Approximate transferability in alkanols

Abstract

The approximate transferability of OH, CH2, and CH3 groups in unbranched primary alkanols has been studied by comparing several atomic and bond properties of the 12 smallest members of this series. These properties were obtained by employing the Atoms in Molecules theory on HF/6-31++G∗∗//HF/6-31G∗ and QCISD/6-31++G∗∗//QCISD/6-31G∗ wave functions. The properties computed at both levels follow parallel evolutions along the series, which allow to conclude that the OH group can be considered approximately transferable along the set of 1-alkanols larger than ethanol, whereas ethanol and methanol present specific hydroxyls. The electron population of the oxygen atom is lower than in ethers, aldehydes, and ketones. The hydroxyl affects significantly the CH3 and CH2 groups that are in positions α, β, γ, or δ, whereas those groups separated from the oxygen by more than 4 bonds can be considered similar to those included in a n-alkane. CH2 groups in the series can be classified into 6 quasi-transferable fragments taking into account their position with regard to the OH (α, β, γ, δ, or beyond (ν)), and respect to the terminal CH3 (attached to it or not). The simultaneous occurrence of both facts gives rise to four specific CH2 fragments: α-CH2 in ethanol, β-CH2 in 1-propanol, γ-CH2 in 1-butanol, and δ-CH2 in 1-pentanol. It has been found that all the CH2 and CH3 fragments that are γ or δ to the OH group do not differ significantly from the corresponding fragments of a dialkyl ether. The energy of oxygen, CH2 and CH3 fragments depend on the molecular size. The effect of the basis set size error on this quantity has been investigated, concluding that the molecular-size dependence is not an artifact due to it. The destabilization experienced by the oxygen atom for a common increase in the molecular size in alcohols is equivalent to that of ethers and smaller than the one displayed by aldehydes and ketones. It was also concluded that the effect due to the variation of the molecular size is independent on the number of alkyl chains that are increased.