Origin of the Large Spin–Orbit Effect in the Vibrational Frequencies of Haloalkane Cations

Abstract

For (CH2)nXI (n = 1,2 and X = Br,Cl) with Cs symmetry,two nearly degenerate nonbonding orbitals of iodine, nApjj) and nAp?) which belong to a’ and a’’ species,respectively,are the highest occupied orbitals. These orbitals are split due to high order interactions such as the inter-halogen interaction. Elimination of an electron from these orbitals would produce a cation in the 2 A’ and 2 A’’ electronic states,respectively. Spin– orbit coupling further mixes these states. Even though the properties of the closed-shell neutrals may not be affected much by this mixing,those of the open-shell cations can be altered significantly. Another way to describe the influence of the spin–orbit coupling on the cation properties is to invoke symmetry reduction induced by this interaction. Treatment of a half-integral spin system,such as radical cations,with strong spin–orbit coupling requires the use of the double group instead of the usual point group. [3] In the double group for Cs, both a’ and a’’ become e1/2. Then,the two states of the cations, 2 A’ and 2 A’’,become 2 E1/2 states in the presence of the spin–orbit coupling and interact with each other. In our previous studies,vibrational spectra of CH 2XI + (X = Br, Cl) in the ground electronic states were obtained by mass-analyzed threshold ionization (MATI) spectrometry. [2,4] In the DFT calculations for the cations,two equilibrium geometries with Cs symmetry were optimized corresponding to the 2 A’ and 2 A’’ states. 2 A’ was the ground state in both cases mainly due to the antibonding character of nApjj) with respect to X and I, which was stronger in CH2BrI + . The calculated vibrational frequencies differed from the experimental data,rather drastically for some modes. [2] Influence of the spin–orbit coupling was in