541
Communications to the Editor
COMMUNICATIONS TO THE EDITOR
Absolute Rate Constants for the Reaction of Oxygen( 3P) Atoms with Methoxybenzene and o-Cresol
2.0r x
7"
Publication cosfsassisfed by the University of California
1.5
2
+
tan
+
+
k2[aromatic]) (I) is the modulation frequency (1200 f 1Hz) and hl, = 2nv/(kl[NO][M]
where hz are the rate constants for the reactions 0
+ +
--
+M NO2 + M (1) aromatic products (2 1 Rate constants k z were determined as described previousl ~Figure . ~ 1 shows examples of plots of 2 ~ v / t a n against [aromatic] for methoxybenzene and o-cresol. The values of 2 d a n 4 in the absence of aromatic were determined ex0
NO
+
1 I
-'
Sir: The reaction of ground state oxygen atoms with aromatic hydrocarbons is known to lead to the formation of phenolic products.1-5 For example, the major volatile product from the reaction of O(3P) atoms with toluene is o-cre~ 0 1 . Although ~ 3 ~ the absolute rate constants for the reaction of O(3P) atoms with a series of aromatic hydrocarbons have been ~ b t a i n e d , there ~ ~ ~ appear -~ to be no reported rate constants for the reaction of phenolic compounds with O(3P) atoms. In this work we have determined the absolute rate constants a t room temperature for the reaction of O(3P) atoms with methoxybenzene and o-cresol. The apparatus and techniques used have been described previously6-s and hence only the pertinent details are given here. O(3P) atoms were generated by the sinusoidally modulated mercury photosensitization of N20 and were monitored by NO2 chemiluminescence. Phase shifts, +, between the NO2 emission and the incident 2537-w radiation were measured using lock-in amplification. The methoxybenzene and o-cresol used had purity levels of 299%. NzO and (9.38 f 0.2)% NO in argon were purified as described previously.6 A known fraction of the NzO flow was saturated with the aromatic vapor at 293.5-295.5'K. The concentration of methoxybenzene in the gas stream was determined before entering the reaction cell by uv absorption. However, for o-cresol the uv absorption was too low to be accurately measured and hence its concentration was calculated assuming saturation of the NzO flow stream (this was observed both p r e v i o u ~ l y and ,~~~ in this work for methoxybenzene, to agree to within f5-10% with the concentrations determined by uv absorption). The aromatic concentrations used were limited by their room temperaM for o-cresol and 51 X ture vapor pressures to 51 X M for methoxybenzene. At the light intensities and flow rates used, conversions of the aromatic compounds were always
