Solvent effects on the infrared spectra of β-alkoxyvinyl methyl ketones

Infrared spectroscopy studies of six β-alkoxyvinyl methyl ketones, with common structure R1O-CR2CH-COR3, where R1 = R3 = CH3, R2 = H (1); R1 = C2H5, R2 = H (2); R3 = CF3; R1 = R2 = CH3, R3 = CF3 (3); R1 = C2H5, R2 = C6H5, R3 = CF3 (4); R1 = C2H5, R2 = 4-O2NC6H4, R3 = CF3 (5); R1 = C2H5, R2 = C(CH3)3, R3 = CF3 (6) in 11 pure organic solvents of different polarity were undertaken to investigate the solute-solvent interactions and to correlate solvent properties by means of linear solvation energy relationships (LSER) with the carbonyl and vinyl stretching vibrations of existing stereoisomeric forms. It was shown that contrary to simple carbonyl-containing compounds where solvent HBD acidity (α) has the largest influence on the ν˜ (CO) band shift to lower wavenumbers, the dipolarity/polarizability (π*) term plays the main role in the interactions of conjugated enones with solvent molecules leading to the ν˜ (CO) and ν˜ (CC) bathochromic band shifts. The trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with solvents. For the ν˜ (CC) band of non-fluorinated enone 1 solvent HBD acidity (α) and solvent HBA basicity term (β) play a perceptible role, whereas for 2 these terms are not significant. β-Substituents in fluorinated enones such as R2 = H, C6H5, and C(CH3)3 assist in the intermolecular hydrogen bond formation of the carbonyl moiety with HBD solvents, while β-substituents such as CH3 and 4-NO2C6H4 prevent the CO group to form the H-bonds with HBD solvents (the solvent HBD acidity term (α) is not significant). The comparison of four conformers of the enone 1 reveals that (EEE) form is the most polarizable conformer; the influences of the solvent dipolarity/polarizability (π*) and solvent HBD acidity (α) term on the bathochromic ν˜ (CO) band shift are opposite to one another.