Solvent dependence of rotational energetics and formyl-proton long-range spin-spin coupling behavior of 2,6-dichloro- and 2,6-dinitrobenzaldehydes using dipolar couplings and temperature dependence of long-range couplings

Abstract

The formyl rotational energetics, solvent effects on the energetics and formyl-proton spin-spin coupling behavior of 2,6-dichlorobenzaldehyde were studied by using dipolar couplings analysis and the temperature dependence of the spin-spin couplings. The general form of the rotational potential was taken from molecular mechanics and the conjugative sin2 Θ-type component (where Θ is the formyl-ring dihedral angle) was then optimized using the dipolar couplings obtained by analyzing 1H NMR and 13C proton satellite spectra in a liquid crystal solvent. The optimization based on the dipolar couplings gave the following rotational free energy potential: V(Θ) = 4.5 cos6 Θ + 10.7 (± 2.0) sin2 Θ, in kJ/mol. The analysis based on the temperature dependence of 6J data allowed estimation of solvent effects on the potential. For example, the potentials in acetone and in acetonitrile differ by a 1.0 sin2 2Θ (in kJ/mol). The chloroform solvent effect can be described by term of -1.0 cos6 Θ. When the formyl-proton six-bond coupling data was fit, assuming it obeyed the expression 6J(Θ) = 6J90 sin2 Θ + 6J0 and assuming 6J0 = 0.015 Hz, the analysis yielded 6J90 of -0.48 (± 0.03) Hz. The scalar coupling temperature dependence method was applied also to the conformational analysis of 2,6-dinitrobenzaldehyde.Key words: spin-spin coupling, dipolar coupling, solvent effect, rotational barrier, benzaldehyde.