Origin of Anomalous Electronic Circular Dichroism Spectrum of RuPt₂(tppz)₂Cl₂(PF₆)₄ in Acetonitrile

We report a theoretical study of the structures, energetics, and electronic spectra of the Pt^II/Ru^II mixed-metal complex RuPt₂(tppz)₂Cl₂(PF₆)₄ (tppz = 2,3,5,6-tetra­(2-pyridyl)­pyrazine) in acetonitrile. The hybrid B3LYP density functional theory and its TDDFT methods were used with a complete basis set (CBS) extrapolation scheme and a conductor polarizable continuum model (C-PCM) for solvation effects. Results showed that the trinuclear complex has four types of stable conformers and/or enantiomers. They are separated by high barriers owing to the repulsive H/H geometrical constraints in tppz. A strong entropy effect was found for the dissociation of RuPt₂(tppz)₂Cl₂(PF₆)_n in acetonitrile. The UV–visible and emission spectra of the complex were also simulated. They are in good agreement with experiments. In this work we have largely focused on exploring the origin of anomalous electronic circular dichroism (ECD) spectra of the RuPt₂(tppz)₂Cl₂(PF₆)₄ complex in acetonitrile. As a result, a new mechanism has been proposed together with a clear illustration by using a physical model

Exploring the Ring-Opening Pathways in the Reaction of Morpholinyl Radicals with Oxygen Molecule

Quantum chemistry calculations using hybrid density functional theory and the coupled-cluster method have been performed to investigate the ring-opening pathways in the oxidation of morpholine (1-oxa-4-aza-cyclohexane). Hydrogen abstraction can form two different carbon-centered radicals, morpholin-2-yl or morpholin-3-yl, or the nitrogen-centered radical, morpholin-4-yl, none of which are found to have low-energy pathways to ring-opening. Extensive exploration of multiple reaction pathways following molecular oxygen addition to these three radicals revealed two competitive low energy pathways to ring-opening. Addition of O₂ to either carbon-centered radical, followed by a 1,4-H shifting mechanism can yield a long-lived cyclic epoxy intermediate, susceptible to ring-opening, following further radical attack. In particular, the second pathway begins with O₂ attack on morpholin-2-yl, followed by a 1,5-H shift and a unimolecular ring-opening without having to overcome a high barrier, releasing a significant amount of heat in the overall ring-opening reaction. The calculations provide valuable context for the development of mechanisms for the low temperature combustion chemistry of nitrogen and oxygen-containing fuels

Theoretical Study of the Reaction of CH₃ with HOCO Radicals

The reaction of HOCO radicals with CH3 radicals is examined using the coupled cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the CH3 + HOCO reaction can produce both the H2O + CH2CO and the CH4 + CO2 products through acetic acid and enediol intermediates. Direct ab initio dynamics calculations determine the thermal rate coefficients to be k(T/K) = 3.24 × 10−11T0.1024 in cm3·molec−1·s−1 at T ≤ 1000 K for the overall reaction. In addition, the product branching ratio of (H2O + CH2CO) to (CH4 + CO2) is predicted to be RH2O/CH4(T/K) = 1.52 + (1.95 × 10−4)T using RRKM theory. Both the thermal rate coefficients and the product branching ratios are weakly temperature dependent

Exploring the Multiple Reaction Pathways for the H + cyc-C₃H₆ Reaction

Reaction pathways for the hydrogen atom plus cyclopropane (cyc-C3H6) reaction are studied using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. For this activated reaction, results reveal two reaction mechanisms, a direct H-abstraction and a H-addition/ring-opening. The hydrogen-abstraction reaction yields the H2 and cyclopropyl (cyc-C3H5) radical products. The vibrationally adiabatic ground-state (VAG) barrier height is predicted to be 13.03 kcal/mol. The isomerization barrier height from the product cyclopropyl to allyl radical is 21.98 kcal/mol via a cyc-C3H5 ring-opening process. In addition, the H-addition and ring-opening mechanism will lead to an n-C3H7 radical, which can result in a variety of products such as CH3 + C2H4, H + CH3CHCH2, and H2 + C3H5, etc. The VAG barrier height of the H-addition reaction is 16.49 kcal/mol, which is slightly higher than that of the direct H-abstraction reaction. Although the H + cyc-C3H6 → CH4 + CH2CH reaction is exoergic by 11.90 kcal/mol, this reaction is unlikely due to a high barrier of 43.05 kcal/mol along the minimum energy path

Full-Dimensional Quantum Calculations of Vibrational Levels of NH₄⁺ and Isotopomers on An Accurate Ab Initio Potential Energy Surface

Vibrational energy levels of the ammonium cation (NH₄⁺) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH₄, the vibrational levels of NH₄⁺ and ND₄⁺ exhibit a polyad structure, characterized by a collective quantum number <i>P</i> = 2­(<i>v</i>₁ + <i>v</i>₃) + <i>v</i>₂ + <i>v</i>₄. The low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm^–1

Ab Initio and RRKM Study of the Reaction of ClO with HOCO Radicals

The reaction pathways for the ClO + HOCO reaction have been explored using the coupled-cluster method to locate and optimize the critical points on the ground-state potential-energy surface. Results show that the ClO + HOCO reaction can produce Cl + HOC(O)O, HOCl + CO2, HCl + CO3, and HClO + CO2 via an addition or a direct hydrogen abstraction reaction mechanism. The reaction kinetics has been studied using the variational RRKM theory. It is found that the ClO + HOCO reaction is fast and has a negative temperature dependence at low temperatures. At room temperature, the thermal rate coefficient is obtained as 4.26 × 10−12 cm3 molecules−1 s−1 with product branching fractions of Cl (0.518), HOCl (0.469), HCl (0.01), and HClO (0.003) at zero pressure. The Cl + HOC(O)O products are major, compared to the HOCl + CO2 products, because of the loose transition state along the dissociation pathway to eliminate Cl. In addition, the RRKM/master equation simulations indicate that the stabilization of the HOC(O)OCl intermediates is noticeable at moderate pressures as its thermal rate constants reach about 6.0 × 10−13 cm3 molecules−1 s−1. In contrast, the other product branching ratios for the ClO + HOCO reaction are weakly dependent on pressure

Active Thermochemical Tables: The Partition Function of Hydroxymethyl (CH₂OH) Revisited

The best currently available set of temperature-dependent nonrigid rotor anharmonic oscillator (NRRAO) thermochemical and thermophysical properties of hydroxymethyl radical is presented. The underlying partition function relies on a critically evaluated complement of accurate experimental and theoretical data and is constructed using a two-pronged strategy that combines contributions from large amplitude motions obtained from direct counts, with contributions from the other internal modes of motion obtained from analytic NRRAO expressions. The contributions from the two strongly coupled large-amplitude motions of CH2OH, OH torsion and CH2 wag, are based on energy levels obtained by solving the appropriate two-dimensional projection of a fully dimensional potential energy surface that was recently obtained at the CCSD­(T)/cc-pVTZ level of theory. The contributions of the remaining seven, more rigid, vibrational modes and of the external rotations are captured by NRRAO corrections to the standard rigid rotor harmonic oscillator (RRHO) treatment, which include corrections for vibrational anharmonicities, rotation-vibration interaction, Coriolis effects, and low temperature. The basic spectroscopic constants needed for the construction of the initial RRHO partition function rely on experimental ground-state rotational constants and the best available experimental fundamentals, additionally complemented by fundamentals obtained from the variational solution of the full-dimensional potential energy surface using a recently developed two-component multilayer Lanczos algorithm. The higher-order spectroscopic constants necessary for the NRRAO corrections are extracted from a second-order variational perturbation treatment (VPT2) of the same potential energy surface. The Lanczos solutions of the fully dimensional surface are validated against available experimental data, and the VPT2 results and the solutions of the reduced dimensionality surface are validated both against the Lanczos solutions and available experiments. The NRRAO thermophysical and thermochemical properties, given both in tabular form and as seven- and nine-coefficient NASA polynomials, are compared to previous results. In addition, the latest ATcT values for the enthalpy of formation of CH2OH at 298.15 K (0 K), −16.75 ± 0.27 kJ/mol (−10.45 ± 0.27 kJ/mol), and of other related CHnOm species (n = 0–4, m = 0,1) are reported, together with a plethora of related bond dissociation enthalpies (BDEs), such as the C–H, O–H, and C–O bond dissociation enthalpies of methanol, 402.16 ± 0.26 kJ/mol (395.61 ± 0.26 kJ/mol), 440.34 ± 0.26 kJ/mol (434.86 ± 0.26 kJ/mol), and 384.85 ± 0.15 kJ/mol (377.14 ± 0.15 kJ/mol), respectively, and analogous BDEs for hydroxymethyl, 343.67 ± 0.37 kJ/mol (339.16 ± 0.37 kJ/mol), 125.54 ± 0.28 kJ/mol (121.11 ± 0.28 kJ/mol), and 445.86 ± 0.29 kJ/mol (438.76 ± 0.29 kJ/mol), respectively. The reasons governing the alternation between strong and weak sequential H atom BDEs of methanol are also discussed

Doppler-Resolved Kinetics of Saturation Recovery

Frequency-modulated laser transient absorption has been used to monitor the ground-state rotational energy-transfer rates of CN radicals in a double-resonance, depletion recovery experiment. When a pulsed laser is used to burn a hole in the equilibrium ground-state population of one rotational state without velocity selection, the population recovery rate is found to depend strongly on the Doppler detuning of a narrow-band probe laser. Similar effects should be apparent for any relaxation rate process that competes effectively with velocity randomization. Alternative methods of extracting thermal rate constants in the presence of these non-thermal conditions are evaluated. Total recovery rate constants, analogous to total removal rate constants in an experiment preparing a single initial rotational level, are in good agreement with quantum scattering calculations, but are slower than previously reported experiments and show qualitatively different rotational state dependence between Ar and He collision partners. Quasi-classical trajectory studies confirm that the differing rotational state dependence is primarily a kinematic effect