Theoretical studies on dications and trications of FH, ClH, and BrH. Properties of the bound 1(5)sigma- states. Electron-spin g-factors and fine/hyperfine constants of the metastable X3Sigma- states of ClH2+ and BrH2+.

This theoretical study reports calculations on the fine and hyperfine structure parameters of the metastable X(3)Sigma(-)(sigma(2)pi(2)) state of ClH(2+) and BrH(2+). Data on the repulsive FH(2+) 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 lambda and spin-rotation coupling gamma) and electron-spin magnetic moments (g-factor) are evaluated in 2nd-order perturbation theory using multireference CI (MRCI) wave functions. Our calculations find for (35)Cl of ClH(2+) A(iso)/A(dip) = 110/-86 MHz; eQq(0) = -59 MHz; 2lambda = 20.4 cm(-1); g( perpendicular)(v = 0) = 2.02217; and gamma = -0.31 cm(-1) (to be compared with the available experimental A(iso)/A(dip)= 162/-30 MHz). For (79)BrH(2+), the corresponding values are 300/-400 MHz; 368 MHz; 362.6 cm(-1); 2.07302; and -0.98 cm(-1) (experimental 2lambda = 445(+/-80) cm(-1)). We find g( perpendicular)(ClH(2+)) to increase by about 0.0054 between v = 0 and 2, whereas the experimental effective g( perpendicular) 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 XH(2+) hyperfine spectra. A novel finding relates to the bound character of the 1(5)Sigma(-)(sigmapi(2)sigma) state in FH(2+), as already known for ClH(2+) and BrH(2+), but having a deeper potential well D(e) approximately 4,000 cm(-1) (versus 1,000 cm(-1) in the heavier radicals). Vertical ionization potentials for formation of XH(3+) trications are also discussed.