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

pot e

n ti a

len

e rgy

r ~ 10-100 Å

pot

entia

l ene

rgy

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

0,4free 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.

CHre

lativ

e an

ion

sign

al d

eple

tion

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 anionsUse 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 1.9

1.8

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

2.02.0

most stable

2.2

2.0

1.8

GZ- W1 anion

1.753.0

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

second 2.1

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

1.91.9

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

1.81.8

1.81.8

1.81.8

µ = 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

1.92.1

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

1.82.0

E = 0.03 eV

1.76

1.9 1.9 2.2

E = 0.03 eV

1.9

2.1 2.11.8

1.9

1.9

E = 0.04 eV

1 2

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 configurationswith 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 = 1,2At 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 forrather 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