Hydrogen bonding: part 78. Ab initio molecular orbital study of intra- andintermolecular hydrogen bonding in choline and betaine and their compoundswith HF and H2O

We previously prepared several compounds of the zwitterions [(CH3)(3)NCH2CH 2O](0) (deprotonated choline, herein named cholaine) and [(CH3)(3)NCH2CO2]( 0) (betaine) and proposed structures based on infrared spectroscopy. We now examine these compounds with use of ab initio molecular orbital methods to further elucidate possible structure. These calculations demonstrate that: (1) cholaine and betaine both have internal CHO hydrogen bonds, and these are retained in some form in all other compounds. (2) Cholaine hydrate and hydrofluoride and betaine hydrofluoride monomers have covalent three-center hydrogen bonds between H2O or HF and negative zwitterion oxygen, and addit ional CHX hydrogen bonds to H2O oxygen or HF fluorine. (3) Cholaine monohyd rate and cholaine hydrofluoride monohydrate form dimers of C-i symmetry whi ch contain planar C-2h ((H2OO)-O-.)(2) and ((HOHF)-F-.)(2) clusters. (4) Ch olaine hydrofluoride forms head-to-tail dimers bound by intermolecular CHX hydrogen bonds; this arrangement could lead to extended linear structures i n the solid state. (5) Betaine hydrofluoride, in contrast, forms a tightly bound discrete dimeric unit in which two molecules join in a head-to-head m anner held together by five intermolecular hydrogen bonds and by the mutual proximities of negative fluorides to positive nitrogens. (C) 2001 Elsevier Science B.V. All rights reserved.