G3. UV Absorption Spectrum of Hydroxycyclohexadienyl
Radical, C6H6OH, and Thermochemistry of the Oxidation Reaction C6H6OH
+ O2 <=> C6H6(OH)OO
Sergey Y. Grebenkin and Lev N. Krasnoperov
Department
of Chemical Engineering, Chemistry
and Environmental Science
New Jersey Institute of Technology
University Heights
Newark, NJ 07102
E-mail:Sergey.y.grebenkin@njit.edu
E-mail: Krasnoperov@adm.njit.edu
Hydroxycyclohexadienyl (HCHD) radical, C6H6OH, is an important intermediate in the oxidation of atmospheric aromatic hydrocarbons1. In this work, UV absorption spectrum, kinetics and thermochemistry of HCHD radical were studied using pulsed excimer laser photolysis combined with UV transient absorption spectroscopy. An imaging spectrograph (Acton, 320i), equipped with a fast gated intensified CCD Camera (ICCD-max, Princeton Instruments) and a photomultiplier tube was used to record the transient spectra and the absorption temporal profiles. Reactant mixtures were pumped through temperature controlled flow cells with either collinear or transverse photolytic light geometry.
Hydroxycyclohexadienyl radicals were produced in a sequence of reactions initiated by the excited oxygen atom, O(1D):
N2O + hn(193nm) --> N2 + O(1D) |
(1) |
O(1D) + H2O --> 2 OH |
(2) |
OH + C6H6 --> ·C6H6OH |
(3) |
Quantitative UV absorption cross-sections were determined based on the absolute measurements of the concentration of hydroxyl radical via transient absorption at 307.206 nm (Zn HCL), sOH(307.206) = (9.36± 0.57)´ 10-17 cm2 molecule-1 (this work). Measured UV absorption cross-sections are shown in Fig. 1 in comparison with the literature data.1 The maximum cross-section is s HCHD(280 nm, max) = (6.21 ± 0.83) ´ 10-18 cm2 molecule-1.
 |
Figure 1. UV absorption cross-sections for hydroxycyclohexadienyl radical. |
 |
Figure 2. "Two-exponential" decay in reaction of cyclohexadienyl radical with molecular oxygen. |
Reaction of hydroxycyclohexadienyl radicals with molecular oxygen (reaction 4) is one of the major sinks of these radicals in the atmosphere:
C6H6OH + O2 <=> C6H6(OH)OO |
(4) |
Theoretical estimates indicate relatively weak chemical bond in the adduct C6H6(OH)OO2. Small bonding energy infers reversibility of reaction 4 near ambient conditions. Fig. 2 shows an example of an "equilibration" type temporal profile of absorption of HCHD radicals when oxygen is added. The two time-domain kinetics is apparent. Fast "equilibration" occurs on the short (ca. 0.5 ms) time scale. Further slow (ca. 3 ms) decay is due to the radical-radical reactions and the further transformation of the adduct. In Fig. 2 a two-time-domain transient absorption with the amplitude of ca. 0.0001 is resolved. The measurements were performed over the temperature range 260 – 300 K. The experimental temporal profiles were processed by a numerical fitting using a small reaction mechanism. Fig. 3 shows the van't Hoff plot of the equilibrium constants obtained in the fits and the third law determination of the Standard Enthalpy and the rate constant of reaction 4 at 280 K:
DH0280K = -76.6 ± 3.7 kJ mol |
(5) |
k4 = (1.1 ± 0.4) ´ 10-15 cm3 molecule-1 s-1 (276 K, 1 bar, He) |
(6) |
The rate constant of reaction 4 measured in this work supports the lower of the two literature values (2.1´ 10-16 cm3 molecule-1 s-1 at 298 K 3 and 5.0´ 10-13 cm3 molecule-1 s-1 at 338 K 1).
|
Figure 3. Van't Hoff plot for the equilibrium constant of the reaction of hydroxycyclohexadienyl radical with molecular oxygen. |
1. Bjergbakke, E; Sillesen, A.; Pagsberg, P. J. Phys. Chem. 1996, 100, 5729.
2. Lay, T.H.; Bozzelli, J.W.; Seinfeld, J.H. J. Phys. Chem. 1996, 100, 6543.
3. R. Knispel, R.; Koch, R.; Siese, M.; Zetzsch, C. Ber. Bunsenges. Phys. Chem. 1990, 94, 1375.