VUV Photofragmentation of Chloroiodomethane: The Iso-CH2I-Cl and Iso-CH2Cl-I Radical Cation Formation

Dihalomethanes XCH₂Y (X and Y = F, Cl, Br, and I) are a class of compounds involved in several processes leading to the release of halogen atoms, ozone consumption, and aerosol particle formation. Neutral dihalomethanes have been largely studied, but chemical physics properties and processes involving their radical ions, like the pathways of their decomposition, have not been completely investigated. In this work the photodissociation dynamics of the ClCH₂I molecule has been explored in the photon energy range 9–21 eV using both VUV rare gas discharge lamps and synchrotron radiation. The experiments show that, among the different fragment ions, CH₂I⁺ and CH₂Cl⁺, which correspond to the Cl- and I-losses, respectively, play a dominant role. The experimental ionization energy of ClCH₂I and the appearance energies of the CH₂I⁺ and CH₂Cl⁺ ions are in agreement with the theoretical results obtained at the MP2/CCSD(T) level of theory. Computational investigations have been also performed to study the isomerization of geminal [ClCH₂I]^•+ into the iso-chloroiodomethane isomers: [CH₂I–Cl]^•+ and [CH₂Cl–I]^•+.

Analytical energy gradients for ionized states using equation-of-motion coupled-cluster theory with spin-orbit coupling

Spin-orbit coupling (SOC) may have a significant effect on the structure and harmonic frequencies of particularly heavy p-block element compounds. However, reports on analytical energy gradients with SOC are scarce, especially for excited states. In this work, we implemented analytical energy gradients for ionized states using the equation-of-motion coupled-cluster (CC) theory at the CC singles and doubles level (EOM-IP-CCSD) with SOC. Effects of SOC on structure and harmonic frequencies as well as properties for both the ground and some excited states of open-shell compounds with one unpaired electron can be investigated efficiently with the present implementation. A closed-shell reference is required in the calculations, and SOC is included in post-Hartree-Fock treatment. Relativistic effective core potentials are employed in dealing with both scalar relativistic effects and SOC, and we treat perturbations that are even under time reversal in this work. Both time-reversal symmetry and double point group symmetry for D_2h ^* and its subgroups are exploited in the implementation. The method is applicable to states which can be reached by removing one electron from a closed-shell reference state. The results of some open-shell cations indicate the importance of SOC on structures and harmonic frequencies of heavy element compounds.