Environmental Toxicology and Chemistry, Vol. 7, pp. 435-442, 1988 Primed in the USA. Pergamon Press pie 0730-7268/88 $3.00 + ,00 Copyright © 1988 SEI AC ESTIMATION OF GAS-PHASE HYDROXYL RADICAL RATE CONSTANTS FOR ORGANIC CHEMICALS Roger Atkinson Statewide Air Pollution Research Center, University of California, Riverside, California 92521 (Received 17 June 1987; Accepted 5 November 1987) Abstract-A structure-activity relationship developed for the estimation of rate constants for the gas-phase reactions of the hydroxyl radical with organic compounds is described and extended to certain organophosphorus compounds. The use of this estimation method is illustrated. Keywords-Hydroxyl radical Organic compounds Atmospheric lifetimes Gas-phase reactions Structure-activity relationship introduction Organic chemicals emitted into the troposphere are removed by chemical reaction with a number of reactive intermediate species, including OH, H02 and NO-, radicals and 03, and by photolysis and wet and dry deposition [1]. For the majority of organic chemicals present in the troposphere, reaction with the OH radical during daylight hours is expected to be the dominant atmospheric removal process [1,2] leading to transformation of the initial chemical to a variety of reaction products. Thus, for example, alkanes and other saturated organic compounds, such as alcohols, esters and ethers, react with the OH radical by abstraction of an H atom from C —H or O — H bonds: OH + CH3CH2CH2CH3 -> H20 + CH3CHCH2CH3 ~> H20 -f- CH2CH2CH2CH3 Alkenes, alkynes and other organic compounds containing /C=C\ and -CSC- bonds react with the OH radical mainly by OH radical addition to the carbon-carbon double or triple bonds: Presented at the Symposium on Structure-Activity Relationships in Environmental Toxicology and Chemistry, 193rd National Meeting of the American Chemical Society, Denver, Colorado, April 6-8, 1987 (co-sponsored by the Society of Environmental Toxicology and Chemistry). The research described in this article was funded by the U.S. Environmental Protection Agency and the California Air Resources Board; however, it has not been subjected to agency review and therefore no official endorsement should be inferred. OH + CH3CH=CH2 -> CH3CHCH2OH and CH3CHOHCH2 Compounds containing aromatic rings react with the OH radical by addition to the ring and H-atom abstraction from the substituent group(s): plus other isomers Subsequent reactions of these initially formed radicals lead to a wide spectrum of product species. In order to allow the dominant tropospheric removal process(es) of organic compounds to be assessed and their atmospheric lifetimes calculated, it is necessary to know the reaction rate constants (over the temperature range appropriate for the troposphere) and the ambient atmospheric concentrations of the reactive intermediates, While rate constants for the gas-phase reactions of a large number (400 or more) of organic chemicals with the OH radical have been measured to date [2], the inherent complexity and concurrent low volatility of many industrially and agriculturally used organic chemicals make such measurements difficult and time- and cost-consuming. 435 436 R. Atkinson With the increasingly large database available, the development and use of estimation techniques for the reliable calculation of OH radical reaction rate constants for a wide number of classes of organic compounds have become feasible. Such estimation techniques can be classified into two general categories: those that rely solely on the structure of the organic chemical (structure-activity relationships, or SARs) and those that utilize a physical or chemical property, such as the ionization potential or bond dissociation energy or correlations between gas- and solution-phase OH-radical reaction rate constants. The intrinsically simplest of these estimation techniques, which should be applicable to all or-ganics, are those utilizing SARs, and these are of interest to regulatory agencies in preparing risk assessments of organic chemicals emitted into the atmosphere. Recently, two SARs have been developed, one by Davenport et al. [3], based upon an update of the earlier Hendry and Kenley SAR [4], and the other by Atkinson [2,5,6]. In this article, the latter SAR [2,5,6] is described and further extended to deal with organophosphorus compounds. The use of this estimation method is illustrated and several environmentally relevant reaction rate constants are estimated. formulation of the sar The estimation method utilizes the fact that there are several separate OH radical reaction pathways that can be dealt with individually [2]. As presently understood, these processes involve (a) H-atom abstraction from C—H bonds in alkanes, carbonyls and other saturated organics, and from O—H bonds in alcohols and glycols, (b) OH radical addition to /C=c( and -C=C~ unsaturated bonds, (c) OH radical addition to aromatic rings and (d) OH radical interaction with nitrogen-, sulfur- and phosphorus-containing groups. The total OH radical reaction rate constant is then given by ^totai = &(H-atom abstraction from C—H and O-H bonds) + k(OU radical addition to )c=c( and — C^C— bonds) + /c(OH radical addition to aromatic rings) + k(OH radical interaction with N-, S- and P-containing groups) (All rate constants in this article have units of cm3 molecule""1 s"1.) These separate processes are dealt with below. H-atom abstraction from C~H and O—H bonds As discussed by Atkinson [2,5,6], the calculation of overall H-atom abstraction rate constants is based on the estimation of — CH3, —CH2—, /CH— and —OH group rate constants. The —CH3, —CH2— and /CH— group rate constants depend on the identity of the substituents around those groups, with Ar(CH3-X) = Arp°riniF(X) Jfc(X-CH2-Y) = Ars°ecF(X)F(Y) and k(x~CH(^) = Ar°,.,F(X)F(Y)F(Z) where k°rim, k°ec and &,°rt are the rate constants per —CH3, — CH2— and /CH— group for a "standard" substituent; X, Y and Z are the substit-uent groups; and F(X), F(Y) and F(Z) are the corresponding substituent factors. The standard substituent group is chosen to be X(= Y = Z) = -CH3, with F(-CH3) = 1.00 by definition [2]. As discussed previously [5,6], the group rate constants k£rim, k°c, /r°rt and k0H are best expressed in the temperature-dependent form, k -AT2e~E/T, rather than the simple Arrhenius form, k = A'e~EVT, and the A factors and values of E are given in Table 1. Thus, for «-butane, CH3CH2CH2CH3, the rate constant is calculated as (starting at the first carbon atom and progressing from carbon atom to carbon atom): Table 1. Temperature-dependent parameters, in the form, k = AT2 e~E/T, for CH3~, -ch-, >CH-and —OH group rate constants [6] 1012 x k (298 K) 10l(lxA (cm3 molecule"1 s ') e(K) "■prim 0.144 4.47 303 lr° "•sec 0.838 4.32 «233 'Mcrt 1.83 1.89 -711 0.036 1.89 460 Gas-phase hydroxy! radical rate constants 437 ^lotal — ^priinF( CH2 ) + *S°WF(-CH,)F(-CH2-) + Ar°ccF(-CH3)F(~CH2~) + *P°r.mF(-CH2-) where F(~-CH3) = 1.00 and the substituent factors F(X) are given in Table 2 for 298 K. Thus *,olBl = (0.144 X 1.29 + 0.838 X 1.00 X 1.29 + 0.838 x 1.00 X 1.29 + 0.144 x 1.29) X 10""'2 cm1 molecule"1 s~' at 298 K. The effects of ring strain are included by factors that account for the ring size; for example, for ethylcyclopentane, Table 2. Substituent factors F(X) at 298 K [6] CHCH2CH3 k= (2Ar°L.[F(-CH2-)]2 + 2^.F(-CH2-)F(>CH-) + Ar,;rl[F(-CH2-)]-,)F3 + ^°ccF(>CH-)F(-CH,) + CimF(-CH2-) where F5 is the factor for a five-membered ring (only the groups involved in the ring are modified by the ring factor [2,6,7]). For polycyclic systems, the ring factors are multiplicative, as shown in the next section. The substituent factors F(X), including factors to take into account the ring strain per ring, Frit,B, are given in Table 2 for 298 K. With the assumption that the effects of a substituent group do not affect the A factor [5], then F(X) = ec*'T. Examples of the calculation of OH radical reaction rate constants for organic molecules of varying complexity follow: For CH3CHO: k = *p°rlmF(-CHO) + *°rtF(-CH1)F(=0) For CH3CH2COCH3: k = *p°rimF(-CH2C(0)-) + *»KF(-CH3)F(-C(0)-) + *°rimF(-C(0)-) X F(X) at 298 K ™CH, 1.00 -CH:- )CH- 1.29 — F 0.099 -CI 0.38 -Br 0.30 -CH3Cl " | -CHCI, 0.57 —CH2Br „ -CCl, 0.090 -CH2F -0.85 — CHF2 -0.10 -CFjCI -0.025 -CF, 0.075 =0 8.8 -CCIO -0.5 -CHO ] -C(O)- j 0.76 ~CH2C(0) )CHC(0)- 4.4 7CC(0)~ J ~CftH, -1.0 /C = C\ ~c«c- „ 1 -1.0 -OH 3.4 -O- 6.1 -C(0)OR 0.0 -OC(0)R 1.5 -CH2ON02 "| )CHONO, 0.21 ^CONO; J -ONO3 0.10 -CN 0.14 -CH2CN 0.5 3-membercd ring 0.017 4-membered ring 0.22 5-membercd ring 0.80 6-membered ring 1.00 7-membered ring -1.0 438 R. Atkinson For CH3CH2OCH2CH2OH: k = A.-°rimF(-CH2-) 4- A:s°JF(-CH3)F(-0-) + Arg°ecF(-0-)F(-CH2-) + Ars°cc(F-CH2-)F(-OH) + kOH For CH3C(0)OCH(CH3)2: k = fcp0rtrnF(-C(0)OR) + ^n[F(~CH3)]2F(-OC(0)R) + 2*p°rimF(>CH-) For cyclohexyl nitrate: ON02 k=[3k°cW(-CM2-)]2 + 2£.°eF(-CH2-)F (>CHON02) + £t°rtF(-CH2-)2F(~ON02))F6 OH radical addition to )c=c( and — c=c— bonds The estimation method used to calculate the rate constants for OH radical addition to /C=C\ and —C=C— bond systems is based on the number of unconjugated double or triple bonds or conjugated double bond systems and the degree, identity and configuration of substitution around these double bonds. For example, for CH2=C(CH3)CH2CH=CH2 the rate constant is given by k = Ar(CH2=C<) + A:(CH2=CH-) + Ars°ecF()C=C<)2 + A:p0rimF(>C=C<) where £(CH2=c() and A:(CH2=CH-) are the rate constants for OH radical addition to 2-methylpropene and propene, respectively, and F(>C=C() = 1.00 (Table 2). For conjugated dialkenes, the )C=C—C=C\ structure is considered as a single unit, with the rate constant depending solely on the number of alkyl substit-uents on this structural unit. The rate constants at 298 K for the various )c=c(, — C=C- and )C—C—C=C\ structural units are given in Table 3. Hydrogen atom abstraction from vinyl hydrogens is negligible at temperatures of 500 K or below [2]. While the contribution of H-atom abstraction from C-H bonds on substituted alkyl groups to the overall room temperature rate constants is generally minor (being, for example, less than 20% for /7-heptene [6]), this is taken into account, as shown above for CH2=C(CH3)CH2CH=CH2. There are no ring strain effects for OH radical addition to /C=C\ bonds [2]. As an example, for bicyclo[2.2.1]-2,5-heptadiene, * = 2*(c«-CH=CH-) + (2/r1°ci,[f()c=c<)]2f(-ch2-) + ^,-[f(>ch-)]2)f52f6 Table 3. Room temperature group rate constants for OH radical addition to /C,= c(, — C«"C~-and )C=C-C=c( structural units [2,6] Structure 10" x k (cm3 molecule-1 $""') CH2=CH- 2.63 CH2«C( 5.14 cis -CH=CH- 5.61 trans -CH-CH- 6.37 -CH=c( )c=c< HCsC- 8.69 11.0 0.64 ~™ ESS 2.9 )c=C-C=c( unit One-alkyl substituent Two-alkyl substituents Three-alkyl substituents Four-alkyl substituents 10.5 13.5 18. 23. Gas-phase hydroxyl radical rate constants i 439 For alkenes containing substituent groups other than alkyl groups, substituent factors, C(X), are used. For example, for CH2=CXY the rate constant is given by /: = *(CH2=C<)C(X)C(Y) + k(H — abstraction from the X and Y substituent groups) The substituent factors C(X) at room temperature [6] are given in Table 4. As noted previously [2,6], a negative temperature dependence of the rate constants for these reactions, equivalent to an Arrhenius activation energy of —1 kcal mol"1, is generally applicable for calculating OH radical addition rate constants at temperatures of 500 K or below. OH radical addition to aromatic rings As discussed elsewhere by Zetzsch [8] and Atkinson [2,6], the linear correlation between the logarithm of the rate constant for OH radical addition to aromatic rings and the sum of the elec-trophilic substituent constants, 2 + (cm3 molecule"1 s" ') 2 a+ is calculated by assuming [8] that (a) steric hindrance can be neglected and a£^h0 is set equal to crp^ra; (b) 2a+ is the sum of all substituent constants of the substituents connected to the aromatic ring; (c) the OH radical adds to the position yielding the most negative value of 2 a+ (preferably to a free position); and (d) if all positions are occupied, the ipso position is treated as a meta position. Of course, the contributions of H-atom abstraction from C—H and O—H bonds on alkyl, —OH and — CHO substituent groups and OH radical interaction with — NH2 and —N(CH3)2 substituent groups (see below) must be taken into account, using F(aromatic ring) = 1.0 (Table 2), For example, for ethylbenzene, for which Sc+S! -0.295 [2], Table 4. Substituent factors C(X) at 298 K [6] Substituent X C(X) ~F -0.4 -CI 0.20 -Br 0.26 ~CH2CI 0.76 ~CN 0.15 -CHO 0.26 -COCH, 0.91 -OCH, 1.3 = 0 1.0 Ar(ethylbenzene) = 10[" -11-69' -i-"(-°-2«s>l + A^t.F(-C6HOF(-CH,) + CimF(-CH2-) As a further example, for 2,4-D CI OCH2C(0)OH the values of crp' (CI) and otf (CI) are 0.114 and 0.399, respectively. For the -OCH2C(0)OH group, the (perhaps gross) assumption is made that its effect is similar to that of an —OCH3 group, with values of o£ and < of —0.778 and 0.047, respectively. The most negative value of S N-, -SHand —S— containing compounds Under atmospheric conditions, the gas-phase reactions of the OH radical with thiols (RSH), sulfides (RSR'), disulfides (RSSR') and amines (RNH2, RR'NH and RR'R"N) proceed, at least in part, via initial OH radical addition to the S- and N-atoms [2]. The values of F(-S-), F(~~SH), F(-NH2), F(-NH-), F(~N<), F(-NNO), F(-NN02), At_sh> *-s-, *-ss-. *-nh2. *>nh. &)N_, &)NNO and A:)NN02 estimated [6] from the available data are given in Table 5. It should be noted that the substituent factors, F(X), given in Table 5 are applicable to H-atom abstraction from C—H bonds only, i.e., for CHxNHy compounds. A comparison of the calculated and experimental room temperature rate constants for the amines and related compounds and for S— containing compounds is given in Figure 2, and again the calculated and experimental data agree to within a factor of 2 (which may not be surprising, since essentially the entire data set was used to derive the necessary parameters). Table 5. Substituent factors F(X) for H-atom abstraction from c-h bonds and group rate constants at 298 K for OH radical reaction with thiols, sulfides, disulfides, amines and related organics [6] Substituent group X F(X) -SH -S- -ss- — nh2 — nh— -N< — nno -nn02 -sh -s--ss- 9.0 -NH2 > > >nn0 :nh nno, 10 10'2 x k (cm3 molecule'1 s""') 31 2.0a -200 20 60 60 0 "Applicable only for 760 torr total pressure of air and 298 K [6]. Phosphorus-containing organics A large number of phosphorus-containing organic compounds are used in industrial and agricultural operations. However, for these compounds, kinetic data currently are available for only six compounds of the general structure: OR I RX~-P=X I XR where X = 0 and/or S and R«CH3 or C2H, [11-13], The experimental data show that £(P=0) ~ zero; fc(P=S) = 5.5 x 10"11 cm3 molecule"1 s"1; and F(-OP-) = F(-SP-) » 20 (which can be compared to F(—0—) =6.1, indicating that the —OCH3 and — SCH3 groups bonded to a P-atom Gas-phase hydroxyl radical rate constants 441 Fig. 2. Plot of calculated versus experimentally measured room temperature rate constants for the gas-phase reaction of the OH radical with sulfur- and nitrogen-containing aliphatic compounds. The solid line indicates perfect agreement; the dashed lines indicate disagreement by a factor of 2. are more reactive than the — OCH3 groups in the aliphatic ethers). A comparison of the calculated and measured room temperature rate constants for these six phosphorus-containing compounds is given in Table 6. The agreement is excellent, and the data show that the inclusion of a P=S system leads to a high reactivity, with a calculated atmospheric lifetime of about 4 h for a daytime OH radical concentration of 1 x 106 molecule cm""' [14]. Table 6. Comparison of experimental and calculated room temperature rate constants for reactions of OH radicals with phosphorus-containing organics 1012 x k (cm3 molecule 1 s 1) Compound Calculated Observed" (CH30)3PO 8.6 7.4 (CH30)2P(0)SCH3 8.6 9.2 (CH^P^OCH, 8.6 9.5