Hydrogen-bond radii

Abstract

The values of intermolecular hydrogen-bond distances, mainly in organic crystals, have been collected in order to invest igate the possibility of expressing these distances as a simple function of values characterist ic of the two groups par t ic ipat ing in the format ion of the hydrogen bond. This compilation represents a fair ly extensive coverage of the l i terature up to 1960, but the individual values and references are not given in view of the previous publication of similar lists (Donohue, 1952; Fuller, 1959; Pimentel & McClellan, 1960). The distances Y . Z have been classified according to the type of donor group Y H and acceptor group Z in the bond Y H . . . Z. Within each class the distances have been averaged and the s t andard deviat ion has been calculated. The da t a so obtained are shown in the first two lines of figures in each port ion of Table 1. Examina t ion of the average values shows tha t , for a given donor group, the hydrogen-bond distances follow roughly the change in the van der Waals radii of the acceptors Z, except where the electronegative a tom in Z is directly a t t ached to hydrogen. For a given acceptor group the hydrogen-bond distances decrease with increasing electronegat ivi ty of the a tom a t t ached to H in the donor group Y-H. I t is found, in fact, t ha t the decrease from the expected van der Waals distance between H and Z brought about by hydrogen-bond format ion is roughly proport ional to the difference in electronegat ivi ty between the hydrogen a tom and the a tom to which it is a t t ached in the donor group Y-I t . These observed t rends suggest t ha t it might be possible to express hydrogen bond distances as the sum of two hydrogen bond radii, one a constant for a given donor ( Y H ) group and one a constant for a given acceptor (Z) group. They also suggest t h a t the best assumpt ion in the appor t ionment of each tota l distance among the two groups might be obtained by taking the hydrogen-bond radius for one acceptor a tom as equal to its van der Waals radius. In view of the preponderance of hydrogen-bond distances with oxygen a toms as acceptors, it was decided to t ake the hydrogen-bond acceptor radius of an oxygen a tom as being equal to 1-40/~. Star t ing from this basis, it was found to be possible to calculate a set of hydrogenbond radii which, when added appropriately, reproduce well the observed average hydrogen-bond distances. These hydrogen-bond radii are given in Table 2 and the expected hydrogen-bond distances calculated from them are shown on the th i rd line of each section in Table 1. Although only the hydrogen-bond acceptor radius