A kinetic study of the reactions of Fe+ with N2O, N2, O2, CO2 and H2O, and the ligand-switching reactions Fe+.X + Y --> Fe+.Y + X (X = N2, O2, CO2; Y = O2, H2O)

A series of reactions involving Fe(+) ions were studied by the pulsed laser ablation of an iron target, with detection of ions by quadrupole mass spectrometry at the downstream end of a fast flow tube. The reactions of Fe(+) with N(2)O, N(2) and O(2) were studied in order to benchmark this new technique. Extending measurements of the rate coefficient for Fe(+) + N(2)O from 773 K to 185 K shows that the reaction exhibits marked non-Arrhenius behaviour, which appears to be explained by excitation of the N(2)O bending vibrational modes. The recombination of Fe(+) with CO(2) and H(2)O in He was then studied over a range of pressure and temperature. The data were fitted by RRKM theory combined with ab initio quantum calculations on Fe(+).CO(2) and Fe(+).H(2)O, yielding the following results (120-400 K and 0-10(3) Torr). For Fe(+) + CO(2): k(rec,0) = 1.0 x 10(-29) (T/300 K)(-2.31) cm(6) molecule(-2) s(-1); k(rec,infinity) = 8.1 x 10(-10) cm(3) molecule(-1) s(-1). For Fe(+) + H(2)O: k(rec,0) = 5.3 x 10(-29) (T/300 K)(-2.02) cm(6) molecule(-2) s(-1); k(rec,infinity) = 2.1 x 10(-9) (T/300 K)(-0.41) cm(3) molecule(-1) s(-1). The uncertainty in these rate coefficients is determined using a Monte Carlo procedure. A series of exothermic ligand-switching reactions were also studied at 294 K: k(Fe(+).N(2) + O(2)) = (3.17 +/- 0.41) x 10(-10), k(Fe(+).CO(2) + O(2)) = (2.16 +/- 0.35) x 10(-10), k(Fe(+).N(2) + H(2)O) = (1.25 +/- 0.14) x 10(-9) and k(Fe(+).O(2) + H(2)O) = (8.79 +/- 1.30) x 10(-10) cm(3) molecule(-1) s(-1), which are all between 36 and 52% of their theoretical upper limits calculated from long-range capture theory. Finally, the role of these reactions in the chemistry of meteor-ablated iron in the upper atmosphere is discussed. The removal rates of Fe(+) by N(2), O(2), CO(2) and H(2)O at 90 km altitude are approximately 0.1, 0.07, 3 x 10(-4) and 1 x 10(-6) s(-1), respectively. The initially formed Fe(+).N(2) and Fe(+).O(2) are converted into the H(2)O complex at approximately 0.05 s(-1). Fe(+).H(2)O should therefore be the most abundant single-ligand Fe(+) complex in the mesosphere below 90 km.     

High-temperature rate constants for CH3OH + Kr --> products, OH + CH3OH --> products, OH + (CH3)(2)CO --> CH2COCH3 + H2O, and OH + CH3 --> CH) + H2O

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the dissociation of methanol between 1591 and 2865 K. Rate constants for two product channels [CH3OH + Kr --> CH3 + OH + Kr (1) and CH3OH + Kr --> 1CH2 + H2O + Kr (2)] were determined. During the course of the study, it was necessary to determine several other rate constants that contributed to the profile fits. These include OH + CH3OH --> products, OH + (CH3)2CO --> CH2COCH3 + H2O, and OH + CH3 --> 1,3CH2 + H2O. The derived expressions, in units of cm(3) molecule(-1) s(-1), are k(1) = 9.33 x 10(-9) exp(-30857 K/T) for 1591-2287 K, k(2) = 3.27 x 10(-10) exp(-25946 K/T) for 1734-2287 K, kOH+CH3OH = 2.96 x 10-16T1.4434 exp(-57 K/T) for 210-1710 K, k(OH+(CH3)(2)CO) = (7.3 +/- 0.7) x 10(-12) for 1178-1299 K and k(OH+CH3) = (1.3 +/- 0.2) x 10(-11) for 1000-1200 K. With these values along with other well-established rate constants, a mechanism was used to obtain profile fits that agreed with experiment to within <+/-10%. The values obtained for reactions 1 and 2 are compared with earlier determinations and also with new theoretical calculations that are presented in the preceding article in this issue. These new calculations are in good agreement with the present data for both (1) and (2) and also for OH + CH3 --> products.     

A kinetic study of the reactions of Ca+ ions with O3, O2, N2, CO2 and H2O

The reactions between Ca(+)(4(2)S(1/2)) and O(3), O(2), N(2), CO(2) and H(2)O were studied using two techniques: the pulsed laser photo-dissociation at 193 nm of an organo-calcium vapour, followed by time-resolved laser-induced fluorescence spectroscopy of Ca(+) at 393.37 nm (Ca(+)(4(2)P(3/2)-4(2)S(1/2))); and the pulsed laser ablation at 532 nm of a calcite target in a fast flow tube, followed by mass spectrometric detection of Ca(+). The rate coefficient for the reaction with O(3) is essentially independent of temperature, k(189-312 K) = (3.9 +/- 1.2) x 10(-10) cm(3) molecule(-1) s(-1), and is about 35% of the Langevin capture frequency. One reason for this is that there is a lack of correlation between the reactant and product potential energy surfaces for near coplanar collisions. The recombination reactions of Ca(+) with O(2), CO(2) and H(2)O were found to be in the fall-off region over the experimental pressure range (1-80 Torr). The data were fitted by RRKM theory combined with quantum calculations on CaO(2)(+), Ca(+).CO(2) and Ca(+).H(2)O, yielding the following results with He as third body when extrapolated from 10(-3)-10(3) Torr and a temperature range of 100-1500 K. For Ca(+) + O(2): log(10)(k(rec,0)/cm(6) molecule(-2) s(-1)) = -26.16 - 1.113log(10)T- 0.056log(10)(2)T, k(rec,infinity) = 1.4 x 10(-10) cm(3) molecule(-1) s(-1), F(c) = 0.56. For Ca(+) + CO(2): log(10)(k(rec,0)/ cm(6) molecule(-2) s(-1)) = -27.94 + 2.204log(10)T- 1.124log(10)(2)T, k(rec,infinity) = 3.5 x 10(-11) cm(3) molecule(-1) s(-1), F(c) = 0.60. For Ca(+) + H(2)O: log(10)(k(rec,0)/ cm(6) molecule(-2) s(-1)) = -23.88 - 1.823log(10)T- 0.063log(10)(2)T, k(rec,infinity) = 7.3 x 10(-11)exp(830 J mol(-1)/RT) cm(3) molecule(-1) s(-1), F(c) = 0.50 (F(c) is the broadening factor). A classical trajectory analysis of the Ca(+) + CO(2) reaction is then used to investigate the small high pressure limiting rate coefficient, which is significantly below the Langevin capture frequency. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.     

CO2 Fixation by Cu2+ and Zn2+ complexes of a terpyridinophane aza receptor. Crystal structures of Cu2+ complexes, pH-metric, spectroscopic, and electrochemical studies

The synthesis of the terpyridinophane-type polyamine 2,6,9,12,16-pentaaza[17]-(5,5' ')-cyclo-(2,2':6',2' ')-terpyridinophane heptahydrobromide tetrahydrate (L.7HBr.4H2O) is described. L presents six protonation constants with values in the range 9.21-3.27 logarithmic units. L interacts with Cu2+ and Zn2+ forming in both cases, neutral, protonated, and hydroxylated mono- and binuclear complexes whose constants have been calculated by potentiometry in 0.15 M NaClO4 at 298.1 K. The crystal structures of the compounds [Cu(HL-carb)(H2O)](ClO4)3.2H2O (1) and [Cu2(H2L)(CO3)]2(ClO4)8.9H2O (2) have been solved by X-ray diffraction. In 1, the metal center presents square pyramidal geometry. The base of the pyramid is formed by the three nitrogen atoms of pyridine and one oxygen atom of a CO2 group which is forming a carbamate bond with the central nitrogen atom of the polyamine chain. The axial position is occupied by a water molecule. In 2, one Cu2+ is bound by the three pyridine nitrogens and the other one by the three central nitrogen atoms of the polyamine chain. The square planar coordination geometry is completed by a carbonate group taken up from the atmosphere that behaves as a bridging mu,mu'-ligand between the two centers. The pH-metric titrations on the ternary Cu2+-L-carbonate and Zn2+-L-carbonate systems show the extensive formation of adduct species which above pH 6 are formed quantitatively in solution. The stoichiometries of the main species formed in solution at pH = 6.8 agree with those found in the crystalline compounds. CO2 uptake by the Zn2+ and Cu2+ 1:1 complexes in aqueous solution has also been followed by recording the variations in the band at ca. 300 nm. The formation of the Zn2+ carbamate moiety has been evidenced by 13C NMR and ESI spectroscopy.     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

A kinetic study of the reactions FeO+ + O, Fe+.N2 + O, Fe+.O2 + O and FeO+ + CO: implications for sporadic E layers in the upper atmosphere

These gas-phase reactions were studied by pulsed laser ablation of an iron target to produce Fe(+) in a fast flow tube, with detection of the ions by quadrupole mass spectrometry. Fe(+).N(2) and Fe(+).O(2) were produced by injecting N(2) and O(2), respectively, into the flow tube. FeO(+) was produced from Fe(+) by addition of N(2)O, or by ligand-switching from Fe(+).N(2) following the addition of atomic O. The following rate coefficients were measured: k(FeO(+) + O --> Fe(+) + O(2), 186-294 K) = (3.2 +/- 1.5) x 10(-11); k(Fe(+).N(2) + O --> FeO(+)+ N(2), 294 K) = (4.6 +/- 2.5) x 10(-10); k(Fe(+).O(2) + O --> FeO(+) + O(2), 294 K) = (6.3 +/- 2.7) x 10(-11); and k(FeO(+) + CO --> Fe(+) + CO(2), 294 K) = (1.59 +/- 0.34) x 10(-10) cm(3) molecule(-1) s(-1), where the quoted uncertainties are a combination of the 1sigma standard errors in the kinetic data and the systematic experimental errors. The surprisingly slow reaction between FeO(+) and O is examined using ab initio quantum calculations of the relevant potential energy surfaces. The importance of this reaction for controlling the lifetime of sporadic E layers is then demonstrated using a model of the upper mesosphere and lower thermosphere.     

The mechanisms of the reactions of W and W+ with COx (x=1, 2): a computational study

The mechanisms of the reactions of W and W+ with COx (x=1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the gas-phase reaction of W with CO2 proceeds with a negligible barrier via an insertion pathway, W(7S)+CO2(1A1)-->W(eta2-OCO)(6A')-->OW(eta1-CO)(1A)-->WO (3Sigma+)+CO(1Sigma). This oxidation process is calculated to be exothermic by 32.4 kcal/mol. Possible intermediates of this reaction are the W(eta2-OCO) and OWCO complexes, among which the latter is 37.4 kcal/mol more stable and lies 39.7 and 7.3 kcal/mol lower than the reactants, W(7S)+CO2(1A1), and the products, WO (3Sigma+)+CO(1Sigma), respectively. The barrier separating W(eta2-OCO) from OWCO is 8.0 kcal/mol (relative to the W(eta2-OCO) complex), which may be characterized as a W+delta-(CO2)-delta charge-transfer complex. Ionization of W does not change the character of the reaction of W with CO2: the reaction of W+ with CO2, like its neutral analog, proceeds via an insertion pathway and leads to oxidation of the W-center. The overall reaction W+(6D) + CO2(1A1)-->W(eta1-OCO)+(6A)-->OW(eta1-CO)+(4A)-->WO+(4Sigma+)+CO(1Sigma) is calculated to be exothermic by 25.4 kcal/mol. The cationic reaction proceeds with a somewhat large (9.9 kcal/mol) barrier and produces two intermediates, W(eta1-OCO)+(6A) and OW(eta1-CO)+(4A). Intermediate W(eta1-OCO)+(6A) is 20.0 kcal/mol less stable than OW(eta1-CO)+(4A), and separated from the latter by a 35.2 kcal/mol barrier. Complex W(eta1-OCO)+(6A) is characterized as an ion-molecular complex type of W+-(CO2). Gas-phase reactions of M=W/W+ with CO lead to the formation of a W-carbonyl complex M(eta1-CO) for both M=W and W+. The C-O insertion product, OMC, lies by 5.2 and 69.3 kcal/mol higher than the corresponding M(eta1-CO) isomer, for M=W and W+, respectively, and is separated from the latter by a large energy barrier.     

Kinetics of the HCCO + NO2 reaction

The kinetics of the HCCO + NO2 reaction were investigated using a laser photolysis/infrared diode laser absorption technique. Ethyl ethynyl ether (C2H5OCCH) was used as the HCCO radical precursor. Transient infrared detection of the HCCO radical was used to determine a total rate constant fit to the following expression: k1= (2.43 +/- 0.26) x 10(-11) exp[(171.1 +/- 36.9)/T] cm3 molecule(-1) s(-1) over the temperature range of 298-423 K. Transient infrared detection of CO, CO2, and HCNO products was used to determine the following branching ratios at 298 K: phi(HCO + NO + CO) = 0.60 +/- 0.05 and phi(HCNO + CO2) = 0.40 +/- 0.05.     

CH(A2Delta) Formation in hydrocarbon combustion: the temperature dependence of the rate constant of the reaction C2H + O2--> CH(A2Delta) + CO2

The temperature dependence of the rate constant of the chemiluminescence reaction C2H + O2 --> CH(A) + CO2, k1e, has been experimentally determined over the temperature range 316-837 K using pulsed laser photolysis techniques. The rate constant was found to have a pronounced positive temperature dependence given by k1e(T) = AT(4.4) exp(1150 +/- 150/T), where A = 1 x 10(-27) cm(3) s(-1). The preexponential factor for k1e, A, which is known only to within an order of magnitude, is based on a revised expression for the rate constant for the C2H + O(3P) --> CH(A) + CO reaction, k2b, of (1.0 +/- 0.5) x 10(-11) exp(-230 K/T) cm3 s(-1) [Devriendt, K.; Van Look, H.; Ceursters, B.; Peeters, J. Chem. Phys. Lett. 1996, 261, 450] and a k2b/k1e determination of this work of 1200 +/- 500 at 295 K. Using the temperature dependence of the rate constant k1e(T)/k1e(300 K), which is much more accurately and precisely determined than is A, we predict an increase in k(1e) of a factor 60 +/- 16 between 300 and 1500 K. The ratio of rate constants k2b/k1e is predicted to change from 1200 +/- 500 at 295 K to 40 +/- 25 at 1500 K. These results suggest that the reaction C2H + O2 --> CH(A) + CO2 contributes significantly to CH(A-->X) chemiluminescence in hot flames and especially under fuel-lean conditions where it probably dominates the reaction C2H + O(3P) --> CH(A) + CO.     

Determination of the CH + O2 product channels

The multichannel CH + O2 reaction was studied at room temperature, in a low-pressure fast-flow reactor. CH radical was obtained from the reaction of CHBr3 with potassium atoms. The overall rate constant was determined from the decay of CH with distance, O2 being introduced in excess. The result, after corrections for axial and radial diffusion, is k = (3.6 +/- 0.5) x 10(-11) cm3 molecule-1 s-1. The OH(A2 sigma +) chemiluminescence was observed, confirming the existence of the OH + CO channel. The vibrational population distribution of OH(A2 sigma +) is 32% in the v' = 1 level and 68% in the v' = 0 level (+/- 5%). The relative atomic concentrations were determined by resonance fluorescence in the vacuum ultraviolet. A ratio of 1.4 +/- 0.2 was found between the H atom density (H atoms being produced from the H + CO2 channel and from the HCO dissociation) and the O atom density (O + HCO). Ab initio calculations of the transition structures have been performed, associated with statistical estimations. The estimated branching ratios are: O + HCO, 20%; O + H + CO, 30%; H + CO2, 30%; and CO + OH, 20%.     

Experimental and theoretical studies of the reactions Y (a2D) + H2CO and Y (a2D) + CH3CHO

The reactions of ground state Y (a(2)D) with H(2)CO and CH(3)CHO were studied at a range of collision energies in crossed molecular beams. For reaction with H(2)CO, three product channels were observed: formation of YH(2) + CO, YCO + H(2), and YHCO + H. Reaction with CH(3)CHO led to three analogous product channels involving formation of HYCH(3) + CO, YCH(2)CO + H(2), and YCH(3)CO + H. The calculated CCSD(T) energetics and DFT geometries for key intermediates in both reactions, together with RRKM theory, are used to calculate a priori the branching ratios between various product channels. These calculated values are compared to those obtained experimentally.     

Vibrational distributions of the CO(v) products of the C2H2 + O(3P) and HCCO + O(3P) reactions studied by FTIR emission

The C2H2 + O(3P) and HCCO + O(3P) reactions are investigated using Fourier transform infrared (FTIR) emission spectroscopy. The O(3P) radicals are produced by 193 nm photolysis of an SO2 precursor or microwave discharge in O2. The HCCO radical is either formed in the first step of the C2H2 + O(3P) reaction or by 193 nm photodissociation of ethyl ethynyl ether. Vibrationally excited CO and CO2 products are observed. The microwave discharge experiment [C2H2 + O(3P)] shows a bimodal distribution of the CO(v) product, which is due to the sequential C2H2 + O(3P) and HCCO + O(3P) reactions. The vibrational distribution of CO(v) from the HCCO + O(3P) reaction also shows its own bimodal shape. The vibrational distribution of CO(v) from C2H2 + O(3P) can be characterized by a Boltzmann plot with a vibrational temperature of approximately 2400 +/- 100 K, in agreement with previous results. The CO distribution from the HCCO + O(3P) reaction, when studied under conditions to minimize other processes, shows very little contamination from other reactions, and the distribution can be characterized by a linear combination of Boltzmann plots with two vibrational temperatures: 2320 +/- 40 and 10 300 +/- 600 K. From the experimental results and previous theoretical work, the bimodal CO(v) distribution for the HCCO + O(3P) reaction suggests a sequential dissociation process of the HC(O)CO++ --> CO + HCO; HCO --> H + CO.     

Silicon-initiated carbonylative carbotricyclization and [2+2+2+1] cycloaddition of enediynes catalyzed by rhodium complexes

The reaction of dodec-11-ene-1,6-diynes or their heteroatom congeners with a hydrosilane catalyzed by Rh(acac)(CO)2 at ambient temperature and pressure of CO gives the corresponding fused 5-7-5 tricyclic products, 5-oxo-1,3a,4,5,7,9-hexahydro-3H-cyclopenta[e]azulenes or their heteroatom congeners, in excellent yields through a unique silicon-initiated cascade carbonylative carbotricyclization (CO-SiCaT) process. It has also been found that the 5-7-5 fused tricyclic products can be obtained from the same type of enediynes and CO through a novel intramolecular [2+2+2+1] cycloaddition process. The characteristics of these two tricyclization processes and the fundamental differences in their reaction mechanisms are discussed. This novel higher-order cycloaddition reaction has also been successfully applied to the tricyclization of undeca-5,10-diyn-1-als, affording the corresponding 5-7-5 fused-ring products bearing a seven-membered lactone moiety. Related [2+2+2] tricyclizations of enediyne and diynal substrates are also discussed. These newly discovered reactions can construct multiple bonds all at once, converting linear starting materials to polycyclic compounds in a single step. Thus, these new processes provide innovative routes to functionalized polycyclic compounds that are useful for the syntheses of natural and unnatural products.     

Experimental Study of C2Cl3+NO2 Reaction

The free radical reaction of C2Cl3 with NO2 was investigated by step-scan time-resolved FTIR (TR-FTIR) emission spectroscopy. Due to the vibrationally excited products of Cl2CO, NO, and CO, strong IR emission bands were observed with high resolution TR-FTIR spectra. Four reaction channels forming C2Cl3O+NO, CCl3CO+NO, CO+NO+CCl3, and ClCNO+Cl2CO were elucidated, respectively. Spectralˉtting showed that the product CO was highly vibrationally excited with the nascent average vibrational energy of 60.2 kJ/mol. Possible reaction mechanism via intermediates C2Cl3NO2 and C2Cl3ONO was proposed.     

New iridacyclohexadienes and iridabenzenes by [2+2+1] cyclotrimerization of alkynes and facile interconversion between iridacyclohexadienes and iridabenzenes

Iridabenzenes [Ir[=CHCH=CHCH=C(CH2R)](CH3CN)2(PPh3)2]2+ (R=Ph 4 a, R=p-C6H4CH3 4 b) are obtained from the reactions of H+ with iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](CO)(PPh3)2]+ (R'=H 3 a, R'=CH3 3 b), which are prepared from [2+2+1] cyclotrimerization of alkynes in the reactions of [Ir(CH3CN)(CO)(PPh3)2]+ with HC[triple chemical bond]CH and HC[triple chemical bond]CR. Iridabenzenes 4 react with CO and CH3CN in the presence of NEt3 to give iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CHR)](CO)2(PPh3)2]+ (6) and [Ir[-CH=CHCH=CHC(=CHR)](CH3CN)2(PPh3)2]+ (7), respectively. Iridacyclohexadienes 6 and 7 also convert to iridabenzenes 4 by the reactions with H+ in the presence of CH3CN. Alkynyl iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](-C[triple chemical bond]CH)(PPh3)2] (8) undergo a cleavage of C[triple chemical bond]C bond by H+/H2O to produce [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](-CH3)(CO)(PPh3)2] (10) via facile inter-conversion between iridacyclohexadienes and iridabenzenes.     

Rhodium-catalyzed [2+2+1+1] cyclocarbonylative coupling of alkynes with carbon monoxide affording tetrasubstituted p-benzoquinones

In this strategy, the tetrasubstituted benzoquinones have been prepared directly by a [2+2+1+1] cyclocarbonylative coupling reaction of internal alkynes with CO in the presence of [RhCl(CO)2]2. The low concentration of CO in the reaction is the crucial point for the chemoselective formation of tetrasubstituted benzoquinones in good to high yields. Functional groups such as chloro, methoxy, cyano, vinyl, fluoro, and carboxylate are tolerated under the reaction conditions.     

Determination of the rate constants for the NH2(X2B1) + NH2(X2B1) and NH2(X2B1) + H Recombination reactions with collision partners CH4, C2H6, CO2, CF4, and SF6 at low pressures and 296 K. Part 2

The recombination rate constants for the reactions NH2(X2B1) + NH2(X2B1) + M → N2H4 + M and NH2(X2B1) + H + M → NH3 + M, where M was CH4, C2H6, CO2, CF4, or SF6, were measured in the same experiment over presseure ranges of 1-20 and 7-20 Torr, respectively, at 296 ± 2 K. The NH2 radical was produced by the 193 nm laser photolysis of NH3. Both NH2 and NH3 were monitored simultaneously following the photolysis laser pulse. High-resolution time-resolved absorption spectroscopy was used to monitor the temporal dependence of both species: NH2 on the (1)2(21) ← (1)3(31) rotational transition of the (0,7,0)A2A1 ← (0,0,0)X2B1 electronic transition near 675 nm and NH3 in the IR on either of the inversion doublets of the qQ3(3) rotational transition of the ν1 fundamental near 2999 nm. The NH2 self-recombination clearly exhibited falloff behavior for the third-body collision partners used in this work. The pressure dependences of the NH2 self-recombination rate constants were fit using Troe’s parametrization scheme, k(inf), k(0), and F(cent), with k(inf) = 7.9 × 10(-11) cm3 molecule(-1) s(-1), the theoretical value calculated by Klippenstein et al. (J. Phys. Chem. A113, 113, 10241). The individual Troe parameters were CH4, k(0)(CH4) = 9.4 × 10(-29) and F(cent)(CH4) = 0.61; C2H6, k(0)(C2H6) = 1.5 × 10(-28) and F(cent)(C2H6) = 0.80; CO2, k(0)(CO2) = 8.6 × 10(-29) and F(cent)(CO2) = 0.66; CF4, k(0)(CF4) = 1.1 × 10(-28) and F(cent)(CF4) = 0.55; and SF6, k(0)(SF6) = 1.9 × 10(-28) and F(cent)(SF6) = 0.52, where the units of k0 are cm6 molecule(-2) s(-1). The NH2 + H + M reaction rate constant was assumed to be in the three-body pressure regime, and the association rate constants were CH4, (6.0 ± 1.8) × 10(-30); C2H6, (1.1 ± 0.41) × 10(-29); CO2, (6.5 ± 1.8) × 10(-30); CF4, (8.3 ± 1.7) × 10(-30); and SF6, (1.4 ± 0.30) × 10(-29), with units cm6 molecule(-1) s,(-1) and the systematic and experimental errors are given at the 2σ confidence level.     

Kinetics of the NCCO + NO2 reaction

The kinetics of the NCCO + NO(2) reaction was studied by transient infrared laser absorption spectroscopy. The total rate constant of the reaction was measured to be k = (2.1 ± 0.1) × 10(-11) cm(3) molecule(-1) s(-1) at 298 K. Detection of products and consideration of possible secondary chemistry shows that CO(2) + NO + CN is the primary product channel. The rate constants of the NCCO + CH(4) and NCCO + C(2)H(4) reactions were also measured, obtaining upper limits of k (NCCO + CH(4)) ≤ 7.0 × 10(-14) cm(3) molecule(-1) s(-1) and k (NCCO + C(2)H(4)) ≤ 5.0 × 10(-15) cm(3) molecule(-1) s(-1). Ab initio calculations on the singlet and triplet potential energy surfaces at B3LYP/6-311++G**//CCSD(T)/6-311++G** levels of theory show that the most favorable reaction pathway occurs on the singlet surface, leading to CO(2) + NO + CN products, in agreement with experiment.     

Atmospheric chemistry of linear perfluorinated aldehydes: dissociation kinetics of CnF2n+1CO radicals

Linear perfluorinated aldehydes (PFALs, CnF2n+1CHO) are important intermediate species in the atmospheric oxidation pathway of many polyfluorinated compounds. PFALs can be further oxidized in the gas phase to give perfluorinated carboxylic acids (PFCAs, CnF2n+1C(O)OH, n = 6, 12) which have been detected in animal tissues and at low parts per billion levels in human blood sera. In this paper, we report ab initio quantum chemistry calculations of the decarbonylation kinetics of CnF2n+1CO radicals. Our results show that CnF2n+1CO radicals have a strong tendency to decompose to give CnF2n+1 and CO under atmospheric conditions: the Arrhenius activation energies for decarbonylation of CF3CO, C2F5CO, and C3F7CO obtained using PMP4/6-311++G(2d,p) are 8.8, 6.6, and 5.8 kcal/mol, respectively, each of which is about 5 kcal/mol lower than the barrier for the corresponding nonfluorinated radicals. The lowering of the barrier for decarbonylation of CnF2n+1CO relative to that of CnH2n+1CO is well explained by electron withdrawal by F atoms that serve to weaken the critical C-CO bond. These results have important implications for the atmospheric fate of PFALs and the atmospheric pathways to PFCAs. The main effect of decarbonylation of CnF2n+1CO is to decrease the molar yield of CnF2n+1C(O)OH; if 100% of the CnF2n+1CO decompose, the yield of CnF2n+1C(O)OH must be zero. There is considerable scope for additional experimental and theoretical studies.     

Rhodium(I)-catalyzed [2+2+1] cycloadditions of 1,3-dienes, alkenes, and CO

Initial examples of a Rh(I)-catalyzed [2+2+1] reaction of diene-enes and CO are described. This method allows for the facile, efficient, and diastereoselective construction of a variety of alkenyl cyclopentanones in good to excellent yields. Control studies show that the diene moiety is required for this process as bis-enes do not give the [2+2+1] products under the same conditions.