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Thermodynamic calculations for molecules with asymmetric internal rotors—application to 1,3-butadiene

✍ Scribed by Bryan M. Wong; Sumathy Raman

Publisher
John Wiley and Sons
Year
2007
Tongue
English
Weight
337 KB
Volume
28
Category
Article
ISSN
0192-8651

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✦ Synopsis

Abstract

We present quantum mechanical partition functions, free energies, entropies, and heat capacities of 1,3‐butadiene derived from ab initio calculations. Our technique makes use of a reaction path‐like Hamiltonian to couple all 23 vibrational modes to the large‐amplitude torsion, which involves heavy asymmetric functional groups. Ab initio calculations were performed at the B3LYP, MP2, and CCSD(T) levels of theory and compared with experimental values as a reference case. By using the ab initio potentials and projected frequencies, simple perturbative expressions are presented for computing the couplings of all the vibrational modes to the large‐amplitude torsion. The expressions are particularly suited for programming in the new STAR‐P software platform which automatically parallelizes our codes with distributed memory via a familiar MATLAB interface. Using the efficient parallelization scheme of STAR‐P, we obtain thermodynamic properties of 1,3‐butadiene for temperatures ranging from 50 to 500 K. The free energies, entropies, and heat capacities obtained from our perturbative formulas are compared with conventional approximations and experimental values found in thermodynamic tables. © 2007 Wiley Periodicals, Inc.J Comput Chem 28: 759–766, 2007

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