Theoretical and experimental conformational preferences of the aldehyde and fluoroaldehyde groups in 2-methylbenzaldehyde, 2-trifluoromethylbenzaldehyde, and 4-chloro-2-methylbenzoyl fluoride. Proximate couplings

Abstract

On the basis of long-range spin–spin coupling constants, the O-syn conformation of 2-methylbenzaldehyde in CCl4 solution at 305 K is favored over the O-anti form by a free energy of 0.53 kJ/mol. This number is compatible with other experiments, as well as with STO-3G MO calculations in which the geometry of the substituents is optimized. The latter yield 0.52 kJ/mol in the internal energy difference. In benzene solution, 2-trifluoromethylbenzaldehyde exists in the O-anti form to the extent of at least 95% at 305 K. In CCl4 solution at this temperature, the population of the O-syn conformer of 4-chloro-2-methylbenzoyl fluoride is likely 75% or more of the total, in semiquantitative agreement with STO-3G optimization procedures. Substantial proximate couplings exist between 1H and 19F nuclei in the sidechains of the latter two compounds and are compared with INDO MO FPT computations. These yield negative values for [Formula: see text] in 2-trifluoromethylbenzaldehyde, whereas the experimental value is 2.23 Hz.