Acetaldehyde–acetaldehyde, formaldehyde–formaldehyde, pyrrole–pyrrole, and acetaldehyde/formaldehyde–pyrrole H-bonds: quantum chemical calculations

The significance of classical N-H…O, and non-classical N-H…π, C-H…O, and C-H…π H-bonds in biological processes is well-known. These H-bonds in the acetaldehyde, formaldehyde, and pyrrole self-associates, and acetaldehyde/formaldehyde–pyrrole heteroassociates have been characterized using quantum chemical parameters such as second-order perturbation energy, electron density, Laplacian of electron density, etc. All the criteria proposed for H-bonds in literature are not found to be strictly adhered by these H-bonds. Significant differences in the computed theoretical parameters between classical N-H…O, and the non-classical C-H…O/N-H… π/C-H… π H-bonds are observed.