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[Display omitted] • Improved ground-state spectroscopic constants of 2-cyanopyridine. • Newly measured spectroscopic constants of Coriolis-coupled dyad, ν 30 and ν 21. • Precise energy separation of ν 30 and ν 21 determined. • a -type and b -type rotational transitions, Coriolis couplings in both manifolds. • Spectroscopic constants of 2-cyanopyridine dyad compared with analogous aromatic nitriles. The millimeter-wave rotational spectrum of 2-cyanopyridine was collected from 130 to 750 GHz, and the ground and two lowest-energy excited vibrational states were analyzed. In total, over 20,000 rotational transitions were least-squares fit for the three vibrational states to partial-octic, distorted-rotor Hamiltonians with low error (σ fit < 50 kHz). For the ground state, the many thousands of newly measured rotational transitions enabled substantial refinement of the rotational constants and determination of the centrifugal distortion constants. The rotational spectrum was collected at room temperature, permitting the observation of the two lowest-energy fundamental modes, ν 30 (A″, 154 cm−1) and ν 21 (A′, 175 cm−1), and determination of their spectroscopic constants. The two excited vibrational states are Coriolis coupled and require a two-state Hamiltonian. Eight Coriolis-coupling parameters (G a , G a J , G a K , G a JJ , F bc , F bc J , G b , and G b J) have been determined, as well as a precise energy difference of 26.524 312 6 (40) cm−1 between the vibrational states. A comparison of the ground-state spectroscopic constants, as well as the Coriolis coupling-related parameters of analogous dyads is presented for multiple cyanoarenes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rotational spectrum (130–360 GHz) measured for chlorobenzene. • Constants determined for ground vibrational states of [35Cl]-, [37Cl]-chlorobenzene. • Constants determined for 7 fundamental states, 8 overtone/combination states of [35Cl]-chlorobenzene. • Constants determined for 4 fundamental states of [37Cl]-chlorobenzene. • Data enable detailed comparisons of spectroscopic constants with structurally related compounds. The rotational spectra of [35Cl]- and [37Cl]-chlorobenzene (C 6 H 5 Cl) have been studied from 2 to 18 GHz and 130–360 GHz, resulting in the measurement, assignment, and least-squares fitting of almost 40,000 transitions of twenty-one vibrational states. Previously measured [35Cl]- and [37Cl]-chlorobenzene ground-state transitions were combined with newly measured transitions and fit to A-reduced, partial sextic Hamiltonian models with low-error (σ total fit <0.05 MHz). Analysis of the 2–18 GHz spectrum allowed for refinement of the nuclear hyperfine coupling constants for the ground-state spectra of both isotopologues, while measurement of the 130–360 GHz spectrum provided sufficient information to determine the sextic centrifugal distortion constants of the ground states for the first time. From these millimeter-wave data collected at room temperature, the spectroscopic constants for the lowest-energy fundamentals of [35Cl]-chlorobenzene (ν 20 , ν 30 , ν 11 , ν 14 , ν 19 , ν 29 and ν 18) and [37Cl]-chlorobenzene (ν 20 , ν 30 , ν 11) were determined. As with previous studies of chloroarenes, the computed (B3LYP/6–311+G(2d,p)) spectroscopic constants show quite close agreement with the experimentally determined values. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rotational transitions from 8 to 38 and 130 to 360 GHz used to study anti- and gauche- 4-cyano-1-butyne (an isomer of pyridine). • Spectroscopic constants for the ground vibrational states determined for the first time. • Hyperfine resolved transitions included to determine quadrupole coupling constants (χ aa , χ bb , and χ cc). • Experimental energy difference determined between the conformers (Δ E anti – gauche). The analysis of the rotational spectrum of 4-cyano-1-butyne, an open-chain isomer of pyridine, is presented herein for the first time. Transitions for the ground vibrational states of both conformational isomers, anti and gauche , were observed in the 8–38 GHz and 130–360 GHz frequency ranges and fit to effective sextic distorted-rotor Hamiltonians with low uncertainty (σ fit < 50 kHz), allowing for their potential detection by radioastronomy or in harsh reaction environments. The microwave region includes hyperfine-split transitions for both conformers, allowing for the experimental determination of the χ aa , χ bb , and χ cc quadrupole coupling constants. In the millimeter-wave region, not every observable ground-state transition could be satisfactorily addressed by a single-state Hamiltonian due to Coriolis coupling with the low-energy vibrationally excited states. Thus, effective single-state fits of the ground states were obtained by including only K a -series transitions without obviously perturbed energy levels. Importantly, the effective fit does include the most intense transitions throughout the observed frequency ranges. The predicted values (MP2/cc-pVTZ) are in close agreement with the experimental spectroscopic constants. These data provide a sufficient foundation for the identification of anti- and gauche- 4-cyano-1-butyne in the interstellar medium by radioastronomy. The gauche conformer is estimated to lie 1.00 (5) kcal/mol higher in energy than the anti conformer, as determined using experimental transition intensities. [ABSTRACT FROM AUTHOR]

• Improved ground-state spectroscopic constants of 3-cyanopyridine. • Newly measured spectroscopic constants of Coriolis-coupled dyad, ν 30 and ν 21. • a -type and b -type rotational transitions, Coriolis couplings in both manifolds. • Quantum interferences between Coriolis couplings. • Observed b -type nominal interstate transitions. The rotational spectrum of 3-cyanopyridine from 130 to 360 GHz was recorded, and an analysis of the ground state and two lowest-energy excited vibrational states was completed. Almost 6700 new transitions were measured for the ground state and fit to a partial octic, distorted-rotor Hamiltonian with low error (σ fit < 0.05 MHz). The first two excited vibrational states, ν 30 and ν 21 , are an isolated dyad that exhibits both a - and b -type Coriolis perturbations and requires a two-state, least-squares fit to fully predict the rotational spectrum and determine accurate spectroscopic constants. Quartic and sextic distortion constants were determined for the dyad, along with seven symmetry-allowed perturbation terms: G a , G a J , F bc , F bc K , G b , G b J , and G b K. Numerous resonances, including those following a -type selection rules, Δ K a = 2 or Δ K a = 4, and b -type selection rules, Δ K a = 3 or Δ K a = 5, were observed and fit. For ν 30 and ν 21 , the energy difference ( Δ E 30 , 21 = 15.7524693 (37) cm−1), both Coriolis coupling constants (ζ 30 , 21 a = 0.8327 (9) and ζ 30 , 21 b = −0.0181 (3)), and vibration–rotation interaction constants were determined experimentally and compared to theoretical values determined computationally. Combined with the work on the vibrationally excited dyads of 4-cyanopyridine, phenyl isocyanide, benzonitrile, and phenylacetylene, the coupling in ν 30 and ν 21 provides an opportunity to compare the Coriolis interactions of these analogous mono-substituted aromatic molecules in unusual detail. Additionally, this work improves the ground-state rotational constants and centrifugal distortion constants of 3-cyanopyridine and provides the fundamental constants needed to support an astronomical search for 3-cyanopyridine in the interstellar medium. [ABSTRACT FROM AUTHOR]

• Improved ground-state spectroscopic constants of 4-cyanopyridine. • Newly measured spectroscopic constants of Coriolis-coupled dyad, ν 20 and ν 30. • Precise energy separation of ν 20 and ν 30 determined. • Spectroscopic constants of 4-cyanopyridine dyad compared with analogous benzonitrile dyad. • Comparison of experimental and computational spectroscopic constants. The rotational spectrum of 4-cyanopyridine (μ a = 1.96 D) was recorded from 130 to 360 GHz, and the analysis of the ground state and two lowest-energy excited vibrational states was completed. Over 3900 new rotational transitions were measured for the ground state, allowing the determination of spectroscopic constants for a partial octic, distorted-rotor Hamiltonian. Over 5600 new transitions were measured for the Coriolis-coupled dyad, ν 20 and ν 30. A coupled-state least-squares fit of the dyad was obtained, which resulted in the precise determination of several parameters addressing the Coriolis coupling between the two states: G a , G a J , F bc , and F bc K. With inclusion of these terms, numerous resonance transitions associated with selection rules Δ K a = 2 or Δ K a = 4 between vibrational states were assigned and fit to low error (σ fit < 50 kHz). The precise energy difference between ν 20 and ν 30 was determined , Δ E = 18.806554 (11) cm−1, along with the Coriolis coupling coefficient ζ 20 , 30 a = 0.8432 (8). Determination of the spectroscopic and perturbation parameters for the vibrational states permits comparison to other arenes bearing –CN or –NC substituents and sharing a similar Coriolis-coupled dyad ~150 cm−1 above the ground state. This new data provides the foundation for an astrochemical search for 4-cyanopyridine in the interstellar medium. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]