Reactions of C2H with 1- and 2-butynes: an ab initio/RRKM study of the reaction mechanism and product branching ratios

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311G** calculations of the C(6)H(7) potential energy surface are combined with RRKM calculations of reaction rate constants and product branching ratios to investigate the mechanism and product distribution in the C(2)H + 1-butyne/2-butyne reactions. 2-Ethynyl-1,3-butadiene (C(6)H(6)) + H and ethynylallene (C(5)H(4)) + CH(3) are predicted to be the major products of the C(2)H + 1-butyne reaction. The reaction is initiated by barrierless ethynyl additions to the acetylenic C atoms in 1-butyne and the product branching ratios depend on collision energy and the direction of the initial C(2)H attack. The 2-ethynyl-1,3-butadiene + H products are favored by the central C(2)H addition to 1-butyne, whereas ethynylallene + CH(3) are preferred for the terminal C(2)H addition. A relatively minor product favored at higher collision energies is diacetylene + C(2)H(5). Three other acyclic C(6)H(6) isomers, including 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne, can be formed as less important products, but the production of the cyclic C(6)H(6) species, fulvene, and dimethylenecyclobut-1-ene (DMCB), is predicted to be negligible. The qualitative disagreement with the recently measured experimental product distribution of C(6)H(6) isomers is attributed to a possible role of the secondary 2-ethynyl-1,3-butadiene + H reaction, which may generate fulvene as a significant product. Also, the photoionization energy curve assigned to DMCB in experiment may originate from vibrationally excited 2-ethynyl-1,3-butadiene molecules. For the C(2)H + 2-butyne reaction, the calculations predict the C(5)H(4) isomer methyldiacetylene + CH(3) to be the dominant product, whereas very minor products include the C(6)H(6) isomers 1,1-ethynylmethylallene and 2-ethynyl-1,3-butadiene.     

PAH formation under single collision conditions: reaction of phenyl radical and 1,3-butadiene to form 1,4-dihydronaphthalene

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.     

An optimized semidetailed submechanism of benzene formation from propargyl recombination

The self-reaction of propargyl (C3H3) radicals has been widely suggested as one of the key routes forming benzene in a variety of aliphatic flames. Currently, in the majority of aromatic models, the C3H3 + C3H3 submechanism often contains one or two C6H6 isomers and a few global reaction steps, which do not adequately represent the actual recombination chemistry. Recent experimental and theoretical studies on the direct propargyl recombination and subsequent C6H6 isomerization have provided sufficient information to revisit and revise the C3H3 + C3H3 reaction submechanism. In the present work, a semidetailed kinetic model consisting of seven isomeric C6H6 species and 14 reaction steps was constructed based on the most recent potential energy surface for this system. The trial model was subjected to systemic optimization by use of a recently developed physically bounded Gauss-Newton (PGN) method against detailed species profiles of direct propargyl recombination and 1,5-hexadiyne (15HD) isomerization obtained from experiments at high temperatures in a shock tube and at low temperatures in a flow reactor, which were all measured at very high pressure (shock tube) or atmospheric (flow reactor) conditions. Predictions of the optimized model were in excellent agreement with all experimental measurements. The optimized C3H3 + C3H3 reaction subset was also tested for flame modeling. Two different aromatic chemistry models that incorporate benzene formation from propargyl radicals as a single step reaction were modified to include the complete submechanism for propargyl recombination. The updated models predict significant percentages of three isomeric species [2-ethynyl-1,3-butadiene (2E13BD), fulvene, and benzene] in premixed fuel-rich acetylene and ethylene flames, reflecting the observed flame structures.     

Isomeric product distributions from the self-reaction of propargyl radicals

We have investigated the isomeric C6H6 product distributions of the self-reaction of propargyl (C3H3) radicals at two nominal pressures of 25 and 50 bar over the temperature range 720-1350 K. Experiments were performed using propargyl iodide as the radical precursor in a high-pressure single-pulse shock tube with a residence time of 1.6-2.0 ms. The relative yields of the C6H6 products are strongly temperature dependent, and the main products are 1,5-hexadiyne (15HD), 1,2-hexadiene-5-yne (12HD5Y), 3,4-dimethylenecyclobutene (34DMCB), 2-ethynyl-1,3-butadiene (2E13BD), fulvene, and benzene, with the minor products being cis- and trans-1,3-hexadiene-5-yne (13HD5Y). 1,2,4,5-Hexatetraene (1245HT) was observed below 750 K but the concentrations were too low to be quantified. The experimentally determined entry branching ratios are: 44% 15HD, 38% 12HD5Y, and 18% 1245HT, which is efficiently converted to 34DMCB. Following the initial recombination step, various C6H6 isomers are formed by thermal rearrangement. The experimentally observed concentrations for the C6H6 species are in good agreement with earlier experiments on 15HD thermal rearrangement.     

Mechanistic studies of the pyrolysis of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d4, 1,2-butadiene, and 2-butyne by supersonic jet/photoionization mass spectrometry

The thermal decomposition of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d(4), 1,2-butadiene, and 2-butyne at temperatures up to 1520 K was carried out by flash pyrolysis on a approximately 20 mus time scale. The reaction products were isolated by supersonic expansion and detected by single-photon (lambda = 118 nm) vacuum-ultraviolet time-of-flight mass spectrometry (VUV-TOFMS). Direct detection of CH(3) and C(3)H(3), as well as C(3)H(4), C(4)H(4), and C(4)H(5) products, provides insight into the initial steps involved in the complex pyrolysis of these C(4)H(6) species below T = 1500 K. The similar pyrolysis product distributions for the C(4)H(6) isomers on such a short time scale support the previously proposed mechanism of facile isomerization of these species. Isomerization of 1,3-butadiene to 1,2-butadiene and subsequent C-C bond fission of 1,2-butadiene to produce CH(3) and C(3)H(3) (propargyl) are most likely the primary initial radical production channel in the 1,3-butadiene pyrolysis.     

The association reaction between C2H and 1-butyne: a computational chemical kinetics study

The potential energy surfaces (PES) for the reaction of the C(2)H radical with 1-butyne (C(4)H(6)) have been studied using the CBS-QB3 method. Density functional B3LYP/cc-pVTZ and M06-2X/6-311++G(d,p) calculations have also been performed to analyze the reaction energetics. For detailed theoretical calculation on the total reaction mechanism, the initial association reactions on more and less substituted C atoms of 1-butyne are treated separately followed by a variational transition state theory (VTST) calculation to obtain reaction rates. The successive unimolecular reactions from the association reaction complexes are subjected to Rice-Ramsperger-Kassel-Marcus (RRKM) calculations for reaction rate constants and product branching ratios. The calculated rate constants in the temperature range 70-295 K for both the association reactions are found to be highly temperature dependent at low temperatures, which is contrary to the experimental findings of temperature independent association rates. We have explained this observation with the help of variational nature of the transition states, and we found a "loose" transition state at low temperatures. The calculated product branching ratios for the unimolecular reactions generally agree with the available experimental data, although some channels show a significant method dependency and therefore the correlation with experiment is lost to some extent. Our detailed reaction energetics calculations confirm that the C(2)H + C(4)H(6) reaction proceeds without an entrance barrier and leads to the important products ethynylallene + CH(3), 1,3-hexadiyne + H, 3,4-hexadiene-1-yne + H, 2-ethynyl-1,3-butadiene + H, 3,4-dimethylenecyclobut-1-ene + H and fulvene + H exothermic by 25-75 kcal mol(-1), with strong dependence of the product distribution on the association mode of C(2)H with C(4)H(6), making these reactions fast under low temperature conditions of Titan's atmosphere. Therefore this study can provide a detailed picture of the complex hydrocarbon formation mechanism in the upper atmosphere.     

Photolysis of organopolysilanes. The reaction of photochemically generated methylphenylsilylene with conjugated dienes

Photolysis of 2-phenylheptamethyltrisilane (I) in the presence of acyclic and cyclic conjugated dienes has been investigated using both a high-pressure mercury lamp with a quartz filter and a low-pressure mercury lamp with a Vycor filter. Irradiation of I in the presence of 1,3-butadiene, isoprene or 2,3-dimethylbutadiene with a high-pressure mercury lamp gave a product arising from photochemical isomerization of a silacyclopropane derivative and a compound apparently formed by 1,4-silylene addition, along with a 1/1 “ene” adduct of the diene to a photo-rearranged intermediate containing the silicon—carbon double bond. Irradiation of I in the presence of the conjugated diene with a low-pressure mercury lamp, followed by treatment of the product with methanol, afforded a methoxysilane arising from methanolysis of the corresponding silacyclopropane, together with the isomerization product, silacyclopentene and rearranged addition product. Irradiation of I in the presence of cyclopentadiene with a high-pressure mercury lamp produced methylphenylsilylcyclopentadiene, while irradiation of a similar mixture with a low-pressure mercury lamp followed by treatment with methanol gave 4-(methoxymethylphenylsilyl)-1-cyclopentene. With 1,3-cyclooctadiene, the photochemically generated methylphenylsilylene afforded many types of addition product. Photolysis of I in the presence of 1,3-cyclohexadiene, however, afforded none of the silylene addition products.     

Photochemistry of benzylallene: ring-closing reactions to form naphthalene

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S(0)-S(1) origin at 37,483 cm(-1). UV-UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump-probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV-vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S(0)-S(1) origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C(10)H(8) isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C(10)H(10) and C(10)H(9) potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.     

Reactions of diacetylene radical cation with ethylene

Ion-molecule reactions and energy-resolved mass spectrometry have been used to determine the structures of the products formed in the reaction of diacetylene radical cation with ethylene in a flowing afterglow-triple quadrupole instrument. The structure of the adduct ion, C(6)H(6)(.+), has been determined to be that of singly ionized benzene. The reaction thus presents a first example of the ability of diacetylene radical cation to undergo an aromatic ring forming reaction. The other products formed in the reaction are m/z 52, C(4)H(4)(.+), and m/z 39, C(3)H(3)(+). Isotopic labeling studies show that C(4)H(4)(.+) and C(3)H(3)(+) are formed with nearly statistical hydrogen incorporation, indicating a complex mechanism that scrambles all protons.     

Low-temperature rate coefficients for the reaction of ethynyl radical (C2H) with benzene

The reaction of the C2H radical with benzene is studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Delta) chemiluminescence from the C2H + O2 reaction. Measurements at very low photolysis energy are performed using CF3C2H as the C2H precursor to study the influence of benzene photodissociation on the rate coefficient. Rate coefficients are obtained over a temperature range between 105 and 298 K. The average rate coefficient is found to be five times greater than the estimated value presently used in the photochemical modeling of Titan's atmosphere. The reaction exhibits a slight negative temperature dependence which can be fitted to the expression k(cm3 molecule(-1) s(-1)) = 3.28(+/-1.0) x 10(-10) (T/298)(-0.18(+/-0.18)). The results show that this reaction has no barrier and may play an important role in the formation of large molecules and aerosols at low temperature. Our results are consistent with the formation of a short lifetime intermediate that decomposes to give the final products.     

A glimpse at the chemistry of GeH2 in solution. Direct detection of an intramolecular germylene-alkene π-complex

The photochemistry of 3-methyl-4-phenyl-1-germacyclopent-3-ene (4) and a deuterium-labeled derivative (4-d(2)) has been studied in solution by steady state and laser flash photolysis methods, with the goal of detecting the parent germylene (GeH(2)) directly and studying its reactivity in solution. Photolysis of 4 in C(6)D(12) containing acetic acid (AcOH) or methanol (MeOH) affords 2-methyl-3-phenyl-1,3-butadiene (6) and the O-H insertion products ROGeH(3) (R = Me or Ac) in yields of ca. 60% and 15-30%, respectively, along with numerous minor products which the deuterium-labeling studies suggest are mainly derived from hydrogermylation processes involving GeH(2) and diene 6. The reaction with AcOH also affords H(2) in ca. 20% yield, while HD is obtained from 4-d(2) under similar conditions. Photolysis of 4 in THF-d(8) containing AcOH affords AcOGeH(3) and 6 exclusively, indicating that the nucleophilic solvent assists the extrusion of GeH(2) from 4 and alters the mechanism of the trapping reaction with AcOH compared to that in cyclohexane. Laser flash photolysis of 4 in hexanes yields a promptly formed transient exhibiting λ(max) ≈ 460 nm, which decays on the microsecond time scale with the concomitant growth of a second, much longer-lived transient exhibiting λ(max) ≈ 390 nm. The spectrum and reactivity of the 460 nm species toward various germylene trapping agents are inconsistent with those expected for free GeH(2); rather, the transient is assigned to an intramolecular Ge(II)-alkene π-complex of one of the isomeric substituted hydridogermylenes derived from a solvent-cage reaction between GeH(2) and its diene (6) coproduct, formed by addition of HGe-H across one of the C=C bonds. These conclusions are supported by the results of DFT calculations of the thermochemistry associated with π-complexation of GeH(2) with 6 and the formation of the isomeric vinylgermiranes and 1,2-hydrogermylation products. A different species is observed upon laser photolysis of 4 in THF solution and is assigned to the GeH(2)-THF complex on the basis of its UV-vis spectrum and rate constants for its reaction with AcOH and AcOD.     

Gas Phase Reaction of Tetrachlorogermane with Conjugated Dienes in the Presence of Hexachlorodisilane Initiator

Studies of the reaction of GeCl₄ with acyclic conjugated dienes (1,3-butadiene, 2-methyl-1,3- butadiene, 2,3-dimethyl-1,3-butadiene) and cyclopentadiene in the presence of Si₂Cl₆ initiator were carried out. With acyclic dienes, corresponding 1,1-dichloro-1-germacyclo-3-pentenes and 1,1-dichloro-1-silacyclo-3- pentenes are preferrably formed. With cyclopentadiene the main reaction products are cyclopentenyltrichlorogermanes and cyclopentenyltrichlorosilanes formed respectively by hydrogermylation and hydrosilylation of the starting diene. The explanation of the established rule is offered.     

Shock tube pyrolysis of 1,2,4,5-hexatetraene

1,2,4,5-Hexatetraene (1245HT) is, according to theory, a key intermediate to benzene from propargyl radicals in a variety of flames; however, it has only been experimentally observed once in previous studies of the C3H3 + C3H3 reaction. To determine if it is indeed an intermediate to benzene formation, 1245HT was synthesized, via a Grignard reaction, and pyrolysized in a single-pulse shock tube at two nominal pressures of 22 and 40 bar over a temperature range from 540 to 1180 K. At temperatures T < 700 K, 1245HT converts efficiently to 3,4-dimethylenecyclobutene (34DMCB) with a rate constant of k = 10(10.16) x exp(-23.4 kcal/RT), which is in good agreement with the one calculated by Miller and Klippenstein. At higher temperatures, various C6H6 isomers were generated, which is consistent with theory and earlier experimental studies. Thus, the current work strongly supports the theory that 1245HT plays a bridging role in forming benzene from propargyl radicals. RRKM modeling of the current data set has also been carried out with the Miller-Klippenstein potential. It was found that the theory gives reasonably good predictions of the experimental observations of 1245HT, 1,5-hexadiyne (15HD), and 34DMCB in the current study and in our earlier studies of 15HD pyrolysis and propargyl recombination; however, there is considerable discrepancy between experiment and theory for the isomerization route of 1,2-hexadien-5-yne (12HD5Y) --> 2-ethynyl-1,3-butadiene (2E13BD) --> fulvene.     

Bond-formation versus electron transfer: C-C-coupling reactions of hydrocarbon dications with benzene

The bimolecular reactions of several hydrocarbon dications C(m)H(n)(2+) (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of C(m)H(n)(2+) with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products C(m)H(n)(+) + C(6)H(6)(+). In addition, also dissociative electron transfer takes place, whereas proton transfer from the C(m)H(n)(2+) dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the C(m)H(n)(2+) dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the C(m)H(n)(2+) dications with neutral benzene under formation of intermediate C(m+6)H(n+6)(2+) species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.     

Absolute photoionization cross-section of the propargyl radical

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.     

Expected and unexpected photochemical reactions of acyl iodides

Photolysis of acyl iodides RCOI (R = Me, Me₂CH, Ph) under UV irradiation in toluene environment for 20–55 h proved to be a simple and efficient method of preparation of symmetrical α-diketones RCOCOR. In contrast, the photolysis under the same conditions of acyl iodides RCOI [R = Me(CH₂)₃, Me₃C] did not lead to the formation of the corresponding diacyls, and the reaction products were unexpected 1,1-bis(4-methylphenyl)pentane and a mixture of isomeric 3- and 4-methyl(tert-butyl)benzenes respectively. The most probable mechanism of their formation is the primary photochemical acylation of toluene in the aromatic ring followed by the photochemical reduction of the arising butyl 4-methylphenyl ketone in the case of the valeroyl iodide or the photochemical Norrish type I cleavage of isomeric 3- and 4-methylphenyl (tert-butyl) ketones in event of the pivaloyl iodide. In the photolysis of acetyl iodide (R = Me) in benzene or toluene alongside the diacetyl formation polyarylation process was observed of acylated and iodinated into the aromatic ring solvents with the formation of polymeric products with semiconductor and paramagnetic properties.     

A crossed molecular beams and ab initio study on the formation of C6H3 radicals. an interface between resonantly stabilized and aromatic radicals

The crossed molecular beams reaction of dicarbon molecules, C(2)(X(1)Σ(g)(+)/a(3)Π(u)) with vinylacetylene was studied under single collision conditions at a collision energy of 31.0 kJ mol(-1) and combined with electronic structure calculations on the singlet and triplet C(6)H(4) potential energy surfaces. The investigations indicate that both reactions on the triplet and singlet surfaces are dictated by a barrierless addition of the dicarbon unit to the vinylacetylene molecule and hence indirect scattering dynamics via long-lived C(6)H(4) complexes. On the singlet surface, ethynylbutatriene and vinyldiacetylene were found to decompose via atomic hydrogen loss involving loose exit transition states to form exclusively the resonantly stabilized 1-hexene-3,4-diynyl-2 radical (C(6)H(3); H(2)CCCCCCH; C(2v)). On the triplet surface, ethynylbutatriene emitted a hydrogen atom through a tight exit transition state located about 20 kJ mol(-1) above the separated stabilized 1-hexene-3,4-diynyl-2 radical plus atomic hydrogen product; to a minor amount (<5%) theory predicts that the aromatic 1,2,3-tridehydrobenzene molecule is formed. Compared to previous crossed beams and theoretical investigations on the formation of aromatic C(6)H(x) (x = 6, 5, 4) molecules benzene, phenyl, and o-benzyne, the decreasing energy difference from benzene via phenyl and o-benzyne between the aromatic and acyclic reaction products, i.e., 253, 218, and 58 kJ mol(-1), is narrowed down to only ～7 kJ mol(-1) for the C(6)H(3) system (aromatic 1,2,3-tridehydrobenzene versus the resonantly stabilized free radical 1-hexene-3,4-diynyl-2). Therefore, the C(6)H(3) system can be seen as a "transition" stage among the C(6)H(x) (x = 6-1) systems, in which the energy gap between the aromatic isomer (x = 6, 5, 4) is reduced compared to the acyclic isomer as the carbon-to-hydrogen ratio increases and the acyclic isomer becomes more stable (x = 1, 2).     

Formal intramolecular photoredox chemistry of meta-substituted benzophenones

[reaction: see text]. Photolysis of 3-(hydroxymethyl)benzophenone (1) in aqueous solution (pH < 3) results in clean formation of 3-formylbenzhydrol (2) at dilute (<10(-4) M) conditions. Evidence suggests that the highly efficient (Phi approximately 0.6) reaction involves a unimolecular mechanism and an overall formal intramolecular photoredox process, which requires electronic communication between the 1,3-positions of the benzene ring, an unprecedented example of the photochemical meta effect. The photoredox reaction was not observed in organic solvents, where only photoreduction of the benzophenone moiety was observed.     

Reaction of <Emphasis Type="Italic">N</Emphasis>-methyl-<Emphasis Type="Italic">N,N</Emphasis>-bis(silatranylmethyl)amine with trichlorosilane

N,N-Bis(silatranylmethyl)methylamine reacts with trichlorosilane to form the amine hydrochloride and perchloropolysilane, the product of polymerization of dichlorosilylene. In the presence of 1,3-butadiene the main products of the reaction are the amine hydrochloride and 1,1-dichlorosilacyclopentene-3, the product of 1,4-addition of dichlorosilylene to 1,3-butadiene.     

Calculational and experimental investigations of the pressure effects on radical-radical cross combination reactions: C2H5+C2H3

Pressure-dependent product yields have been experimentally determined for the cross-radical reaction C2H5 + C2H3. These results have been extended by calculations. It is shown that the chemically activated combination adduct, 1-C4H8*, is either stabilized by bimolecular collisions or subject to a variety of unimolecular reactions including cyclizations and decompositions. Therefore the "apparent" combination/disproportionation ratio exhibits a complex pressure dependence. The experimental studies were performed at 298 K and at selected pressures between about 4 Torr (0.5 kPa) and 760 Torr (101 kPa). Ethyl and vinyl radicals were simultaneously produced by 193 nm excimer laser photolysis of C2H5COC2H3 or photolysis of C2H3Br and C2H5COC2H5. Gas chromatograph/mass spectrometry/flame ionization detection (GC/MS/FID) were used to identify and quantify the final reaction products. The major combination reactions at pressures between 500 (66.5 kPa) and 760 Torr are (1c) C2H5+C2H3-->1-butene, (2c) C2H5 + C2H5-->n-butane, and (3c) C2H3+C2H3-->1,3-butadiene. The major products of the disproportionation reactions are ethane, ethylene, and acetylene. At moderate and lower pressures, secondary products, including propene, propane, isobutene, 2-butene (cis and trans), 1-pentene, 1,4-pentadiene, and 1,5-hexadiene are also observed. Two isomers of C4H6, cyclobutene and/or 1,2-butadiene, were also among the likely products. The pressure-dependent yield of the cross-combination product, 1-butene, was compared to the yield of n-butane, the combination product of reaction (2c), which was found to be independent of pressure over the range of this study. The [1-C4H8]/[C4H10] ratio was reduced from approximately 1.2 at 760 Torr (101 kPa) to approximately 0.5 at 100 Torr (13.3 kPa) and approximately 0.1 at pressures lower than about 5 Torr (approximately 0.7 kPa). Electronic structure and RRKM calculations were used to simulate both unimolecular and bimolecular processes. The relative importance of C-C and C-H bond ruptures, cyclization, decyclization, and complex decompositions are discussed in terms of energetics and structural properties. The pressure dependence of the product yields were computed and dominant reaction paths in this chemically activated system were determined. Both modeling and experiment suggest that the observed pressure dependence of [1-C4H8]/[C4H10] is due to decomposition of the chemically activated combination adduct 1-C4H8* in which the weaker allylic C-C bond is broken: H2C=CHCH2CH3-->C3H5+CH3. This reaction occurs even at moderate pressures of approximately 200 Torr (26 kPa) and becomes more significant at lower pressures. The additional products detected at lower pressures are formed from secondary radical-radical reactions involving allyl, methyl, ethyl, and vinyl radicals. The modeling studies have extended the predictions of product distributions to different temperatures (200-700 K) and a wider range of pressures (10(-3)-10(5) Torr). These calculations indicate that the high-pressure [1-C4H8]/[C4H10] yield ratio is 1.3+/-0.1.