Ab Initio Calculation of Entropy and Heat Capacity of Gas-Phase n-Alkanes Using Internal Rotations

Abstract

This work attempts to improve the theoretical reproduction of thermodynamic properties, such as entropies and heat capacities of gas-phase n-alkanes, by using a more precise quantum-mechanical treatment of the internal rotations. Present ab initio methods all handle the internal modes in the harmonic oscillator approach. It has already been noted that this approach underestimates the microscopic partition functions (Van Speybroeck et al., J. Phys. Chem. A 2000, 104, 10939). In this work, an uncoupled scheme for internal rotations is applied to a large number of n-alkanes within the DFT formalism at the B3LYP/6-311g** level. The method being examined in this paper drastically improves the agreement between theoretical and experimental thermodynamic properties. Moreover, the method has been shown to be efficient and to be easily implemented in each ab initio software package.

First‐principles based group additivity values for thermochemical properties of substituted aromatic compounds

“…To take thermochemical effects due to rotation of single bonds into consideration, the one-dimensional hindered rotor (1-D-HR) approach of Van Speybroeck and coworkers [38][39][40] is applied. It is well known that such a treatment significantly improves the calculated thermochemical properties, 39 in particular the entropy results.…”

Ab Initio Study of Free‐Radical Polymerization: Polyethylene Propagation Kinetics

“…This method is referred to as the onedimensional hindered rotor scheme (1D-HR). The procedure to calculate the internal rotor partition functions was already discussed in references [13,16] and we refer to these articles for further details. We only mention that a rotational potential is determined for each rotor m by fitting a Fourier expansion to the energies of the fully optimized stable conformers and rotational transition state structures [Eq.…”

Thermochemical Properties of Hydroxycyclohexadienyl Peroxy Isomers from Reaction of O2 with the Benzene-OH adduct

“…Speybroeck also studied effects of coupling between internal rotors and report that the coupling has only, a minor influence on the kinetics, relative to treating all internal rotors as uncoupled. Speybroeck et al reported overall good agreement between experimental and calculated entropy and heat capacity values for n-alkanes, treating internal rotors without coupling [22]. Chen and Bozzelli [23] have also shown very good agreement between calculated and experimental o ( ) and ( ) for ethers and ether radicals when internal rotation contributions were included.…”