Conformers of gaseous glycine

Abstract

9568-9575error. The following gives the choices made for the BDEs.CH. The CH BDEs of several molecules have been determined.70 However, Me3Si-CHJ1 (BDE of 99.2 kcal/mol) is the most similar to the present systems. Therefore, the value of 99 kcal/mol is used for all C H BDEs.SiH. The closest analogue to the systems of interest is the H3Si-SiH, molecule, which has a BDE of 86.3 kcal/m~l.~' The value of 86 kcal/mol is used for all SiH BDEs.CeH. The BDE of Ge-H for CH3GeH3 is 83 k c a l / m~l .~~ The BDE for GeH4 is given as 8473 and 8972 kcal/mol by different experimental researchers. Also, a theoretical value of 84.8 kcal/mol has been determined by Binning and C~r t i s s . '~ The BDE for Me3GeH has been determined to be 82 kcal/mol. The value of 82 kcal/mol is chosen for all G e H BDEP except for that of CH3GeH3, since the electronegativity of Si, Ge, and Sn should have similar effects as that of the bulky methyl groups of Me3GeH.SnH. The BDE of SnH4 is 71.6 kcal/m~l.'~ The value for Me3Sn-H76s77 and Bu3Sn-H7* is 74 kcal/mol. Therefore, the value of 74 kcal/mol is used for all Sn-H BDEs. (73) Noble, P. N.; Walsh, R. Inr.Abstract: Correlated level ab initio calculations (large basis set MP2, and MP4, CCSD, and CCSD(T) computations) have been performed for 13 conformers of neutral glycine, including all 8 possible conformers with planar heavy-atom arrangements. These calculations resulted in accurate geometric structures, relative energies, harmonic vibrational frequencies, and infrared intensities for all conformers. The structural results obtained support the rotational constants measured for the two lowest-energy forms of glycine, and their high accuracy should be profitable in the search for other conformers by rotational spectroscopy. Energetic, structural, and quadratic force field results indicate possible model improvements for an existing gas-phase electron-diffraction study of the lowest-energy conformer. Predictions, probably accurate to within about 100 cm-I, are made for the order and relative energy of all conformers considered. (1) (a) Sellers, H. L.; Schifer, L. J. Am. Chem. Soc. 1978, 100, 7728. (b) Schifer, L.; Sellers, H. L.; Lovas, F. J.; Suenram, R. D. J. Am. Chem. SOC. 1980, 102, 6566. (2) Frey, R. F.; Coffin, J.; Newton, S. Q.; Ramek, M.; Cheng, V. K. W.; Momany, F. A.; Schifer, L. J. Am. Chem. SOC. 1992, 124, 5369. (3) Sam, K.; Klimkowski, V. J.; Ewbank, J. D.; Van Alsenoy, C.; Schifer, L.

Electronic and vibrational polarizabilities of the twenty naturally occurring amino acids

“…The energy of 1 relative to 2 (-0.84 kcal/mol, when corrected for ZPVE) is somewhat overestimated with respect to the experimental value of -1.4 kcal/mol [21] and to the CCSD value of -1.43 kcal/mol [20]. Note that single point CCSD(T)/DZP calculations [20] gave E R = -1.06 kcal/mol, while MP2/6311++G** calculations gave E R = -0.59 [22][23][24] and -1.05 [25] kcal/mol vs. a DFT-B3LYP/6-311G** datum of -0.82 kcal/mol [25].…”

Matrix-Isolation Infrared and Theoretical Studies of the Glycine Conformers

“…[10][11][12][13][14][15][16][17][18][19][20] All calculations have been consistent in predicting that the conformer I is the most stable form. However the stability order and the structures of the other glycine conformers depended on the level of theory and the basis set used in the calculations.…”

“…The entry-level HF calculations with a small basis sets 19 predicted the planar backbone structure for all three conformers. Increasing the basis, by additional polarization functions and accounting for the electron correlation effects, led to nonplanar structures of the conformers II and III, 10,12,16,17 while conformer I still remains planar in all calculations. As it is seen from Table 1, at both the DFT/ B3LYP and MP2 levels we obtain planar structures for conformer I and nonplanar structures for conformer II.…”

“…For example, the energy difference between the more stable non-planar form of the conformer II and its planar counterpart was predicted to be 0.21 kJ mol -1 at the B3LYP/ DZP level, 11 0.24 kJ mol -1 at the MP2/6-31++g** level, 16 0.55 kJ mol -1 at the CISD/DZP level, 15 and 0.42 kJ mol -1 at the CCSD (T)/DZP level. 15 For conformer III, the calculation carried out at the CISD/DZP 15 and DFT/B3LYP/DZP levels 11 predicted the planar conformation to be the true minima, but at the same time the calculation at the MP2/6-311++G** level, 16 as well as our calculations at the MP2/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels, predict that a nonplanar conformation is the true minimum. The flat potential energy surface near the equilibrium point predicted for the conformer III requires tighter thresholds in the geometry optimization.…”

“…12, 17 The DFT calculations of the structure and relative stabilities of the glycine conformers 11,14 produced results which are very similar to the results of the MP2 method. The sophisticated investigations of the potential energy surface of glycine 12,15,16,19 allowed identification of as many as eight minimum-energy conformers and the estimation of the barriers of the conformer interconversions. Some of these barriers were predicted to be quiet small.…”