The product of acrolein dimerization (3,4-Dihydro-2H-Pyran-2-carboxaldehyde), relevant to atmospheric chemistry in context of secondary aerosol formation, was investigated using molecular jet Fourier-transform microwave spectroscopy. r0pt Figure A new spectral assignment tool 'Analysis Program Extension for XIAM and more' (APEX) has been developed that allowed for swift identification of two conformers, one with axial aldehyde group (ax) and one with equatorial aldehyde group (eq). The abstract figure shows a screenshot of APEX’s user interface together with the structures of the two conformers. Based on a second order Taylor expansion, the user can change rotational constants and observe the effect on the predicted spectrum (ax: blue, eq: red) compared to the experimental spectrum (black). Efforts for the determination of precise equilibrium structures will be shown as well.

This study reports the microwave spectra of 1-fluoronaphthalene (1FN) and its water clusters. Previous measurements conducted in 1973H. Karlsson, Acta Chem. Scand., 1973, 27, 1435–1436.nd 2014S. J. Carey, M. Sun and S. G. Kukolich, Journal of Molecular Spectroscopy, 2014, 304, 25–27.n 1FN monomer did not study any isotopologues, which is crucial for precise structural elucidation. We used a chirped-pulse Fourier transform microwave spectrometer in the present study. We assigned all the singly substituted ¹³C isotopologues of the 1FN monomer, revealing a nearly uniform inertial defect (-0.14 amu Ų) across all isotopologues. The negative inertial defect is attributed to the lowest out-of-plane bending mode of the 1FN ring. A formula based on empirical data was utilised to compute the lowest out-of-plane bending modeT. Oka, Journal of Molecular Structure, 1995, 352, 225-233. which was then compared with the bending mode calculated using the MP2/aug-cc-pVTZ level of theory. Additionally, the rotational spectrum of the 1FN···H₂O complex was investigated, unveiling an O-H···F hydrogen bond where H₂O acts as a H-bond donor alongside a C-H···O weak interaction where H₂O serves as a weak H-bond acceptor. Experimentally obtained inertial defect (-1.30 amu Ų) of 1FN···H₂O complex indicated an effective planar geometry. Atoms in Molecules (AIM), Natural Bond Orbital (NBO), and Non-covalent Interactions (NCI) analyses supported the presence of hydrogen bonds, while energy decomposition analysis highlighted the dominant role of electrostatic interactions in stabilising the 1FN···H₂O complex. The preliminary results of the dihydrate complex of 1FN have confirmed the structure in which the water dimer is forming O-H···F and O-H···C hydrogen bonds while interacting from above the 1FN plane. The dispersion plays a significant role in stabilising the aforementioned structure in the 1FN···(H₂O)₂ complex.

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Footnotes:

H. Karlsson, Acta Chem. Scand., 1973, 27, 1435–1436.a S. J. Carey, M. Sun and S. G. Kukolich, Journal of Molecular Spectroscopy, 2014, 304, 25–27.o T. Oka, Journal of Molecular Structure, 1995, 352, 225-233.,

We have recorded the rotational spectra of 2-methylcyclopentanone and 2,2-dimethylcyclopentanone in the 26.5-40 GHz frequency range, using chirped pulse Fourier transform microwave spectroscopy (CP-FTMW) of the molecules cooled in a supersonic expansion. Both molecules are useful synthetic intermediates for a wide variety of organic molecules. By determining their molecular structures, we seek to gain a better understanding of their functionality. We have determined the rotational constants, centrifugal distortion constants, and puckering barriers for both molecules. Moreover, the spectra of both molecules were obtained with sufficient signal-to-noise that we could detect and assign ¹³C transitions of all C-atoms in the two molecules in natural abundance. In this talk we will compare the structure of these two molecules to see how the substitution of a hydrogen for a methyl group affects puckering.

Caprolactams are important structural motifs for polymers, natural products, and pharmaceuticals. The size and flexibility of the ring structure in these lactams result in a very rich conformational landscape. Here we discuss three caprolactams, two of which are N-substituted: ϵ-caprolactam, N-vinylcaprolactam, and N-acetylcaprolactam. 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 several conformers for each molecule and CH₃ internal rotation barrier for N-acetylcaprolactam. Each lactam shows nitrogen quadrupole hyperfine splitting. Extra splittings in addition to those from N quadrupole are present in the experimental spectrum of ϵ-caprolactam. This is suspected to come from a source of large amplitude motion in the ring. Analysis of the rotational spectra and structural comparisons among the lactams and with comparable species will be discussed.

Succinimides and maleimides are highly polar cyclic organics that play important roles in biological chemistry, biotechnology, and organic synthesis. In this talk, we will describe our broadband rotational studies of N-methyl succinimide, N-methylmaleimide, N-ethyl succinimide, and N-ethyl maleimide. Microwave spectroscopy provides high-resolution rotational spectra of these molecules in the gas phase and when combined with electronic structure calculations, the rotational fits can be used to extract the molecular structures. The broadband spectra of four succinimide derivatives have been investigated in the 26.5-40 GHz frequency range region using a chirped pulse Fourier transform microwave spectrometer coupled to a supersonic expansion. The structures, molecular parameters, and methyl internal rotation dynamics of these four species will be discussed and compared.

Pyridine (C_2v; κ = 0.85, μ_a = 2.1 D) is a fundamental organic molecule and the simplest analogue of benzene containing a heteroatom. We have analyzed the rotational spectra of the normal and all singly substituted heavy-atom isotopologues at natural abundance, as well as those of the perdeuterio- and two singly deuteriated isotopologues (2-²H and 3-²H). The rotational constants of these isotopologues, along with vibration-rotation interaction and electron-mass corrections predicted using the CCSD(T) level of theory, can be used to determine a highly precise semi-experimental equilibrium structure (r_e^SE) of pyridine. Although a single substitution of each atom is sufficient for a complete r_e^SE determination, we intend to synthesize and analyze additional deuteriated isotopologues in order to obtain an even more precise and accurate structure, and to bring the current r_e^SE of pyridine to the same standard as those achieved for benzene and pyridazine.

Cyclopropanone, an astrochemically relevant ketone, has recently been synthesized via the reaction of ketene and diazomethane, and its rotational spectrum studied from 85-750 GHz. Transitions of the ground vibrational state of the main isotopologue, as well as those of two ¹³C heavy-atom isotopologues, were detectable at natural abundance and least-squares fit to sextic centrifugally distorted rotor Hamiltonians. The synthetic methodology has also allowed for convenient synthesis of all possible deuterium-substituted cyclopropanones. The deuterium-enriched samples were measured from 235-500 GHz and their transitions similarly least-squares fit to distorted-rotor Hamiltonians. Computed (CCSD(T) and B3LYP rotational and centrifugal distortion constants are compared to their corresponding experimental values. The experimental rotational constants (B₀) have been corrected for vibration-rotation interactions and electron mass distributions to obtain equilibrium rotational constants (B_e). The B_e constants from a relatively large number of isotopologues yield highly accurate and precise, semi-experimental equilibrium (r_e^SE) structure of cyclopropanone. Our best theoretical estimate of the structure will be compared to the r_e^SE structure.

The rotational spectrum of proline methyl ester (PrOMe) has been recorded using a cavity-based Fourier transform microwave spectrometer in the range of 10 GHz - 20 GHz. Previous work on neutral proline identified spectra arising from two structures with different ring-puckering configurations.¹ The rotational spectrum of the lowest energy structure of prolinamide has also been previously assigned.² In this work, 15 conformers of PrOMe were modeled using Density Functional Theory calculations and ab initio calculations at the ωB97XD/aug-cc-pVDZ and MP2/aug-cc-pVDZ. 51 rotational transitions were fit to a Watson's A-reduced Hamiltonian: A = 3678.44 (7) MHz, B = 1037.56 (3) MHz, and C = 944.21 (3) MHz. 293 ¹⁴N nuclear quadrupole hyperfine components (144 of A components and 149 of E components) were assigned and fit to χ_aa = 2.302 (6) MHz and χ_bb - χ_cc = 5.487 (9) MHz. The measured spectrum of PrOMe is assigned to the lowest energy structure which has intramolecular hydrogen bond between imino hydrogen and carbonyl oxygen. The structural similarities and differences between PrOMe, neutral proline and prolinamide will be discussed. (1) Lessarri, A; Mata, S.;Cocinero, E. J.;Blanco, S.;Lopez, J. C.;Alonso, J. L. The Structure of Nuetral Proline. Angew. Chem. 2002, 41, 4673-4676. (2) Kuhls, K. A.; Centrone, C. A.;Tubergen, M. J. Microwave Spectroscopy of the Twiest C-Exo/C-Endo Conformation of Prolineamide. J. Am. Chem. Soc. 1998, 120, 10194-10198.

A conformational analysis was performed on methyl heptanoate and methyl octanoate utilizing cavity-based Fourier transform microwave spectroscopy. Spectra from the two lowest energy conformers were observed, the C_s conformer and the C₁ conformer, for each species. Preliminary fits of the spectra gave A = 6331.82 (1) MHz, B = 336.20 (1) MHz, and C = 323.94 (1) MHz for the C_s conformer of methyl heptanoate, A = 3261.062 (1) MHz, B = 424.09 (1) MHz, and C = 401.90 (1) MHz for the C₁ conformer, A = 5488.820 (4) MHz, B = 245.340 (3) MHz, and C = 236.17 (2) MHz for the C_s conformer of methyl octanoate, and A = 2657.353 (1) MHz, B = 306.875 (4) MHz, and C = 290.476 (4) MHz for the C₁ conformer. The spectrum of the C_s conformer of methyl heptanoate contains 5 rotational transitions and the spectrum of the C₁ conformer contains 25 rotational transitions. The spectrum of the C_s conformer of methyl octanoate contains 14 rotational transitions and the spectrum of the C₁ conformer contains 22 rotational transitions. Because of the small values for the rotational constants, the spectra of both conformers were composed of transitions between states with very high J values. All rotational transitions were split by methyl internal rotation tunneling into A and E components. These tunneling splittings were assigned and fit using XIAM. Discussion on the molecular structure and V₃ barriers will be discussed.

The rotational spectrum of iodoacetonitrile was reinvestigated^a using high-resolution cavity-based Fourier transform microwave spectroscopy to better understand the nuclear quadrupole coupling tensors of iodine and nitrogen. Theoretical calculations at the CCSD(T), B3LYP, and MP2 levels of theory using the 6-311++G basis set were used to obtain modeled rotational constants, centrifugal distortion constants, and hyperfine constants, which aided in spectral assignment. A semi-rigid rotor Hamiltonian perturbed by hyperfine interactions was used to fit the spectrum. 24 rotational transitions composed of 364 hyperfine components were fit to an RMS error of 6 kHz, and the fitted molecular constants are A = 20032.6182 (7) MHz, B = 1749.048 (6) MHz, C = 1623.843 (6) MHz, ∆_J = 0.635 (1) kHz, ∆_JK = -26.65 (4) kHz, δ_J = 0.094 (2) kHz, δ_J = -0.12 (3) kHz, (¹⁴N)χ_aa = -1.83 (1) MHz, (¹⁴N)χ_bb - χ_cc = -2.34 (2) MHz, (¹²⁷I)χ_aa = -1367.066 (1) MHz, (¹²⁷I)χ_bb - χ_cc = -808.976 (2) MHz, (¹²⁷I)χ_ab = -1396.20 (1) MHz. ^a R. C. Claytor, G. M. Ault, J. D. Graybeal, Proceedings of Talk TG12, 40th Ohio State University Symposium on Molecular Spectroscopy, Columbus, Ohio, 1985 ( unpublished).