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

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 C-13 NMR and proton H-1 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.