Collision‐Induced Intersystem Crossing in CH₂ (ã ¹A₁): A Quantitative Analysis Using the Mixed‐State Model

Abstract

Carbenes / Chemical Kinetics / Energy Transfer / Intersystem Crossing Laser Magnetic ResonanceThe mechanism of collision-induced inte_rsystem crossing (ISC) in CH2 from the % 'A, first excited singlet to the 3BI triplet ground electronic state, CH, (2) + M -t CHI ( X ) + M, is investigated. Collisional ISC rate constants are calculated for four selected collider gases, M = He, Ar, N2, and CH4, using the "mixed-state" model of Freed and Gelbart. The most important triplet-perturbed rotational levels in the %-state are identified from the spectroscopic data reported in the literature. The rates of rotational relaxation processes, Yhich according to the model effectively govern the collision-induced ISC, are determined by studying the collision broadening of CH2 (X) Far Infrared Laser Magnetic Resonance (FIR-LMR) transitions. The collisional ISC rate constants for CH2 in the ortho nuclear spin modification are calculated to be klsc = 1.4 . lo'*, 4.0 . and 6.6 . 10l2 cm3/mol . s for M = He, Ar, N2, and CH4, respectively. The results are in good agreement with published experimental rate constants.7.8 .

Temperature Dependences of Singlet Methylene Removal Rates

“…The most successful model for CIISC in methylene is the “mixed-state” model of Gelbart and Freed . It attributes the rate for this process to relaxation within the triplet manifold from states of mixed singlet−triplet parentage. , Thus the rate constants for CIISC are largely those for rotational relaxation within the triplet and are relatively insensitive to the collision partner. Quantitative analysis of this mixed-state model for 1 CH 2 to 3 CH 2 transfer without the need for adjustable parameters has been carried out by Bley et al , Hancock and co-workers 9,10 have considered the effect of temperature on the mixed-state model estimates of the CIISC rate constants for Ar collider and compared the results with their experimental observations.…”

“…It attributes the rate for this process to relaxation within the triplet manifold from states of mixed singlet−triplet parentage. , Thus the rate constants for CIISC are largely those for rotational relaxation within the triplet and are relatively insensitive to the collision partner. Quantitative analysis of this mixed-state model for 1 CH 2 to 3 CH 2 transfer without the need for adjustable parameters has been carried out by Bley et al , Hancock and co-workers 9,10 have considered the effect of temperature on the mixed-state model estimates of the CIISC rate constants for Ar collider and compared the results with their experimental observations. They pointed out that provided the redistribution of population within the singlet manifold is fast and that vibrational relaxation within the triplet manifold is not rate determining, then the effect of temperature will simply be to alter the relative proportion of singlet molecules in the perturbed levels and to change the rotational relaxation rate constant out of the perturbed triplet levels.…”

Influence of Temperature on the Removal Rates of CH2(ã1A1 ) by Inert Gases and Hydrocarbons

“…The error limits for the *CH 2 reactions are 3 cr of the In k versus 1/T plots, because of the small number of points. Those for the *NH and 2 CH reactions are quoted by the authors of [42,45] as 2 o. Rate constant measurements for chemical reactions of 0( X D 2 ) were performed at 200-350 K. The second order rate coefficients for H 2 and CH 4 , and also for NH 3 , H 2 0, and HCl were found to be independent of temperature in these experiments [35,46].…”

“…[42]) were carried out within similar temperature domains. The error limits for the *CH 2 reactions are 3 cr of the In k versus 1/T plots, because of the small number of points.…”

Singlet Methylene Removal Rate Constants from the (0,1,0) Vibrational Level: Enhancement via Complex-Mediated Vibrational Relaxation

“…Since CIISC is the only channel available for these partners, this observation shows that (a) the rate constant for CIISC is the same for the (0,0,0) and (0,1,0) states, and (b) vibrational relaxation from (0,1,0) to (0,0,0) is inefficient compared with CIISC for these species. Conclusion (a) is expected to apply to other species since the most successful model for CIISC rates in methylene is based on the "mixed state" model of Gelbart and Freed which associates this process with rotational relaxation within the triplet manifold [42][43][44][45][46]. Thus if the CIISC rates are the same for the (0,0,0) and (0,1,0) levels for He and Ar, they are likely to be so for other species.…”