Double-Layered Composite Methods Extrapolating to Complete Basis-Set Limit for the Systems Involving More than Ten Heavy Atoms: Application to the Reaction of Heptafluoroisobutyronitrile with Hydroxyl Radical

An Optimized Force Field for Vapor-Liquid Equilibria and Molecular Dynamics Simulations of Eco-Friendly Dielectric Fluid Perfluoronitriles

A classical all-atom force field for perfluoronitriles (PFN-AA) is proposed for simulating the phase equilibria and dynamic transport properties of perfluoronitrile compounds that are a promising chemical family as a novel eco-friendly replacement for SF₆ in various applications. The force-field parameters are developed primarily by fitting to molecular structures, vibrational frequencies, energetic profiles of the conformational rotation, and intermolecular interactions of the dimeric complexes from ab initio calculations. The performance of the PFN-AA force field is examined by simulating the vapor-liquid coexistence and physical properties of heptafluoro-iso-butyronitrile (C4) using the Gibbs ensemble simulation with the hybrid configurational-bias Monte Carlo technique and the molecular dynamics simulations. Theoretical vapor pressures and the boiling point of the pure C4 compound are in excellent agreement with available experimental data. The physical properties of C4 in the phase envelope including critical properties, self-diffusion coefficients, dielectric constants, shear viscosity, thermal conductivity, and thermodynamic properties are predicted computationally for the first time. In addition, the transferability of the PFN-AA force field with respect to other force fields, i.e., EPM2 for CO₂, is validated by the successful description of the fluoronitrile/CO₂ mixture. The current PFN-AA force field outperforms the generic potential models (e.g., COMPASS and CVFF) in the understanding of the fundamental properties of the novel perfluoronitrile dielectric fluids and their mixtures.

Atmospheric Chemistry of Perfluoro-3-methyl-2-butanone [CF3C(O)CF(CF3)2]: Photodissociation and Reaction with OH Radicals

Perfluoro-3-methyl-2-butanone (CF₃C(O)CF(CF₃)₂, abbreviated as C5) is a potentially excellent fire suppression alternative to halons and a promising dielectric gas for SF₆ replacement. As a prototypical perfluorinated asymmetrical ketone, photodissociation and reaction with hydroxyl radicals of C5 have been investigated theoretically to gain insights into its atmospheric chemistry and environmental impact. C5 has a broad UV absorption band in the range 260-360 nm with a maximum at 302 nm and the maximal photolysis rate coefficient is 8.3 × 10^-5 s^-1. Photoexcitation from S₀ through the perpendicular n → π* transition produces the excited S₁ species, which can either dissociate straightforwardly via the bifurcated α-CC bond cleavage or be trickled down to T₁ via the S₁/T₁ intersystem crossing (ISC) pathway. In the Franck-Condon region of the S₁ surface, the long-lived S₁ species exists and the slow ISC pathway is dominant, followed by the α-cleavage through T₁ barriers to form perfluoroalkyl and perfluoroacetyl radicals. While the excitation energy exceeds 286 nm, the direct dissociation of C5 though the S₁ barriers takes over before the ISC occurs. Several pathways for regeneration of the ground-state S₀ from S₁ and T₁ via seams of crossing or internal conversion were revealed. The C5 + OH reaction occurs via direct carbonyl addition mechanism followed by the rapid displacement of one of the alkyl groups. Although it can be accelerated considerably by the H₂O-mediated catalysis or the intercepted vibrationally excited quantum states in the hot S₀*, the degradation of C5 by OH radicals is too slow to compete with the photolysis pathways.

A quantum theory investigation on atmospheric oxidation mechanisms of acrylic acid by OH radical and its implication for atmospheric chemistry

The hydroxyl radical, as the most important oxidant, controls the removal of some volatile organic compounds (VOCs) in the atmosphere. In this work, the atmospheric oxidation processes of acrylic acid by OH radical have been investigated by density functional theory (DFT). The energetic routes of the reaction of CH₂CHCOOH with OH radical have been calculated accurately at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level. It is implicated that the oxidation has five elementary reaction pathways mostly hinging on how hydroxyl radical approaches to the carbon skeleton of acrylic acid. The atmospheric degradation mechanisms of the CH₂CHCOOH by OH radical are the formation of reactive intermediates IM1 and IM2. Meanwhile, the further oxidation mechanisms of IM1 and IM2 by O₃ and NO are also investigated. The rate coefficients have been computed using tight transition state theory of the variflex code. The calculated rate coefficient is 2.3 × 10^-11 cm³ molecule^-1 s^-1 at standard pressure and 298 K, which is very close to the laboratory data (1.75 ± 0.47 × 10^-11 cm³ molecule^-1 s^-1). Moreover, the atmospheric lifetime of acrylic acid is about 6 h at 298 K and 1 atm, implying that the fast sinks of acrylic acid by hydroxyl radical.