The structure of hydrazoic acid

Abstract

The structure of hydrazoic acid is described in terms of bond order, formal charge and hybridization. Calculations by the molecular orbital method show that the orders of N 1 —N 2 and N 2 —N 3 bonds in hydrazoic acid H —N 1 —N 2 —N 8 are 1.65 and 2.64 respectively. The distribution of charge on N 1 , N 2 and N 8 atoms is found to be —0.29, +0.61 and —0.31 respectively. The orders of these bonds have also been derived by the valence-bond method. In accordance with Penney’s definition the orders of bonds between N 1 and N 2 , and N 2 and N 3 atoms have been found to be 1.67 and 2.67 respectively. In order to obtain an insight into the relative importance of the three factors, the heat of formation of hydrazoic acid has been calculated. With the help of the principle of additivity of bond energies and the method of localized electron pairs the heat of formation of hydrazoic acid E HN 3 can be expressed in terms of cr-bond energy K , exchange integral J , repulsion energy between two lone pairs of electrons R , and promotional energy P . The expression obtained is of the form E HN 3 = E N—H + ( K 1 - 1/2 J 1 ) + ( K 2 + 1/2 J 2 ) + √(J 2 1 - J 1 J 2 ) - 1/2 R-P , where E N—H is the energy of the N—H bond and the suffixes 1 and 2 refer to bonds between the N 1 and N 2 , and N 2 and N 3 atoms respectively. The numerical calculations show that E HN3 has a minimum value of energy of 315.8 kcal for the lengths of N 1 —N 2 and N 2 —N 3 bonds equal to 1.26 and 1.13 Å respectively. If allowance is made for the increase in energy arising from the presence of formal charge and increased hybridization on the central nitrogen atom, E HN3 has a minimum value of 319.1 kcal for lengths of N 1 —N 2 and N 2 —N 3 bonds equal to 1.24 and 1.12 Å respectively. These calculated values of interatomic distances and the heat of formation agree with the experimental values.