Non-linear hydrogen bonds and rotational spectroscopy: measurement and rationalisation of the deviation from linearity

Abstract

When the rotational spectra of hydrogen-bonded dimers B⋯HX (X = Cl or Br) of suitable symmetry are investigated under high resolution by the pulsed-nozzle, Fourier-transform technique, it is possible to determine the complete halogen nuclear quadrupole coupling tensor referred to the principal inertial axis system of the molecule. This tensor can then be diagonalised to give the components χ_xx, χ_yy and χ_zz in the HX axis system and the angle α_az between the direction of the principal inertial axis a and the direction of the HX axis z. A consideration of the values of χ_xx and χ_yy establishes that the HX angular oscillation is close to isotropic with respect to the x and y directions. It can then be shown that the angle α_az obtained is unaffected by the zero-point angular oscillation of the HX subunit. This result allows the deviation θ of the hydrogen bond B⋯H—X from linearity to be determined.

Values of θ determined for the series of hydrogen-bonded dimers B⋯HX, where B is furan, sulfur dioxide, 2,5-dihydrofuran, oxirane, thiirane and formaldehyde and X is either ³⁵Cl or ⁷⁹Br, are considered. θ is found to vary significantly along this series and this variation is discussed in terms of a simple model of the complex involving a secondary interaction between the X atom and the subunit B.