C-H...O=C AND C-H...H-C INTERACTIONS IN THE SIDE-CHAINS OF 2-ISOPROPYLBENZALDEHYDE - A NEGATIVE 5J(CHO,CH(CH3)2)

The H-1 nuclear magnetic resonance spectra of 2-isopropylbenzaldehyde in CS2/C6D12 and acetone-d6 solutions provide the chemical shifts and coupling constants of all the protons. The long-range coupling constants involving the side-chain protons yield certain sums of the populations of the four putatively planar conformations. The o-anti conformers have a fractional population of 0.55(3) in the polar and of 0.49(3) in the nonpolar solvent. The conformers in which the methine C-H bond lies cis to the aldehyde group have a fractional population of 0.83(3) in both solutions. The close approach of the methine and aldehydic hydrogen atoms in one conformer is indicated by a negative proximate coupling constant between their protons of -0.39(1) Hz. The chemical shifts of the ring and of the side-chain protons are consistent with the conformer populations deduced from the long-range coupling constants and also with the indications that the side chains do not, on average, deviate from ''coplanarity'' with the ring by much more or less than in the parent compounds. The C-H...H-C and C-H...O = C interactions in the o-syn and o-anti conformers are most likely repulsive and of very similar magnitude and lead to a significant deshielding of the protons in these moieties. Molecular orbital computations are also reported and are an aid in estimating the populations of the individual conformers. The STO-3G MO structures have H...H and H...O distances well below the sums of the van der Waals radii of hydrogen and oxygen atoms in the conformers with the methine C-H bond placed cis to the aldehyde group, yet these are computed to be by far the most abundant by the STO-3G as well as by AM1 algorithms.