Prediction of the abiotic degradability of organic compounds in the troposphere

Abstract

A statistically relevant correlation between the reaction rate coefficient, koH , for the OH radical reaction with 161 organic compounds in the gas phase at 300 K, and the corresponding vertical ionisation energies Ei, v, reveals two classes of compounds: aromatics where -log(koH/cm3s -1) -~ 3/2 Ei, v(eV) -2 and aliphatics where -log(7;OH/Cm 3 s-l) ~_ 4/5 El, v(eV) + 3. The prediction of the rate coefficient, koH , for the reaction of OH with organic molecules from the above equations has a probability of about 90%. Assuming a global diurnal mean of the OH radical concentration of 5 x l0 s cm s, the upper limit of the tropospheric half-life of organic compounds and their persistence can be estimated.

Estimations of OH radical rate constants from H‐atom abstraction from CH and OH bonds over the temperature range 250‐1000 K

“…More recent approaches to estimating (or rationalizing) H-atom abstraction rate constants by OH radicals [ 10-17~9-211 have extended this earlier method to take into account the dependence of differing C -H bond dissociation energies on the particular C -H bonds from which H-atom abstraction occurs, using literature C -H bond dissociation energies [10,15,16,20] or empirical methods [11,[12][13][14][17][18][19]211.…”

Correlations between experimental and theoretical adiabatic ionization energies for organic compounds and rate constants for atmospheric reactions with hydroxyl radicals

“…With the exception of the work by Gusten et al [7] on aliphatics (n=129), the datasets to date have been limited in size and scope, and have not considered in detail the use of theoretical methods for estimating IEs within these types of models. This latter consideration is critical, as the majority of compounds for which estimated k OH values are desired have either not been synthesized experimentally, or do not have experimental IEs available in the open literature.…”

“…Gaffney and Levin [5] found high correlation coefficients between experimental IEs and log k OH for a small set of 18 alkenes and dienes that react via hydroxyl radical addition (r=0.97, m=-0.61±0.04, b=-4.68±0.37), as well as a separate minimal set of 4 oxygenated and halogenated alkenes that also react via the addition mechanism (r=0.95, m=-0.57±0.13, -5.42±1.24). Subsequently, Gusten et al [7] reported separate aromatic and aliphatic based correlations between experimental vertical IEs and experimental -log k OH for larger sets of organic compounds: aromatics, n=32, r=0.95, s=0.29, m=1.52±0.10, b=-2.06±0.84; and aliphatics, n=129, r=0.95, s=0.36, m=0.79±0.02, b=3.06±0.24. More recently, Grosjean and Williams [12] published a correlation between the log k OH for various unsaturated organic compounds and IE (n=36, r=0.89, m=-0.44±0.04, b=-6.23±0.34), along with a separate correlation for a small subset of chlorinated olefins (n=7, r=0.81, m=1.44±0.47, b=-25.6±4.6).…”

“…These two properties often correlate well against each other, but the IE is the easier to obtain -particularly as molecules become more complex. For example, even a relatively small semi-volatile organic compound may have >10 different BDEs, but only a single IE value.For this reason, several studies have investigated univariate correlations between IEs and k OH [5,7,12,15]. Gaffney and Levin [5] found high correlation coefficients between experimental IEs and log k OH for a small set of 18 alkenes and dienes that react via hydroxyl radical addition (r=0.97, m=-0.61±0.04, b=-4.68±0.37), as well as a separate minimal set of 4 oxygenated and halogenated alkenes that also react via the addition mechanism (r=0.95, m=-0.57±0.13, -5.42±1.24).…”

“…In addition to experimental approaches, theoretical methods for predicting the rate constants for reaction of organic compounds with the hydroxyl radical (k OH ) have been the focus of a number of studies. In general, these predictive investigations have taken one of three general tactics [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]: (1) rigorous and time-intensive medium-through high-level computational studies (e.g., density functional theory, composite methods) on all possible mechanistic pathways for the reaction of a particular compound with the hydroxyl radical (i.e., addition to unsaturated groups, hydrogen abstractions, etc. ); (2) regression based quantitative structure-activity relationship (QSAR) models employing a range of two-and threedimensional molecular descriptors; and (3) univariate correlations with physicochemical properties (e.g., ionization energies [IEs], bond dissociation enthalpies [BDEs]).…”