Origin of the large spin-orbit effect in the vibrational frequencies of haloalkane cations.

Spin–orbit coupling often causes fine structure splittings in atomic and molecular spectra and induces spin-forbidden transitions by breaking symmetry selection rules. Recently, it was found that the vibrational frequencies of molecular cations such as CH2XI + (X=Br, Cl) are also affected dramatically by this interaction. Hyperconjugative inter-halogen interaction was thought to play a part. Investigation has been extended to anti-1,2-C2H4ClI + in this work for better understanding of the phenomenon. For (CH2)nXI (n=1, 2 and X=Br, Cl) with Cs symmetry, two nearly degenerate nonbonding orbitals of iodine, n ACHTUNGTRENNUNG(I5pjj) and nACHTUNGTRENNUNG(I5p? ) 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 A’ and 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. In the double group for Cs, both a’ and a’’ become e1/2. Then, the two states of the cations, A’ and A’’, become E1/2 states in the presence of the spin–orbit coupling and interact with each other. In our previous studies, vibrational spectra of CH2XI + (X=Br, Cl) in the ground electronic states were obtained by mass-analyzed threshold ionization (MATI) spectrometry. 4] In the DFT calculations for the cations, two equilibrium geometries with Cs symmetry were optimized corresponding to the A’ and A’’ states. A’ was the ground state in both cases mainly due to the antibonding character of n ACHTUNGTRENNUNG(I5pjj) 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. Influence of the spin–orbit coupling was investigated through spin–orbit DFT (SODFT) calculations, which employ the spin–orbit operators from the start. Only one geometry was optimized for each cation, which was noticeably different from the DFT geometries. More importantly, the frequencies calculated by SODFT were in much better agreement with the experimental data than the DFT results. The frequencies of CH2ClI + were found to be more affected by the spin– orbit coupling than those of CH2BrI + . To see if weaker interhalogen interaction would cause larger spin–orbit effect on vibrational frequencies, 1,2-C2H4ClI + has been chosen for further study because the inter-halogen interaction in this cation is expected to be even weaker than in CH2ClI + . The vibrational spectrum of 1,2-C2H4ClI + in the ground electronic state was obtained by MATI. According to our DFT calculations, the neutral with the gauche conformation was higher in energy than the anti-form by around 800 cm . Hence, the peaks in the spectrum were thought to be exclusively due to the latter form. The frequencies of the stretching modes n(CI) and n ACHTUNGTRENNUNG(CCl) and the bending modes dACHTUNGTRENNUNG(ICC) and dACHTUNGTRENNUNG(CCCl) are shown in Table 1 together with some data for CH2BrI + and CH2ClI + reported previously. The full spectrum and the vibrational assignments are included in Supporting Information.