Quantum dynamics study on multichannel dissociation and isomerization reactions of formaldehyde

We study quantum dynamics of the multichannel reactions of H(2)CO including the molecular and radical dissociation channels as well as the isomerization ones, H(2)CO-->trans-HCOH and trans-HCOH-->cis-HCOH. For this purpose, the previously developed potential energy function [T. Yonehara and S. Kato, J. Chem. Phys. 117, 11131 (2002)] is refined to give accurate transition state energies and to describe the radical dissociation channel. The cumulative reaction probabilities for the molecular dissociation and two isomerization channels are calculated by using the full Watson Hamiltonian. We also carry out wave packet dynamics calculations starting from the transition state region for the molecular dissociation. A contracted basis set for the angular coordinates is constructed to reduce the size of dynamics calculations. The intramolecular vibrational relaxation dynamics is found to be fast and almost complete within 300 fs. Using the energy filtered wave functions, the time propagation of HCOH population is obtained in the energy range from 81 to 94 kcal/mol. The branching ratio of the radical product is estimated by calculating the time dependent reactive fluxes to the molecular and radical dissociation products.     

A theoretical study of the photochemical isomerization reactions of furans from the triplet state

The mechanisms of photoisomerization reactions were investigated theoretically using a model system of 2-methylfuran with the CASSCF (10-electron/8-orbital active space) and MP2-CAS methods and the 6-311(d,p) basis set. After 2-methylfuran molecules are produced in the T(1) state by photoexcitation at 254 nm, intersystem crossing to the S(0) surface is the most probable pathway for deactivation. Relaxing to the S(0) state, the 2-methylfuran molecules can dissociate into 3-methylcyclopropene and carbon monoxide products. Otherwise, they may revert to singlet 2-methylfuran or undergo photorearrangement to produce 3-methylfuran. These stepwise mechanisms are consistent with the available experimental observations.     

Mesomorphic flexible chain polymers based on silicon

Poly(dialkylsiloxane)s and poly(dialkylsilane)s form a similar type of columnar mesophase. Although, the polysilanes are stiffer than polysiloxanes, both classes of polymers may be considered to be flexible due to the ability to form chain-folded crystals. Chain flexibility rather than the presence of chain stiffness determines whether the columnar mesophase is formed. A certain amphiphilic character does not appear to be required, as polysiloxanes with short side groups, e.g. polydiethylsiloxane display the same mesophase behaviour as polydialkylsilanes with long side chains and other nonpolar flexible chain molecules. The importance of the entropy gain upon conformational disordering is reflected in the increase in temperature stability with increasing alkyl side group length and the absence of mesophase behaviour in the case of the dimethyl substituted polymers     

Highly efficient triplet chain isomerization of Dewar benzenes: adiabatic rate constants from cage kinetics

Quantum yields as high as 120 were achieved for triplet-sensitized photoisomerizations of several Dewar benzene reactants, R, to the corresponding benzene products, P. Considerable chain amplification is maintained even at high conversion. All relevant rate constants of this triplet chain reaction were extracted from laser flash photolysis plus steady-state photolysis experiments. The crucial rate constant ka for adiabatic isomerization of the triplet reactant to triplet product (R* --> P*) cannot be directly measured because it is so large relative to the bimolecular rate of R* formation via sensitization. However, ka was obtained indirectly using a cage/encounter complex model to analyze the competition between the dissociation of encounter pairs with the sensitizer, e.g., S/R* --> S + R*, and the in-cage processes, S/R* --> S/P* --> S*/P, in nonviscous and viscous solvents. These measurements yielded ka values of (approximately 4-9) x 10(9) s(-1), which suggests that only a small (approximately 3 kcal/mol) energy barrier exists along the potential energy surface from R* to P*. Steady-state data indicated that the chain-terminating rate constant R* --> R is negligibly small, an ideal condition for chain amplification. Triplet energy transfer from a series of sensitizers to the Dewar benzene derivatives shows a nonclassical falloff in rate constants with decreasing sensitizer triplet energy, suggesting energy transfer to thermally distorted configurations having lower singlet-triplet energy gaps. As a result of distorted geometries of R* and P*, the chain-propagating energy transfer from P* to R proceeds with a rate constant of only approximately 2 x 10(7) M(-1) s(-1), despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of absorbed photons due to the high-energy content of the reactant together with the large chain length.     

Concerted chain isomerization in protonated alkylbenzenes

The first step in the unimolecular reaction of metastable protonated alkylbenzenes is the stretching of the C(benzene)—C(α) bond. Therefore, the intermediacy of a π-complex [C₆H₆, alkyl⁺] has often been proposed. In this work, the kinetic energy releases associated with the [tert-alkyl]⁺ product were measured for a large number of [C₆H₆–CnH] (n > 3) ions. At least for β-branched alkylbenzenes, it is shown that the chain isomerization which occurs prior to dissociation involves neither the [C₆H₆, CnH] π-complex nor a [C₆H₇⁺, alkene] ion–neutral complex. The data are explained by a concerted process in which the stretching of the C(benzene)—C(α) bond is accompanied by the migration of the tertiary β-hydrogen from C(β) to C(α).     

Precise isomerization polymerization of alkenylcyclohexanes: stereoregular polymers containing six-membered rings along the polymer chain

Pd and Ni diimine complexes catalyze the isomerization polymerization of alkenylcyclohexanes to afford polymers composed of alternating trans-cyclohexane-1,4-diyl rings and oligomethylene spacers with high selectivity. The melting points of the polymers vary from 130 to 226 °C depending on length of the oligomethylene spacer.     

Kinetics of carbonyl triplet reactions in water

The presence of urea during the gelling of AlPO₄–Al₂O₃ catalysts (AlPO₄/Al₂O₃ weight ratio =3) has been studied with respect to its effect on the porous texture and surface acidity of the resulting gels. An increase in pore volume and mean pore radius with no modification in surface acidity is found when the AlPO₄–Al₂O₃ catalyst is modified by the addition of 5 mol% urea in the AlPO₄ solution.

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, AlPO₄/Al₂O₃ ( AlPO₄/Al₂O₃=3) . , AlPO₄/Al₂O₃ 5 .% AlPO₄ - .

     

One-way isomerization and internal rotation in anthrylethylenes in the excited triplet state

1-(2-Anthryl)-2-(bromophenyl)ethylene and 1-(2-anthryl)-2-(4-methoxyphenyl)ethylene undergo cis→trans one-way isomerization in the excited triplet state through an adiabatic process from the cis-triplet to the trans-triplet states. The trans-isomers of these compounds undergo one-way internal rotation in the excited triplet state with an activation barrier of }7 kcal mol^?1 and a frequency factor of }10¹² s^?1, while no internal rotation takes place in the excited singlet state.     

Quantum chemical study on isomerization and transformation of hexabromocyclododecanes

Structural Chemistry - Hexabromocyclododecanes (HBCDs) have been frequently detected in environment. The isomerization between HBCDs and thermodynamic properties of unimolecular and bimolecular...     

The role of triplet repulsion in elementary steps of cyclic chain termination of oxidizing polymers and other substances

Parabolic model of the transition state was used for analysis of activation energies of reduction of benzoquinone (Q), aminyl (Am^*), and nitroxyl (AmO^*) radicals by peroxyl radicals of different structure. It was proved that radicals HO₂^* and R₂C(OH)O₂^* reduce Am^*, and AmO^* with zero activation energy due to low triplet repulsion in the transition state. On the contrary alkylperoxyl radicals reduce Am^* and AmO^* slowly due to high triplet repulsion and sufficiently high activation energy. That is why AmH and AmOH perform cyclic chain termination in oxidizing substances where peroxyls with OH-group are generated.     

Sensitive electrochemical assaying of DNA methyltransferase activity based on mimic-hybridization chain reaction amplified strategy

A mimic-hybridization chain reaction (mimic-HCR) amplified strategy was proposed for sensitive electrochemically detection of DNA methylation and methyltransferase (MTase) activity In the presence of methylated DNA, DNA-gold nanoparticles (DNA-AuNPs) were captured on the electrode by sandwich-type assembly. It then triggered mimic-HCR of two hairpin probes to produce many long double-helix chains for numerous hexaammineruthenium (III) chloride ([Ru(NH₃)₆]³⁺, RuHex) inserting. As a result, the signal for electrochemically detection of DNA MTase activity could be amplified. If DNA was non-methylated, however, the sandwich-type assembly would not form because the short double-stranded DNAs (dsDNA) on the Au electrode could be cleaved and digested by restriction endonuclease HpaII (HapII) and exonuclease III (Exo III), resulting in the signal decrement. Based on this, an electrochemical approach for detection of M.SssI MTase activity with high sensitivity was developed. The linear range for M.SssI MTase activity was from 0.05 U mL⁻¹ to 10 U mL⁻¹, with a detection limit down to 0.03 U mL⁻¹. Moreover, this detecting strategy held great promise as an easy-to-use and highly sensitive method for other MTase activity and inhibition detection by exchanging the corresponding DNA sequence.     

Functional monomers and polymers 159 synthesis and photochemical reactions of polymers containing thymine photodimer units in the main chain

Polyamides containing thymine photodimer units in the main chain were synthesized, and their photolysis by ultraviolet irradiation below 260 nm were studied in film state. Photodimers of thymine derivatives were obtained by photochemical reaction of the carboxylic acid derivatives of thymine in aqueous solution irradiated above 270 nm. An attempt was made to resolve the isomers of the photodimers, and the two kinds of cis isomers [cis–syn(head to head), and cis–anti(head to tail)] were isolated successfully. The polyamides were prepared by condensation of the photodimers with diamine using an activated ester method. The photodissociation of the thymine photodimer in the polymer main chain caused the breakage of the polymer chains, leading to the production of oligomers and dimer compounds containing thymine bases at the ends of the molecule. The dissociation rate of the polymer did not depend on the kind of the thymine photodimer which was in the main chain of the polymer.     

Syntheses and reactions of functional polymers. XLII. Preparation and use of polymers having N-hydroxy-succinimide unit in the chain

Styrene-maleic anhydride alternating copolymer was converted to N-hydroxymaleimide-styrene copolymer by reaction with hydroxylamine in pyridine at room temperature. The conversion was more than 90%. From this copolymer, N-acetoxy- or N-benzoyloxymaleimide-styrene copolymers were derived by action of acetic anhydride or benzoyl chloride in dimethylformamide at room temperature. Acylation of several primary amines was carried out effectively by use of these N-acyloxyimide-styrene copolymers. The reaction of the acetylated copolymer with diethylamine at room temperature afforded N-hydroxyimide copolymer.     

Radical transfer reactions in polymers

Free-radical transfer in polymers have been studied by pulse radiolysis and product analysis with the water-soluble polymers poly(vinyl alcohol), poly(acrylic acid), poly(methacrylic acid), polynucleotides and DNA. When OH radicals react with polymers the lifetime of the polymer radical thus created strongly depends on the number of radicals per polymer chain. Moreover, in negatively charged polymers the increased stiffness at high pH results in a remarkable increase of the lifetime of the radicals with respect to recombination. This allows a number of radical transfer reactions to occur (e.g. intramolecular H-transfer, β-fragmentation, depolymerization, reactions with additives).     

Synthesis and reactions of cysteine‐based telechelic polymers

The radical polyaddition of N‐4‐vinylbenzoyl‐L ‐cysteine methyl ester (VCM) was carried out in the presence of 2,2′‐azobisisobutyronitrile (AIBN, 3 mol %) as an initiator in dimethyl formamide (DMF) with monomer concentrations of 0.5 and 1.0 M at 60 °C for 20 h under nitrogen atmosphere to afford the corresponding polymers [poly(VCM), PVCM] with number‐average molecular weights (M_n)'s of 5300 and 18,000 in 92 and 95% yields, respectively. The obtained polymers had a heterotelechelic structure with thiol and olefin end moieties. The radical polymerization of methyl methacrylate and trityl methacrylate was carried out in the presence of PVCM with AIBN (3 mol %) as an initiator in DMF at 60 °C for 20 h to afford the block copolymers with M_n values in the range of 13,000–26,800 in good yields. PVCM [M_n = 18,000; polydispersity (M_w/M_n) = 1.56] was treated with 4 equiv of NaOH aq. (1.0 M) to afford the polymer having carboxyl groups in the side chain with a M_n of 17,300 and M_w/M_n of 1.88 in 95% yield and was also oxidized to polysulfoxide and polysulfone with 4 equiv of H₂O₂ per sulfide unit in CH₂Cl₂ (1.0 M) for 20 h. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 23–31, 2001     

Hole-induced quenching of triplet and singlet excitons in conjugated polymers

Quantitative information on the mechanisms and rates of hole (radical cation)-induced quenching of triplet and singlet excitons in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] has been acquired by a new technique, fluorescence-voltage time-resolved single molecule spectroscopy (FV-TR-SMS). FV-TR-SMS measures the fluorescence intensity of a single conjugated polymer molecule that is embedded in a capacitor-like device while simultaneously modulating the bias on the device and the irradiation intensity. The results demonstrate that triplet excitons are efficiently quenched by holes in conjugated polymers for hole densities >10(16) charges/cm(3), while singlet excitons are quenched with a much lower efficiency. Detailed kinetic analysis shows that the greater efficiency for quenching of triplets by holes (compared to that for singlets) is due to a >10(6) times longer exciton lifetime for triplets. In fact, the results suggest that while singlet quenching is less efficient due to a much shorter singlet lifetime, the rate constant for the quenching of singlets by holes actually exceeds that for triplets by several orders of magnitude.     

On the triplet nature of paramagnetic centers in conducting polymers

Temperature dependences of the magnetic susceptibility of solutions and powders of polyaniline synthesized by oxidative polymerization using two methods were measured by ESR in the temperature range from 123 to 423 K. The dependences observed can be described by the integral of susceptibility of the polymer fragments in the triplet state over the singlet—triplet splitting from E ₁ to E ₂ with constant weight. The susceptibility of the fragments was accepted to obey the Bleaney—Bowers equation. The most part of the experimental dependences can be presented as the sum of the temperature-independent susceptibility and the susceptibility obeying the Curie law. The both susceptibilities are described in a single manner at E ₁ < 0. In some cases, the comparison of the calculated and experimental dependences makes it possible to determine the length of the fragments L. The conditions of polymer synthesis, heating, and water vapors affect the E ₁ and E ₂ values. A similar analysis can be applied to other conducting polymers. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 316–321, February, 2008.     

Acid-catalyzed deprotection mechanism of tert-butyloxycarbonyloxy polymers in chemically amplified resists

A mechanism of acid-catalyzed deprotection of poly(4-tert-butyloxycarbonyloxy-styrene), PBOCST, in chemically amplified resists has been elucidated in terms of elementary processes by means of semiempirical molecular orbital calculations. It is concluded that the overall deprotection of tert-butyloxycarbonyl (t-BOC) group proceeds stepwise; i.e., (a) the first products are an acid carbonate and a tert-butyl cation; (b) a phenolic compound is the secondary and final product from the acid carbonate, which is realized by assistance with a counter anion accompanied by acid; (c) the counter anion also assists acid regeneration from the tert-butyl cation to produce isobutylene. The yield rate of the phenol is proportional to the product of concentrations of the polymer, the catalytic acid, and the counter anion. The activation energy (21 kcal/mol) calculated for the rate-determining step (a) is in good agreement with an experiment. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1035–1042, 1998