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Received Apr 1, 2017; Accepted Jul 4, 2017
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1. Introduction
Torsional potential energy calculations provide conformational information and allow finding the barriers to the rotations of bonds. Early applications were performed in organic compounds phosphates, 1,3-butadiene, polypeptides, and dimethyl groups [1–4]. These initial works showed that if steric interactions are not strong, a rigid rotor model applies. Furthermore, Bongini and Bottoni used density functional theory (DFT) and found that correlation energy effects are to be taken into account for accurate calculations of the conformal properties of organic compounds [4]. The topologies of the PES in the ground state and a selected excited state are useful in understanding the active site of reactions and photoreactivity [1]. PESs were employed to investigate excited-state reaction mechanisms and proton transfer in a complex molecule (2-(2′-hydroxyphenyl)benzimidazole) and its amino derivatives [2].
The calculations by Bongini and Bottoni compared Hartree-Fock and second-order Møller-Plesset methods [3–5] to DFT results (at the BLYP and B3LYP levels) [6, 7] and determined the structure of the two molecules considered (3,3′-dimethyl-2,2′-bithiophene and 3,4′-dimethyl-2,2′-bithiophene) and to experimental data to quantify the reliability of the models. Both BLYP and B3LYP computations correctly predict both of the rotational minima observed in experiment to an accuracy of 5°. Bongini and Bottoni produced torsional potential energy plots, but their interest was in the nature of the minima, not the characteristics of the energy surface itself [8]. Raman spectra of multiple physical phases of a compound can be used in conjunction with computational ab initio and DFT methods to determine the torsional potential of a two-rotor system and are capable of determining the potential energy surface (PES) with a 3% accuracy [9]. However, this method requires multiple Raman spectra, several single-point energy computations for representative molecular conformations, and theoretical knowledge of the general form of the PES [9]. The PES of ethoxybenzene, a molecule where the coupling between rotors is small, has been calculated using DFT at the...