Ab Initio NMR Study of the Isomeric Hydrogen-Bonded Methanol–Water Complexes EUDES E. FILETI, SYLVIO CANUTO Instituto de Fı ´sica, Universidade de Sa ˜o Paulo, CP 66318, 05315-970, Sa ˜o Paulo, SP, Brazil Received 2 July 2004; accepted 25 October 2004 Published online 7 January 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.20437 ABSTRACT: Isotropic and anisotropic chemical shifts for all atoms of complexes CH 3 HO ... H 2 O and CH 3 OH ... OH 2 have been calculated at the Hartree–Fock, second- order Møller-Plesset (MP2) and density functional (B3LYP) theoretical levels using the 6-311++G(2d,2p) basis set. The influence of the hydrogen bond formation on the nuclear magnetic resonance chemical shifts in all atoms is analyzed. The basis set superposition error was taken into account, and its effects were more significant for the anisotropic shieldings on the oxygen atoms of the proton donor CH 3 OH ... OH 2 and proton acceptor methanol CH 3 HO ... H 2 O. Using counterpoise correction to the MP2 results, our best estimate for the calculated isotropic and anisotropic shifts of the H atom involved in the hydrogen bond are -2.98 and 11.95 ppm for CH 3 HO ... H 2 O and -2.91 and 11.48 ppm for the CH 3 OH ... OH 2 isomer, respectively. For the O atom of the OH hydrogen bonded, these calculated shifts are -5.21 and -5.35 ppm for CH 3 HO ... H 2 O and -6.32 and -8.75 ppm for CH 3 OH ... OH 2 . The effect of monomer relaxation was also considered and was found to be appreciable only for the oxygen atom of the proton donor OH due to the distance increase after complex formation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem 102: 554 –564, 2005 Key words: NMR; methanol–water; hydrogen bond; chemical shift; ab initio Introduction W ater–alcohol mixtures exhibit anomalous behavior compared with the properties of the pure components. When alcohol and water are mixed, the entropy of the mixture increases less than expected. This effect was first pointed out by Frank and Evans [1] and was explained in terms of a hydrophobic interaction with the nonpolar alco- hol headgroups, which induces an ice-like structure in the surrounding water. There is extensive litera- ture on this topic [2–12]. Spectroscopic studies [2–9] and computer simulations [8, 10 –12] have been per- formed to study the structure of water–alcohol mix- Correspondence to: S. Canuto; e-mail: canuto@if.usp.br Contract grant sponsors: CNPq; FAPESP (Brazil). International Journal of Quantum Chemistry, Vol 102, 554 –564 (2005) © 2005 Wiley Periodicals, Inc.