Comparison of models for treating angular momentum in RRKM calculations with vibrator transition states. Pressure and temperature dependence of Cl + C[sub 2]H[sub 2] association

• Published in: Journal of physical chemistry (1952) Vol. 97:2
• Main Authors: Zhu, L., Chen, W., Hase, W.L., Kaiser, E.W.
• Format: Journal Article
• Language: English
• Published: United States 14.01.1993
• Subjects: ALKENES; ANGULAR MOMENTUM; CHEMICAL REACTION KINETICS; COUPLING; DISSOCIATION; ENERGY LEVELS; ENTHALPY; ETHYLENE; EXCITED STATES; HYDROCARBONS; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; KINETICS; MATHEMATICAL MODELS; ORGANIC COMPOUNDS; PHYSICAL PROPERTIES; QUANTUM NUMBERS; RADICALS; REACTION HEAT; REACTION KINETICS; ROTATIONAL STATES; TEMPERATURE DEPENDENCE; THERMODYNAMIC PROPERTIES 400201 > Chemical & Physicochemical Properties; VIBRATIONAL STATES
• DOI: 10.1021/j100104a010
• ISSN: 0022-3654

The RRKM expression for the microcanonical unimolecular rate constant depends on how the K rotational quantum number is treated. The limiting cases are to treat K as either adiabatic or active. The former occurs when K is a good quantum number for describing the vibrational/rotational energy levels. The latter case arises when coriolis coupling extensively mixes the (2J + 1) K levels. It is not necessary that K be simultaneously active or adiabatic for both the transition state and reactant molecule. Thus, four models which explicitly treat K with its proper limits, i.e.[minus]J[le]K[le]J, can be formulated for the RRKM theory. For example, to fit the association rate constant at low pressure using the model with K adiabatic in the transition state and active for the molecule requires a value for the collision efficiency [beta][sub c] twice the size needed when K is treated as active in both the molecule and transition state. These results indicate that until the proper model for treating K is identified it will be difficult to determine unambiguous values for [beta][sub c] by fitting experimental data. To assist in interpreting the experimental kinetics for the Cl + C[sub 2]H[sub 2] [r arrow] C[sub 2]H[sub 2]Cl system, ab initio calculations were performed to determine the structure, vibrational frequencies, and energy for C[sub 2]H[sub 2]Cl. The ab initio G2 value for the O K heat for Cl + C[sub 2]H[sub 2] association is [minus]16.855 kcal/mol. To fit the low-pressure experimental rate constants for Cl + C[sub 2]H[sub 2] association in air requires values for [beta][sub c] which range from 0.4 to 1.0. However, none of the RRKM models for treating K provides an adequate fit to all the temperature- and pressure-dependent experimental rate constants. 86 refs., 9 figs., 4 tabs.