N-methylation of peptides has been shown to be a useful way for tuning their physicochemical properties for medicinal use. This interest has led us to investigate the structure and rotational spectra of N-methylated lactams, cyclic molecules containing an H₃C-N-C=O moiety. Here we discuss three lactam species of increasing ring size (5 – 7 atoms): 1-methyl-2-pyrrolidone, 1-methyl-2-piperidone, and 1-methylcaprolactam. The rotational spectrum of each lactam has been acquired using a K_a-band (26.5 - 40 GHz) chirped-pulse Fourier transform microwave spectrometer equipped with a heated source and a pulsed supersonic expansion to achieve rotational cooling. The experimental spectroscopy is complemented with MP2/cc-pVDZ calculations of the structures and CH₃ internal rotation barriers. Each lactam shows nitrogen quadrupole hyperfine splitting as well as a clear signature of internal rotation, consistent with the calculations which show a progressive decrease in the internal rotation barrier as the ring size increases. Extra splittings in addition to those from N quadrupole and CH₃ internal rotation are present in the experimental spectrum of 1-methyl-2-pyrrolidone. Analysis of the rotational spectra and structural comparisons among the lactams and with comparable species will be discussed.

Safranal (2,6,6-trimethyl-1,3-cyclohexadien-1-carboxaldehyde) is a cyclical terpenic aldehyde isolated from saffron, the spice consisting of the stigmas of crocus flowers. The molecular formula of safranal is C₁₀H₁₄O. Being famous as a flavoring and odorant chemical, safranal is also known for its anticonvulsant and antidepressant properties.
The rotational spectrum of safranal was measured in the 18-26 GHz frequency range using chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy. The presence of a methyl rotor attached to the sp² hybridized carbon leads to the splitting of each rotational transition into two states, A and E. The barrier to methyl internal rotation was calculated to be around 170 cm⁻¹at B3LYP-D3(BJ)/aug-cc-pVTZ level of theory. Global fits (simultaneous fitting of both A- and E- states transitions) have been performed using XIAMH. Hartwig and H. Dreizler, "The microwave spectrum of trans-2,3-dimethyl oxirane in torsional excited states", Zeitschrift für Naturforschung A 51, 923–932 (1996).llowing the determination of the rotational constants and the centrifugal distortion constants as well as the V₃ barrier height to methyl internal rotation.
In addition to the analysis of the methyl internal rotation, we could also determine the gas phase structure of safranal via the analysis of the rotational signatures of singly substituted ¹³C isotopologues resulting in the structure determination using Kraitchman equationsJ. Kraitchman, "Determination of Molecular Structure from Microwave Spectroscopic Data", American Journal of Physics 21, 17–24 (1953).

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Footnotes:

H. Hartwig and H. Dreizler, "The microwave spectrum of trans-2,3-dimethyl oxirane in torsional excited states", Zeitschrift für Naturforschung A 51, 923–932 (1996).a J. Kraitchman, "Determination of Molecular Structure from Microwave Spectroscopic Data", American Journal of Physics 21, 17–24 (1953)..

Methoxy-methanol (CH₃OCH₂OH), an Oxygenated Volatil Organic Compound of low stability detected in the interstellar medium, represents an example of non-rigid biological molecule showing more than two interacting and inseparable large amplitude motions. The species discloses a relevant coupling among torsional modes, strong enough to prevent assignments of groups of lines using effective Hamiltonians of reduced dimensionality. This paper analyzes the Far Infrared Region using highly correlated ab initio methods and a three-dimensional vibrational model. The molecule presents two Gauche-gauche and one Trans-gauche conformers, which relative energies are very small and are separated by relatively low barriers (1200-1500 cm-1). Low-energy levels and their tunneling splittings are determined variationally up to 700 cm⁻¹. The A/E splitting of the ground vibrational state has been computed to be 0.003 cm⁻¹, as was expected given the methyl torsional barrier (600 cm⁻¹).

Nitrogen-containing cyclic molecules have been found on carbonaceous chondride metorites, and are theorized to have contributed to the emergence of prebiotic chemical systems on Earth 3.8-3.5 billion years ago. A potential formation pathway for meteoric nitrogen heterocycles is believed to occur via reactions in the interstellar medium at low temperatures, and have been of significant astrochemical interest. We have investigated the rotational spectra of select γ-lactam molecules; 2-pyrrolidone, N-vinyl-2-pyrrolidone and N-acetyl-2-pyrrolidone with a combination of ab initio quantum chemical calculation and chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 26.5-40 GHz region. Calculations at the MP2/cc-pVDZ level of theory predict that 2-pyrrolidone and N-vinyl-2-pyrrolidone have low ring puckering barriers (787.96 and 607.79 cm⁻¹, respectively). The experimental spectra accordingly exhibit splittings due to large-amplitude motion. Additionally, preliminary analysis for N-acetyl-2-pyrrolidone revealed splittings from methyl rotation. The derived spectroscopic parameters and effective barrier heights will be discussed.

We recorded the first pure rotational spectrum of 3-methylstyrene (3MS) to study quantum tunneling effects and how they differ between the cis- and trans-conformations of the molecule. The rotational spectrum (10 - 22 GHz) was recorded using the cavity molecular beam FT-microwave spectrometer at Kent State University. We used computational geometry optimizations at the B3LYP-GD3BJ/Def2TZVP level to differentiate between the cis- and trans-conformers. The combination of low dipole moment (μ_a= -0.51 D, μ_b= - 0.16 D, μ_c= 0.00 D) and low methyl torsional barrier resulted in weak and widely split rotational lines for cis-3MS. We used BELGI-Cs and XIAM internal rotor fitting programs to fit 115 transition frequencies of cis-3MS, up to J=11, to obtain its torsional and rotational parameters. Our fits show a low barrier to methyl torsion in cis-3MS, V₃= 30.35 (5) cm⁻¹, aligning with the behavior observed in toluene and its derivatives lacking significant steric hindrance.

Here we report the pure rotational spectral data and derived quantum tunneling information for the trans conformer of 3-methylstyrene (trans-3MS). Trans-3MS is a challenging but interesting problem for rotational spectroscopists due to its weaker dipole moment (μ_a = 0.53 D, μ_b = -0.19 D, μ_c = 0.00 D) and methyl torsion near the free-rotor limit. The very low methyl torsional barrier in trans-3MS led to wider A-E tunneling splitting, with some splitting exceeding 2.5 GHz. Quantum mechanically assigned spectroscopic data were combined with ab inito results to confirm the trans-geometry, which is only 0.025 kJ/mol higher than the cis-3MS per our calculations at B3LYP-GD3BJ/Def2TZVP level. BELGI-Cs and XIAM fitting programs were used to fit over 60 transitions (up to J=10) in the 10-22 GHz range and obtain vibrational ground state torsional and rotational parameters. A comparison of cis-3MS (previous talk) and trans-3MS data will reveal the sensitivity of quantum tunneling to conformational changes.

l0pt Figure 2,6-dimethylphenol has three internal rotors, with two methyl rotors taking up positions ortho to the OH group on the phenol ring. Previous work from Welzel et al.¹ showed that 2-methylphenol has two isomers with very different barriers to methyl internal rotation depending on whether the OH hydrogen is pointing towards (V₃ = 660 cm⁻¹) or away from (V₃  ∼  370 cm⁻¹) the methyl group. On the other hand, in 2,6-dimethylfluorobenzene, Khemissi et al.² have fit the internal rotor structure to obtain a V₃ potential for both rotors of 237 cm⁻¹. The question we seek to address is how the coupled methyl and OH motions will appear in the microwave spectrum when all three rotors are present and OH tunneling is possible. We recorded the broadband microwave spectrum of 2,6-dimethylphenol over the 7.5-17.5 GHz range under jet-cooled conditions. The spectrum is replete with evidence for OH tunneling; however, it appears in a form that is initially hard to interpret, as many of the rotational transitions appear as triplets, with a splitting between the outer two members of the triplet of  ∼  50 MHz, and a central transition half-way between. We will present our spectrum, give a qualitative interpretation for its appearance in which internal rotor states that have the two methyl groups in the same methyl rotor state can tunnel, but those that are in different methyl rotor states cannot. We complement our experimental data with calculations of the three-rotor potential energy surface and fit this surface to a symmetrized potential that shows the major coupling terms that are present. This work was supported by the Gas Phase Chemical Physics program of the Department of Energy, Office of Science, Basic Energy Sciences. (1) Welzel, A.; Hellweg, A.; Merke, I.; Stahl, W. Structural and torsional properties of o-cresol and o-cresol-OD as obtained from microwave spectroscopy and ab initio calculations. J. Mol. Spectrosc. 2002, 215, 58-65. (2) Khemissi, S.; Nguyen, H. V. L. Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6-Dimethylfluorobenzene. ChemPhysChem 2020, 21, 1682-1687.