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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.


The upper trace is the spectrum of the mixture of the stereoisomers of menthone and isomenthone in the region of 4700–5650 MHz while the lower traces are simulations based on fitted molecular parameters. Three different conformers of menthone and one conformer of isomenthone were identified in the spectrum and successfully assigned. A rotational temperature of 1.5 K for all four species gives the best match between the simulated and the experimental intensities. A few residual lines with SNR ratios of about 3:1 remain unassigned, which might originate from contaminants, clusters or instrument noise. Line splittings due to internal rotation of one of the methyl groups of menthone or isomenthone were not observed.
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Figure 6: The upper trace is the spectrum of the mixture of the stereoisomers of menthone and isomenthone in the region of 4700–5650 MHz while the lower traces are simulations based on fitted molecular parameters. Three different conformers of menthone and one conformer of isomenthone were identified in the spectrum and successfully assigned. A rotational temperature of 1.5 K for all four species gives the best match between the simulated and the experimental intensities. A few residual lines with SNR ratios of about 3:1 remain unassigned, which might originate from contaminants, clusters or instrument noise. Line splittings due to internal rotation of one of the methyl groups of menthone or isomenthone were not observed.

Mentions: The spectrum of the mixture of menthone isomers is presented in the frequency range from 4700 to 5650 MHz in the positive trace of Figure 6. The high number and density of lines of molecular origin and various background lines complicated the assignment of the individual lines to the particular species. Therefore, a computer-aided assignment routine was developed to facilitate the assignment process. This routine is described in Section 3.3. It was possible to assign most of the lines to four different species. For all species, most of the assigned transitions are b-type, with fewer a-type transitions and even fewer c-type transitions. This observation suggests that for all four species μb > μa > μc applies. The final fits were executed employing the SPFIT/SPCAT suite of programs using the Ir representation and the Watson A reduction. The results are compiled in Table 3.


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

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

The upper trace is the spectrum of the mixture of the stereoisomers of menthone and isomenthone in the region of 4700–5650 MHz while the lower traces are simulations based on fitted molecular parameters. Three different conformers of menthone and one conformer of isomenthone were identified in the spectrum and successfully assigned. A rotational temperature of 1.5 K for all four species gives the best match between the simulated and the experimental intensities. A few residual lines with SNR ratios of about 3:1 remain unassigned, which might originate from contaminants, clusters or instrument noise. Line splittings due to internal rotation of one of the methyl groups of menthone or isomenthone were not observed.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: The upper trace is the spectrum of the mixture of the stereoisomers of menthone and isomenthone in the region of 4700–5650 MHz while the lower traces are simulations based on fitted molecular parameters. Three different conformers of menthone and one conformer of isomenthone were identified in the spectrum and successfully assigned. A rotational temperature of 1.5 K for all four species gives the best match between the simulated and the experimental intensities. A few residual lines with SNR ratios of about 3:1 remain unassigned, which might originate from contaminants, clusters or instrument noise. Line splittings due to internal rotation of one of the methyl groups of menthone or isomenthone were not observed.
Mentions: The spectrum of the mixture of menthone isomers is presented in the frequency range from 4700 to 5650 MHz in the positive trace of Figure 6. The high number and density of lines of molecular origin and various background lines complicated the assignment of the individual lines to the particular species. Therefore, a computer-aided assignment routine was developed to facilitate the assignment process. This routine is described in Section 3.3. It was possible to assign most of the lines to four different species. For all species, most of the assigned transitions are b-type, with fewer a-type transitions and even fewer c-type transitions. This observation suggests that for all four species μb > μa > μc applies. The final fits were executed employing the SPFIT/SPCAT suite of programs using the Ir representation and the Watson A reduction. The results are compiled in Table 3.

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.