Systematic errors in DFT calculations of haloalkane heats of formation

Abstract

The B3LYP and B3PW91 density functionals were employed with a large [BS1 = 6-311+G(3df,2p)] and small [BS2 = 6-311G(d,p)] basis set to compute enthalpies of formation (at optimized MP2/6-31G(d) geometries and with scaled HF/6-31G(d) frequencies) in the following series of haloalkanes: (1) the 15 fluoro-, chloro-, and chlorofluoromethanes, (2) the 18 fluorinated and chlorinated ethanes. Similar to earlier higher level calculations on haloalkanes, the computed enthalpies exhibited very large, systematic deviations from experiment. It was found that these errors could be largely eliminated using a very simple Bond Additivity Correction (BAC) formula, Delta(f)H degrees (BAC) = Delta(f)H degrees (calc) - n(CX). Delta(CX) [X = F, Cl], in which the BAC parameters, Delta(CF) and Delta(CCl) were determined by fitting the equation to experimental data on the four fluoroethanes and chloroethanes, respectively. The resultant BAC corrected enthalpies of formation are in excellent agreement with experiment, with RMS deviations in the same range as quoted RMS errors in measured enthalpies. Therefore, this simple BAC procedure may be utilized to provide reliable semiquantitative estimates of enthalpies of formation in larger haloalkanes, for which higher level ab initio calculations are not feasible.

Comparison of theoretical methods for assessing the heat of formation of C1 and C2 chlorofluorocarbons and hydrochlorofluorocarbons

“…Based on it, Cloud and Schwartz obtained relatively accurate results by applying bond additive correction (BAC) method [14]. The essence of the BAC procedure is that it enables energies to be calculated accurately without the need to resort to expensive basis sets.…”

Thermodynamic functions of conformational changes, part IV: Functional analysis of conformational entropy of substituted ethane and methanol

“…Most often, single independent variable ( x ) conformational curves are used in chemistry: Typical examples are those that involve rotation about a CC bond. Potential energy curves (PEC) of internal rotation of ethane and substituted ethanes, including butane, were one of the most frequently studied systems 1–23 during the past several years. All these and other earlier studies were aimed to generate the PEC of internal rotors in order to describe internal molecular motions.…”

Kinetic modeling of 1,2‐dichloropropane (PDC) free‐radical chlorination

“…To estimate the rate constants and equilibrium constants used in the kinetic model, we first estimated standard heats of formation, standard entropy and heat capacity, both for gas phase and liquid phase, of the various chloropropane isomers formed during the chlorination of PDC. A comparison between the estimated and experimental (from PPDS thermodynamic property database) heats of formation indicate good agreement, generally within about 3 kcal/mol which is typical for such calculations (See Figure and Table in the Appendix). Surprisingly, the agreement between experimental and calculated values in the liquid phase appears to be slightly better than those in gas phase.…”