Ž . Vibrational Spectroscopy 17 1998 173–182 Vibrational spectroscopy and conformational studies of 1,4-benzodioxan: ab initio and density functional calculations Jaebum Choo a, ) , Soonjoo Yoo a , Seongho Moon b , Younghi Kwon b , Hoeil Chung c a Department of Chemistry, Hanyang UniÕersity, Ansan 425-791, South Korea b Department of Chemistry, Hanyang UniÕersity, Seoul 133-791, South Korea c NIR Project TFT, Production Technology Center, SK, Ulsan 680-130, South Korea Received 15 May 1998; revised 24 July 1998; accepted 27 July 1998 Abstract The gas-phase infrared and pure-liquid Raman spectra of 1,4-benzodioxan have been recorded and analyzed. The infrared vibrational frequencies, absolute intensities, and ring-inversion potential energy profile of the molecule have been also Ž . Ž . predicted using Becke’s three-parameter hybrid B3LYP method in the density functional theory DFT method, as well as Ž . Ž . the Hartree–Fock HF and molecular mechanics MM3 methods. The vibrational frequencies calculated at the B3LYP levels agree much better with the observed frequencies than those predicted by the HF or MM3 methods. The enlargement of basis sets at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. The ring-inversion process of 1,4-benzodioxan between the twist and bent conformers has also been investigated using the DFT, HF, and MM3 methods. The calculated results at the B3LYPr6-31G) level indicate that the twisted conformer has a lower energy than the bent conformer and that the energy difference between the two forms is 7.5 kcalrmol. This value is 1.3 kcalrmol lower than the barrier of nonbenzene-fused ring 1,4-dioxene. q 1998 Elsevier Science B.V. All rights reserved. Keywords: 1,4-Benzodioxan; Ab initio method; Density functional theory; Vibrational frequencies; Ring-inversion process 1. Introduction The conformational structures of nonrigid ring molecules have attracted considerable attention since the capability to change conformation easily is very important to determine the chemical reactivities and kinetics of w x a molecule 1–3 . Among various nonrigid ring systems, benzene-fused ring compounds have been of particular interest since they are major building blocks of nature’s bioorganic compounds and their conformational behaviors play an important role to determine the biological activity of a system. In order to elucidate the conformational behaviors and structural properties of a benzene-fused ring compound, various spectroscopic methods including infrared, Raman, laser-induced fluorescence, electron diffraction, and NMR have been w x extensively utilized 4,5 . ) Corresponding author. Tel.: q82-345-400-5505; Fax: q82-345-407-3863; E-mail: jbchoo@mail.hanyang.ac.kr 0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-2031 98 00029-0