The gas phase SN2 reaction transition state structures for nine [XCZ_3 Y]^- systems, where X,Y=H,F,Cl and Z = H,F were optimized and their normal modes of vibrations were determined at the QCISD/aug-cc-pVTZ level of theory. Using Quantum Theory of Atoms in Molecules (QTAIM), the atomic charges and atomic dipoles were obtained and used to calculate the Charge – Charge Transfer – Dipolar Polarization (CCTDP) contributions to the imaginary normal mode intensity of transition states. The results show that the imaginary bands are strong, ranging from 1217 to 16086 〖km∙mol〗^(-1), much higher than occurs for most bands found in molecules. For all systems, the CT contribution is responsible for 80% of the total intensity on average. The Charge contributions are slightly higher for transitions states with Z = F. Dipolar polarization contributions are always small. The contributions from the Z atoms are negligible, thus only atoms aligned with the reaction axis X-C-Y contribute to total intensity. All charge transfers were evaluated taking the carbon atom as reference, implying that almost all infrared intensity is determined by electron transfers from the nucleophile and carbon and from carbon to the leaving group. The mechanism of charge transfer revealed by the CCTDP model is consistent with the reaction mechanism itself, which points towards the connection between the imaginary normal mode and the reaction coordinate.
Infrared intensities of imaginary frequencies: Gas-Phase SN2 Transition States