Current teamMikhail Ryazanov Dr. Chirantha Rodrigo

Overtone-induced dissociation and isomerization of the hydroxymethyl (CH2OH) radical

First team:Lin Feng Dr. Jie Wei, Boris Karpichev

Support: US Department of Energy

CH2OH + 4nOH CH2O + H

OSU 66th MSS, June 2011

Theory collaboratorsDr. Eugene Kamarchik, Prof. Anna Krylov, Prof. Joel Bowman

CH2OH radical: dissociation on the ground state

11,600

~ 15,000 cm-1

~ 14,000

~3,000

0

Wav

enu

mb

ers 1

03 cm

-1

2

1

(X 2A″)CH2OH

CH3O

H + CH2O (X)~

H + CH2O (X)~

~

1

2

3

4

5

O-H

IR

10,484

13,600

~16,000

7,158

3,675

Previous work on the hydroxymethyl radical

•Characterized ionization and excited Rydberg states, including conical intersections

• Studied excited state photochemistry

Demonstrated the breaking of the weak O-H bond in CH2OH following IR overtone excitation

Today: direct dissociation dynamics vs. isomerization

CH2OH IE = 7.56 eV

Feng, Wei, and Reisler, J. Phys. Chem. A, 108, 7903 (2004).Wei, Karpichev, and Reisler, J. Chem. Phys. 125 (3): 34303-34303 (2006).

Challenges

– OH-stretch is the reaction coordinate for direct OH bond cleavage: How high can we go?

– Isomerization vs. direct O-H fission; relative barrier heights

– OH-overtone pumping is always hard: CH3OH, NH2OH, HOOH, etc. (Crim, Rizzo, Perry, etc.);

– Radical is a minor species and hard to make

– CH2OH has low-lying electronic states: Will we get substantial vibrationally mediated dissociation?

Detection of CH2OH via 1+1 REMPI

10

30

40

0

Wav

enu

mb

ers 1

03 cm

-1

(X 2A″)~

CH2OH

CH3OH + CH2O (X)

~

14,000

11,000

15,000 cm-1

H + CH2O (X)~

Cs3pz (

2A")

3px

3s = 26,000 cm-1

Barrier heights:Radom, Schaeffer;Harding, Klippenstein;Temps;Walch…

OH stretch vibration: 1-3nOH

-10 -5 0 5 10 Energy - 0 (cm-1)

OH

= 3674.8 cm-1, = 0.4 cm-1

3OH

= 10484.3 cm-1, = 0.4 cm-1

2OH

= 7158.0 cm-1, = 0.8 cm-1

3nOH ; 10484 cm-1; linewidth = 0.4 cm-1

2nOH ; 7158 cm-1; linewidth = 0.8 cm-1

1nOH; 3678 cm-1; linewidth = 0.4 cm-1

T = 10-13 K

10470 10475 10480 10485 10490 10495 10500

Simulation qP(N") qQ qR(N")

5 4 3 2 1 0 1 2 3 4 5(b)

Inte

nsi

ty (

a.u.

)

Pump Frequency (cm-1)

Pump on Pump off

(a)

CH

2O

H+ (

a.u

.)

Simulation of 2nd overtone spectrum:a/b–type transition

Wei, Karpichev, Reisler, J. Chem. Phys.(2006)

No H atom detected

3OH ;

Laser linewidth: 0.1 cm-1

Observed linewidth = 0.4 cm-1

New imaging arrangement: Slice imaging of H photofragments

• Lenses in field free region control VMI conditions and image size• 5 ns FWHM pulser allows slicing of H fragment

Design based on: A. Suits, K. Liu

Br

Br*

3685 cm-1

5ns

X10 improvement in resolution and sensitivity

CH2OHH-photofragment yield spectrum in the region of 4nOH

Accidental resonance between 4nOH and 3nOH + 1nasymCH

[3nOH + 1nasymCH ]*731.75 nm13662 cm-1

[4nOH]*735 nm13602 cm-1

Methanol overtone spectrum shows a similar accidental resonance between 5nOH and 4nOH + 1nasymCH . [Rizzo, Perry, Boyarkin]

300

250

200

150

100

50

H+ s

igna

l (co

unts

)

13680136601364013620136001358013560

vac (cm

–1)

GroundExcited

A /cm–1 6.51 6.0

B /cm–1 1.01 1.01

C /cm–1 0.88 0.88

90% : 10% : 0% (a:b:c)

T: 13 K

Linewidths:Laser: 0.1 cm-1

Lorenzian width (FWHM) Strong band: ~ 1.6 cm–1

Weak band: ~ 0.6 cm–1

bright state

dark states

moleculareigenstates

spectrum

Coupling between a zeroth-order bright state and a dark state

H product

GroundExcited

A /cm–1 3.78 3.56

B /cm–1 0.84 0.83

C /cm–1 0.69 0.73

80% : 20% : 0% (a:b:c)

T: 13K

D product (small)

H+ s

igna

l (co

unts

)

13680136601364013620136001358013560

vac (cm

–1)

300

250

200

150

100

50

H+ s

igna

l (co

unts

)

13680136601364013620136001358013560

vac (cm

–1)

D signal is is very small4nOH =13621 cm-1

No other bands observed

Lorenzian Linewidths: Both bands: ~ 1.6 cm–1

H and D products from CD2OH

0 500 1000 1500 2000 2500 3000 35000

5

0

C–H symC=O

CH2 scis

CH2 wag

C–H asym CH2 rock

C–H sym

C=O

CH2 scis

CH2 wag

C–H asym

CH2 rock

Inte

nsi

ty,

a.u

.

KER, cm

0.0

0.5

From maximum KER :D0 = 10,166 ± 70 cm-1

Compared to calculated D0 = 10,188 cm-1

(Marenich and Boggs)

Vibrational labels of CH2O co-fragment are marked. Excited levels = 8%.Rotational temperature of CH2O co-fragment: ~ 140 K

CH2OH: Slice image of H-photofragments from 4nOH peak

0

C–H symC=O

CH2 scis

CH2 wag

C–H asym CH2 rock

C–H sym

C=O

CH2 scis

CH2 wag

C–H asym

CH2 rock

0 500 1000 1500 2000 2500 3000 35000

1

Kinetic Energy Release, cm-1

Inte

nsit

y, a

rb. u

nits

0.0

0.1

CH2OH + hn → H + CH2O, n = 13662 cm-1

KER of H-photofragments from 3nOH + 1nasymCH peak

Vibrational labels of CH2O co-fragment are marked. Excited levels = 13%.

0

C–D symC=O

CD2 scisC–D asym

CD2 rock

CD2 wag

0

C–D

sym

C=

O

CD

2 scis

C–D

asym

CD

2 rock

CD

2 w

ag

0 500 1000 1500 2000 2500 3000 35000

1

Intensity, arb. units

Kinetic Energy Release, cm1

CD2OH: Monitoring H + CD2O hn = 13621 cm-1

0.04

0

C–H symC=O

CH2 scis

CH2 wag

C–H asym CH2 rock

C–H sym

C=O

CH2 scis

CH2 wag

C–H asym

CH2 rock

0 500 1000 1500 2000 2500 3000 35000

1

Kinetic Energy Release, cm1

In

ten

sity

, arb

. u

nits

0.00

0.05

CH2OH

0

C–D symC=O

CD2 scisC–D asym

CD2 rock

CD2 wag

0

C–D

sym

C=

O

CD

2 scis

C–D

asym

CD

2 rock

CD

2 w

ag

0 500 1000 1500 2000 2500 3000 35000

1

Intensity, arb. units

Kinetic Energy Release, cm1

CD2OH

Monitoring H

0.04

Formaldehyde cofragment:No CH stretchRatio of CO str/CHH(D) bends is larger in CD2OH

Higher rotational and CHD vibrational excitation; No CO stretch

0

C–H C–D

C=O

CHD scis

CHD rock

CHD wag

0 500 1000 1500 2000 2500 3000 35000

1

In

ten

sity

, arb

. u

nits

Kinetic Energy Release, cm1

CD2OH: Monitoring D + CHDO hn = 13621 cm-1

Summary

The OH overtone spectrum of CH(D)2OH has been characterized in the 4nOH

region. Mixed levels (most likely involving levels 4nOH and 3nOH + 1nasymCH ) are seen in CH2OH but not in CD2OH.

Dissociation, probably by tunneling through the O—H barrier, has been demonstrated.

The observation of formaldehyde levels with skeletal CH(D)excitation appears to be associated with isomerization, whereas C=O stretch excitation accompanies O-H bond fission.

Isomerization yield is < 10%. It appears to be the result of statistical IVR due to weak high-order resonance couplings.

Electronic structure and dynamical calculations are in progress in collaborative work with Dr. Eugene Kamarchik, Joel Bowman (Emory) and Anna Krylov (USC).

10

0

Wav

enu

mb

ers 1

03 cm

-1

H + CH2O (X)~

H + CH2O (X)~

Calculated Geometries; CCSD(T)/aug-cc-pVTZ Dr. Eugene Kamarchik

355nm

Photolyticaly initiated reaction

Cl2 2Cl

Cl + CH3OH CH2OH + HCl k1 = 3.7x1013 (cm3.mole-1.s-1)

Radical production

CH2OH + Cl CH2O + HCl k2= 4x1014 (cm3.mole-1.s-1)…..

+

CH3OH/Cl2/He CH2OH

355nm

CH2OH diagnostics:3pz REMPI or H-photofragment spectrum

0nlmπco*

3pz (22A”) 12A”

C-O Stretch (n6) Progression00 1

60

260

C-O

Detection schemes for IR spectroscopy

Double Resonance

CH2OH (X2A")

CH2OH+

3pz

Probe

PumpnOH

Scheme 2

Or: Look directly at H atom when the radical dissociates

39800 40000 40200 40400 40600

60

1

80

2

13

390

2

Pump on Pump off

CH

2OH

+ sig

nal (

a.u.

)

2-photon probe frequency (cm-1)

Birge-Sponer plot

OH stretch AB, Anharmonicity

CH3OH 3769 86.1

CH2OH 3766 91.4

NH2OH 3743 90.6

HOOH 3701 90.5

potential

Anharmonic oscillator

/ = A – B

0 1 2 3 4 53300

3400

3500

3600

3700

/

(cm

-1)

Vibrational level,

Ground state

A=3766.3±2.7cm-1

B=91.4±1.0 cm-1

Dissociation barrier > 4ν1H-atom generated via tunneling

Tunneling: Eckart Potential

• Linewidth of 1.3 cm-1 corresponds to a tunneling probability of 0.2%; Imaginary frequency= 1712 cm-1 (Larry Harding)

• Dissociation barrier height of 15,200 cm-1

• Theoretical estimation: 14000 – 16000 cm-1

CH2OH

9557

cm-

1

1359

8 cm

-1 ?1712 cm-1

CH2O + H

13000 13500 14000 14500 15000 15500

1E-3

0.01

0.1

1E=E(4

1),13600 cm-1

D0(CH

2O-H)=9557 cm-1,

c=1712 cm-1

Tunneling probability

P=0.5, E0=13 600 cm-1

P=2*10-3, E0=15 200 cm-1

Tu

nn

elin

g p

rob

ab

ility

Barrier, E0, (cm-1)

Next goal: Excite above barrier to dissociation

11,000

16,000cm-1

14,000

~2,000

0

Wav

enu

mb

ers 1

03 cm

-1

2

1

(X 2A″)CH2OH

CH3O

H + CH2O (X)~

H + CH2O (X)~

~

1

2

3

4

5

O-H

IR

11,000

14,600

18,000

7,158

3,675

735 nm 732 nmmode descr. freq. rel. pop. (fit)

- ground 0 1 11 C-H sym. 2783 - -2 C=O 1746 0.0203 0.02923 CH2 bend 1500 0.0187 0.02934* CH2 wag 1167 0.0291 0.05615 C-H asym. 2843 - -6* CH2 rock 1249 0.0166 0.0348

% excited (fit) 7.8 13% excited (int.) 7.7 12.8

T_rot (fit), K 139 158

bright state

dark states

moleculareigenstates

spectrum

Coupling between a zeroth-order bright state and a bath of dark states

Exciting the 3rd overtone, 4n1: Monitoring H photofragments from CH2OH and CD2OH

13600 13610 13620 1363013580 13590 13600 13610Pump frequency (cm-1)

= 1.3 cm-1

H+

sign

al (

a.u.

)

Pump frequency (cm-1)

Pump-on Simulation

CH2OH

= 1.3 cm-1

Pump-on Simulation

CD2OH

CH2OH and CD2OH have the same linewidth: 1.3 cm-1

Could not detect deuterium from CD2OHIsomerization is at best a minor channel.