Thanh Lam Nguyen

Theoretical study of the gas-phase ozonolysis of β-pinene (C10H16)

The O3-initiated oxidation of β-pinene, a monoterpene emitted in forested areas, was theoretically characterized using DFT, CBS-QB3 and CASPT2 quantum chemical calculations combined with statistical kinetic RRKM/master equation analysis and transition state theory. The first-principles based rate coefficient of the initial O3 attack on the exocyclic double bond shows a slight positive temperature dependence, ktot(T) = 1.27 × 10−22×T2.64× exp(−714 K/T) cm3 molecule−1 s−1, and is in close agreement with experiment. The resulting primary ozonides are found to give rise to two distinct, non-interconvertible conformers of the predominant Criegee intermediate (CI-1 and CI-2), with subsequent chemistries that are shown to be very different; this crucial aspect of β-pinene ozonolysis was not taken into account in earlier studies. One of the conformers CI-2—carrying nearly half the total reaction flux—cannot undergo the usual “hydroperoxide channel”, thus rationalizing why the OH yield from β-pinene is barely half of that from α-pinene. The predicted first-generation product distribution for atmospheric conditions is consistent with the available experimental data on the overall products. Our final results predict 5% of nopinone formation, 28% OH radicals with 2-oxo-alkyl radical coproducts, 37% of stabilized Criegee intermediates (SCI), 17% lactones, 10% CO2 formed after an intersystem crossing, and 3% of a newly proposed biradical formed from prompt ring opening in the CI. In atmospheric conditions, additional OH production is expected from the stabilized CI-1 conformer via the thermal unimolecular “hydroperoxide channel”, whereas the stabilized CI-2 can react with H2O and its dimer, to produce additional nopinone. The expected subsequent chemistries of the large coproduct radicals formed from reactions of the two CI are also addressed in extenso.

On the heats of formation of formyl cyanide and thioformyl cyanide

Ab initio molecular orbital calculations have been applied to determine the standard enthapies of formation of formyl cyanide and thioformyl cyanide. Using electronic energies at the coupled cluster single double (triple)/ 6-311++G(3df,2p) level in conjunction with different working reactions, the following values have been obtained: ΔHf,2980(HCOCN)=56u2002kJ/mol and ΔHf,2980(HCSCN)=271u2002kJ/mol, with a probable error of ±8 kJ/mol. These values differ significantly from the experimental ones of 26±20 and 222±30 kJ/mol, respectively, recently derived from mass spectrometric measurements by Born et al. [J. Phys. Chem. 100, 17662 (1996)].

Absolute rate coefficients over extended temperature ranges and mechanisms of the CF(X2Π) reactions with F2, Cl2 and O2

The absolute rate coefficients of the reactions of the carbyne-radical CF(X(2)Pi, nu = 0) with O(2), F(2) and Cl(2) have been measured over extended temperature ranges, using pulsed-laser photodissociation-laser-induced fluorescence (PLP-LIF) techniques. The CF(X(2)Pi) radicals were generated by KrF excimer laser 2-photon photolysis of CF(2)Br(2) at 248 nm and the real-time exponential decays of CF(X(2)Pi, nu = 0) at varying coreactant concentrations, in large excess, were monitored by LIF (A(2)Sigma(+), nu = 1 <-- X(2)Pi, nu = 0 transition). The experimental bimolecular rate coefficients of the CF(X(2)Pi) reactions with F(2) and Cl(2) can be described by simple Arrhenius expressions: k(F2)(295-408 K) = (1.5 +/- 0.2) x 10(-11) exp[-(370 +/- 40)K/T] cm(3) molecule(-1) s(-1); and k(Cl2)(295-392 K) = (6.1 +/- 2.1) x 10(-12) exp[+(280 +/- 120)K/T]. The k(F2)(T) and k(Cl2)(T) results can be rationalized in terms of direct halogen-atom abstraction reactions in which the radical character of CF dominates; a quantum chemical CBS-Q//BHandHLYP/6-311G(d,p) study confirms that the ground state reactants CF(X(2)Pi) + F(2)(X(1)Sigma) connect directly with the ground-state products CF(2)(X(1)A(1)) + F((2)P) via a nearly barrierless F-atom abstraction route. The rate coefficient of CF(X(2)Pi) + O(2) can be represented by a two-term Arrhenius expression: k(O2)(258-780 K) = 1.1 x 10(-11) exp(-850 K/T) + 2.3 x 10(-13) exp(500 K/T), with a standard deviation of 5%. The first term dominates at higher temperatures T and the second at lower T where a negative temperature dependence is observed (<290 K). Quantum chemical computations at the CBS-QB3 and CCSD(T)/aug-cc-pVDZ levels of theory show that the k(O2)(T) behaviour is consistent with a change of the dominant rate-determining mechanism from a carbyne-type insertion into the O-O bond at high T to a radical-radical combination at low T.