Structure and spectroscopy of (HCN)n clusters: Cooperative and electronic delocalization effects in C–H⋅⋅⋅N hydrogen bonding

Abstract

We investigate ab initio energetic, structural, dielectric, and ir spectroscopic properties of linear (HCN)_n clusters (extending the theoretical levels and cluster sizes previously considered) to quantitatively characterize cooperative effects in C–H...N hydrogen bonding that may be amenable to experimental detection. Our results indicate that large cooperative effects should be evident in H‐bond energies (∼90% increase), intermolecular separations (∼0.10–0.15 Å shrinkage), average dipole moments (∼25% increase), and, particularly, in CH stretching frequencies (∼100 cm⁻¹ shift) and intensities (∼300%–400% increase per monomer) as cluster size increases. Such non‐pairwise‐additive effects lie outside the scope of empirical potentials in common usage, and thus reflect fundamental inadequacies of these potentials and the underlying ‘‘electrostatic’’ picture of H bonding. We employ natural bond orbital (NBO) analysis to examine the detailed electronic origins of cooperative effects, particularly the dramatic ir intensity enhancements that may provide a unique spectroscopic signature of concerted intermolecular charge shifts. NBO analysis suggests how the nonlinear cooperativity effects can be rationalized in terms of the fundamental n_N→σ_CH^* ‘‘charge transfer’’ (‘‘resonance’’) nature of H bonding, manifested even in low‐polarity H bonds involving CH groups.