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-RH06-PROTON DONOR/ACCEPTOR PROPENSITIES OF

AMMONIA ROTATIONAL STUDIES OF ITS MOLECULAR COMPLEXES WITH ORGANIC MOLECULES

64th OSU International Symposium on Molecular Spectroscopy - - June 22-26, 2009

Dipartimento di Chimica ”G.Ciamician” - Universita’di Bologna

B.M. Giuliano, A. Maris, S. Melandri, L.B. Favero, L. Evangelisti, W. Caminati

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Bologna Molecular Beam Fourier Transform MW Spectrometer: Overview

Updated&operating with J.-U. Grabow hardware&software, http://www.pci.uni-hannover.de/~lgpca/spectroscopy/ftmw

W.Caminati, A.Millemaggi, J.L.Alonso, A. Lesarri, J.C.Lopez, S. Mata, Chem.Phys.Letters, 392, 1 (2004)

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Complexes of NH3 with organic molecules

Motivation: Many complexes involving one or more molecules of water linked to organic or biomolecules have been investigated by MW . Water participates in several kinds of hydrogen bonds, acting either as a proton acceptor or a proton donor. The most common hydrogen bonds in these systems are of the type O-Hw···O, O-H···Ow, O-Hw···N, N-H···Ow with interaction energy in the range 15-25 kJmol-

1.

Less information is available on molecular complexes of ammonia with organic molecules. The investigated systems where ammonia could play the double proton donor/acceptor role are just F3CH···NH3, CH3OH···NH3, pirrole···NH3 and tert-butanol···NH3. All rotational spectra of its complexes with organic molecules revealed only conformers where ammonia, differently from water, acts exclusively as a proton acceptor.A few rotationally resolved electronic spectroscopy studies on adducts of ammonia with larger molecules confirm this behaviour, such as in the case of 1-naphtol···NH3, hydroquinone···NH3, and aniline···NH3.*

Results: We studied the molecular complexes of NH3 with 5 organic molecules, tert-butanol, glycidol, ethanol, 1,4-difluorobenzene and anisole. We observed quite different shapes and interactions among them. We compare the results to the complexes with water.

*J. Chem. Phys. 1986, 84, 5983, Chem. Phys. 1988, 125, 31, J. Mol. Spectrosc. 2008, 251, 224, Chem.Phys.Letters, 2008, 463, 330, J.Chem.Phys. 1996, 104, 8332, J.Chem.Phys. 1997, 106, 908.

4B.M.Giuliano, M.C.Castrovilli, A.Maris, S.Melandri, W.Caminati and E.A.Cohen, Chem.Phys.Letters, 463 330–333 (2008).

TBA-NH3

Structural Parameters

R (Å) 2.03

Θ (°) 171

Rθ

5

MP2/6-311++G** Exptl.

I II

aa/MHz 1.58 -3.74 -3.188(5)

bb-cc/MHz 5.38 0.12 0.040(8)

E’0/kJmol-1 9 0 -

I II

TBA-NH3

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TBA-NH3: Internal motions

Ene

rgia

V

α

453 cm-1

28 cm-1

V3 for the internal rotation of NH3 is almost 0, according to the Pbb value and to the missing of nearby “E” lines

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TBA-H2O

The TBA-hydroxyl hydrogen is

tunnelling also in the complex, while water does not practically

move

TBA-OD species small tunnelling splittingsTBA-OH species large tunnelling splittings

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Comparing TBA-H2O to TBA-NH3

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Monomer 1 (G1)

Monomer 2 (G2)

[1] K.-M. Marstokk, H. Møllendal and Y. Stenstrøm, Acta Chem. Scand., 46, 432 (1992).

MW spectra of glycidol revealed hydrogen bonded conformers 1 and 2 [1]. Conformer 1 is more stable by 3.4 (4) kJ/mol.

Glycidol-NH3: Monomeric

Glycidol

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Glycidol-NH3

Transition 212-101

G1-NH3

G2-NH3

We assigned the MW spectra of both adducts, G1-NH3 and G2-NH3. This is one of the few cases in which adducts with two different conformers have been rotationally observed.

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Glycidol-NH3

G1-NH3 G1-15NH3 G2-NH3 G2-15NH3 A/MHz 4006.550(2) 3992.850(1) 4079.158(1) 4035.447(2) B/MHz 2502.7685(8) 2431.3359(4) 2382.1668(6) 2326.6815(4) C/MHz 1857.2265(4) 1814.7855(5) 1630.5642(3) 1597.6225(4)

DJ/kHz 2.11(3) 2.04(2) 1.52(2) 1.52(1) DJK/kHz -0.3(1) [-0.3] -0.16(6) [-0.16] DK/kHz 2.9(2) [2.9] 3.8(3) [3.9] d1/kHz -0.66(2) [-0.66] -0.57(1) [-0.57] d2/kHz [0] [0] 0.09(1) [0.09] Dc3J/MHz 1.95(4) [1.95] -1.57(1) [-1.57] Dpi2K/MHz -0.336(4) [-0.336] 0.299(2) [0.299]

aa/MHz 1.092(4) 1.535(3)

- /MHz -4.200(3) -5.056(5)

V3/ kJmol-1 3.517(2) 3.522(2) 2.4382(3) 2.4369(4)

I/ uÅ2 2.620(1) 2.620(1) 2.7849(3) 2.7885(5) (a,i)/° 69.287(7) 68.11(1) 83.7341(7) 82.076(8) (b,i)/° 24.55(1) 25.57(1) 7.1555(9) 8.623(8)

(c,i)/° 77.402(7) 77.45(1) 86.5584(7) 86.621(3)

/ kHz 3 3 3 3

N 66 16 90 26

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Glycidol-NH3

G1-NH3 G2-NH3

0.034

0.027

0.014

t-BuOH-NH3 Electron density (au)

The electron density at the O-H···N Hydrogen Bond in GLY-NH3 is higher than in t-Buthanol. The small density at the N-H···O contact suggests ammonia to behave as a protic wire (both a hydrogen bond donor and acceptor).

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Glycidol-NH3

From the rotational measurements, we could derive a variety of precise chemical data difficult, if not impossible, to obtain with other techniques:

(i) In both complexes NH3 is linked to the alcohol molecule through an O-H···N (stronger) and an N-H···O (weaker) hydrogen bond;

(ii) The energies of the N-H···O interactions, from the determined V3 barriers to internal rotation of the ammonia moiety, are 3.522(2) and 2.270(1) kJ mol-1 for the G1-NH3, and G2-NH3, respectively;

(iii) The conformational energy difference between the two G2-NH3 on G1-NH3 species is almost zero;

(iv) An incipient charge transfer takes place at the O-H···N bonds;

(v) Precise Hbond structural parameters are obtained;

(vi) Finally, the experimental observation of the complexes of ammonia with both conformers of glycidol can be interpreted in terms of “molecular recognition”, where two locks (G1 and G2) accept the same master-key (NH3).

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Ethyl alcohol-NH3

EAtrans-NH3 EAgauche-NH3

R

R

Also in this case the monomer exists in two different forms. We assigned the MW spectra of both adducts, EAtrans-NH3 and EAgauche-NH3. Again, one of the few cases in which adducts with two different conformers have been rotationally observed.

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Ethyl alcohol-NH3

EAtrans∙∙∙NH3 EAgauche∙∙∙NH3

Parent 15NH3 Parent 15NH3

A/MHz 28499.909(2)b 28472.139(3) 10513.331(1)b 10491.098(1) B/MHz 2093.9444(5) 2030.3617(9) 2879.9819(7) 2787.8611(8)

C/MHz 1999.2946(5) 1941.1160(9) 2432.5544(8) 2365.3906(8)

DJ/kHz 3.286(7) 3.10(1) 13.47(1) 12.62(2)

DJK/kHz -137.70(5) -135.03(7) -54.66(8) -51.00(9)

d1/kHz 0.387(9) 0.34(1) 3.72(3) 3.38(3)

d2/kHz [0.] [0.] 0.32(1) 0.26(2)

aa/MHz -3.134(2) - -2.675(2) - (bb-cc)/MHz -0.104(4) - -0.564(4) -

/kHz 5 2 4 3 N 53 13 114 36

E0+

0-/MHz 0.0197(7) 0.026(1)

V3/kJmol-1 13.5(2) 13.5(2) (a,i)/° 157(3) 156(4)

(b,i)/° 68(3) 67(4)

(c,i)/° 83(1) 83(1)

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Anisol-NH3

I: 15.98

II: 15.25

III: 15.15

IV: 14.11

V: 12.98

VI: 12.66

VII: 11.31

VIII: 11.10

IX: 11.09

X: 10.56

XI: 10.41

XII: 9.28

XIII: 9.17

XIV: 9.04

XV: 8.94

From a distributed multipole analysis (DMA) of electron charge distribution.Dissociation energies in kJ/mol.

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Anisol-NH3

I II III V

-E / Eh

a 402.434918 402.434219 402.434546 402.433444 ZPE/ Eh

b 0.168123 0.168387 0.168320 0.167585 -E0 / Eh

c 402.266796 402.265832 402.266226 402.265859 De / kJ mol-1d 12.80 10.96 11.92 8.93 D0 / kJ mol-1e 9.02 6.49 7.52 6.56 A/MHz 2138 2206 3304 2014 B/MHz 1090 1038 792 996 C/MHz 997 711 644 672 a / D 0.12 0.70 0.49 -3.20 b / D 0.56 2.52 -0.03 -1.57 c / D 0.06 0.00 0.00 0.00 aa / MHz 0.08 -0.43 1.18 -1.58 bb / MHz 1.67 -1.79 -3.39 -0.49 cc / MHz -1.75 2.22 2.21 2.06

The data obtained with the model potential calculations, were refined for the six more stable conformers of ANI-NH3 with full geometry optimization on the MP2 counterpoise corrected PES (MP2CP(full))/6-311++G** level of calculation. Only four stationary points were found, since species IV and VI appeared not to be minima, and relaxed to species I and V, respectively.

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Anisol-NH3

ANI-14NH3 ANI-15NH3

A/MHz 2132.9433 (8)a 2087.5873(7) B/MHz 1091.7811 (4) 1082.7776(4) C/MHz 1000.7962 (3) 983.5337(3)

DJ/kHzb 0.861 (6) 0.903(5) DJK/kHz 2.32 (4) 1.95(2) DK/kHz 1.62 (5) 2.08(6) d1/kHz -0.146 (4) -0.167(4) d2/kHz 0.036 (4) 0.036(3)

aa / MHz -0.063 (6) bb / MHz 1.096 (5) cc / MHz -1.033 (5)

/kHz 2 1 Nc 46 26

Isotopomeric Conformational Change in Anisole-Water

The deuteration of water produces a conformational change in the AnisoleWater complex, as shown by the scheme. The value of the angle decreases from 138 to 128, while the secondary interaction OHMe is replaced by the OHPh one.

B.M.Giuliano and W.Caminati, Angew.Chem.Int.Ed.Engl. 44 603-605 (2005).

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1,4-diFluorobenzene-NH3

A/MHz 3048.228(3)

B/MHz 808.1610(4)

C/MHz 639.5514(2)

DJ/kHz 0.149(2)

DJK/kHz 2.75(6)

DK/kHz 20.6(6)

dJ/kHz -0.03792(8)

dK/kHz -0.51(7)

aa/MHz -2.00(1)

(bb-cc)/MHz 1.128(8)

/kHz 2

N 83

/uÅ2 -0.8

The σ rather than the complex was observed.

Similar to 1,4-diFluorobenzene-H2O.

(H. Mäder, personal communication)

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Conclusions

Ammonia does not show any propensity to behave as a proton donor in complexes with ethers.

Ammonia acts primarily as a proton acceptor in its complexes with alcohols. However, it can play a protic wire role when the partner molecule has a favourable configuration (glycidol). In this case an incipient charge transfer effect is observed.

DIFFERENTLY RESPECT TO WATER

SIMILARLY TO WATER

Ammonia forms σ rather than complexes with fluoroderivatives of benzene.

In all cases, plenty of data on the internal dynamics