Theoretical study of the internal rotational barriers of fluorine, chlorine, bromine, and iodine-substituted ethanes

Brueckner Doubles variation of W1 theory (W1BD) adapted to pseudopotential: W1BDCEP theory

Composite methods are the combination of ab initio calculations used to achieve high precision in the face of a computational reduction. Weizmann-n theories (n = 1, 2, 3, and 4) stand out for presenting a high precision, and a version of the W1 theory is the W1BD theory that uses ab initio Brueckner Doubles (BD) methods. One way to reduce the computational cost of composite methods and maintain accuracy is to use pseudopotentials in the calculation steps; in this context, W1BDCEP composite method was developed from the respective W1BD all-electron version by considering the implementation of compact effective pseudopotential (CEP). The test set used to evaluate the theory were 8 proton affinities (PA₀), 46 electron affinities (EA₀), 54 ionization energies (IE₀), 80 enthalpies of formation (Δ_fH⁰), and 10 bond dissociation energies (BDE). The mean absolute deviation values (MADs) for W1BD and for the version adapted to the pseudopotential, W1BDCEP, were similar, with values of 0.97 kcal mol^-1 and 1.03 kcal mol^-1, respectively, when the properties PA₀, EA₀, IE₀, and Δ_fH⁰ were evaluated together. Comparing the versions of the theories that employ ab initio Brueckner Doubles calculations with the W1 and W1CEP theories, it is possible to observe that the W1BD and W1BDCEP theories are more accurate than the W1 theory (MAD_W1 = 1.25 kcal mol^-1) and W1CEP (MAD_W1CEP = 1.44 kcal mol^-1), proving the accuracy of using the BD method. Pseudopotential reduces computational time by up to 30% and thus enables more accurate calculations with less computational time.

Determination of the standard enthalpy of formation of iodine compounds through the G2 and G3(MP2)//B3-SBK theories

The domain of application of the G3(MP2)//B3-SBK theory was expanded, and its efficiency was evaluated to determinate enthalpies of formation of forty-one iodine compounds. The results were compared to those obtained with the G2 theory for the same set of molecules. The G3(MP2)//B3-SBK theory showed a mean deviation and deviation standard equal to 3.7 kcal mol^-1 and 6.0 kcal mol^-1, respectively. The G2 theory (mean deviation = 3.1 kcal mol^-1 and standard deviation = 4.9 kcal mol^-1) presented a lower error and standard deviation, but at a significantly higher computational cost. For a more complete evaluation, as a secondary part of the work, it also used different functionals B3LYP, M06-2X, WB97XD, and MP2 method with four different basis sets 6-311G(d,p), LANL2DZ, jorge-ADZP, and CEP-31G(d). The best density functional/basis set combination was obtained with M06-2X/CEP-31G(d) among the three mentioned functionals. However, the produced mean deviation is significant and equal to 17.3 kcal mol^-1, with a standard deviation equal to 23.0 kcal mol^-1. The 6-311G(d,p) basis achieved the best performance with the MP2 method, generating an equally significant mean deviation of 12.8 kcal mol^-1 with a standard deviation equal to 18.7 kcal mol^-1.

Theoretical study of internal rotational barriers of electrons donating and electrons withdrawing groups in aromatic compounds

The presence of internal rotation in sigma bonds is essential for conformational analysis of organic molecules and its understanding is of great relevance in chemistry, as well as in several other areas. However, for aromatic compounds that have substituent groups, withdrawers or donors of electron, there are no data in the literature to explain their rotational barriers. In this context, the work studied the internal rotational barriers of electron donating and withdrawing groups in aromatic compounds using the MP3, MP4, and CCSD(T) methods and the influence of substituents' nature on barrier heights was investigated through calculations based on the theory of Natural Bond Orbitals (NBO) and Quantum Theory of Atoms in Molecules (QTAIM). The results obtained showed that the CCSD(T) method is the one that best describes the internal rotational barriers, followed by MP4 and MP3 and the electron donating groups decrease the barrier, whereas electron withdrawing groups increase. Through the NBO analysis it was possible to observe that for withdrawing groups the interaction of the molecular orbitals is more accentuated promoting the increase of the rotational barrier of these compounds. Through the QTAIM analysis it was possible to show that, for electron donating groups, the internal rotation is influenced by the loss of electronic density when the substituents is perpendicular to the ring plane, however, for withdrawing groups the density is little influenced, regardless of the two conformations (minimum and maximum energy). Two molecules showed free rotation, trichloromethylbenzene and methylbenzene, and the theoretical calculations NBO and QTAIM showed that for these species there is no difference in the properties studied when there is rotation of the dihedral angle.

Theoretical and experimental investigation on decomposition mechanism of eco-friendly insulation gas HFO1234zeE

Decomposition experiments under corona discharge and theoretical calculations using the density functional theory (DFT) method were accomplished to clarify the dissociation behavior and decomposition mechanism of HFO1234zeE (trans-1,3,3,3-tetrafluoropropene), an eco-efficient SF₆ alternative gas. The discharge decomposition products of HFO are mainly fluorocarbon, unsaturated hydrocarbon and saturated hydrocarbons, which are containing no more than three carbons. Free radicals, CF₃·, F· and H·, generated via bond-cleavage reaction are important structures to promote the decomposition, and HFO is more likely to dissociate with them by abstraction reaction to form CF₄ and CF₃H. Long-chain radicals, such as CF₃CHCF·, CF₃CHCH· and CF₃CCHF·, will be decomposed into CF₃CCH and CF₃CCF, and small intermediates would be easier to combine to form HF, C₂F₆, C₂F₄, C₂HF₅, CF₃HCF₃H and C₃F₈. It is also likely to be converted to Z-isomer. Due to the high discharge intensity and ion bombardment, solid by-products appeared on the electrode surface may contain carbon dust and metal compounds. The solid attached to the surface has little effect on the electric field distribution, and most gas decomposition products still maintain the insulation strength, so the air-gap breakdown voltage only dropped by about 6.2% after long-term corona discharge. The obtained results not only reveal the decomposition mechanism in a comprehensive way, but also present useful reference for exploring the application potentials of HFO1234zeE.