Stabilities, excitation energies, and dissociation reactions of CF2Cl2 and CF2Br2:: Quantum chemical computations of heats of formation of fluorinated methanes, methyls, and carbenes

The heats of formation of a range of chlorofluoro-, bromofluoro-, and iodofluoromethanes, methyls, and carbenes were computed using the Gaussian-3 (G3) and Gaussian-2 (G2) methods. From a number of computed isodesmic reaction enthalpies, the heats of formation (at 298 K) of the key methyl species in the decomposition reactions of difluoromethanes, namely CF2Cl, CF2Br, and CF2I, were determined as -64.7 +/- 2.0, -52.3 +/- 2.0, and -39.3 +/- 2.0 kcal mol(-1), respectively, while our recommended heat of formation for CL is 77.0 +/- 3.0 kcal mol(-1). Using the available thermochemistry the energetics of the dissociation reactions of CF2X2 (X = Cl, Br, I) corresponding to the product channels CF2X + X and CF2 + X-2 (as well as CF2 + 2X) were thus computed and compared. The energetics favor the molecular channel but only by similar to 10-12 kcal mol(-1) in the case of CF2Cl2 and CF2Br2. However, no transition states were found in these systems for the molecular elimination of Cl-2 and Br-2. By contrast, Hz elimination in CF2H2 was found to be favored by similar to 40 kcal mol(-1) (at the G2 level) with the corresponding transition state predicted to be 17.3 kcal mol(-1) below the radical products CHF2 + H. The vertical excitation energies of CF2Cl2 and CF2Br2 (to the lowest singlet and triplet B-1, A(2), B-2, and A(1) excited states) were computed using a range of methods including CASPT2 and EOM-CCSD in conjunction with the cc-pVTZ basis set. The results obtained for the lowest energy, viz., (A) over bar(B-1(1)) <-- transitions are consistent with the available experimental data. The excited state potential energy surfaces, as calculated at the CASPT2 level of theory, are found to be repulsive with respect to a single C-CI or C-Br bond stretch, but have shallow local minima in C-2v symmetry, i.e., when both C-Cl or C-Br bonds are stretched to <similar to>2.3 Angstrom, that suggest the existence of weakly bound metastable states.