Introduction

This paper, specifically prepared for the orkshop on Chemical Kinetic Data Needs for cdeling the Lower Troposphere, is built around everal key questions proposed to the speakers by he organizers.

Why is this topic important with respect
to the chemistry of the lower troposphere?

A simplified general scheme for understanding e chemistry of the lower troposphere is given figure 1. We see the role of organic free dical chemsitry in those mechanisms, and we e quickly led to understand that modeling of is complex chemistry will require knowledge of any rate constants involving organic radical ecies, both aliphatic and aromatic. In fact, can readily see that the numbers of individual te constants which will be needed is enormous. us, we need to be able to make reliable predicons and estimations based on a carefully lected data base.

ΔΕ

For the disproportionation reaction

RNN'R SNN'S RNN'S + SNN'R

=

σ

any additivity approximation assumes that A = ΔΦ where is any molecular property, and A is the contribution to that property due to symmetry changes and optical isomerism. For the molecular properties of interest here, AHT → 0, 0, and AST S = R In K where 'K a(RRNN'R)o(SNN'S)/o (RNN'S)o, SNN'R), o(X) being the symmetry number including both internal and external symmetry. An additional term for entropy of mixing, due to the existence of optical isomers, must also be included.

p,T

σ

ΝΟ

OH + NO2

H02

ig. 1. A simplified scheme for the chemistry of the lower troposphere.

1

Figures in brackets indicate literature references at the end of this paper.

If the molecular framework NN' is two atoms or greater, these relationships imply the additivity of group properties, which include all nearestneighbor interactions, since a group is defined as an atom together with its ligands (e.g., in the group C-(H)3(C), the central C atom is bonded to three H atoms and one C atom). Thus, the equation

CH3OH + CH3CH2OCH3 CH3CH2OH + CH3OCH 3 implies the additivity of the properties of the groups C-(H)3(C), C-(H),(O), O-(C) (H), C-(H)2(C) (0), and 0-(C)2, if the appropriate ▲ =

We have developed group additivity methods that permit the estimation, for many organic chemicals in the gas phase, of heats of formation ± 1 kcal/mol, and of entropies and heat capacities to + 1 cal/(mol-K), from which free energies of formation can be derived to better than ± 2 kcal/ mol.

It should be noted that entropy and heat capacity are molecular properties that can be accurately estimated under much less stringent conditions than energy (or enthalpy). Thus, the method of bond additivity seems to work quite well (± 1 cal/(mol-K)) for estimating the former properties, but not at all well (± 4 kcal/mol) for the latter.

Structural Considerations and Model Compounds. If sufficient thermochemical data is lacking for the estimation of group properties, entropy, and heat capacity can often be adequately estimated from structural parameters of the molecule. (Enthalpy estimates are more difficult, requiring a better knowledge of potential functions than are usually available). The methods of statistical thermodynamics may be used to calculate Co and So directly for those molecules where a complete vibrational assignment can be made or estimated.

Also, "reasonable" structural and vibrational frequency "corrections" to the corresponding established thermodynamic properties of "reference" compounds may be made. A suitable choice of reference compound, i.e., one similar in mass size and structure to the unknown, assures that the external rotational and translational entropies and heat capacities of the reference and unknown compounds will be the same and that many of the vibrational frequencies will be similar. The basic assumption is that S° and Co difference can be closely estimated by considering only lowfrequency motions thought to be significantly changed in the unknown. Fortunately, entropies and heat capacities are not excessively sensitive to the exact choice of these vibrational frequencies, and estimates of moderate accuracy may be made with relative ease.

Kinetics. The extension of thermochemical estimation techniques to the evaluation of kinetic data rests largely on the validity of transition state theory.

The transition state theory expression for a thermal rate constant is:

of interest to us will be energy transfer

controlled bimolecular processes).

In hydrocarbon reactions in the troposphere, ncluding those of aromatic compounds, we may xpect that most direct metathesis reactions ill involve the exchange of a hydrogen atom etween larger groups. A simple, semi-empirical rescription exists for estimating the value of S for these types of reactions. First, one ealizes that these values are limited between le "loosest" possible model (A-factor equals gas inetic collision frequency) and the "tightest" ssible model in which R...H...R' is represented the molecular R-R'. Experience using data in le 300 < T/K < 700 has taught us that generally le S value corresponds to a transition state ly slightly looser than the tightest possible lue.

Since the other two classes of bimolecular ocesses are the reverse of unimolecular reacons, we may consider them in that direction. he equilibrium constant is either known or timable). Once again using experimental results our guide, we note that model transition state ich correspond to the values of AS are generay "tight". That is, we may visualize them as nor modifications of the reactant molecule, ually involving some increase in rotational tropy due to slight enlargement of certain nds. The dominant entropic feature is usually e stiffening of internal rotations as a result multiple bond formation or ring formation [2].

Bond scission reactions present a particular blem, since it is particularly difficult to cate a transition state. Recent work [2,4], h experimental and theoretical, indicates that se reactions can be modeled with a transition

rises.

=

The recalculated data are presented in figures 2 through 5, and the corresponding Arrhenius parameters are presented in table 1. The data for t-BuO are the most extensive (fig. 2), covering nearly four orders of magnitude. The individual sets of experimental data taken independently show a rather wide range of Arrhenius parameters and appear to be inconsistent, but taken together, the actual data give a reasonably good straight line with parameters, log k/s1 = 15.1 - 16.2/0. Given the entropy change of the reaction, AHO 41.2 Gibbs/mole, the A-factor for the reverse reaction is A = 107.9 M1 s1, a value very close to that for the reaction of methyl radicals with isobutene (log A = 8.0) [10]. This suggests a self-consistent method for evaluating and codifying the limited data available for the other alkoxyl radical reaction: choose an A-factor for the reverse reaction and find the corresponding activation energy. If this unified scheme is used, the alkoxyl decompositions can be considered together as a class, rather than individually.

-1

The decomposition of an alkoxyl radical is the reverse of the addition of an alkyl radical to the carbon atom of a carbonyl group, which is analogous to alkyl radicals adding to the 2-position of a primary olefin. Šince data are only available for alkyl radicals adding to the 1-position of primary olefins, the assumption was