Copper-exchanged zeolites are useful materials for step-wise methane-to-methanol conversion (MMC). However, methanol yields on copper-exchanged zeolites are often modest, spurring interest in the development of active-site species that are activated at moderate temperatures, afford greater yields, and provide excellent methanol selectivities. Ultraviolet–visible (UV–vis) spectroscopy is a major tool for characterizing the active-sites and their evolution during the step-wise MMC process. However, computation of the UV–vis spectra of the copper-oxo active sites using Tamm–Dancoff time-dependent density functional theory (TDA-DFT) can be quite problematic. This has led to utilization of expensive methods based on multireference approaches, Green functions, and the Bethe–Salpeter equation. In this work, we examined the optical spectra of [CuO]+, [Cu2O]2+, [Cu2O2]2+, and [Cu3O3]2+ species implicated in MMC in zeolites. For the larger species, we examined how agreement with experimental data is improved with increasingly larger cluster models. For [CuO]+, we compared TDA-DFT against restricted active space 2nd-order perturbation theory, RASPT2. We found that signature peaks for [CuO]+ have multireference behavior. The excited states have many configuration state functions with a double excitation character. These effects are likely responsible for the poor utility of conventional TDA-DFT methods. Indeed, we obtain good agreement with experimental data and RASPT2 after accounting for 2h/2p excitations within TDA-DFT with a previously described configuration interaction singles and doubles, CIS(D)-style scheme. This was the case for [CuO]+, [Cu2O]2+, as well as a [Cu2O2]2+ species. Using a long-range corrected double-hybrid, ωB2PLYP, we provide for the first time computational evidence for the experimental UV–vis spectrum of the [Cu3O3]2+ active site motif.
Improved values for two acidity constants of 2,3-dihydroxypyridine have been computed using program PITMAP. The composition of the complex with iron(111) and its conditional and overall stability constants in I M hydrochloric acid have been evaluated.On a calcule par ordinateur les valeurs ameliorees des deux constantes d'acidite du 2.3-dihydroxypyridine en utilisant le programme PITMAP. On a evalue aussi la composition du complexe avec le fer(I11) et ses constantes de stabilite globales dans I'acide chlorhydrique I IM.Canadian Journal of Chemistry. 50. 1649 (1972) In a previous report from this laboratory (1 ), 2,3-dihydroxypyridine (DHP) was shown to react with iron(II1) in mineral acid media 1.0 M in hydrogen ion to form blue complex species of metal-to-ligand ratios of I : I . A spectrophotometric procedure for the analysis of iron(I11) in 1.0 M hydrochloric acid has been developed (2). This work reinvestigates the acidity constants of DHP and reports on the coinplex composition and the conditional and overall stability constants of the complex with iron(II1) in 1.0 M hydrochloric acid solution. Experimental Relrget~lsIron(111) chloride solutions were prepared by dilution with 1.0 M hydrochloricacid solution froma 9.1 10 x M stock solution prepared and standardized as before (I). Solutions of D H P were prepared by dissolving accurately weighed amounts of the sublimed reagent in a known volume of the appropriate solvent. Other chemicals were reagent grade and were tested as necessary for the absence of interfering impurities. Water used was deionized, distilled, and deaerated with nitrogen. Appuro /usAbsorbance measurements were made at 25°C with a double beam Cary 14 recording spectrophotometer using fused quartz cells (I0 mm) throughout. An Orion Model 801 pH meter with a glass and calomel electrode pair calibrated as a concentration probe was used to measure hydrogen ion concentration (3). All volumetric ware was Class A and all pipettes were calibrated.Computations were performed on an IBM 360,'75 computer with graphical output plotted by a Calcomp 763 digital incremental plotter.De/ertiiino/ion oj'rlle Acidiry Consronrs oj'DHP The pH of twenty-two solutions of D H P concentration 1.500 x lo-" M was adjusted with standardized potassium hydroxide and/or hydrochloric acid solutions to give a range of [HC] = 9.50 M to a p H of 13.12. The ionic strength of solutions between p H 0 and 14 was maintained at 1.0 by means of potassium chloride solution but constancy was sacrificed at higher acidities in order to determine the "approximate protonation constant" (4) of DHP. The spectrum of each of these solutions was recorded between 220 and 315 nm against a reference solution of a solvent blank, corrected for baseline, and absorbance values at every 5 nm tabulated. The pH of each solution between 0 and 14 was measured while the spectrum was being recorded, while those of the solutions of higher acidities were calculated from known hydrogen ion concentrations. These data were then submitted to the computer progr...
It is demonstrated that a double hybrid density functional approximation (DH-DFA), ωB88PTPSS, that incorporates equipartition of density functional theory (DFT) and non-local correlation however with a meta-GGA correlation functional as well as with the range-separated exchange of ωB2PLYP provides accurate excitation energies for conventional systems as well as correct prescription of negative singlet-triplet gaps for non-conventional systems with inverted gaps, without any necessity for parametric scaling of the same-spin and opposite-spin non-local correlation energies. Examined over " safe" excitations of the QUESTDB set, ωB88PTPSS performs quite well for open-shell systems, correctly and fairly accurately (relative to EOM-CCSD reference) predicts negative gaps for 50 systems with inverted singlet-triplet gaps, is one of the leading performers for intramolecular charge-transfer excitations and achieves near-CC2 and ADC(2) quality for the Q1 and Q2 subsets. Subsequently, we tested ωB88PTPSS on two sets of real-life examples from recent computational chemistry literature; the low energy bands of chlorophyll a (Chl a) and a set of thermally activated delayed fluorescence (TADF) systems. For Chl a, ωB88PTPSS quantitatively and quantitatively achieves DLPNO-STEOM-CCSD-level performance and provides excellent agreement with experiment. For TADF systems, ωB88PTPSS agrees quite well with SCS-CC2 excitation energies as well as experimental values for the gaps between the S1 and T1 excited states.
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.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.