Coming to Grips with N−H···N Bonds. 2. Homocorrelations between Parameters Deriving from the Electron Density at the Bond Critical Point

The equilibrium geometries of 54 small molecules containing linear or near-linear N−H···N bonds (sample M ) have been optimized at the MP2/6-31G(d,p) level and the values of p ‘ and p ‘ ‘ of the parameters p c at the bond-critical points ( p ‘ in the N−H, p ‘ ‘ in the H···N bond) have been computed from the results of these optimizations. Because the N−H and the H···N part of an N−H···N bond system have different character, the trends of p ‘ and of p ‘ ‘ in M are described by different functions. With the p c as descriptors (the electron density ρ c, the curvatures λ c, i, the Laplacian ∇ 2 c, the kinetic energy densities G c and K c, and the potential energy density V c ), we have searched for correlations of p ‘ and p ‘ ‘ ( homocorrelations ) in M . A high degree of correlation has been found for all the parameters. With the exception of the linear ρ‘,ρ‘ ‘ correlation the homocorrelations of the other p c are nonlinear and some of them nonmonotonic. The homocorrelations permit estimates of the p c values, p s, in symmetric N−H−N bonds, where estimates from experiment are not without problems. They also answer some of the questions concerning limiting values of the p c . With the exception of G c, correlations between unlike p c 's (heterocorrelations, p ‘, q ‘ and p ‘ ‘, q ‘ ‘) will be reported in a subsequent paper, now in preparation. The heterocorrelations involving G c are included here because of the prominence of G c in recent discussion of hydrogen bonds in the literature.