A note on the torsional potential functionV(φ) in the dipeptide model

The studies made on N-methyl acetamide (NMA), N-ethyl acetamide (NEA), N-isopropyl acetamide (NIA), using semi-empirical quantum chemical methods have indicated that NMA is not the proper model compound for arriving at the form ofV(φ), whereas NEA and NIA molecules are suggested as model systems for arriving at the form of the potential functionV(φ). The present calculations have indicated thatV(φ) is of the form 1/2 and not (1 — cos3φ); however the value of barrier heightV(φ) was found to be very small. So it is suggested that there is no need of separately adding theV(φ) term in empirical potential energy calculations.