Thermochemical Values of Oxygen-Containing Compounds from ab Initio Calculations. 1. Enthalpies of Formation of Ethers and Alcohols

Abstract

A semitheoretical calculation of enthlapies of formation has been applied to alcohols and ethers. The calculation involves two steps. In the first step the ab initio energy for the conformer of lowest energy is converted into an estimate of the formal steric enthalpy (FSE). In the second step the FSE is combined with the formal bond enthalpy (FBE) to generate an estimate of ΔHf°. The group increments for calculating the FBE values are derived from experimental enthalpies of formation. FSE values and FBE values are defined in terms of standard molecules. Calibration requires minimal calculation, and the procedure is readily generalized to other classes of compounds. The calculation is group isodesmic. Three basis sets were used: 3-21G, 6-31G*, and 6-31G**. Electron correlation was performed with single point estimates using MP2 (Moeller−Plesset, truncated at the quadratic expansion) with geometry optimized with HF 6-31G**. For a selection of molecules geometry optimizations were also performed with the MP2/6-31G** procedure. ΔHf° values derived using the 6-31G** basis set with a single point MP2 estimation of electron correlation agree with experimental ΔHf° values within the reasonable standard deviation of 0.55 kcal/mol for 14 molecules, including strained cyclic ethers and highly strained acyclic examples. Examination of the conformer families provides information useful for interpretation of steric effects in synthetic reactions. Average relative energy differences for four important torsional sequences expressed as gauche minus trans energy differences are 0.8 kcal/mol for C−C−C−C (literature), 1.4 for C−C−O−C, 0.6 for O−C−C−O, and −0.5 for C−C−C−O but −1.2 for the last sequence if the terminal carbon atom has an attached oxygen atom. Intramolecular hydrogen bonding effects are important (3 kcal/mol) for some conformers of 1,2-diols and reach 5 kcal/mol for 1,3-diols.