Structural determination of menthol and isomenthol, a minty compound and its nonminty isomer, by means of gas electron diffraction augmented by theoretical calculations
Egawa, T; Sakamoto, M; Takeuchi, H; Konaka, S
The molecular structures of menthol and isomenthol were determined by means of gas electron diffraction. The nozzle temperatures were 90 °C and 137 °C, for menthol and isomenthol, respectively. The results of RHF and DFT calculations were used as supporting information. FTIR spectra were measured and used to refine the theoretical force constants. It was found that the electron diffraction data of menthol can be reproduced by assuming essentially one conformer in which all the substituents of the cyclohexane ring, isopropyl, methyl, and hydroxyl groups are in the equatorial position. On the other hand, isomenthol has two stable conformers in the gas phase with comparable abundance. One conformer has its isopropyl and hydroxyl groups in the equatorial position, and the other conformer has them in the axial position. The abundance of the former is 63 ± 30% and that of the latter is 37%. The determined structural parameters (rg and ∠α) of menthol are as follows: 〈r(C-C)〉 = 1.534(2) Å, r(C-O) = 1.408(10) Å, 〈r(C-H)〉 = 1.117(3) Å, 〈∠C-C-Cring〉 = 112.2(5)°, 〈∠C-C-Cexo〉 = 111.6(6)°, 〈∠C-C-O〉 = 110.0(11)°, 〈∠C-C-H〉 = 109.2(11)°, φ = 57(7)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit. Angles C-C-Cexo are the C-C-C angles other than the six C-C-C angles in the cyclohexane ring. Angle φ is the torsional angle around the Ci-pr-Cring bond. Those of the main conformer of isomenthol whose isopropyl and hydroxyl groups are in the equatorial position are as follows: 〈r(C-C)〉 = 1.538(1) Å, r(C-O) = 1.427(13) Å, 〈r(C-H)〉 = 1.112(3) Å, 〈∠C-C-Cring〉 = 112.6(25)°, 〈∠C-C-Cexo〉 = 111.5(9)°, 〈∠C-C-O〉 = 109.0(17)°, 〈∠C-C-H〉 = 108.9(15)°, φ = -63(17)°. The relationships between the minty odor and conformations are briefly discussed.