Conformational Analysis of -Alkyl--[2-(diphenylphosphoryl)ethyl]amides of Diphenylphosphorylacetic Acid: Dipole Moments, IR Spectroscopy, DFT Study.

Experimental and theoretical conformational analysis of -methyl--[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide, -butyl--[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide, and -octyl--[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide was carried out by the methods of dipole moments, IR spectroscopy, and Density Functional Theory (DFT) B3PW91/6-311++G(df,p) calculations. In solution, these ,-dialkyl substituted bisphosphorylated acetamides exist as a conformational equilibrium of several forms divided into two groups-with - or -configuration of the carbonyl group and alkyl substituent, and or arrangement of the phosphoryl-containing fragments relative to the amide plane. The substituents at the phosphorus atoms have eclipsed - or staggered -orientation relative to the P=O groups, and orientation of the substituents is due to the presence of intramolecular H-contacts P=O...H-C or p,π conjugation between the phosphoryl group and the phenyl ring. Preferred conformers of acetamides molecules are additionally stabilized by various intramolecular hydrogen contacts with the participation of oxygen atoms of the P=O or C=O groups and hydrogen atoms of the methylene and ethylene bridges, alkyl substituents, and phenyl rings. However, steric factors, such as a flat amide fragment, the bulky phenyl groups, and the configuration of alkyl bridges, make a significant contribution to the realization of preferred conformers.