Structure, conformations, and internal hydrogen bonding in gaseous 4-fluorobutan-1-ol. Gas-phase electron diffraction and ab initio study

Excluding enantiomers, there are 14 possible rotomeric conformations of 4-f luorobutan-1-ol, of which two are capable of forming an internal O-(HF)-F-. .. hydrogen bond. The composition of the gaseous system is of special inter est because it is determined by the energies of the conformers which reflec t the energies of the hydrogen bonds. We have investigated the conformation al composition of the gaseous system and the molecular structures of the co nformers at 83 degreesC by gas-phase electron diffraction (GED) augmented b y molecular orbital calculations. Because of the complexity of the 4-fluoro butan-1-ol system, the parameters of the several models tested were simplif ied by various constraints taken from the theoretical work. With these cons traints, the best agreement with the GED data was obtained with a model con sisting of about equal amounts of hydrogen-bonded and non-hydrogen-bonded c onformers. Because the curled-up shape of the two forms capable, in princip le, of forming internal hydrogen bonds is expected to be energetically unfa vorable in the absence of such bonding, the experimental results are interp reted as strong evidence for its existence. Weighted average values, with e stimated 2 sigma uncertainties, of the more important bond distances (r(a)/ Angstrom) and bond angles (angle (a)/deg) for the preferred model are r(C-O -C) = 1.529(2), r(C-C-C-C) = 1.537(2), r(C-C-F) = 1.520(2), r(C-O) = 1.430( 5), r(C-F) = 1.401(5), angle (C-O-C-C) = 112.5(33), angle (C-C-O)(H bond) = 112.3(55), angle (C-C-O)(no bond) = 108.2(45), and angle (C-C-C-F) = 109.8 (12). The two hydrogen-bonded conformers comprise 48.5% of the mixture with an estimated 2 sigma, uncertainty of 14.0%. A rough estimate of the energy of the O-(HF)-F-... hydrogen bond is 3 kcal mol(-1). The average (OF)-F-.. . separation in this bond for the H-bonded conformers is 2.46(4) Angstrom, about 0.3 Angstrom less than the sum of the van der Waals radii.