SynopsisThe properties of the electron distribution in amides, peptides, and carboxylic acids, obtained from ab-initio molecular orbital calculations using both minimal and extended basis sets have been studied. These properties are discussed in terms of some of the common assumptions made in empirical conformational calculations of biomolecules. In particular, population analyses of 15 compounds in these families were carried out with both the minimal and extended basis sets, and compared with results of CND0/2 calculations. It is suggested that population analysis is a useful tool for recognizing patterns of charge distributions, and investigating the transferability of parameters of different functional groups. However, its use for providing partial charges for conformational analysis is a questionable procedure.A more detailed analysis of the charge distribution was carried out by calculating the spatial electron distribution in the four compounds, N-methylacetamide, acetic acid, diketopiperazine, and N-acetyl-W-methylalanine. Both total electron-density maps and differencedensity maps are presented. The properties of the overall shape of the molecule and the atoms in the molecule, are discussed in terms of the former along with three-dimensional shape plots of the total density. The distortion accompanying molecular formation, resulting in such features as the lone pair orbital and "bonding deensities" is discussed in terms of the difference maps. Semiquantitative estimates of the bonding and orbital densities resulting from the integration of the densities are also presented. Finally, one of the novel features of the study is the presentation of three-dimensional surfaces of constant difference densities from which the shapes of the orbitals and bonding densities emerge. METHOD Molecular OrbitalsThe quantum-mechanical treatment is performed within the framework of the Hartree-Fock approximation. The 2N electron wave function x of the molecular ground state is written as an antisymmetrized product of one electron molecular orbitals 4;:The $i are constructed as linear combinations of atomic orbitals 6, as:The coefficients Cij of the j t h atomic orbital in the ith molecular orbital are obtained by solving Roothaan's equation^.^ Two different basis sets were used: a minimal STO-3G5 and an extended 6-31G6 basis. In the STO-3G basis set the $j(r) are represented by a linear combination of three Gaussian functions, which gives a best least square fit to a Slater-type orbital with exponent lU:

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