Studies of 1,2-Dihalo Shifts in Carbon–Carbon, Carbon–Silicon, and Silicon–Silicon Systems: A Computational Study

Computational study of the threshold energy for the 1,2-interchange of X and R (X, R = halogens, pseudohalogens, and monovalent hydrocarbon groups) on CH2XCH2R

Transition state geometries and threshold energies, E0, were computed for an unusual unimolecular isomerization reaction that exchanges two groups (X, R) on CH2XCH2R. An objective is to determine the most energetically feasible interchanges to guide experimental investigations. The interchanging species included halogens (F, Cl, Br) and pseudohalogens and monovalent hydrocarbons (H, SH, CH3, NH2, OH, OCF3, OCH3, CH=CH2, CH2CH3, CH2OH, C≡CH, CH2CF3, CCl3, CF3) attached to a two carbon backbone. Ground state and transition state geometries were optimized with the B3PW91 level of theory and 6-311+G(2d,p) basis set. The Br–Br interchange had the lowest E0 (141 kJ/mol), and CH3–H had the highest E0 (582 kJ/mol). In general, larger atoms or groups with lone pairs of electrons such as halogens, SH, OH, OCH3, OCF3, and NH2 tend to lower the E0 barrier for interchange, making them the most likely to be experimentally observed.

Unimolecular isomerization of CH2FCD2Cl via the interchange of Cl and F atoms: assignment of the threshold energy to the 1,2-dyotropic rearrangement

The room-temperature gas-phase recombination of CH2F and CD2Cl radicals was used to prepare CH2FCD2Cl molecules with 91 kcal mol(-1) of vibrational energy. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2Cl; HCl and DF elimination to give CHF═CD2 and CH2═CDCl plus isomerization to give CH2ClCD2F by the interchange of F and Cl atoms. The Cl/F interchange reaction was observed, and the rate constant was assigned from measurement of CHCl═CD2 as a product, which is formed by HF elimination from CH2ClCD2F. These experiments plus previously published results from chemically activated CH2ClCH2F and electronic structure and RRKM calculations for the kinetic-isotope effects permit assignment of the three rate constants for CH2FCD2Cl (and for CH2ClCD2F). The product branching ratio for the interchange reaction versus elimination is 0.24 ± 0.04. Comparison of the experimental rate constant with the RRKM calculated rate constant permitted the assignment of a threshold energy of 62 ± 3 kcal mol(-1) for this type-1 dyotropic rearrangement. On the basis of electronic structure calculations, the nature of the transition state for the rearrangement reaction is discussed. The radical recombination reactions in the chemical system also generate vibrationally excited CD2ClCD2Cl and CH2FCH2F molecules, and the rate constants for DCl and HF elimination were measured in order to confirm that the photolysis of CD2ClI and (CH2F)2CO mixtures was giving reliable data for CH2FCD2Cl.