Gas-phase IR spectroscopic studiesof polar neutral mass-selected complexes
A new infrared spectroscopy technique for structural studies
of mass-selected neutral polar molecules or complexes
(without chromophore), using dipole-bound anion formation.
Charles DesfrançoisJ.C. Gillet, F. Lecomte, G. Grégoire, J.P. SchermannLab. de Physique des Lasers, U. Paris-Nord, France
Dipole-bound anions: « neutral » ionsDipole-bound anions: « neutral » ions
D
dipolar attraction-µ/r2 -Q/4r3 -/2r4
repulsion
bound state
distance e- - dipole
very diffuseorbital
pote
ntia
len
ergy
r ~ 10-100 Å
pot
enti
al e
nerg
y
inter- or intra- molecular coordinate
Eb ~ 0.01 eV
M
M-
e-M
Dipole-bound anion formation: Rydberg Electron Transfert (RET)Dipole-bound anion formation: Rydberg Electron Transfert (RET)
Collision region
Atomic Xe beam Electron gun:metastable Xe
Ion detectorMass spectrum
Electrostatic lens
Field-detachment grids
Molecular or cluster supersonic beam: valve + oven + carrier gas
Extraction andacceleration grids
Pulsed dye laser460-540 nm; n = 6-50
Anion time-of-flight
Xe(nf) + M(µ) Xe+ + M-(Eb) we measure k(M-) as a function of n
k (M-)
The RET technique is selective with respect to the excess electronbinding energy Eb that depends on the total dipole moment µ andthus on the neutral parent geometry.
A simple system: formamide – water complexA simple system: formamide – water complex
isolated formamide: µ = 3.9 D; Eb
th = 15 meV Eb
exp = 16 meVformamide-water complex:lowest configurationµ = 2.7 D; Eb
th = 2.8 meV Eb
exp = 3.1 meV
10 12 14 16 18 20 22 24 26 28 30 32 340,0
0,2
0,4
0,6
0,8
1,0
1,2
isolatedformamideEb = 16 meV
formamide waterEb = 3.1 meV
Rel
ativ
e an
ion
form
atio
n ra
tes
Rydberg quantum number n
µ(D)
177 6.46
3.94
D0
(meV)
Ebexp
= 3.1 meV
113
0 2.69
Ebth
= 2.8 meV
Ebth
= 30 meV
Ebth
= 130 meV
Less simple: N-methylformamide - water complexLess simple: N-methylformamide - water complex
5 10 15 20 25 300
1
2
3N-Methylformamide-water Eb = 29 and 3.5 meV
Rel
ativ
e an
ion
form
atio
n ra
tes
Rydberg quantum number n
µ(D)
100 6.25
4.36
De
(meV)
12
0 4.02
Ebth
= 39 meV
Ebth
= 28 meV
Ebexp
= 29 and 3.5 meV
Ebth
= 125 meV
µ(D)
1114.45
De
(meV)
03.24
Ebth
= 4.2 meV
Ebth
= 29 meV
trans NMF cis
Need for more experimental data !
Coupling dipole-bound anion formation and IR Coupling dipole-bound anion formation and IR spectroscopy spectroscopy
• Dipole-bound anion formation by RET is a unique ionisation method that is almost totally non-perturbative: almost no internal energy is provided upon ionisation. It allows rigorous mass-selection and partial structure-selection. It is an alternative technique to REMPI when polar molecules without chromophore are involved.
• If resonant IR absorption occurs, for C-H N-H or O-H bonds, the neutral molecule or complex will acquire a lot of internal energy (2800-3800 cm-1) and then:* molecular dipole-bound anions will autodetach* neutral non-covalent complexes will predissociateIn both cases, anion signal will decrease.
Rydberg Electron Transfer (RET) + IR SpectroscopyRydberg Electron Transfer (RET) + IR Spectroscopy
Typical anion signal: 0.1/laser shotRydberg laser: 20 Hz; IR OPO laser: 10 HzReal-time anion signal depletion but still shot-to-shot fluctuations
h = 0.42 eV
IR laser
D0 = 0.28 eVneutral vibrationalpredissociation
dipole-bound anionautodetachment
IR OPO laser ~ 100 µs before the dye laser Scan: 2500-4000 cm-1
Atomic Xe beam Electron gun:metastable Xe
Ion detectorMassspectrum
Electrostatic lens
Field-detachment grids
Molecular or cluster supersonic beam: valve + oven + carrier gas
Extraction andacceleration grids
Pulsed dye laser460-540 nm; n = 6-50
Anion time-of-flight
: IR laser
IR spectrumwater dimer
MP2 / 6-31++G(2d,2p) calculations: full bars: absolute anharmonic valuesdash bars: scaled harmonic values
3500 3550 3600 3650 3700 3750-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
?
freeOH
bonded OH
rela
tive
anio
n si
gnal
dep
letio
n
frequency (cm-1)
sym.
IR spectrum formamide -water complex
full bars: absolute anharmonic valuesdash bars: scaled harmonic values
: IR laser2850 2900 2950
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
free CH
rela
tive
anio
n si
gnal
dep
letio
n
frequency (cm-1)
3400 3450
bondedNH / OH
3500 3550
?
free NH
3700 3750
free OH
MP2 / 6-31++G(2d,2p) calculations:
IR spectrumNMF – waterhigh dipoleconformers
full bars: absolute anharmonic valuesdash bars: scaled harmonic values
MP2 / 6-31++G(2d,2p) calculations:
red
blue
Isolated formamide and dimer
2800 2900 3000 3100 3200 3300 3400 3500 3600-0,6
-0,4
-0,2
0,0
0,2
NHasym.
NHsym.
CH
rela
tive
anio
n si
gnal
dep
letio
n
frequency (cm-1)
2800 2900 3000 3100 3200 3300 3400 3500 3600
-0,4
-0,2
0,0
0,2freeNH
bonded NH
CH
rela
tive
anio
n si
gnal
dep
letio
nshifts Exp. Th. anh.
Th. harm. scaled
NH free -39 -36 -47
NH bonded
-220
-260-269 -240
CH
-50 ?
+160 ?+15 +23
vibrational autodetachment is less efficientthan vibrational predissociation
Formamide
Dimer
F: dipole-boundµ = 3.72 DEb = 16.5 meV
F2: quadrupole-boundµ = 0 D; Q = +48 DÅEb = 11.3 meV
D0 = 0.47 eV
Ongoing workOngoing work
• To improve the dipole-bound anion signals: Rubidium Rydberg source (2-photon excitation).
• To switch to a laser desorption source for the molecular beam.
• To extend the IR OPO to the 4-10 µm region (AgGaSe2).
Calculations of glycine – water Calculations of glycine – water clusters :clusters :
neutrals, zwitterions and anions neutrals, zwitterions and anionsCharles Desfrançois, Lab. de Physique des Lasers, Charles Desfrançois, Lab. de Physique des Lasers,
Université Paris 13Université Paris 13Sungyul Lee, Kyung Hee University, KoreaSungyul Lee, Kyung Hee University, KoreaGoals
To understand Bowen’s and Johnson’s PES results:DB anions for GWn, n=0,1,2 with Eb= 0.095, 0.195, 0.14/0.33
eVThreshold for stable (valence ?) anions: 4-5 waterVDE = 0.62 eV EAad = 0.4 eVTo follow the hydration transition from neutrals to zwitterionsIs it 4-5 water molecules (Bowen exp.) ? Or 7-8 (Gordon calc.)To check for dipole-bound anions vs “zwitteranions”What are the dipole moments of the lowest neutrals ?
PES dataPES data
EA ~ 0.4 eVVDE ~ 0.6 eV
0.4 eV
Gly- VDE = 95 meV
Gly-W2
VDE = 0.14 eVVDE = 0.33 eV
Gly-W1
VDE = 0.195 eV
MethodsMethodsSearch for G(H2O)n equilibrium structures and energiesFor GI, GII, (GIII), GZ and GZ- and for n = 0, 1, 2, 3, 4, 5…
Use of home-made force-field and genetic algorithm (MINGEN)
In order to get starting structures within 0.2 eV above the minimum.
B3LYP/6-31++G** calculations with full optimizationsGood enough to obtain a good H-bond representation and rather
accuratevalence anions energies (anion stabilities may be overestimated).
MP2/6-31++G** full optimizations on all B3LYP minimaIn order to check for energy ordering, especially for valence anions
Use of semi-empirical calculations for dipole-bound anionsSame program and same parameters as for previous studies: dipolemoment, Q moments, polarizabilities, empirical repulsive parameter;Cylindrical symmetry; angular algebra, 1D Schrodinger equation.
µ = 5.7 DEb= 100±15 meV
µ = 1.2 D
E = 0.045 eV
GZunstable
µ = 11.2 D µ = 10.5 D
E = 1.02 eV
GIIsecond
GIlowest
E = -0.52 eV
1.82.0
2.81.9
GZ-
stable
GIW1
µ = 2.1 D, E = 0
most stable structure
µ = 1.8 D, E = 0.12 eV
µ = 2.4 D, E = 0.14 eV1.8
2.1
2.22.01.9
third structure
second
No DBA
second
GIIW1
µ = 3.9 DE = 3 meVEb= 16 meV
µ = 8.8 D; E = 58 eVEb= 300 meV
2.1
1.9
µ = 3.9 DEb= 20 meV 2.3
2.4
2.5
2.1
µ = 5.7 DE = 57 eVEb= 90 meV
1.91.8
2.7
most stable
fourth
third
Ebexp= 195 meV
GZW1 neutralAll starting structuresare not stable: they alldecay towards GIIW1µ = 9.3 D
De= 0.70 eVstable only at the HF level
2.0
2.0
most stable
2.2
2.0
1.8
GZ- W1 anion
1.75
3.0
µ = 14.2 D
VDE ≈ 0.6 eV
second stable
2.0
GIW2
µ = 1.7 D, no DBA
most stable structure
µ = 3.3 D, E = 0.18 eV, Eb= 7 meV
µ = 4.0 D, E = 0.14 eV Eb= 20 meV
1.7
1.9
2.22.0
1.9
third structure
second2.1
1.81.8
second
GIIW2
µ = 2.3 DE = 5 meVEb= 0.1 meV
µ = 3.4 DE = 13 meVEb= 6 meV 1.9
1.9
µ = 3.4 DEb= 6 meV
2.0
2.2
2.2 2.15
µ = 3.6 DE = 11 meVEb= 24 meV
2.1
1.852.1
most stable
fourth
1.9
1.9
1.85
third
6th: µ = 6.8 D; E = 0.38 eV Eb= 120 meV
7th: µ = 8.3 D; E = 0.41 eV Eb= 200 meV
Ebexp= 140/330 meV
GZW2 neutrals5 stable structures
µ = 7.6 D De= 1.55 eV
1.8
1.8
1.8
1.8
1.8
1.8µ = 7.6 DE = 0.07 eV
1.75
1.7 1.75
1.7µ = 6.6 DE = 0.04 eV
1.81.8
µ = 10.1 DE = 0.26 eV
1.9
2.1
1.91.7
µ = 7.2 DE = 0.11 eV
1.71.76
1
5
2 3
4
GZ- W2 anions
µ = 15.5 D; E/GII = 0.170 eV
2.0
2.0
1.8
1.9
1.8
2.0
E = 0.03 eV
1.76
1.9 1.9 2.2
E = 0.03 eV
1.9
2.1 2.1
1.8
1.9
1.9
E = 0.04 eV
12
3 4
VDE ≈ 0.72 eV
Relative stabilities of the 4 Relative stabilities of the 4 conformersconformers
µ = 2.1 DD0= 0.36 eV
µ = 1.7 DD0= 0.80 eV
µ = 1.6 DD0= 1.14 eV
µ = 3.4 DD0=1.49 eV
µ = 2.4 DD0= 1.81 eV
GIWn: water clusters bound to COOH
µ = 2.3-3.4 DD0= 0.61 eV
µ = 2.9-3.2 DD0= 1.02 eV
µ = 4.9 DD0= 1.72 eV
µ = 3.3-4.4 DD0= 1.38 eV
GIIWn: water chains between NH and CO
µ = 3.9 DD0= 0.25 eV
µ = 2.6 DD0= 1.73 eV
µ = 7.0 DDe= 2.16 eV
µ = 4.2-4.4 DDe= 3.29 eV
µ = 4.1 DDe= 2.73 eV
GZWn: water between NH3+ and COO-
µ = 7.6 DDe= 1.55 eV
D0= 2.25 eV
GZ-Wn: water clusters on COO-
D0= 1.86 eV
D0= 0.55 eV
D0= 1.01 eV
D0= 1.45 eV
-
NH4 withno fieldIP = 4.8 eV
NH4 in the field of apoint -e charge at ~ 3 AIP = 1.0 eV (µ = 14 D)
VDE = 0.30 eVCCSD(T): 0.39 eV
VDE = 0.59 eV VDE = 0.75 eV
VDE = 0.90 eV VDE = 1.00 eV VDE = 1.12 eV
MP2 calculations
Provisional conclusionsProvisional conclusionsUp to n = 5, lowest neutral structures correspond to GIWn configurations
with rather low dipole moments for n = 1,2,3.No dipole-bound anions from cold neutrals up to n = 3. Bowen’s data ? But stable DBAs can be form from higher energy neutral GIIWn isomers.
The calculated Eb fit hardly with Johnson’s data for n = 1,2
At n = 4-5, GZ- Wn valence anions become more stable than the GIIWn
neutrals and their possible dipole-bound anions.Does this explain the exp. threshold for anions at n = 4-5 ?E between lowest GZ- Wn and corresponding GZWn:
For n = 4,5 VDEGZ-Wn ~ 1 eV (too high/0.6 eV measured by Bowen).
GZWn zwitterions are minima at n = 2 but become more stable than GIWn
and GIIWn neutrals only at n ≈ 7. See Gordon’s calculations.
GIIWn neutrals become more stable than GIWn neutrals probably only for
rather large clusters (n > 10).
A non-typical dipole-bound anion: water dimer A non-typical dipole-bound anion: water dimer (H(H22O)O)22
neutralgeometryangle = 120 ° µ = 2.6 D
Ebcalc = 20 meV
Ebth = 35 meV
Aniongeometryangle = 215 ° µ = 4.2 DEb
exp = 30 meV
The neutral geometry correspondsto a total dipole moment close to thethreshold for dipole-binding
In the anion, the geometryrearrangement increases thetotal dipole moment
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