Investigation of vibrational theory of olfaction with variously labelled benzaldehydes

Vibrational theory of olfaction was investigated with the following three l abelled analogues of benzaldehyde; C-13(6)(ring)-benzaldehyde, (CHO)-C-13-b enzaldehyde and benzaldehyde-d(6). Sensory analysis, by a trained panel of 30 subjects, using a due-trio test, showed that the benzaldehyde-d(6) gave a statistically significant difference in odour perception (P = 0.002) rela tive to the unlabelled benzaldehyde, The odour changes in the other substit uted analogues were found to be less significant (P = 0.058 for ring-C-13(6 ) and P = 0.017 for (CHO)-C-13). Analysis of the infrared spectra revealed that benzaldehyde-d(6) also experienced the most drastic shifts (Delta lamb da similar to 700 cm(-1)) in the absorption Frequencies of the aromatic and aldehydic C-H stretching bands (3085, 2819 and 2738 cm(-1)), in addition t o smaller shifts (Delta lambda similar to 25-100 cm(-1)) in the C-H bending or deformation bands (828, 745, 688 and 650 cm(-1)). In addition, molecula r modelling studies indicated that the shape of the molecule was retained a fter isotopic substitution but the molecular volume increased by less than 1%. The data suggested that one of the required vibrational bands for the p erception of bitter almond aroma lies in the 3000 to 2500 cm(-1) frequency range. (C) 2001 Elsevier Science Ltd. All rights reserved.