The sensation of smell has been a topic of scholarly discussion since the time of ancient Greece. Two general areas of investigation have developed over the years; the problems of empirical odor classification and the process of odor reception. A number of related empirical classification schemes have been proposed. Perhaps the most complete odor classification is that of Zwaardemaker, which is listed in the first two columns of Table 1. While there is a fair degree of unanimity among such empirical schemes, theories of how odors are sensed are exceedingly diverse. There are presently four prominent theories of odor reception: shape sensors, vibration sensors, neural penetration, and acid-base interaction.
Amoore’s shape sensor theory has been the most widely discussed of the four. It postulates that there are templates present in the olfactory area which sense the shape of the molecules in the passing air by fitting the molecules in a lock-and-key manner. This theory developed from the observation that for certain categories compounds of similar odor have similar shapes when constructed from space-filling models. Seven categories have been postulated, but no unifying shapes have been found for pungent or putrid odors, so interactions other than shape must be involved.
The vibration sensor theory stems from the observation of Dyson, expanded by Wright, that compounds of similar odor absorb in similar Raman spectral regions. This led to tbe postulation of vibrational coupling between the molecules and the neurons in the nose. Recently, this theory has lost support because of the difficulty of postulating vibrational coupling in aqueous medium.