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The kinetics and mechanism for the reaction of HCO with NO occurring by both singlet and triplet electronic state potential-energy surfaces (PESs) have been studied at the modified Gaussian-2 level of theory based on the geometric parameters optimized by the Becke-3 Lee–Yang–Parr/6-311G(d,p) method. There are two major reaction channels on both singlet and triplet PESs studied: one is direct H abstraction producing CO+HNO and the other is association forming a stable HC(O)NO (nitrosoformaldehyde) molecule. The dominant reaction is predicted to be the direct H abstraction occurring primarily by the lowest-energy path via a loose hydrogen-bonding singlet molecular complex, ON...HCO, with a 2.9-kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol). The commonly assumed HC(O)NO intermediate, predicted to lie below the reactants by 27.7 kcal/mol, has a high HNO-elimination barrier (34.5 kcal/mol). Bimolecular rate constants for the formation of the singlet products and their branching ratios have been calculated in the temperature range of 200–3000 K. The rate constant for the disproportionation process producing HNO+CO, found to be affected strongly by multiple reflections above the well of the complex at low temperature, is predicted to be k(HNO)=3.08×10-12 T0.10 exp(242/T) for 200–500 K, and 1.72×10-16 T1.47 exp(888/T) for 500–3000 K in units of cm3 molecule-1 s-1. The high- and low-pressure rate constants for the association process forming HC(O)NO can be represented by k∞=4.42×10-11 T0.25 exp(-28/T) cm3 molecule-1 s-1 (200–3000 K) and k0=7.30×10-16 T-5.75 exp(-719/T) (200–1000 K) and 1.82×102 T-11.92 exp(1846/T) (1000–3000 K) cm6 molecule-2 s-1 for N2-buffer gas. The absolute values of total rate constant, predicted to be weakly dependent negatively on temperature but positively on pressure, are in close agreement with most experimental data within their reported errors. [ABSTRACT FROM AUTHOR]

The rate constants for the NCN+NO reaction have been measured by laser photolysis/laser-induced fluorescence technique in the temperature range of 254–353 K in the presence of He (40–600 Torr) and N2 (30–528 Torr) buffer gases. The NCN radical was produced from the photodissociation of NCN3 at 193 nm and monitored with a dye laser at 329.01 nm. The reaction was found to be strongly positive-pressure dependent with negative-temperature dependence, as was reported previously. The experimental data could be reasonably accounted for by dual-channel Rice–Ramsperger–Kassel–Marcus calculations based on the predicted potential-energy surface using the modified Gaussian-2 method. The reaction is predicted to occur via weak intermediates, cis- and trans-NCNNO, in the 2A″ state which crosses with the 2A′ state containing more stable cis- and trans-NCNNO isomers. The high barriers for the fragmentation of these isomers and their trapping in the 2A′ state by collisional stabilization give rise to the observed positive-pressure dependence and negative-temperature effect. The predicted energy barrier for the fragmentation of the cis-NCNNO (2A′) to CN+N2O also allows us to quantitatively account for the rate constant previously measured for the reverse process CN+N2O→NCN+NO. [ABSTRACT FROM AUTHOR]

Rate coefficients of the reaction S+O2 with Ar under 50 Torr in the temperature range 298–878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. Our measurements show that k(298 K)=(1.92±0.29)×10-12 cm3 molecule-1 s-1, in satisfactory agreement with previous reports. New data determined for 505–878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T)=(9.02±0.27)×10-19T2.11±0.15 exp[(730±120)/T] cm3 molecule-1 s-1 for 298≤T≤3460 K. Theoretical calculations at the G2M (RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO(1A′) dominates at T<500 K, whereas channels involving formation of SOO(3A″) followed by isomerization to SO2 before dissociation, and formation of SOO(1A″) followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper reports the first quantitative ab initio prediction of the disproportionation/combination ratio of alkyl+alkyl reactions using CH[sub 3]+C[sub 2]H[sub 5] as an example. The reaction has been investigated by the modified Gaussian-2 method with variational transition state or Rice–Ramsperger–Kassel–Marcus calculations for several channels producing (1) CH[sub 4]+CH[sub 2]CH[sub 2], (2) C[sub 3]H[sub 8], (3) CH[sub 4]+CH[sub 3]CH, (4) H[sub 2]+CH[sub 3]CHCH[sub 2], (5) H[sub 2]+CH[sub 3]CCH[sub 3], and (6) C[sub 2]H[sub 6]+CH[sub 2] by H-abstraction and association/decomposition mechanisms through singlet and triplet potential energy paths. Significantly, the disproportionation reaction (1) producing CH[sub 4]+C[sub 2]H[sub 4] was found to occur primarily by the lowest energy path via a loose hydrogen-bonding singlet molecular complex, H[sub 3]C··HC[sub 2]H[sub 4], with a 3.5 kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol), instead of a direct H-abstraction process. Bimolecular reaction rate constants for the formation of the above products have been calculated in the temperature range 300–3000 K. At 1 atm, formation of C[sub 3]H[sub 8] is dominant below 1200 K. Over 1200 K, the disproportionation reaction becomes competitive. The sum of products (3)–(6) accounts for less than 0.3% below 1500 K and it reaches around 1%–4% above 2000 K. The predicted rate constant for the disproportionation reaction with multiple reflections above the complex well, k[sub 1]=5.04×T[sup 0.41] exp(429/T) at 200–600 K and k[sub 1]=1.96×10[sup -20] T[sup 2.45] exp(1470/T) cm[sup 3] molecule[sup -1] s[sup -1] at 600–3000 K, agrees closely with experimental values. Similarly, the predicted high-pressure rate constants for the combination reaction forming C[sub 3]H[sub 8] and its reverse dissociation reaction in the temperature range 300–3000 K, k[sub 2][sup ∞]=2.41×10[sup -10] T[sup -0.34] exp(259/T) cm[sup 3] molecule[sup -1] s[sup -1] and k[sub -2][sup ∞]=8.89×10[sup 22] T[sup -1.67]exp(-46 037/T) s[sup -1], respectively, are also in good agreement with available experimental data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The mechanism for the CH[sub 3]+C[sub 2]H[sub 5]OH reaction has been investigated by the modified Gaussian-2 method based on the geometric parameters of the stationary points optimized at the B3LYP/6-311+G(d,p) level of theory. Five transition states have been identified for the production of CH[sub 4]+CH[sub 3]CHOH (TS1), CH[sub 4]+CH[sub 3]CH[sub 2]O (TS2), CH[sub 4]+CH[sub 2]CH[sub 2]OH (TS3), CH[sub 3]OH+CH[sub 3]CH[sub 2] (TS4), and CH[sub 3]CH[sub 2]OCH[sub 3]+H (TS5) with the corresponding barriers 12.0, 13.2, 16.0, 44.7, and 49.9 kcal/mol, respectively. The predicted rate constants and branching ratios for the three lower-energy H-abstraction reactions were calculated using the conventional and variational transition state theory with quantum-mechanical tunneling corrections for the temperature range 300–3000 K. The predicted total rate constant, k[sub t]=8.36×10[sup -76] T[sup 20.00] exp(5258/T) cm[sup 3] mol[sup -1] s[sup -1] (300–600 K) and 6.10×10[sup -25] T[sup 4.10]exp(-4058/T) cm[sup 3] mol[sup -1] s[sup -1] (600–3000 K), agrees closely with existing experimental data in the temperature range 403–523 K. Similarly, the predicted rate constants for CH[sub 3]+CH[sub 3]CD[sub 2]OH and CD[sub 3]+C[sub 2]H[sub 5]OD are also in reasonable agreement with available low temperature kinetic data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The mechanisms for HO[sub 2]+NO and its reverse reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the B3LYP/6-311+G(3df,2p) level and their energies refined by single-point calculations with the highest scheme of the modified Gaussian-2 method. Ab initio results show that formation of HO+NO[sub 2] from HO[sub 2]+NO by the direct fragmentation of the peroxynitrous acid, HOONO intermediate, is predominant; the alternative path occurring by the isomerization of HOONO to HONO[sub 2] is 5.2 kcal/mol less favorable. The stepwise formation of HNO+O[sub 2] from HOONO is energetically unfavorable; the barriers for the direct H abstract reactions via singlet and triplet paths are found to be rather high also. Rate constant calculations show that the forward reaction is pressure independent below 10 atm; the reverse OH+NO[sub 2] reactions producing HONO[sub 2] and HOONO appear to be strongly pressure dependent; below 1 atm, the yield of HOONO from HO+NO[sub 2] is <2.5% at 300–400 K, it reaches 12.2% and 9.1%, respectively, at 300 and 400 K at 3.8×10[sup 4] Torr pressure. The low- and high-pressure rate constants with He as a third-body for the formation of HOONO and HONO[sub 2] from the HO+NO[sub 2] reaction can be expressed by k[sup 0] (HOONO)=3.15×10[sup 2] T[sup -12.3] exp(-585/T), k[sup 0] (HONO[sub 2])=3.32×10[sup -6] T[sup -8.8] exp(-1569/T)cm[sup 6] molecule[sup -2] s[sup -1] and k[sup ∞] (HOONO)=1.71×10[sup -10] T[sup -0.24] exp(100/T) and k[sup ∞] (HONO[sub 2])=4.74×10[sup -9] T[sup -0.82] exp(21/T) cm[sup 3] molecule[sup -1] s[sup -1], respectively, in the temperature range of 200–2000 K. The unimolecular decomposition rate constant (in Ar) of HNO[sub 3] can be expressed as k[sub d][sup ∞] (HNO[sub 3])=2.30×10[sup 23] T[sup -2.27] exp(-26317/T) s[sup -1] and k[sub d][sup 0] (HNO[sub 3])=1.27×10[sup 15] T[sup -6.55]exp(-26038/T) cm[sup 3] molecule[sup -1] s[sup -1], respectively. The predicted values are all in close agreement with experimental data for both forward and reverse processes. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The isomerization and decomposition of ClOO and OClO radicals and related Cl + O[sub 2] and O + ClO reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the PW91PW91/6-311+G(3df) level and their energies refined by single-point calculations with the modified Gaussian-2 method. Predicted bond-dissociation energies of ClOO and OClO, D[sub 0](Cl-OO) = 4.6 and D[sub 0](O-ClO) = 58.5 kcal/mol, agree well with experimental values. Calculated rate constants for the Cl + O[sub 2] → ClOO reaction in 160-1000 K at the high- and low-pressure limits can be expressed by k[sub 1, sup ∞] = 1.8 ± 0.1 × 10[sup -10] cm³ molecule[sup -1] s[sup -1] and k[sub 1](He) = 1.66 × 10[sup -19] T[sup -5.34] exp(-675/T) and k[sub 1, sup 0](O[sub 2]) = 1.26 × 10[sup -16] T[sup -6.22] exp(-943/T) cm[sup 6] molecule[sup -2] s[sup -1]. For Ar and N[sub 2], theory underpredicts k[sub 1, sup 0](M) below room temperature due to significant contributions from the "chaperon" mechanism involving Cl-M complexes. The corresponding rate constants for O + ClO→OClO are predicted to be: k[sub 2, sup ∞] = 4.33 × 10[sup -11] T[sup -0.03] exp(43/T) cm³ molecule[sup -1] s[sup -1] and k[sub 2, sup 0] = 8.60 × 10[sup -21] T[sup -4.1] exp(-420/T) cm[sup 6] molecule[sup -2] s[sup -1] for 200-1000 K with N[sub 2] as the third body. The O + ClO reaction producing Cl + O[sub 2] via ClOO was found to be pressure-independent with k[sub 3] = 4.11 × 10[sup -11] T[sup -0.06] exp(42/T) cm³ molecule[sup -1] s[sup -1]. For the dissociation of ClOO, the rate constants are predicted to be: k[sub -1, sup ∞] = 6.17 × 10[sup 15] T[sup -0.46] 1.89 × 10[sup 7] exp(-2570/T) s[sup -1] and k[sub -1, sup 0] = T[sup -5.88] × exp(-3280/T) cm³ molecule[sup -1] s[sup -1] for 160-500 K with O[sub 2] as the third-body. The corresponding... [ABSTRACT FROM AUTHOR]

The kinetics and mechanism for the H + C[SUB2]H[SUB5]OH reaction, a key chain-propagation step in the high temperature decomposition and combustion of ethanol, have been investigated with the modified GAUSSIAN -2 (G2M) method using the structures of the reactants, transition states and products optimized at the B3LYP/6-311 + G(d,p) level of theory. Four transition states have been identified for the production of H[SUB2] + CH[SUB3](CHOH)(TS1), H[SUB2] + CH[SUB2]C[SUB2]OH (TS2), H[SUB2] + C[SUB2]H[SUB5]O (TS3), and H[SUB2]O + C[SUB2]H[SUB5] (TS4) with the corresponding barriers, 7.18, 13.30, 14.95, and 27.10 kcal/mol. The predicted rate constants and branching ratios for the three H-abstraction reactions have been calculated over the temperature range 300-3000 K using the conventional and variational transition state theory with quantum-mechanical tunneling corrections. The predicted total rate constant, k[SUBt] = 3.15 × 10[SUP3]T[SUP3.12] exp(-1508/T) cm[SUP3] mol[SUP-1]s[SUP-1], agrees reasonably with existing experimental data; in particular, the result at 423 K was found to agree quantitatively with an available experimental value. The small deviation between the predicted k[SUBt] and another set of experimental data measured at 295-700 K has been examined by kinetic modeling; the deviation is attributable to insufficient corrections for contributions from secondary reactions. [ABSTRACT FROM AUTHOR]

The mechanism for the reaction ClO+OClO has been investigated by ab initio molecular orbital and transition state theory calculations. Nine stable isomers of Cl[sub 2]O[sub 3] (including optical isomers) are located at the B3LYP/6-311+G(3df ) level. The transition states between pairs of isomers are explored and the stability of the isomers and their dissociation mechanisms are discussed. The relative stability predicted by the modified Gaussian-2 (G2M) method at the G2M//B3LYP/6-311+G(3df ) level is ClOCl(O)O>ClOOOCl(C[sub 2])>ClOOOCl(C[sub s])>ClClO[sub 3](C[sub 3v])>cyc-ClOOCl(O)>ClOOClO. The formation of ClOCl(O)O is dominant at low temperatures, taking place barrierlessly with the second and third order rate constant, k[sub 1][sup ∞]=3.0×10[sup -10] cm[sup 3] molecule[sup -1] s[sup -1] and k[sub 1][sup 0]=1.1×10[sup -17] T[sup -5.5] exp(-398/T) cm[sup 6] molecule[sup -2] s[sup -1] in the temperature range of 200–400 K for N[sub 2] as the third-body. Over 500 K, formation of ClOO+ClO becomes dominant and ClOCl+[sup 1]O[sub 2] is also competitive. Their overall rate constants can be given by k[sub ClOO]=1.0×10[sup -22] T[sup 2.8] exp(-78/T) and k[sub ClOCl]=9.6×10[sup -22] T[sup 2.4] exp(-1670/T)cm[sup 3] molecule[sup -1] s[sup -1], respectively, in the temperature range of 500–2500 K. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The self-reaction of ClO radicals has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces were predicted by the modified Gaussian-2 method. The reaction was shown to take place mainly over the singlet surface by forming ClOOCl (k[sub 1]) and ClOClO (k[sub 1[sup ′]]). These association processes were found to be strongly pressure dependent and the predicted total rate constants are in good agreement with the experimental data. The predicted second- and third-order rate constants for formation of ClOOCl and ClOClO can be expressed, respectively, in units of cm3 molecule-1 s-1 and cm6 molecule-2 s-1 by k[sub 1][sup ∞]=1.6×10[sup -9]T[sup -0.67] exp(-64/T), k[sub 1[sup ′]][sup ∞]=6.4×10[sup -9]T[sup -0.78] exp(-76/T), and k[sub 1][sup 0]=8.31×10[sup -20]T[sup -4.96] exp(-336/T), k[sub 1[sup ′]][sup 0]=1.72×10[sup -14]T[sup -6.99] exp(-926/T) in the temperature range 180–500 K for N[sub 2] as the third body. The observed T, P-dependent data could be best accounted for with the heat of formation of ClOOCl, Δ[sub f]H[sub 0][sup o](ClOOCl)=29.4±1 kcal/mol. The formation of Cl[sub 2]+O[sub 2] (2), Cl+ClOO (3), and Cl+OClO (4) products have been confirmed, with the predicted pressure-independent rate constants: k[sub 2]=1.09×10[sup -13]T[sup 0.66] exp(-1892/T); k[sub 3]=1.36×10[sup -13]T[sup 0.77]exp(-2168/T); k[sub 4]=6.26×10[sup -11]T[sup 0.005] exp(-2896/T), respectively, in units of cm3 molecule-1 s-1, covering the temperature range 200–1500 K. These results are also in reasonable agreement with existing experimental kinetic data. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The unimolecular decomposition of iso-C[sub 3]H[sub 7]OH has been studied with a modified GAUSSIAN-2 method. Among the six low-lying product channels identified, the H[sub 2]O-elimination process (2) via a four-member-ring transition state is dominant below 760 Torr over the temperature range 500-2500 K. At higher pressures and over 1200 K, the cleavage of a C-C bond by reaction (1) producing CH[sub 3]+CH[sub 3]C(H)OH is predicted to be dominant. The predicted low- and high-pressure limit rate constants for these two major product channels can be given by k[sup 0, sub 1] = 6.3 × 10[sup 42] T[sup -16.21] exp(-47 400/T), k[sup 0, sub 2] = 7.2 × 10[sup 44] T[sup -14.70] exp(--35 700/T) cm³ molecule[sup -1] s[sup -1], k[sup ∞, sub 1] = 8.0 × 10[sup 29] T[sup -3.75] exp(-45 800/T), and k[sup ∞, sub 2] = 2.0 × 10[sup 6] T[sup 2.12] exp(-30 700/T) s[sup -1], respectively. Predicted k[sup l] values compare reasonably with available experimental data; however, k[sub 2] values are lower than the experimentally determined apparent rate constant for C[sub 3]H[sub 6] formation, which may derive in large part from secondary radical reactions. Other minor decomposition products were predicted to have the barriers H[sub 2]+CH[sub 3]C(O)CH[sub 3], E[sup 0, sub 3]=82.8kcal/mol; H[sub 2]O+¹CH[sub 3]CCH[sub 3], E[sup 0, sub 4] = 77.9 kcal/mol; CH[sub 4] + CH[sub 3] C(H)O, E[sup 0, sub 5] = 84.3 kcal/mol; and CH[sub 4] + ¹CH[sub 3] COH, E[sup 0, sub 6] = 81.9 kcal/ mol. The triplet-singlet energy gap for CH[sub 3]CCH[sub 3] was predicted to be 5.2 kcal/mol, favoring the singlet state. [ABSTRACT FROM AUTHOR]

The unimolecular decomposition of CH[sub 3]CH[sub 2]OH has been investigated at the G2M (RCC2) level of theory. The decomposition reaction was found to be dependent strongly on pressure and temperature. Among the eight product channels identified, the H[sub 2]O-elimination process (1) via a four-member-ring transition state is dominant below 10 atm in the temperature range of 700-2500 K. At the high—pressure limit and over 1500 K, cleavage of the C-C bond by reaction (2) producing CH[sub 3] + CH[sub 2]OH is predicted to be dominant while the CH[sub 3]CH[sub 2]+OH channel (8) also becomes competitive. The predicted high-pressure rate constants for the two major product channels can be given by k[sub l] =7.0 × 10[sup 13] exp(-34200/T) and k[sub 2]=3.7 × 10[sup 26] T[sup -2.95] exp(-45 600/T) s[sup -1], which compare reasonably with earlier data and with our preliminary experimental result obtained in a shock tube and static cell study. At the internal energy corresponding to the O(¹D)+C[sub 2]H[sub 6] reaction (140.7 kcal/mol above C[sub 2]H[sub 5]OH), the predicted branching ratios for the production of CH[sub 3], C[sub 2]H[sub 5], and H[sub 2] are in qualitative agreement with the result of a recent cross-molecular beam experiment. [ABSTRACT FROM AUTHOR]

The reaction of OH with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces predicted by the G2M method are presented. The reaction was shown to take place primarily over the singlet surface by two main channels producing HO[sub 2]+Cl and HCl+O[sub 2]([sup 1]Δ), with the former being dominant. The predicted total rate constant, k[sub t]=5.27×10[sup -9] T[sup 1.03] exp (-40/T) cm[sup 3] molecule[sup -1] s[sup -1], and product branching ratios in the temperature range 200–500 K at P<200 atm agree satisfactorily with experimental values. The computed branching ratios, k[sub 2]/(k[sub 1]+k[sub 2])=0.073 for HCl+[sup 1]O[sub 2] and 0.045–0.048 for DCl+[sup 1]O[sub 2] in the temperature range 200–500 K based on the recent experimental heat of formation for HO[sub 2] (4.0±0.8 kcal/mol) compare closely with the experimental values, 0.07±0.03 and 0.05±0.02, respectively. At higher temperatures (1000–2500 K), the branching ratios increase slightly to 0.084–0.137 and 0.061–0.111 for the OH and OD reactions, respectively. The rate constant for HO[sub 2]+Cl and HCl+O[sub 2] production from OH+ClO in the temperature range, 500-2500 K, can be given by k[sub 1]=3.4×10[sup -13] T[sup 0.3] exp (725/T) and k[sub 2]=5.85×10[sup -19] T[sup 1.67] exp (1926/T) cm[sup 3] molecule[sup -1] s[sup -1], respectively. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The potential energy surface for the unimolecular decomposition of benzene and H+C[sub 6]H[sub 5] recombination has been studied by the ab initio G2M(cc, MP2) method. The results show that besides direct emission of a hydrogen atom occurring without an exit channel barrier, the benzene molecule can undergo sequential 1,2-hydrogen shifts to o-, m-, and p-C[sub 6]H[sub 6] and then lose a H atom with exit barriers of about 6 kcal/mol. o-C[sub 6]H[sub 6] can eliminate a hydrogen molecule with a barrier of 121.4 kcal/mol relative to benzene. o- and m-C[sub 6]H[sub 6] can also isomerize to acyclic isomers, ac-C[sub 6]H[sub 6], with barriers of 110.7 and 100.6 kcal/mol, respectively, but in order to form m-C[sub 6]H[sub 6] from benzene the system has to overcome a barrier of 108.6 kcal/mol for the 1,2-H migration from o-C[sub 6]H[sub 6] to m-C[sub 6]H[sub 6]. The bimolecular H+C[sub 6]H[sub 5] reaction is shown to be more complicated than the unimolecular fragmentation reaction due to the presence of various metathetical processes, such as H-atom disproportionation or addition to different sites of the ring. The addition to the radical site is barrierless, the additions to the o-, m-, and p-positions have entrance barriers of about 6 kcal/mol and the disproportionation channel leading to o-benzyne+H[sub 2] has a barrier of 7.6 kcal/mol. The Rice-Ramsperger-Kassel-Marcus and transition-state theory methods were used to compute the total and individual rate constants for various channels of the two title reactions under different temperature/pressure conditions. A fit of the calculated total rates for unimolecular benzene decomposition gives the expression 2.26x10[sup 14] exp(-53 300/T)s[sup -1] for T=1000-3000 K and atmospheric pressure. This finding is significantly different from the recommended rate constant, 9.0x10[sup 15] exp(-54 060/T) s[sup -1], obtained by kinetic modeling assuming only the H+C[sub 6]H[sub 5] product channel. At T=1000 K, the branching rat... [ABSTRACT FROM AUTHOR]

Dynamics of the thermal desorption of CD4 molecularly and dissociatively adsorbed on Pt(111) has been studied by laser-induced thermal desorption (LITD). CD4 adsorbates were prepared at the surface temperature of 55 K and the coadsorbates of CD3 and D were prepared by UV photolysis of CD4 adsorbates. The thermal desorption was induced by irradiating the surface with pulses of a frequency-doubled Nd:YAG laser and time-of-flight distributions of CD4 in each desorption channel were measured as a functions of the desorption angle (θ). The desorption of molecularly adsorbed CD4 shows a broad angular distribution (cos2.5 θ) and its translational energy distribution can be approximately described by the Maxwell–Boltzmann distribution with the temperature close to the estimated maximum surface temperature. In contrast, associatively formed CD4 desorbs sharply along the surface normal (cos31 θ) and its average translational energy is almost constant at 0.43 eV in the range of θ=0°–25°. The experimental results are compared with the calculated ones obtained from the statistical model developed by Ukraintsev and Harrison [J. Chem Phys. 101, 1564 (1994)]. Although the statistical model apparently succeeds in describing some of adsorption/desorption features by fitting the data of sticking coefficient measurements with molecular beams, it fails to predict the LITD data for the reverse process when their fitted data are applied in total. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rate constants for the formation of various products in the CH3+C2H2 reaction have been computed by multichannel RRKM calculations using the molecular and transition-state parameters predicted by the BAC-MP4 method. The results of the calculations agree quantitatively with experimental data obtained under varying conditions: T=300–2200 K, P=30–2500 Torr. At low temperatures (T<1300 K), the CH3+C2H2 reaction is dominated by the addition-stabilization process producing CH3C2H2. Under high-temperature (T>=1400 K) and atmospheric-pressure conditions, the reaction occurs primarily by the CH3-for-H displacement process producing CH3C2H, a likely source of the C3H3 radical (which has recently been shown to be a key precursor of C6H6 in hydrocarbon combustion reactions). [ABSTRACT FROM AUTHOR]

Rotational energy distributions of OD and OH radical reaction products desorbing from a smooth Pt(111) single crystal surface have been measured under closely similar experimental conditions by using the laser-induced fluorescence technique in conjunction with an UHV apparatus. Over the temperature ranges studied, nearly full rotational energy accommodation was observed for OD with a rotational to surface temperature ratio, Tr/Ts =0.96±0.04, and a somewhat lower ratio of 0.86±0.05 was obtained for OH. For both OD and OH the spin–orbit temperatures were the same as the rotational temperatures and no preference for any lambda-doublet state was observed. [ABSTRACT FROM AUTHOR]