Insights into the mechanism, selectivity, and substituent effects in the Diels-Alder reaction of azatrienes with electron-rich dienophiles: An MEDT study.

The reactivity and mechanistic intricacies of azatrienes in Diels-Alder reactions have been relatively unexplored despite their intriguing potential applications. In this study, we employ Molecular Electron Density Theory to theoretically investigate the hetero-Diels-Alder reaction involving azatrienes with ethyl vinyl ether and allenyl methyl ether. Analysis of Conceptual Density Functional Theory, energetic profiles, and the topological characteristics is conducted to elucidate the reactions. The revealed mechanism manifests as a polar one-step two-stages process under kinetic control. We establish a clear relationship of between the periselectivity, regioselectivity, and stereoselectivity on one hand and the characteristics of the reactions mechanism on the other hand. The influence of weak interactions on reaction activation barriers and bonding evolution are discussed in detail. We demonstrate that substituents enhancing the reverse electron density flux facilitate the feasibility of the reactions. The results lay ground for a meticulous control of the reaction of azatriene in similar synthetic scenarios.

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.

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.

Unveiling the mechanism and selectivity of [3+2] cycloaddition reactions of benzonitrile oxide to ethyl trans-cinnamate, ethyl crotonate and trans-2-penten-1-ol through DFT analysis.

The mechanism and regioselectivity of [3+2] cycloaddition (32CA) reactions of benzonitrile oxide with ethyl trans-cinnamate, ethyl crotonate and trans-2-penten-1-ol has been studied in gas phase and in acetonitrile, ethyl acetate and tetrahydrofuran using the B3LYP functional in connection with 6-31G(d) basis set. The 32CA reactions followed one-step mechanism with asynchronous TSs. The calculated global electron density transfer (GEDT) at the TSs showed electronic flux from benzonitrile oxide to ethyl trans-cinnamate and ethyl crotonate, while the electronic flux from trans-2-penten-1-ol to benzonitrile oxide was predicted, in complete agreement with the Conceptual Density Functional Theory (CDFT) indices. The regioselectivity is correctly described in coherence with the experiment data. The intermolecular interaction at the TSs was realized through visualization and quantification by means of independent gradient model (IGM) analysis based on promolecular density.

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.

Computational study of the 1,3-dipolar cycloaddition between methyl 2-trifluorobutynoate and substituted azides in terms of reactivity indices and activation parameters.

The 1,3-dipolar cycloaddition of methyl 2-trifluorobutynoate with various azides has been studied in terms of several theoretical approaches at DFT/B3LYP/6-311++G(d,p) level of theory. The mechanism of regioselectivity of these reactions was investigated through the evaluation of the potential energy surface of the cycloaddition process calculations and density DFT-based reactivity indices. These approaches were successfully applied to prediction of preferable regio-isomers for various reactions of 1,3-dipolar cycloadditions. The reactions were followed by performing transition state optimization, calculation of Intrinsic Reaction Coordinate and activation energies. Analysis of the geometries of the corresponding transition structures shows that the cycloaddition takes place along a single elementary step (one-step mechanism) but asynchronous mechanism. The calculation of the activation energies and reaction energies show that the 1,5-regioisomer for substituted phenyl azides as dipoles and the 1,4-regioisomer for substituted benzyl azides as dipoles are thermodynamically in all the cycloadditions reactions. The solvent effect was also studied in the solvent tert-butyl alcohol using self-consistent reaction field model. The observed regioselectivity was explained by using developed DFT-based reactivity descriptors, such as Fukui and Parr functions. The results were compared with experimental data to find a good agreement.

Ab†Initio and DFT Studies on CO2 Interacting with Zn(q+)-Imidazole (q=0, 1, 2) Complexes: Prediction of Charge Transfer through σ- or π-Type Models.

Using first-principles methodologies, the equilibrium structures and the relative stability of CO2 @[Zn(q+) Im] (where q=0, 1, 2; Im=imidazole) complexes are studied to understand the nature of the interactions between the CO2 and Zn(q+) -imidazole entities. These complexes are considered as prototype models mimicking the interactions of CO2 with these subunits of zeolitic imidazolate frameworks or Zn enzymes. These computations are performed using both ab initio calculations and density functional theory. Dispersion effects accounting for long-range interactions are considered. Solvent (water) effects were also considered using a polarizable continuum model approach. Natural bond orbital, charge, frontier orbital and vibrational analyses clearly reveal the occurrence of charge transfer through covalent and noncovalent interactions. Moreover, it is found that CO2 can adsorb through more favorable π-type stacking as well as σ-type hydrogen-bonding interactions. The inter-monomer interaction potentials show a significant anisotropy that might induce CO2 orientation and site-selectivity effects in porous materials and in active sites of Zn enzymes. Hence, this study provides valuable information about how CO2 adsorption takes place at the microscopic level within zeolitic imidazolate frameworks and biomolecules. These findings might help in understanding the role of such complexes in chemistry, biology and material science for further development of new materials and industrial applications.

Solvation effects and stabilization of multicharged ions: a case study of Ar(m)BeO(q+) complexes.

Using first-principle methods, we characterized Ar(m)BeO(q+) (0 ≤ m ≤ 3 and 0 ≤ q ≤ 2) and BeO(q+) (3 ≤ q ≤ 5) multicharged ions (MCIs). This includes their structural parameters, relative stabilities and vibrational wavenumbers. Our calculations confirm the existence of the ArBeO complex. In addition, we found (meta-)stable neutral and ionic complexes such as ArBeO(+), ArBeO(2+), Ar(2)BeO, Ar(2)BeO(+), Ar(2)BeO(2+), Ar(3)BeO, Ar(3)BeO(+) and Ar(3)BeO(2+). The analysis of the structural parameters and of the electron density differences shows that a strong perturbation in the electronic structure of the BeO(q+) (q = 1,2) moiety occurs upon complexation, resulting in a major increase in covalency of these multicharged ions (MCIs). The consequences of solvation of MCIs in argon matrices are pointed out. More generally, the effects on the spectroscopy of MCIs trapped on cold matrices are discussed.