No abstract

An effective one-electron quantum chemical method was applied to enumerate the conformers of unbranched aliphatic alkanes. The results obtained for butane, pentane, hexane, and heptane were utilized to derive four rules with which the number and sequences of the existing conformers up to undecane could be reproduced. The validity of the rules was confirmed at Hartree-Fock and second-order Moeller-Plesset levels too. Full ab initio conformational analyses were performed for the butane, pentane, hexane, heptane, and octane molecules. The rules demonstrate that the most important factors governing the conformational behavior of unbranched aliphatic alkanes are the nonbonded repulsive-attractive (van der Waals) interactions between the hydrogen atoms attached to the carbon atoms at positions 1,4; 1,5; 1,6; and 1,7. The calculated gasphase standard heats of formation of the unbranched aliphatic alkanes closely matched the experimental values.

Update required! The differences between dependable computed and experimental enthalpies of formation of larger hydrocarbons, growing systematically with size, are traced to an about 0.5 kJ mol−1 error in the canonical best estimate of the enthalpy of formation of atomic carbon (see figure). The results obtained in this study call for an update of Δf${{{\rm H}_0^{\rm{{\rm o}}} }}$(Cgas).

The proton affinity and the enthalpy of formation of the prototypical carbonyl, formaldehyde, have been determined by the first-principles composite focalpoint analysis (FPA) approach. The electronic structure computations employed the allelectron coupled-cluster method with up to single, double, triple, quadruple, and even pentuple excitations. In these computations the aug-cc-p(C)VXZ [X ϭ 2(D), 3(T), 4(Q), 5, and 6] correlation-consistent Gaussian basis sets for C and O were used in conjunction with the corresponding aug-cc-pVXZ (X ϭ 2-6) sets for H. The basis set limit values have been confirmed via explicitly correlated computations. Our FPA study supersedes previous computational work for the proton affinity and to some extent the enthalpy of formation of formaldehyde by accounting for (a) electron correlation beyond the "gold standard" CCSD(T) level; (b) the non-additivity of core electron correlation effects; (c) scalar relativity; (d) diagonal Born-Oppenheimer corrections computed at a correlated level; (e) anharmonicity of zero-point vibrational energies, based on global potential energy surfaces and variational vibrational computations; and (f) thermal corrections to enthalpies by direct summation over rovibrational energy levels. Our final proton affinities at 298.15 (0.0) K are ⌬ pa H o (H 2 CO) ϭ 711.02 (704.98) Ϯ 0.39 kJ mol Ϫ1 . Our final enthalpies of formation at 298.15 (0.0) K are ⌬ f H o (H 2 CO) ϭ Ϫ109.23 (Ϫ105.42) Ϯ 0.33 kJ mol Ϫ1 .The latter values are based on the enthalpy of the H 2 ϩ CO 3 H 2 CO reaction but supported by two further reaction schemes, H 2 O ϩ C 3 H 2 CO and 2H ϩ C ϩ O 3 H 2 CO. These values, especially ⌬ pa H o (H 2 CO), have better accuracy and considerably lower uncertainty than the best previous recommendations and thus should be employed in future studies.

Due to its crucial importance, numerous studies have been conducted to determine the enthalpy difference between the conformers of butane. However, it is shown here that the most reliable experimental values are biased due to the statistical model utilized during the evaluation of the raw experimental data. In this study, using the appropriate statistical model, both the experimental expectation values and the associated uncertainties are revised. For the 133-196 and 223-297 K temperature ranges, 668 ± 20 and 653 ± 125 cal mol(-1), respectively, are recommended as reference values. Furthermore, to show that present-day quantum chemistry is a favorable alternative to experimental techniques in the determination of enthalpy differences of conformers, a focal-point analysis, based on coupled-cluster electronic structure computations, has been performed that included contributions of up to perturbative quadruple excitations as well as small correction terms beyond the Born-Oppenheimer and nonrelativistic approximations. For the 133-196 and 223-297 K temperature ranges, in exceptional agreement with the corresponding revised experimental data, our computations yielded 668 ± 3 and 650 ± 6 cal mol(-1), respectively. The most reliable enthalpy difference values for 0 and 298.15 K are also provided by the computational approach, 680.9 ± 2.5 and 647.4 ± 7.0 cal mol(-1), respectively.