We estimated rate constants for the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical, a critically important reaction in the combustion of biofuel. We employed the recently developed multi-structural variational transition-state theory (MS-VTST), which utilizes a multifaceted dividing surface that allows us to include the contributions of multiple structures for reacting species and transition states. First, multiconfigurational Shepard interpolation-based on molecular-mechanics-guided interpolation of electronic-structure Hessian data obtained by the M08 HX/jun-cc-pVTZ electronic model chemistry-was used to obtain the portion of the potential energy surface needed for single-structure variational transition-state theory rate constants including multidimensional tunneling; then, the M08-HX/MG3S electronic model chemistry was used to calculate multi-structural torsional anharmonicity factors to complete the MS-VTST rate constant calculations. The lowest-energy structures of the transition state have strongly bent hydrogen bonds. Our results indicate that neglect of multi-structural anharmonicity would lead to errors of factors of 0.3, 46, and 171 at 200, 1000, and 2400 K for this reaction.
In the present work, we study the H atom abstraction reactions by hydroxyl radical at all five sites of 1-butanol. Multistructural variational transition state theory (MS-VTST) was employed to estimate the five thermal rate constants. MS-VTST utilizes a multifaceted dividing surface that accounts for the multiple conformational structures of the transition state, and we also include all the structures of the reactant molecule. The vibrational frequencies and minimum energy paths (MEPs) were computed using the M08-HX/MG3S electronic structure method. The required potential energy surfaces were obtained implicitly by direct dynamics employing interpolated variational transition state theory with mapping (IVTST-M) using a variational reaction path algorithm. The M08-HX/MG3S electronic model chemistry was then used to calculate multistructural torsional anharmonicity factors to complete the MS-VTST rate constant calculations. The results indicate that torsional anharmonicity is very important at higher temperatures, and neglecting it would lead to errors of 26 and 32 at 1000 and 1500 K, respectively. Our results for the sums of the site-specific rate constants agree very well with the experimental values of Hanson and co-workers at 896-1269 K and with the experimental results of Campbell et al. at 292 K, but slightly less well with the experiments of Wallington et al., Nelson et al., and Yujing and Mellouki at 253-372 K; nevertheless, the calculated rates are within a factor of 1.61 of all experimental values at all temperatures. This gives us confidence in the site-specific values, which are currently inaccessible to experiment.
We present a time-resolved broadband cavity-enhanced UV-absorption spectrometer apparatus that we have constructed and utilized for temperature- and pressure-dependent kinetic measurements of formaldehyde oxide (CH2OO) reactions.
The photoredox catalysts pheophorbide a (PheoA) and zinc tetraphenylporphine (ZnTPP) under illumination display strong selectivity toward reversible addition-fragmentation chain transfer (RAFT) agents containing thiocarbonylthio groups, namely dithiobenzoates, xanthates, and trithiocarbonates. The underlying mechanism for the process-whether via energy or electron transfer from the photoexcited catalyst to RAFT agent-has remained unclear, as has the reason for the remarkable selectivity. Quantum chemistry and molecular dynamics calculations are utilized to provide strong evidence that none of the common energy-transfer mechanisms (Förster resonance energy transfer; Dexter electron exchange; or internal conversion followed by vibrational energy transfer) are likely to facilitate polymerization, let alone explain the observed selectivities. In contrast, extensive quantum chemical characterizations of the excited-state orbitals associated with the catalyst-RAFT agent complexes uncover a clear selectivity pattern associated with charge-transfer states that is highly consistent with experimental findings. The results shed light on the intrinsic catalytic role of the photocatalysts and provide a strong indication that a reversible electron/charge-transfer mechanism underpins the remarkable photocatalytic selectivity.
Density functional theory as well as Møller-Plesset investigations has been carried out on tetrafluoro cyclobutadiene, C4F4, to explore the origin of its nonplanarity. Although Petersson et al. (Petersson, E. J.; Fanuele, J. C.; Nimlos, M. R.; Lemal, D. M.; Ellison, G. B.; Radziszewski, J. G. J. Am. Chem. Soc. 1997, 119, 11122-11123) had earlier predicted a nonplanar geometry of this compound on the basis of spectral and bond orbital analysis, the explanation of the same from a more fundamental point of view is still missing. In the present study, we provide a heuristic explanation for the origin of nonplanarity of C4F4. The two major driving forces behind this nonplanar geometry are the unusual aromaticity of this cyclic homoatomic 4pi electron system and the second-order Jahn-Teller effect (SOJTE). These driving forces can well be explained by various energy and density parameters and also by nucleus-independent chemical shift (NICS) values. Aromaticity of a cyclic homoatomic 4pi electron system is quite remarkable. The enhancement of pi- delocalization as evidenced from molecular orbital analysis may be attributed to s-ppi mixing in nonplanar C4F4.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.