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1.
J Phys Chem A ; 123(34): 7454-7462, 2019 Aug 29.
Artigo em Inglês | MEDLINE | ID: mdl-31414807

RESUMO

The ionization energy (IE) of VCH, the 0 K V-CH/VC-H bond dissociation energies (D0s), and the heats of formation at 0 K (ΔHf0°) and 298 K (ΔHf298°) for VCH/VCH+ are predicted by the wave function-based CCSDTQ/CBS approach. This composite-coupled cluster method includes full quadruple excitations in conjunction with the approximation to the complete basis set (CBS) limit. The contributions of zero-point vibrational energy, core-valence (CV) correlation, spin-orbit coupling, and scalar relativistic corrections are taken into account. The present calculations show that adiabatic IE(VCH) = 6.785 eV and demonstrate excellent agreement with an IE value of 6.774 7 ± 0.000 1 eV measured with two-color laser-pulsed field ionization-photoelectron spectroscopy. The CCSDT and MRCI+Q methods which include CV correlations give the best predictions of harmonic frequencies: ω2 (ω2+) (bending) = 534 (650) and 564 (641) cm-1 and the V-CH stretching ω3 (ω3+) = 835 (827) and 856 (857) cm-1 compared with the experimental values. In this work, we offer a streamlined CCSDTQ/CBS approach which shows an error limit (≤20 meV) matching with previous benchmarking efforts for reliable IE and D0 predictions for VCH/VCH+. The CCSDTQ/CBS D0(V+-CH) - D0(V-CH) = -0.012 eV and D0(VC+-H) - D0(VC-H) = 0.345 eV are in good accord with the experimentally derived values of -0.028 4 ± 0.000 1 and 0.355 9 ± 0.000 1 eV, respectively. The present study has demonstrated that the CCSDTQ/CBS protocol can be readily extended to investigate triatomic molecules containing 3d-metals.

2.
Phys Chem Chem Phys ; 20(44): 27725-27729, 2018 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-30383056

RESUMO

We report the formation of H2+ and C2+ from dissociation of acetylene induced by α-particle irradiation. The unusual dissociation channel [C2H2]2+ → H2+ + C2+ is unambiguously identified by measuring the time-of-flight of both fragmented ions in coincidence. Our quantum chemical calculation confirms the existence of this dissociation pathway. It shows that [HCCH]2+ is firstly populated to the 3Π excited electronic state, followed by acetylene-vinylidene isomerization, and finally the vinylidene-like intermediate dissociates to H2+ and C2+. This dissociation channel is the simplest prototypical reaction involving H migration, H-H combination, and C-H bond cleavage. The current study plays an important role for understanding the H2+/H3+ formation reactions from organic di-cations in an interstellar medium.

3.
Chemistry ; 24(70): 18735-18742, 2018 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-30259584

RESUMO

The kinetics of water oxidation by K2 FeO4 has been reinvestigated by UV/Vis spectrophotometry from pH 7-9 in 0.2 m phosphate buffer. The rate of reaction was found to be second-order in both [FeO4 2- ] and [H+ ]. These results are consistent with a proposed mechanism in which the first step involves the initial equilibrium protonation of FeO4 2- to give FeO3 (OH)- , which then undergoes rate-limiting O-O bond formation. Analysis of the O2 isotopic composition for the reaction in H2 18 O suggests that the predominant pathway for water oxidation by ferrate is intramolecular O-O coupling. DFT calculations have also been performed, which support the proposed mechanism.

4.
Phys Chem Chem Phys ; 20(32): 20756-20765, 2018 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-29989120

RESUMO

Adenine, a DNA base, exists as several tautomers and isomers that are closely lying in energy and that may form a mixture upon vaporization of solid adenine. Indeed, it is challenging to bring adenine into the gas phase, especially as a unique tautomer. The experimental conditions were tuned to prepare a jet-cooled canonical adenine (9H-adenine). This isolated DNA base was ionized by single VUV photons from a synchrotron beamline and the corresponding slow photoelectron spectrum was compared to ab initio computations of the neutral and ionic species. We report the vibronic structure of the X+ 2A'' (D0), A+ 2A' (D1) and B+ 2A'' (D2) electronic states of the 9H adenine cation, from the adiabatic ionization energy (AIE) up to AIE + 1.8 eV. Accurate AIEs are derived for the 9H-adenine (X[combining tilde] 1A') + hν → 9H-adenine+ (X+ 2A'', A+ 2A', B+ 2A'') + e- transitions. Close to the AIE, we fully assign the rich vibronic structure solely to the 9H-adenine (X 1A') + hν → 9H-adenine+ (X+ 2A'') transition. Importantly, we show that the lowest cationic electronic states of canonical adenine are coupled vibronically. The present findings are important for understanding the effects of ionizing radiation and the charge distribution on this elementary building block of life, at ultrafast, short, and long timescales.


Assuntos
Adenina/química , Cátions , Isomerismo , Cinética , Modelos Moleculares , Estrutura Molecular , Transição de Fase , Processos Fotoquímicos , Fenômenos Físicos , Teoria Quântica , Termodinâmica
5.
Dalton Trans ; 47(1): 240-245, 2017 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-29210406

RESUMO

The ferrate(vi) ion, [FeO4]2-, has attracted much interest in recent years because of its potential use as a green oxidant in organic synthesis and water treatment. Although there have been several reports on the use of ferrate(vi) for the oxidation of alcohols to the corresponding carbonyl compounds, the mechanism remains unclear. In this work, the kinetics of the oxidation of a series of alcohols with α-C-H bond dissociation energies ranging from 81 to 95 kcal mol-1 have been studied by UV/Vis spectrophotometry. The reactions are first-order in both [FeO4]2- and [alcohol]. The deuterium isotope effects for the oxidation of methanol/d4-methanol, ethanol/d6-ethanol and benzyl alcohol/d7-benzyl alcohol are 18.0 ± 0.1, 4.1 ± 0.1 and 11.2 ± 0.1, respectively. A linear correlation is found between the second-order rate constants and the α-C-H bond dissociation energies (BDEs) of the alcohols, consistent with a hydrogen atom transfer (HAT) mechanism. The proposed HAT mechanism is supported by DFT calculations.

6.
Inorg Chem ; 56(21): 12699-12702, 2017 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-29027799

RESUMO

We have previously reported that the oxidation of SO32- to SO42- by a trans-dioxoruthenium(VI) complex, [RuVI(TMC)(O)2)]2+ (RuVI; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazcyclotetradecane) in aqueous solutions occurs via an O-atom transfer mechanism. In this work, we have reinvestigated the effects of the pH on the oxidation of SIV by RuVI in more detail in order to obtain kinetic data for the HSO3- pathway. The HSO3- pathway exhibits a deuterium isotope effect of 17.4, which indicates that O-H bond breaking occurs in the rate-limiting step. Density functional theory calculations have been performed that suggest that the oxidation of HSO3- by RuVI may occur via a concerted or stepwise proton-coupled O-atom transfer mechanism.

7.
Angew Chem Int Ed Engl ; 56(40): 12260-12263, 2017 09 25.
Artigo em Inglês | MEDLINE | ID: mdl-28734083

RESUMO

The OsVI nitrido complex, OsVI (N)(quin)2 (OTs) (1, quin=2-quinaldinate, OTs=tosylate), is a highly selective and efficient catalyst for the ring hydroxylation of alkylbenzenes with H2 O2 at room temperature. Oxidation of various alkylbenzenes occurs with ring/chain oxidation ratios ranging from 96.7/3.3 to 99.9/0.1, and total product yields from 93 % to 98 %. Moreover, turnover numbers up to 6360, 5670, and 3880 can be achieved for the oxidation of p-xylene, ethylbenzene, and mesitylene, respectively. Density functional theory calculations suggest that the active intermediate is an OsVIII nitrido oxo species.

8.
Phys Chem Chem Phys ; 19(15): 9778-9789, 2017 Apr 12.
Artigo em Inglês | MEDLINE | ID: mdl-28352920

RESUMO

We report detailed absolute integral cross sections (σ's) for the quantum-rovibrational-state-selected ion-molecule reaction in the center-of-mass collision energy (Ecm) range of 0.05-10.00 eV, where (vvv) = (000), (100), and (020), and . Three product channels, HCO+ + OH, HOCO+ + H, and CO+ + H2O, are identified. The measured σ(HCO+) curve [σ(HCO+) versus Ecm plot] supports the hypothesis that the formation of the HCO+ + OH channel follows an exothermic pathway with no potential energy barriers. Although the HOCO+ + H channel is the most exothermic, the σ(HOCO+) is found to be significantly lower than the σ(HCO+). The σ(HOCO+) curve is bimodal, indicating two distinct mechanisms for the formation of HOCO+. The σ(HOCO+) is strongly inhibited at Ecm < 0.4 eV, but is enhanced at Ecm > 0.4 eV by (100) vibrational excitation. The Ecm onsets of σ(CO+) determined for the (000) and (100) vibrational states are in excellent agreement with the known thermochemical thresholds. This observation, along with the comparison of the σ(CO+) curves for the (100) and (000) states, shows that kinetic and vibrational energies are equally effective in promoting the CO+ channel. We have also performed high-level ab initio quantum calculations on the potential energy surface, intermediates, and transition state structures for the titled reaction. The calculations reveal potential barriers of ≈0.5-0.6 eV for the formation of HOCO+, and thus account for the low σ(HOCO+) and its bimodal profile observed. The Ecm enhancement for σ(HOCO+) at Ecm ≈ 0.5-5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO+ at low Ecm < 0.4 eV may involve a complex mechanism, which is mediated by the formation of a loosely sticking complex between HCO+ and OH. The direct collision and complex mechanisms proposed also allow the rationalization of the vibrational inhibition at low Ecm and the vibrational enhancement at high Ecm observed for the σ(HOCO+).

9.
J Phys Chem A ; 121(3): 669-679, 2017 Jan 26.
Artigo em Inglês | MEDLINE | ID: mdl-28075604

RESUMO

The ionization energies (IEs) of TiO and TiO2 and the 0 K bond dissociation energies (D0) and the heats of formation at 0 K (ΔH°f0) and 298 K (ΔH°f298) for TiO/TiO+ and TiO2/TiO2+ are predicted by the wave-function-based CCSDTQ/CBS approach. The CCSDTQ/CBS calculations involve the approximation to the complete basis set (CBS) limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy (ZPVE), high-order correlation (HOC), core-valence (CV) electronic, spin-orbit (SO) coupling, and scalar relativistic (SR) effect corrections. The present calculations yield IE(TiO) = 6.815 eV and are in good agreement with the experimental IE value of 6.819 80 ± 0.000 10 eV determined in a two-color laser-pulsed field ionization-photoelectron (PFI-PE) study. The CCSDT and MRCI+Q methods give the best predictions to the harmonic frequencies: ωe (ωe+) = 1013 (1069) and 1027 (1059) cm-1 and the bond lengths re (re+) = 1.625 (1.587) and 1.621 (1.588) Å, for TiO (TiO+) compared with the experimental values. Two nearly degenerate, stable structures are found for TiO2 cation: TiO2+(C2v) structure has two equivalent TiO bonds, while the TiO2+(Cs) structure features a long and a short TiO bond. The IEs for the TiO2+(C2v)←TiO2 and TiO2+(Cs)←TiO2 ionization transitions are calculated to be 9.515 and 9.525 eV, respectively, giving the theoretical adiabatic IE value in good agreement with the experiment IE(TiO2) = 9.573 55 ± 0.000 15 eV obtained in the previous vacuum ultraviolet (VUV)-PFI-PE study of TiO2. The potential energy surface of TiO2+ along the normal vibrational coordinates of asymmetric stretching mode (ω3+) is nearly flat and exhibits a double-well potential with the well of TiO2+ (Cs) situated around the central well of TiO2+(C2v). This makes the theoretical calculation of ω3+ infeasible. For the symmetric stretching (ω1+), the current theoretical predictions overestimate the experimental value of 829.1 ± 2.0 cm-1 by more than 100 cm-1. This work together with the previous experimental and theoretical investigations supports the conclusion that the CCSDTQ/CBS approach is capable of providing reliable IE and D0 predictions for TiO/TiO+ and TiO2/TiO2+ with error limits less than or equal to 60 meV. The CCSDTQ/CBS calculations give the predictions of D0(Ti+-O) - D0(Ti-O) = 0.004 eV and D0(O-TiO) - D0(O-TiO+) = 2.699 eV, which are also consistent with the respective experimental determination of 0.008 32 ± 0.000 10 and 2.753 75 ± 0.000 18 eV.

10.
J Am Chem Soc ; 138(51): 16596-16599, 2016 12 28.
Artigo em Inglês | MEDLINE | ID: mdl-27992194

RESUMO

Biological entities, such as DNA bases or proteins, possess numerous tautomers and isomers that lie close in energy, making the experimental characterization of a unique tautomer challenging. We apply VUV synchrotron-based experiments combined with state-of-the-art ab initio methodology to determine the adiabatic ionization energies (AIEs) of specific gas-phase cytosine tautomers produced in a molecular beam. The structures and energetics of neutral and cationic cytosine tautomers were determined using explicitly correlated methods. The experimental spectra correspond to well-resolved bands that are attributable to the specific contributions of five neutral tautomers of cytosine prior to ionization. Their AIEs are experimentally determined for the first time with an accuracy of 0.003 eV. This study also serves as an important showcase for other biological entities presenting a dense pattern of isomeric and tautomeric forms in their spectra that can be investigated to understand the charge redistribution in these species upon ionization.


Assuntos
Citosina/química , Fótons , Isomerismo , Processos Fotoquímicos
11.
Chem Commun (Camb) ; 52(76): 11430-11433, 2016 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-27711289

RESUMO

Hydroquinone is readily oxidized by a (salen)ruthenium(vi) nitrido complex in the presence of pyridine to give benzoquinone. Experimental and computational studies suggest that the reaction occurs via a novel mechanism that involves an initial electrophilic attack at the aromatic ring of the hydroquinone by the nitrido ligand.

12.
Chem Commun (Camb) ; 52(74): 11056-9, 2016 Sep 25.
Artigo em Inglês | MEDLINE | ID: mdl-27523385

RESUMO

A novel µ-C4R2H2 core structure (formed by an unprecedented regioselective, redox-neutral C(sp(2))-C(sp(2)) coupling process) in binuclear group 4 complexes displays adaptable coordination and accommodates different metal sizes, and is sufficiently robust to promote interesting catalytic reactivity at the bimetallic centers.

13.
Chemistry ; 22(31): 10754-8, 2016 Jul 25.
Artigo em Inglês | MEDLINE | ID: mdl-27246832

RESUMO

The kinetics and mechanism of the reaction of S(IV) (SO3 (2-) +HSO3 (-) ) with a ruthenium(VI) nitrido complex, [(L)Ru(VI) (N)(OH2 )](+) (Ru(VI) N, L=N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion), in aqueous acidic solutions are reported. The kinetic results are consistent with parallel pathways involving oxidation of HSO3 (-) and SO3 (2-) by Ru(VI) N. A deuterium isotope effect of 4.7 is observed in the HSO3 (-) pathway. Based on experimental results and DFT calculations the proposed mechanism involves concerted N-S bond formation (partial N-atom transfer) between Ru(VI) N and HSO3 (-) and H(+) transfer from HSO3 (-) to a H2 O molecule.

14.
Chemistry ; 22(33): 11537-42, 2016 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-27123884

RESUMO

The use of light to control the course of a chemical/biochemical reaction is an attractive idea because of its ease of administration with high precision and fine spatial resolution. Staudinger ligation is one of the commonly adopted conjugation processes that involve a spontaneous reaction between azides and arylphosphines to form iminophosphoranes, which further hydrolyze to give stable amides. We designed an anthracenylmethyl diphenylphosphinothioester (1) that showed promising Staudinger ligation reactivity upon photo-excitation. Broadband photolysis at 360-400 nm in aqueous organic solvents induced heterolytic cleavage of its anthracenylmethyl-phosphorus bond, releasing a diphenylphosphinothioester (2) as an efficient traceless Staudinger-Bertozzi ligation reagent. The quantum yield of such a photo-induced heterolytic bond-cleavage at the optimal wavelength of photolysis (376 nm) at room temperature is ≥0.07. This work demonstrated the feasibility of photocaging arylphosphines to realize the photo-triggering of the Staudinger ligation reaction.

15.
Angew Chem Int Ed Engl ; 55(9): 3012-6, 2016 Feb 24.
Artigo em Inglês | MEDLINE | ID: mdl-26798981

RESUMO

Although FeO4(2-) (ferrate(IV)) is a very strong oxidant that readily oxidizes water in acidic medium, at pH 9-10 it is relatively stable (<2 % decomposition after 1 h at 298 K). However, FeO4(2-) is readily activated by Ca(2+) at pH 9-10 to generate O2. The reaction has the following rate law: d[O2]/dt=kCa [Ca(2+) ][FeO4(2-)](2). (18)O-labeling experiments show that both O atoms in O2 come from FeO4(2-). These results together with DFT calculations suggest that the function of Ca(2+) is to facilitate O-O coupling between two FeO4 (2-) ions by bridging them together. Similar activating effects are also observed with Mg(2+) and Sr(2+).

16.
Dalton Trans ; 45(1): 70-3, 2016 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-26610053

RESUMO

The oxidation of phenols by HFeO4(-) proceeds via a hydrogen atom transfer (HAT) mechanism, as evidenced by a large deuterium isotope effect and a linear correlation between the log(rate constant) and bond dissociation free energy (BDFE) of phenols. The Marcus cross relation has been applied to predict the rate constant of HAT from hydroquinone to HFeO4(-).

18.
Chem Commun (Camb) ; 51(71): 13686-9, 2015 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-26229990

RESUMO

The osmium(VI) nitrido complex, [Os(VI)(N)(L)(CH3OH)](+) (1, L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes under ambient conditions using H2O2 as the oxidant. Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover numbers up to 2230. Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H2O2 to [Os(VI)(N)(L)](+) to generate an [Os(VIII)(N)(O)(L)](+) active intermediate.

19.
J Am Chem Soc ; 137(34): 10918-21, 2015 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-26267016

RESUMO

Molecular catalysis of carbon dioxide reduction using earth-abundant metal complexes as catalysts is a key challenge related to the production of useful products--the "solar fuels"--in which solar energy would be stored. A direct approach using sunlight energy as well as an indirect approach where sunlight is first converted into electricity could be used. A Co(II) complex and a Fe(III) complex, both bearing the same pentadentate N5 ligand (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), were synthesized, and their catalytic activity toward CO2 reduction was investigated. Carbon monoxide was formed with the cobalt complex, while formic acid was obtained with the iron-based catalyst, thus showing that the catalysis product can be switched by changing the metal center. Selective CO2 reduction occurs under electrochemical conditions as well as photochemical conditions when using a photosensitizer under visible light excitation (λ > 460 nm, solvent acetonitrile) with the Co catalyst. In the case of the Fe catalyst, selective HCOOH production occurs at low overpotential. Sustained catalytic activity over long periods of time and high turnover numbers were observed in both cases. A catalytic mechanism is suggested on the basis of experimental results and preliminary quantum chemistry calculations.

20.
Inorg Chem ; 54(13): 6582-93, 2015 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-26087119

RESUMO

In this Article, we present a series of cyclometalated iridium(III) polypyridine complexes of the formula [Ir(N^C)2(N^N)](PF6) that showed dual emission under ambient conditions. The structures of the cyclometalating and diimine ligands were changed systematically to investigate the effects of the substituents on the dual-emission properties of the complexes. On the basis of the photophysical data, the high-energy (HE) and low-energy (LE) emission features of the complexes were assigned to triplet intraligand ((3)IL) and triplet charge-transfer ((3)CT) excited states, respectively. Time-dependent density functional theory (TD-DFT) calculations supported these assignments and indicated that the dual emission resulted from the interruption of the communication between the higher-lying (3)IL and the lower-lying (3)CT states by a triplet amine-to-ligand charge-transfer ((3)NLCT) state. Also, the avidin-binding properties of the biotin complexes were studied by emission titrations, and the results showed that the dual-emissive complexes can be utilized as ratiometric probes for avidin. Additionally, all the complexes exhibited efficient cellular uptake by live HeLa cells. The MTT and Annexin V assays confirmed that no cell death and early apoptosis occurred during the cell imaging experiments. Interestingly, laser-scanning confocal microscopy revealed that the complexes were selectively localized on the cell membrane, mitochondria, or both, depending on the nature of the substituents of the ligands. The results of this work will contribute to the future development of dual-emissive transition metal complexes as ratiometric probes and organelle-selective bioimaging reagents.


Assuntos
Corantes Fluorescentes/química , Modelos Moleculares , Compostos Organometálicos/química , Teoria Quântica , Morte Celular , Ciclização , Corantes Fluorescentes/síntese química , Células HeLa , Humanos , Irídio/química , Microscopia Confocal , Compostos Organometálicos/farmacocinética , Piridinas/química
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