Thermochemical Values of Oxygen-Containing Compounds from Ab Initio Calculations: 2. High Accuracy Enthalpies of Formation of Alcohols and Ethers

The objective of this series of studies is to develop procedures for calculating high quality enthalpies of formation and differences of enthalpies of typical organic compounds. Can this be achieved using basis set/ electron correlation methods (BSECMs) of modest size so that the calculations are routinely applicable to molecules having 12 or more heavy atoms? The answer is a qualified "yes." The procedure I have explored is based on conversion of ab initio energies into formal steric enthalpy (FSE) values. FSE is the difference of the energy of a target molecule and Sigman(i)d(i), the sum of the energies of its constituent structural groups as defined by a set of standard molecules. FSE values are group isodesmic because the same numbers of each structural group appear in both the target molecule and in the summation. To a considerable extent, the isodesmic calculation cancels out errors due to limitations of BSECMs. FSE values can be converted to estimates of gas phase DeltaH(f)(omicron) values by a method related to the group increment method developed extensively by Benson and others. Energies derived with a number of BSECMs were explored. Of those evaluated, the most successful were MP2/6-31+G(d,p)//6-31G(d,p) and MP2/6-311+G(2df,2p)//6-3 1G(d,p). For 21 alcohols and ethers having a range of ring strain and steric congestion, the former gave differences between calculated DeltaH(f)(omicron)(g,298) Values and experimental values with a standard deviation of 0.56 kcal/mol and with a maximum deviation of 1.35, whereas the latter gave a standard deviation of 0.62 with a maximum deviation of 1.74. These numbers can be compared with an estimated standard deviation of the experimental DeltaH(f)(omicron) data of 0.44 kcal/mol and a maximum deviation of 1.22 for the same data set. Energies derived with the popular density functional B3LYP/G-31G(d,p) gave poorer results. The standard deviation was 1.25 and maximum deviation was 2.49. An important use of FSEs is in comparing enthalpies. While comparison of ab initio energies is restricted to conformers of the same molecule, comparison of FSEs gives a valid estimate of the difference of enthalpies of isomers as well. The difference of FSEs of unrelated molecules provides an estimate of the difference of strain enthalpies, and this may be converted to an estimate Of the difference of total enthalpies by correcting for the bond enthalpy terms as described in the text. These comparisons pertain to hypothetical compounds that consist solely of the molecules being compared.