The simplest S-nitrosothiol, thionitrous acid (HSNO), is a reactive molecule of both biological and astronomical interest. Here we report the first detection of both cis- and trans-HSNO by means of Fourier-transform microwave spectroscopy and double resonance experiments. Surprisingly, HSNO is readily produced in a gas expansion of H₂S and NO, i.e. without applying any discharge. Once formed, HSNO appears quite stable, as evidenced by its high steady-state concentration. A precise empirical molecular equilibrium structure was derived from a combination of theory and experiment.

H₂NNO plays a central role as the initial intermediate in the NH₂ + NO reaction. As the simplest N-nitrosamine, it is also the basis for understanding how specific substituents subtly change the structure of the NNO unit in larger nitrosamines, an important set of compounds that can be produced in foods by nitrites, but which are commonly carcinogenic to humans. Due to its perceived instability, H₂NNO has never been isolated and spectroscopically characterized in the gas phase, but, by means of Fourier transform microwave spectroscopy in combination with millimeter-wave double resonance techniques, the rotational spectrum of the normal and six of its rare isotopic species have been measured to high accuracy between 15 and 90 GHz. For each isotopic species, all three rotational constants have been determined to a fractional accuracy of 10 ppm or better; nitrogen quadrupole coupling constants have also been derived to very high accuracy. By correcting the experimental rotational constants for vibrational corrections calculated theoretically, a precise semi-experimental structure has been derived. These findings are consistent with new CCSD(T) calculations which predict that the equilibrium geometry of H₂NNO is planar, but that it possesses an extremely flat H₂N-X potential, like NH₃. Other aspects of this joint work, including the bond order inferred from the eQq(N) coupling constants, and the issue of planarity in substituted derivatives of the form R₁R₂NNO, will be discussed.

The rotational spectrum of 1-bromobutane measured by the 480 MHz bandwidth chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. In this paper, the ab initio calculation and the analysis of rotational spectrum were performed, and the properties of gas molecule are reported. 1-bromobutane have five confomers; aa, ag, ga, gg, gg'. The transitions were assigned to three different conformers which are most stable forms; aa, ag, ga. The spectra for the normal isotopic species and ⁸¹Br substitution were observed and assigned. The rotational spectrum of 2-bromobutane has been observed in the frequency region 7-18 GHz. 2-bromobutane has the three possible conformers; G+, A, G-. The difference of their energy is very small, so the spectra of all conformers were found in the full range of our spectrum. Consequentially, the rotational constants, nuclear quadrupole constants, and centrifugal distortion constants were determined and the dipole moment of the aa conformer with ⁷⁹Br were measured. All the experimental data is in good agreement with the calculated data.

An accurate equilibrium structure of trans-hexatriene has been determined by the mixed estimation method with rotational constants from 8 deuterium and carbon isotopologues and high-level quantum chemical calculations. In the mixed estimation method bond parameters are fit concurrently to moments of inertia of various isotopologues and to theoretical bond parameters, each data set carrying appropriate uncertainties. The accuracy of this structure is 0.001 Å and 0.1^°. Structures of similar accuracy have been computed for the cis,cis, trans,trans, and cis,trans isomers of octatetraene at the CCSD(T) level with a basis set of wCVQZ(ae) quality adjusted in accord with the experience gained with trans-hexatriene. The structures are compared with butadiene and with cis-hexatriene to show how increasing the length of the chain in polyenes leads to increased blurring of the difference between single and double bonds in the carbon chain. In trans-hexatriene r(“C₁=C₂”) = 1.339 Å and r(“C₃=C₄”) = 1.346 Å compared to 1.338 Å for the “double” bond in butadiene; r(“C₂–C₃”) = 1.449 Å compared to 1.454 Å for the “single” bond in butadiene. “Double” bonds increase in length; “single” bonds decrease in length.

r0pt Figure A systematic study on the ring puckering potentials of three fluorinated cyclopentenes has been performed using Fourier transform microwave spectroscopy in tandem with quantum chemical calculations. Spectra between 8 GHz and 16 GHz have been measured for octafluorocyclopentene, 1H-heptafluorocyclopentene, and 1H,2H-hexafluorocyclopentene, where the hydrogens sequentially replace the fluorines on the sp² hybridized carbons. Rotational constants and centrifugal distortion constants have been determined for the parent species and all ¹³C isotopologues. In regards to the ring puckering, double minimum potential, both cross state and intra-state transitions were observed for all molecules except the 1H,2H-hexafluorocyclopentene. Experimental Coriolis coupling constants and ∆E₀₁ values will be presented and discussed. The ring puckering barrier heights for C₅F₈, C₅HF₇, and C₅H₂F₆, have been calculated to be 222 cm⁻¹, 302 cm⁻¹, and 367 cm⁻¹, respectively.

2-Methyltetrahydrofuran (2-MeTHF) is a promising environmentally friendly solvent and biofuel component which is derived from renewable resourcesV. Pace, P. Hoyos, L. Castoldi, P. Domínguez de María, A. R. Alcántara, ChemSusChem 5 (2012), 1369−1379. Following the principles of Green Chemistry, 2-MeTHF has been evaluated in various fields like organometallics, metathesis, and biosynthesis on the way to more eco-friendly synthesesa) D. F. Aycock, Org. Process Res. Dev. 11 (2007),156−159. b) M. Smole\'n, M. K edziorek, K. Grela, Catal. Commun. 44 (2014), 80−84. Cyclopentane as the prototype of five-membered rings is well-known to exist as twist or envelope structures. However, the conformational analysis of its heterocyclic derivative 2-methyl-tetrahydrothiophene (MTTP) yielded two stable twist conformers and two envelope transition statesV. Van, C. Dindic, H.V.L. Nguyen, W. Stahl, ChemPhysChem 16 (2015), 291−294. Here, we report on the heavy atom r_s structure of the oxygen-analog of MTTP, 2-MeTHF, studied by a combination of molecular beam Fourier transform microwave spectroscopy and quantum chemistry. One conformer of 2-MeTHF was observed and highly accurate molecular parameters were determined using the XIAM programH. Hartwig, H. Dreizler, Z. Naturforsch. A 51 (1996), 923−932. In addition, all ¹³C-isotopologues were assigned in natural abundance of 1%. A structural determination based on the r_s positions of all carbon atoms was achieved via Kraitchman’s equationsJ. Kraitchman, Am. J. Phys. 21 (1953), 17−24. The methyl group in 2-MeTHF undergoes internal rotation and causes A–E splittings of the rotational lines. The barrier was calculated to be 1142 cm⁻¹at the MP2/6-311++G(d,p) level of theory, which is rather high. Accordingly, narrow A–E splittings could be observed for only a few transitions. However, the barrier height could be fitted while the angles between the internal rotor axis and the principal axes of inertia were taken from the experimental geometry.

---

Footnotes:

V. Pace, P. Hoyos, L. Castoldi, P. Domínguez de María, A. R. Alcántara, ChemSusChem 5 (2012), 1369−1379.. a) D. F. Aycock, Org. Process Res. Dev. 11 (2007),156−159. b) M. Smole\'n, M. K edziorek, K. Grela, Catal. Commun. 44 (2014), 80−84.. V. Van, C. Dindic, H.V.L. Nguyen, W. Stahl, ChemPhysChem 16 (2015), 291−294.. H. Hartwig, H. Dreizler, Z. Naturforsch. A 51 (1996), 923−932.. J. Kraitchman, Am. J. Phys. 21 (1953), 17−24..

Dimers of aromatic molecules with weak proton donors such as acetylene are prototypical systems for investigating weak CH…π interactions. A logical progression from our recent rotational spectroscopic studies of benzene…HCCH and fluorobenzene…HCCH was to study 1,2-difluorobenzene(1,2-dfbz)…HCCH, so the effect of increasing the number of electronegative substituents could be investigated. In this talk, structures of benzene, fluorobenzene, and 1,2-difluorobenzene complexed with HCCH will be compared, and the challenges and pitfalls encountered during assignment of the very rich chirped-pulse Fourier-transform microwave (CP-FTMW) spectrum will be discussed. The spectrum of a mixture of 1,2-dfbz and HCCH in a neon carrier was initially recorded using the CP-FTMW spectrometer at the University of Virginia. Transitions matching the patterns and approximate rotational constants predicted for 1,2-dfbz…HCCH were readily identified; however, efforts to fit the observed frequencies to an asymmetric top Hamiltonian were unsuccessful. A second CP-FTMW scan of only 1,2-dfbz monomer revealed that the transitions initially believed to be 1,2-dfbz…HCCH were actually present in both scans. Subtraction of lines common to both data sets revealed a previously unidentified pattern of transitions that have now been confirmed by isotopic substitution to belong to 1,2-dfbz…HCCH. The originally identified transitions are likely 1,2-dfbz…²⁰Ne, which has a similar mass to the HCCH complex. Ab initio calculations for 1,2-dfbz…HCCH and 1,2-dfbz…Ne lead to several possible orientations for each dimer with similar energies and rotational constants, and efforts to improve the computational methods and to reliably identify stationary points on the dimer potential energy surfaces are ongoing.

The structure of the weakly bound dimer between fluoroethylene (FE) and 1,1-difluoroethylene (DFE) has been determined using a combination of chirped-pulse and resonant-cavity Fourier-transform microwave spectroscopy over a 7.5 to 19 GHz range. The rotational constants of the most abundant isotopomer were determined to be A = 6601.14(35) MHz, B = 833.3336(5) MHz and C = 744.0217(5) MHz, and are in excellent agreement with ab initio predictions at the MP2/6-311++G(2d,2p) level. Observation of all four unique ¹³C isotopologues in natural abundance allowed for a full structure determination, showing that the dimer takes on a planar configuration with the H-C-F end of FE aligned with one of the F-C=C-H sides of DFE, forming two inequivalent CH…F contacts. The dipole moment components (μ_a = 0.9002(18) D, μ_b = 0.0304(80) D) were determined using Stark effect measurements and confirm the observed structure.

Pyrimidine, the meta substituted dinitrogen analog of benzene, has been studied in the mm-wave region from 260 – 360 GHz, expanding on previous studies up to 337 GHz.Z. Kisiel, L. Pszczolkowski, I. R. Medvedev, M. Winnewisser, F. C. De Lucia, E. Herbst, J. Mol. Spectrosc. 233, 231-243 (2005).G. L. Blackman, R. D. Brown, F. R. Burden, J. Mol. Spectrosc. 35, 444-454 (1970).W. Caminati, D. Damiani, Chem. Phys. Lett. 179, 460-462 (1991).he spectra of all four of the singly-substituted ¹³C and ¹⁵N isotopologues were observed in natural abundance. Samples of deuterium enriched pyrimidine were synthesized, giving access to several deuterium-substituted isotopologues. The experimental rotational constants have been corrected for vibration-rotation coupling and electron mass. The vibration-rotation corrections were calculated with an anharmonic frequency calculation at the CCSD[T]/ANO1 level using CFOUR. An equilibrium structure determination has been performed using the corrected rotational constants with the xrefit module of CFOUR. Several vibrational satellites of pyrimidine have also been studied. Their rotational constants have been compared to those obtained computationally.

---

Footnotes:

Z. Kisiel, L. Pszczolkowski, I. R. Medvedev, M. Winnewisser, F. C. De Lucia, E. Herbst, J. Mol. Spectrosc. 233, 231-243 (2005).

---

Footnotes:

W. Caminati, D. Damiani, Chem. Phys. Lett. 179, 460-462 (1991).T

Phenyl isocyanate (PhNCO) has been studied in the frequency range of 250-360 GHz, improving on rotational and centrifugal distortion constants based on previous spectroscopic studies between 4.7 and 40 GHz.A. Bouchy and G. Roussy, Journal of Molecular Spectroscopy. 65 (1977), 395-404.W. Kasten and H. Dreizler, Z. Naturforsch. 42a (1987), 79-82. Using the rigid rotor/centrifugal distortion model, many transitions have been assigned for the ground state (approximately 2200 transitions) and the fundamental of the -NCO torsional vibration (approximately 1500 transitions) for J values ranging between 140 and 210 and K_prolate values from 0 to 42. Beyond these K values, these two spectra show effects of perturbations with other vibrational states. Vibrational energy levels and vibration-rotation interaction constants were predicted using CFOUR at the CCSD(T)/ANO0 level. The two lowest energy excited vibrational modes are predicted to have energies of 47 cm⁻¹ (-NCO torsion) and 95 cm⁻¹ (in-plane -NCO wag). Fermi resonance between the first overtone of the -NCO torsional vibration (94 cm⁻¹) and the fundamental of the in-plane -NCO wag has been observed in the spectra of these two states. Analysis for vibrationally excited states up to  190 cm⁻¹ is in progress.

---

Footnotes:

A. Bouchy and G. Roussy, Journal of Molecular Spectroscopy. 65 (1977), 395-404.

---

Footnotes:

The rotational spectrum of trans-cinnamaldehyde ((2E)-3-phenylprop-2-enal) has been obtained with chirped-pulse microwave spectroscopy in the frequency range of 2 - 8.5 GHz. The odorant molecule is the essential component in cinnamon oil and causes the characteristic smell. In the measured high-resolution spectrum, we were able to assign the rotational spectra of two conformers of trans-cinnamaldehyde as well as all singly ¹³C-substituted species of the lowest-energy conformer in natural abundance. Two different methods were used to determine the structure from the rotational constants, which will be compared within this contribution. In addition, the current progress of studying ether-alcohol complexes, aiming at an improved understanding of the interplay between hydrogen bonding and dispersion interaction, will be reported. Here, a special focus is placed on the complexes of diphenylether with small aliphatic alcohols.

The pure rotational spectra of 9-fluorenone (C₁₃H₈O) and benzophenone (C₁₃H₁₀O) were observed using chirped-pulse Fourier transform microwave spectroscopy (cp-FTMW). The 9-fluorenone spectrum was collected between 8 and 13 GHz, which allowed for the assignment of 124 rotational transitions. A separate spectrum spanning from 8 to 14 GHz was collected for benzophenone, allowing for the assignment of 133 rotational transitions. Both aromatic ketones exhibited strong b-type spectra with little to no centrifugal distortion, indicating highly rigid molecular structures. A comparison of the experimentally determined spectral constants of 9-fluorenone to those calculated using both ab initio and density functional theory strongly suggest the molecule conforms to a planar C_2v symmetric geometry as expected for its polycyclic structure. Whereas, a comparison of the experimental benzophenone constants to those predicted by theory suggests a molecule with non-planar C₂ symmetry, where the two phenyl groups are rotated approximately 32^° out-of-plane to form a paddlewheel like geometry.

2-Aminoisobutyric acid (Aib) is an achiral, α-amino acid having two equivalent methyl groups attached to C_α. Extended Aib oligomers are known to have a strong preference for the adoption of a 3₁₀-helical structure in the condensed phase.Toniolo, C.; Bonora, G. M.; Barone, V.; Bavoso, A.; Benedetti, E.; Di Blasio, B.; Grimaldi, P.; Lelj, F.; Pavone, V.; Pedone, C., Conformation of Pleionomers of α-Aminoisobutyric Acid. Macromolecules 1985, 18, 895-902.ere, we have taken a simplifying step and focused on the intrinsic folding propensities of Aib by looking at a series of capped Aib oligomers in the gas phase, free from the influence of solvent molecules and cooled in a supersonic expansion. Resonant two-photon ionization and IR-UV holeburning have been used to record single-conformation UV spectra using the Z-cap as the UV chromophore. Resonant ion-dip infrared (RIDIR) spectroscopy provides single-conformation IR spectra in the OH stretch and NH stretch regions. Data have been collected on a set of Z-(Aib)_n-X oligomers with n = 1, 2, 4, 6 and X = -OH and -OMethyl. The impacts of these capping groups and differences in backbone length have been found to dramatically influence the conformational space accessed by the molecules studied here. Oligomers of n=4 have sufficient backbone length for a full turn of the 3₁₀-helix to be formed. Early interpretation of the data collected shows clear spectroscopic markers signaling the onset of 3₁₀-helix formation as well as evidence of structures incorporating C7 and C14 hydrogen bonded rings.

---

Footnotes:

Toniolo, C.; Bonora, G. M.; Barone, V.; Bavoso, A.; Benedetti, E.; Di Blasio, B.; Grimaldi, P.; Lelj, F.; Pavone, V.; Pedone, C., Conformation of Pleionomers of α-Aminoisobutyric Acid. Macromolecules 1985, 18, 895-902.H

IR/UV double resonance spectroscopy has shown that the structure of the capped dipeptide Ac-Trp-Tyr-NH₂ is dominated by a hydrophobic interaction between the aromatic rings on the side chains. Using the same method, a similar molecule, Ac-Phe-Phe-NH₂, had three experimentally observed conformers including one similar to that of Ac-Trp-Tyr-NH₂. In this work, calculations were performed on additional dipeptides containing two aromatic rings to determine if the dispersion-dominated structure was among the lowest energy structures in all such cases. The B3LYP-DCP method developed by DiLabio and Torres was used to calculate the conformations of each dipeptide. Appending dispersion correction potentials (DCP) to B3LYP input files improves results for systems containing dispersion interactions without significantly increasing the calculation time. This method was used first on Ac-Trp-Tyr-NH₂ and Ac-Phe-Phe-NH₂ to confirm that it successfully identified the experimentally observed structures among the lowest energy results and was then applied to other capped dipeptides containing two aromatic rings including Ac-Phe-Tyr-NH₂ and Ac-Tyr-Phe-NH₂.