Unusual Regioselective C-H Difluoroalkylation of Heteroarenes under Photoredox Catalysis.

We disclose a new general strategy for the site-selective difluoroalkylation of nonprefunctionalized heteroarenes, such as quinoxaline at the C-8 position, and benzothiadiazole, benzoxadiazole, and benzothiazole at the C-4 position via consecutive organophotoredox-catalyzed radical-radical cross-coupling and base-assisted hydrogen abstraction reactions. The current methodology represents a site-selective direct difluoroalkylative strategy to allow broad functional group tolerance and a wide substrate scope in good to excellent yields. Careful experimental investigations and detailed DFT calculations revealed the exact site-selectivity of the heteroarenes and a possible mechanistic pathway.

First-Principles Insights into the Mechanism of CO2 Hydrogenation Reactions by Fe-PNP Pincer Complex.

Using the state of the art theoretical methods, we have provided a comprehensive mechanistic understanding of the CO2 hydrogenation into HCOOH, H2CO, and CH3OH by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated Fe pincer complex, featuring one CO and two H as co-ligands. For the computational investigation, a verified structural model containing methyl groups in place of the experimental isopropyl groups was used. Three catalytic conversions involving hydrogenation of CO2 into formic acid (HCOOH), HCOOH into formaldehyde and methanol were studied in different solvent medium. Our modelled complex appears to be a viable base-free catalyst for the conversion of CO2 into HCOOH and HCOOH into H2CO, based on the free energy profiles, which show apparent activation energy barriers of 16.28 kcal/mol and 23.63 kcal/mol for the CO2 to HCOOH and HCOOH to H2CO conversion, respectively. However, the computed results show that, due to the huge energy span of H2CO to CH3OH conversion, complete hydrogenation of CO2 into methanol could not occur under moderate conditions. Morpholine co-catalyst, which can lower the hydrogenation barrier by taking part in a simultaneous H-atom donation-acceptance process, could have assisted in completing this step.

Single B-vacancy enriched α1-borophene sheet: an efficient metal-free electrocatalyst for CO2 reduction.

By employing first principles calculations, we have studied the electronic structures of pristine (α1) and different defective (α1-t1, α1-t2) borophene sheets to understand the efficacy of such systems as metal-free electrocatalysts for the CO2 reduction reaction. Among the three studied systems, only α1-t1, the defective borophene sheet created by removal of a 5-coordinated boron atom, can chemisorb and activate a CO2 molecule for its subsequent reduction processes, leading to different C1 chemicals, followed by selective conversion into C2 products by multiple proton coupled electron transfer steps. The computed onset potentials for the C1 chemicals such as CH3OH and CH4 are low enough. On the other hand, in the case of the C2 reduction process, the C-C coupling barrier is only 0.80 eV in the solvent phase which produces CH3CHO and CH3CH2OH with very low onset potential values of -0.21 and -0.24 V, respectively, suppressing the competing hydrogen evolution reaction.

Encapsulation and Stabilization of a Donor-Acceptor Stenhouse Adduct Isomer in Water Inside the Blue Box: A Combined Experimental and Theoretical Approach.

We synthesized two types of donor-acceptor Stenhouse adducts (DASAs), a new type of photochromic molecules showing dual color in two different isomeric forms in solution phase, using Meldrum acid (DASA-Mel) and barbituric acid (DASA-Bar), along with a naphthalimide derivative to obtain interesting fluorescence properties. DASA-Mel was found to have fast photochromic conversion in comparison to DASA-Bar, evident from ultraviolet-visible (UV-vis) and fluorescence spectroscopic studies. The colored form of DASA-Mel was encapsulated inside the water-soluble Stoddart's blue box and became soluble in water much faster than DASA-Bar. Interestingly, the competitive encapsulation experiment showed that DASA-Mel was selectively encapsulated inside the blue box in water whereas DASA-Bar was mostly separated out from the solution after centrifugation, and this phenomenon was confirmed by 1H and DOSY NMR and mass spectroscopies. Moreover, we found through density functional theory (DFT) optimization that the open form of DASA-Mel was more stable during the encapsulation reaction in a water medium in comparison to DASA-Bar. The calculated binding energies of encapsulated DASA-Mel and DASA-Bar are -10.2 and -9.9 kcal/mol, respectively, clearly showing that the former is more stable by 0.3 kcal. Consequently, the organic macrocycle selectively separating one kind of DASA from a mixture by encapsulation in water is reported for the first time with experimental and theoretical support in the literature.

Towards H2O catalyzed N2-fixation over TiO2 doped Run clusters (n = 5, 6): a mechanistic and kinetic approach.

H2O driven N2 fixation is known as the best alternative pathway to synthesise NH3 under ambient conditions. The thermodynamic non-spontaneous reaction can be accomplished by a photocatalytic water splitting reaction over a TiO2 supported surface with oxygen vacancies. Previous experiments have also shown N2 activation over a neutral Ru cluster whose catalytic activity was remarkably enhanced by TiO2 doping. In this article, we have investigated the detailed mechanism and kinetics of the H2O catalyzed nitrogen reduction reaction (NRR) over bare and TiO2 doped Ru5 clusters in conjunction with DFT and TST calculations. The lack of photochemical activity of the small model cluster provoked us to explore an alternative route of NH3 formation via H2O catalysis. For this, we have considered H2 as co-reactant. The partial reduction of N2 into NH3 or N2H4 could be achieved by a H2O oxidation reaction, however, catalytic regeneration requires additional H2 which effectively makes the overall reaction catalyzed by H2O. Above all, the present investigation suggests that NH3 is most favorably produced through the distal mechanism. Analysis of the rate constants demonstrates that the doping with TiO2 accelerates the kinetics of NRR by a few orders of magnitude. Furthermore, an increase of the size of the metal cluster would not significantly enhance the overall performance of NRR.

Theoretical Investigations on the Possibility of Prebiotic HCN Formation via O-Addition Reactions.

Until now, reactions between methane photolysis products (CH3•, CH2) and active N atom or reactive NO radical are proposed as routes of HCN formation in the prebiotic Earth. Scientists think that the reducing atmosphere of primitive Earth was made of H2, He, N2, NO, CH4, H2O, CO2, etc., and there was no molecular oxygen. However, it has been evident from experiments that the vacuum ultraviolet (VUV) photolysis of CO2 can produce atomic oxygen. Therefore, it can be presumed that atomic oxygen was likely present in early Earth's atmosphere. Was there any impact of atomic oxygen in production of early atmospheric HCN for the emergence of life? To hunt for the answer, we have employed computational methods to study the mechanism and kinetics of CH3NO + O(1D) and CH2NO• + O(3P) addition reactions. Current study suggests that the addition of O(1D) into nitrosomethane (CH3NO) and the addition of O(3P) into nitrosomethylene radical (CH2NO•) can efficiently produce HCN through an effectively barrierless pathway. At STP, Bartis-Widom phenomenological loss rate coefficients of O(1D) and O(3P) are obtained as 2.47 × 10-12 and 4.67 × 10-11 cm3 molecule-1 s-1, respectively. We propose that addition reactions of atomic oxygen with CH3NO and CH2NO• might act as a potential source for early atmospheric HCN.

CH3NO as a potential intermediate for early atmospheric HCN: a quantum chemical insight.

Hydrogen cyanide (HCN) has played a central role in the production of several biological molecules under prebiotic conditions on primitive Earth. Previously, K. J. Zahnle (J. Geophys. Res.: Atmos., 1986, 91, 2819) and Tian et al. (Earth Planet. Sci. Lett., 2011, 308, 417) emphasized that HCN production in the early Earth's CH4-rich atmosphere could have been possible through the reaction between active nitrogen atoms (N) and methane photolysis products. Here, we have proposed alternative pathways for the formation of early atmospheric HCN via the decomposition of CH3NO as an intermediate. In the early Earth's O2-free atmosphere, CH3˙ could preferentially attach to NO, which was generated via early atmospheric volcanism or lightning and photochemical processes. We have quantum chemically explored both unimolecular and bimolecular decomposition pathways of CH3NO via the assistance of another CH3NO molecule and via H2O, NH3, HCl, HCOOH, HNO3 and H2SO4 catalysis. Both energetic and kinetic analyses reveal that H2SO4 is more efficient in this regard than other atmospheric species. Overall, it has been suggested that the proposed bimolecular decomposition pathways might have been alternative pathways for the formation of HCN under certain conditions on prebiotic Earth, while the unimolecular decomposition of CH3NO could lead to the formation of HCN in the high temperature volcanic environment on early Earth.

Small molecular organic nanocrystals resemble carbon nanodots in terms of their properties.

The most commonly observed phenomena in carbon nanodots (CNDs) are the strong excitation wavelength dependent multicolor fluorescence emission and the particle size distribution between 3-5 nm observed using a transmission electron microscope (TEM). However, it is not evident yet whether the emission originates from the particles observed using a TEM. In this article, we show that hydrothermal treatment of citric acid produces methylenesuccinic acid, which gives rise to hydrogen-bonded nano-assemblies with CND-like properties. While single crystal X-ray crystallography confirms the structure of methylenesuccinic acid, fluorescence correlation spectroscopy (FCS) confirms the presence of a molecular fluorophore with an average hydrodynamic diameter of ∼0.9 nm. This size is much smaller than the size of the particles observed using a TEM. We conclude that the particles observed using a TEM are the drying mediated nanocrystals of methylenesuccinic acid.

Impact of OH Radical-Initiated H2CO3 Degradation in the Earth's Atmosphere via Proton-Coupled Electron Transfer Mechanism.

The decomposition of isolated carbonic acid (H2CO3) molecule into CO2 and H2O (H2CO3 → CO2 + H2O) is prevented by a large activation barrier (>35 kcal/mol). Nevertheless, it is surprising that the detection of the H2CO3 molecule has not been possible yet, and the hunt for the free H2CO3 molecule has become challenging not only in the Earth's atmosphere but also on Mars. In view of this fact, we report here the high levels of quantum chemistry calculations investigating both the energetics and kinetics of the OH radical-initiated H2CO3 degradation reaction to interpret the loss of the H2CO3 molecule in the Earth's atmosphere. It is seen from our study that proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) are the two mechanisms by which the OH radical initiates the degradation of the H2CO3 molecule. Moreover, the PCET mechanism is potentially the important one, as the effective barrier, defined as the difference between the zero point vibrational energy (ZPE) corrected energy of the transition state and the total energy of the isolated starting reactants in terms of bimolecular encounters, for the PCET mechanism at the CCSD(T)/6-311++G(3df,3pd) level of theory is ∼3 to 4 kcal/mol lower than the effective barrier height associated with the HAT mechanism. The CCSD(T)/6-311++G(3df,3pd) level predicted effective barrier heights for the degradations of the two most stable conformers of H2CO3 molecule via the PCET mechanism are only ∼2.7 and 4.3 kcal/mol. A comparative reaction rate analysis at the CCSD(T)/6-311++G(3df,3pd) level of theory has also been carried out to explore the potential impact of the OH radical-initiated H2CO3 degradation relative to that from water (H2O), formic acid (FA), acetic acid (AA) and sulfuric acid (SA) assisted H2CO3 → CO2 + H2O decomposition reactions in both the Earth's troposphere and stratosphere. The comparison of the reaction rates reveals that, although the atmospheric concentration of the OH radical is substantially lower than the concentrations of the H2O, FA, AA in the Earth's atmosphere, nevertheless, the OH radical-initiated H2CO3 degradation reaction has significant impact, especially toward the loss of the H2CO3 molecule in the Earth's atmosphere. In clean environments, which exist in greater numbers in comparison to the polluted environments of the Earth's atmosphere, the impact of the OH radical-initiated H2CO3 degradation reaction is seen to be comparable to that from a competing pathway which utilizes hydrogen bonded molecules such as H2O, FA or AA to catalyze the H2CO3 decomposition. Similarly, in the polluted environments, and especially in the Earth's troposphere, although the reactions rates for the OH radical-initiated H2CO3 degradation and FA-assisted H2CO3 decomposition are comparable within a factor of ∼15, nevertheless, the AA-assisted H2CO3 decomposition reaction is appeared to be the dominant channel. This follows only because of slightly greater catalytic efficiency of the AA over FA upon the H2CO3 → CO2 + H2O decomposition reaction. In contrary, although the catalytic efficiencies of SA, FA, and AA upon the H2CO3 decomposition reaction are similar to each other and the concentrations of both the SA and OH radical in the Earth's atmosphere are more-or-less equal to each other, but nevertheless, the SA-assisted H2CO3 decomposition reaction cannot compete with the OH radical-initiated H2CO3 degradation reaction.

Autocatalytic isomerizations of the two most stable conformers of carbonic acid in vapor phase: double hydrogen transfer in carbonic acid homodimers.

The cis-cis [(cc)] and cis-trans [(ct)] conformers of carbonic acid (H2CO3) are known as the two most stable conformers based on the different orientations of two OH functional groups present in the molecule. To explain the interconversion of the (cc)-conformer to its (ct)-conformer, the rotation of one of the two indistinguishable OH functional groups present in the (cc)-conformer has been shown until now as the effective isomerization mechanism. Moreover, the (ct)-conformer, which is slightly energetically disfavored over the (cc)-conformer, has been considered as the starting point for the decomposition of H2CO3 into CO2 and H2O molecules. Experimentally, on the other hand, the infrared (IR) and Raman spectroscopy of the crystalline H2CO3 polymorphs suggest that the most possible basic building blocks of H2CO3 polymorphs consist of only and exclusively the (cc)-conformers. However, the sublimations of these crystalline H2CO3 polymorphs result both the (cc)- and (ct)-conformers in the vapor phase with the (cc)-conformer being the major species. In this article, we first report the high level ab initio calculations investigating the energetics of the autocatlytic isomerization mechanism between the two most stable conformers of carbonic acid in the vapor phase. The calculations have been performed at the MP2 level of theory in conjunction with aug-cc-pVDZ, aug-cc-pVTZ, and 6-311++G(3df,3pd) basis sets. The results of the present study specifically and strongly suggest that double hydrogen transfer within the eight-membered cyclic doubly hydrogen-bonded (H-bonded) ring interface of the H2CO3 homodimer formed between two (cc)-conformers is ultimately the starting mechanism for the isomerization of the (cc)-conformer to its (ct)-conformer, especially, during the sublimation of the H2CO3 polymorphs, which result in the vapor phase concentration of the (cc)-conformer at the highest levels.

New mechanism for autocatalytic decomposition of H2CO3 in the vapor phase.

In this article, we present high level ab initio calculations investigating the energetics of a new autocatalytic decomposition mechanism for carbonic acid (H2CO3) in the vapor phase. The calculation have been performed at the MP2 level of theory in conjunction with aug-cc-pVDZ, aug-cc-pVTZ, and 6-311++G(3df,3pd) basis sets as well as at the CCSD(T)/aug-cc-pVTZ level. The present study suggests that this new decomposition mechanism is effectively a near-barrierless process at room temperature and makes vapor phase of H2CO3 unstable even in the absence of water molecules. Our calculation at the MP2/aug-cc-pVTZ level predicts that the effective barrier, defined as the difference between the zero-point vibrational energy (ZPE) corrected energy of the transition state and the total energy of the isolated starting reactants in terms of bimolecular encounters, is nearly zero for the autocatalytic decomposition mechanism. The results at the CCSD(T)/aug-cc-pVTZ level of calculations suggest that the effective barrier, as defined above, is sensitive to some extent to the levels of calculations used, nevertheless, we find that the effective barrier height predicted at the CCSD(T)/aug-cc-pVTZ level is very small or in other words the autocatalytic decomposition mechanism presented in this work is a near-barrierless process as mentioned above. Thus, we suggest that this new autocatalytic decomposition mechanism has to be considered as the primary mechanism for the decomposition of carbonic acid, especially at its source, where the vapor phase concentration of H2CO3 molecules reaches its highest levels.