The conformational equilibria for 1,3-difluoro- (DFA), 1,3-dichloro-(DCA), and 1,3-dibromo-acetone (DBA) have been determined through IR spectroscopy, ab initio calculations at the B3LYP/6-311++G(2df,2p) level, and solvation calculations. The C=O stretching frequencies of DFA, DCA, and DBA in the vapor phase were calculated theoretically and compared to the experimental C=O frequencies in solvents with varying polarities. The theoretical calculations also gave the energy minima and thus the conformational equilibria of these molecules in the vapor phase in terms of the rotamers 1 (cis/cis), 2 (cis/gauche), 3 (cis/trans), 4 (gauche/gauche), 5 (gauche/gauche'), and 6 (trans/trans). For DFA, the stable conformers in the vapor phase are 1, 3, and 5, conformer 3 being the most stable. The conformer ratio in solution changes from circa 12% for 1, 85% for 3, and 3% for 5 in CCl4 to 34% for 1 and 66% for 3 in CH3CN. For DCA, the most stable forms are the same as those for DFA, but in this case conformer 5 is the most stable form in the vapor phase. In solution the equilibrium changes from circa 58% for 3 and 42% for 5 in CCl4 to 37% for 1, 57% for 3, and 6% for 5 in CH3CN. For DBA, the most stable conformers in the vapor phase are 1, 2, and 5, conformer 5 again being the most stable form. In solution the population changes from circa 36% for 2 and 64% for 5 in CCl4 to 29% for 1, 57% for 2, and 14% for 5 in CH3CN. The conformer populations in both the vapor phase and the solvents studied can be explained quantitatively from the ab initio and solvation calculations.