Conformation and hydrogen bond in 1,2-propanediol

Abstract The high-resolution microwave spectrum of two conformations, each with an internal hydrogen bond has been observed for the four possible H D isotopic species of 1,2-propanediol. In both rotamers the hydroxyl in position 1 acts as proton donor to the oxygen in position 2, whose hydrogen is trans with respect to the C 1 C 2 bond. In one case the methyl group is trans with respect to the C 1 O 1 bond ( T form) and in the other one is gauche ( G form). The former is the more stable with Δ E T  G = −0.58 kcal/mole. The dipole moment components have been obtained as: μ a T = 2.524, μ b T = 0.00, μ c T = 0.471, μ a G = 2.247, μ b G = 0.258, μ c G = 0.518 D. Evidence of the O ⋯ O “shrinkage” upon deuteration has been observed as the r s (HH) distances between the hydroxyl hydrogens are not consistent within the experimental errors with respect to the choice of the parent molecule. The spectra of some excited states have been measured for the T form.

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