Intramolecular Hydrogen Transfer Research Articles

Dynamic multiphase semi-crystalline polymers based on thermally reversible pyrazole-urea bonds

Constructing responsive and adaptive materials by dynamic covalent bonds is an attractive strategy in material design. Here, we present a kind of dynamic covalent polyureas which can be prepared from the highly efficient polyaddition reaction of pyrazoles and diisocyanates at ambient temperature in the absence of a catalyst. Owing to multiphase structural design, poly(pyrazole-ureas) (PPzUs) show excellent mechanical properties and unique crystallization behavior. Besides, the crosslinked PPzUs can be successfully recycled upon heating (~130 °C) and the molecular-level blending of polyurea and polyurethane is realized. Theoretical studies prove that the reversibility of pyrazole-urea bonds (PzUBs) arises from the unique aromatic nature of pyrazole and the N-assisting intramolecular hydrogen transfer process. The PzUBs could further broaden the scope of dynamic covalent bonds and are very promising in the fields of dynamic materials.

Intramolecular Hydrogen Transfer from the Alpha-Carbon (Cα) and Backbone Amide Nitrogen (Nb) to Form c- and y-Ions in Negative-Ion CID of Peptides.

The source of hydrogen in the formation of c- and y-ions produced by intramolecular hydrogen transfer in negative-ion CID experiments with peptides has been examined using Cα-, Cβ-, and backbone amide (Nb)-deuterated peptides AAA(d3)AA, AAG(d2)AA, AAAG(d2)A, and AAAAA-d7, as well as five other peptides. The c- and y-ions produced by deuterium transfer from the deuterated residues were detected and identified by the exact m/z values obtained with a high-resolution orbitrap mass spectrometer. The rate of deuterium transfer obtained indicates that over 50% of the hydrogen was originated from the backbone amide nitrogen, with the residual hydrogen coming from the backbone Cα. It is clear that the hydrogen does not originate from the side chain Cβ. It is hypothesized that the intramolecular hydrogen transfer to form negative c- and y-ions takes place via 3-, 4-, 6-, 7-, 8-, and 9-membered ring transition states.

Thermochemistry and kinetics of the 2‐butanone‐4‐yl CH3C(=O)CH2CH2• + O2 reaction system

AbstractThermochemistry and kinetic pathways on the 2‐butanone‐4‐yl (CH3C(=O)CH2CH2•) + O2 reaction system are determined. Standard enthalpies, entropies, and heat capacities are evaluated using the G3MP2B3, G3, G3MP3, CBS‐QB3 ab initio methods, and the B3LYP/6‐311g(d,p) density functional calculation method. The CH3C(=O)CH2CH2• radical + O2 association reaction forms a chemically activated peroxy radical with 35 kcal mol−1 excess of energy. The chemically activated adduct can undergo RO−O bond dissociation, rearrangement via intramolecular hydrogen transfer reactions to form hydroperoxide‐alkyl radicals, or eliminate HO2 and OH. The hydroperoxide‐alkyl radical intermediates can undergo further reactions forming ketones, cyclic ethers, OH radicals, ketene, formaldehyde, or oxiranes. A relatively new path showing a low barrier and resulting in reactive product sets involves peroxy radical attack on a carbonyl carbon atom in a cyclic transition state structure. It is shown to be important in ketones when the cyclic transition state has five or more central atoms.

Thermal degradation of waste plastics under non-sweeping atmosphere: Part 1: Effect of temperature, product optimization, and degradation mechanism

Continuous generation of plastic waste has prompted substantial research efforts in its utilization as a feedstock for energy generation. Pyrolysis has emerged as one of the best waste management technique for energy extraction from the plastic waste. The objective of this work is to investigate the effect of operating temperature on the liquid product yields in the pyrolysis process by non-isothermal heating. Non-catalytic thermal pyrolysis of waste polyethylene (PE) [high density polyethylene (HDPE)], waste polypropene (PP), waste polystyrene (PS), waste polyethylene terephthalate (PET) and mixed plastic waste (MPW) was carried out in a non-sweeping atmosphere in a semi-batch reactor at four different temperatures 450, 500, 550, and 600 °C. The minimum degradation temperature of the mixed and individual plastics was obtained using a thermogravimetric apparatus (TGA) at a heating rate of 20 °C/min. The TGA results show that all plastics degrade in a single step and the degradation temperatures of PS > PET > PP > HDPE, while mixed plastic degradation indicates two distinct degradation steps. Further, a waste polymer shows a lower degradation temperature than the virgin polymer. The degradation of HDPE is found to produce the maximum oil yield with minimum solid residue. The degradation of PET results in the highest amount of solid and benzoic acid as crystals and gas with no oil. Degradation of mixed plastic causes oil yield in the intermediate range of pyrolysis of individual plastic wastes. Overall, 500 °C is observed to be an optimum temperature for the recovery of low-density pyrolytic oil with the highest liquid yield. The degradation of PE and PP is found to be caused by random chain scission followed by inter and intramolecular hydrogen transfer. The degradation of PS occurs by side elimination or end chain scission followed by β-scission mechanism. The degradation of mix plastics results from random chain scission followed by β-scission mechanism. The effect of temperature on oil and gas recovery as well as recovery time was also assessed.

Current–voltage characteristics of β-ketoenamines molecular switches induced by intramolecular hydrogen transfer

Using non-equilibrium Green’s function formalism combined with first-principles density functional theory, we analyzed the transport properties of (Z)-4-aminopent-3-en-2-one (which can be transformed into two enol and keto forms) as molecular switches. In addition to H atom, we obtained the transport properties for CH3 and C2H5 groups at R position. The current–voltage curves, ON–OFF ratio, electronic transmission coefficients, HOMO–LUMO gaps, three different adsorption types (hollow, top, and bridge), and the effect of electrode materials, Y (Y = Au, Ag, and Pt) on electronic transport properties of keto and enol forms of the mentioned molecules are discussed in detail. According to our obtained results, the enol and keto forms are ON and OFF states, respectively.

Re(I)-catalyzed hydropropargylation of enamides: a useful method for the preparation of 4-pentynylamine derivatives.

Hydropropargylation of enamides was achieved in good yields through the 1,4-addition of enamides to the α,β-unsaturated carbene complex intermediate, followed by intramolecular hydrogen transfer to the iminium carbon. This method is useful for the synthesis of 4-pentynylamine derivatives from enamides and easily available propargyl ether. Moreover, tandem cyclization reaction was achieved to afford a pyrrolidine derivative in a single operation by using a secondary enamide.

Constructing surface synergistic effect in Cu-Cu2O hybrids and monolayer H1.4Ti1.65O4·H2O nanosheets for selective cinnamyl alcohol oxidation to cinnamaldehyde

A multifunctional photocatalyst is constructed based on Cu-Cu2O hybrid and monolayer H1.4Ti1.65O4·H2O nanosheets via an in situ photo-deposition process (Cu-Cu2O/TNS). The prepared sample enables the oxidation of cinnamyl alcohol to cinnamaldehyde with over 95% of selectivity under visible light irradiation using molecule O2 as the oxidant. In situ FTIR spectra suggest that the Lewis acid sites in TNS would selectively adsorbed the hydroxyl of cinnamyl alcohol forming surface CO∙∙∙TiO coordination species. UV-DRS spectra reveal that light adsorption of TNS can be extended to visible light region due to the interface charges transfer from COH to TiO via the surface coordination, resulting in the activation of OH in alcohol. In situ ESR further indicates that directional interface charge transfer induces the charge enrichment in Ti atoms of TiO6, facilitating O2 adsorption. XAFS results reveal that the hybrid electronic states of Cu-Cu2O efficiently improve the interface electronic transfer kinetics to inhibit the intramolecular hydrogen transfer for dominating the selectivity towards cinnamaldehyde. This work highlights the synergistic effect of surface coordination-activation-photocatalysis for selective photosynthesis of fine chemicals.

Elucidating the Nuclear Quantum Dynamics of Intramolecular Double Hydrogen Transfer in Porphycene.

We address the double hydrogen transfer (DHT) dynamics of the porphycene molecule, a complex paradigmatic system in which the making and breaking of H-bonds in a highly anharmonic potential energy surface require a quantum mechanical treatment not only of the electrons but also of the nuclei. We combine density functional theory calculations, employing hybrid functionals and van der Waals corrections, with recently proposed and optimized path-integral ring-polymer methods for the approximation of quantum vibrational spectra and reaction rates. Our full-dimensional ring-polymer instanton simulations show that below 100 K the concerted DHT tunneling pathway dominates but between 100 and 300 K there is a competition between concerted and stepwise pathways when nuclear quantum effects are included. We obtain ground-state reaction rates of 2.19 × 1011 s–1 at 150 K and 0.63 × 1011 s–1 at 100 K, in good agreement with experiment. We also reproduce the puzzling N–H stretching band of porphycene with very good accuracy from thermostated ring-polymer molecular dynamics simulations. The position and line shape of this peak, centered at around 2600 cm–1 and spanning 750 cm–1, stem from a combination of very strong H-bonds, the coupling to low-frequency modes, and the access to cis-like isomeric conformations, which cannot be appropriately captured with classical-nuclei dynamics. These results verify the appropriateness of our general theoretical approach and provide a framework for a deeper physical understanding of hydrogen transfer dynamics in complex systems.

Highly Selective Fluorescent Probe Based on 2‐(2′‐Dansylamidophenyl)‐Thiazole for Sequential Sensing of Copper(II) and Iodide Ions

A novel highly selective fluorescent probe based on 2‐(2′‐dansylamidophenyl)‐4‐phenylthiazole (1) is developed for sequential sensing of copper(II) and iodide ions in acetonitrile. The fluorescence mechanism is based on cation‐induced inhibition of excited‐state intramolecular hydrogen transfer, intramolecular charge transfer, and metal–ligand electron/charge transfer. Subsequent iodide‐induced extrusion of copper(II) results in partial revival of fluorescence. Probe 1 and its ensemble with copper (II) show high selectivity for copper(II) and iodide ions, respectively, in acetonitrile solution.

Molecular dynamic simulation on the oxidation process of coal tar pitch

Molecular dynamic (MD) simulations based on the reactive force field (ReaxFF) were successfully used for the study of coal tar pitch (CTP) oxidation process under medium temperature. Three CTP components were divided into three fractions: pitch acetone solubles (PAS), pitch pyridine soluble-acetone insolubles (PPS), and pitch pyridine insolubles (PPI). A series of ReaxFF MD simulations were conducted to study the influence of temperature and oxygen content on CTP oxidation mechanism. The results demonstrate that the oxidation process is initiated by generation of CO bonds and OO bonds following intramolecular hydrogen transfer. The phenoxyl free radical which is initially formed by the cleavage of hydroxyl and the oxidization of benzene ring is an important intermediate product, it plays a key role in the formulation of H2O, CO2 and glyoxal, and it also promotes the combination of two small molecules. The cleavage of CH bonds is more likely to happen when temperature is larger than 650 K. The PPI fraction is more stable compared with PAS and PPS components. The major reaction pathways are analyzed, and it’s observed that the reaction pathways are able to reproduce and explain the experimental findings in the CTP oxidation process.

Reversible intramolecular hydrogen transfer: a completely new mechanism for low impact sensitivity of energetic materials.

The intramolecular H transfer of energetic NO2-compounds has been recognized as a possible primary step in triggering molecular decomposition for a long time. Nevertheless, studies on H transfer in different complex situations are limited, lacking a comprehensive understanding of its role in NO2-compounds. In this work, twenty intramolecular H transfer reactions are studied for eighteen nitro compounds and compared with the NO2 partition in thermodynamics and kinetics. Three factors, including the high planarity of molecules, the short transfer distance between the target H and O atoms and the high protonation of the H atom are identified to facilitate H transfer. If H transfer is more kinetically favorable than NO2 partition, and if a reverse H transfer occurs with a barrier less than 30 kcal mol-1, we define it as a reversible one. In our study, for those impact insensitive nitro compounds with H50 larger than that of 2,4,6-trinitrotoluene, all of them are found to be accompanied with reversible H transfer, while the impact sensitive compounds are not. Accordingly, we propose that the reversible H transfer can effectively buffer the external stimuli against the molecular decomposition through chemical energy absorption/release. Beyond the conventional understanding that H transfer triggers molecular decomposition, this work builds a new correlation between reversible H transfer and the low impact sensitivity of energetic nitro-compounds.

Iron(II) Chloride‐Catalyzed Nitrene Transfer Reaction for Dearomative Amination of β‐Naphthols with Aryl Azides

AbstractA first example of Fe‐catalyzed dearomative amination of β‐naphthols with aryl azides is described. The method constructs a nitrogen‐containing quaternary carbon center catalyzed by cheap and readily available iron salt FeCl2, and affords a series of α‐amino‐β‐naphthalenones in good to excellent yields under simple and mild reaction conditions. Preliminary experimental and computational studies suggest that the reaction is probably initiated by iron‐iminyl diradical and followed by intramolecular hydrogen transfer to generate iron‐aminyl radical and finally an intramolecular radical combination reaction.magnified image

Theoretical investigation of cyromazine tautomerism using density functional theory and Møller–Plesset perturbation theory methods

ABSTRACTA computational chemistry analysis of six unique tautomers of cyromazine, a pesticide used for fly control, was performed with density functional theory (DFT) and canonical second-order Møller–Plesset perturbation theory (MP2) methods to gain insight into the contributions of molecular structure to detection properties. Full geometry optimisation using the 6-311++G** basis set provided energetic properties, natural charges, frontier orbitals and vibrational modes. Excitation energies were obtained using time-dependent DFT. Hydrogen location and bond order contribute significantly to the electronic properties. The common cyromazine tautomer possesses the lowest energy, highest band gap energy and highest excitation energy. B3LYP/6-31G** dynamics simulations indicate each tautomer possesses a stable structure with limited rotation about the single bonds. Tautomerisation involving intramolecular hydrogen transfer influences the natural charges of neighbouring atoms and the frontier orbital properties. The excitation energies are highly correlated with band gap energies of the frontier orbitals. The calculated infrared and Raman spectra are suitable for vibrational assignments associated with the chemical structure. The tautomeric forms of cyromazine possess similar spatial properties and significant variation in electronic properties.

Solvent mediated catalysis and proton-shuttling in the formation of 3-methylphthalide from a ketene intermediate

Photoexcitation of o-acetylbenzaldehyde (oABA) in acetonitrile yields, via photoinduced intramolecular hydrogen transfer, the lactone 3-methylphthalide (3MP) with a quantum yield of ca. 0.3. Femtosecond stimulated Raman spectroscopy (FSRS) measurements revealed the formation of a Z-ketene-enol ground state intermediate during this process ca. 2–3 ps after excitation [Phys. Chem. Chem. Phys. 2015, 7, 376]. UV traces furthermore indicate the occurrence of a cyclic enol form shortly before the reaction proceeds towards the lactone. To obtain insight into the reaction mechanism and to clarify the potential role of a cyclic enol, we have identified stationary points along the oABA – 3MP ground-state reaction path with DFT methods applying implicit/explicit solvation models. The computations show that the reaction can efficiently proceed with or without involvement of the cyclic enol by a) catalysis through traces of water in the solvent or b) directly via acetonitrile proton shuttling.

Intersystem Crossing Drives Photoisomerization in o-Nitrotoluene, a Model for Photolabile Caged Compounds.

o-Nitrobenzyl (oNB) derivatives are widely used photolabile caged compounds in chemical and biological applications. The primary step in the photoinduced deprotection is an excited state intramolecular hydrogen transfer (ESIHT) leading to tautomerization of the oNB compound and subsequent release of the protecting group. The prototype molecule for studying such ESIHT is o-nitrotoluene (oNT), where hydrogen transfers from the methyl to the nitro group. Using the complete active space self-consistent field (CASSCF) method with second-order perturbative energy corrections (CASPT2), we have comprehensively investigated the photoisomerization and photo decay mechanisms in oNT. We have obtained the minimum energy crossing points (MECPs) between relevant electronic states and identified the singlet and triplet pathways. There is a barrierless path for oNT to relax to the lowest triplet state. In this T1 state, the ESIHT products are more stable than T1 oNT. Hydrogen-transfer occurs on the T1 state followed by relaxation to the ground state to give the isomerized product. A biradical intermediate proposed by previous studies is characterized to be the hydrogen-transferred T1 product. On the singlet pathway, in contrast to the triplet, the ground state tautomer is formed from the S1 oNT through a geometrically distant and energetically higher S1/S0 conical intersection. Although nonadiabatic dynamical studies are essential for determining branching ratios, our study, which considers the accessibility of different MECPs based on geometry and energy, and the magnitude of spin-orbit coupling at singlet-triplet MECPs, suggests that a significant fraction of the isomerization yield is due to the triplet channel.

Hydrogen Promoted Decomposition of Ammonium Dinitramide: an ab initio Molecular Dynamics Study

Thermal decomposition of a famous high oxidizer ammonium dinitramide (ADN) under high temperatures (2000 and 3000 K) was studied by using the ab initio molecular dynamics method. Two different temperature-dependent initial decomposition mechanisms were observed in the unimolecular decomposition of ADN, which were the intramolecular hydrogen transfer and N—NO2 cleavage in N(NO2)−. They were competitive at 2000 K, whereas the former one was predominant at 3000 K. As for the multimolecular decomposition of ADN, four different initial decomposition reactions that were also temperature-dependent were observed. Apart from the aforementioned mechanisms, another two new reactions were the intermolecular hydrogen transfer and direct N—H cleavage in NH4+. At the temperature of 2000 K, the N—NO2 cleavage competed with the rest three hydrogen-related decomposition reactions, while the direct N—H cleavage in NH4+ was predominant at 3000 K. After the initial decomposition, it was found that the temperature increase could facilitate the decomposition of ADN, and would not change the key decomposition events. ADN decomposed into small molecules by hydrogen-promoted simple, fast and direct chemical bonds cleavage without forming any large intermediates that may impede the decomposition. The main decomposition products at 2000 and 3000 K were the same, which were NH3, NO2, NO, N2O, N2, H2O, and HNO2.

Density functional tight-binding and infrequent metadynamics can capture entropic effects in intramolecular hydrogen transfer reactions.

Infrequent metadynamics uses biased simulations to estimate the unbiased kinetics of a system, facilitating the calculation of rates and barriers. Here the method is applied to study intramolecular hydrogen transfer reactions involving peroxy radicals, a class of reactions that is challenging to model due to the entropic contributions of the formation of ring structures in the transition state. Using the self-consistent charge density-functional based tight-binding (DFTB) method, we applied infrequent metadynamics to the study of four intramolecular H-transfer reactions, demonstrating that the method can qualitatively reproduce these high entropic contributions, as observed in experiments and those predicted by transition state theory modeled by higher levels of theory. We also show that infrequent metadynamics and DFTB are successful in describing the relationship between transition state ring size and kinetic coefficients (e.g., activation energies and the pre-exponential factors).

Computational study of the decomposition mechanisms of ammonium dinitramide in the gas phase

ABSTRACTCBS-QB3 method has been employed to determine the geometries, the vibrational frequencies of the reactants, the products and the transition states involved in intramolecular hydrogen-transfer and decomposition reactions of the free gas-phase H3N···HN(NO2)2 (ADN*). The results show that the intramolecular hydrogen-transfer reaction of ADN* is more feasible than that of HDN. ADN* and its hydrogen-transfer isomers ADN*-IIa,b,c decompose along four channels to form NH3 + HONO + 2NO (PI), ȮH + ṄO3 + N2 + NH3 (PII), ȮH + ṄO2 + N2O + NH3 (PIII), and HNO3 + N2O + NH3 (PIV), respectively. It has been found that the dominant decomposition channels are PI and PIII. The hydrogen-transfer reaction can reduce the barrier of elimination of NO2 and forming N2O reactions in ADN* and HDN. The decomposition of ADN*-IIc to form NO2 and N2O is more feasible than that of the gas-phase HDN. The rate constants (k) of rate-determining step of ADN* show that kPI and kPIII are higher than kPIV and kPII. Compared with HDN-IIc → N2O+ȮH+ṄO2, kPIII of ADN*-IIc is significantly higher than that of kHDN-IIc. These results reveal that NH3 (as a chaperon) has a certain influence on the decomposition mechanisms and kinetics of ADN*.

Selective Production of Renewable para-Xylene by Tungsten Carbide Catalyzed Atom-Economic Cascade Reactions.

Tungsten carbide was employed as the catalyst in an atom-economic and renewable synthesis of para-xylene with excellent selectivity and yield from 4-methyl-3-cyclohexene-1-carbonylaldehyde (4-MCHCA). This intermediate is the product of the Diels-Alder reaction between the two readily available bio-based building blocks acrolein and isoprene. Our results suggest that 4-MCHCA undergoes a novel dehydroaromatization-hydrodeoxygenation cascade process by intramolecular hydrogen transfer that does not involve an external hydrogen source, and that the hydrodeoxygenation occurs through the direct dissociation of the C=O bond on the W2 C surface. Notably, this process is readily applicable to the synthesis of various (multi)methylated arenes from bio-based building blocks, thus potentially providing a petroleum-independent solution to valuable aromatic compounds.

Selective Production of Renewable para‐Xylene by Tungsten Carbide Catalyzed Atom‐Economic Cascade Reactions

AbstractTungsten carbide was employed as the catalyst in an atom‐economic and renewable synthesis of para‐xylene with excellent selectivity and yield from 4‐methyl‐3‐cyclohexene‐1‐carbonylaldehyde (4‐MCHCA). This intermediate is the product of the Diels–Alder reaction between the two readily available bio‐based building blocks acrolein and isoprene. Our results suggest that 4‐MCHCA undergoes a novel dehydroaromatization–hydrodeoxygenation cascade process by intramolecular hydrogen transfer that does not involve an external hydrogen source, and that the hydrodeoxygenation occurs through the direct dissociation of the C=O bond on the W2C surface. Notably, this process is readily applicable to the synthesis of various (multi)methylated arenes from bio‐based building blocks, thus potentially providing a petroleum‐independent solution to valuable aromatic compounds.