A novel site-specific structure-activity relationship was developed for the site-specific addition of OH radicals to (poly)alkenes at 298 K. From a detailed structure-activity analysis of some 65 known OH + alkene and diene reactions, it appears that the total rate constant for this reaction class can be closely approximated by a sum of independent partial rate constants, ki, for addition to the specific (double-bonded) C atoms that depend only on the stability type of the ensuing radical (primary, secondary, etc.), that is, on the number of substituents on the neighboring C atom in the double bond. The (nine) independent partial rate constants, ki, were derived, and the predicted rate constants (kOH,pred = Sigmak(i)) were compared with experimental k(OH,exp) values. For noncyclic (poly)alkenes, including conjugated structures, the agreement is excellent (Delta < 10%). The SAR-predicted rate constants for cyclic (poly)alkenes are in general also within <15% of the experimental value. On the basis of this SAR, it is possible to predict the site-specific rate constants for (poly)alkene + OH reactions accurately, including larger biogenic compounds such as isoprene and terpenes. An important section is devoted to the rigorous experimental validation of the SAR predictions against direct measurements of the site-specific addition contributions within the alkene, for monoalkenes as well as conjugated alkenes. The measured site specificities are within 10-15% of the SAR predictions.

By normalizing the excimer emission profile with respect to the monomer emission profile, the &method provides a novel and an unambiguous method for determining kE, the reciprocal decay time of the pyrene excimer. With this value and those of k-' and 8, one can compute values of k,(t), point by point, to assess its time evolution. In this analysis, values of k-' are determined over a range of temperatures where the transient effects are small and then extrapolated via an Arrhenius expression to nearby temperatures where these effects are significant.We make two very important observations about the time profile of k,(t) under conditions (high temperatures and pyrene concentrations) where excimer dissociation followed by excimer reformation is expected to be important. We see first that kl(t) decreases, passes through a minimum, and then increases to a steady-state value. We attribute this increase to excimer reformation from dissociated pairs. The second important amsideration is that the steady-state value of kl found under these conditions is larger than that predicted for the case of irreversible processes and indicates that the local steady-state concentration is intermediate between that of irreversible pracesses and the initial bulk concentration of reactants. Acknowledgment. J.D. and M.A.W. thank NSERC (Canada) for its support of this research. J.M.G.M. also thanks JNICT (Portugal) for financial support and INVOTAN for a fellowship. Registry No. Pyrene, 129-00-0. References and Notes(1) This paper no. 11 in a series on transient effects on diffusionantrolled reactions by Toronto group and no. 25 in a similar series by the group in Nancy.(2) Rice, S. (9) Periasamy, N.; Doraiswamy, S.; Venkataraman, B.; Fleming, G. R. J. (21) (a) Martinho, J. M. G.; Campos, V. M.; Tencer, M.; Winnik, M. A. Macromolecules 1987,20, 1582-7. (b) Martinho, J. M. G.; Sienicki, K.; Blue, D.; Winnik, M. A. Macromolecules 1988,110,7773-7. (c) Martinho, J. M. G.; Tencer, M.; Campos, M.; Winnik, M. A. Macromolecules 1989, 22, 322-7. (d) Stukelj, M.; Martinho, J. M. G.; Winnik, M. A.; Quirk, R. P.The branching ratios of the two dominant methylene sources in C2H2/O/H systems C2H2 + 0 -CH#BI) + CO (r2a) or HCCO + H (r2b) and HCCO + H -CH2('AI) + CO (r3a) or CH#BI) + CO (r3b) have been determined, at 285 K, using discharge-flow/molecular beam mass spectrometry techniques (D-F/MBMS). The ratios were derived from the observed decreases of MBMS CH2 signals upon substituting 25% of the helium bath gas by 0.5 Torr of methane, which selectively scavenges CH2('Al); in the absence of CH, the singlet CH2 is mainly collisionally converted to CH2(!Bl). In this way, the ratios of the total formation rates of singlet and triplet CH2 are obtained. As the rate of reaction r3 is linked to the rate of reaction r2b by the known fraction of HCCO reacting with H, in competition with 0, both the ratios kt/kzb and k3s/k3b can be. extracted from the data. Thus, including also probably systematic errors, the HCCO yield of C2H2 + 0 is found to be k2,/kz = 85?% and...

The HCCO + N O reaction (r5) was investigated in C2H2/0/NO systems at a pressure of 2 Torr (He bath gas) using discharge flow-molecular beam mass spectrometry techniques (DF-MBMS). The first rate coefficient data at temperatures > 300 K are presented. The coefficient was measured relative to the known k(HCCO+O) from the changeof steady-state HCCO signals upon adding increasing amounts of NO. Thus, in the temperature range from 290 to 670 K, the k(HCCO+NO) coefficient was found to exhibit a slight but significant temperature dependence: k(T) = (1.0 f 0.3) X 10-lo e-(350*150)/(TIQ cm3 molecule-' (T = 290-670 K). The product distribution was determined at 700 K. The experiments relied on the fact that all reaction pathways yield either CO or C02. Ketenyl radicals, generated by quantitative reaction of a known amount of 0 atoms with C2H2 in high excess, were reacted with a large excess of NO, ensuring quantitative conversion into the products C02 and CO. The product distribution was essentially deduced from the ratios [CO2]fomd/[O]input and [CO]for,d/ [O]inPut. Formation of CO together with CHz(3B1) in a minor (25 f 15%) channel of the C2H2 + 0 reaction was taken into account. Small corrections for secondary reactions such as HCCO + 0 -H + 2 CO were made by kinetic modeling. Thus, the following yields were obtained: for HCCO + N O -(CHNO) + CO, 77 f 9%; for HCCO + N O -(CHN) + C02, 23 f 9%. Strong product signals were also observed at m / e = 43 and m / e = 27, confirming that CHNO isomers and C H N isomers are formed along with CO and COz, respectively.Theoretical predictions regarding the CO/CO2 yield ratio, presented in a companion paper in this issue, can be reconciled with the experimental product distribution only when an as yet unidentified entrance pathway to the formyl isocyanate intermediate is assumed to exist and to be thermally accessible.

In earlier work on the room temperature oxidation of CzHz by 0 atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel l b of the CzHz + 0 reaction, and (ii) delayed formation via the secondary reaction 3 involving the products HCCO and H of the other primary channel la:Presently, it was confirmed by a detailed sensitivity analysis that the precise shapes of the resulting total methylene concentration-versus-time profiles in C2H2/0 systems depend strongly on the kla/klb branching ratio. Along that line, the important parameter kla/klb was determined from relative CH2 concentration-versus-time profiles measured in a variety of CzH2/0/H systems using Discharge FlowMolecular Beam sampling Mass Spectrometry techniques (DF-MBMS). The data analysis was carried out by deductive kinetic modelling; the method, as applied to profile shapes, is discussed at length. Via this novel, independent approach, the C H Z (~B~) yield of the two-channel C2Hz + 0 reaction was determined to be k l b / k 1 = 0.17 5 0.08. The indicated 2v error includes possible systematic errors due to uncertainties in the rate constants of other reactions that influence the shapes of the CH2 profiles. The present result, which translates to an HCCO yield kl,/kl = 0.83 i-0.08, is in excellent agreement with other recent determinations. The above mechanism, with the subsequent reactions that it initiates, also reproduces the measured absolute [CzHz], [O], and [HI profiles with an average accuracy of 5%, thus validating the consistency of the CzHz/O/H reaction model put forward here.

In this work, the novel reaction mechanism initiated by the fast CH( 2 Π) + C 2 H 2 f C 3 H 2 + H reaction (r13a) and followed by C 3 H 2 + O f C 2 H + HCO (or H + CO) (r25a) was established as the dominant C 2 H formation pathway in low-pressure acetylene/atomic oxygen flames at 600 K. The C 2 H 2 /O/H flames were investigated in an isothermal discharge-flow reactor at a pressure of 2 Torr, with He as bath gas. Concentration vs reactiontime data were obtained by molecular beam sampling and threshold ionization mass spectrometry. The crucial role of CH( 2 Π) was evidenced by CH 4 -addition experiments on room-temperature C 2 H 2 /O/H systems, where CH( 2 Π) and CH 2 ( 1 A 1 ) are the sole intermediates that react rapidly with CH 4 . The observed strong reduction of [C 3 H 2 ], [C 2 H], and [C 4 H 2 ] upon CH 4 addition could be correlated quantitatively with the known removal of CH( 2 Π) by CH 4 . The reaction channels r13a and r25a as sources of C 3 H 2 and C 2 H, respectively, were each established by quasi-steady-state analyses of the pertaining radicals in C 2 H 2 /O/H mixtures at 600 K. By a similar method, the observed C 3 H radicals could be attributed to a minor channel of the CH( 2 Π) + C 2 H 2 reaction (r13) parallel to that producing C 3 H 2 . At 600 K and 2 Torr, the following approximate product yields of reaction r13 were derived: 85 -19 +9 % C 3 H 2 plus H, and 15 -9 +19 % C 3 H plus H 2 . Concomitantly with the identification of reaction r25a as dominant C 2 H source, the rate constant of the reaction C 2 H + O (r19) was determined relative to the well-known kinetic coefficients of C 2 H + C 2 H 2 and C 2 H + O 2 : k 19 ) (9 ( 4) × 10 -11 cm 3 molecule -1 s -1 at 600 K. It is suggested that a sizeable fraction of the ethynyl radicals formed in fuel-rich hydrocarbon flames is produced likewise by oxidation of C 3 H x radicals (x ) 1-3) that arise in the fast reactions of CH(X 2 Π) and CH 2 (a 1 A 1 ) with C 2 H 2 .