Investigation of vibrational theory of olfaction with variously labelled benzaldehydes

Vibrational theory of olfaction was investigated with the following three labelled analogues of benzaldehyde; 13C6(ring)-benzaldehyde, 13CHO-benzaldehyde and benzaldehyde-d6. Sensory analysis, by a trained panel of 30 subjects, using a duo-trio test, showed that the benzaldehyde-d6 gave a statistically significant difference in odour perception (P=0.002) relative to the unlabelled benzaldehyde. The odour changes in the other substituted analogues were found to be less significant (P=0.058 for ring-13C6 and P = 0.017 for 13CHO). Analysis of the infrared spectra revealed that benzaldehyde-d6 also experienced the most drastic shifts (Δλ ∼ 700 cm−1) in the absorption frequencies of the aromatic and aldehydic C–H stretching bands (3085, 2819 and 2738 cm−1), in addition to smaller shifts (Δλ ∼ 25–100 cm−1) in the C–H bending or deformation bands (828, 745, 688 and 650 cm−1). In addition, molecular modelling studies indicated that the shape of the molecule was retained after isotopic substitution but the molecular volume increased by less than 1%. The data suggested that one of the required vibrational bands for the perception of bitter almond aroma lies in the 3000 to 2500 cm−1 frequency range.