Hydrogen bonds in crystals - X. The isotope effect and thermal expansion of non-co-operative hydrogen bonds in furoic acid

Abstract

Thermal effects in the hydrogen bonds in crystalline furoic acid have been studied by isotope substitution and by measuring the thermal expansion. In furoic acid, hydrogen bonds join molecules of the acid in closed pairs instead of forming a network extending throughout the lattice as in crystals previously studied. On substituting deuterium for hydrogen, lattice expansion is observed as in other crystals containing short hydrogen bonds. Though it is smaller in magnitude than for extended networks of bonds, the direction of this expansion coincides approximately with the directions of the short hydrogen bonds. The thermal expansion is, however, sharply differentiated from the isotope effect and has its maximum roughly at right angles to the hydrogen bonds. These observations confirm that short hydrogen bonds even in isolated pairs show the usual isotope effects; but when isolated they do not control the thermal expansion. In crystals which contain networks of bonds, it seems likely that co-operative effects make significant contributions to hydrogen bonding.