br Electronic Distribution of S(N)2 IRC and TS Structures: Infrared Intensities of Imaginary Frequencies

A novel IRC-TS-CCTDP method to investigate transitionstates (TS) is proposed in which changes in the molecular geometry followatomic displacements corresponding to the imaginary frequency normalcoordinate. Electronic charge structure changes can be analyzed using thecharge-charge-transfer-dipolar polarization (CCTDP) model. Anapplication is presented for the gas-phase SN2 reaction transition statestructures for nine NuCX3LG-systems, with Nu and LG = H, F, Cl and X= H, F. Using quantum theory of atoms in molecules (QTAIM) at theQCISD/aug-cc-pVTZ level, atomic charges and atomic dipoles wereobtained and applied to calculate the CCTDP contributions to theirimaginary normal mode intensities. The results show that the imaginary bands are exceptionally strong, ranging from 1217 to 16 086kmmiddotmol-1, much higher than the stretching intensities found in the methyl halides (that are all less than 100 kmmiddotmol-1). For allsystems, the CT contributions are responsible for 63% of the total dipole moment derivatives. The charge contributions are slightlyhigher for transition states where X = F. Dipolar polarization contributions are always small and only reflect the molecularorientation change when the nucleophile displaces the leaving group and, therefore, can be neglected. The same occurs forcontributions from the X atoms. Only atoms aligned with the reaction axis Nu--C-LG contribute to the total intensity. Almost allof the infrared intensities are determined by electron transfers from the nucleophile to carbon and subsequently from carbon to theleaving group. The mechanism of charge transfer revealed by the CCTDP model is consistent with the well-accepted reactionmechanism. Open-access codes for performing the IRC-TS-CCTDP analysis are described and provided for potential users in theSupporting Information.