Matrix-isolation FT-IR spectra and molecular orbital calculations on neutral N,N-dimethylglycine

Abstract

The structures and vibrational spectra of the preferred conformers of the neutral form of N,N-dimethylglycine (DMG) were studied by a combined approach, using DFT(B3LYP)/6-311++G** and MP2/6-31++G** calculations and low temperature matrix isolation IR spectroscopy. The conformational ground state was found to be the intramolecularly O-HÁ Á ÁN hydrogen-bonded GAT form, where the (lone pair)-N-C-C and N-C-C=O dihedral angles are 30 (gauche; G) and ca. 180 (anti; A), respectively, and the carboxylic group assumes the trans (T ) configuration (O=C-O-H dihedral angle equal to 180). The presence in the matrices of two additional conformers, where the carboxylic moiety assumes the most commonly found cis (C) conformation and the N-C-C=O axis adopts the syn arrangement (the two conformers differ only in the positions of the methyl groups), could also be established. Observation of these conformers is in consonance with the theoretical predictions, which indicate that the observed conformers should differ in energy by less than 7 kJ mol À1. Full assignment of the observed infrared spectra of both DMG and its -OD isotopomer in Ar and Xe matrices was carried out on the basis of comparison with the theoretically predicted spectra and temperature variation experiments.

Importance of Entropy in the Conformational Equilibrium of Phenylalanine: A Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Study

“…27,28 Most of these studies also reported theoretical calculations at various levels of theory. [5][6][7][8][9][10][11][12][13]15,17,[19][20][21][22][23][24][25][26][27][28] In general, the gas-phase results are qualitatively consistent with the matrix-isolation data, despite van der Waals forces appearing in the solid matrix between the sample and inner gas molecules. The most stable structures of aliphatic amino acids have been found to be stabilized by a bifurcated H-bond linking the carboxylic oxygen atom and the hydrogen atoms of the amino group.…”

“…48 In matrixes, conformational cooling has been demonstrated to occur in glycine, sarcosine and other related R-substituted carbonyl compounds possessing barriers to intramolecular rotation of only a few kJ mol -1 . 15,19,49,50 Therefore, the conformational interconversion barriers between the relevant conformers of phenylalanine (in particular those conformers whose geometries differ essentially by the value of one torsion angle) were calculated to evaluate the possibility of this phenomenon to take place also in this molecule. The computed interconversion energy barriers are presented in Table 4.…”

“…[1][2][3] Amino acids adopt their neutral form also in the gas phase [4][5][6][7][8][9][10][11][12][13] and in low-temperature inert matrixes. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Therefore, the structures of these compounds have been extensively studied in both these media. A number of aliphatic amino acids as well as some of their derivatives have been investigated experimentally up to date: glycine, 4,5,[14][15][16][17][18] sarcosine (N-methylglycine), 19 N,N-dimethylglycine, 20 alanine, 6,[21][22][23] valine, 7,24 proline 8,25,26 and serine.…”

“…[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Therefore, the structures of these compounds have been extensively studied in both these media. A number of aliphatic amino acids as well as some of their derivatives have been investigated experimentally up to date: glycine, 4,5,[14][15][16][17][18] sarcosine (N-methylglycine), 19 N,N-dimethylglycine, 20 alanine, 6,[21][22][23] valine, 7,24 proline 8,25,26 and serine. 27,28 Most of these studies also reported theoretical calculations at various levels of theory.…”

“…DFT methods have been shown to be reliable enough to provide good estimations of energy barriers for this kind of molecules, being successfully used in the past, for instance, in the investigation of serine, 27 valine 24 and glycine derivatives. 19,20 Contrary to aliphatic amino acids, the structures and preferred conformations of aromatic amino acids are still quite unexplored fields. There are three naturally occurring aromatic amino acids: phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp).…”