The stable conformers of glycine and the inter-conversions between them were studied theoretically at various levels of theory, B3LYP, MP2, CCSD and CCSD(T), in the gas phase and in aqueous solution. In aqueous solution, the structures examined by use of the conductor-like polarizable continuum model (CPCM) with various cavity models, UA0, UAHF, UAKS, UFF, BONDI and PAULING, and by use of a discrete/continuum solvation model with eight water clusters. The Gibbs free energy differences between the neutral (NE) and zwitterionic conformers (ZW), ΔG Z-N [= G ZW -G NE ], in aqueous solution were well reproduced by using the BONDI and PAULING cavity models. However the ΔG Z-N values were underestimated in other cavity models, although the ZW conformers existed as stable species in aqueous solution. In the studies of a discrete/ continuum solvation model with eight water clusters, gas phase results are still insufficient to reproduce the experimental findings. However the ΔG Z-N values calculated by use of CPCM method in aqueous solution agreed well with the experimental ones.
In this work, the reactions of carbamyl and thiocarbamyl halides with NH 3 were studied in the gas phase at the MP2(FC)/6-31+G(d) level of theory. Single point calculations were performed at the QCISD/6-311+G(3df,2p) to refine the energetics. The reaction mechanisms were also studied in aqueous solution. The structures were fully optimized at the CPCM-MP2(FC)/6-31+G(d) and refined by a single point CPCM-QCISD/6-311+G(3df,2p) calculations. The reaction mechanisms for the title compounds were compared with those for the acetyl and thioacetyl halides. The lower reactivity of carbamyl (and thiocarbamyl) groups was explained by comparing the C=O and C=S π-bond strengths as well as resonance contributions in the ground state.
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