Thermodynamic Properties of Cysteine in Harmonic and Anharmonic Approximations Taking Into Account Mixing

The geometric structure, total electron energies, and harmonic and anharmonic frequencies of vibration are found for cysteine conformers by means of M06/6-311++G(3df,3pd). The internal rotation in the main conformer is studied, and the contribution of internal rotation to the thermodynamic functions is considered. The conformational space, main conformers, and mole fractions of cysteine are determined. Thermodynamic functions of cysteine in the gas phase are found in the 298−1500 K range of temperatures with allowance for symmetry and isomerism, mole fractions, and contributions of mixing in harmonic and anharmonic approximations. It is shown that the entropy and free energy must necessarily include the contribution of mixing.