Theoretical Studies on Dications and Trications of FH, ClH, and BrH. Properties of the Bound 15Σ- States. Electron-Spin g-Factors and Fine/Hyperfine Constants of the Metastable X3Σ- States of ClH2+ and BrH2+

Abstract

This theoretical study reports calculations on the fine and hyperfine structure parameters of the metastable X³Σ^-(σ²π²) state of ClH²⁺ and BrH²⁺. Data on the repulsive FH²⁺ system are also included for comparison purposes. The hyperfine structure (hfs) coupling constants for magnetic (A_iso, A_dip) and quadrupole (eQq) interactions are evaluated using B3LYP, MP4SDQ, CCSD, and QCISD methods and several basis sets. The fine structure (fs) constants (zero-field splitting λ and spin-rotation coupling γ) and electron-spin magnetic moments (g-factor) are evaluated in 2nd-order perturbation theory using multireference CI (MRCI) wave functions. Our calculations find for ³⁵Cl of ClH²⁺ A_iso/A_dip = 110/−86 MHz; eQq₀ = −59 MHz; 2λ = 20.4 cm^-1; g_⊥(v = 0) = 2.02217; and γ = −0.31 cm^-1 (to be compared with the available experimental A_iso/A_dip= 162/-30 MHz). For ⁷⁹BrH²⁺, the corresponding values are 300/−400 MHz; 368 MHz; 362.6 cm^-1; 2.07302; and −0.98 cm^-1 (experimental 2λ = 445(±80) cm^-1). We find g_⊥(ClH²⁺) to increase by about 0.0054 between v = 0 and 2, whereas the experimental effective g_⊥ changes drastically with vibrational excitation. Nuclear quadrupole coupling constants for halogen atoms X are found to be as large as corresponding A_dip(X)'s, indicating that both terms may have to be included in the Hamiltonian used to interpret XH²⁺ hyperfine spectra. A novel finding relates to the bound character of the 1⁵Σ^-(σπ²σ*) state in FH²⁺, as already known for ClH²⁺ and BrH²⁺, but having a deeper potential well D_e ≈ 4000 cm^-1 (versus 1000 cm^-1 in the heavier radicals). Vertical ionization potentials for formation of XH³⁺ trications are also discussed.