Theoretical Analysis of Molecular Structure, Hydrogen Bond Strength, and Proton Transfer Energy in O-H‚‚O Aromatic Compounds J. Palomar,* ,† J. L. G. De Paz, and J. Catala ´ n Departamento de Quı ´mica Fı ´sica Aplicada, UniVersidad Auto ´ noma de Madrid, E-28049 Cantoblanco, Madrid, Spain ReceiVed: NoVember 16, 1999; In Final Form: February 28, 2000 Molecular geometries for a set of 2-hydroxybenzoyl compounds were obtained at B3LYP/6-31G** level and analyzed in view of a parametric model of intrinsic substituent effects by Taft and Topsom. The structural study of the non- and hydrogen-bonded species, together with proton transferred forms, resulted as very useful in understanding the different factors determining the intramolecular hydrogen bond strength and the proton transfer process in this family of molecules. In addition, the previous study was extended to a sequence of other related six-membered hydrogen-bonded structures (alkane, naphthalene, and alkene derivatives) with increasing aromaticity. The results clearly showed the influence of the covalent and electrostatic (acid-base) nature of the hydrogen bond system on its commonly related chemical properties, hydrogen bond strength, and proton-transfer energy. A significant finding in this paper is the approach between the oxygens that yields the internal hydrogen bond, which occurs in the midpoint of the proton transfer, depends on the acid- base characteristics of the proton donor and acceptor groups, and it is not substantially affected by the aromaticity of the system. 1. Introduction Organic molecules as 2-hydroxybenzoyl compounds (see Scheme 1) possess a strong intramolecular hydrogen bond (IMHB) (Chart 1) as a result of they bearing a hydroxyl group and a carbonyl group that act as a proton donor (acid) and acceptor (base), respectively, in adjacent positions. It is widely accepted that the presence of this strong IMHB is partly responsible for the characteristic photophysics properties of these compounds. 1,2 According to the mechanism proposed by Weller, 3,4 the changes in the acid-base properties of the hydroxyl and carbonyl groups on the aromatic ring by the effect of electronic transitions may give rise to the mechanism shown in Scheme 1. Proton transfers (PT) take place via the IMHB, and therefore, its characteristics should mainly influence these processes. The nature of the intramolecular hydrogen bond of 2-hy- droxybenzoyl derivatives has been studied by several experi- mental techniques such as infrared (IR), 5-14 Raman, 15 nuclear magnetic resonance (NMR), 16-18 and X-ray and neutron diffraction 19-23 spectroscopies. Theoretical calculations nowa- days provide a complementary way to study these molecular systems containing an IMHB. The development of density functional theory (DFT) at present has allowed one to obtain accurate information on these medium-size IMHB compounds. In fact, they predict molecular properties, such as geometries, IR or Raman frequencies, and chemical shifts, which match outstandingly well the available experimental data. 24-34 Historically, IR spectroscopy has been the most used method to detect and characterize hydrogen bonds.This technique has shown that 2-hydroxybenzoyl compounds only form strongly internal hydrogen-bonded isomers in inert media. 5-14 In addition, IR measurements proved that the IMHB strength for a series of related aromatic compounds increased with the double bond character of the bond that connects the functional groups (C 3 d C 4 ; see Chart 1). 6 In this sense, IR data have been commonly used for evaluating the strength of the hydrogen bond of the type -O-H‚‚Od, taking into account the frequency shift of the characteristic donor (O-H) or acceptor (CdO) stretching bands, Δν ˜ ) ν ˜ non-IMHB - ν ˜ IMHB , by the influence of the IMHB. 5,8 The problem of both frequently used criteria is the † Telefax: (internat.) +91-3974-187. E-mail: pepeo@tendilla.qfa.uam.es. SCHEME 1 ν ν CHART 1 6453 J. Phys. Chem. A 2000, 104, 6453-6463 10.1021/jp994067o CCC: $19.00 © 2000 American Chemical Society Published on Web 06/20/2000