10150
J. Phys. Chem. 1996, 100, 10150-10158
Quantitative Structure-Stability Relationships for Oxides and Peroxides of Potential Atmospheric Significance M. A. Grela and A. J. Colussi* Department of Chemistry, UniVersity of Mar del Plata, 7600 Mar del Plata, Argentina ReceiVed: September 15, 1995; In Final Form: April 1, 1996X
A critical analysis of the most recent experimental and ab initio thermochemical data on gas-phase species reveals that (1) isodesmic metathetical reactions among simple oxides, 1/2XOX + 1/2YOY T XOY, and those exclusively involving peroxides 1/2XOOX + 1/2YOOY T XOOY, are nearly thermoneutral (∆Hr ) 0 ( 3 kcal/mol), but exchange reactions between oxides and peroxides, XOX + YOOY T YOY + XOOX, are generally not, (2) bond additivity values BAV[X-(O-O)-X]’s for the contributions of the peroxide bond to the heats of formation of H-, C-, N-, S-, F-, and Cl-containing peroxides decrease linearly with Pauling electronegativities χ(X)’s and (3) semiempirically evaluated (MOPAC) thermochemical datasets, even when inaccurate in absolute terms, are internally consistent with such rules. These findings happen to proVide the basis for the rapid and systematic assessment of the stability of noVel oxides and peroxides. In this paper, that focuses on species that could be formed in the terrestrial atmosphere, we briefly discuss XOO-H bond energy trends in hydroperoxides, the existence of stable and metastable peroxy XOO• radicals, the thermochemistry of peroxyacyl nitrates, of Br- and S-containing peroxides, and the mechanism of the (FO + ClO), (FO + NO2), (OH + OClO), (HO2 + ClO), (ClO + NO2), and (ClO + NO3) reactions, among other issues.
Introduction The interpretation of rates and products of bimolecular reactions between polyatomic radicals often rests on the involvement of weakly bound intermediates.1 For example, the reaction between (HO + OClO) seems to proceed via asnever detectedsHOOClO adduct en route to the observed (HOCl + O2) products.2 Chemical models of the dramatic loss of polar stratospheric ozoneswhich takes place in a fairly dilute environment, ca. 200 Kscrucially rely on properties of poorly characterized compounds such as ClOOCl.3 Thus, fleeting species not readily amenable to isolation and identification in the laboratory, i.e., likely to be ignored in a preliminary analysis, may be nevertheless essential in understanding important atmospheric processes and phenomena.4-8 These considerations suggest the need for simple, reliable rules for predicting the thermal stability of conceivable molecular structures. The emergence of improved theoretical calculation methods promises help in the task of postulating possible intermediates. However, even high-level ab initio energy calculations can fall short of chemical accuracy if precautions are not taken to control systematic errors.9-11 It is current practice to assume uniform convergence above certain levels of theory in estimating absolute errors arising from basis set deficiencies and limitations in the treatment of electron correlation effects. More often, the desired ∆Hf is obtained on a relative basis by evaluating the heats of isodesmic reactions involving the compound of interest, in conjunction with calibrated thermochemical data for the other species.4,9a In this case systematic errors incurred in calculations for individual molecules are expected to cancel out provided that the same number of bonds of each particular type appears on both sides of the reaction. Such assumption is reminiscent of bond additivity schemes; it actually suggests that an intelligent selection of the participating speciessbased on a judicious definition of bond similaritysmight render the chosen isodesmic reactions not only error free but thermoneutral.5,12 X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(95)02763-8 CCC: $12.00
In this paper we expand the scope, improve the predictive abilities, and define the limits of our recently developed procedure for the estimation of heats of formation of simple oxides and peroxides from bond contributions.5 Because nonbonded interactions tend to fall off sharply with distance,12 we hoped that bond additivity would hold reasonably well for peroxides.13 Actually, we found that isodesmic exchange reactions among peroxides (eq 1), or among oxides (eq 2):
/2XOOX + 1/2YOOY T XOOY
(1)
/2XOX + 1/2YOY T XOY
(2)
1
1
are indeed thermoneutral to within (3 kcal/mol, but that formally isodesmic reactions such as those interconverting oxides and peroxides (eq 3):
XOOX + YOY T XOX + YOOY
(3)
can release as much as 25 kcal/mol (e.g., if X, Y ≡ H, F). In other words, peroxide bonds are unique, i.e., their enthalpic contributions are not generally transferable.5 However, we verified that they correlate well with the Pauling electronegativities of attached atoms. In this manner we arrive at a small set of remarkably simple rules that allow us to estimate the heats of formation of a wide variety of oxides and peroxides systematically and efficiently. When available, the agreement with ab initio or independently evaluated data is within the stated errors, with the conspicuous exceptions of CF3-containing species,14-16 and some compounds of hypervalent chlorine.17,18 It is widely agreed that the omission of key intermediates and processes is the most serious flaw of postulated reaction mechanisms. Our pragmatic approach, essentially a learning exercise that takes full advantage of existing information, lets one carry out global searches for adducts of potential atmospheric relevance based on trends established by previous data, rather than on case-by-case theoretical calculations.19 Present results may provide plausible hints about the thermal stability © 1996 American Chemical Society
Quantitative Structure-Stability Relationships
J. Phys. Chem., Vol. 100, No. 24, 1996 10151
TABLE 1: Heats of Formation (kcal/mol, at 300 K) A. Gaseous Species speciesa
∆Hfb
H 2O HOOH HOCl HOClO HOClO2 HOClO3 ClOOH ClOOCl ClOClO Cl2O OClOOH O2ClOOH O3ClOClO3 ClOClO2 OClOClO FOH FOOH FOOF F2O FOCl FOOCl FClO FOClO FClO2 FClO3 FONO FOONO CH3OH CH3OCH3 CH3OOCH3 CH3OOH CH3OONO2 CH3ONO2 CH3(CO)OONO2 HONO HONO2 HOONO2 HOONO
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
-19.2
-0.1 40.8 19.8 14.6 64.8 59.8 -25.9 -13.0
12.7 18.5 -9.2 32.9 12.9 19.4
-31.3 -11.2 -60.7
∆Hfc
refs
-57.8 -32.6 -18 1.0, 11.9 -4.2, 4.2 -1.5, 10.8 0.2 31.0 42.1 19.5 25.3 33.1 65.0 37
6 6 6 5, 18a 5, 18c 5, 18b 22 6 30a 6 18c 18b 12b 6
-23.4 -9.3 5.0 5.9 13.4 36.8 7.4
6 23a 6 6 48 27 17
-8.0 -6.0 12.0
6 6 5, 17c
-48.2 -44.0 -30.0 -31.3 -10.6 -28.6 -62 -19.0 -32.3 -12.5
49 50 35 6 6 6 6 6 6 6
-2.2
speciesa 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76
O2NONO2 ONONO ONNO2 ONOONO FONO2 ClONO2 OClNO2 OClONO ClOONO BrOClO3 BrOONO2 BrONO2 OClONO2 ClOONO2 HOS(O2)OH SF6 F2SO2 Cl2SO2 HOS(O)OH HOS(O)OOH FS(O2)OH FS(O2)OOF FS(O2)OOH [HOS(O2)]2O [FS(O2)O]2 [FS(O2)]2O [HOS(O2)O]2 HOS(O2)OOH HOS(O2)ONO HOS(O2)ONO2 HOS(O2)OONO HOS(O2)OONO2 BrOH Br2O BrOOBr BrOOH BrOOCl BrOClO
∆Hfb 3.2 19.8 31.6 -0.4 6.4 4.8 39.8 32.3 42.3 25.6 11.5 26.5 19.0
-103.9 -178.2 -136.4 -155.1 -284.2 -278.3 -288.6 -273.9 -152.9 -137.2 -140.5 -120.3 -133.6 -15.8
∆Hfc 2.7 19.8 33.3 2.5 5.5
refs 6 5, 51 6 25 6, 52 6
