Axial Asymmetry of the Charge- and Spin-Density Distributions in Π States. Molecular Quadrupole Moments and Hyperfine Coupling Constants of CH, NH, OH, CF, LiO, NO, and FO

The axial asymmetry of the charge- and spin-density distributions in Π states is studied via second-rank traceless tensors Pii (ii = xx, yy, zz), namely, quadrupole moments (Θii), electric field gradients (qii), and magnetic dipolar (Tii) hyperfine coupling constants (hfcc’s). In linear molecules, it holds that Pxx ≠ Pyy ≠ Pzz for Π, but Pxx = Pyy ≠ Pzz for Σ, Δ, Φ,..., states. Thus, traceless Pii in Π states have two independent parameters, P∥ = Pzz ∝ [rm(3 cos2 θ − 1] and δP⊥ = |Pxx − Pyy| ∝ [rm sin2 θ], with m = 2(Θii) or −3(qii, Tii). All linear states have P∥ ≠ 0, but only Π states exhibit δP⊥ ≠ 0, as shown by hfcc’s like c = (3/2)Tzz, and d = |Txx − Tyy|, as well as q0 = (−qzz) and |q2/2| = |qxx − qyy|. Little is known about Θzz and δΘ⊥ = |Θxx − Θyy| in Π states since most experimental values (gas-phase) are rotational averages, and several theoretical studies have reported Θzz but assumed δΘ⊥ = 0. The diatomics studied here have X2Π1/2(π1) ground states, like CH and NO, or are of type X2Π3/2(π3), l...