Coming to grips with N-H center dot center dot center dot N bonds. 1. Distance relationships and electron density at the bond critical point

In an attempt to discover and analyze trends in distance relationships and properties at the bond critical point (BCP) in linear or near-linear N-H . . .N hydrogen bonds, the geometry of such bonds in a large number of suitab le simple chemical species was optimized at the RHF/6-31G** level. The resu lts for 67 of these are reported here; the geometry of 19 of them was optim ized also at the MP2/6-31G** level. Correlations between the internuclear N -H, H . . .N, and N . . .N separations as well as between the N BCP and H B CP distances for these data sets and for different model functions are desc ribed in detail. The special case of symmetric N-H-N bonds is discussed; co mparison with available experimental evidence shows that the correlation fu nctions derived from the-ab initio data have useful predictive value for cr ystallographic determinations involving short N-H-N bonds. Analysis of the correlation between the d(N-H) distance and the electron density rho (c) at the BCP has shown that although acceptable d,rho (c) representations are o btained when rho (c) is fitted over the entire d(N-H) range by a single mod el function, significantly better fits result for both the 6-31G** and the MP2/6-31G** set when two separate regression functions of the same type are used, one for the covalent and another for the H . . .N bonds. The implica tions of these findings are discussed. The results of the correlation analy sis of the curvatures lambda (i), the Laplacians del (2)rho (c), and the ki netic energy densities at the BCP, based on the data presented in this pape r, will be reported in a subsequent paper, together with some aspects of th e energy of formation of the N-H . . .N bonds.