The Microwave Rotational Electric Resonance (RER) Spectrum of Benzothiazole.

The microwave spectra of benzothiazole were measured in the frequency range 2-26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. Hyperfine splittings arising from the quadrupole coupling of the 14N nucleus were fully resolved and analyzed simultaneously with the rotational frequencies. In total, 194 and 92 hyperfine components of the main species and the 34S isotopologue, respectively, were measured and fitted to measurement accuracy using a semi-rigid rotor model supplemented by a Hamiltonian accounting for the 14N nuclear quadrupole coupling effect. Highly accurate rotational constants, centrifugal distortion constants, and 14N nuclear quadrupole coupling constants were deduced. A large number of method and basis set combinations were used to optimize the molecular geometry of benzothiazole, and the calculated rotational constants were compared with the experimentally determined constants in the course of a benchmarking effort. The similar value of the χcc quadrupole coupling constant when compared to other thiazole derivatives indicates only very small changes of the electronic environment at the nitrogen nucleus in these compounds. The small negative inertial defect of -0.056 uÅ2 hints that low-frequency out-of-plane vibrations are present in benzothiazole, similar to the observation for some other planar aromatic molecules.

Discovery of a Missing Link: First Observation of the HONO-Water Complex.

The still unexplained daytime HONO concentration in the Earth's atmosphere and the impact of water on the HONO chemistry have been a mystery for decades. Several pathways and many modeling methods have failed to reproduce the atmospheric measurements. We reveal in this study the first spectroscopic observation and characterization of the complex of HONO with water observed through its rotational signature. Under the experimental conditions, HONO-water is stable, particularly straightforward to form, and features intense absorption signals. This could explain both the influence of water on the HONO chemistry and the missing HONO sources, as well as the missing contribution of many other molecules of atmospheric relevance that skew the accuracy of field measurements and the full account of partitioning species in the atmosphere.

The effects of proton tunneling, 14N quadrupole coupling, and methyl internal rotations in the microwave spectrum of ethyl methyl amine.

The spectra of N-ethyl methyl amine, CH3(NH)CH2CH3, were measured using a molecular jet Fourier transform microwave spectrometer in the frequency range of 2 GHz-26.5 GHz. Splittings due to proton inversion tunneling, Coriolis coupling, 14N quadrupole coupling, and methyl internal rotation were fully resolved. The experimentally deduced rotational constants are A = 25 934.717(21) MHz, B = 3919.8212(23) MHz, and C = 3669.530(21) MHz. The proton tunneling causes (+) ↔ (-) splittings of about 1980.9 MHz for all c-type transitions between the lowest symmetric and the higher anti-symmetric energy levels. The splittings of the (+) ← (+) and (-) ← (-) levels, mainly influenced by Coriolis coupling, were also observed and assigned for b-type transitions, yielding the coupling constants Fbc = 0.3409(71) MHz and Fac = 163.9(14) MHz. The 14N quadrupole coupling constants were determined to be χaa = 2.788 65(55) MHz and χbb - χcc = 4.630 45(91) MHz. Fine splittings arising from two inequivalent methyl rotors are in the order of 150 kHz, and the torsional barriers are determined to be 1084.62(41) cm-1 for the CH3NH methyl group and 1163.43(80) cm-1 for the CH2CH3 methyl group. The experimental results are in good agreement with those of quantum chemical calculations.