l0pt Figure New high resolution (0.0014 cm⁻¹) infrared spectra of neopentane (2,2-dimethylpropane) have been recorded at 203 K by use of synchrotron radiation (Canadian Light Source). Spectra were recorded in the IR (8.3-6.4 μm) with 8 m of optical path. Neopentane is a spherical top molecule with T_d (tetrahedral) symmetry. A low temperature is favorable for a detailed rotational analysis of its fundamental bands. This study focuses on the analysis of 3 band systems of neopentane. The first system is a dyad consisting of a ν₁₅ fundamental band and a ν₇ + ν₁₉ combination band located at 1472.5 and 1489 cm⁻¹, respectively. The second system is a ν₁₆ fundamental band located at 1369.4 cm⁻¹with uncertain band origin. The third band system is a dyad of a ν₁₇ fundamental and a ν₈ + ν₁₈ combination band. Only the ν₁₇ fundamental band was analyzed in this last system due to the weaker structure of the combination band. For all spectral simulations and calculations the ‘XTDS’ program based on the tensorial formalism by V. Boudon was used. A subprogram ‘SPVIEW’, paired with XTDS, was used for spectral assignment. A special emphasis will be given to the first dyad located at 1472.5 and 1489 cm⁻¹.

r0pt Figure The rotational spectrum of 1-ethylsilacyclopentane (1ESCP) and 1-ethyl-1-fluorosilacyclopentane (1E1FSCP) were collected and assigned in the 5.0 - 19.0 GHz region of the electromagnetic spectrum. In collaboration with Dr. Gamil Guirgis of the College of Charleston, the title molecules were synthesized in Charleston, SC and rotational spectra were recorded on a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer at the Missouri University of Science and Technology in Rolla, MO. The substitution of a fluorine atom in place of the hydrogen on the silicon atom inside the five member ring has an influence on the types of transitions being observed. Differences in the two structures and their comparisons to theoretical calculations will be discussed.

Fluorinated amino acids are rare bio-organic compounds gaining great relevance in the pharmaceutical field. 4-fluoro-threonine (4FT) is the only natural fluoro amino acid discovered so far. The fluorination of the methyl group significantly increases the complexity of the already intricate conformational panorama of threonine Alonso J. L, Pérez C, Sanz M. E, López J. C, Blanco S. Phys Chem Chem Phys. 2009 Jan 6;11(4):617–27 making any conformational study even more challenging. Guided by state-of-the-art machine-learning methodologiesLeón I, Fusè M, Alonso ER, Mata S, Mancini G, Puzzarini C, et al. J Chem Phys. 2022;157(7).nd using a tandem of laser ablation rotational spectroscopies, up to twelve rotameric species have been discovered in the supersonic expansion of our experiments. These species have been ascribed to twelve of the thirteen low-energy predicted conformers based on rotational constants, nuclear quadrupole coupling constants, and dipole moment selection rules. These results again proved the potential of the laser ablation rotational spectroscopy techniques in structural studies of non-volatile solid biomolecules.

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Footnotes:

Alonso J. L, Pérez C, Sanz M. E, López J. C, Blanco S. Phys Chem Chem Phys. 2009 Jan 6;11(4):617–27, León I, Fusè M, Alonso ER, Mata S, Mancini G, Puzzarini C, et al. J Chem Phys. 2022;157(7).a

Cyclohexylamine was re-investigated by high-resolution Fourier transform microwave spectroscopy. In order to interpret the microwave spectrum, high-level first-principle quantum chemical calculations were carried out at the B3LYP and MP2 methods using the aug-cc-pVTZ basis set. Two potential energy surface scans (one for the axial and another for equatorial conformer) were performed along the H-N-C-C dihedral angle at the B3LYP/6-311++G level of theory. The scan identified four conformers which correlates with previously reported vibrational spectra and theoretical calculations. Eighteen rotational transitions were assigned to the gauche-equatorial conformer and fit to A= 4264.343(57) MHz, B=2222.446 (39) MHz, and C=1604.877(31) MHz. Ring strain and spectral assignment for additional conformers will be discussed.

The rotational spectrum of cyclopropylchloromethyldifluorosilane has been investigated in the 5 –- 19 GHz region of the electromagnetic spectrum using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The molecule was synthesised by collaborators in the Guirgis group at the College of Charleston. Owing to sample limitations, it was only possible to acquire $\sim$80,000 free induction decays (FIDs), although this was sufficient to yield a spectrum with appreciable signal to noise. As a result, both $^{35}$Cl and $^{37}$Cl isotopologues have been observed, primarily arising from the high relative natural abundances of the Cl isotopes, and their spectra have been assigned. The presence of the Cl nuclei, both with \emph{I} = 3/2, leads to transitions splitting into multiple hyperfine components, and thus a good estimation of the nuclear electric quadrupole coupling constants is essential for an accurate assignment. The observation of the Cl isotopologues allows some conclusions to be made pertaining to the electronic and geometric structure of the molecule, and shall be compared to the equilibrium structures obtained from both DFT and \emph{ab inito} methods. Additionally, a conformer arising from a rotation of the chloromethyl group has also been observed.

Succinimide is a heterocyclic organic compound whose derivatives have anticonvulsant applications. We measured its molecular parameters using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 26-40 GHz frequency range, which is highly sensitive to the molecular structure. The spectrometer is equipped with a heated reservoir to increase the vapor pressure of solid samples, and is coupled to a supersonic expansion which results in rotational temperatures on the order of 1-5 K. The rotational spectra were analyzed with the help of state-of-the-art ab initio calculations. We have determined the rotational constants, inertial defect, and centrifugal distortion constants of succinimide. In this talk we will discuss our findings on the molecular structure and comparisons to structurally similar molecules.

N-halosuccinimides are important reagents in organic synthesis and in the chemistry of natural products, they can mediate or catalyze many reactions including halocyclizations and formation of heterocyclic systems. Surprisingly, these molecules have not been previously investigated by rotational spectroscopy. In organic chemistry, halogen substituents are known to be inductively electron withdrawing and resonance donating. To explore the effect of the halogen substituents on the structure and electronics of succinimide, N-chlorosuccinimide, N-bromosuccinimide, and N-iodosuccinimide have been studied in the 26.5-–40 GHz range using the chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy technique. The samples were heated and coupled to a pulsed supersonic expansion to achieve rotational cooling. Experimental details and the determined molecular parameters will be discussed, along with structural comparisons among the halogenated series.

Cyclobutenone has been characterized by high-resolution Fourier transform microwave spectroscopy for the first time. High level first-principles quantum chemical calculations at the B3LYP, MP2, and CCSD levels of theory were employed to better understand the molecular structure and obtain model rotational and centrifugal distortion constants to aid spectral assignment. Results from the different levels of theory are compared. Nine rotational transitions were measured and fit to a Watson A-reduced Hamiltonian (3 kHz error) and S-reduced Hamiltonian (4.2 kHz error). Cyclobutane and its derivatives are known to undergo ring-puckering inversion and were observed to have tunneling splittings of their rotational transitions. No tunneling splittings were observed for cyclobutenone indicating no significant ring-puckering tunneling in this four-membered ring system.

Diketones are a family of compounds extensively investigated in chemistry, mainly due to their keto-enol tautomerism. Rotational spectroscopy is a uniquely powerful tool to probe tautomerization processes owing to its direct sensitivity to molecular structure. The broadband microwave spectra of three cyclic diketone molecules, 4-cyclopentene-1,3-dione, 1,3-cyclopentanedione, and 2-methyl-1,3-cyclopentanedione, have been investigated in the 26-40 GHz frequency range region using a chirped pulse Fourier transform microwave spectrometer coupled to a supersonic expansion. The spectroscopic data were analyzed with the aid of ab-initio calculations to yield complete sets of molecular parameters, including rotational constants and centrifugal distortion constants for all the observed molecules. Details on the molecular structure and the equilibrium between keto- and enol- forms will be discussed.

The rotational spectrum of 1,3-oxazole (c-C₃H₃NO, C_s) has been studied from 40 GHz to 750 GHz. Transitions of the main isotopologue and five heavy-atom isotopologues (¹⁵N, ¹⁸O, and three ¹³C) were fit to S- and A-reduced, sextic distorted-rotor Hamiltonians. Rotational spectra have been obtained for several deuterium-enriched samples from 235 to 360 GHz, and rotational constants for more than a dozen deuterium-containing isotopologues have been determined. A highly accurate and precise semi-experimental (r_e^SE) equilibrium structure of oxazole has been determined using spectroscopic constants for all available isotopologues and CCSD(T) corrections to the rotational constants. Comparison will be made to a CCSD(T)/cc-pCV5Z equilibrium (r_e) structure and the r_e^SE structures of other heteroaromatic compounds.

Cyclophanes are an interesting family of strained organic molecules that contain aromatic rings and aliphatic units that bridge the aromatic rings.S. Kotha, M. E. Shirbhate, and G. T. Waghule, Beilstein journal of organic chemistry, 2015, 11, 1274-1331.2,2]-Paracyclophanes are an example of cyclophanes with two co-facially stacked benzene rings linked through ethylene chains. Z. Hassan, E. Spuling, D. M. Knoll, J. Lahann, and S. Bräse, Chemical Society Reviews, 2018, 47(18), 6947-6963.he rigid connectivity and close positioning of the aromatic units in paracyclophane scaffolds makes them fundamentally important in supramolecular chemistry, while they find applications in material sciences due to their chiroptical and optoelectronic properties.^b,Y. Morisaki and Y. Chujo, Bulletin of the Chemical Society of Japan, 2019, 92(2), 265–274.tructural homologues of [2,2]-paracyclophanes and different substitutions have features of chemical and industrial importance as they alter the photophysical and electrochemical properties of molecular scaffolds. Gas-phase studies of such molecular systems can provide the necessary isolated conditions to elucidate their structures and dynamics. One of the most powerful techniques to provide accurate gas-phase structures is high-resolution rotational spectroscopy.

We report an in-depth study of [2,2]-paracyclophanes using our broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer COMPACT in the 2-8 GHz frequency range.D. Schmitz, V. A. Shubert, T. Betz, and M. Schnell, Journal of Molecular Spectroscopy, 2012, 280, 77–84.e have explored the conformations and the intramolecular interactions of an alcohol-, an aldehyde- and, a ketone-substituted [2,2]-paracyclophane. The most stable conformations of all three derivatives obtained from quantum-chemical calculations and observed in experiments will be discussed along with the singly-substituted ¹³C-isotopologues that facilitated the determination of the experimental structure of these molecules from the paracyclophane family. S. Kotha, M. E. Shirbhate, and G. T. Waghule, Beilstein journal of organic chemistry, 2015, 11, 1274-1331.[ Z. Hassan, E. Spuling, D. M. Knoll, J. Lahann, and S. Bräse, Chemical Society Reviews, 2018, 47(18), 6947-6963.T Y. Morisaki and Y. Chujo, Bulletin of the Chemical Society of Japan, 2019, 92(2), 265–274.S D. Schmitz, V. A. Shubert, T. Betz, and M. Schnell, Journal of Molecular Spectroscopy, 2012, 280, 77–84.W