Abstract

The carbonyl stretching vibrations decrease in frequency in the order of solvent polarity (acetonitrile and nitromethane are exceptions) and in the increasing mole concentration of the more polar solvent of a solvent pair. Intermolecular hydrogen bonding between the chloroform proton and the carbonyl oxygen atom of the solute shifts <i>v</i>C=O to lower frequency. Polar solvents such as dimethyl sulfoxide cause <i>v</i>C=O to shift to lower frequency by interacting with the solute in a manner which induces a canonical form which lengthens and weakens the C=O bond. Steric factors of α-halo or oxy groups of carboxylic acids or ketones prevent close solute/solvent interaction in the <i>gauche</i> rotational isomer compared to the <i>cis</i> rotational isomer; thus, <i>gauche v</i>C=O has a lesser contribution from the induced canonical form due to solute/solvent interaction, and <i>gauche v</i>C=O shifts less to lower frequency than does <i>cis</i> <i>v</i>C=O. The rotational isomer concentration distribution also changes with change in solvent polarity. Thus, physical factors of the solute play a role in solvent-induced chemical shifts of IR group frequencies.

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