The Jahn-Teller effect in ${\rm CH}_{\rm 3} {\rm Cl}^{\rm + } ( {{\rm \tilde X}{}^{\rm 2}{\rm E}} )$ CH 3 Cl +(X̃E2): A combined high-resolution experimental measurement and ab initio theoretical study

Abstract

The energy levels of CH(3)Cl(+)X̃(2)E showing strong spin-vibronic coupling effect (Jahn-Teller effect) have been measured up to 3500 cm(-1) above the ground vibrational state using one-photon zero-kinetic energy photoelectron and mass-analyzed threshold ionization spectroscopic method. Theoretical calculations have been also performed to calculate the spin-vibronic energy levels using a diabatic model and ab initio adiabatic potential energy surfaces (PESs). In the theoretical calculations the diabatic potential energy surfaces are expanded by the Taylor expansions up to the fourth-order including the multimode vibronic interactions. The calculated spin-orbit energy splitting (224.6 cm(-1)) for the ground vibrational state is in good agreement with the experimental data (219 ± 3 cm(-1)), which indicates that the Jahn-Teller and the spin-orbit coupling have been properly described in the theoretical model near the zero-point energy level. Based on the assignments predicted by the theoretical calculations, the experimentally measured energy levels were fitted to those from the diabatic model by optimizing the main spectroscopic parameters. The PESs from the ab initio calculations at the level of CASPT2/vq(t)z were thus compared with those calculated from the experimentally determined spectroscopic parameters. The theoretical diagonal elements in the diabatic potential matrix are in good agreement with those determined using the experimental data, however, the theoretical off-diagonal elements appreciably deviate from those determined using the experimental data for geometric points far away from the conical intersections. It is also concluded that the JT effect in CH(3)Cl(+) mainly arises from the linear coupling and the mode coupling between the CH(3) deform (υ(5)) and CH(3) rock (υ(6)) vibrations. The mode couplings between the symmetric C-Cl stretching vibration υ(3) with υ(5) and υ(6) are also important to understand the spin-vibronic structure of the molecule.

The Renner-Teller effect in HCCCl+(X̃2Π) studied by zero-kinetic energy photoelectron spectroscopy and ab initio calculations

“…The standard deviation of the theoretical calculations relative to the experimental data is about 20 cm −1 , similar to those of our previous theoretical calculations on BrCN + and CH 3 Cl + . 14,39,40 Each vibronic level is assigned to a term symbol 2S+1 K P , where K is the sum of the projection of the electronic and vibrational angular momentum on the molecular axis, K = |Λ + l 4 + l 5 |, and P is the projection of the total angular momentum on the molecular axis, P = |Λ + l 4 + l 5 + Σ|. In this work, the rotational excitation was not considered and the quantum number P is thus a good quantum number.…”

“…On the other hand, the ground state SO energy splitting of CH 3 Cl + (X 2 E) is 219 cm −1 , which is due to a combination result of the SO and vibronic couplings. 39,40 In fact, the SO coupling strength of CH 3 Cl + (X 2 E) is −613 cm −1 , as determined from the fitting of the experimental data. 39,40 The SO energy splitting is reduced by the Jahn-Teller coupling in CH 3 Cl + (X 2 E) and not by delocalization of the p electrons of the Cl atom, as in the case of HCCCl + (X 2 Π).…”

“…The experimental apparatus has been described previously, 14,17,[39][40][41] so only a brief description is provided here. The coherent VUV (vacuum ultraviolet) radiation was generated by employing resonance enhanced four-wave difference mixing in a pulsed Kr jet.…”

“…43 Recently, this model has been successfully developed to calculate the vibronic energy levels of polyatomic molecules with C 3v symmetry. 39,40,[44][45][46][47][48][49][50] One of the key features of this method is that the diabatic potential energy surfaces are expressed by Taylor expansions with normal coordinates as variables. This method has been extended to linear molecule in previous work, which enables us to compute the spin-vibronic energy levels of linear BrCN + (X 2 Π).…”

“…This is due to the large second-order vibronic coupling between the symmetric and the degenerate vibrational modes, the so-called bilinear coupling effect. 39,40,44 For HCCCl + , the second order vibronic couplings between the stretching and degenerate vibrations can be neglected due to the symmetry of linear molecules.…”