NONLINEAR HYDROGEN-BONDS AND ROTATIONAL SPECTROSCOPY - MEASUREMENT AND RATIONALIZATION OF THE DEVIATION FROM LINEARITY

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 tenser referred to the principal inertial axis system of the molecule. This tenser can then be diagonalised to give the components chi(xx), chi(yy) and chi(zz) in the HX axis system and the angle alpha(az) between the direction of the principal inertial axis a and the direction of the HX axis z. A consideration of the values of chi(xx) and chi(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 alpha(az) obtained is unaffected by the zero-point angular oscillation of the HX subunit. This result allows the deviation theta of the hydrogen bond B...H-X from linearity to be determined. Values of theta 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 Cl-35 Or Br-79, are considered. theta 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.