Nuclear Magnetic Resonance Studies Concerning the Effect of Adjacent Hydrogen Atoms on the Hydrogen-Bond Properties

Abstract

The proton-resonance absorption spectrum has been studied in several hydrogen-bonded solids with different O–H···O bond lengths ranging from 2.45 to 3.30 Å. The resonance linewidth increases with the O–H···O bond length and shows a tendency to flatten out when the bond length exceeds 2.72 Å. The proton-resonance absorption linewidth in molecular solids is due primarily to nuclear dipole—dipole interaction, and hence it is proportional to Σk(rjk)—3, where rjk is the distance between Protons j and k, where j is any particular chosen proton. Because of the inverse-cube dependence on r, appreciable contribution to the line broadening comes from nearest neighbors only. The dependence of the proton-resonance linewidth on the hydrogen-bond length, therefore, points out that the hydrogen-bond length gets smaller when the protons are far apart and larger when they are close. This result was verified independently by calculating the nearest hydrogen—hydrogen distances for compounds where the positions of hydrogen atoms are known from neutron-diffraction studies and in other cases by estimating these distances from the usual values of the bond lengths and bond angles. The observed result is explained by postulating that the interaction between the two O–H bonds involved in two adjacent O–H···O bonds is essentially electrostatic in origin; the interaction potential consists of attractive terms due to dipole—dipole, induced dipole, and dispersion interactions in addition to a repulsive term. The calculated interaction energy between two O–H···O hydrogen bonds is attractive up to O–H···O bond lengths of 2.80 Å and has a minimum value around 2.70 Å. The O–H···O bond length of 2.70 Å seems to be most preferred from the point of view of this energy. This is consistent with the fact that there are a large number of hydrogen-bonded solids with bond lengths around 2.70 Å.