Structure Determination, Conformational Flexibility, Internal Dynamics, and Chiral Analysis of Pulegone and Its Complex with Water

Abstract

In the current work we present a detailed analysis of the chiral molecule pulegone, which is a constituent of essential oils, using broadband rotational spectroscopy. Two conformers are observed under the cold conditions of a molecular jet. We report an accurate experimentally determined structure for the lowest energy conformer. For both conformers, a characteristic splitting pattern is observed in the spectrum, resulting from the internal rotation of the two non-equivalent methyl groups situated in the isopropylidene side chain. The determined energy barriers are 1.961911(46) kJ mol and 6.3617(12) kJ mol for one conformer, and 1.96094(74) kJ mol and 6.705(44) kJ mol for the other one. Moreover, a cluster of the lowest energy conformer with one water molecule is reported. The water molecule locks one of the methyl groups by means of a hydrogen bond and some secondary interactions, so that we only observe internal rotation splittings from the other methyl group with an internal rotation barrier of 2.01013(38) kJ mol . Additionally, the chirality-sensitive microwave three-wave mixing technique is applied for the differentiation between the enantiomers, which can become of further use for the analysis of essential oils.

Analysis of thyme essential oils using gas-phase broadband rotational spectroscopy

“…For linalool, we recorded the 8-18 GHz range using chirps of 2.5 GHz spectral width. The microwave chirps were created by an arbitrary waveform generator (AWG), amplified with a 300 W traveling wave tube amplifier (2-8 GHz) or a 50 W solid-state amplifier (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) and then broadcasted into the vacuum chamber using a horn antenna. The molecular signal was collected in the time domain as a free induction decay (FID).…”

“…The EE component is further split into EE + and EEdue to toptop interactions. 16 Note that this last splitting is only resolved for c type transitions with K a = 3.…”

“…High-resolution rotational spectroscopy in the gas phase (also denoted as microwave or molecular rotational resonance (MRR) spectroscopy) has been used to study the structures of several monoterpenes, for example linalool, 10 carvone, 11,12 fenchone, 13 camphor 14,15 or pulegone. 16 Additionally, the chiral analysis of conformational mixtures 12 and mixtures of different terpenes 17 has been demonstrated with a combination of broadband rotational spectroscopy and the chirality-sensitive microwave three-wave-mixing (M3WM) technique. Recently, two studies were reported on using this last experimental approach to analyze commercially available peppermint oil 7,18 and buchu oil, 19 identifying the terpenes present in the oils and performing a chiral analysis of menthone and its diastereomer isomenthone in the oils.…”

“…Moreover, the samples can be analyzed without preparation and separation, and because of the recent developments in microwave spectroscopy, the introduction of the chirp technique, spectra can be recorded in just a few hours. 16 As a requirement, the molecules need to have a permanent dipole moment and have to be brought into the gas phase.…”

The role of secondary interactions on the preferred conformers of the fenchone–ethanol complex

“…[7][8][9][10][11] ). The development of chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy 12 , which allows fast collection of broad segments of the spectrum at once, facilitating the identification of spectral patterns, has dramatically extended the range and complexity of molecular systems that can be tackled, including large molecules 13 , odorants 14,15 , and intricate clusters and complexes [16][17][18] .…”