ROTATIONAL ISOMERS OF N-(BETA-PHENYLPROPIONYL)ALANINE ETHYL DITHIOESTER - A RAMAN-SPECTROSCOPIC AND MO STUDY

Raman spectra of N-(beta-phenylpropionyl)alanine ethyl dithioester (C6 H5CH2CH2C(=O)NHCH(CH3)C(=S)SC2H5) in CCl4 and CH3CN solutions were mea sured as a function of temperature and the enthalpy differences (DELTA H) between rotational isomers differing by internal rotation around th e NH-CH(CH3) and CH(CH3)-C(=S) bonds (forms A, B and C5) were evaluate d from relative band intensities. The spectroscopic results are consis tent with a greater thermodynamical stability of the B-type conformer, where the N and S(thiol) atoms are in close contact. In addition, a c omparison of the measured DELTAH(A-B) for the present molecules with p reviously reported values for a series of similar glycine-based ethyl dithioesters shows that the presence of the extra CH3 group at the alp ha-carbon atom leads to a stabilization of the B-type conformer relati ve to the A-type form in the alanine-based dithioester. Semiempirical AM1 molecular orbital calculations were also performed on the title mo lecule and on its glycine analogue, N-(beta-phenylpropionyl)glycine et hyl dithioester. In general terms, the results of these calculations a gree with the experimental findings, thus providing good theoretical s upport for the experimental data.