Ab initio calculations have been carried out at the MP2/6-3 l+G(d,p) level of theory to determine the equilibrium structures of complexes formed between HF and the chloromethanes, HF:CH4-,,Cln, for n = 1-4. The equilibrium complexes present a variety of structural types including cyclic structures stabilized by two distorted hydrogen bonds, open structures stabilized by a traditional linear hydrogen bond, dipoledipole bifurcated and trifurcated hydrogen-bonded structures, and a van der Waals complex. Two equilibrium structures have been found on each surface except for HF:CH&l, where there are three. Binding energies have been computed at MP2/ cc-pVTZ+//MP2/6-31+G(d,p), and binding enthalpies at 13 K range from 0.3 to 3.2 kcal/mol. The order of increasing stability of the most stable complexes on these surfaces is HF:CCl4 < HF:CHC13 < HF:CH2C12 < HF:CH&l, which parallels the order of increasing electron density on the C1 atom. Because of the structural diversity of these complexes, the shifts of the H-F stretching frequency observed experimentally do not correlate with the stabilities of these complexes. However, when HF is a proton donor, the H-F frequency shifts do correlate with binding energies provided that complexes of the same structural type are compared. The computed results provide insight into the origin of the H-F frequency shifts and detailed structural and energetic information about these complexes which is not available experimentally.

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The competitive intran~olecular O H . .

Of the five conformers predicted for 2-haloethanols, only tm70 (halogen and OH trails and gauche) can be positively identified by high resolution infrared spectroscopy. From the temperature depelldence of the relative intensities of the fundamental OH stretching bands of the trans and gauche forms of the compouilds XCHZCHaOH, where X = F, C1, Br, and I, the gauche conformers are shown to be energetically favored in dilute carbon tetrachloride solution, with enthalpy differences between the two conforlners of 2.072~0.53, 1.20&0.09, 1.26f 0.08, and 0.8110.09 Ircal/mole respectively. Equilibrium constants and the conformational free energy and eiitropy differences are reported. Inter-and intra-molecular OH. . .X hydrogen-bond interactions are found to be similar in that the enthalpy increases in the order X = I < Br < C1 < F, and is linearly related to a decrease in the fundamental OH band shift, in contrast to the Badger-Bauer relationship. The ~nolecular and thermodyliamic factors governing conformatioiial equilibria involving intramolecular hydrogen bonds are discussed. IYTRODUCTIONSince 1936, when the thermodynamic properties of hydrocarbons calculated on the basis of restricted internal rotation about a single bond were first shown to be in agreement with the experimental values ( I ) , a large amount of inforination has been accumulated about the nature of the potential ininirna which exist for those molecular conforinations in which the bonds to the atonls on the axis of rotation have taken up a staggered orientation (2). As the barriers t o rotation are usually relatively low, aliphatic hydrocarbons and their derivatives exhibit a high degree of conformational heterogeneity in the gaseous or liquid state. In the crystalline state such rotational freedom is lost, and the rnolecule is frozen in its no st stable form. RIuch of the early work in this field involved halogenated ethanes.Recently Xuhn, Luettke, and Aleclte (3).showed that in 2-haloethanols in CC14 solution only two conforn~ers (trans and gauche) could be identified by means of infrared spectra in the OH stretching region obtained with a pris~n spectrophotometer. The two gauche forms in the equilibria are inirror inlages and thus spectros~opically equivalent. The low frequency vex con~ponent is assigned to the gauche forms. NL'Funck's general treatment of rotational isoinerisill in linear polymethylene chains X-(CH2),-Y (4) predicts nine rotational isomers for XCHzCH20H, taking into V a p e r presented at the @tlz Annual