Ab initio study on the six lowest energy conformers of iso-octane: conformational stability, barriers to internal rotation, natural bond orbital and first-order hyperpolarizability analyses, UV and NMR predictions, spectral temperature sensitivity, and scaled vibrational assignment

Mouhi Eddine Hachim; Karima Sadik; Said Byadi; Christian Van Alsenoy; Aziz Aboulmouhajir

doi:10.1007/s00894-019-4105-5

Ab initio study on the six lowest energy conformers of iso-octane: conformational stability, barriers to internal rotation, natural bond orbital and first-order hyperpolarizability analyses, UV and NMR predictions, spectral temperature sensitivity, and scaled vibrational assignment

J Mol Model. 2019 Jul 30;25(8):254. doi: 10.1007/s00894-019-4105-5.

Authors

Mouhi Eddine Hachim¹, Karima Sadik¹, Said Byadi¹, Christian Van Alsenoy², Aziz Aboulmouhajir^{3

4}

Affiliations

¹ Team of Molecular Modelling and Spectroscopy, Faculty of Sciences, University of Chouaib Doukkali, El Jadida, Morocco.
² Structural Chemistry Group, Department of Chemistry, University of Antwerp, Antwerp, Belgium.
³ Team of Molecular Modelling and Spectroscopy, Faculty of Sciences, University of Chouaib Doukkali, El Jadida, Morocco. aboulmouhajir@gmail.com.
⁴ Organic Synthesis, Extraction and Valorization Laboratory, Team of Extraction, Spectroscopy and Valorization, Sciences Faculty of Ain Chock, University of Hassan II, Casablanca, Morocco. aboulmouhajir@gmail.com.

PMID: 31363855
DOI: 10.1007/s00894-019-4105-5

Abstract

In this paper, we present the quantum electronic study of iso-octane, based on MP2 and B3LYP methods using the 6-311++G(d,p) basis set. In addition to conformational stability and internal rotation barriers studies, the delocalization energies associated with the internal charge transfer (ICT) within each of the six lowest energy conformers were evaluated using NBO analysis. With the aim to differentiate even more between these conformers, the energy gap between HOMO and LUMO orbitals, chemical softness, and first-order hyperpolarizability (nonlinear optics property) were evaluated. Similarly, their spectral behavior was investigated at different levels; the ultraviolet (UV) absorption bands were assigned using molecular orbitals data obtained by TD-B3LYP calculations with 6-311++G(d,p) basis set, while carbon ¹³C NMR and proton ¹H signal peaks were assigned using the GIAO-B3LYP/6-311++G(d,p) method. In addition, the normal mode calculations of the most and least stable conformers using a scaled force field in terms of nonredundant local symmetry coordinates were carried out to approach the vibrational spectra temperature dependency.

Keywords: 13C and 1H NMR spectra; Ab initio; Conformational isomerism; First-order hyperpolarizability; Natural bond orbital; Scaled vibrational analysis; UV absorption bands.