Intramolecular Hydrogen Transfer Process Research Articles

Atomic insights into the thermal decomposition mechanism and cluster growth law of nanoscale HMX and LLM-126 mixture: A ReaxFF-lg molecular dynamics study

In this paper, the thermal decomposition process and cluster growth law of the HMX/LLM-126 nanoscale mixture system were studied by ReaxFF-lg combined with DFT, and the thermostability of the mixture system with a molar ratio of 1:1 was investigated. The results show that clusters can be formed between HMX and LLM-126 in the mixture system, which effectively delays the cracking speed of the HMX structure. Meanwhile, NO2 generated from the initial decomposition of HMX will accelerate the denitration process of LLM-126. The decomposition process of HMX is mainly a continuous denitration until the structural ring-opening disintegration, the initial decomposition step is C4H8O8N8 => C4H8O6N7 + NO2. In contrast, the initial decomposition of LLM-126 is dominated by intramolecular hydrogen transfer reactions and the generation of dimer clusters, followed by the detachment of nitro and bitter amino groups, and finally the cleavage of the pyridine ring. The intramolecular hydrogen transfer process of LLM-126 is the transfer of H on -NH- to the adjacent nitro group. After LLM-126 was added to the HMX system, the oxygen balance of the system increased, and the N content, exothermic rate, and the number of final products decreased significantly compared with the HMX pure component system. Also, the number of clusters generated, and the maximum cluster weight increased significantly. These phenomena are important reasons for the improved thermostability of the mixture system compared to the pure HMX system. This work can provide a theoretical basis for the design and application of nanoscale high-energy thermostable mixed explosives.

The Role of Imidazolium-Based Surface-Active Ionic Liquid to Restrain the Excited-State Intramolecular H-Atom Transfer Dynamics of Medicinal Pigment Curcumin: A Theoretical and Experimental Approach.

The naturally occurring polyphenolic compound curcumin has shown various medicinal and therapeutic effects. However, there are various challenges associated with curcumin, which limits its biomedical applications, such as its high degradation rate and low aqueous solubility at neutral and alkaline pH. In the present study, efforts have been directed towards trying to resolve such issues by encapsulating curcumin inside the micelles formed by imidazolium-based surface-active ionic liquid (SAIL). The shape and size of the micelles formed by the SAIL have been characterized by using DLS analysis as well as TEM measurements. The photo-physics of curcumin in the presence of ionic liquid (IL) and also with the addition of salt (NaCl) has been explored by using different optical spectroscopic tools. The time-dependent absorption studies have shown that there is relatively higher suppression in the degradation rate of curcumin after encapsulation by the imidazolium-based SAIL in an aqueous medium. The TCSPC studies have revealed that there is deactivation in the nonradiative intramolecular hydrogen transfer process of curcumin in the presence of IL micelles as well as with the addition of salt. Furthermore, the time-dependent fluorescence anisotropy measurement has been carried out to figure out the location of curcumin inside the micellar system. In order to correlate all experimental findings, density functional theory (DFT) and classical molecular dynamics (MD) simulations at neutral pH media have been performed. It has been found that the van der Waals force of interactions plays a major role in the stabilization of curcumin in the micelles rather than the coulombic forces. It also has been observed that the van der Waals interactions remain unaffected in the presence of salt. However, as revealed by the MD simulation results, the micelles are found to be more compact in size after the addition of salt. The RMSD results show that the micelles formed by the SAIL achieve greater stability after a particular time constraint. Our results have divulged that the SAIL could act as a promising drug delivery system.

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.

Easy access to enamides: a mild nickel-catalysed alkene isomerization of allylamides

The first Ni-catalysed alkene isomerization of allylamides for the synthesis of enamides was demonstrated. Various substituted N-allylamides were found to be suitable substrates for this isomerization. Isotopic labelling experiments showed that it is an intramolecular hydrogen transfer process.

Radiationless decay of excited states of tetrahydrocannabinol through the S 1–S 0 (conical) intersection

The ground and electronically excited singlet states of tetrahydrocannabinol have been studied theoretically using density functional and time-dependent density functional methods. The vertical excitation energies, the equilibrium geometries as well as the adiabatic excitation energies have been determined. Opening of the six-membered ring between the oxygen and carbon atoms has been considered as photochemical reaction path. This mechanism leads to a typical excited-state intramolecular hydrogen-transfer process and produces low-lying S 0–S 1 intersection (possible conical intersection, CI) which provides a channel for effective radiationless deactivation of the electronically excited state.

Conformation-Specific Pathways of β-Alanine: A Vacuum Ultraviolet Photoionization and Theoretical Study

We report a photoionization and dissociative photoionization study of beta-alanine using IR laser desorption combined with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Fragments at m/z = 45, 44, 43, and 30 yielded from photoionization are assigned to NH(3)CH(2)CH(2)(+), NH(2)CHCH(3)(+), NH(2)CHCH(2)(+), and NH(2)CH(2)(+), respectively. Some new conformation-specific dissociation channels and corresponding dissociation energies for the observed fragments are established and determined with the help of ab initio G3B3 calculations and measurements of photoionization efficiency (PIE) spectra. The theoretical values are in fair agreement with the experimental results. Three low-lying conformers of the beta-alanine cation, including two gauche conformers G1+, G2+ and one anti conformer A+ are investigated by G3B3 calculations. The conformer G1+ (intramolecular hydrogen bonding N-H...OC) is found to be another precursor in forming the NH(3)CH(2)CH(2)(+) ion, which is complementary to the previously reported formation pathway that only occurs with the conformer G2+ (intramolecular hydrogen bonding O-H...N). Species NH(2)CHCH(2)(+) may come from the contributions of G1+, G2+, and A+ via different dissociation pathways. The most abundant fragment ion, NH(2)CH(2)(+), is formed from a direct C-C bond cleavage. Intramolecular hydrogen transfer processes dominate most of the fragmentation pathways of the beta-alanine cation.

Theoretical study on intramolecular hydrogen transfer involving amino-substituted perylenequinone

2-Amino-perylenequinone (2-NH2-PQ) and 3,10-diamino-perylenequinone (3,10-diNH2-PQ) were used as model compounds for investigating the intramolecular hydrogen transfer (IHT) reactions involving amino-substituted perylenequinonoid photosensitizers (PQP). A semi-empirical MO method (AM1) was employed to calculate the theoretical values associated with the IHT process. It was found that amino-substitution enhanced the entropy of a specific isomer of PQ, causing it to be more stable than other possible isomers. Amino-substitution raised the HOMO energy levels (EHOMO) of PQ, by increasing its electron-donating ability. It was also found that placing an NH2 group in the 2-position lowered the energy barriers to IHT in PQ in the ground state and the singlet excited state, and 3,10-diNH2-substitution increased the IHT energy barriers in PQ in both states. The energy barrier for the double IHT was almost two times the energy barrier for the single IHT, making the double IHT a less important process. The IHT energy barrier for NH2-PQ decreased in the singlet excited state, while the energy barrier decrease from the excited state to ground state was larger for 2-NH2-PQ than for 3,10-diNH2-PQ. The charge on the hydrogen atom and the molecular dipole moment varied during the IHT process, and the IHT energy barriers were governed by the differences in the charges on the hydrogen atom in the transition state and initial state. The dipole moment for the molecular singlet excited state changed considerably during the IHT process, indicting that IHT would be accelerated by strongly polar solvents. Like the PQ case, there was no correlation between changes in the charge on the hydrogen atom and dipole moment changes in the IHT process.

Tunnel effects on inter- and intramolecular hydrogen transfer reactions of transient dihydro- and hexahydrocarbazoles

We examine inter- and intramolecular hydrogen-transfer processes in two related metastable dihydrocarbazoles in nonpolar solvents of different viscosity and compare them with similar transfer processes in transient hexahydrocarbazoles. N-ethyldiphenylamine (A′) and N-ethyl-2,6-dimethyldiphenylamine (A) photocyclize in their triplet states, yielding the triplet states of the zwitterionic dihydrocarbazoles 3Z′* and 3Z*, respectively, which subsequently relax to their metastable singlet ground states 1Z′ and 1Z′. In spite of their similarity, the two transients 1Z′ and 1Z stabilize by completely different pathways: the unsubstituted transient 1Z′ is converted into N-ethylcarbazole (C) and an N-ethyltetrahydrocarbazole (THC) by a bimolecular disproportionation reaction. The methylsubstituted intermediate 1Z is converted into a stable dihydrocarbazole (D) by a sigmatropic, intramolecular [1,8]-H-shift and by an intermolecular, mutual hydrogen-exchange reaction within the encounter complex 1(ZZ) which yields two molecules of D. The rates of the intra- and of the intermolecular transfer reaction of 1Z are governed by tunnel effects. The rate of the intramolecular tunnel process does not depend on solvent friction and becomes temperature independent at low temperatures. The rate of the intermolecular, reaction-controlled exchange reaction 1(ZZ) → 2 1D becomes also temperature independent if the solvent is fluid enough. In more viscous solvents the reaction becomes diffusion controlled and, therefore, strongly temperature dependent. The intermolecular disproportionation reaction 2 1Z′ → C + THC is also reaction controlled but no tunnel effects are observed.

The decomposition of trumethylgallium, triethylgallium and trimethylaluminum on Si(100)

This paper summarizes the results of a series of previous papers where the decomposition of trimethylgallium (TMGa), triethylgallium (TEGa), and trimethylaluminum (TMAI) on Si(100) wasstudied using TPD, XPS, EELS, and molecular beam techniques. The results show that a low coverges, the TMGa adsorbs molecularly. It then undergoes a series of intramolecular hydrogen transfer processes to yield methane, hydrogen, gallium, and adsorbed carbon. At higher exposures it is also possible to transiently populate a weakly bound state on top of the first monolayer of TMGa. The weakly bound TMGa decomposes above 450 K to yield dimethylgallium (DMGa) and CH x ( X = 3,4) groups. The DMGa and some of the CH x groups also desorb. However, some CH x groups remain bound to the substrate. There also is evidence for formation of traces of monomethylgallium. TMAI adsorbs as dimers. The dimers decompose upon heating to 400 K. Thereafter, the chemistry of TMAI decomposition is similar to that for TMGa except that methyl groups in the first monolayer of TMAI have a tendency to migrate to the silicon substrate. TEGa adsorbs as a monomer. It then decomposes via a β-hydrogen elimination process, producing ethylene. This pathway does not deposit carbon. At low temperatures and high incident TEGa fluxes there is a reaction pathway involving trapping of C 2H x groups which does deposit carbon. However, this is a minor reaction pathway. These results explain why TEGa is a much better source gas than TMGa when one needs to produce films with low carbon levels.

Mass spectral characterization of polymers. Primary thermal fragmentation processes in polyureas

AbstractThe primary fragmentation processes in the thermal decomposition of polymers were studied in detail on a series of structurally related polyureas by direct pyrolysis with a mass spectrometer. Our results indicate that polyureas I–III undergo a quantitative depolycondensation process analogous to that observed for N‐monosubstituted polyurethanes. The thermal decomposition of polyureas IV–VI proceeds by intramolecular hydrogen transfer processes that occur at higher temperatures with respect to depolycondensation. Polycarboxypiperazine VI is decomposed by a single‐stage decomposition mechanism that leads to fragments with amino end groups and carbon oxide.

Polymer reactions—Part VII: Thermal pyrolysis of polypropylene

Polypropylene has been pyrolysed in a carrier stream of helium from 388° to 900°C in both the programmed heating and flash pyrolysis modes. The products were on-line identified and quantitatively analysed by an interfaced GC peak identification system. The first order rate constants for pyrolysis are 3·7 × 10 −4 sec −1 and 4·0 × 10 −4 sec −1, respectively, for atactic and isotactic polypropylene at 388°C; the corresponding overall activation energies are 56 ± 6 and 51 ± 5 kcal mole −1. The main products in decreasing yields are 2,4-dimethyl-1-heptene, 2-pentene, propylene, 2 methyl-1-pentene and 2,4,6-trimethyl-1-nonene. Also isolated, but in much smaller quantities, are: ethane, isobutylene, 4,6-dimethyl-2-nonene, 2,4,6-trimethyl-1-heptene, 3-methyl-3,5-hexadiene and methane. Propylene is the product of an unzipping reaction. Most of the other products can be accounted for by a mechanism involving first, random scission of carbon-carbon bonds to produce methyl, primary and secondary alkyl radicals, followed by intramolecular hydrogen transfer processes. Methane and ethane are formed from the methyl radicals. All the products found in high yields are derived from the secondary alkyl radicals.

An ESR Study of Light- and Temperature-Induced Free Radical Conversions in Single Crystals of Dihydrouracil

In dihydrouracil a reversible free radical conversion between the two hydrogen-abstraction radicals 5-yl and 6-yl has been found. Thus, the 6-yl to 5-yl transformation takes place if an irradiated sample is cooled below 220 K and exposed to light with wavelengths below 450 nm. The reverse process is initiated by a subsequent heating of the sample. The kinetics for both the light- and temperature-induced processes have been studied. The results indicate that an intramolecular hydrogen transfer process is more probable than a corresponding intermolecular one. Comparison is made with other unsaturated and saturated pyrimidine derivatives.

Kinetics of ethylene polymerization and butadiene polymerization catalyzed by solutions of hydridochlorotris(triphenylphosphine)ruthenium(II)

The kinetics of ethylene polymerization and butadiene polymerization, catalyzed homogeneously by solutions of HRuCl(PPh 3) 3 in N,N-dimethylacetamide at 50–85 °C, are reported. The reaction rates are first order in catalyst, and are first order in substrate for the ethylene system, but are independent of the substrate concentration in the diene system. The data are interpreted in terms of a common mechanism analogous to that of Ziegler catalysts, where, following coordination of the monomer to metal, the polymer chain grows by insertion of the monomer at a polarized metal-carbon bond. The differences in kinetics are accounted for by the stronger complexing ability of the diene. The activation parameters obtained show that the butadiene polymerization rate is higher because of a more favorable entropy of activation. The catalyst becomes deactivated due to decarbonylation of the solvent, and possibly due to an intramolecular hydrogen transfer process involving a phenyl ring of the phosphine ligand.