J. Phys. Chem. 1995, 99, 10802-10807
10802
Radiative Association Kinetics of Methyl-Substituted Benzene Ions Yu-Wei Cheng and Robert C. Dunbar* Chemistly Department, Case Westem Reserve University, Cleveland, Ohio 44106 Received: December 1, 1994; In Final Form: March 31, 1995@
Dimerization of molecular ions with parent neutrals was observed for the series benzene, methylbenzene, p-dimethylbenzene (all at 196 K), and 1,2,4,5-tetramethylbenzene(at 230 and 247 K). From the pressure dependence of the observed rates the rate constants k3 for collision-stabilized association and k,, for radiative association were assigned. The results are analyzed along with the prior results for 1,3,5-trimethylbenzeneS The radiative association rate constants are all relatively low at these temperatures, ranging from 1.5 x (toluene) to 7.1 x cm3 s-l molecule-’ (mesitylene). The radiative association rate for benzene ion was below the limits of observation. The results all agree within a factor of 2 with the semiquantitative predictions of the “standard hydrocarbon” model. With additional assumptions, separate rate constants for photon emission from the collision complex and redissociation of the collision complex were derived. The photon emission rates (10-30 s-I) increase modestly with increasing complex size and are slightly larger than expected for typical hydrocarbon ions. The redissociation rates are of the magnitude expected from simple RRKM modeling, but correct modeling of the size dependence requires some modification of the simplest picture of substituent effects.
Introduction Association of ions with neutrals is increasingly recognized as an important constructive process for building larger aggregates in low-pressure ionized environments, notably interstellar clouds and, in the laboratory, low-pressure ion traps.’-” At low pressures, association normally proceeds by infrared radiative stabilization of the metastable complex ion. For small systems the process is inefficient except in exceptional cases, but for medium-sized ion-molecule systems (say more than 50 degrees of freedom) it has been found to be significantly efficient for a variety of systems. For yet larger systems a regime is approached where every collision is likely to result in sticking through radiative stabilization. We have a continuing interest in studying the kinetics of increasingly larger systems as the transitions among these three regimes occur. The present study of the dimerization of a series of methylbenzene ions explores the kinetics for cases which are all quite inefficient near room temperature, but span a range from low to high efficiency at interstellar cloud temperatures (typically -50 K). In order for radiative association to be observable in these systems, it is necessary to work at temperatures substantially below room temperature (196 K for most of these experiments.)The kinetics of radiative association of a gas-phase ion with a neutral molecule is controlled by a delicate interplay of effects of extemal temperature, binding energy, molecular size, and infrared radiative strengths. It is interesting to watch these different effects play out across a series of closely related molecules like the methyl-substituted benzenes. The similarity of complex structure and binding energy gives help in sorting out the influence of the different effects on the kinetics. Results are discussed for five methylbenzene systems: benzene, methylbenzene, 1,4-dimethylbenzene, 1,3,5trimethylbenzene, and 1,2,4,5tetramethylbenzene.These are shown below with their trivial names:
* To whom correspondence @
should be addressed. Abstract published in Advance ACS Absrracrs, May 15, 1995
0022-365419512099-10802$09.00/0
benzene
I
toluene
p-xylene
mesitylene
durene
Of these, mesitylene was described in a previous study.’ For benzene, radiative association was below the limit of detection, and only collisionally stabilized association was measured. For the others, both radiative and collisional association kinetics are reported and discussed here. Mautner et a1.I8measured dissociation enthalpies for the dimer ions from benzene to mesitylene by equilibrium methods. These values are included in Table 1. They reported a surprising trend, with the binding energies decreasing from benzene to p-xylene and then jumping back up at mesitylene. However, this odd trend was not convincingly greater than their experimental error and could be easily dismissed as data scatter. Radiative association rates are strongly sensitive to binding energy differences, and one of the main points of interest in the present work was whether this “mesitylene anomaly” could be observed and verified by following the radiative association rates across this series. At any particular pressure, ion-neutral association is observed as an apparent bimolecular reaction
where the apparent bimolecular rate constant k2 is a function of pressure. If k2 is plotted versus density of neutral molecules, the intercept gives the (second-order) radiative association rate constant k,,, and the slope gives the (third-order) collisional association rate constant k3. 0 1995 American Chemical Society
Kinetics of Methyl-Substituted Benzene Ions
J. Phys. Chem., Vol. 99, No. 27, 1995 10803
TABLE 1: Dissociation Thermochemistry and RRKM Parameters Eo" (cm-I) &?I, K (Cd K-') benzene 5960 3.1 toluene 5610 8.5 p-xylene 5470 13.4 mesitylene 6030 18.2 durene 6200 23.1 All the EO values were taken equal to the A&,, values from ref 18, except durene, which was fitted to the present results as described in the text. The following kinetic scheme is often taken as the basis for analyzing association results: A+
+M
kb
( A+M )* -!!- ( A+M ) \
k0lM1
f
The metastable complex (AfM)* is stabilized both by collisional energy transfer (kc) and by emission of infrared photons (kr) and can also redissociate back to reactants (kb). As pointed out by Kofel and McMahon,'* assuming this mechanism permits the photon emission rate and the redissociation rate to be calculated from the observed quantities via the equations
and
(4) In these equations the first equality corresponds to the exact low-pressure solution of the kinetic scheme. The second, approximate, equalities hold when k,
