On the basis of long-range spin–spin coupling constants, theO-synconformation of 2-methylbenzaldehyde in CCl4solution at 305 K is favored over theO-antiform 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 theO-antiform to the extent of at least 95% at 305 K. In CCl4solution at this temperature, the population of theO-synconformer 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 between1H and19F nuclei in the sidechains of the latter two compounds and are compared with INDO MO FPT computations. These yield negative values forin 2-trifluoromethylbenzaldehyde, whereas the experimental value is 2.23 Hz.