Theoretical study of HF elimination kinetics of ethane fluorides and derivatives [C₂H_6<i>-n</i>F<i>_n</i> (<i>n </i>= 1- 4)]

1579-1585A theoretical study of the thermal decomposition kinetics of ethane fluorides, viz., ethyl fluoride, 1,1-difluoroethane, 1,1,1-trifluoroethane and 1,1,2,2-tetrafluoroethane, has been carried out at the B3LYP/6-31++G**, B3PW91/6-31++G** and MP2/6-31++G** levels of theory. Among these methods, data for activation parameters obtained with the B3PW91/ 6-31++G** method is in good agreement with the experimental data. The calculated data demonstrate that in the HF elimination reaction of the studied compounds, the polarization of the C1-F3 bond is rate determining. Analysis of bond order, NBO charges, bond indexes and synchronicity parameters suggest the HF elimination occurs through a concerted and asynchronous four-membered cyclic transition state type of mechanism

Theoretical Study of the Oxidation Mechanisms of Naphthalene Initiated by Hydroxyl Radicals: The H Abstraction Pathway

Reaction mechanisms for the initial stages of naphthalene oxidation at high temperatures (<i>T</i> ≥ 600 K) have been studied theoretically using density functional theory along with various exchange-correlation functionals, as well as the benchmark CBS-QB3 quantum chemical approach. These stages correspond to the removal of hydrogen atoms by hydroxyl radical and the formation thereby of 1- and 2-naphthyl radicals. Bimolecular kinetic rate constants were estimated by means of transition state theory. The excellent agreement with the available experimental kinetic rate constants demonstrates that a two-step reaction scheme prevails. Comparison with results obtained with density functional theory in conjunction with various exchange-correlation functionals also shows that DFT remains unsuited for quantitative insights into kinetic rate constants. Analysis of the computed structures, bond orders, and free energy profiles demonstrates that the reaction steps involved in the removal of hydrogen atoms by OH radicals satisfy Hammond’s principle. Computations of branching ratios also show that these reactions do not exhibit a particularly pronounced site-selectivity

Ab initio chemical kinetics of Isopropyl acetate oxidation with OH radicals

Global reactivity descriptors of Isopropyl acetate (IPA) and thermo-kinetics aspects of its oxidation via the ground state (X2Π) and the first excited state (A2Σ+) •OH radicals have been studied computationally using the moderate ab initio composite method, restricted open-shell complete basis setquadratic Becke3 (ROCBSQB3), the accurate thermo-kinetic density functional method (DFT) M06-2X/cc-pVTZ and the time-dependent density functional TDDFT-M06-2X/cc-pVTZ//M06-2X/cc-pVTZ levels. Ten oxidation pathways have been investigated all of them are exothermic. The potential energy diagram has been sketched using different pre- and post-reactive complexes for all reactions. Rate constants calculations were obtained directly by connecting the separated reactants with different transition states and via an effective approach using the unimolecular Rice-Ramsperger-Kassel-Marcus (RRKM) and the transition state (TST) theories. The results indicate that the reaction of IPA with •OH radicals occurs in the ground state rather than the excited state and the rate constants obtained directly and from the effective approach are the same which confirmed the accuracy of the estimated pre-reactive complexes and the reaction mechanism. Rate constants and branching ratios show that H- atom abstraction from iso C-H (C2 atom) bond is the most kinetically preferable route up to 1000 K, while at higher temperature H-atom abstraction from the out-plane CH3 group (C3 atom) became the most dominant route with the high competition with that of in-plane CH3 group (C4 atom)

Computational analysis of substituent effects on proton affinity and gas-phase basicity of TEMPO derivatives and their hydrogen bonding interactions with water molecules

Abstract The study investigates the molecular structure of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives in the gas phase using B3LYP and M06-2X functional methods. Intermolecular interactions are analyzed using natural bond orbital (NBO) and atoms in molecules (AIM) techniques. NO2-substituted TEMPO displays high reactivity, less stability, and softer properties. The study reveals that the stability of TEMPO derivatives is mainly influenced by LP(e) → σ ∗ electronic delocalization effects, with the highest stabilization observed on the oxygen atom of the nitroxide moiety. This work also considers electron density, atomic charges, and energetic and thermodynamic properties of the studied NO radicals, and their relative stability. The proton affinity and gas-phase basicity of the studied compounds were computed at T = 298 K for O-protonation and N-protonation, respectively. The studied DFT method calculations show that O-protonation is more stable than N-protonation, with an energy difference of 16.64–20.77 kcal/mol (22.80–25.68 kcal/mol) at the B3LYP (M06-2X) method. The AIM analysis reveals that the N–O…H interaction in H2O complexes has the most favorable hydrogen bond energy computed at bond critical points (3, − 1), and the planar configurations of TEMPO derivatives exhibit the highest EHB values. This indicates stronger hydrogen bonding interactions between the N–O group and water molecules

Following the Molecular Mechanism of Decarbonylation of Unsaturated Cyclic Ketones Using Bonding Evolution Theory Coupled with NCI Analysis

The synergetic use of bonding evolution theory (BET) and noncovalent interaction (NCI) analysis allows to obtain new insight into the bond breaking/forming processes and electron redistribution along the reaction path to understand the molecular mechanism of a reaction and recognize regions of strong and weak electron pairing. This viewpoint has been considered for cheletropic extrusion of CO from unsaturated cyclic ketones cyclohepta-3,5-dien-1-one <b>CHD</b>, cyclopent–3-en-1-one <b>CPE</b>, and bicyclo[2.2.1]­hept-2-en-7-one <b>BCH</b> by using hybrid functional MPWB1K in conjugation with aug-cc-pVTZ basis set. Decarbonylation of <b>CHD</b>, <b>CPE</b>, and <b>BCH</b> are nonpolar cyclo-elimination reactions that are characterized by the sequence of turning points (TPs) as <b>CHD</b>, 1–11-C­[CC]­C^†C^†FFF^TSC^†C^†C^†–0:<b>HT</b> + CO; <b>CPE</b>, 1–8-CC­[C^†C^†F^†]­[FF]­[FF]­F^TS[C^†C^†]–0:<b>BD</b> + CO; and <b>BCH</b>, 1–8-CC­[C^†C^†]­F­[FF]­F^TS[C^†C^†]–0:<b>CD</b> + CO. Breaking of C–C bond between the terminal carbon atoms of diene/triene framework and carbon atom of CO fragment starts at a distance of ca. 1.9–2.0 Å in the vicinity of the transition structure where the transition states are not reached yet. NCI analysis explains that the noncovalent interactions between two fragments appeared after the breaking of C–C bonds

Theoretical Study of the Oxidation Mechanisms of Naphthalene Initiated by Hydroxyl Radicals: The OH-Addition Pathway

The oxidation mechanisms of naphthalene by OH radicals under inert (He) conditions have been studied using density functional theory along with various exchange–correlation functionals. Comparison has been made with benchmark CBS-QB3 theoretical results. Kinetic rate constants were correspondingly estimated by means of transition state theory and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Comparison with experiment confirms that, on the OH-addition reaction pathway leading to 1-naphthol, the first bimolecular reaction step has an effective negative activation energy around −1.5 kcal mol^–1, whereas this step is characterized by an activation energy around 1 kcal mol^–1 on the OH-addition reaction pathway leading to 2-naphthol. Effective rate constants have been calculated according to a steady state analysis upon a two-step model reaction mechanism. In line with experiment, the correspondingly obtained branching ratios indicate that, at temperatures lower than 410 K, the most abundant product resulting from the oxidation of naphthalene by OH radicals must be 1-naphthol. The regioselectivity of the OH^•-addition onto naphthalene decreases with increasing temperatures and decreasing pressures. Because of slightly positive or even negative activation energies, the RRKM calculations demonstrate that the transition state approximation breaks down at ambient pressure (1 bar) for the first bimolecular reaction steps. Overwhelmingly high pressures, larger than 10⁵ bar, would be required for restoring to some extent (within ∼5% accuracy) the validity of this approximation for all the reaction channels that are involved in the OH-addition pathway. Analysis of the computed structures, bond orders, and free energy profiles demonstrate that all reaction steps involved in the oxidation of naphthalene by OH radicals satisfy Leffler–Hammond’s principle. Nucleus independent chemical shift indices and natural bond orbital analysis also show that the computed activation and reaction energies are largely dictated by alterations of aromaticity, and, to a lesser extent, by anomeric and hyperconjugative effects