1. Quantum study on the branching ratio of the reaction NO2+OH
Christopher F Williams, Sergei K Pogrebnya, David C Clary J Chem Phys. 2007 Apr 21;126(15):154321. doi: 10.1063/1.2714511.
A reduced dimensionality (RD) approximation is developed for the title reaction which treats the angle of approach of the hydroxyl radical to the nitrogen dioxide molecule and the radial distance between the two species explicitly. All other degrees of freedom are treated adiabatically. Electronic structure calculations at the complete active space self-consistent field level are used to fit a potential energy surface (PES) in these two coordinates. Within this RD model the adiabatic capture centrifugal sudden approximation is used to calculate the high pressure limit rate constant. A correction for reflection from the PES due to rotationally nonadiabatic transitions is applied using the wave packet capture approximation. The branching ratio for the title reaction is calculated for the atmospherically significant temperature range of 200-400 K at 20 Torr without distinguishing between the conformers of HOONO. The result is k(HOONO)k(HNO(3) )=0.051 at 20 Torr and 300 K, which is in good agreement with the measured branching ratio between cis-cis-HOONO and nitric acid. This suggests that most of the different conformers of HOONO were converted to the most stable cis-cis conformer on the time scale of the measurements made.
2. The flexibility of modified-linker MIL-53 materials
Alexis S Munn, Renjith S Pillai, Shyam Biswas, Norbert Stock, Guillaume Maurin, Richard I Walton Dalton Trans. 2016 Mar 14;45(10):4162-8. doi: 10.1039/c5dt03438h.
The flexibility of eight aluminium hydroxo terephthalates [Al(OH)(BDC-X)]·n(guest) (BDC = 1,4-benzene-dicarboxylate; X = -H, -CH3, -Cl, -Br, -NH2, -NO2, -(OH)2, -CO2H) crystallising in the MIL-53-type structure was investigated upon thermal dehydration of as-made samples, superhydration and methanol adsorption/desorption using in situ powder X-ray diffraction (PXRD). Profile fitting was used to determine lattice parameters as a function of time and/or temperature to describe their structural evolution. It has thus been shown that while methanol vapour adsorption induces an opening of all the modified frameworks, except the -NH2 material, superhydration only leads to open structures for Al-MIL-53-NO2, -Br and -(OH)2. All the MIL-53 solids, except Al-MIL-53-(OH)2 are present in the open structures upon thermal dehydration. In addition to the exploration of the breathing behavior of this MIL-53 series, the issue of disorder in the distribution of the functional groups between the organic linkers was explored. As a typical illustration, density functional theory calculations were carried out on different structures of Al-MIL-53-Cl, in which the distribution of -Cl within two adjacent BDC linkers is varied. The results show that the most energetically stable configuration leads to the best agreement with the experimental PXRD pattern. This observation supports that the distribution of the selected linker substituent in the functionalised solid is governed by energetics and that there is a preference for an ordering of this arrangement.
3. Reflected shock tube studies of high-temperature rate constants for OH + NO2 --> HO2 + NO and OH + HO2 --> H2O + O2
Nanda K Srinivasan, Meng-Chih Su, James W Sutherland, Joe V Michael, Branko Ruscic J Phys Chem A. 2006 Jun 1;110(21):6602-7. doi: 10.1021/jp057461x.
The motivation for the present study comes from the preceding paper where it is suggested that accepted rate constants for OH + NO2 --> NO + HO2 are high by approximately 2. This conclusion was based on a reevaluation of heats of formation for HO2, OH, NO, and NO2 using the Active Thermochemical Table (ATcT) approach. The present experiments were performed in C2H5I/NO2 mixtures, using the reflected shock tube technique and OH-radical electronic absorption detection (at 308 nm) and using a multipass optical system. Time-dependent profile decays were fitted with a 23-step mechanism, but only OH + NO2, OH + HO2, both HO2 and NO2 dissociations, and the atom molecule reactions, O + NO2 and O + C2H4, contributed to the decay profile. Since all of the reactions except the first two are known with good accuracy, the profiles were fitted by varying only OH + NO2 and OH + HO2. The new ATcT approach was used to evaluate equilibrium constants so that back reactions were accurately taken into account. The combined rate constant from the present work and earlier work by Glaenzer and Troe (GT) is k(OH+NO2) = 2.25 x 10(-11) exp(-3831 K/T) cm3 molecule(-1) s(-1), which is a factor of 2 lower than the extrapolated direct value from Howard but agrees well with NO + HO2 --> OH + NO2 transformed with the updated equilibrium constants. Also, the rate constant for OH + HO2 suitable for combustion modeling applications over the T range (1200-1700 K) is (5 +/- 3) x 10(-11) cm3 molecule(-1) s(-1). Finally, simulating previous experimental results of GT using our updated mechanism, we suggest a constant rate for k(HO2+NO2) = (2.2 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1) over the T range 1350-1760 K.
