| Abstract: | Thioamide, 1, is shown to exist as two rotameric forms in solution, rotamer A and B. Conformations of the rotamers are defined to be trans for rotamer A and cis for rotamer B. Energy barriers restricting rotation about the C--N bond of the thioamide group are examined as a function of pH, temperature, buffer concentration, ionic strength, and dielectric constant. Kinetic parameters, kobs, k1, k2, Ea, delta S not equal to, delta H not equal to, and Keq are obtained. No significant buffer effect is observed. Ionic strength (0.2-0.8) does not markedly affect the rate of interconversion. The extent of reaction (Keq) is related to dissociation of the carboxylic acid function. The reaction rate decreases with increasing polarity of solvent which retards interconversion. The energy barriers (Ea = 25-26 kcal/mol) are almost invariant with changing pH values. When protected from light, rotamer A is thermodynamically more stable than rotamer B. The rotamer ratio is dependent on pH and temperature. Under 500 foot-candle light exposure, the ratio of rotamer A to B is close to one at 30 degrees C. |
