Theoretical study on N2+, P2+, As2+, NP+, NAs+, and PAs+: Hyperfine coupling constants for 12Σ(g)+, and electron-spin g-factors for 12Σg+/1,22Σu+(X2+) and 1,22Σ+ (XY+) states

ESR parameters are reported for N2+, P2+, As2+, NP+, NAs+, and PAs+ radicals. Hyperfine coupling constants (Aiso, Adip) are given for the lowest-lying 2Σg+/2Σ+ state of each ion (which is the ground state in the three nitrides but the first excited state in others). Aiso/Adip are calculated with ab initio and density functional theory methods. The Adip (X)'s for 12Σ+ (2σ23σ1π4) show that the 3σ(2σg) singly occupied MOs have large p (X)-character (45-50%). All Aiso (X) and Adip (X) are positive, except for Aiso(P)/Aiso(As) ≈ −200 MHz in NP+ and NAs+. Electron-spin g-shifts, calculated with multireference CI methods via sum-over-states expansions, are studied for several 2Σ+ states-two for XY+ (1,22Σ+) and three for X2+(12Σg+/1,22Σu+). They are negative for 12Σg+/12Σ+(2σ23σ1π4), increasing in magnitude from 2700 to 15 600 to 17 500 to 36 200 to 103 000 ppm for N2+ → NP+ → P2+ → PAs+ → As2+, as expected due to the parallel rise in 2Σ+-2Π spin-orbit mixing. NAs+ has an anomalously large |Δg⊥ (12Σ+)| of 153 000 ppm, due to a small ΔEvert (12Π) = 0.27 eV. At equilibrium, 12Σu+/22Σ+ are described mainly by a three-open-shell configuration leading to positive Δg⊥'s (800-77 000 ppm). Only for N2+ is 12Σu+ a one-open-shell state, with a negative Δg⊥ (−6000 ppm). Thus, according to Curl's equation, the spin-rotation coupling constant γ is always positive for 12Σg+/12Σ+(σ2σπ4) but positive or negative for 12Σu+/22Σ+, depending on whether its composition is σσ2π4 or σ2σπ3π∗ (one- versus three-open-shells). Our findings rationalize why the experimental γ (22Σ+) of NP+ is vanishingly small: a mixed composition σσ2π4 + σ2σπ3π∗ at Re (22Σ+) results in the mutual cancellation of second-order contributions. Spin-orbit (αel) and orbital (βel) parameters describing 2Σ+-2Π perturbations are provided. The calculated ESR and γ values compare well with experimental numbers as far as available.