%0 Journal Article
%T Conformational Analysis in Solution of a Chiral Bisoxazoline Molecule: Vibrational Circular Dichroism Spectroscopy and Density Functional Theory Study
%A A. Aamouche
%A P. J. Stephens
%J International Journal of Spectroscopy
%D 2011
%I Hindawi Publishing Corporation
%R 10.1155/2011/905045
%X The conformations of the chiral bisoxazoline: 2,2¡ä-methylenebis[3a,8a-dihydro-8H-indeno[1,2-d]oxazole] (also named IndaBOX), have been studied. Density functional theory (DFT) calculations identify four inequivalent stable conformations. Two, I and II have, C2 symmetry; two, III and IV, have C1 symmetry. The electronic energies of I¨CIV are ordered: . The span in energy is <1.0£¿kcal/mole. Vibrational unpolarised absorption and circular dichroism spectra have been predicted for the four conformations using DFT. Comparison of population-weighted spectra to experimental spectra of CHCl3 and CDCl3 solutions in mid-IR region strongly supports the DFT predictions of the number, structures, and relative energies of the conformations of IndaBOX. This shows that DFT predicts spectra with a high degree of reliability. We will undoubtedly illustrate the advantage added by vibrational circular dichroism spectroscopy in conformational analysis and in the absolute configuration determination. 1. Introduction We report a study of the conformations of a chiral bisoxazoline:£¿£¿2,2¡ä-methylenebis[3a,8a-dihydro-8H-indeno[1,2-d]oxazole], 1, in solution using ab initio vibrational spectroscopy. The molecular structure of 1 is represented on Scheme 1. Scheme 1 In combination with metal salts, chiral bisoxazolines catalyse a wide variety of asymmetric transformations, in many cases with high enantioselectivity [1¨C5]. To date, although the X-ray structures of several metal complexes of chiral bisoxazolines have been reported [6¨C10], there appear to have been no structural studies of chiral bisoxazolines alone. Here, we report a study of the bisoxazoline, 1, using ab initio density functional theory (DFT) [11, 12], in combination with vibrational unpolarised absorption (IR) and vibrational circular dichroism (VCD) [13¨C16] spectroscopies. 1 is a flexible molecule; internal rotation can occur about the CC1 and CC2 bonds. We seek to establish the structures and relative energies of the stable conformations of 1. DFT has had a major impact on molecular structure calculations. For many years, wavefunction functional theory (WFT; i.e., HF/SCF and MPs) has been the method of choice. The DFT approach expresses ground-state proprieties, such as total energies, equilibrium positions dipole, and magnetic moments, in terms of electronic density and spin density. The need to include the electron correlation in calculations and the discovery of accurate approximations to exchange-correlation energy density functional raise the DFT popularity. The computational advantage of DFT originates from
%U http://www.hindawi.com/journals/ijs/2011/905045/