Conformational analysis of N-methylglycine and N,N-Dimethylglycine (cas 1118-68-9) by ab initio calculations

Add time:09/27/2019    Source:sciencedirect.com

Eight of the most stable conformers of N-methylglycine (NMG) and five of N,N-Dimethylglycine (cas 1118-68-9) (DMG) were analyzed by high level ab initio calculations. Since NMG has only one amino hydrogen and a carboxylic acid hydrogen, it is capable of the formation of various types of hydrogen-bonded conformers and as a result is ideally suited to studying the importance of hydrogen-bonding on the relative stabilities of the various types of conformers of glycine and N-alkylated glycines. Comparisons of the relative energies of the various NMG and DMG conformers that have different types and number of hydrogen bonds (H-bonds) reveal the importance of hydrogen bonds to the stability of the different types of conformers. For NMG, conformer Ib which has two types of H-bonds and a dipole moment of 1.2 debyes is the most stable. Conformer Ib is similar to that of the most stable conformer of glycine. For DMG, on the other hand, IIc is the most stable conformer. IIc has a dipole moment of 5.6 debyes (compared to a value of 1.1 debyes for another of its conformers, Ic) and only one H-bond which involves the carboxylic acid and amino functionalities. The stability of IIc is attributed to the relative strength of the type H-bond formed — a similar type H-bond of glycine and NMG is predicted to be weaker. Thus, for a particular conformer, the relative strength and number of possible H-bonds that can be formed, and not necessarily the magnitude of the dipole moment, play key roles in the relative stability of amino acid conformers in the gas phase.

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