asterisks designate residual bands due tomonomeric H2O.

Table 1. Relative Electronic Energies (ΔE), Relative ZPVECorrected Energies (ΔEZPVE), and Relative Gibbs Energiesat 298 K (ΔG298K) Calculated at the B3LYP/6-311++G(d,p)Level within Harmonic and Anharmonic Models, and theEquilibrium Populations of PA Conformers Estimated fromthe Relative Gibbs Energies at 298 K (P298)

a

harmonic (HO+RR)c anharmonic (HRAO)d

structure ΔE ΔEZPVE ΔG298K P298 ΔEZPVE ΔG298K P298Tc 0.0 0.0 0.0 93.1 0.0 0.0 93.8Tt 9.5 8.8 6.8 5.9 8.9 6.7 6.2Ct 15.2 14.3 11.3 1.0Ccb 46.2

aRelative energies are given in kJ mol−1, populations in %. Theabsolute calculated E is −342.514415 au for the most stable Tc form.The graphical representation of the PA structures is given in Figure 1.bThe Cc form was characterized as a first-order saddle point.cHarmonic oscillator and rigid rotor model. dHindered rotor +anharmonic oscillator model. Contributions computed by means ofthe HDCPT2 model62 in conjunction with simple perturbation theory(SPT).61,62 The two lowest vibrations have been described byhindered-rotor contributions computed using an automatic proce-dure.63,75

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increased only up to 10% of the total conformational mixture,and only the eight strongest IR modes could be assigned. In thepresent work, the maximum amount of the Tt form could beincreased up to 75% of the total conformational mixture(Figure 4a−d), owing to the selectivity of the NIR-inducedconformational transformation.76

The observed changes in the IR spectrum of PA indicate thatthe intramolecular hydrogen bond existing in the initiallypresent Tc conformer is disrupted in Tt after the conforma-

tional change. For example, the observed shifts in the νOHmode to a higher frequency (from 3432 to 3556 cm−1, Figure4a), and in the τOH mode to a lower frequency (from the 664to 588 cm−1, Figure 4d), confirm this conclusion.The comparison of the experimental difference spectrum

(Figure 5a), with that simulated on the basis of the calculatedanharmonic spectra for the two conformers (Tc and Tt; Figure5b), undoubtedly confirms that the NIR-generated speciescorresponds to the Tt form. This permitted a detailed

Table 2. Observed (Ar and N2 Matrix, 15 K) and Theoretical Anharmonic B3LYP/6-311++G(d,p) Wavenumbers (ν, cm−1) and

Infrared Intensities (I, km mol−1) of the Pyruvic Acid Conformer Tc

observed calculated

mode assignmenta N2b Arb,c gasd,e ν I

2ν1 6643 6630.0b 6696 6671 3.30ν1+ν4 5235 5228.3b 5241 0.27ν1+ν7 4823 4819.7b 4851 4807 0.43ν1+ν8 4794 4795.4b 4770 2.03ν3+ν5 4647 4643.7b 4668 4677 0.09ν1+ν9 4635 4631.8b 4620 2.10ν1+ν10 4573 4565.9b 4589 4554 1.172ν4+ν11 4549 4540.7b

ν1+ν12 4198 4194.4b 4200 4183 0.26ν1+ν20 4158.8b 4145 4164 0.41ν1+ν21 4113 4119.3b 4112 0.56ν1+ν13 4037.5b 4039 0.17ν2+ν11 4000b 3968 0.17ν1+ν15 3823 3818.5b 3820 0.862ν4 3578 3584.2c 3594 4.40ν1 ν(OH) 3434.6 3432.0c 3463 3438 107.35ν2 ν(CH3) as 3020 3032.3

c 3025 3005 5.29ν17 ν(CH3) as 2982.0

c 2948 0.92ν3 ν(CH3) s 2932 2936.0

c 2941 2928 0.57ν5+ν11 2725b 2711 0.65ν8+ν9 2522 2515b 2565 2513 1.03ν9+ν12 1959 1957.2b 1966 1938 0.65ν6+ν15 1809 1804.7b 1800 8.54ν4 ν(C3O) 1797 1799.5c 1804 1809 245.39ν18+ν22 1794 1797.6b 1809 0.54ν9+ν13 1790 1795.7b 1794 0.22ν9+ν14 1737 1730.2c 1714 17.27ν5 ν(C2O) 1733 1727.9c 1737 1751 94.51ν6 δ(CH3) as 1424 1423.7

c 1424 1425 15.40ν18 δ(CH3) as 1406 1408.3

c 1415 7.24ν7 ν(CC) as 1388 1384.5c 1391 1367 5.66ν8 δ(CH3) s 1355 1354.6

c 1360 1350 313.90ν9 δ(COH) 1216 1214.4c 1211 1193 107.32ν10 ν(CO) 1140 1136.8c 1133 1120 59.05ν19 γ(CH3) 1017 1017.8

c 1030 1014 2.13ν11 γ(CH3) 969 968.4

c 970 964 21.52ν12 ν(CC) s 767 762.2c 761 747 7.38ν20 γ(C3O) 678.8b 718 0.01ν21 τ(OH) 662 664.2c 668 659 117.11ν13 δ(C2O) 605 603.8c 604 601 16.08ν14 δ(C3O) 534.9c 527 3.27ν22 γ(C2O) 394 393 18.16ν15 δ(CCO) 389 384 8.33ν16 δ(CCC) 258 255 24.78ν23 τ(CH3) 134 120 0.04ν24 τ(CC) 90 95 9.38

aApproximate description: ν, stretching; δ, bending; γ, rocking; τ, torsion. bThis work. cFrom ref 24. dGas phase results below 1000 cm−1 from ref23. eGas phase results above 1000 cm−1 from ref 16.

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assignment of many additional bands in the experimental Ttspectrum (Table 3). The mean unsigned error (MUE) resultingfrom the comparison between experimentally observed andnonscaled anharmonic B3LYP/6-311++G(d,p) wavenumbers(in the range below 1900 cm−1, for two conformers) amountsto 8.6 cm−1. This outperforms the scaled harmonic MUE value(10.9 cm−1) obtained from the B3LYP/aug-cc-pVDZ calcu-lations used in the previous study on pyruvic acid24 and MUEsobtained from scaled harmonic calculations for other func-tionals and methods.77 Moreover, the anharmonic IR spectraprovide also intensities for nonfundamental transitions,vanishing at the harmonic level, allowing for the assignmentof overtone and combination bands, nonaccessible from any

computations based on the double-harmonic approximation.51

Furthermore, a low value of obtained MUE demonstrates thatthe argon matrix is inert enough and spectra of pyruvic acidisolated in an argon matrix are in good agreement with resultsof anharmonic calculations that were carried for monomericmolecules in vacuo. That is also confirmed by comparison withthe available gas-phase results reported by Vaida and co-workers.16 We note that the larger discrepancy is only observedfor νOH, which is known to be more sensitive to the matrixenvironment.43,46,49,50,78−80 A low value of MUE for fullyanharmonic calculations at B3LYP level is also in line withextended benchmark studies,51 giving us confidence in usage ofthe present theoretical approach for interpretation of the

Figure 4. Spectral indication of a large-scale conformational change in pyruvic acid isolated in an Ar matrix upon irradiation at 6630 cm−1 for 90 min:blue trace, spectrum taken immediately after deposition; red trace, spectrum after NIR irradiation.

Figure 5. (a) Experimental difference IR spectrum, the spectrum obtained after irradiation at 6630 cm−1 minus the spectrum of the freshly depositedPA in argon matrix at 15 K. (b) Simulated difference anharmonic IR spectrum at the B3LYP/6-311++G(d,p) level considering the quantitativeconversion of the Tc into the Tt form (ratio 1:1). See section 3 for the details of simulation.

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experiment. The results of anharmonic vibrational analysis ofPA are presented in Tables 2 and 3.The observed experimental NIR-induced rotamerization in

PA suggests a quantitative transformation from the Tc to Ttconformer. In a recent study on matrix isolated glycolic acid46 itwas shown that the conformational transformations may occurstepwise. Monochromatic near-IR excitation of the most stableSSC conformer of glycolic acid (GA) produces directly only

one of the possible minor forms (GAC). The generation ofadditional minor GA form (AAT) needs excitation withanother near-IR photon, acting on the GAC, which is thentransformed into AAT.46 Following the same logic, it isplausible to assume that a third conformer of pyruvic acid mightbe generated in a stepwise process similar to glycolic acid, byselective NIR-excitation of the minor Tt form in PA.

Table 3. Observed (Ar and N2 Matrix, 15 K) and Theoretical Anharmonic B3LYP/6-311++G(d,p) Wavenumbers (ν, cm−1) and

Infrared Intensities (I, km mol−1) of the Pyruvic Acid Conformer Tt

observed calculated

mode assignmenta N2b Arb,c gasd ν I

2ν1 6900/6892.9 6944/6940b 6975 6977 5.15ν1+ν4 5325.5b 5349 0.15ν1+ν5 5293.2 5316.0b 5337 0.63ν1+ν7 4934.0b 4934 0.14ν1+ν8 4907.2b 4891 0.97ν1+ν9 4721 4750.3b 4737 0.29ν3+ν4 4702.5b 4706 0.06ν1+ν10 4660 4670.6b 4682 0.07ν2+ν6 4441.1b 4416 0.53ν2+ν18 4400.2b 4409 0.23ν2+ν7 4381.0b 4367 0.21ν17+ν18 4381.0b 4365 0.26ν3+ν18 4329.8b 4347 0.19ν1+ν20 4288.0b 4295 0.09ν1+ν12 4288.0b 4290 0.16ν1+ν21 4158.8 4143.7b 4177 1.022ν4 3501.8 3504.3b 3536 3.502ν5 3486.5 3484b 3514 3.24ν1 ν(OH) 3535.8/3532.8 3556.2/3554.1c 3579 3571 65.93ν2 ν(CH3) as 3028.6 3030.4

b 3007 6.20ν17 ν(CH3) as 2992.0 2982.4

b 2954 2.78ν3 ν(CH3) s 2929 0.004ν5+ν10 2878b 2875 0.88ν8+ν9 2514.7 2496b 2496 1.151ν10+ν12 1868.5 1818.6b 1829 1.784ν4 ν(C3O) 1765 1763.7c 1780 200.38ν9+ν13 1762 1761.4c 1762 2.91ν5 ν(C2O) 1751 1750.8c 1767 209.47ν7+ν15 1749.4c 1746 1.31ν19+ν20 1749.4c 1741 1.36ν18 δ(CH3) as 1430 1426.1

b 1420 10.03ν6 δ(CH3) as 1414 1422.3

b 1422 11.21ν7 ν(CC) as 1363 1387.0b 1365 10.11ν8 δ(CH3) s 1338 1356.9

b 1354 34.33ν9 δ(COH) 1197 1205.0b 1208 22.94ν10 ν(CO) 1127.7 1118.8c 1110 226.02ν19 γ(CH3) 1120 1116.6

c 1018 1.61ν11 γ(CH3) 966 961.9

c 954 44.77ν20 γ(C3O) 726 722.7c 723 31.34ν12 ν(CC) 716.4c 722 11.54ν21 τ(OH) 626.6/618.6 588.2c 605 97.15ν13 δ(C2O) 594.1 592.1c 590 69.71ν14 δ(C3O) 513 1.65ν15 δ(CCO) 382 1.30ν22 γ(C2O) 378 0.05ν16 δ(CCC) 257 10.01ν23 τ(CH3) 121 0.0002ν24 τ(CC) 39 7.10

aApproximate description: ν, stretching; δ, bending; γ, rocking; τ, torsion. bThis work. cFrom ref 24. dGas phase results from ref 16.

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The matrix previously subjected to irradiation at 6630 cm−1

(at the 2νOH wavenumber of the Tc conformer), thuscontaining an increased population of the Tt form, wasirradiated at the 2νOH wavenumber of the latter conformer.This band appears at 6940 cm−1 in the experiment. It is alreadydiscernible in the NIR spectrum of the nonirradiated matrix(Figure 3) and increases considerably when the Tt form isproduced. Its spectral position agrees well with the B3LYP/6-311++G(d,p) calculated anharmonic value (6977 cm−1). As aresult of this irradiation, no bands ascribable to a new, thirdconformer of PA (a putative Ct form) could be identified in thespectrum. Instead, the Tt form was efficiently converted back toTc. Therefore, after Tt has been generated in the matrix (uponirradiation at 6630 cm−1), the reverse Tt → Tc transformationcould be successfully induced by NIR irradiation at 6940 cm−1

(Scheme 1). The possibility of efficient bidirectional optical

control of the Tc ↔ Tt interconversion process (i.e., of therelative populations of the two PA conformers) makes thismolecule a system satisfying the criterion of molecular opticalswitch.38,81

Following again the analogy with the recently reported caseof glycolic acid,46 we studied the influence of the matrix hostmaterial on the conformational behavior of pyruvic acid. Forglycolic acid, the NIR-induced photochemistry was different inargon and in nitrogen matrixes. A successful generation of anew, fourth GA conformer (SST) was achieved in solidnitrogen, whereas the minor forms of GA accessible in theargon matrix were not populated in nitrogen (and vice versa).The detailed description of the experiments with PA isolated ina nitrogen matrix will be given later in this work. Briefly, wehave isolated PA in solid N2 and characterized its vibrationalspectra in the mid-infrared and near-infrared domains. Theprocesses occurring for PA in solid nitrogen were very similarto those just described for PA in solid argon. The minor Ttform was produced from Tc, and bidirectional Tc ↔ Tttransformations were successfully induced by irradiations at therespective 2νOH overtones (at 6643 and 6894 cm−1). Therewas no indication of generation of a putative third PAconformer (Ct) in the solid nitrogen matrix either.4.3. Experimental Vibrational Signatures of Pyruvic

Acid Isolated in Argon and in Nitrogen Matrixes. Usingnitrogen as a matrix material was found previously to produce astabilizing effect on the minor conformers of several matrix-isolated molecules, such as formic,82,83 acetic,82 tetrazole−acetic,84 squaric,43 and glycolic46 acids, glycine,49 alanine,41,50

and cysteine,85 among others. The minor conformers of thesecompounds are stable in nitrogen but hard or impossible todetect by steady-state spectroscopic techniques in argon. Asimilar stabilization effect was found to occur also in pyruvicacid. As will be shown in section 4.4, the optically generated Ttconformer decayed back to Tc in argon matrixes, whereas nosuch relaxation occurred for the Tt form isolated in nitrogenmatrixes.

Because of the long lifetime of the NIR-generated Ttconformer of PA in an argon matrix (several hours, see below),and indefinitely stable Tt form in a nitrogen matrix, it waspossible to produce the Tt species in a large amount andcharacterize this conformer spectroscopically in detail. For PAisolated in a nitrogen matrix, the 2νOH overtone of Tc appearsat ∼6643 cm−1, and selective irradiation at this frequencyefficiently promotes its transformation to the Tt conformer,similarly to argon. The spectral indications of the NIR-inducedrotamerization in argon and in nitrogen matrix are shown in theform of difference spectra in Figure 6a−d. As can be seen fromthe figure, the spectral manifestations of the Tc conformer arequite similar in Ar and in N2 (note negative blue and red bandsin Figure 6). However, for the NIR-generated Tt conformer,the spectral signatures exhibit much larger discrepancies upongoing from argon to nitrogen. Especially striking are thesediscrepancies related with positions of vibrations involving theOH group. In the Tc conformer, the OH group is “hidden”inside the molecule, the intramolecular hydrogen bondinteraction is essentially the same (judging from the observedvibrational frequencies) in both types of matrix (Figure 6). Inthe Tt conformer, the intramolecular H-bond is disrupted, theOH group is oriented to the outside of molecule and mustinteract with the environment. Apparently, in argon such aninteraction is weak (note a good agreement between calculationand experiment, and low value of MUE’s), as discussed above.In the nitrogen matrix, the vibrational frequencies of the Tt

vibrational modes involving the OH group are strongly shifted(comparing with the Tt bands in argon), and these shifts occurin the direction of the Tc form (note shifts by 22 and 38 cm−1

for the νOH and τOH modes). This indicates that the OHgroup of the Tt form establishes a stabilizing (hydrogen-bond-like) interaction with nitrogen matrix. Such an interaction isstrong enough to stabilize the Tt conformer in N2 indefinitelylong against relaxation back to Tc.In addition to the mid-infrared spectra, the spectral

signatures of the matrix-isolated PA were also recorded in thenear-infrared domain, before and after the NIR-inducedrotamerizations. The performed anharmonic calculationsaided the assignments. The experimental difference spectrashowing changes in the overtone range (7200−6400 cm−1) andin the range of combination bands (5500−3800 cm−1) arecompared in Figure 7 with the anharmonic simulated spectra.Noteworthy, these shifts in frequencies from argon to

nitrogen matrix observed for the fundamental bands of Ttconformer are also reproduced in the near-infrared domain. Forinstance, a shift of νOH fundamental Tt mode by −22 cm−1corresponds to the shift of 2νOH overtone Tt by −46 cm−1.The shift of the (νOH + νCO) combination Tt mode by−23 cm−1 (appearing around 5300 cm−1) is well explained bythe sum of shifts of the corresponding two fundamentals, wherethe νCO essentially does not change the spectral positionfrom argon to nitrogen. The shift of the (νOH + τOH)combination mode by +15 cm−1 is also in agreement with theshifts of the fundamental modes by −23 and +38 cm−1 in theopposite directions (Figure 7a,b).Note that the intensities of the overtone and combination

bands are by 2 orders of magnitude lower than thecorresponding fundamental bands (compare the ordinate scalesof Figures 6 and 7). The observation of the combination bandswas made possible by the large scale of the NIR-inducedtransformation, and their assignment was facilitated bycomparison between the experiments in argon and nitrogen,

Scheme 1. Bidirectional NIR-Induced RotamerizationObserved for Matrix-Isolated PA

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Figure 6. Difference spectra showing effects of near-IR irradiations on PA isolated in Ar (irradiation at 6630 cm−1, blue line) and N2 (irradiation at6643 cm−1, red line) matrixes. Positive bands are due to the growing Tt form generated at the expense of Tc form (negative bands). Numbers showshifts of vibrations related to the OH group upon going from argon to nitrogen.

Figure 7. (a, b) Experimental difference NIR spectra in the range of the OH str overtone (left) and combination bands (right) showing effects ofNIR irradiations at (a) 6643 cm−1 and (b) 6630 cm−1 of PA isolated in (a) nitrogen and (b) argon matrixes at 15 K. Negative and positive bands aredue to the Tc and Tt conformers, respectively; assignments are given in Tables 2 and 3. (c) Simulated difference anharmonic IR spectrumconsidering the quantitative conversion of the Tc into the Tt form (ratio 1:1). See section 3 for the details of the simulation. The region below 4000cm−1 in (b) contains a few residual positive bands due to the not entirely compensated atmospheric water vapors. The gray bar with blue asterisk in(c) is discussed in section 4.4.

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and also by comparison with the fully anharmonic calculationsin this range of spectrum, including the infrared intensities(Figure 7c).The assignment of the full mid-infrared spectra of Tt is

presented in Table 3. It completes the previously proposedassignments for this conformer presented in the RSAF paper.24

The assignment of several overtone and combination Tt bandsis also included. Table 2 provides the corresponding data forconformer Tc and suggests some reassignment of combinationbands reported by Vaida et al.16 Some of the combinationbands observed in the present work may play an important rolein the conformational relaxation of PA, as discussed below.4.4. Conformational Tt → Tc Relaxation of Pyruvic

Acid in the Dark and in the Presence of Broad-BandMid-Infrared and Near-Infrared Radiation. For manypreviously studied carboxylic acids and amino acids, the NIR-generated higher-energy conformers quickly (in the course of afew minutes) convert back in an argon matrix to the startingconformer by quantum-mechanical tunneling. For the matrix-isolated PA in argon, the Tt conformer also converted to themost stable form Tc, but this process took a much longer time(many hours) compared to the other systems.40,43,86−88 Theexplanation for this fact relies on the structural characteristics ofthe relevant conformers. For most of the previously studiedsystems, the lower-energy conformers had their carboxylicfragment in the intrinsically more stable trans CCOH (same ascis OCOH) orientation, whereas the NIR-generated formshad their carboxylic moiety in the less stable cis CCOH (sameas trans OCOH) arrangement. Because of this, the CO cis↔ trans energy barriers in these molecules are lower (by 10−20kJ mol−1) than in case of PA, where the NIR-generatedconformer has the carboxylic fragment in the intrinsically morestable trans arrangement. The increase of the barrier height inPA thus reduces its permeability to tunneling and increases thelifetime of the higher energy conformer.It has been reported previously that exposure of the matrix

isolated molecules to the broad-band IR light produced by theIR spectrometer source may also lead to cis ↔ trans COrotamerization, through excitation of the νOH fundamental,like in cytosine.40 The Tt → Tc spontaneous relaxation processin PA was also investigated for the sample exposed to suchradiation. Once the Tt form was generated by irradiation at6630 cm−1, several kinetic observations, in independentexperiments, were performed. In one case, the sample wasexposed to the unfiltered IR beam of the spectrometer. In othercases, a long-pass cutoff infrared filter was placed between thespectrometer source and the sample. Two different cutoff filterswere applied: transmitting only light with wavenumbers (a) upto 4200 cm−1 (∼50 kJ mol−1) and (b) up to 2200 cm−1 (∼25 kJmol−1). In all cases the Tt → Tc decay process was followedspectroscopically over the time. The decay rates in theseexperiments depended on the applied filter range (Figure 8). Inthe case of the filter transmitting only light up to 2200 cm−1 thedecay was considerably slow: it took ∼17 h to convert a half ofTt form back to Tc. In the experiments without filter or withfilter transmitting in the whole mid-IR range (transparent up to4200 cm−1), the decay rates were much faster (∼5 and ∼6 h).In the statistical analysis of the observed decay kinetics, the

experimental integrated intensities of the two strongest bandsof conformer Tt (1764 and 1119 cm−1) were used as themeasures of the evolution. The intensity of each of these bandsat the beginning of measurement was normalized to unity, andthe average value over two bands is plotted in Figure 8. The

kinetic analyses of the amount of conformer Tt, designated asn(Tt), were carried out with the classical monoexponentialmodel:

= −=n n ktTt Tt( ) ( ) exp( )t 0 (1)The corresponding decay rates k obtained from the

monoexponential fit were 2.34 × 10−3 min−1 (no filter), 1.97× 10−3 min−1 (filter 4200 cm−1), and 6.94 × 10−4 min−1 (filter2200 cm−1). These decay rates yield the correspondingoptimized classical half-life times t1/2 of 297, 351, and 999min, respectively.89 The R2 correlation coefficients obtained forthe above fits are 0.99948, 0.99971, and 0.99514, respectively.From these R2 values, it is obvious that the classical fit (blackline in Figure 8) does not reproduce the kinetics observed withfilter 2200 cm−1 as well as the two other fits. Initially, the decayprocess with this filter is faster than the best fit to classicalkinetics equation. At later stages, the decay clearly slows down.This suggests that the probability of the Tt → Tc relaxation isdependent on slight differences in the matrix microenviron-ments. Some matrix cavities allow for a faster conversion of thetrapped molecules that takes place at the initial stages of theexperiment. At later stages, the conversion becomes slower.Such behavior is typical of transformations of moleculesembedded in inhomogeneous media.90 Usually, the timeevolution of such processes follows the equations of thedispersive kinetics:91−93

= − α=n n BtTt Tt( ) ( ) exp( )t 0 (2)where k(t) = Btα−1 (B, α are constants; B ≡ α/ταdisp). Thedispersive kinetics fit for the decay observed in PA with filter2200 cm−1 resulted in a much better correlation coefficient R2 =0.99941, with the fitted parameters B = 0.00143, α = 0.892. Thelower the numerical value of α, the greater the dispersivity ofthe process. The limit of classical kinetics corresponds to α = 1.The value of α ≈ 0.89, obtained for the Tt→ Tc decay at 15 K,suggests that, although the Ar matrix environment is not very

Figure 8. Decay kinetics of Tt conformer in Ar matrix at 15 K. Thespectra were recorded: without filter (red squares); with a cutoff filtertransmitting only up to 4200 cm−1 (green triangles); with a cutoff filtertransmitting only up to 2200 cm−1 (blue circles). The red, green, andblack lines represent fits using the classic single exponential kinetics.The blue line represents the fit with equations of dispersive kinetics.

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disordered, the inhomogeneous character of this mediumcannot be neglected. Earlier, the inhomogeneous character ofargon matrixes was also confirmed during the studies oftunneling in matrix-isolated cytosine.40 In that case, thedispersive kinetic fit resulted in α ≈ 0.8. The dispersive kineticsdecay cannot be characterized by one numeric value relatedwith the half-life of the reacting species, because the speed ofconversion constantly changes with time. Instead, the dispersivekinetics can be characterized by the time constant ταdisp, whichcan be derived from the B ≡ α/ταdisp expression. For B =0.00143 and α = 0.892, the resulting ταdisp = 1360 min.The experimental observations for different Tt → Tc

relaxation times can be explained in terms of the rotamerizationbarrier, separating the Tt and Tc forms, and the existingvibrational transitions in two conformers. The calculatedrelaxed potential energy profile connecting the two PAconformers via the intramolecular torsion of the OH group isdepicted in Figure 9. The value of barrier in the Tt → Tc

direction was calculated to be ∼47 kJ mol−1. The usage of afilter with cutoff at 2200 cm−1 is equivalent to irradiations of thesample with energies no more than 25 kJ mol−1. Such energiesare not sufficient to promote transitions over the barrier.Therefore, in a slow relaxation Tt → Tc process observed withthis filter the quantum-mechanical tunneling mechanism mustdominate.Exposing the matrix-isolated pyruvic acid to the infrared light

up to 4200 cm−1, allows excitation of transitions due to Ttconformer appearing in an argon matrix at 3556.2 cm−1

(equivalent of 42.5 kJ mol−1) and some combination bands,such as at 4143.7 cm−1 (equivalent of 49.5 kJ mol−1). They canbe ascribed to the νOH fundamental transition and the νOH +τOH combination, respectively (Table 3). These transitions ofthe Tt form appear near the top or above the torsional barrierof pyruvic acid (Figure 9, red arrows). When the matrix isolated

PA molecules are exposed to the spectrometer source withoutfilter, or with filter transmitting up to 4200 cm−1, the Ttconformer may absorb at these frequencies. Then thevibrationally excited PA molecules may isomerize via over-the-barrier mechanism, provided there are levels of correspond-ing energy on the side of Tc conformer. Indeed, there is a poolof densely spaced combination bands in the Tc form at 4400−4300 cm−1 (gray rectangle in Figure 7c and in Figure 9), whichon the common energy scale are isoenergetic with the νOHfundamental transition of Tt. These combination bands mayensure an efficient vibrational coupling between Tt and Tc. Itmust be noted that the energy diagram shown in Figure 9 isconstructed on the basis of the experimentally observedwavenumbers, whose vertical transitions are built startingfrom the electronic energies calculated at the B3LYP/6-311++G(d,p) level (Tt−Tc difference of 9.5 kJ mol−1). Based oncomparison with reported CCSD(T) results,24,34 it is plausibleto assume that the calculated energy gap between the groundstates of Tc and Tt is in reality slightly larger (by 1−2 kJmol−1). Also in such cases, the qualitative picture should remainthe same, as there are still many energy levels of the Tc formthat can be responsible for the vibrational coupling, for examplesuch as those appearing at 4194.4 and 4819.7 cm−1 (Figures 7and 9, note the gray bar).The νOH fundamental transition of the Tc form appears at

3432 cm−1 and is accessible to the infrared irradiation withoutfilter. However, this vibrational level of the Tc form does nothave a counterpart level on the other side of the barrier (tosatisfy the coupling condition, the Tt form would need to havean energy level of about 2682 cm−1). The excitation of Tc at3432 cm−1 should not be effective in conformational isomer-ization (crossed arrow in Figure 9). First, it would imply aprocess below the barrier, and second, there are no matchingvibrational levels on the Tt side of the molecule. Altogether, theenergy level scheme of vibrational fundamental and combina-tion transitions in pyruvic acid suggests that it should relax fromTt to Tc more efficiently, when the excitations above thebarrier are allowed. As discussed above, it is confirmed by theaccelerated decay kinetics (Figure 8) in the experimentswithout filter, or with filter transmitting up to 4200 cm−1.This direction of isomerization is also consistent with theabsorption cross sections reported by Vaida et al. for the OHstretching fundamental transitions, the overtones, and thecombinations bands in PA.16 It provides additional insight intowhy the relaxation occurs in the direction from Tt to Tc.

5. CONCLUSIONSIn this investigation, the Tt conformer of pyruvic acid has beensuccessfully populated by narrow-band selective pumping of thefirst νOH overtone of most stable Tc conformer isolated inargon and nitrogen matrixes. The ground-state potential energysurface of the molecule has been investigated in detail usingDFT calculations, and the located minima characterizedstructurally and vibrationally also by performing fullyanharmonic vibrational calculations, which allow for simulationof band position and infrared intensities, for the fundamentalmodes, overtones, and combinations up to two quanta.Because, contrary to what happens most frequently, the

conformational ground state of PA (Tc) has its carboxylicgroup in the intrinsically less stable cis CCOH configurationand the conformer produced after NIR-induced conversion ofthis form (Tt) has this group in the trans CCOH arrangement,the stability of the NIR-produced form has been found to be

Figure 9. Relaxed potential energy scan for the intramolecular OHtorsion in PA calculated at the B3LYP/6-311++G(d,p) level. Thehorizontal colored lines (Tc, blue; Tt, red) designate energy levelsobtained from the experimental values of the τOH and νOH modes intwo conformers, as observed for monomers of PA in Ar matrix.Selected energy levels for some observed combination bands are alsoshown. The gray rectangle designated with a blue asterisk iscommented in the text. Dashed curved arrows (black) indicatepossible couplings between the closely spaced energetic levels in Tcand Tt. The vertical blue and red arrows are plotted for transitionswith energies lower than 4200 cm−1. Note that the ordinate scale isgiven in cm−1 (left) and in kJ mol−1 (right).

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considerably higher than in other carboxylic acids. Moreover,the used strategy allowed promoting a large scale conforma-tional Tc → Tt conversion (not reachable either by thermalpopulation of the Tt form in the gas phase or by in situ UVirradiation of the initially deposited in a cryogenic matrix Tcform). Owing to this fact, the experimental spectra of bothconformers, especially the minor Tt form, could be thoroughlystudied in every detail.After the higher-energy Tt form was generated in the

matrixes, it could be converted back to Tc using a similarstrategy, i.e., by selective NIR irradiation at the respective firstOH overtone. In addition, it was also observed that the Ttform, once produced in argon matrixes, spontaneously decaysback to the most stable Tc form in the dark, by tunneling, in aprocess obeying the dispersive-type kinetics, with a character-istic lifetime of more than 16 h. Noteworthy, in the presence ofnear-infrared broad-band light, the speed of relaxation of the Ttform back to Tc considerably increases, as a result of acontribution of the over-the-barrier mechanism. This con-clusion is supported by a deep analysis of the vibrationalmanifold characteristics of the ground-state potential energysurface region interconnecting the two conformers. In an N2matrix, the Tt form, which has its OH group not involved inany intramolecular H-bond (in Tc, this group establishes anintramolecular H-bond to the carbonyl oxygen), was found tobe considerably stabilized at the cost of interaction between theOH group and the matrix N2 molecules. This stabilizationmanifested itself in the absence of Tt → Tc relaxation and in aconsiderable change of the vibrational Tt signature upon goingfrom argon to nitrogen.

■ AUTHOR INFORMATIONCorresponding Author*R. Fausto. E-mail: rfausto@ci.uc.pt. Telephone: +351-239-854-483.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThese studies were partially funded by the Portuguese“Fundaca̧õ para a Cien̂cia e a Tecnologia” (FCT), FEDER,and project PTDC/QUI-QUI/118078/2010, FCOMP-01-0124-FEDER-021082, cofunded by QREN-COMPETE-UE.C.M.N. acknowledges the FCT for the Postdoctoral GrantNo. SFRH/BPD/86021/2012. The Coimbra Chemistry Centre(CQC) is supported by the FCT through the project PEst-OE/QUI/UI0313/2014. This work was also supported by ItalianMIUR (under the project PON01-01078/8). M.B. gratefullythanks Prof. Vincenzo Barone and Prof. Cristina Puzzarini forhelpful discussions, and the high-performance computerfacilities of the DREAMS center (http://dreamshpc.sns.it) forproviding computer resources. The support of the COSTCMTS-Action CM1002 “COnvergent Distributed Environ-ment for Computational Spectroscopy (CODECS)” is alsoacknowledged.

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mailto:rfausto@ci.uc.pthttp://dreamshpc.sns.it

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