Quantum Mechanics Calculations on PASH: Attempts to Correlate Thermodynamic Properties to Chromatographic Behaviour and/or Relative Abundance or Presence in Petroleum Samples

Abstract

Polycyclic Aromatic Compounds containing one sulfur atom i.e. Polycyclic Aromatic Sulfur Heterocycles (PASH) form a very diversified subclass of compounds and the number of compounds increases with alkylation. Nonsubstituted as well as mono- and dimethylated PASH are found in crude oils. Identification and analysis of these compounds is a difficult challenge that can be facilitated by theoretical calculations up-side.

The purpose of this study was to use quantum mechanics calculations to generate properties such as molecular shape or thermodynamic stability that could be related to chromatographic indices. Tricyclic (dibenzothiophene, naphthothiophenes) and tetracyclic (phenanthrothiophenes, anthrathiophenes and benzonaphthothiophenes) parent molecules and mono- and di-methylated analogues have been studied. A special attention has been paid to compounds with a crowded bay region that results in a twisted molecular geometry.

The results from semi-empirical methods (AMPAC 6.51-MNDO, AM1) and ab initio Hartree-Fock methods (RHF/6-31G) have been compared to X-ray determined geometries when available and attempts have been made to correlate enthalpies of formation and chromatographic behaviour and/or relative abundance or presence in petroleum samples.

Key Words:

• PASH
• sulfur atom
• retention time
• enthalpy of formation
• MNDO
• AM1 RHF calculations