Distribution of hydrogen bond angles in molecular crystals

Abstract

THE structures of hydrogen-bonded molecular crystals have been studied extensively, and the resulting histograms of the distributions of the O–H ċ O bond angles θ show maxima at approximately 15° from the linear configuration^1–3. At first sight this seems odd, because theoretically a linear hydrogen bond is stable^4,5. Kroon and Kanters³ have indicated, however, that because the number of possible hydrogen bond configurations at any value of θ is proportional to sinθ, these histograms should not be interpreted to indicate that the configuration of θ = 15° is, energetically, the most stable. They applied the sinθ correction to the statistics of a series of 196 values of hydrogen bond angles in molecular crystals, and showed that the experimental distributions are not inconsistent with an assumed preference for linear hydrogen bonds.