Microwave spectrum, structure, dipole moment, quadrupole coupling constant, and internal motion of thioformamide

The r_s structure of thioformamide has been determined from the microwave spectra of the normal as well as isotopic species of the molecule. The structural parameters obtained assuming the planarity of the molecule are NH_c = 1.001 ₈ ± 0.006 Å, NH_t = 1.006 ₅ ± 0.003 Å, CN = 1.358 ₂ ± 0.003 Å, CS = 1.626 ₂ ± 0.002 Å, CH_a = 1.09 ₆ ± 0.08 Å, ∢ H _cNH _t, = 121°42' ± 40', ∢ H_cNC = 117°55' ± 40', ∢ H_tNC = 120°22' ± 30', ∢ NCS = 125°16' ± 15' ∢ NCH_a = 108° _5' ± 5°, and ∢ SCH_a = 126° _39' ± 5° . The dipole moment is calculated from the Stark effects of the three transitions to be μ_a = 3.99 ± 0.02 D, μ_b = 0.13 ± 0.25 D, and μ_total = 4.01 ± 0.03 D, where the c component is assumed to be zero. The quadrupole coupling constant of the ¹⁴N nucleus is estimated using the doublet splittings observed for six Q-branch transitions; χ_cc - χ_bb = -5.39 ± 0.15 MHz and χ_aa = 2.9 ± 1.2 MHz. Two sets of vibrational satellites are observed and assigned to the first excited state of the amino wagging and the NCS bending vibrations, respectively. The relative intensity measurement gives the vibrational energies of 393±40 cm ^-1 and 457 ± 50 cm ^-1 for NH ₂CHS and 293 ± 30 cm ^-1 and 393 ± 40 cm ^-1 for ND ₂CHS. The amino wagging inversion vibration in the molecule is discussed in comparison with that in formamide. It is most probable that the thioformamide molecule is also planar without any potential hump to the amino inversion at the planar configuration.