Electronic structure theory study of the F(-) + CH(3)I → FCH(3) + I(-) potential energy surface.

MP2 and DFT electronic structure theories, with the functionals OPBE, OLYP, HCTH407, BhandH, and B97-1 for the latter, were used to investigate stationary point properties on the F(-) + CH(3)I → FCH(3) + I(-) potential energy surface (PES). The aug-cc-pVDZ and aug-cc-pVTZ basis sets for C, H, and F, with Wadt and Hay's 3s3p valence basis and an effective core potential (ECP) for iodine, were employed for both MP2 and DFT. Single-point CCSD(T) calculations were also performed to obtain the complete basis set (CBS) limit for the stationary point energies. The CCSD(T)/CBS reaction exothermicity is only 5.0 kJ/mol different than the experimental value. MP2 and DFT do not predict the same stationary points on the PES. MP2 predicts the C(3v) F(-)-CH(3)I and FCH(3)-I(-) ion-dipole complexes and traditional F-CH(3)-I central barrier as stationary points, as well as a C(s) hydrogen-bonded F(-)-HCH(2)I complex and a F-HCH(2)-I transition state connecting this latter complex to the F(-)-CH(3)I complex. A CCSD(T)/CBS relaxed potential energy curve, calculated for the MP2 structures, shows that going from the F(-)-CH(3)I complex to the F-CH(3)-I TS is a barrierless process, indicating these two structures are not stationary points. This is also suggested by the DFT calculations. The structures and frequencies for CH(3)I and CH(3)Cl given by MP2 and DFT are in overall good agreement with experiment. The calculations reported here indicate that the DFT/B97-1 functional gives the overall best agreement with the CCSD(T) energies, with a largest difference of only 7.5 kJ/mol for the FCH(3)-I(-) complex.