Derivation of carbon-nitrogen, hydrogen-nitrogen and nitrogen-nitrogen non-bonded potential parameters in molecular crystals

Potential function parameters for CN, HN, and NN non-bonded atom-atom pair interactions were derived from the heats of sublimation of cyanogen and dicyanoacetylene and from the crystal structures of cyanogen, dicyanoacetylene, tetracyanoethylene, s-tetrazine, pteridine and pyridazino[4,5-d]pyridazine. Procedure and CC, CH, and HH parameters used were those of Williams [J. Chem. Phys. (1967). 47, 4680-4684]. The exponents of CN, HN, and NN repulsion terms were taken equal to those of CC, CH, and CC repulsion terms, respectively. The lattice summation was cut off at 6, 5.5 and 6 Å for the CN, HN, and NN terms, respectively. The coefficients of the attractive and repulsive terms were fitted by weighted least-squares calculations to 45 observational equations. It proved to be necessary to estimate externally the parameters of the HN attractive term. The derived parameters show rough agreement with the values found by other investigators. The experimental values of the heats of sublimation of cyanogen and dicyanoacetylene are reproduced within 4% with the calculated parameters. The parameters predict reasonable values for the heats of sublimation of tetracyanoethylene, s-tetrazine, pteridine and pyridazino[4,5-d]pyridazine. A difference of about 6 kcal mole^-1 between the heats of sublimation of the latter two isomers was calculated.