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B3LYP/6-311+G(2d,p), the density functional theory method of gaussian 98 package, is applied to study the hydrogen bonding of a series of linear (HCN) n and (HNC) n molecular clusters (for n =1–10). By the localization analysis methods we developed, pair-wised σ type H-bond orders and bond energies are calculated for each pair of the two near-by molecules in both (HCN) n and (HNC) n clusters. The calculated results are checked well with the shortening of N–H or C–H distance, the elongation of CH or NH bond distance, and the red shift of stretching frequencies of CH or NH. All pieces of evidence show that the central pair of the two molecules forms the strongest H bond when n of (HCN) n or (HNC) n is even, and the two middle pairs form the two strongest H bonds when n is odd. Two terminal pairs of HCN or HNC molecules always form the two weakest H-bonds in each molecular cluster. When comparing molecular cluster energies between (HCN) n and (HNC) n for various values of n , the well-known (HCN) n is found more stable than the related (HNC) n from energy calculation. However, if outcomes of H-bond local analysis are contrasted, our analysis significantly shows that inter-molecular H-bonds inside of (HNC) n clusters are much stronger than the corresponding H-bonds in (HCN) n with the same n . In comparing energy differences between these related clusters per monomer, [ E (HNC) n − E (HCN) n ]/ n is found decreasing monotonically as n increases. All pieces of evidence from this theoretical prediction indicate that (HNC) n with large n is probably constructed by its relative strong H-bonds. |
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