Deuterium nuclear quadrupole coupling constants in vibrationally excited C2HD: Evidence for electron reorganization

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Abstract
The l-doubling spectra of monodeuteroacetylene have been obtained in the ν4=1 (C–D bend) and ν5=1 (C–H bend) states of the molecule with resolution sufficient to determine the deuterium nuclear quadrupole coupling constants. The spectroscopic constants obtained from the J=1 rotational level are UFRULE2 ν4=1 ν5=1 UFRULE1 eqQDaa (kHz) 207.(6) 221.(2) eqQDbb−eqQD cc (kHz) −31.(22) −6.(4) CD (kHz) −6.(2) −1.5(3) CH (kHz) ⋅⋅⋅ −20.0(5) μ (D) 0.023 59(11) 0.056 24(3) q (MHz) 133.050 6(9) 105.699 3(1) UFRULE2 These results are interpreted in terms of the electron distribution surrounding the deuterium nucleus and are shown to be inconsistent with the assumption of a cylindrical distribution about the C–D bond axis in these states. This deviation from the symmetry appropriate for the molecule at equilibrium is shown to arise from a distortion in the cross section of the electron distribution that is unaccompanied by changes along the bond axis.