Bond Fission Pathways Research Articles

Theoretical re-investigation on the N–N bond breaking of N2O+ cations in the A2Σ+ and B2Π states at the CASPT2 level

Two-dimensional multistate potential energy surfaces along N–NO bond length and N–N–O bond angle of N2O+ in the A2Σ+ and B2Π states were calculated at the CASPT2/cc-pVQZ level. In comparison to the known decomposition mechanisms in linear structure, a new N–NO bond fission pathway was proposed in bent geometry for the A2Aʹ(A2Σ+) state with a lower barrier, leading to rotationally excited NO+(X1Σ+) and N(2D) fragments. Likewise, the respective contributions of the Aʹ and A″ components split from the B2Π state were clarified. Considering avoided crossing and the coupling of spin states, ro-vibrational distributions of the NO+ fragment ion observed in experiments are elucidated.

Experimental and theoretical analysis on decomposition and by-product formation process of (CF3)2CFCN mixture

Although sulfur hexafluoride (SF6) is widely used in gas-insulated electrical equipment due to the excellent electrical insulation and arc-interruption behaviors, its strong greenhouse effect with high glow warming potential (GWP) has been driving the research on environmentally friendly gases for SF6 alternatives. Recently, the heptafluoro-isobutyrontrile (CF3)2CFCN (3M NovecTM 4710) is reported to have the potential to replace SF6. The decomposition characteristic of gas insulating medium is one of the basic conditions to measure whether it has the feasibility of replacing SF6. However, the decomposition characteristic of (CF3)2CFCN and the formation mechanism of by-products are still unclear. In this paper, a series of AC corona discharge experiments were performed, and the gas by-products of (CF3)2CFCN mixed with CO2, N2 and air were qualitatively analyzed by gas chromatography mass spectrometry (GC/MS) method, respectively. Then, the decomposition mechanism of (CF3)2CFCN molecule and the formation mechanism of gas by-products were studied via density functional theory (DFT) method. The results show that the major gas by-products are CO, CO2, C2O3F6, CF4, C2F6, C3F6, C3F8, C2F4, (CF3)3CF, CF3CN, C2F5CN and CNCN, in addition to trace amounts of F3CC≡CCF3 and CF3CF=CFCF3. Among the three initial dissociation pathways of (CF3)2CFCN molecule, a C-CF3 bond fission pathway that produces CF3CFCN and CF3 radicals is identified as the thermodynamically favorable channel. The bond energy of C–CF3 bond calculated at M06-2X/def2-QZVP//M06-2X/6-311G(d,p) level is 94.7 kcal/mol.

Nucleophilic Substitution Reactions of 2,4‐Dinitrophenyl X‐Substituted‐Benzenesulfonates and Y‐Substituted‐Phenyl 4‐Nitrobenzenesulfonates with Azide Ion: Regioselectivity and Reaction Mechanism

The second‐order rate constants for reactions of 2,4‐dinitrophenyl X‐substituted‐benzenesulfonates (4a–4f) and Y‐substituted‐phenyl‐4‐nitrobenzenesulfonates (5a–5f) with ion have been measured spectrophotometrically. The reactions of 4a–4f proceed through SO and CO bond fission pathways competitively. Fraction of the SO bond fission decreases rapidly as the substituent X in the benzenesulfonyl moiety changes from an electron‐withdrawing group to an electron‐donating group. The Hammett plots for reactions of 4a–4f are linear with ρX = 1.87 and 0.56 for the SO and CO bond fission, respectively. The fact that the substituent X is further away from the reaction site of the CO bond fission than that of the SO bond fission is one reason for the smaller ρX value. The nature of the reaction mechanism (i.e., a stepwise mechanism in which expulsion of the leaving group occurs after the rate‐determining step) is also responsible for the smaller ρX value obtained from the CO bond fission. The Brønsted‐type plot for the reactions of 5a–5f is linear with βlg = −0.63, which is typical for reactions reported previously to proceed through a concerted mechanism. Effects of substituents X and Y on regioselectivity and reaction mechanism are discussed in detail.

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ↔ CH(3)O (CD(2)OH ↔ CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).

Low-temperature combustion chemistry of biofuels: pathways in the initial low-temperature (550 K–750 K) oxidation chemistry of isopentanol

The branched C(5) alcohol isopentanol (3-methylbutan-1-ol) has shown promise as a potential biofuel both because of new advanced biochemical routes for its production and because of its combustion characteristics, in particular as a fuel for homogeneous-charge compression ignition (HCCI) or related strategies. In the present work, the fundamental autoignition chemistry of isopentanol is investigated by using the technique of pulsed-photolytic Cl-initiated oxidation and by analyzing the reacting mixture by time-resolved tunable synchrotron photoionization mass spectrometry in low-pressure (8 Torr) experiments in the 550-750 K temperature range. The mass-spectrometric experiments reveal a rich chemistry for the initial steps of isopentanol oxidation and give new insight into the low-temperature oxidation mechanism of medium-chain alcohols. Formation of isopentanal (3-methylbutanal) and unsaturated alcohols (including enols) associated with HO(2) production was observed. Cyclic ether channels are not observed, although such channels dominate OH formation in alkane oxidation. Rather, products are observed that correspond to formation of OH viaβ-C-C bond fission pathways of QOOH species derived from β- and γ-hydroxyisopentylperoxy (RO(2)) radicals. In these pathways, internal hydrogen abstraction in the RO(2)⇄ QOOH isomerization reaction takes place from either the -OH group or the C-H bond in α-position to the -OH group. These pathways should be broadly characteristic for longer-chain alcohol oxidation. Isomer-resolved branching ratios are deduced, showing evolution of the main products from 550 to 750 K, which can be qualitatively explained by the dominance of RO(2) chemistry at lower temperature and hydroxyisopentyl decomposition at higher temperature.

Thermal decomposition of t-amyl methyl ether (TAME) studied by flash pyrolysis/supersonic expansion/vacuum ultraviolet photoionization time-of-flight mass spectrometry

Thermal decomposition of the oxygenated fuel component tert-amyl methyl ether (TAME) has been studied by flash pyrolysis up to 1250 K in a 20–100 μs time scale. Pyrolysis was followed by supersonic expansion to isolate intermediates and products, which are monitored by vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). The species detected, such as CH 3, C 2H 4, C 2H 5, C 4H 8, C 5H 10, C 3H 6O, and C 4H 8O, show competition between molecular elimination and bond fission pathways. The alkenes 2-methyl-1-butene ( 1) and 2-methyl-2-butene ( 2), the primary molecular elimination products of TAME, were separately pyrolyzed to evaluate the extent of secondary decompositions, as were the ketones (acetone and 2-butanone) produced by losses of two alkyl radicals. While vicinal elimination of methanol from TAME to form 1 and 2 in an approximate 3:1 ratio begins around 600 K and continues to dominate at higher temperatures, homolysis of TAME to form radicals onsets >825 K, yielding more acetone than 2-butanone. Contributions from secondary dissociations of the ketone and alkene products are evaluated.

High Resolution Photofragment Translational Spectroscopy Studies of the Ultraviolet Photolysis of Phenol-d5

The dissociation dynamics of gas phase phenol-d(5) molecules (C(6)D(5)OH) following excitation at numerous wavelengths in the range 275 > or = lambda(phot) > or = 193.3 nm have been investigated using the techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance enhanced multiphoton ionization spectroscopy. The results are compared with those from recent studies of the fully hydrogenated and fully deuterated isotopologues (C(6)H(5)OH and C(6)D(5)OD), and various halo- and methyl-substituted phenols. Analysis of the vibrational energy disposal within the phenoxyl-d(5) dissociation products identifies three distinct O-H bond fission pathways, involving nonadiabatic coupling to dissociative states of (1)pisigma* character, following initial pi* <-- pi excitation. Dissociation at lambda(phot) > 248 nm involves internal conversion (IC) to high vibrational levels of the electronic ground ((1)pipi) state and subsequent coupling to the lowest (1)pisigma* potential energy surface (PES) via a conical intersection (CI) between the (1)pipi/(1)pisigma* PESs at extended O-H bond lengths (R(O-H)). Once lambda(phot) < or = 248 nm, dissociation proceeds directly, via a (1)pipi*/(1)pisigma* CI. Both pathways yield phenoxyl-d(5) products in selected vibrational levels of the ground (X(2)B(1)) electronic state. The detailed energy disposal within the phenoxyl-d(5)(X) products shows many parallels with that deduced from companion studies of other phenol isotopologues and various substituted phenols, but a notable isotope effect is identified, thus providing yet greater insights into the factors controlling the vibrational energy disposal in the phenoxyl products. A hitherto unobserved O-H bond fission channel yielding phenoxyl-d(5) fragments in the electronically excited B(2)A(2) state is identified at the shortest excitation wavelength (lambda(phot) = 193.3 nm) and rationalized in terms of nonadiabatic coupling to, and subsequent dissociation on, the second excited (1)pisigma* PES. Selective deuteration as in phenol-d(5) causes little reduction in the intensity of the "slower" H atom products that are observed from all phenol systems, suggesting that C-H/D bond fission makes at most a minor contribution to this feature.

Flash Pyrolysis of Ethyl, n-Propyl, and Isopropyl Iodides as Monitored by Supersonic Expansion Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometry

The thermal decomposition of ethyl and propyl iodides, along with select isotopomers, up to 1300 K was performed by flash pyrolysis with a 20-100 mus time scale. The pyrolysis was followed by supersonic expansion to isolate the reactive intermediates and initial products, and detection was accomplished by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). The products monitored, such as CH(3), CH(3)I, C(2)H(5), C(2)H(4), HI, I, C(3)H(7), C(3)H(6), and I(2), provide for the simultaneous and direct observation of molecular elimination and bond fission pathways in ethyl and propyl iodides. In the pyrolysis of ethyl iodide, both C-I bond fission and HI molecular elimination pathways are competitive at the elevated temperatures, with C-I bond fission being preferred; at temperatures >or=1000 K, the ethyl radical products further dissociate to ethene + H atoms. In the pyrolysis of isopropyl iodide, both HI molecular elimination and C-I bond fission are observed and the molecular elimination channel is more important at all the elevated temperatures; the isopropyl radicals produced in the C-I fission channel undergo further decomposition to propene + H at temperatures >or=850 K. In contrast, bond fission is found to dominate the n-propyl iodide pyrolysis; at temperatures >or=950 K the n-propyl radicals produced decompose into methyl radical + ethene and propene + H atom. Isotopomer experiments characterize the extent of surface reactions and verify that the HI molecular eliminations in ethyl and propyl iodides proceed by a C1, C2 elimination mechanism (the 1,2 intramolecular elimination).

Theoretical Study of Isomerization and Decomposition Reactions for Methyl-nitramine

The complex potential energy surface and reaction mechanisms for the unimolecular isomerization and decomposition of methyl-nitramine (CH3NHNO2) were theoretically probed at the QCISD(T)/6-311+G*//B3LYP/6-311+G* level of theory. The results demonstrated that there are four low-lying energy channels: (i) the NN bond fission pathway; (ii) a sequence of isomerization reactions via CH3NN(OH)O; (IS2a); (iii) the HONO elimination pathway; (iv) the isomerization and the dissociation reactions via CH3NHONO (IS3). The rate constants of each initial step (rate-determining step) for these channels were calculated using the canonical transition state theory. The Arrhenius expressions of the channels over the temperature range 298-2000 K are k6(T)=1014.8e46.0/RT, k7(T)=1013.7e42.1/RT, k8(T)=1013.6e51.8/RT and k9(T)=1015.6e54.3/RT s1, respectively. The calculated overall rate constants is 6.9104 at 543 K, which is in good agreement with the experimental data. Based on the analysis of the rate constants, the dominant pathway is the isomerization reaction to form CH3NN(OH)O at low temperatures, while the NN bond fission and the isomerization reaction to produce CH3NHONO are expected to be competitive with the isomerization reaction to form CH3NN(OH)O at high temperatures.

Effect of Substituent on Regioselectivity and Reaction Mechanism in Aminolysis of 2,4-Dinitrophenyl X-Substituted Benzenesulfonates

[reaction: see text] We report on a kinetic study for the nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzensulfonates (X = 4-MeO, 1a, and X = 4-NO(2), 1c) with a series of primary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 degrees C. The reactions proceed through S-O and C-O bond fission pathways competitively. The fraction of the S-O bond fission increases as the attaching amine becomes more basic and the substituent X changes from 4-MeO to 4-NO(2), indicating that the regioselectivity is governed by the electronic nature of the substituent X as well as the basicity of amines. The S-O bond fission has been suggested to proceed through an addition intermediate with a change in the rate-determining step (RDS) at pK(a) degrees = 8.9 +/- 0.1. The electronic nature of the substituent X influences k(N)(S-O) and k(1) values, but not the k(2)/k(-1) ratios and the pK(a) degrees value significantly. Stabilization of the ground state (GS) through resonance interaction between the electron-donating substituent and the electrophilic center has been suggested to be responsible for the decreased reactivity of 1a compared to 1c. The second-order rate constants for the C-O bond fission exhibit no correlation with the electronic nature of the substituent X. The distance effect and the nature of the reaction mechanism have been suggested to be responsible for the absence of the correlation.

Thermal decomposition of peroxy acetyl nitrate CH3C(O)OONO2.

The thermal decomposition of peroxy acetyl nitrate (PAN) is investigated by low pressure flash thermolysis of PAN highly diluted in noble gases and subsequent isolation of the products in noble gas matrices at low temperatures and by density functional computations. The IR spectroscopically observed formation of CH3C(O)OO and H2CCO (ketene) besides NO2, CO2, and HOO implies a unimolecular decay pathway for the thermal decomposition of PAN. The major decomposition reaction of PAN is bond fission of the O-N single bond yielding the peroxy radical. The O-O bond fission pathway is a minor route. In the latter case the primary reaction products undergo secondary reactions whose products are spectroscopically identified. No evidence for rearrangement processes as the formation of methyl nitrate is observed. A detailed mapping of the reaction pathways for primary and secondary reactions using quantum chemical calculations is in good agreement with the experiment and predicts homolytic O-N and O-O bond fissions within the PAN molecule as the lowest energetic primary processes. In addition, the first IR spectroscopic characterization of two rotameric forms for the radical CH3C(O)OO is given.

Effect of amine nature on reaction rate and mechanism in nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with alicyclic secondary amines.

Second-order rate constants have been measured for reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with a series of alicyclic secondary amines. The reaction proceeds through S-O and C-O bond fission pathways competitively. The S-O bond fission occurs more dominantly as the amine basicity increases and the substituent X in the sulfonyl moiety becomes more strongly electron withdrawing, indicating that the regioselectivity is governed by the amine basicity as well as the electronic nature of the substituent X. The S-O bond fission proceeds through an addition intermediate with a change in the rate-determining step at pK(a) degrees = 9.1. The secondary amines are more reactive than primary amines of similar basicity for the S-O bond fission. The k(1) value has been determined to be larger for reactions with secondary amines than with primary amines of similar basicity, which fully accounts for their higher reactivity. The second-order rate constants for the S-O bond fission result in linear Yukawa-Tsuno plots while those for the C-O bond fission exhibit poor correlation with the electronic nature of the substituent X. The distance effect and the nature of reaction mechanism have been suggested to be responsible for the poor correlation for the C-O bond fission pathway.

Theoretical predictions of the decomposition mechanism of 1,3,3-trinitroazetidine (TNAZ)

A theoretical study of the decomposition pathways of 1,3,3-trinitroazetidine (TNAZ) is described. Possible decomposition transition-states, intermediates, and products are identified and structures, energies, and vibrational frequencies are determined at the B3LYP/6-31G(d,p) level for these species. Four major pathways are apparent. Two pathways are initiated by the fission of the N–NO2 and C–NO2 bonds to yield radical intermediates, while the other two pathways involve the molecular elimination of HONO. Energy profiles for the pathways and possible routes to some of the experimentally observed species of TNAZ decomposition are presented. The energy required to initiate the NO2 bond fission pathways are 4–8 kcal/mol lower than the HONO elimination pathways. In the gas phase, the NO2 elimination pathways will be the dominant routes for TNAZ decomposition. In the condensed phase, however, this trend may be reversed.

Regioselectivity and the nature of the reaction mechanism in nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with primary amines.

Second-order rate constants have been measured for the reaction of 2,4-dinitrophenyl X-substituted benzenesulfonates with a series of primary amines. The nucleophilic substitution reaction proceeds through competitive S-O and C-O bond fission pathways. The S-O bond fission occurs dominantly for reactions with highly basic amines or with substrates having a strong electron-withdrawing group in the sulfonyl moiety. On the other hand, the C-O bond fission occurs considerably for the reactions with low basic amines or with substrates having a strong electron-donating group in the sulfonyl moiety, emphasizing that the regioselectivity is governed by both the amine basicity and the electronic effect of the sulfonyl substituent X. The apparent second-order rate constants for the S-O bond fission have resulted in a nonlinear Brønsted-type plot for the reaction of 2,4-dinitrophenyl benzenesulfonate with 10 different primary amines, suggesting that a change in the rate-determining step occurs upon changing the amine basicity. The microscopic rate constants (k(1) and k(2)/k(-)(1) ratio) associated with the S-O bond fission pathway support the proposed mechanism. The second-order rate constants for the S-O bond fission result in good linear Yukawa-Tsuno plots for the aminolyses of 2,4-dinitrophenyl X-substituted benzenesulfonates. However, the second-order rate constants for the C-O bond fission show no correlation with the electronic nature of the sulfonyl substituent X, indicating that the C-O bond fission proceeds through an S(N)Ar mechanism in which the leaving group departure occurs rapidly after the rate-determining step.

C–C bond fission pathways of chloroalkenyl alkoxy radicals

Density-functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energetics of the chloroalkenyl alkoxy radicals arising from Cl-initiated reactions of isoprene as well as the transition states and products of their decomposition reactions. Geometry optimizations of the various species were performed at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-31G(d,p) level, and single-point energies were computed using second-order Møller–Plesset and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations. The activation and reaction energies of C–C bond scission of the alkoxy radicals are in the ranges of 12–25 and −3–22 kcal mol−1, respectively. Using the obtained activation barriers and transition state structures, we have calculated the high-pressure limit decomposition rates of the chloroalkenyl alkoxy radicals using transition state theory, ranging from 1×10−5 to 2×104 s−1. The results indicate that C–C bond decomposition of the chloroalkenyl alkoxy radicals is rather slow and likely plays a minor role in the Cl-isoprene reactions. Implications of the present results on the formation yields of methyl vinyl ketone, methacrolein, and 1-chloro-3-methyl-3-buten-2-one are discussed.

Photodissociative spectroscopy of the hydroxymethyl radical (CH2OH) in the 3s and 3px states

The absorption spectrum and photodissociation dynamics of the hydroxymethyl radical via its two lowest excited electronic states, 3s and 3px, are investigated in a supersonic molecular beam by the depletion, resonance enhanced multiphoton ionization, and photofragment yield spectroscopy methods. The measured origins of the electronic transitions to the 3s and 3px states agree with the most recent ab initio calculations. The vibronic bands of the 2 2A′(3px)←1 2A″ transition are much broader than those of the transition terminating in the 2 2A″(3pz) state, while the transition to the 1 2A′(3s) state appears structureless. The investigation of the deuterated analog CH2OD shows that near the onset of the transition to the 3s state, only the O–D bond fission pathway is important, while both H and D products are detected following excitation to the 3px state. The progressive broadening of the absorption features from the uppermost 3pz to the lowest 3s excited state is explained based on recent calculations of surface couplings to lower electronic states. These couplings also control the photodissociation dynamics and the reaction outcomes.

Dissociation Channels of the 1-Propenyl Radical and Its Photolytic Precursor cis-1-Bromopropene

The primary photodissociation dynamics of cis-1-bromopropene upon excitation at 193 nm and the unimolecular dissociation dynamics of the nascent 1-propenyl radical are investigated in a crossed laser−molecular beam apparatus. The lowest-energy dissociation barrier of the 1-propenyl radical is experimentally determined for the first time and is found to be 31.5 ± 2.0 kcal/mol, in substantial agreement with the theoretical calculations of Davis et al. There are three dissociation channels of the 1-propenyl radical seen in these experiments: C−C bond fission to give C2H2 + CH3, C−H bond fission to give propyne + H, and isomerization to the allyl radical followed by H atom loss to give allene + H. The data show that the C−C fission channel dominates the product branching for the dissociative radicals with the lowest internal energies. The branching to the H + allene channel at higher energies suggests that the calculated isomerization barrier is too high with respect to the H + propyne barrier. The data for the precursor molecule, cis-1-bromopropene, indicate that there are at least three competing primary product channels, HBr elimination and two C−Br bond fission pathways leading to 1-propenyl + Br formation. In addition, there is a product channel described by the total reaction C3H5Br → Br + H2 + C3H3. We also show evidence that the Br atom is formed in the excited (2P1/2) spin−orbit state from the C−Br bond fission product channels.

Decomposition Pathways of Peroxynitrous Acid: Gas-Phase and Solution Energetics

It was recently suggested that HOONO, which forms after protonation of the ONOO- anion under biological conditions, decomposes into HNO and (1Δg)O2. Subsequent workers argued that the mechanism for HOONO decomposition proceeds via homolytic bond fission, producing the radical pair OH and NO2, and another recent study argued that a cyclic form of the peroxynitrous acid results in the products H+ + O2(1Δg) + NO-. Calculations on the reaction pathway for the process showed that it required a high activation energy, and is thus implausible. High level ab initio molecular orbital theory including extrapolation to the complete basis set limit has been used to calculate the heats of formation of reactants and products for the homolytic bond fission pathways for decomposition of HOONO and the molecular pathway that yields HNO and 1O2 as well as the transition state for the latter process. These data are used to evaluate the probability of whether the decomposition of peroxynitrous acid can produce HNO and 1O2. Is...

Multiple‐Well, multiple‐path unimolecular reaction systems. II. 2‐methylhexyl free radicals

Vibrationally excited 2-methylhexyl radicals formed by shock wave activation or by chemical activation can isomerize by multiple pathways to form any of six stable isomers, can fragment by multiple CH and CC bond fission pathways, and can be collisionally stabilized. Master equation simulations of chemical activation and of shock wave activation are used to explore the generic behavior of this complicated coupled system. Selecting the argon pressure in chemical activation systems that produce the 2-methyl-1-hexyl radical isomer (1) can control the yield of specific isomers. Shock heating of 1 also shows a highly regular sequence of isomer formation. This regular behavior is because the first isomerization steps are faster than subsequent steps. Other radical isomers, such as 2-methyl-3-hexyl (3), do not show such regular behavior, because the first isomerization step is slower than subsequent steps. Incubation and unimolecular rate-constant fall-off are observed in the shock wave simulations. The unimolecular rate-constant fall-off for the coupled system produces low-pressure limiting rate constants proportional to [M]n, where n can be greater than unity. The fact that n can be greater than unity is a natural feature of multichannel coupled unimolecular reaction systems, but detection of the effect in experiments may be very demanding. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 246–261, 2001

Multiple‐Well, multiple‐path unimolecular reaction systems. II. 2‐methylhexyl free radicals

AbstractVibrationally excited 2‐methylhexyl radicals formed by shock wave activation or by chemical activation can isomerize by multiple pathways to form any of six stable isomers, can fragment by multiple CH and CC bond fission pathways, and can be collisionally stabilized. Master equation simulations of chemical activation and of shock wave activation are used to explore the generic behavior of this complicated coupled system. Selecting the argon pressure in chemical activation systems that produce the 2‐methyl‐1‐hexyl radical isomer (1) can control the yield of specific isomers. Shock heating of 1 also shows a highly regular sequence of isomer formation. This regular behavior is because the first isomerization steps are faster than subsequent steps. Other radical isomers, such as 2‐methyl‐3‐hexyl (3), do not show such regular behavior, because the first isomerization step is slower than subsequent steps. Incubation and unimolecular rate‐constant fall‐off are observed in the shock wave simulations. The unimolecular rate‐constant fall‐off for the coupled system produces low‐pressure limiting rate constants proportional to [M]n, where n can be greater than unity. The fact that n can be greater than unity is a natural feature of multichannel coupled unimolecular reaction systems, but detection of the effect in experiments may be very demanding. © 2001 John Wiley &amp; Sons, Inc. Int J Chem Kinet 33: 246–261, 2001