A THEORETICAL STUDY OF DIETHYL KETONE - VARIATION OF A CARBONYL FREQUENCY WITH GEOMETRY STERIC EFFECT AND INDUCTIVE EFFECT

The IR spectrum of diethyl ketone has been computed starting with the force constants (both diagonal and off-diagonal elements) obtained by normal coordinate analysis of acetone and a normal paraffin. The position of the carbonyl frequency, νCO, is accurately computed in the gas phase. The molecule has then been deformed (G-matrix variation) and the spectrum computed. It is concluded that purely geometric effects can shift the νCO by sizable amounts, upwards of 12 cm-1. In other calculations the maximum amount of steric effect on the carbonyl vibration has been estimated to be almost 30 cm-1. This estimate has been made by using only the diagonal elements of the F-matrix assuming that the off-diagonal elements represent non-bonded interactions. This is a reasonable assumption. It has been concluded that the carbonyl stretching frequency must be used with extreme caution as a model for carbonium ion reactions. In particular the position of νCO is determined by numerous factors other than just simple angle strain. © 1969.