Coming to Grips with N-H…N Bonds, 2. homocorrelations between Parameters Deriving from the Electron Density at the Bond Critical Point

摘要

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.