Why betaine crystallizes in high local Cs symmetry. An ab initio MO and DFT study of anhydrous betaine and betaine monohydrate |
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Authors: | Tommi H Nyrönen Reijo Suontamo Ilkka Pitkänen |
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Institution: | CSC – Center for Scientific Computing, P.O. Box 405, FIN-02101 Espoo, Finland, FI Department of Chemistry, University of Jyv?skyl?, P.O. Box 35, FIN-40351 Jyv?skyl?, Finland, FI
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Abstract: | A theoretical study of the structure, charge distribution, rotational barrier and fundamental vibrations of anhydrous betaine
(CH3)3NCH2COO (trimethylglycine) was carried out and compared with available experimental data. Calculations were carried out at HF,
MP2 and B3LYP levels using a 6-31+G(d,p) basis set. The calculated rotational barrier of the betaine carboxylic group is 40.5 kJ/mol at the MP4(SDQ)/6-311G(d,p)//HF/6-31+G(d,p) level of theory. The rotation of the carboxylic group changes the molecule from a highly symmetric (C
s
) conformation into a twisted conformation resulting in shortening of the molecule by about 50 pm. Natural population analysis
(NPA) indicates intramolecular interaction between the carboxylic oxygen and the nearest methyl hydrogens resulting in internal
hydrogen bonding. MP4(SDQ)/6-311G(d,p) single-point NPA calculations on a betaine monohydrate model taken from the X-ray geometry show an expected weakening in
the internal hydrogen bond. Calculations explain why betaine preferentially crystallizes in high local C
s
symmetry.
Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 7 December 1998 |
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Keywords: | : Ab initio molecular orbital and density functional theory calculations Trimethylglycine Anhydrous betaine Betaine monohydrate Charge distribution Rotational barrier |
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