Glycine Conformational Analysis

Ab initio quantum mechanical methods, including the self-consistent field (SCF), single and double excitation configuration interaction (CISD), the single and double excitation coupled cluster (CCSD), and the single, double, and perturbative triple excitation coupled cluster [CCSD(T)] have been applied to five C₅ conformers and four of their C₁ counterparts on the potential energy hypersurface of glycine. A large basis set TZ2P+f designated H(5s2p1d/ 3s2p1d) and C,N,O(10s6p2d1f/5s3p2d1f) was chosen to evaluate the importance of d functions on hydrogen and f functions on carbon, nitrogen, and oxygen. A very subtle feature of the glycine potential energy hypersurface is the out-of-plane bending of the C₅ structures II (to C\ structure III) and IV (to C₁ structure V). Conformer I is the global minimum at all levels of theory. Two of our results are different from previous ab initio predictions: structure II/III is lower in energy than structure IV/V at our highest level of theory, and should be identified as the second minimum. Secondly, although the C₁ structure V lies slightly below its C₅ counterpart IV at the DZP SCF and TZ2P SCF levels of theory, this ordering is reversed with the TZ2P+f SCF and DZP CISD methods.