Transition state vibrational level thresholds for the dissociation of triplet ketene
- Department of Chemistry, University of California at Berkeley and Chemical Sciences Division of the Lawrence Berkeley Laboratory, Berkeley, California 94720 (United States)
Rate constants for the unimolecular dissociation of ketene (CH[sub 2]CO) and deuterated ketene (CD[sub 2]CO) have been measured at the threshold for the production of CH[sub 2] ([ital [tilde X]] [sup 3][ital B][sub 1]) or CD[sub 2] ([ital [tilde X]] [sup 3][ital B][sub 1]) and CO ([ital [tilde X]] [sup 1][Sigma][sup +]) by photofragmentation in a cold jet. The rate constant increases in a stepwise manner as energy increases. This is in accord with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds at each energy level for vibrational motion orthogonal to the reaction coordinate. The first step in rate constant and/or photofragment excitation (PHOFEX) spectrum gives accurate values for the barrier to dissociation above the zero-point energy of the products, 1281[plus minus]15 cm[sup [minus]1] for CH[sub 2]CO and 1071[plus minus]40 cm[sup [minus]1] for CD[sub 2]CO. The measured rate constants are fit by Rice--Ramsperger--Kassel--Marcus (RRKM) theory. The vibrational frequencies at the transition state obtained from the fits are compared with [ital ab] [ital initio] results. Vibrational motions at the transition state orthogonal to the reaction coordinate are also revealed in CO product rotational distributions. Calculations using an impulsive model which includes vibrational motions at the transition state reproduce the experimental dependence of the PHOFEX spectra on the CO [ital J] state quite well. The small dependence of rate constant on jet temperature (4--30 K) indicates that the [ital K][sub [ital a]] quantum number for rotation about its symmetry axis is conserved in the energized ketene molecule.
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 6602005
- Journal Information:
- Journal of Chemical Physics; (United States), Journal Name: Journal of Chemical Physics; (United States) Vol. 102:8; ISSN JCPSA6; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400500* -- Photochemistry
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
CORRELATIONS
DISSOCIATION
ENERGY
ENERGY LEVELS
EXCITED STATES
HEAT RATE
IONIZATION
JETS
KETENES
MULTIPLETS
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOTOIONIZATION
QUANTIZATION
QUANTUM NUMBERS
ROTATIONAL STATES
THRESHOLD ENERGY
TRIPLETS
VIBRATIONAL STATES
YIELDS