skip to main content


Title: Improved computational modeling of the kinetics of the acetylperoxy + HO 2 reaction
The acetylperoxy + HO 2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O 2 (reaction (1a)) competing with singlet pathways leading to acetic acid + O 3 (reaction (1b)) and acetoxy + OH + O 2 (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways ( α 1a , α 1b , and α 1c ) from 229 K to 294 K. We constructed a theoretical model for predicting α 1a , α 1b , and α 1c using quantum chemical and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) simulations. Our main quantum chemical method was Weizmann-1 Brueckner Doubles (W1BD) theory; we combined W1BD and equation-of-motion spin-flip coupled cluster (SF) theory to treat open-shell singlet structures. Using RRKM/ME simulations that included all conformers of acetylperoxy–HO 2 pre-reactive complexes led to a 298 K triplet rate constant, k 1a = 5.11 × 10 −12 cm 3 per molecule per s, and values of α 1a in excellent agreement with experiment. Increasing the energies of all singlet structures by 0.9 kcal mol −1 led to a combined singlet rate constant, k 1b+1c = 1.20 × 10 −11 cm 3 per molecule per s, in good agreement with experiment. However, our predicted variations in α 1b and α 1c with temperature are not nearly as large as those measured, perhaps due to the inadequacy of SF theory in treating the transition structures controlling acetic acid + O 3 formation vs. acetoxy + OH + O 2 formation.  more » « less
Award ID(s):
2108202 1919571
NSF-PAR ID:
10422847
Author(s) / Creator(s):
; ;
Publisher / Repository:
Royal Society of Chemistry
Date Published:
Journal Name:
Faraday Discussions
Volume:
238
ISSN:
1359-6640
Page Range / eLocation ID:
589 to 618
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    The products of the Cl‐atom‐initiated oxidation of hydroxyacetone (HYAC, CH3C(O)CH2OH) have been examined under conditions relevant to the earth's lower atmosphere. Over the range of temperatures studied (252‐298 K), in the absence of NOx, methylglyoxal (CH3C(=O)CH=O, MGLY) was formed with a primary yield >84% (96 ± 9% at 298 K), while in the presence of elevated NOx, MGLY and formic acid were both formed as major primary products. In contrast to a previous study, acetic acid was not identified as a major primary product under the conditions studied. The results are quantitatively interpreted from a consideration of the formation of a stabilized CH3C(O)CH(OH)OO• radical, either in a ≈50% yield from the addition of O2to CH3C(O)CH•(OH) or in 100% yield from the addition of HO2to MGLY. At high temperature and low NOx, decomposition of the stabilized CH3C(O)CH(OH)OO• radical to MGLY is favored, while lower temperatures and conditions of high NOxfavor bimolecular reactions of the stabilized radical, with subsequent production of formic acid. Analysis of the data allows for a semiquantitative determination ofK3 = (2.9 ± 0.4) × 10−16cm3molecule−1, for the HO2+ MGLY ↔ CH3C(O)CH(OH)OO• equilibrium process at 298 K and a roughly order of magnitude increase inK3at 252 K.

     
    more » « less
  2. The reactions of thioformaldehyde (H 2 CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H 2 CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be −0.8 kcal mol −1 , relative to the separated reactants, which has a ∼1.0 kcal mol −1 lower barrier than the abstraction channel. The H 2 CS + ˙OH reaction assisted by a single water molecule reduces the barrier heights significantly for both the addition and abstraction channels, to −5.5 and −6.7 kcal mol −1 respectively, compared to the un-catalyzed H 2 CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol −1 for both reaction paths. The rate constants for the H 2 CS⋯H 2 O + ˙OH and OH⋯H 2 O + H 2 CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H 2 CS + ˙OH reaction paths for comparison with the H 2 CS + ˙OH + H 2 O reaction in the same temperature range were also computed. The results suggest that the rate of the H 2 CS + ˙OH + H 2 O reaction is slower than that of the H 2 CS + ˙OH reaction by ∼1–4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H 2 CS + ˙OH + H 2 O and H 2 CS + ˙OH reactions were found to be 2.2 × 10 −8 s −1 and 6.4 × 10 −6 s −1 , respectively, calculated using [OH] = 1.0 × 10 6 molecules cm −3 , and [H 2 O] = 8.2 × 10 17 molecules cm −3 (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H 2 C(OH)S˙ product in the presence of 3 O 2 were also performed. The results show that H 2 CS is removed from the atmosphere primarily by reacting with ˙OH and O 2 to form thioformic acid, HO 2 , formaldehyde, and SO 2 as the main end products. 
    more » « less
  3. null (Ed.)
    We present a single molecular polyoxometalate cluster (K 41 [(P 2 W 12 Nb 6 O 62 ) 6 {Mn 3 (OH) 3 (H 2 O) 6 } 4 {Mn 3 Na(H 2 O) 16 }]·26H 2 O) with controllable release of a large number of protons (∼40 per molecule) in its aqueous solution upon addition of a base. The deprotonation/protonation process is reversible with the clusters remaining intact. This molecule can also absorb up to 11 protons per cluster when an acid, HCl, was added to its original aqueous solution. To the best of our knowledge, such large proton absorption/release capacity along with excellent stability is unprecedented. 
    more » « less
  4. Abstract

    Ising superconductivity, observed in NbSe2and similar materials, has generated tremendous interest. Recently, attention was called to the possible role that spin fluctuations (SF) play in this phenomenon, in addition to the dominant electron–phonon coupling (EPC); the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations. However, these hypotheses have not been put to a quantitative test. In this paper, we report first principle calculations of the EPC and also estimate coupling with SF, including full momentum dependence. We find that: (1) EPC is strongly anisotropic, largely coming from the$$K-{K}^{{\prime} }$$KKscattering, and therefore excludes triplet symmetry even as an excited state; (2) superconductivity is substantially weakened by SF, but anisotropy remains as above; and, (3) we do find the possibility of a Leggett mode, not in a singlet–triplet but in ans++s±channel.

     
    more » « less
  5. Singlet fission (SF) is a photophysical process considered as a possible scheme to bypass the Shockley–Queisser limit by generating two triplet-state excitons from one high-energy photon. Polyacene crystals, such as tetracene and pentacene, have shown outstanding SF performance both theoretically and experimentally. However, their instability prevents them from being utilized in SF-based photovoltaic devices. In search of practical SF chromophores, we use many-body perturbation theory within the GW approximation and Bethe–Salpeter equation to study the excitonic properties of a family of pyrene-stabilized acenes. We propose a criterion to define the convergence of exciton wave-functions with respect to the fine k-point grid used in the BerkeleyGW code. An open-source Python code is presented to perform exciton wave-function convergence checks and streamline the double Bader analysis of exciton character. We find that the singlet excitons in pyrene-stabilized acenes have a higher degree of charge transfer character than in the corresponding acenes. The pyrene-fused tetracene and pentacene derivatives exhibit comparable excitation energies to their corresponding acenes, making them potential SF candidates. The pyrene-stabilized anthracene derivative is considered as a possible candidate for triplet–triplet annihilation because it yields a lower SF driving force than anthracene. 
    more » « less