Microwave optical double resonance spectrum of NH2. VII. Hyperfine coupling constants (14N and 1H) in Σ(0, 9, 0) and Π (0, 10, 0) of a Ã2A1

Abstract

Magnetic dipole allowed transitions N_KaKc J = N−1/2 ↔ J = N+1/2 have been observed in two vibrational states Σ(0,9,0) and Π (0,10,0) of ? ²A₁ NH₂ using microwave optical double resonance spectroscopy. Extensive hyperfine structure involving both ortho and para rotational states has been analyzed to yield ¹⁴N and ¹H magnetic hyperfine coupling constants for the Π (0,10,0) vibronic state; the Σ (0,9,0) data were not sufficient alone to determine all these constants. The hyperfine structure of Σ (0,9,0) and Π (0,10,0) could not be fitted simultaneously with a single set of hyperfine coupling constants. However, contrary to our expectations and some previous analyses, no significant difference between the Fermi contact terms for ¹⁴N in the Σ (0,9,0) and Π (0,10,0) could be identified. Instead the discordance between the two states appears to arise from different magnetic dipole–dipole coupling constants. The fine and hyperfine structure clearly reflect the presence of perturbations which limit the precision attainable for the derived hyperfine coupling constants. The coupling constants obtained for Π (0,10,0) are a_N = 153.0(13), T_aa^N = −39.5(33), T_cc^N = −37.4(19), ξ_aa^N = 1.4(10), ξ_cc^N = −7.4(10), a_H = 52.2(14), T_aa^H = 59.5(34), and T_cc^H = −50.0(20), where all constants are in MHz and the uncertainties listed are 1 standard deviation. The magnetic constants for ¹⁴N are quite consistent with a simple sigma molecular orbital picture for the unpaired electron.