Thermodynamic molecular mechanics force field: Modified QCFF program

✍ Scribed by Mangalya Kar; Terry G. Lenz; John D. Vaughan

• Publisher: John Wiley and Sons
• Year: 1994
• Tongue: English
• Weight: 859 KB
• Volume: 15
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.540151107

✦ Synopsis

The QCFF program originated by Warshel and Karplus4" was modified to compute accurate thermodynamic properties So, C;, (H; -Hi)/T, and AH; for various acyclic and cyclic alkenes and alkadienes. Modifications consisted of adjusted bond angle, dihedral angle, bond stretch, and bond energy parameters that improved calculated vibrational frequencies, zero point energies, and thermodynamic functions. Supplemental torsional potential energy functions that were added to existing torsional functions led to greatly improved relative conformer energies and AH! values. It was shown that inclusion of hindered internal rotation leads to significantly better agreement of calculated thermodynamic functions with observed values for acyclic alkenes at high temperatures. The calculated thermodynamic properties of the alkenes and alkadienes were deemed sufficiently accurate for calculation of standard enthalpies and Gibbs free energies of gas phase chemical reactions at various temperatures.