REGRESSION AND PRINCIPAL COMPONENT ANALYSES OF INTERNAL COORDINATES FOR THE CARBOXAMIDE AND CARBOXYLATE GROUPS - DIVERSITY OF AMIDE BONDING IN PRIMARY CARBOXAMIDES, OLIGOPEPTIDES, AND LACTAMS

A new model of electronic structure of the carboxamide and carboxylate bonds is proposed to account for the diversity of the patterns of structural variation displayed by such bonds in the crystal structures. The geometries of the amide and carboxylate ester fragments retrieved from the Cambridge Crystallographic Data Centre database were examined by means of the regression and principal component analyses. Correlations of the C=O and C-O/C-N bond distances and correlations of the bond distances with the out-of-plane distortions are consistent with the predictions of Pauling's resonance model only in the more extensively substituted esters, secondary amides, and common ring lactams. Surprisingly, in the unsubstituted methyl esters and primary amides, correlations between the bond distances are positive (e.g., both C-N and C=O increase or decrease). Furthermore, for the majority of the less substituted amides and lactams, an increase in pyramidalization at the nitrogen is associated with the shortening of the C-N bond instead of the expected lengthening. Consequently, factor analyses of the r(C=O), r(C-N), \theta(N)\ Coordinates for the 42 subclasses of amides and lactams reveal three patterns of coupling of structural parameters; these patterns appear to be related to the major types of the amide substitution. A hypothesis explaining this diversity is based on the assumption that structural variation observed in each of the narrowly defined subclasses of amides maps out initial stages of rehybridization accompanying internal rotation; that is, the amide bonds in the crystal structures deform along the rehybridization/rotation path. It is proposed that the positions of the minimum and the saddle point along this path depend on the alkyl substitution of the bond and the size of the embedding ring.