The important role played by the hydroperoxyl radical, HO*, as an inter- mediary in chemical reactions has long been known. HOz is being increasingly.
Journal of Molecular Structure, THE OCHEM Elsevier
Science
Publishers
108 (1984)
B.V.,
Amsterdam
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AN AB INITIO MOLECULAR ORBITAL STUDY OF THE INTERACTIONS AND REACTIONS OF THE HYDROPEROXYL RADICAL, HO*, AND RELATED SPECIES*
C. N. R. RAO**, and U. CHANDRA
G. V. KULKARNI,
A. MURALIKRISHNA
RAO
SINGH
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012 (India) (Received
8 September
1983)
ABSTRACT Ab initio molecular orbital calculations show that the dimer of HO, may possess a symmetric cyclic hydrogen bonded structure. Energy changes accompanying a number of reactions of HO, of interest in atmospheric chemistry have been evaluated. H,O, is shown to be stable with respect to HO, and OH radicals. Ionization of HO, to 0; as well as the reaction of HO, with 0; and ethylene have been examined. HO, and 0; are shown to form stable hydrogen bonded complexes with H,O. INTRODUCTION
The important role played by the hydroperoxyl radical, HO*, as an intermediary in chemical reactions has long been known. HOz is being increasingly recognized as a key species in atmospheric chemistry [l, 21 and combustion chemistry [3]. Typical primary reactions involving HO, in atmospheric chemistry can be seen in the recent papers of Thrush and Wilkinson [4] , Cox et al. [ 51 and Sander et al. [6]. Of particular importance is the dimerization of HO* followed by disproportionation as also the reaction of HO2 with species such as OH and 03. Action of HO* in biological systems has been investigated by several workers [ 7, 81, especially in relation to lipid peroxidation. It has been recently suggested [9] that HO* may indeed be responsible for some of the biological effects presently attributed to the superoxide ion, O;, the concentration of the latter being directly related to that of HO*. Although the HO* radical has been characterized spectroscopically [ 10,111 and its structure investigated by quantum mechanical methods [12, 131, energies of the interaction of HO, with other species have not been investigated except for the 1:l linear hydrogen bond interaction of HO* with water and ammonia [ 141. Based on thermodynamic considerations [ 151, Hamilton [2] rules out the possibility of dimerization of HO,. Giguere [16], on the *Contribution No. 190 from **To whom all correspondence 0166-1280/84/$03.00
the Solid State & Structural should be addressed.
o 1984
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contrary, has recently pointed out that HOz may indeed form a stable sixmembered cyclic dimer. We have investigated the dimerization of HO, as well as its interaction with H,O, 0; and ethylene by carrying out ab initio MO calculations. More importantly, we have examined the energetics of some of the important reactions involving HO, such as its generation from the reaction of H,O, or 0, with OH radicals, its disproportionation to H,Oz and 0,, its reaction with OH and O3 and its ionization reaction in water. As part of the study, we have also examined the interaction of HO, and 0; with H,O. The reaction between HO? and OH has been investigated in some detail recently [ 17-191 and H,O, has been considered to be the transition state in this reaction [ 191. We have examined four possible structures of H,03 in the present study. RESULTS
AND DISCUSSION
Calculations on the HO2 radical employing both the STO-3G [ 201 and the 4-31G [21] methods gave optimized structural parameters and energies (Table 1) were similar to those reported in the literature [13, 141. We next carried out calculations by both the methods on linear as well as cyclic dimers of HOz. Among the two possible linear structures II and III (Fig. l), the singlet species (II) was unstable with respect to the monomer radicals. The symmetric, planar six-membered hydrogen bonded structure, IV, was more stable than the linear triplet dimer III (Table 1). The dissociation energy of the cyclic structure IV found by 4-31G calculations is about 31.6 kJ mol-’ compared to 15.7 kJ mol-’ of the stable linear dimer III (Table 1). This stability probably arises from the presence of two hydrogen bonds in the cyclic structure. It is interesting that Giguere [16] has in fact suggested that the cyclic dimeric structure of H,04 is likely to be stable from the structural standpoint. Our calculations on nonplanar cyclic structures of the dimer [22] showed them to be unstable compared to the planar cyclic dimer IV. The value of 31.6 kJ mol-’ calculated by us for the dissociation energy of the cyclic dimer is, however, close to the estimates of Nangia and Benson [ 231 and of Diem et al. [22]. Earlier MO calculations [ 141 show that HO,? forms stable complexes with H,O and NH3. Our STO-3G calculations on HO, * HzO, V (Fig. l), yield a dissociation energy of 36.8 kJ mol-’ (Table 1). This hydrogen bond is considerably stronger than that found in the HO* dimer, providing some indirect justification for considering the H,04 dimer as a Van der Waals complex
r221. The importance of the HO, radical in atmospheric reactions was mentioned earlier. The most important reactions involving the HO,! radical are OH + H,Oz + HO* + H,O
(1)
OH + O3 + HOz + O2
(2)
115 TABLE 1 Structural parameters, total energies and dissociation energies of various species related to HO, radical* Structural parametersb
H,O,(III)c
= 1.00 (0.951) r(O-0) = 1.35 (1.391) LO-O-H = 104 (196) r(O-0) = 1.31 (1.59) r(0.a.H) = 2.00
H,O,(IV)C
LO--O...H = 123 LO-He..0 = 150 r(O...H) = 2.10
HO,(I)
0;
HO, - H,O(V) H,O* O;(VI)
HO, - O;(VII)
HO, - O;(VIII)
H,O,(IX)
HO,. H,O(XIV) C,H, * HO&XVI)
C,H, - O,(XVII)
r(O-H)
LO-H...O LO-O-.-H r(0.e.H) = LO-Ho..0 r(0.a.H) = LO-H...O LO--O...H
= 125 = 130 1.64 = 180 1.98 = 124 = 124 r(O...O) = 1.89 LO-O...0 = 98.9 LO*.*O--O = 104 r(O...H) = 1.17 r(O...O) = 2.05 ~0-0.a.H = 93.7 LO-H...O = 159 r(O-0) = 1.396 r(O-H) = 1.00 LO-O-O = 105 LO-O-H = 102 Dihedral angle = 85 r(O...H) = 1.95 LO-H-. .O = 180 r(C-C) = 1.52 r(C...O) = 1.43 r(O-0) = 1.40 r(C-C) = 1.56 r(C-0) = 1.47 r(O-0) = 1.36 LC-O-O = 107
Total energy (a.u.)
Dissociation energy (kJ molI1)d
-148.197 (-149.963)
-
-147.384 (-149.363) -296.397 (-299.932)
-
-296.398 (-299.938)
10.8e (31.6)
-223.176
36.8
-222.386
97.7
-295.606
65.6
-295.651
183.8
-222.580 (-225.260)
48.8f (26.4)
-296.983
29.8
-225.282
62.0
-225.362
(1;:;)
273
aResults from 4-31G calculations are given in parenthesis.bOptimized STO-3Gparameters are shown; distances are in a and angles are in degrees. cf_O--O-H remains the same as in HO,. dCorresponds to dissociation to component species. eCorresponds to two hydrogen bonds instead of one hydrogen bond as in III. fCorresponds to dissociation to HO, and OH radicals.
116 O-H
O-H
/
o-o
o-o
H---_-O
o-o
/ H
/ o-o
/ H-O
O-_H-__--0 4
i”p
o(H-llp
‘H
0-
It--O (P)
(lx)
(Xl)
(II)
(II
(XI)
(ml
-0
(IZIlT)
Fig, 1. Structures of HO,, HO, dimers, HO, . H,O, 0;. H,O and HO, * 0; investigated in the present study.
HO2 + HO, -+ H,Oz + 0,
(3)
HOz+OH+HzO+O,
(4)
HOz+03+OH+202
(5)
Of these reactions, (1) and (2) generate HO, while (3)-(5) use up the HO* formed. We have obtained the total energies of all the species involved in reactions (l)---(5) and these are given in Table 2. Some of these energies were also available in the literature and our values agree with them. We have evaluated the energy changes involved in reactions (l)-(5) and find them all to be exothermic and hence energetically favourable. Thus, the STO-3G energy changes in reactions (l)--(5) are -84, -704, -13, -98 and 625 kJ mol-’ respectively. The corresponding values from 4-31G calculations are -66, -377, -26, -92 and -440 kJ mol-’ respectively. It is to be noted that of the five reactions considered, reactions (2) and (5) involving 0, are especially exothermic. We have investigated whether the species Hz03 supposedly formed in reaction (4) between HO* and OH radicals is a stable one [17-191. We have considered four possible structures of H,03, IX-XII (Fig. 2). Of these, IX is a singlet while the other three are hydrogen bonded triplet structures. We have found only the singlet structure IX to be stable with respect to the component HO* and HO radicals. The dissociation energy of H,03 into the radicals estimated by the STO-3G method is quite large (49 kJ mall’) and that by 4-31G is 26.4 kJ mall’. The molecule is nonplanar, the two HO0 planes making an angle of around 85”. HO2 may be considered to be the acid of the superoxide radical anion, 0; the equilibrium between 0; and HO, being of importance in biological reactions [ 91 HO2 + H,O + O;+
H30+
(6)
The optimized O-O distance in 0; is found to be 1.31 and 1.59 ,4 by the STO-3G and the 4-31G calculations respectively, the total energy being larger
117 TABLE 2 Structural parameters and total energies of the various component involving HO,
species in reactions Total energyb (a.u.)
Structural parameter@
OH-
r(O-H)
= 1.068
OH’
r(O-H)
= 1.014
H*O
r(O-H)
= 0.989
H,O+
LH-O-H = 100 r(O-H) = 0.991) LH-O-H
-74.065 (-75.230) -74.365 (-75.287) -74.966 (-75.909) -75.330 (-76.201)
= 114
0,
r(O-0)
= 1.217
H*O*
r(O-0)
= 1.398
-147.634 (-149.376) -148.765 (-150.560)
r(O-H) = 0.999 ~H-0-0 = 101.7 @HOOH = 160 r(0-0) = 1.321 LO-G-O
= 152
r(O,-0,)
= 1.35
-221.199 (-223.909) -222.007 (-224.614)
= 1.41 r(O-H) = 0.999 Lo-o-o = 105 LO-O-H = 99 r(O,-O,)
aOptimized STO-3G parameters are shown; distances are in W and angles are in degrees. bResults from 4-31G calculation are given in parenthesis.
/” y’O\
p-”
‘,6-i
J--J
o_o,H
. ..-__
i
,_~o-”
.H-0
/” \H
/
01O (XIX)
1
‘\,H
4
(Xm)
Hi\,
;>c_C;_y
