The gas-phase rotational spectrum from 8 to 750 GHz and the high-resolution infrared (IR) spectrum of pyridazine (o-C4H4N2) have been analyzed for the ground and four lowest-energy vibrationally excited states. A combined global fit of the rotational and IR data has been obtained using a sextic, centrifugally distorted-rotor Hamiltonian with Coriolis coupling between appropriate states. Coriolis coupling has been addressed in the two lowest-energy coupled dyads (nu(16), nu(13) and nu(24), nu(9)). Utilizing the Coriolis coupling between the vibrational states of each dyad and the analysis of the IR spectrum for nu(16) and nu(9), we have determined precise band origins for each of these fundamental states: nu(16) (B-1) = 361.213 292 7 (17) cm(-1), nu(13) (A(2)) = 361.284 082 4 (17) cm(-1), nu(24) (B-2) = 618.969 096 (26) cm(-1), and nu(9) (A(1)) = 664.723 378 4 (27) cm(-1). Notably, the energy separation in the nu(16)-nu(13) Coriolis-coupled dyad is one of the smallest spectroscopically measured energy separations between vibrational states: 2122.222 (72) MHz or 0.070 789 7 (24) cm(-1). Despite nu(13) being IR inactive and nu(24) having an impractically low-intensity IR intensity, the band origins of all four vibrational states were measured, showcasing the power of combining the data provided by millimeter-wave and high-resolution IR spectra. Additionally, the spectra of pyridazine-d(x) isotopologues generated for a previous semi-experimental equilibrium structure (r(e)(SE)) determination allowed us to analyze the two lowest-energy vibrational states of pyridazine for all nine pyridazine-d(x) isotopologues. Coriolis-coupling terms have been measured for analogous vibrational states across seven isotopologues, both enabling their comparison and providing a new benchmark for computational chemistry.

The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130-375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (re SE) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration-rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a "best theoretical estimate" (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and re SE values is discussed.

The pure rotational spectrum of pyrimidine (m-C4H4N2), the meta-substituted dinitrogen analog of benzene, has been studied in the millimeter-wave region from 235 GHz to 360 GHz. The rotational spectrum of the ground vibrational state has been assigned and fit to yield accurate rotational and distortion constants. Over 1700 distinct transitions were identified for the normal isotopologue in its ground vibrational state and least-squares fit to a partial sextic S-reduced Hamiltonian. Transitions for all four singly substituted 13C and 15N isotopologues were observed at natural abundance and were likewise fit. Deuterium-enriched samples of pyrimidine were synthesized, giving access to all eleven possible deuterium-substituted isotopologues, ten of which were previously unreported. Experimental values of rotational constants and computed values of vibration-rotation interaction constants and electron-mass corrections were used to determine semi-experimental equilibrium structures (re SE) of pyrimidine. The re SE structure obtained using coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] corrections shows exceptional agreement with the re structure computed at the CCSD(T)/cc-pCV5Z level (≤0.0002 Å in bond distance and ≤0.03° in bond angle). Of the various computational methods examined, CCSD(T)/cc-pCV5Z is the only method for which the computed value of each geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The exceptionally high accuracy and precision of the structure determination is a consequence of the large number of isotopologues measured, the precision and extent of the experimental frequency measurements, and the sophisticated theoretical treatment of the effects of vibration-rotation coupling and electron mass. Taken together, these demanding experimental and computational studies establish the capabilities of modern structural analysis for a prototypical monocyclic aromatic compound.

The pure rotational spectrum of diketene has been studied in the millimeter-wave region from ∼240 to 360 GHz. For the ground vibrational state and five vibrationally excited satellites (ν24, 2ν24, 3ν24, 4ν24, and ν16), the observed spectrum allowed for the measurement, assignment, and least-squares fitting a total of more than 10 000 distinct rotational transitions. In each case, the transitions were fit to single-state, complete or near-complete sextic centrifugally distorted rotor models to near experimental error limits using Kisiel's ASFIT. Additionally, we obtained less satisfactory least-squares fits to single-state centrifugally distorted rotor models for three additional vibrational states: ν24 + ν16, ν23, and 5ν24. The structure of diketene was optimized at the CCSD(T)/ANO1 level, and the vibration-rotation interaction (αi) values for each normal mode were determined with a CCSD(T)/ANO1 VPT2 anharmonic frequency calculation. These αi values were helpful in identifying the previously unreported ν16 and ν23 fundamental states. We obtained a single-crystal X-ray structure of diketene at -173 °C. The bond distances are increased in precision by more than an order of magnitude compared to those in the 1958 X-ray crystal structure. The improved accuracy of the crystal structure geometry resolves the discrepancy between previous computational and experimental structures. The rotational transition frequencies provided herein should be useful for a millimeter-wave or terahertz search for diketene in the interstellar medium.

The millimeter-wave spectrum of hydrazoic acid (HN3) was analyzed in the frequency region of 235-450 GHz. Transitions from a total of 14 isotopologues were observed and fit using the A-reduced or S-reduced Hamiltonian. Coupled-cluster calculations were performed to obtain a theoretical geometry, as well as rotation-vibration interaction corrections. These calculated vibration-rotation correction terms were applied to the experimental rotational constants to obtain mixed theoretical/experimental equilibrium rotational constants (Ae, Be, and Ce). These equilibrium rotational constants were then used to obtain an equilibrium (Re) structure using a least-squares fitting routine. The Re structural parameters are consistent with a previously published Rs structure, largely falling within the uncertainty limits of that Rs structure. The present Re geometric parameters of HN3 are determined with exceptionally high accuracy, as a consequence of the large number of isotopologues measured experimentally and the sophisticated (coupled-cluster theoretical treatment (CCSD(T))/ANO2) of the vibration-rotation interactions. The Re structure exhibits remarkable agreement with the CCSD(T)/cc-pCV5Z predicted structure, validating both the accuracy of the ab initio method and the claimed uncertainties of the theoretical/experimental structure determination.

Millimeter-wave spectra for formyl azide (HC(0)N-3) were obtained from 240 to 360 GHz at ambient temperature. For the ground state of syn formyl azide, over 1500 independent rotational transitions were measured and least-squares fit to a complete S-reduced 8th order centrifugal distortion/rigid rotor Hamiltonian. The decomposition of formyl azide was monitored over a period of several hours, the half-life (t(1/2) = 30 min) was determined, and its decomposition products were investigated. Transitions from five vibrational satellites of syn formyl azide (v(9), v(12), 2v(9), v(9) + v(12), and v(11)) were observed, measured, and least-squares fit to complete or nearly complete octic centrifugally-distorted, single-state S-reduced models. A less complete single-state fit of 3v(9) (509.3 cm(-1)) was obtained from an unperturbed subset of its assignable transitions. This state is apparently coupled to the fundamental v(8) (489.4 cm(-1)) and the overtone 2v(12) (503.6 cm-1), but the coupling remains unanalyzed. Anharmonic CCSD(T)/ANO1 estimates of the vibrational frequencies of syn formyl azide were in close agreement with previously published experimental and computational values. Experimentally determined vibration-rotation interaction (alpha(i)) values were in excellent agreement with coupled-cluster predicted alpha(i) values for the fundamentals v(9), v(12), and v(11). (C) 2016 Elsevier Inc. All rights reserved.

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.ab 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 Kprolate 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.

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.abc The spectra of all four of the singly-substituted 13 C and 15 N 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.

The rotational spectra for hydrazoic acid (HN 3 ), its isotopologues, and its vibrational satellites have been reexamined using millimeter-wave rotational spectroscopy in the range of 240-360 GHz. Treating sodium azide (NaN 3 ) or the commercially available singly 15 N-labeled NaN 3 with phosphoric acid or deuterated phosphoric acid yielded 6 different isotopologues. From these samples, we were also able to observe all of the isotopologues containing one additional 15 N at natural abundance. In total, we assigned rotational transitions to 14 different species; only H 15 N 3 and D 15 N 3 were not accessible. With the large number of rotational constants determined for these isotopologues, an excellent equilibrium structure determination was performed with CFOUR’s xrefit routine. This structure shows a bent azide sub-unit, and is in excellent agreement with the geometry optimization performed at the CCSD(T)/ANO2 level of theory. The Coriolis perturbation of the ground and first two vibrationally excited states of HN 3 will also be discussed.

Millimeter-wave absorption spectra for carbonyl diazide (OC(N-3)(2)) are reported in the frequency range of 243-360 GHz, at both 293 K and 213 K. Transitions for two of the three possible conformations, one with both of the azide groups syn to the carbonyl group, or with one syn and the other anti, were observed in the spectra. Theoretical calculations at the CCSD(T)/ANO1 level do an excellent job of predicting the ground state rotational constants and 4th order centrifugal distortion terms for both conformers. Relative line intensities, along with theoretically predicted dipole moments, were used to estimate the energy difference of the two observed forms, yielding a result in good agreement with the ab initio potential energy surface. The spectra of the v(12), v(7), v(9) and 2v(12) excited vibrational states for the more abundant syn-syn conformer have been assigned, and a great many transitions for each of them have been fit using partial 6th and 8th order centrifugal distortion Hamiltonians. Anharmonic vibration-rotation interaction constants from the CCSD(T)/ANO1 calculations are in excellent agreement with the experimentally determined constants in the case of v(7) and v(9), but not for v(12). (C) 2013 Elsevier Inc. All rights reserved.

The rotational spectrum of pyridazine (o-C4H4N2), the ortho disubstituted nitrogen analog of benzene, has been measured and analyzed in the gas phase. For the ground vibrational state of the normal isotopolog, over 2000 individual rotational transitions have been identified between 238 and 360 GHz and have been fit to 13 parameters of a 6th-order centrifugal distortion Hamiltonian. All transitions in this frequency region can now be predicted from this model to near experimental accuracy, i.e., well enough for the purpose of any future radio-astronomical search for this species. Three isotopologs, [3-13C]-C4H4N2, [4-13C]-C4H4N2, and [1-15N]-C4H4N2, have been detected in natural abundance, and several hundred lines have been measured for each of these species and fit to 6th-order Hamiltonians. Ten additional isotopologs were synthesized with enhanced deuterium substitution and analyzed to allow for a complete structure determination. The equilibrium structure (Re) of pyridazine was obtained by correcting the experimental rotational constants for the effects of vibration-rotation coupling using interaction constants predicted from CCSD(T) calculations with an ANO0 basis set and further correcting for the effect of electron mass. The final Re structural parameters are determined with excellent accuracy, as evidenced by their ability to predict 28 independent moments of inertia (Ia and Ib for 14 isotopologs) very well from 9 structural parameters. The rotational spectra of the six lowest-energy fundamental vibrational satellites of the main isotopolog have been detected. The rotational spectra of the five lowest-energy vibrational satellites have been assigned and fit to yield accurate rotational and distortion constants, while the fit and assignment for the sixth is less complete. The resultant vibration-rotation interaction (α) constants are found to be in excellent agreement with ones predicted from coupled-cluster calculations, which proved to be the key to unambiguous assignment of the satellite spectra to specific vibration modes.

Author Institution: The University of Wisconsin - Madison Department of Chemistry, 1101 University Avenue, Madison, WI 53705

Guided by ab initio predictions, the structure of the gas-phase complex formed between cis-1,2-difluoroethylene and an argon atom in a pulsed molecular jet is determined using microwave spectroscopy in the 5.7-21.5 GHz region of the spectrum. This is a non-planar, symmetric species, with the argon atom located in the FCCF cavity of the difluoroethylene. The transitions in the microwave spectrum are observed to be split by an interconversion tunneling motion between the two equivalent configurations for the complex with the argon atom located either above or below the difluoroethylene molecular plane. Both one- and two-dimensional discrete variable representation calculations of the tunneling splitting using the ab initio interaction potential for the complex suggest that the barrier to interconversion is overestimated by theory.

Author Institution: The University of Wisconsin - Madison Department of Chemistry, 1101 University Avenue, Madison, WI 53705