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Exploring the conformational landscape of menthol, menthone, and isomenthone: a microwave study.

Schmitz D, Shubert VA, Betz T, Schnell M - Front Chem (2015)

Bottom Line: For menthol only one conformation was identified under the cold conditions of the molecular jet, whereas three conformations were observed for menthone and one for isomenthone.The conformational space of the different molecules was extensively studied using quantum chemical calculations, and the results were compared with molecular parameters obtained by the measurements.Finally, a computer program is presented, which automatically identifies different species in a dense broadband microwave spectrum using calculated ab initio rotational constants as initial input parameters.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for the Structure and Dynamics of Matter Hamburg, Germany ; The Center for Free-Electron Laser Science Hamburg, Germany.

ABSTRACT
The rotational spectra of the monoterpenoids menthol, menthone, and isomenthone are reported in the frequency range of 2-8.5 GHz, obtained with broadband Fourier-transform microwave spectroscopy. For menthol only one conformation was identified under the cold conditions of the molecular jet, whereas three conformations were observed for menthone and one for isomenthone. The conformational space of the different molecules was extensively studied using quantum chemical calculations, and the results were compared with molecular parameters obtained by the measurements. Finally, a computer program is presented, which automatically identifies different species in a dense broadband microwave spectrum using calculated ab initio rotational constants as initial input parameters.

No MeSH data available.


Microwave spectrum of menthol in the region of 2–8 GHz. The upper trace shows the experimental spectrum, the lower trace is a simulation based on fitted molecular parameters. The spectrum is dominated by a-type and c-type transitions. A rotational temperature of 0.5 K gives the best match between the simulated and the experimental intensities. A line splitting due to the internal rotation of any of the methyl groups is not observed in the spectrum.
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Figure 3: Microwave spectrum of menthol in the region of 2–8 GHz. The upper trace shows the experimental spectrum, the lower trace is a simulation based on fitted molecular parameters. The spectrum is dominated by a-type and c-type transitions. A rotational temperature of 0.5 K gives the best match between the simulated and the experimental intensities. A line splitting due to the internal rotation of any of the methyl groups is not observed in the spectrum.

Mentions: The experimental spectrum of menthol is presented in the positive trace of Figure 3. The spectrum is typical for a prolate asymmetric rotor with μa as the largest dipole moment component. We assigned and fitted 32 a-type and 21 c-type lines, but no b-type lines were observed in the spectrum. The spectrum arises from only a single species, indicating that only one conformer of menthol is significantly populated in the supersonic expansion, in line with the findings of a recent FTIR study (Albrecht et al., 2010). Furthermore, it confirms that menthol is indeed the main stereoisomer in the purchased sample, since isomenthol, neomenthol and neoisomenthol all have different rotational constants and thus can be easily differentiated by their rotational spectra. For fitting the spectrum, we used Pickett's SPFIT/SPCAT program suite employing the Ir representation and the Watson A reduction (Pickett, 1991). Table 1 summarizes the results of the fit. We observed and assigned lines including rotational quantum numbers J = 1 to J = 7 and a maximum Ka of 4.


Exploring the conformational landscape of menthol, menthone, and isomenthone: a microwave study.

Schmitz D, Shubert VA, Betz T, Schnell M - Front Chem (2015)

Microwave spectrum of menthol in the region of 2–8 GHz. The upper trace shows the experimental spectrum, the lower trace is a simulation based on fitted molecular parameters. The spectrum is dominated by a-type and c-type transitions. A rotational temperature of 0.5 K gives the best match between the simulated and the experimental intensities. A line splitting due to the internal rotation of any of the methyl groups is not observed in the spectrum.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4355985&req=5

Figure 3: Microwave spectrum of menthol in the region of 2–8 GHz. The upper trace shows the experimental spectrum, the lower trace is a simulation based on fitted molecular parameters. The spectrum is dominated by a-type and c-type transitions. A rotational temperature of 0.5 K gives the best match between the simulated and the experimental intensities. A line splitting due to the internal rotation of any of the methyl groups is not observed in the spectrum.
Mentions: The experimental spectrum of menthol is presented in the positive trace of Figure 3. The spectrum is typical for a prolate asymmetric rotor with μa as the largest dipole moment component. We assigned and fitted 32 a-type and 21 c-type lines, but no b-type lines were observed in the spectrum. The spectrum arises from only a single species, indicating that only one conformer of menthol is significantly populated in the supersonic expansion, in line with the findings of a recent FTIR study (Albrecht et al., 2010). Furthermore, it confirms that menthol is indeed the main stereoisomer in the purchased sample, since isomenthol, neomenthol and neoisomenthol all have different rotational constants and thus can be easily differentiated by their rotational spectra. For fitting the spectrum, we used Pickett's SPFIT/SPCAT program suite employing the Ir representation and the Watson A reduction (Pickett, 1991). Table 1 summarizes the results of the fit. We observed and assigned lines including rotational quantum numbers J = 1 to J = 7 and a maximum Ka of 4.

Bottom Line: For menthol only one conformation was identified under the cold conditions of the molecular jet, whereas three conformations were observed for menthone and one for isomenthone.The conformational space of the different molecules was extensively studied using quantum chemical calculations, and the results were compared with molecular parameters obtained by the measurements.Finally, a computer program is presented, which automatically identifies different species in a dense broadband microwave spectrum using calculated ab initio rotational constants as initial input parameters.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for the Structure and Dynamics of Matter Hamburg, Germany ; The Center for Free-Electron Laser Science Hamburg, Germany.

ABSTRACT
The rotational spectra of the monoterpenoids menthol, menthone, and isomenthone are reported in the frequency range of 2-8.5 GHz, obtained with broadband Fourier-transform microwave spectroscopy. For menthol only one conformation was identified under the cold conditions of the molecular jet, whereas three conformations were observed for menthone and one for isomenthone. The conformational space of the different molecules was extensively studied using quantum chemical calculations, and the results were compared with molecular parameters obtained by the measurements. Finally, a computer program is presented, which automatically identifies different species in a dense broadband microwave spectrum using calculated ab initio rotational constants as initial input parameters.

No MeSH data available.