OBSERVATION OF VIBRATIONALLY HOT CH2CHO IN THE 351 NM PHOTODISSOCIATION OF XCH2CH2ONO

(X=F,Cl,Br,OH)

Rabi Chhantyal-Pun, Ming-Wei Chen, Dianping Sun, Terry A. Miller

Motivation• XRONO are chemical precursors for XRO (alkoxy) radicals • OH substituted alkoxy radicals are important intermediate in atmospheric

oxidation of alkenes like ethene, butadiene and isoprene • HOCH2CH2O radical is a prototypical hydroxyalkoxy radical

• Halogen substituted ethoxy can be a model for the study of HOCH2CH2O radical

Ethene Emission sources

74% Natural sources

26% Human made

Total emission: 18-45 Tg/yr

Ethene sink processes

89% OH

8% O3

3% to the Stratosphere

S. Sawada and T. Totsuka, Atmos. Environ. 20, 821 (1986)

C C

H

H

H

H

OH

C CH

H H

O

H

H

C CH

H

O

H

O

H

H O

O2

C CH

H

O

H

O

H

H

NO

Experimental technique

~ ~• Laser Induced Fluorescence (LIF) method has been used in the past to

study the B-X transition of alkoxy radicals• LIF coupled with supersonic free jet expansion produces rotationally cold

spectrum; high resolution LIF spectrum can be used to obtain rotational constants and geometry of alkoxy radicals.

Gopalakrishnan et. al. JCP 118 4954

1-Propoxy

XCH2CH2ONO/ He

General Valve ControllerDG535 Pulse Generator

XeF Excimer Laser

Nd:YAG Laser Sirah Dye Laser

Nozzle

T0

PMT

Q-Switch

Flash Lamp

T0 / GPIB

T0

Lens

Frequency Doubler

Precursor preperation: XCH2CH2OH/H2SO4/NaNO2

Experimental apparatus

XCH2CH

2O

NO

ClCH2CH2ONO / FCH2CH2ONO

0

10000

20000

30000

40000

50000

60000

70000

80000

Inte

nsi

ty a

.u.

ClCH2CH

2ONO

27000 27500 28000 28500 29000 29500 30000 30500 31000 31500 320000

30000

60000

90000

120000

150000

180000

210000

240000

Inte

nsi

ty a

.u.

wavenumber

FCH2CH

2ONO

1Gopalakrishnan et. al J. Chem. Phys. 118 (2003) 49–542 MSS 2011 FE09

HCHO

HCHO

Alkoxy Exp.1-Propoxy G1 286341-Propoxt T1 29219

FEO-G2 29869FEO-T2 30519ClEO-G 28786ClEO-T

FCH2CH2OAlkoxy CO str.Ethoxy 603

Prpoxy-G 596FEO-G 604

546

A

EC

D

BG

F

Unknown species identifiedL. R. Brock and E. A. Rohlfing JCP 106 10048 (1997)

-ClCH2CH2ONO

28500 28750 29000 29250 29500 29750 30000 30250 30500

Inte

ns

ity

(a

.u.)

Frequency (cm-1)

HCHO from different XCH2CH2ONO

28290 28300 28310 28320 28330 28340 28350

freq / cm-1

5K 4K 3K 2K 1K

Simulation Experiment

28290 28300 28310 28320 28330 28340

Frequency (cm-1)

F Cl Br OH

28770 28773 28776 28779 28782 28785 28788 28791 28794 28797

Inte

nsity

(a.

u.)

Frequency (cm-1)

28770 28773 28776 28779 28782 28785 28788 28791 28794 28797

Inte

nsity

(a.

u.)

Frequency (cm-1)

Cl

Br

F

OH

Vinoxy from XCH2CH2ONO

Exp.

Sim.

-Larger power broadening in F and OH substituted nitrite due to higher laser power used (to overcome lower S/N)

Rotational temperature

• HCHO and CH2CHO fragments show similar rotational temperature pattern for different substituted nitrites.

Rotational temperature (K)

Precursor HCHO CH2CHO

HOCH2CH2ONO 2.0 1

BrCH2CH2ONO 2.6 <2

ClCH2CH2ONO 4.5 4

FCH2CH2ONO 7.5 5

28400 28500 28600 28700 28800

Inte

nsi

ty (

a.u

.)

Frequency (cm-1)

a

g

f e

dc bi

jh

k

*

*

F

Cl

Br

OH

Vibrational hot bands of Vinoxy from various XCH2CH2ONO

* Bands not assgined to Vinoxy

Hot band assignments

Exp.Mode Description X B B-X

7 C1H1H3 rock 1143 1122 -218 C1C2 st 957 917 -409 C1C2O bend 500 449 -51

12 C1C2 torsion 404 274(x2=548) -130

L. R. Brock and E. A. Rohlfing JCP 106 10048 (1997)

Relative number density of HCHO/Vinoxy

φ = Quantum yield for fluorescence = Probability of transition N = Number density S = Fluorescence signalF = FormadehydeV = Vinoxy

R. G. Miller and E. K. C. Lee J. Chem. Phys. 68, 4448, (1978) (φ for HCHO)L. R. Brock and E. A. Rohlfing J. Chem. Phys. 106, 10048, (1997) (φ for Vinoxy, Lifetime for B state)D. T. Co et. al. J. Phys. Chem. A 109, 10675, (2005) (σ for HCHO)J. M. F. V. Dijk et. al. J. Chem. Phys. 69, 2453, (1978) ( value for HCHO)CASSCF/ 6-31G(d,P) ( value for Vinoxy)theo.

exp.

Precursor SV/SF (LIF signal)

NV/NFX103 (exp.)

NV/NFX103 (theo.)

HOCH2CH2ONO 0.29±0.01 0.09 0.03

FCH2CH2ONO 1.49±0.22 0.48 0.13

ClCH2CH2ONO 4.55±0.45 1.46 0.40

BrCH2CH2ONO 9.09±1.36 2.91 0.79

Photo-fragments formation mechanism

C C O

N

O

X

H

H H

H

C C OX

H

H H

H

N O

C C

H

H

H

O

C

H

H

XC

O

H H

351 nm

H X

351 nmC

C

O

NO

H

H

H

H

C C

H

H

H

O

X

N O

Rxn 1

Rxn 2

Rxn 3

HCHO formation (Rxn 1)

Radical Calculated barrier2 for Rxn I

HOCH2CH2O 9.2

FCH2CH2O 15.1

ClCH2CH2O 14.2

BrCH2CH2O 15.9

-1J. Heicklen Advances in Photochemistry Volume 14 Pg. 177-2CBS-QB3 method (most stable confomeric geometry used)

• RO-NO BDE: 40 kcal/mol1 • Maximum energy available after 351nm photo-

dissociation: 41.5 kcal/mol• Dissociation barrier for Rxn 1: ~15 kcal/mol• HCHO should be formed primarily due to the

secondary dissociation of XCH2CH2O radical

Vinoxy formation (Rxn 2 and Rxn 3)

• Calculation of barrier for HX elimination from XCH2CH2O (Rxn: 2)

• Preliminary result from collaboration with Laurie Butler group shows the internal energy left over in the radical would be less than the barrier calculated

• Non classical mechanisms like roaming could be in effect (Roaming have been observed in a similar radical HOCH2CH2 dissociating to H2O and CH2CH)2

• Calculation of barrier for Rxn 3 (HX elimination from XCH2CH2ONO) is ongoing

Radical Calculated barrier1 for Rxn 2

HOCH2CH2O 36.13

FCH2CH2O 39.74

ClCH2CH2O 38.2

BrCH2CH2O 34.62

1B3LYP/6-311+g(d,p) method (most stable confomeric geometry used)2 E. Kamarchik et. al. J. Phys. Chem. Lett. 1, 3058, (2010)

Conclusion• CH2CHO and HCHO fragments are produced

following 351nm photo-dissociation of XCH2CH2ONO (X= OH, F, Cl, Br); CH2CHO fragments produced are vibrationally hot

• HCHO is produced from the ground state dissociation of XCH2CH2O radical

• CH2CHO formation mechanism is still being studied

AcknowledgementMiller group members

-Prof. Terry Miller-Dr. Dmitry Melnik-Dr. Dianping Sun-Dr. Mourad Roudjane-Dr. Takashige Fujiwara-Neal Kline-Terrance Codd-Meng Huang