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

Studies of the vibrational unpolarized absorption and vibrational circular dichroism (VCD) spectra of the bicyclic chiral molecules: 4,4a,5,6,7,8-Hexa hydro-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 compa red to spectra calculated using ab initio density functional theory (DFT), hybrid functionals (B3PW91 and B3LYP), gauge-invariant atomic orbitals (GIA Os), and the 6-31G* basis set. 1 and 2 are flexible molecules. Three confor mations -"trans-chair" (I), "cis-chair" (II) and "trans-boat" (III)- of 1 a nd 2 have been studied. Predicted relative energies are I < II < LII. Compa rison of predicted spectra for the three conformations to experimental mid- IR spectra in CCl4 and CS2 solutions unambiguously demonstrates that for bo th 1 and 2 conformation I is the preferred conformation in these solvents. The predicted energy differences of conformations I and II are similar to 1 .8 and similar to 1.2 kcal/mol in 1 and 2 respectively. Weak bands in the v ibrational spectra of 2 can be attributed to conformation II. Their intensi ties are consistent with the predicted energy difference. Bands attributabl e to conformation II are not definitively observed in the vibrational spect ra of 1, consistent with the greater energy difference predicted. X-ray cry stallography 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 i n solution t(in CCl4 and CS2) and in the crystalline solid state.