Conformations of Chiral Molecules in Solution: Ab Initio Vibrational Absorption and Circular Dichroism Studies of 4,4a,5,6,7,8-Hexahydro-4a-methyl-2(3H)-naphthalenone and 3,4,8,8a-Tetrahydro-8a-methyl-1,6(2H,7H)-naphthalenedione

Abstract

Studies of the vibrational unpolarized absorption and vibrational circular dichroism (VCD) spectra of the bicyclic chiral molecules: 4,4a,5,6,7,8-Hexahydro-4a-methyl-2(3H)-naphthalenone (1) and 3,4,8,8a-Tetrahydro-8a-methyl-1,6(2H,7H)naphthalenedione (2) are reported. Experimental spectra are compared to spectra calculated using ab initio density functional theory (DFT), hybrid functionals (B3PW91 and B3LYP), gauge-invariant atomic orbitals (GIAOs), and the 6-31G* basis set. 1 and 2 are flexible molecules. Three conformations -“trans-chair” (I), “cis-chair” (II) and “trans-boat” (III)- of 1 and 2 have been studied. Predicted relative energies are I < II < III. Comparison of predicted spectra for the three conformations to experimental mid-IR spectra in CCl4 and CS2 solutions unambiguously demonstrates that for both 1 and 2 conformation I is the preferred conformation in these solvents. The predicted energy differences of conformations I and II are ∼1.8 and ∼1.2 kcal/mol in 1 and 2 respectively. Weak bands in the vibrational spectra of 2 can be attributed to conformation II. Their intensities are consistent with the predicted energy difference. Bands attributable to conformation II are not definitively observed in the vibrational spectra of 1, consistent with the greater energy difference predicted. X-ray crystallography has shown that conformation I is the conformation of 2 in the solid state; our work demonstrates that the conformation of 2 is the same in solution (in CCl4 and CS2) and in the crystalline solid state.