Theoretical study of the hyperfine coupling constants and electron-spin magnetic moments (g-factors) of X2Σ+ alkali-metal radicals XY± (X, Y=Li, Na, K) Original Research Article

Ab initio calculations on the electron-spin resonance (ESR) parameters of the title radicals are reported. The Aiso and Adip constants are studied using 6-311 + G(2df) basis sets and several methods (UHF, CISD, MP2, B3LYP, PW91PW91). The hyperfine spectra for X2Σ+(σ) of XY+ and X2Σ+(σ2σ) of XY− are governed by the Aiso terms, as expected for s-rich radicals. The electron-spin g-factors (magnetic moment μS) are calculated using perturbation expansions (up to second-order), a Hamiltonian based on Breit–Pauli theory, and correlated (MRDCI) wavefunctions. The present XY± ions have negative values of Δg∥ and Δg⊥. The sum-over-states expansions for Δg⊥ are dominated by the coupling with just one 2Π excited state. The Δg⊥ʹs of homonuclear X2+ ions are small (−60 to −220 ppm) but somewhat larger for heteronuclear XY+ (−290 to −1200 ppm). The magnetic coupling is generally stronger for the anions, as pointed out by Δg⊥ values of −2600±100 ppm for both LiK− and NaK−, and of −3300 ppm for K2−. Of all radicals studied, an ESR spectrum is only available for Na2+ in Ne matrices. Our study supports the experimental Aiso and Adip while calculated values of −30 ppm for Δg∥ and of −220 ppm for Δg∥ lie within the broad range reported experimentally, −1700(±2000) and −620(±900) ppm, respectively.