Polar and stereoelectronic effects on the structural and spectroscopic properties of halomethanols

Halogenated methanol derivatives (halogen = F, Cl and Br) were theoretically studied and the gauche conformer relative to the X–C–O–H torsional angle was the single energy minimum found at the MP2/6-311G++(d,p) and B3LYP/6-311G++(d,p) levels in the gas phase, implicit and explicit aqueous solution. The generalized anomeric effect is operative, since the structure (gauche) with an electron lone pair in the anti orientation relative to the halogen is the stable conformation. Natural bond orbital calculations show that this conformational preference, in comparison with the stationary anti structure that exhibits imaginary frequency, is due to hyperconjugation rather than electrostatic effects, while internal hydrogen bond OH⋯X does not operate. However, the 1JC,F coupling constant for the fluorinated derivative is exponentially dependent on the molecular dipole moment and linearly correlated with the C–F distance, but a correlation with the nO → σ∗CF hyperconjugation was not observed. Thus, the generalized anomeric effect in some model systems agrees with the hyperconjugation model, while the Perlin-like effect does not appear to have hyperconjugative dependence, but a polar instead.