The di-π-methane rearrangement of systems with simple vinyl moieties : Mechanistic and exploratory organic photochemistry

3,3-Dimethyl-1,1-diphenyl-1,4-pentadiene and two 5-substituted derivatives were synthesized and studied. The regioselectivity, stereochemistry, quantum efficiency, multiplicity, and excited state reaction rates were studied in each case. The parent hydrocarbon, 5-MeO-derivative, and 5-cyano-diene—all rearranged on direct irradiation to give vinylcyclopropanes. The first compound led to 3,3-dimethyl-2,2-diphenyl-1-vinylcyclopropane. The second afforded 3,3-dimethyl-2,2-diphenyl-1-(2'-methoxyvinyl)cyclopropane. The last gave 1-cyano-3,3-dimethyl-2-(2',2'-diphenylvinyl)cyclopropane. Thus, the vinyl and methoxyvinyl groups survive in the products intact, while the cyanovinyl group is incorporated in the three-ring. In the two substituted dienes, cis-reactant gave cis-product and trans-reactant gave trans-product, both where the substituent was on the vinyl group of the product and where it became a ring substituent. The substituted di-π-methane systems underwent only cis-trans isomerization on sensitization, while the parent, unsubstituted diene led to di-π-methane product on sensitized as well as direct photolysis. While the quantum yields for the hydrocarbon diene were the same at room temperature for the direct and sensitized runs, only the sensitized runs showed a temperature dependence of efficiency with a dramatic, 5-fold increase on a 46° temperature increase. Thus, evidence was obtained for a singlet rearrangement in all cases and a triplet process only in the case of the unsubstituted diene. A sizable activation energy was seen for the triplet but not for the singlet. The room temperature quantum yields in the direct irradiations were: φ(parent diene)=0.011, φ(trans-methoxydiene)=0.051, φ(cis-methoxy-diene)= 0.050, φ(trans-cyanodiene)=0.36, and φ(cis-cyano-diene) = 0.20. A competing side reaction was cis-trans isomerization but these quantum yields were lower. Single photon counting was employed to obtain excited singlet reaction and decay rates at low temperature (i.e. 77°K) and the method of magic multipliers was used to obtain room temperature rates. These were: k_r(parent diene) = 4.7 × 10⁸ sec^?1, k_r(trans-cyano-diene)= 1.5 ×10¹⁰ sec^?1, k_r(cis-cyano-diene)= 8.0 × 10⁹sec^?1, and k_r(trans-methoxy-diene) = 1.9 × 10⁹ sec^?1. The results are discussed in terms of excited state molecular structure.An SCF-CI molecular orbital treatment of the reaction was developed. This used a cyclopropyldicarbinyl diradical species, with Walsh cyclopropane basis orbitals, as representing the half-reacted species. The energy of formation of this species from vertical excited state reactant was calculated for all three dienes and an excellent correlation with observed excited singlet rates was obtained. Similarly, dissection of the excited diradical energy into bond components led to a correlation between regioselectivity and weakness of the three-ring bond broken in the regioselectivity-determining step. Evidence was adduced for localization of the excitation energy in S₁ of reactant in the diphenylvinyl chromophore with migration of electronic excitation into the cyclopropyldicarbinyl diradical moiety during the vinyl-vinyl bridging process. A general method for quantitatively partitioning excitation energy was developed and applied to the case in hand. Finally, there was predicted a greater probability of di-π-methane three-ring fission in the excited state compared to the diradical ground state where Grob fragmentation proved energetically more favorable.     

The Lifetime and Reactivity of Singlet Tetrachloropyridyl-4-nitrene

The principal direction in the photolytic decomposition of 4-azidotetrachloropyridine in methylene chloride solution involves the intermediate formation of singlet tetrachloropyridyl-4-nitrene, the lifetime of which amounts to 50 nsec. The nitrene reacts readily with the pyridine (k _pyr = 2.67·10⁷ mole^-1·sec^-1) with the formation of the corresponding pyridinium ylide, which has a characteristic absorption band in the UV spectrum with a maximum at 406 nm.     

Liquid phase catalytic oxidation of butene to methyl ethyl ketone: a kinetic study

The kinetics of oxidation of butene to methyl ethyl ketone was studied using a homogeneous PdCl₂-CuCl₂ catalyst in aqueous medium. The effect of PdCl₂ concentration, butene partial pressure and CuCl₂ concentration on the rate of reaction was investigated over a temperature range of 308–358 K, and a rate expression has been proposed. The activation energy of the reaction was found to be 3.022 × 10⁴ J mol⁻¹.     

Gas‐phase ozonolysis: Rate coefficients for a series of terpenes and rate coefficients and OH yields for 2‐methyl‐2‐butene and 2,3‐dimethyl‐2‐butene

The kinetics of the gas‐phase reactions of O₃ with a series of selected terpenes has been investigated under flow‐tube conditions at a pressure of 100 mbar synthetic air at 295 ± 0.5 K. In the presence of a large excess of m‐xylene as an OH radical scavenger, rate coefficients k(O₃+terpene) were obtained with a relative rate technique, (unit: cm³ molecule^?1 s^?1, errors represent 2σ): α‐pinene: (1.1 ± 0.2) × 10^?16, ³Δ‐carene: (5.9 ± 1.0) × 10^?17, limonene: (2.5 ± 0.3) × 10^?16, myrcene: (4.8 ± 0.6) × 10^?16, trans‐ocimene: (5.5 ± 0.8) × 10^?16, terpinolene: (1.6 ± 0.4) × 10^?15 and α‐terpinene: (1.5 ± 0.4) × 10^?14. Absolute rate coefficients for the reaction of O₃ with the used reference substances (2‐methyl‐2‐butene and 2,3‐dimethyl‐2‐butene) were measured in a stopped‐flow system at a pressure of 500 mbar synthetic air at 295 ± 2 K using FT‐IR spectroscopy, (unit: cm³ molecule^?1 s^?1, errors represent 2σ ): 2‐methyl‐2‐butene: (4.1 ± 0.5) × 10^?16 and 2,3‐dimethyl‐2‐butene: (1.0 ± 0.2) × 10^?15. In addition, OH radical yields were found to be 0.47 ± 0.04 for 2‐methyl‐2‐butene and 0.77 ± 0.04 for 2,3‐dimethyl‐2‐butene. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 394–403, 2002     

Decomposition of chemically activated ethyltrimethylgermane the arrhenius A-factors for rupture of group IVA—methyl bonds

The decomposition rate of chemically activated ethyltrimethylgermane from the reaction ¹CH₂ + (CH₃)₄Ge, where ¹CH₂ was produced from diazomethane photolysis at 3660 Å, is 8.6 × 10⁵ sec^?1. This result combined with RRKM theory and critical energy estimates yields an Arrhenius A factor of log[A (sec^?1)/methyl] = 14.7 ± 0.8 for methyl rupture from germanium. Log A values for methyl rupture from carbon, silicon, and germanium linearly correlate with the vibrational-rotational entropies of the corresponding tetramethyls. Extrapolation predicts log[A (sec^?1)/methyl] = 14.4 and 14.3 for methyl rupture from tin and lead, respectively.     

Nuclear quadrupole interaction at133Cs in some barium compounds

Nuclear quadrupole interaction frequencies at¹³³Cs following the electron-capture decay of¹³³Ba for BaSO₄, Ba/BrO₃/2 and Ba/NO₃/2 were obtained by measuring the perturbation of 356–81 keV cascade of¹³³Ba. Nuclear quadrupole interaction frequencies for BaSO₄ and Ba/BrO₃/2 are 17.2 Mrad sec^–1 and 14.6 Mrad sec^–1, respectively, while no perturbation of 356–81 keV cascade was observed in case of Ba/NO₃/2. Further the possibility of any after-effects of electron-capture decay is ruled out through the measurement of 276–161 keV gamma-gamma directional correlation.     

Photophysical insights on quantum dots-zinc porphyrazine system studied in solution

The present study reports spontaneous interaction of a quantum dots, namely, CdS_xSe_1-x/ZnS (QD) with zinc porphyrazine (1) in toluene. It is observed from steady state fluorescence measurements that photoluminescence of QD suffers quenching by 1. Time resolved fluorescence measurements reveal small change in the lifetime of QD (16.10 ns) following it interaction with 1 (15.77 ns). The magnitude of k_q for QD-1 system, i.e., k_q ​= ​5.25 ​× ​10¹² ​L⋅mol⁻¹⋅sec⁻¹ (evaluated from the stern-volmer plot) establishes that photoexcited QD undergoes decay by 1 according to static quenching mechanism. The results emerging from above study confirm that QD-1 system may be judiciously applied as an energy storage material in near future.     

On the photolysis of homo- and copolymers of phenylisopropenylketone

Poly(phenylisopropenylketone (PPIK) and copolymers of PIK and methylmethacrylate (MMA) or styrene (St) were irradiated in benzene solution at 30° with 313 nm light (stationary irradiations) or with 347 nm light (flash photolysis experiments). Homo PPIK undergoes main chain degradation (β-scission) with ø(S) ≈ 0.05. The quantum yield for α-scission is ø(α) = 0.3. For copolymers of MMA and PIK (1 to 15 mol%), ø(S) is 0.04 independent of the copolymer composition. With copolymers of St and PIK, ø(S) was found to increase with decreasing PIK content and to approach 0.15 for very small PIK contents. The flash photolysis experiments showed: (a) the carbonyl triplet decay rate constant k_T(6 × 10⁶ sec^?1) for CP-MMA-PIK samples is independent of copolymer composition but is lower than for homo PPIK (1 × 10⁷ sec^?1). In CP-St-PIK samples k_T decreases with decreasing PIK content [from 8 × 10⁶ sec^?1 (12 mol% PIK) to 3 × 10⁶ sec^?1 (1 mol% PIK)]; (b) the transient spectra of triplet decay products indicate the formation of benzoyl radicals in the cases of PPIK and CP-MMA-PIK, and the formation of various different species in the case of CP-St-PIK.The results are consistent with the following concept. In homo PPIK and CP-MMA-PIK, α-scission (Norrish type I) is the dominant chemical route of triplet deactivation. In CP-St-PIK, however, type II processes become more and more important as the PIK content decreases.     

Arrhenius parameters for the gas‐phase reactions of O3 with two butenes and two methyl‐substituted butenes over the temperature range of 295–351 K

Gas‐phase reactions of ozone with two butenes (1‐butene and isobutene) and two methyl‐substituted butenes (2‐methyl‐1‐butene and 3‐methyl‐1‐butene) have been studied in an indoor chamber at 295–351 K. The O₃ concentrations were monitored by Model 49C‐Ozone analyzer. The butene concentrations were measured by gas chromatography–flame ionization detector. The Arrhenius expressions of k=3.50×10^?15e^{(?1756±84)/T} cm³ molecule^?1 s^?1, k=3.39×10^?15e^{(?1697±52)/T} cm³ molecule^?1 s^?1, k=6.18×10^?15e^{?(1822±80)/T} cm³ molecule^?1 s^?1, and k=7.24×10^?14e^{?(2741±139)/T} cm³ molecule^?1 s^?1 were obtained for the ozonolysis reactions of 1‐butene, isobutene, 2‐methyl‐1‐butene, and 3‐methyl‐1‐butene, respectively. Both the reaction rate constant and activation energy obtained in this work are in good agreement with those reported by using different techniques in the literature. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 238–246, 2011     

Kinetic study on the reaction of cis-dioxo-(N-(5-X-salicylidene)-2-aminobenzenethiolato) molybdenum(VI) with ethyldiphenylphosphine

The reactions of ethyldiphenylphosphine with a number of cis-dioxomolybdenum(VI) Schiff base coordination complexes are described. These molybdenum complexes incorporate tridentate Schiff base ligands obtained from the condensation of 5-X-salicylaldehyde (X = Cl, Br, H, CH₃O) with o-aminobenzenethiol. Oxomolybdenum(IV) Schiff base complexes were observed as products of the reaction of these Mo(VI) complexes with PEtPh₂. The kinetics for these reactions were followed spectrophotometrically and the applicable rate law is ? d[MoO₂L]/dt = k₁[MoO₂L][PEtPh₂]. The k₁'s were shown to vary systematically as the X-substituent on the ligand was changed. For MoO₂(5-X-SSP), the specific rate constants at 30°C span the range from 19.6 × 10^?4 M^?1 sec^?1 (X = Br) to 8.4 × 10^?4 M^?1 sec^?1 (X = CH₃O). It was also observed that a correlation exists between the cathodic reduction potentials (E_pc) and the k₁'s within the series. The rate of reaction of MoO₂(5-X-SSP) with PEtPh₂ was altered and systematically controlled through ligand design.     

The addition of benzocyclobutenylidene to benzene : The facile rearrangement of spiro[benzocyclobutene-1,7'-cyclohepta-1',3',5'-triene] to 9a,10-dihydrobenz[α]azulene

Thermal decomposition of the sodium salts of benzocyclobutenone tosylhydrazone and 2-methylbenzocyclobutenone tosylhydrazone in benzene affords 9a,10-dihydrobenz[α]azulene 4 and trans-10-methyl-9a, 10-dihydrobenz[α]azulene 3, respectively. A mechanism involving initially the addition of the carbene benzocyclobutenylidene, or its 2-Me derivative, to the benzene ring is postulated. A proposed intermediate in the reaction, spiro [benzocyclobutene 1,7' cyclohepta-1',3',5'-triene] 12 has been synthesised, and shown to give rise to 4 under the reaction conditions. The rate of rearrangement of 12 → 4 has been measured, and the activation energy determined: E_a = 125.9 ± O.8 KJmol^?1 and A = 1.38 × lO¹⁴sec^?1. The mechanism for the rearrangement must involve ring opening of the benzocyclobutene moiety of 12 to give an o- xylylene intermediate which is postulated to possess considerable diradical character. At 71.8 °, this ring opening is 2.7 × 10⁶ times faster than the ring opening of the parent benzocyclobutene molecule. The decomposition of the sodium salt of 2-(7' -cyclohepta-1',3',5' trienyl)benzaldehyde tosylhydrazone has also been investigated and is shown to yield 4a,10-dihydrobenz[α]azulene, 9,10-dihydrobenz[α]azulene and 8,9-benzotricyclo [5.3.0.0^2.10]deca-3,5,8-triene. A mechanism involving intramolecular 1,3-dipolar addition of a diazo grouping to a cycloheptatriene Π-bond, followed by decomposition of the resulting pyrazoline intermediate, is proposed.     

Reversible intersystem crossing,fluorescence lifetime lengthening and collisionally induced phosphorescence in biacetyl

The emissions of biacetyl excited at 4200 Å were studied at pressures down to 10^?3 torr. Apart from the well-known nanosecond fluorescence, a new emission of the same spectral composition was found with a non-exponential decay in the microsecond range. Furthermore the phosphorescence, as defined by its spectral composition, was found to be collisionally induced.The results imply that after excitation, the molecule rapidly transfers (rate constant k_S→T) to the triplet state, giving rise to the nanosecond decay time; and can then transfer back to the singlet state (rate constant k_T→S), giving rise to the microsecond emission. At the same time internal conversion can occur (k_S→S0). From an analysis of the data we find for k_S→S0 = 2.4 × 10⁷ sec^?1, k_S→T = 7.6 × 10⁷ sec^?1, k_T→S = 1.9 × 10⁵ sec^?1. The kinetic treatment can be transformed to a quantum mechanical one, yielding values for the triplet level density (?_T), the coupling element V_ST and the number of triplet states (N) coupled to the singlet excited. At 4200 Å we find ?_T = 6.3 × 10⁵cm, V_ST = 1.0 × 10^?5 cm^?1, N = 400.Phosphorescence occurs only when the molecule is deactivated by collisions to a vibronic triplet state below the vibrationless excited singlet state. The efficiency of biacetyl collisions is 0.54.     

Delocalization energy of the cyanomethyl radical: Kinetics of thermal diastereoisomerization of cis- and trans-1,2-dicyano- and 1 ,2-dicyano-1-methylcyclopropanes

Arrhenius parameters for the thermal first-order geometrical isomerization of 1,2-dicyanocylopropanes(I) have been determined in naphthalene solution over the range 208.0–259.5° in both directions:

where θ = 4.575T × 10^?3 and k is in sec^?1. Since this enthalpy of activation is lower than that of the geometrical isomerization of 1,2 - dideuterocyclopropane by 17.8±0.4 kcal, it may be concluded that replacement of hydrogen by the cyano group leads to an energy lowering of 8.9$\text{kcal}\text{mol}$.Kinetic parameters have been determined in the gas-phase at two temperatures, 217.8° and 259.5°: log k_t,c = 13.73– 45.64/θ; log k_c,t =13.86–44.43/θ.The rates of cis-trans interconversion of 1,2 - dicyano - 1 - methyl - cyclopropane(II) relative to those of I have been obtained by examination of mixtures of both substances in t-butylbenzene solution at 259.5°: 1.2-dicyano, k_t,c= 1.25 and k_c,t = 3.53; 1,2 - dicyano - 1 - methyl, k_t,c = 8.09 and k_c,t = 22.35 × 10^?5 sec^?1. The rate acceleration by methyl amounts to a factor of 6.4, corresponding to ΔΔG^≠ = 1.96$\text{kcal}\text{mol}$. A preliminary examination of optically active material leads to a minimum R_A = 1.37 favoring rotation of (CN)(H) over (CN)(CH₃).     

The 1B1u ← 1A1g 0—0 transition in crystal benzene

The transition linewidth ΔE in crystal C₆H₆, C₆D₆ and sym-C₆H₃D₃ has been measured as a function of temperature T from 4.2 to 135°K, and it extrapolates to a common value of ΔE_o = 50 cm^? at O°K. In C₆H₆ ΔE = (50 + 7T^{$\text{1}\text{2}$}) cm^?1, indicative of strong exciton—phonon coupling, and there is a line shift of +40 cm^?1 per substituent deuteron. Fluorescence excitation spectral data are used to separate the ¹B_1u(= S₂) decay rate k_H = 9.4 × 10¹² sec^?1, derived from ΔE₀, into S₂S₁ internal conversion (rate ≈ 6.6 × 10¹² sec^?1) and S₂S_x (channel 3) internal conversion (rate ≈ 2.8 × 10¹² sec^?1. A similar value of k_H = 9.9 × 10¹² sec^?1 is obtained from the S₂S_o fluorescence quantum yield of liquid benzene.     

Quaternization of 2-aziridino-5-chlorobenzophenone,an efficient synthesis of medazepam

Quaternization of 2-aziridino-5-chlorobenzophenone (1) with methyl iodide resulted in formation of 2-(N-β-iodoethyl-N-methyl)aminobenzophenone ( 2 ), via an unstable quaternary compound. Rate constants for 1 → 2 conversion, as determined by an nmr method at 35 ± 0.1°, varied between 0.22 × 10^?3 sec^?1 in DMSO-d₆, and 0.95 × 10^?6 sec^?1 in methanol-d₄. Ammonolysis with hexamine, and subsequent cyclization afforded 7-chloro-l-methyl-5-phenyl-2,3-dihydro-lH-1,4-benzodiazepine (3, generic name medazepam) in 92% over-all yield.     

KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S₁(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S₁(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S₁(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S₁(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [¹ππ*→¹nπ*→³ππ*→S₀] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate.     

Investigation of the rate coefficient for O(3p) + NO2 → O2 + NO

Measurements of the rate coefficient of the reaction (O³P) + NO₂ → O₂ + NO have been made at 296°K and 240°K, using the technique of NO₂* chemiluminescent decay. Values of 9.3 × 10^?12 cm³ molec^?1 sec^?1 at 296°K and 10.5 × 10^?12 cm³ molec^?1 sec^?1 at 240°K were obtained, in excellent agreement with the recent results of Davis, Herron, and Huie [1]. The earlier lower values may have resulted from loss of NO₂ on surfaces.     

The triplet state of 4-nitro-N,N-dimethynaphthylamine

Nanosecond laser photolytic studies of 4-nitro-N,N-dimethylnaphthylamine (4-NDMNA) in nonpolar and polar solvents at room temperature show a transient species with an absorption maximum in the 500-510-nm range. This species is assigned to the lowest triplet excited state of 4-NDMNA. The absorption maximum of this state is independent of solvent polarity, and its lifetime is a function of the hydrogen donor efficiency of the solvent. In n-hexane the lifetime 1/k of the triplet state is 9.1 × 10^?6 sec, while in acetonitrile 1/k is 2.0 × 10^?7 sec. The hydrogen abstraction rate constant k_H of the triplet state with tributyl tin hydride (Bu₃SnH) in n-hexane is 1.7 × 10⁷M^?1·sec^?1, while in the case of isopropyl alcohol as hydrogen donor, k_H is 4.0 × 10⁷M^?1·sec^?1. The activation energy for the hydrogen abstraction by the triplet state from Bu₃SnH in deaerated n-hexane is 0.6 kcal/mol. The lack of spectral shift with increasing solvent polarity, and the appreciable hydrogen abstraction reactivity of the triplet state, also independent of solvent polarity, seem to indicate that this excited state is an n-π* state which retains its n-π* character even in polar media.     

Structure–reactivity relationships for the self‐ reactions of linear secondary alkylperoxy radicals: An experimental investigation

The self‐reactions of the linear pentylperoxy (C₅H₁₁O₂) and decylperoxy (C₁₀H₂₁O₂) radicals have been studied at room temperature. The technique of excimer laser flash photolysis was used to generate pentylperoxy radicals, while conventional flash photolysis was used for decylperoxy radicals. For the former, the recombination rate coefficients were estimated for the primary 1‐pentylperoxy isomer (n‐C₅H₁₁O₂) and for the secondary 2‐ and 3‐pentylperoxy isomers combined (“sec‐C₅H₁₁O₂”) by creating primary and secondary radicals in different ratios of initial concentrations and simulating experimental decay traces using a simplified chemical mechanism. The values obtained at 298 K were: k(n‐C₅H₁₁O₂+n‐C₅H₁₁O₂→Products)=(3.9±0.9)×10⁻¹³ cm³ molecule⁻¹ s⁻¹; k(sec‐C₅H₁₁O₂+sec‐C₅H₁₁O₂→Products)=(3.3±1.2)×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. Quoted errors are 1σ, whereas the total relative combined uncertainties correspond to an estimated uncertainty factor around 1.65. For decylperoxy radicals, the kinetics of all the types of secondary peroxy isomers reacting with each other were considered equivalent and grouped as sec‐C₁₀H₂₁O₂ (as for sec‐C₅H₁₁O₂). The UV absorption spectrum of these secondary radicals was measured, and the combined self‐reaction rate coefficients then derived as: k(sec‐C₁₀H₂₁O₂+sec‐C₁₀H₂₁O₂)=(9.4±1.3)×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 298 K. Again, quoted errors are 1σ and the total uncertainty factor corresponds to a value around 1.75. The sec‐dodecylperoxy radical was also investigated using the same procedure, but only an estimate of the rate coefficient could be obtained, due to aerosol formation in the reaction cell: k(sec‐C₁₂H₂₅O₂+sec‐C₁₂H₂₅O₂)≡1.4×10⁻¹³ cm³ molecule⁻¹ s⁻¹, with an uncertainty factor of about 2. Despite the fairly high uncertainty factors, a relationship has been identified between the room‐temperature rate coefficient for the self‐reaction and the number of carbon atoms, n, in the linear secondary radical, suggesting: log(k(sec‐RO₂+sec‐RO₂)/cm³ molecule⁻¹ s⁻¹)=−13.0–3.2×exp(−0.64×(n‐2.3)). Concerning primary linear alkylperoxy radicals, no real trend in the self‐reaction rate coefficient can be identified, and an average value of 3.5×10⁻¹³ cm³ molecule⁻¹ s⁻¹ is proposed for all radicals. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet: 31: 37–46, 1999