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 χ_cc 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 uŲ hints that low-frequency out-of-plane vibrations are present in benzothiazole, similar to the observation for some other planar aromatic molecules.

Theoretical spectroscopic study of acetyl (CH 3CO), vinoxy (CH 2CHO), and 1-methylvinoxy (CH 3COCH 2) radicals. Barrierless formation processes of acetone in the gas phase

Background: Acetone is present in the earth´s atmosphere and extra-terrestrially. The knowledge of its chemical history in these environments represents a challenge with important implications for global tropospheric chemistry and astrochemistry. The results of a search for efficient barrierless pathways producing acetone from radicals in the gas phase are described in this paper. The spectroscopic properties of radicals needed for their experimental detection are provided.   Methods: The reactants were acetone fragments of low stability and small species containing C, O and H atoms. Two exergonic bimolecular addition reactions involving the radicals CH 3, CH 3CO, and CH 3COCH 2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH 3COCH 2 and CH 3CO, as well as the CH 2CHO isomer, was performed. Rovibrational parameters, anharmonic vibrational transitions, and excitations to the low-lying excited states are provided. For this purpose, RCCSD(T)-F12 and MRCI/CASSCF calculations were performed. In addition, since all the species presented non-rigid properties, a variational procedure of reduced dimensionality was employed to explore the far infrared region. Results: The internal rotation barriers were determined to be V 3=143.7 cm -1 (CH 3CO), V 2=3838.7 cm -1 (CH 2CHO) and V 3=161.4 cm -1 and V 2=2727.5 cm -1 (CH 3COCH 2).The splitting of the ground vibrational state due to the torsional barrier have been computed to be 2.997 cm -1, 0.0 cm -1, and 0.320 cm -1, for CH 3CO, CH 2CHO, and CH 3COCH 2, respectively. Conclusions: Two addition reactions, H+CH 3COCH 2 and CH 3+CH 3CO, could be considered barrierless formation processes of acetone after considering all the possible formation routes, starting from 58 selected reactants, which are fragments of the molecule. The spectroscopic study of the radicals involved in the formation processes present non-rigidity. The interconversion of their equilibrium geometries has important spectroscopic effects on CH 3CO and CH 3COCH 2, but is negligible for CH 2CHO.

Theoretical study of the 1,3-DC reaction between fluorinated alkynes and azides: Reactivity indices, transition structures, IGM and ELF analysis

DFT/B3LYP/6-31 + g(d,p) calculations were performed and reactivity indices, transition structures theory were applied to elucidate the molecular mechanism of the reaction between fluorinated alkynes and different azides. The process of cycloaddition operates through a one-step mechanism and an asynchronous transition states. Examination of the energy profile in the gas phase show two possible 1,4 and 1,5 addition channels for amid, the former is preferred. Furthermore, the Parr functions have used to elucidate the reaction's regioselectivity. It was noted that the addition of solvent (DMSO) does not affect the regioselectivity of the reactions. Analysis of charge transfer in the transition structures shows a moderate polar character for the majority of the studied reactions. Moreover, the independent gradient model based on promolecular density (IGM) has been used to evaluate the asynchronicity of the formation of the two new sigma bonds. Finally, the ELF calculation has been performed to clearly describe the formation of the previous bonds along the IRC path.