The mechanism and kinetics of the atmospheric oxidation of CF3(CF2)2CHCH2 (HFC-1447fz) by hydroxyl radicals: ab initio investigation.

The oxidation of 3,3,4,4,5,5,5-heptafluoro-1-pentene (HFC-1447fz) by hydroxyl radicals plays a crucial role in atmospheric conditions. By employing the CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level of theory, the detailed reaction mechanism, kinetics and atmospheric implications of the degradation of HFC-1447fz by hydroxyl radicals were investigated. Compared to H-abstraction channels, the OH addition reaction is determined to be more favorable initial pathways in the degradation processes of HFC-1447fz. The overall rate coefficient of the degradation of HFC-1447fz by OH radicals is estimated to be 1.66 × 10-12 cm3 molecule-1 s-1 and the lifetime of HFC-1447fz is found to be 7 days at 298 K, which are in good agreement with the reported experimental results. The global warming potential (GWP) for HFC-1447fz on the 50, 100 and 500-year time horizons is estimated using the calculated rate coefficient. Furthermore, the mechanisms of the subsequent reactions of two OH-addition adducts have also been investigated. By TD-DFT calculations, it was found that eleven species can undergo photodissociation, while ten other species are photolytically stable under sunlight.

Size-controlled growth of ZnSe nanocrystals for high-performance photocatalytic H2O2 production in pure water.

Semiconductor photocatalysis is deemed as a novel and promising process that can produce H2O2 from earth-abundant water and gaseous dioxygen using sunlight as the energy supply. The searching of novel catalysts for photocatalytic H2O2 production has received increasing attention in the last few years. Herein, size-controlled growth of ZnSe nanocrystals was realized via a solvothermal method by varying the amount of Se and KBH4. The performance of the as-obtained ZnSe nanocrystals towards photocatalytic H2O2 production depends on the mean size of the synthesized nanocrystals. Under O2-bubbling, the optimal ZnSe sample presented an excellent H2O2 production efficiency (8.596 mmol g-1 h-1), and the apparent quantum efficiency for H2O2 production reaches as high as 2.84% at λ = 420 nm. Under air-bubbling, the accumulation of H2O2 was as high as 1.758 mmol L-1 after 3 h irradiation at the ZnSe dosage of 0.4 g L-1. The photocatalytic H2O2 production performance is far superior to the most investigated semiconductors such as TiO2, g-C3N4, and ZnS.

The Mechanism and Kinetics Model of Degradation of Dicarboxylic Acids by Hydroxyl Radicals under Atmospheric Conditions.

The atmospheric degradation mechanism of dicarboxylic acids (DCAs) initiated by hydroxyl radicals has been theoretically investigated at the DLPNO-CCSD(T)/def2-TZVP//BH&HLYP/6-311++G(d,p) level of theory. In the presence of O2, the degradation of DCAs by hydroxyl radicals takes place through a two-step mechanism: the α-H elimination and the degradation of the peroxyl radical intermediate. The latter degradation mechanism is easy to proceed for the exothermic process of radical recombination. Therefore, the degradation rate of DCAs is determined by an α-H elimination step, which is accelerated in the case of long carbon-chain DCAs with a lower energy barrier. Canonical variational transition state theory has been employed to estimate the rate constants of the H-elimination step of the DCA degradation reaction by hydroxyl radicals over the temperature range of 220-1000 K.

True Grit in Learning Math: The Math Anxiety-Achievement Link Is Mediated by Math-Specific Grit.

In this study, we tested a possible mechanism of the association between math anxiety and math achievement: the mediating role of math-specific grit (i.e., sustaining effort in the face of adversity when learning math). In Study 1, a sample of 10th grade students (N = 222) completed a battery of personality and attitude questionnaires, and math achievement was indexed by curriculum-based examination scores. Mediation analyses indicated that math-specific grit, but not domain-general grit, mediated the relationship between math anxiety and math achievement. In Study 2, we replicated and extended the above findings with another sample of 11th grade students (N = 465). Mediation analyses indicated that math-specific grit and math-specific procrastination played sequential mediating roles in the relationship between math anxiety and math achievement. That is, individuals with higher math anxiety were less gritty in math learning, possibly further leading them to be more procrastinated in performing math work, which may finally result in worse math achievement. In summary, the current study provides the first evidence that math-specific grit may mediate the relationship between math anxiety and math achievement. Furthermore, it also demonstrated the value of math-specific grit over domain-general grit in predicting math success, which invites a broader investigation on subject-specific grit.

Evidence and evolution of Criegee intermediates, hydroperoxides and secondary organic aerosols formed via ozonolysis of α-pinene.

α-Pinene, the most abundant monoterpene in the atmosphere, accounts for more than 50% of global monoterpene emission. Though its reaction with ozone has been generally perceived as a major source of secondary organic aerosols (SOAs), direct evidence of its reaction intermediates (RI) and their evolution remain lacking. Here we study the ozonolysis of α-pinene between 180 and 298 K using a long-path, temperature-variable aerosol cooling chamber coupled to a rapid-scan time-resolved Fourier transform infrared spectrometer. The spectroscopic signatures of large Criegee intermediates (CIs) and hydroperoxides (HPs) were found for the first time. The aerosol size evolution during the reaction was also measured. In contrast to a previous perception, we show that temperature plays a determinant role in the ozonolysis kinetics. Finally, we show that the formation of HPs is an energetically favorable pathway to dissipate CIs. This study provides new insights into the ozonolysis of α-pinene and its contribution to SOA formation.

Phthalide Derivatives from Angelica Sinensis Decrease Hemoglobin Oxygen Affinity: A New Allosteric-Modulating Mechanism and Potential Use as 2,3-BPG Functional Substitutes.

Angelica sinensis (AS), one of the most versatile herbal medicines remains widely used due to its multi-faceted pharmacologic activities. Besides its traditional use as the blood-nourishing tonic, its anti-hypertensive, anti-cardiovascular, neuroprotective and anti-cancer effects have been reported. Albeit the significant therapeutic effects, how AS exerts such diverse efficacies from the molecular level remains elusive. Here we investigate the influences of AS and four representative phthalide derivatives from AS on the structure and function of hemoglobin (Hb). From the spectroscopy and oxygen equilibrium experiments, we show that AS and the chosen phthalides inhibited the oxygenated Hb from transforming into the high-affinity "relaxed" (R) state, decreasing Hb's oxygen affinity. It reveals that phthalides cooperate with the endogenous Hb modulator, 2,3-bisphosphoglycerate (2,3-BPG) to synergetically regulate Hb allostery. From the docking modeling, phthalides appear to interact with Hb mainly through its α1/α2 interface, likely strengthening four (out of six) Hb "tense" (T) state stabilizing salt-bridges. A new allosteric-modulating mechanism is proposed to rationalize the capacity of phthalides to facilitate Hb oxygen transport, which may be inherently correlated with the therapeutic activities of AS. The potential of phthalides to serve as 2,3-BPG substitutes/supplements and their implications in the systemic biology and preventive medicine are discussed.

Molecular Basis of the Antioxidant Capability of Glutathione Unraveled via Aerosol VUV Photoelectron Spectroscopy.

Glutathione (GSH), the most abundant nonenzymatic antioxidant in living systems, actively scavenges various exogenous/endogenous oxidizing species, defending important biomolecules against oxidative damages. Although it is well established that the antioxidant activity of GSH originates from the cysteinyl thiol (-SH) group, the molecular origin that makes the thiol group of GSH a stronger reducing agent than other thiol-containing proteins is unclear. To gain insights into the molecular basis underlying GSH's superior antioxidant capability, here we report, for the first time, the valence electronic structures of solvated GSH in the aqueous aerosol form via the aerosol vacuum ultraviolet photoelectron spectroscopy technique. The pH-dependent electronic evolution of GSH is obtained, and the possible correlations between GSH and its constituting amino acids are interrogated. The valence band maxima (VBMs) for GSH aqueous aerosols are found at 7.81, 7.61, 7.52, and 5.51 ± 0.10 eV at a pH of 1.00, 2.74, 7.00, and 12.00, respectively, which appear to be lower than the values of their corresponding hybrid counterparts collectively contributed from the three isolated constituting amino acids of GSH. An additional photoelectron feature is observed for GSH aqueous aerosols at pH = 12.00, where the thiol group on its Cys residue becomes deprotonated and the relatively well-separated feature allows its vertical ionization energy (VIE) to be determined as 6.70 ± 0.05 eV. Compared to a VIE of 6.97 ± 0.05 eV obtained for a similar thiolate feature observed previously for isolated Cys aqueous aerosols ( Su et al. VUV Photoelectron Spectroscopy of Cysteine Aqueous Aerosols: A Microscopic View of Its Nucleophilicity at Varying pH Conditions . J. Phys. Chem. Lett. 2015 , 6 , 817 - 823 ), a 0.27 eV reduction in the VIE is found for GSH, indicating that the outermost electron corresponding to the nonbonding electron on the thiolate group can be removed more readily from the GSH tripeptide than that from Cys alone. The possible origins underlying the decrease in the VBM of GSH with respect to that of each corresponding hybrid counterpart and the decrease in the VIE of the thiolate feature of GSH with respect to that of the isolated Cys are discussed, providing hints to understand the superior antioxidant capability of GSH from a molecular level.

VUV Photoelectron Spectroscopy of Cysteine Aqueous Aerosols: A Microscopic View of Its Nucleophilicity at Varying pH Conditions.

Cysteine (Cys) is unique due to its highly reactive thiol group. It often regulates the biological function of proteins by acting as the redox site. Despite its biological significance, however, the valence electronic structure of Cys under the aqueous environments remains unavailable. Here, we report the VUV photoelectron spectroscopy of Cys aqueous aerosols via a newly built aerosol VUV photoelectron spectroscopy apparatus. The photoelectron spectra of Cys show distinct band shapes at varying pH conditions, reflecting the altered molecular orbital characters when its dominating form changes. The ionization energy of Cys is determined to be 8.98 ± 0.05 eV at low pH. A new feature at a binding energy of 6.97 ± 0.05 eV is observed at high pH, suggesting that the negative charge on the thiolate group becomes the first electron to be removed upon ionization. This work implies that when Cys is involved in redox processes, the charge transfer mechanism may be entirely altered under different pH conditions.

Time-resolved and mechanistic study of the photochemical uncaging reaction of the o-hydroxycinnamic caged compound.

The o-hydroxycinnamic derivatives represent efficient caged compounds that can realize quantification of delivery upon uncaging, but there has been lack of time-resolved and mechanistic studies. We used time-resolved infrared (TRIR) spectroscopy to investigate the photochemical uncaging dynamics of the prototype o-hydroxycinnamic compound, (E)-3-(2-hydroxyphenyl)-acrylic acid ethyl ester (HAAEE), leading to coumarin and ethanol upon uncaging. Taking advantage of the specific vibrational marker bands and the IR discerning capability, we have identified and distinguished two key intermediate species, the cis-isomers of HAAEE and the tetrahedral intermediate, in the transient infrared spectra, thus providing clear spectral evidence to support the intramolecular nucleophilic addition mechanism following the trans-cis photoisomerization. Moreover, the product yields of coumarin upon uncaging were observed to be greatly affected by the solvent polarity, suppressed in CH2Cl2 but enhanced in D2O/CH3CN with the increasing volume ratio of D2O. The highly solvent-dependent behavior indicates E1 elimination of the tetrahedral intermediate to give rise to the final uncaging product coumarin. The photorelease rate of coumarin was directly characterized from TRIR (3.6 × 10(6) s(-1)), revealing the promising application of such o-hydroxycinnamic compound in producing fast alcohol jumps. The TRIR results provide the first time-resolved detection and thus offer direct dynamical information about this photochemical uncaging reaction.

Mechanism of the deamination reaction of isoguanine: a theoretical investigation.

Mechanisms of the deamination reactions of isoguanine with H2O, OH(-), and OH(-)/H2O and of protonated isoguanine (isoGH(+)) with H2O have been investigated by theoretical calculations. Eight pathways, paths A-H, have been explored and the thermodynamic properties (ΔE, ΔH, and ΔG), activation energies, enthalpies, and Gibbs energies of activation were calculated for each reaction investigated. Compared with the deamination reaction of isoguanine or protonated isoguanine (isoGH(+)) with water, the deamination reaction of isoguanine with OH(-) shows a lower Gibbs energy of activation at the rate-determining step, indicating that the deamination reaction of isoguanine is favorably to take place for the deprotonated form isoG(-) with water. With the assistance of an extra water, the reaction of isoguanine with OH(-)/H2O, pathways F and H, are found to be the most feasible pathways in aqueous solution due to their lowest Gibbs energy of activation of 174.7 and 172.6 kJ mol(-1), respectively, at the B3LYP/6-311++G(d,p) level of theory.

Formation of guanine-6-sulfonate from 6-thioguanine and singlet oxygen: a combined theoretical and experimental study.

As an end metabolism product of the widely used thiopurine drugs, 6-thioguanine (6-TG) absorbs UVA and produces (1)O2 by photosensitization. This unusual photochemical property triggers a variety of DNA damage, among which the oxidation of 6-TG itself by (1)O2 to the promutagenic product guanine-6-sulfonate (G(SO3)) represents one of the major forms. It has been suspected that there exists an initial intermediate, G(SO), prior to its further oxidation to G(SO2) and G(SO3), but G(SO) has never been observed. Using density functional theory, we have explored the energetics and intermediates of 6-TG and (1)O2. A new mechanism via G(SOOH) → G(SO2) → G(SO4) → G(SO3) has been discovered to be the most feasible energetically, whereas the anticipated G(SO) mechanism is found to encounter an inaccessibly high barrier and thus is prevented. The mechanism through the G(SOOH) and G(SO4) intermediates can be validated further by joint experimental measurements, where the fast rate constant of 4.9 × 10(9) M(-1) s(-1) and the reaction stoichiometry of 0.58 supports this low-barrier new mechanism. In addition to the dominant pathway of G(SOOH) → G(SO2) → G(SO4) → G(SO3), a side pathway with higher barrier, G(SOOH) → G, has also been located, providing a rationalization for the observed product distributions of G(SO2) and G(SO3) as major products and G as minor product. From mechanistic and kinetics points of view, the present findings provide new chemical insights to understand the high phototoxicity of 6-TG in DNA and point to methods of using 6-TG as a sensitive fluorescence probe for the quantitative detection of (1)O2, which holds particular promise for detecting (1)O2 in DNA-related biological surroundings.

[2+2] Photocycloaddition reaction dynamics of triplet pyrimidines.

Taking the 266 nm excited pyrimidine (uracil or thymine) with cyclopentene as model reaction systems, we have examined the photoproduct formation dynamics from the [2 + 2] photocycloaddition reactions of triplet pyrimidines in solution and provided mechanistic insights into this important DNA photodamage reaction. By combining two compliment methods of nanosecond time-resolved transient IR and UV-vis laser flash-photolysis spectroscopy, the photoproduct formation dynamics as well as the triplet quenching kinetics are measured. Characteristic IR absorption bands due to photoproduct formation have been observed and product quantum yields are determined to be ∼0.91% for uracil and ∼0.41% for thymine. Compared to the measured large quenching rate constants of triplet uracil (1.5 × 10(9) M(-1)s(-1)) or thymine (0.6 × 10(9) M(-1)s(-1)) by cyclopentene, the inefficiency in formation of photoproducts indicates competitive physical quenching processes may exist on the route leading to photoproducts, resulting in very small product yields eventually. Such an energy wasting process is found to be resulted from T(1)/S(0) surface crossings by the hybrid density functional calculations, which compliments the experiments and reveals the reaction mechanism.

Reaction mechanisms of a photo-induced [1,3] sigmatropic rearrangement via a nonadiabatic pathway.

Time-resolved Fourier transform infrared absorption spectroscopy measurements and B3LYP/cc-pVDZ calculations have been conducted to characterize the reaction dynamics of a remarkable photoinduced 1,3-Cl sigmatropic rearrangement reaction upon 193 or 266 nm excitation of the model systems acryloyl chloride (CH(2)CHCOCl) and crotonyl chloride (CH(3)CHCHCOCl) in solution. The reaction is elucidated to follow nonadiabatic pathways via two rapid ISC processes, S(1) --> T(1) and T(1) --> S(0), and the S(1)/T(1) and T(1)/S(0) surface intersections are found to play significant roles leading to the nonadiabatic pathways. The S(1) --> T(1) --> S(0) reaction pathway involving the key participation of the T(1) state is the most favorable, corresponding to the lowest energy path. It is also suggested that the photoinduced 1,3-Cl migration reaction of RCHCHCOCl (R = H, CH(3)) proceeds through a stepwise mechanism involving radical dissociation-recombination, which is quite different from the generally assumed one-step concerted process for pericyclic reactions.

Theoretical investigation of direct amination of beta-ketoesters catalyzed by copper(II)-bisoxazoline(BOX).

The mechanism of the direct amination of beta-keto esters catalyzed by copper(II)-bisoxazoline has been studied by means of density functional theory of B3LYP method. The computational results support the present mechanism, which involves (i) the generation of the enol from beta-keto esters, which coordinates to copper(II)-bisoxazoline. The coordination step appears to be fast, exothermic, and irreversible. (ii) The formation of the sigma(N-C) bond via a six-membered ring transition state after azo dicarboxylate coordination with the chiral catalyst. This step is chirality-control step. (iii) Intramolecular hydrogen migration generates a catalyst-product complex, which can finally yield product. The hydrogen shift is the rate-determining step, which affords the experimentally observed (R)-product. The stereochemical predictions have been rationalized in terms of steric repulsions, showing good agreement with experimental data.