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Structure and conformation in molecular peroxides
of Molecular Structure, 67 (1980) 35-44 Elsevier Scientific Publishing Company, Amsterdam
Journal
STRUCTURE
AND CONFORMATION
CHRISTOPHER
GLIDEWELL
Chemistry Department, (G t. Btitain)
University
(Received
1979)
18 December
-
Printed in The Netherlands
IN MOLECULAR
of St. Andrews,
St. Andrews,
Fife
PEROXIDES
KY 16 SST
ABSTRACT Molecular structures and energies have been calculated in the MIND0 approximation for fifteen neutral and anionic peroxides: fully optimized torsional potential functions have been calculated for twelve of these, and torsional potential functions, subject to constrained optimizations, for a further two peroxides. Bond dissociation energies D(R,O-ORJ were also calculated. Equilibrium structures and energies were calculated for the polyoxo species H,O,, HO,F, F,O,, HO, and FO, (TV< 4), and a complete set of bond dissociation energies derived for H,O,, HO,F, and F,O,. INTRODUCTION
In molecular peroxides, R1OORz, the dihedral angle 6 (R100R2) has been observed as large as 180” and as small as 87.5” [l] ,and the factors which determine the value of 6 in a particular peroxide are obviously of interest. Calculations on (Me3C)202 and (Me3Si)202 in the CNDO approximation [Z] , using experimental values for all the geometrical parameters except 6, indicated that in (Me3Q202 the coulombic repulsion was dominant in determining the torsional minimum, but in neither molecule did the torsional potential achieve the correct minimum. A torsional potential function has been calculated [ 31 in the MIND0 approximation for (PhC0)202: in this calculation the phenyl groups were treated as fixed hexagons and all other parameters, except for d(0-0) and 6, were taken from the optimization of (CH3C0)202. p this instance, the barriers will almost certainly be calculated somewhat too large, as a consequence of the non-optimization of the whole geometry. In the present paper, MIND0 calculations are reported on the structures, conformations and torsional potentials of fifteen molecular peroxides, ranging from HzOz and 0d2- to (CH3)&OOC3H5, together with calculations on the structures and conformations of the polyoxo species HO,H, FO,F, HO,F (n < 4) and their associated radicals HO, and FO,. METHOD
Energy calculations were performed with the IBM version of MIND0/3 [4] implemented on a Honeywell 66/80 computer. Except where specifically 0022-2860/80/000~0000/$02.25
0 1980
Elsevier Scientific Publishing Company
36
stated otherwise, no assumptions of any kind were made and the energy of each molecule was minimized with respect to all geometric variables simultaneously. Torsional potential energy calculations were made at fixed values of 6, with simultaneous optimization of all other parameters. Torsional potential functions were calculated in the form AH?=
Xn a, cos"B
Seven terms were usually employed, and the deviations between calculated and input AfIT values were generally of the order of 10-2-10-3 kJ mol-' . After calculation of the coefficients an, the functions were then recast into the form AH?
= z b, cos n6
In this form the value of AHj for the trans
AH? (tram)
= G (-1)”
conformer is given by
b,
and that for the cis by AH? (cis) = ;
b,
Equilibrium molecular properties are given in Table 1, and equilibrium geometries for peroxides in Table 2. The coefficients b, for the torsional potentials are available from BLLD as Sup Pub No. SUP 26163 (7 pages), and the rotation barriers for cis and tram conformers are given in Table 3. TABLE1 Molecularproperties at equilibrium Molecuie
H,O, HOOF F,O, CH,OOH CH,OOF CF,OOH CF,OOF (C%),O, (C,H,W, C(CH,),CH120, (CH,),COOC,H, (CH,CO),O, (1) (CH,CO),O, (11) W-WOW, (111)
=Seetext.
Totalenergy (eW -650.2043 -1110.9976 -1571.8824 -806.5054 -1267.3474 -2197.4404 -2658.0386 -962.7407 -1276.8307 -1589.9267 -1715.3021 -1840.7687 -1840.2552 -1840.4613 -1232.1785 -2118.5317 -2242.3905
AH?
6 (R,OOR,)
(kJmol_') (")
-132.22 -107.12 -90.84 -121.81 -101.41 -871.95 -328.26 -105.06 -227.80 -254.62 -109.41 -583.82 -534.28 -554.16 +620.63 -661.42 +33.35
84.60 81.31 89.73 83.90 78.86 90.65 71.84 110.66 Ill.78 123.63 154.40 107.69 180.00 117.74 120.21 139.43 99.09
D(R,O-OR,) (kJmol-')
273.4 215.7a 166.8= 240.4 187.5= 268.0 191.8a 201.2 218.0 230.2 127.6 213.1 163.3 183.4 -570.3 -191.0 203.6
d(O-0) (A)
1.380 1.319 It.253 1.383 1.319 1.383 1.322 1.391 1.393 1.389 1.403 1.399 1.398 1.394 1.453 1.402 1.363
37 TABLE
2
Molecular geometries at equilibrium (Bond lengths, XY in A ; bond angle WXY, HOOH HOOF FOOF CH,OOH
CH,OOF CF,OOH CF,OOF CH,OOCH, (C,H,)*O,
(C,) (C,)
[(CH, ),CH
1~0~ (c, 1
(CH,COLO,
(1) (Cd
v%com
(11)(C,)
WH,CO),O,
(III)
o,*-(C,) (o,c),o:-
(W’JW,
(C,)
and dihedral angle WXYZ
in “)
HO, 0.965; 00,1.380; HOO, 106.9; HOOH, 84.6 HO, 0.970; 00, 1.319; OF, 1.490; HOO, 109.9; OOF, 109.9; HOOF, 81.3 FO, 1.492; 00, 1.253; FOO, 113.3; FOOF, 89.7 HC, 1.121; CO, 1.359; 00,1.383; OH, 0.965; HCO, 112.7; COO, 113.9; OOH, 107.4; HCOO, 55.9, 175.8, 294.8; COOH, 83.9 HC, 1.118; CO, 1.368; 00,1.319; OF, 1.490; HCO, 112.0; COO, 116.8; OOF, 111.3; HCOO, 56.3,175.6, 295.2; COOF, 78.9 FC, 1.332; CO, 1.321; 00, 1.383; OH, 0.965; FCO, 116.2; COO, 114.7;OOH, 106_5;FCOO, 57.7, 176.0, 293_5;COOH, 90.6 FC, 1_332;CO, 1.322; 00, 1.338; OF, 1.479; FCO, 115.5; COO, 118.5; OOF, 109.1;FCOO; 55.8, 175.1, 291_9;COOF, 71.8 HC, 1.120; CO, 1.360;00,1.391; HCO, 112_8;COO, 113.1; HCOO, 59.9,179.4,298.7; COOC, 110.7 H,C, 1.112; CC, 1.493; H,C, 1.132; CO, 1.363; 00,1.393; H,CC, 113.5; CCH,, 114.2; CCO, 107.3; COO, 115.2; H,CCO, 60.6, 179.8,299.2; H,CCH,, 56.6, 300.9; COOC, 111.8 H,C, 1_113;CC, 1.512; HC, 1.137; CO, 1.386;00,1.389; H&C, 113.7; CCC, 116.8; HCO, 113.5; CCO, 108.7; COO, 115.4; CCOO, 113.3,241.4; HCOO, -2.5; COOC, 123.6 H,C, 1.112; CC (x 3), 1.536; CO, 1.396; 00,1.403; OC, 1.361; CH, 1.122;CC (x2), 1.504;CC (xl), 1.499;CH,, 1.107; CC0 (X 3), 108.5;CCC (X 3), 112.7;COO, 115_O;OOC, 109.6; OCH, 116.9; OCC (x 2), 118.5;CCOO, 59.3,177-g, 296.6; COOC, 154_4;OOCH, 0.9;OOCC, 145.9,215.0 HC, l.lll;CC, 1.497; CO!, 1.212; COU, 1.340; 01011, 1.395; HCC, 113.7; CCOI, 124.4; CCOII, 102.5; OfiOn, 133.1; CO,,OI,, 121.5; HCCO,, 59.3,179.3,299.2; O$OnOII, 0.0; co,o,c, 107.7 HC, 1.110; CC, 1_492;CO,, 1.219; CO*, 1.345;0,,0,1, 1.398; HCC, 113.9; CCOI, 121.9; CCOII, 122.6; O&On, 115.5; COIIOI,, 119.7; HCCO,, 60.2,180.3, 300.4; O,CO,O,, 179.1; COnO&, 180.0 HC, 1.110; CC, 1.493; COI, 1.219; COII, 1.343; OnOn, 1.394; Ofi, 1_346;CO,, 1.211;CC, 1.494;CH, l.lll;HCC, 114.0; CCO,, 121.4; CCOII, 122.5; O&OII, 116.1; COIIOn, 123.0; OIIO&, 120.9; O&O,, 132.8; O&C, 102.6; O&C, 124.6; CCH, 113_7;HCCO,, 60.3, 180.5,300.5; OfiOIIOII, 174.8; COIIOI$, 117.4; 01101~01, -10.9; OjXH, 60.5, 180.5, 300.5 o,o,, 1.400; o,o,, 1.453;0,0,0,, 117.1;o,o,030,, 120.2 O,C, 1.273; CO,*, 1.411: 0,,011, 1.402: O$O,, 116.4: O$O,, 127.2;COnOU, 122.4; OICOIIOn, -0.8,177.4; COIIO,C, 139.4 OIN, 1.217; NOII, 1.441; OuOII, 1.363; OINOm, 114.5; O,NO,, 131.0; NOIIOII, 119.8; O,NO,,O,, -8.3,171.8; NOIIOnN, 99.1
38 TABLE
3
Torsional
barriers A V( trclns) (kJ mol-’ )
Molecule
Molecule
P V(d) (kJ mol-I)
H:O,
10.38
13.89
W-I&O,
HOOF F:O, CH,OOH
42.86 64.18 10.09
28.37 88.3 5 12.34
(C,W,O,
CH,OOF
40.85
25.12
(CH,CO)zO,
CF,OOH CF,OOF
6.99 26.96
=Constrained
[(CJ%),CHl A=
(CH,)&OOC,H,=
10.32 -
optimizations;
(I)
A v( tmns)
A v(h)
(kJ mol-‘)
(kJ mol-’ )
3.76 3.54 0.85 0.86 3.80 3.11 12.06 22.06
22.88 20.18 25.63 41.23 24.78 38.64 36.36 41.91
see text.
For each peroxide the dissociation energy D(RIO--OR*) describing the reaction RlOORz + RI0 + OR* was calculated from the AHFvahles of the fragments RI0 and R20 and that of the parent peroxide: the energies and structures calculated for these fragments are given in SUP 26163 (7 pages), and the derived values of D(R,O-0R2) are in Table 1. RESULTS
Molecular
AND
DISCUSSION
structures
and energies
Free optimizations of the simpler peroxides RIOOR in which R, = Rz yielded structures of exact C2 symmetry: this was so for R, = H, F, CH3, and C2HS, and also for O,‘-; polyoxo species H20, and FzO, (n = 3,4) also optimized to C1 symmetry. As a consequence of this, the larger peroxides and I(CH3)zCHlzOz, (CH3COM’z ( isomers I and II; see below), (OzN)ZOz (O,C),O,*- were constrained to C2 in order to economize on computer time. The only other constraint imposed was that in iso-propyl and t-butyl groups, the CH3 fragments were constrained to be equivalent_ Satisfactory optimization was then achieved for all peroxides investigated with the exception of (CF3)*02: several different starting geometries were tried, but the optimization always failed to achieve self-consistence, despite use of the Pople bondorder matrix. For (CH3C0)202; three conformers are possible, I-III, shown for convenience in the tram, (COOC) = 180” form: CH,-C
(1)
,o \
o=c O-O
\ ,pCH, 0
/ \
CH, o=c O--O
\
cH/c=o 3 (11)
/ \
CH, O-O
\ 0”
(III)
C-CH,
39
At equilibrium, I is the most stable and II the least stable. In general, equilibrium dihedral angles are in good agreement with those observed experimentally in the cases where the molecular structure has been determined [l] ; in particular, as aIky1 groups became larger, the dihedral angles also became bigger. The largest system which the present version of MIND0 can handle is limited to 50 atomic orbitals: it was therefore not possible to investigate [(CH,),C] 202 which contains 58 valence shell orbitals. The largest symmetrical peroxide investigated was [ (CH,),CH] 20, (46 orbitals), and the largest unsymmetrical peroxide was t-butylcyclopropyl peroxide (CH3)&!OOCJH5 (50 orbitals). In dialkyl peroxides, the dihedral angle increases from 110.7” in (CH&02 through 111.8” in (C2H5)202 and 123.6” in [ (CH&CH] 20, to 154.4” in (CH,),COOC,H,. Peroxides in which one of the substituents is either H or F are calculated to have dihedral angles below loo”, as observed experimentally. The two anionic species considered, 0e2- and (O2C),O,2-, both have dihedral angles substantially below the value of 180” observed for (OjS)2022- in its ammonium and caesium salts [ 51. The equilibrium structures of (O,(Z), 022- and (02 N)202 (Table 2) may be compared with those of (0,C!),02(O,C, 1.276 BL;Con, 1.371 a ; OICOn, 116.9”; OICOI, 126.2” ; CO1,C, 171.9”; OICOIIC, 90.5”, 269.6”) and of (O,N),O (OIN, 1.210 a; NOI1, 1.410 a; O,NOII, 114.1”; OiNOr, 131.8O ; NOII N, 149.1O; OINOIIN, 90.7”, 269.3”). The MOIi distances are appreciably longer in the peroxo compounds, which are both markedly non-planar, having dihedral angles of 139-4” and 99.1” : in contrast, the 0x0 compounds are quite near planarity, the carbonate especially. Oxygen-oxygen distances are in general systematically underestimated in the peroxides considered here: this reflects the known underestimate in MIND0/3 of repulsions between adjacent atoms bearing non-bonding electrons [6]. However the ordering of the O-O distances is certainly correct: of those which have been determined experimentally that in FOOF is the shortest (1.217 A, talc. 1.253 A) and that in CF300F the next smallest (1.366 BL, talc. 1.322 a ), while those in diaikyl or disilyl peroxides are the longest (1.480 a, talc. 1.39 a). Noteworthy amongst the calculated values is the very long (1.453 A ) central O-O distance in Of-: this molecule is in fact thermodynamically unstable with respect to 20; (see below). In order to determine the bond dissociation energies D(R,O-OR,), the energies of the fragments RO were required: unconstrained optimization converged satisfactorily for all fragments except CF30 and OF. For CF30, free optimization gave a C,, structure with FC, 1.356 A ; CO, 1.184 a; FCO, 141.7”; FCF, 64.9”; aH7, - 886.23 kJ mol-‘. In this structure the FCF angle is far too small, presumably as a consequence of the underestimate in MIND0/3 of F - - - F repulsions [7], and the AH? value is probably also far too negative. Consequently an optimization was carried out in which all the FCO angles were fixed at 115.9”) the mean of the equilibrium values in CF300H and CF,OOF, when MF was calculated as -674.48 kJ mol-‘.
40
All attempts to calculate AHI * for the OF radical were unsuccessful, regardless of the starting bond length chosen. Attempts to calculate bond dissociation energies in F202 and HOOF directly by increasing the O-O distances progressively were also unsuccessful: at O-O distances greater than ca. 3.5 a, the optimizations failed to achieve self-consistence, but the AH”f values were still rising rapidly as a function of d(O-0). Consequently, a value of + 38 kJ mol-’ was adopted for AH?, based on the extensive ab initio calculations of O’Hare and Wahl [8] _ These authors computed a value of 1.321 a for the equilibrium bond length and a dissociation energy of 3.0 eV, associated with the rather wide limits of uncertainty of +0.3 eV, -0.8 eV. Their central value yields a AH? value of + 38 kJ mol-‘, but the extreme values yield a AH? value of + 10 kJ mol-* or + 106 kJ mol-I. Taking the value of 38 kJ mol-‘, D(RIO-OR2) for HOOF, F202, CHsOOF and CFsOOF to be 216, 167, 188, and 192 kJ mol-‘, respectively, these are rather low values, certainly lower than the 273 kJ mol-’ in HzOz, and do not seem consistent with the short O--O bond distances in these molecules. Values calculated taking A HF(OF) as + 106 kJ mol-’ are 284,303,256, and 260 kJ mol-‘, respectively, which correlate rather better with the bond lengths. For comparison the dissociation energy of 0, is 497.3 kJ mol-*, and in terms of the three-centre bond description of Fz02 [9] this compound would be expected to have both a high torsional barrier, as calculated {see below), and a high FC-OF dissociation energy. The anionic species 04’- and (O2C)2O22- are unstable with respect to 20; amd 2Ca, respectively, and consequently the calculated RIO-OR2 bond dissociation energies are negative: the ions are presumably prevented from optimizing to the dissociated radical-ions by a potential barrier, although this was not investigated. In contrast with (OZC)202- and (02C)2022-, the orUzo-dicarbonate (OsC)206- when freely optimized dissociates: (o&)*0+ Torsional
-+ 2co32- + 02potentials
Complete torsional potential functions were calculated for most of the peroxides considered in this paper. For the Iarger molecules, the computation was extremely time-consuming when the geometry was fully optimized at each point on the potential curve, and consequently only one conformer of ( CH3C0)202 was investigated : for the two largest molecules, [( CH3 )2 CH] 2 O2 and (CH3)&OOC3H5, only the COOC geometry was optimized, all other parameters being fixed at their equilibrium values. Since the potential minima in these examples correspond to full optimization, and the AH? value at 0” and 180” to constrained optimization, the torsional barriers (see Table 3) are somewhat too high for these molecules. For CF,OOF, the calculations at values of 6 (COOF) below 60” were seriously perturbed by the usual MIND0 underestimate of F - - - F rep& sions: as the dihedral angle was reduced below 60”) the OOF angle became
41
progressively smaller, and at very small dihedral angles the AH? value was calculated to be more negative that the equilibrium value. It should be noted that the calculated equilibrium geometry of CF,OOF, when compared with those of CH300F and CF300H, shows no anomalies: these only appear at small dihedral angles when the unique fluorine atom and the CF3 group are close to one another. On the other hand, no geometrical anomalies were observed at low dihedral angles for any other fluoro species although it is perhaps significant in this respect that HOOF and CH300F are the only two species in which A V(cis) is calculated to be less than A V( tram). With the exception of HOOF and CH300F all peroxides have A V(b) values greater than A V(trans); the difference in barrier heights calculated for H202 is smaller than the experimental value [lo], although the observed barriers for HzOz, A V(trans), 4.62 kJ mol-‘, and A V(cis), 29.4 kJ mol-‘, are very similar to those calculated for both dialkyl peroxides, and for diacetyl peroxide. As expected, the barriers.in FzOz are substantially greater than for any other peroxide. The absolute necessity for full optimization of the geometry at each point if meaningful results are to be obtained (cf. ref. 3) is apparent from SUP 26163 where molecular geometries at S = 0” and S = 180” are recorded for comparison with equilibrium geometry (Table 2). Even in the simplest peroxides, HzOz, HOOF, and F202, major changes in geometry are apparent between cis and trans conformers. In species containing CH, or CF3 groups, although C-X distances and OCX angles (X = H, F) are essentially unchanged, the torsional conformation of the CX3 group with respect to the COO plane differs between cis and conformers. In (0,C),0Z2- and (02N)202, the whole molecule is planar when 6 = 180”, but when 6 = 0” the two OzMO planes are twisted away from the MOOM plane by ca. 53” when M = C and by ca. 43” when M = N, although no such twist is observed at 6 = 0” for the conformer I of (CH,CO),O, which is formally analogous to the (01M)202 species. If the total energy is partitioned between electronic energy (attractive) and coulombic energy (repulsive) then the variation of these energies show three distinct patterns for molecular peroxides and representative examples are given in SUP 26163. For Fz02, the negative (attractive) energy shows a maximum at 6 = ca. 40”, while the positive (repulsive) energy shows a minimum also at 6 = ca. 40”; the total energy shows a minimum well removed from the component energy extrema at 6 = 89.7”. For (C2H5)202, the extrema are the opposite of those for F,O,; the attractive energy shows a minimum, and the repulsive energy a maximum, again both near 6 = 40”, again well removed from the minimum in total energy at 6 = 111.8”. For (CH3)202 both component energies vary monotonically with 6, the attractive and repulsive energy both decreasing in magnitude as S varies from 0” to 180”) although the total energy exhibits a clear minimum at 6 = 110.7°. In each case cited, (and most others also), the overall variation in total energy is very much
42
smakr than the variations in the two component energies, which generally vary in contrary senses: the resultant dihedral angIe is consequently dependent on a rather small variation in total energy with 6 which is itself the difference between two large contrary variations; given the disparate patterns
of the variation of attractive and repulsive energies shown in SUP 26163, simple rationalization of the equilibrium dihedral angles can be made.
no
Energies and equilibrium structures of the species HZO,, HO,F and FZO, (n = 1-4) are given in Table 4, and of the analogous radicals HO, and FO, in Table 5. It is known [7f that free optimization of F20 leads, because of the underestimated F . . . F repulsion, to an absurdly low FOF angle and to a LWF value which is too low: however if the FOF angle is fixed at the experimental value and only the FO distances are optimized the restitiig AH? value is very close to the experimental value 1111. AU the diamagnetic TABLE
4
Energies and structures of polyoxo Molecule
HO&f
Total energy (eV)
aHfG (kJ mol-‘)
Geometry (bond lengths in A; bond angles and dihedral angles in o )
fall haue c z symmefryf
WA HzO,
-650.2043 -959.2900
-132.22 -77.51
H*O,
-1268.3192
-17.36
FOs F,O F,O, F,O,
species XO,U
(ait fiave C, symmetry) -1261.5921 - 29.31 - 157 1.8824 -90.84 - 1880,7lao -11.23
FzO.,
-2189.8708
-i-36.24
HOrP HOF HO,F
-801.4594 -1110.9976
-118.l6 -107.12
HO,F
--1420,0629
-50.44
HO,F
- 1729.0943
t-9.50
HO, 0.965; 00,1.380; HOO, 106.9; HOOH, 84.6 HO, 0.964; 00, 1.384; HOO, 109.3;000,116.5; HOOO, 77.5 HO, 0.964; O,O,, 1.384; O,O,, 1.391; HOO, 109.2; 000,118.2; HOOO, 74.1; 0000, ‘77.0
FO, 1.447; FOF, 103.3 (fixed) FO, 1.492; 00,1.253; FOO, 113.3; FOOF, 89.7 FO, 1.481;00,1.331; FOO, 112.7;000,120.2; FOOO, 71.1 FO, 1.487; O,O,, 1.312; O,O,, 1.421; FOO, 112.0; 000,121 .O; FOOO, 79.6; 0000, 53.3
HO, 0.949; OF, 1.471; HOF, 95.5 HO, 0.970; 00, 1.319; OF, l-490; HOO, 109.9; OOF, 109.9; HOOF, 81.3 HO,, 0.961; O,O,, 1.389; O,O,, 1.316; O,F, 1.492; HO,O,, 110.9; O,O,O,, 119.2; O,O,F, 112.9; HO,O,O,, 61.6; O,O,O,F, 80.6 HO,, 0.965; O,O,, 1.374; O,O,, 1.406; O,O,, 1.316; O,F, 1.490; HO,O,, 109.2; O,O,O,, 118.8; O,O,O,, 120.4; O,O,F, 112.0; HO,O,O,, 78.4; O,O,O,O,, 66.5; O,O,O,F, 77.6
43 TABLE
5
Energies and structures of radicals HO, and FO, Radical
Total energy
(eV HO(’ n) HO,
Aff?
(kJ mol-‘)
HO,
-323.6856 -634.1906 -943.4075
+ 70.57 -11.67 + 30.37
HO,
-1252.2732
+ 106.31 a
FO FO, FO,
--1095_4976 ..-1403.6597
-36.13 + 107.69
F0.I
.-X713.0396
i- 134.01
aSee
Geometry (bond lengths in A ; bond angles and dihedral angles in ” )
HO, 0.948 HO, 0.977; 00,1.275; HOO, 111.7 HO,, 0.960;0,0,, 1.389; O,O,, 1.261; HO,O,, 114.3; O,O,O,, 122.7; HO,O,O,, 0.7 HO,, 0.966; O,O,, 1.376; O,O,, 1.431; 0,0,, 1.261; HO,O,, 108.7; 0,020,, 118.0;010,04, 120.3; HO,O,O,, 78.8; O,O,O,O,, 74.0 FO, 1.496; 00,1.2X.1; FOO, 117.1 FO,, 1.489; O,O,, 1.314; O,O,, 1.257; FO,O,, 116.2; O,O,Os, 124.2; F0,0,09, 53.1 FO,, 1.488: O,O,, 1.312; OzO,, 1.439; O,O,, 1.253; FO,O,, 110.1; O,O,O,, 122.0;0,0,0,, 123.3; FO,O,O,, 80.4; O,O,O,O,, 47.8
text.
polyoxo species except H&F and F,O, are endothermic compounds. In contrast all the HO, and FO, radicals except HOn and FOz are exothermic: the problem of the FO radical has been discussed above. As usual, O-O bonds adjacent to O-F bonds are shorter than those adjacent to O-H bonds, or those in the middle of an 0, chain: in diamagnetic species having n = 3 or 4, the 000 angles range from 116.5” in H203 via 119.2” in H03F to 120.2” in F203, and from 1182” in Hz04 via 118.8” (closest to H) and 120.4” (remote from H) in HOgF to 121.0” in dihedral angle in partiF204. Dihedral angles are all below 90“) the 0000 cular becoming smaller with increasing fluorine substitution in both X,0, and X0+ HO3 is Wlculated to be almost planar in the cis conformation_ No exploration of torsional barriers in these species was undertaken. By use of the AH? values for X20, and X0,, bond dissociation energies for each fra~enta~ion of X20, can be derived, and these are set out in SUP 26163. When n = 4, the cleavage of the central O-O bond is always exothermic (cf. 0,2-). Similarly the bond dissociation energy D( FO-02F) is extremely small; on the other hand the bond dissociation energies D( F--O,F) are all substantial. REFERENCES 1 C. B, 2 D. 3 0. 4 R.
Glidewell, D. C. Liles, D. J. Walton and G. M. Sheldrick, Acta Crystallogr., Sect. 35 (1979) 500. Kiss, H. Oberhammer, D. Brandes and A. Blaschette, 3. Mol. Struct., 40 (1977) 65. Kikuchi, A. Hiyama, H. Yoshida and K. Suzuki, Bull. Chem. Sot. Jpn., 51 (1978) 11. C. Bingham, M. 3. S. Dewar and D. H. Lo, QCPE, XI, ( 1975) 309.
44 5 6 7 8 9 10
W. H. Zachariasen and R. C. L. Mooney, 2. Krist., 88 (1934) 63. R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1302. R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1307. P. A. G. O’Hare and A. C. WahI, J. Chem. Phys., 53 (1970) 2469, R. H. Jackson, J. Chem. Sot., (1962) 4585. R. H:Hunt, R. A. Leacock, C. W. Peters and K. T. Hecht, J. Chem. Phys., 42 (1965) 1931. 11 J. L. Franklin, J. G. Diilard, H.M. Rosenstock, Y.T. Hemon, K. Draxl and F. H. Field, Nat. Stand. Ref. Data Ser., Nat. Bur- Stand., No. 26 (1969). 12 H. A. Skinner and L. Pilcher, Q. Rev. Chem. Sot., 17 (1963) 264.
Journal
STRUCTURE
AND CONFORMATION
CHRISTOPHER
GLIDEWELL
Chemistry Department, (G t. Btitain)
University
(Received
1979)
18 December
-
Printed in The Netherlands
IN MOLECULAR
of St. Andrews,
St. Andrews,
Fife
PEROXIDES
KY 16 SST
ABSTRACT Molecular structures and energies have been calculated in the MIND0 approximation for fifteen neutral and anionic peroxides: fully optimized torsional potential functions have been calculated for twelve of these, and torsional potential functions, subject to constrained optimizations, for a further two peroxides. Bond dissociation energies D(R,O-ORJ were also calculated. Equilibrium structures and energies were calculated for the polyoxo species H,O,, HO,F, F,O,, HO, and FO, (TV< 4), and a complete set of bond dissociation energies derived for H,O,, HO,F, and F,O,. INTRODUCTION
In molecular peroxides, R1OORz, the dihedral angle 6 (R100R2) has been observed as large as 180” and as small as 87.5” [l] ,and the factors which determine the value of 6 in a particular peroxide are obviously of interest. Calculations on (Me3C)202 and (Me3Si)202 in the CNDO approximation [Z] , using experimental values for all the geometrical parameters except 6, indicated that in (Me3Q202 the coulombic repulsion was dominant in determining the torsional minimum, but in neither molecule did the torsional potential achieve the correct minimum. A torsional potential function has been calculated [ 31 in the MIND0 approximation for (PhC0)202: in this calculation the phenyl groups were treated as fixed hexagons and all other parameters, except for d(0-0) and 6, were taken from the optimization of (CH3C0)202. p this instance, the barriers will almost certainly be calculated somewhat too large, as a consequence of the non-optimization of the whole geometry. In the present paper, MIND0 calculations are reported on the structures, conformations and torsional potentials of fifteen molecular peroxides, ranging from HzOz and 0d2- to (CH3)&OOC3H5, together with calculations on the structures and conformations of the polyoxo species HO,H, FO,F, HO,F (n < 4) and their associated radicals HO, and FO,. METHOD
Energy calculations were performed with the IBM version of MIND0/3 [4] implemented on a Honeywell 66/80 computer. Except where specifically 0022-2860/80/000~0000/$02.25
0 1980
Elsevier Scientific Publishing Company
36
stated otherwise, no assumptions of any kind were made and the energy of each molecule was minimized with respect to all geometric variables simultaneously. Torsional potential energy calculations were made at fixed values of 6, with simultaneous optimization of all other parameters. Torsional potential functions were calculated in the form AH?=
Xn a, cos"B
Seven terms were usually employed, and the deviations between calculated and input AfIT values were generally of the order of 10-2-10-3 kJ mol-' . After calculation of the coefficients an, the functions were then recast into the form AH?
= z b, cos n6
In this form the value of AHj for the trans
AH? (tram)
= G (-1)”
conformer is given by
b,
and that for the cis by AH? (cis) = ;
b,
Equilibrium molecular properties are given in Table 1, and equilibrium geometries for peroxides in Table 2. The coefficients b, for the torsional potentials are available from BLLD as Sup Pub No. SUP 26163 (7 pages), and the rotation barriers for cis and tram conformers are given in Table 3. TABLE1 Molecularproperties at equilibrium Molecuie
H,O, HOOF F,O, CH,OOH CH,OOF CF,OOH CF,OOF (C%),O, (C,H,W, C(CH,),CH120, (CH,),COOC,H, (CH,CO),O, (1) (CH,CO),O, (11) W-WOW, (111)
=Seetext.
Totalenergy (eW -650.2043 -1110.9976 -1571.8824 -806.5054 -1267.3474 -2197.4404 -2658.0386 -962.7407 -1276.8307 -1589.9267 -1715.3021 -1840.7687 -1840.2552 -1840.4613 -1232.1785 -2118.5317 -2242.3905
AH?
6 (R,OOR,)
(kJmol_') (")
-132.22 -107.12 -90.84 -121.81 -101.41 -871.95 -328.26 -105.06 -227.80 -254.62 -109.41 -583.82 -534.28 -554.16 +620.63 -661.42 +33.35
84.60 81.31 89.73 83.90 78.86 90.65 71.84 110.66 Ill.78 123.63 154.40 107.69 180.00 117.74 120.21 139.43 99.09
D(R,O-OR,) (kJmol-')
273.4 215.7a 166.8= 240.4 187.5= 268.0 191.8a 201.2 218.0 230.2 127.6 213.1 163.3 183.4 -570.3 -191.0 203.6
d(O-0) (A)
1.380 1.319 It.253 1.383 1.319 1.383 1.322 1.391 1.393 1.389 1.403 1.399 1.398 1.394 1.453 1.402 1.363
37 TABLE
2
Molecular geometries at equilibrium (Bond lengths, XY in A ; bond angle WXY, HOOH HOOF FOOF CH,OOH
CH,OOF CF,OOH CF,OOF CH,OOCH, (C,H,)*O,
(C,) (C,)
[(CH, ),CH
1~0~ (c, 1
(CH,COLO,
(1) (Cd
v%com
(11)(C,)
WH,CO),O,
(III)
o,*-(C,) (o,c),o:-
(W’JW,
(C,)
and dihedral angle WXYZ
in “)
HO, 0.965; 00,1.380; HOO, 106.9; HOOH, 84.6 HO, 0.970; 00, 1.319; OF, 1.490; HOO, 109.9; OOF, 109.9; HOOF, 81.3 FO, 1.492; 00, 1.253; FOO, 113.3; FOOF, 89.7 HC, 1.121; CO, 1.359; 00,1.383; OH, 0.965; HCO, 112.7; COO, 113.9; OOH, 107.4; HCOO, 55.9, 175.8, 294.8; COOH, 83.9 HC, 1.118; CO, 1.368; 00,1.319; OF, 1.490; HCO, 112.0; COO, 116.8; OOF, 111.3; HCOO, 56.3,175.6, 295.2; COOF, 78.9 FC, 1.332; CO, 1.321; 00, 1.383; OH, 0.965; FCO, 116.2; COO, 114.7;OOH, 106_5;FCOO, 57.7, 176.0, 293_5;COOH, 90.6 FC, 1_332;CO, 1.322; 00, 1.338; OF, 1.479; FCO, 115.5; COO, 118.5; OOF, 109.1;FCOO; 55.8, 175.1, 291_9;COOF, 71.8 HC, 1.120; CO, 1.360;00,1.391; HCO, 112_8;COO, 113.1; HCOO, 59.9,179.4,298.7; COOC, 110.7 H,C, 1.112; CC, 1.493; H,C, 1.132; CO, 1.363; 00,1.393; H,CC, 113.5; CCH,, 114.2; CCO, 107.3; COO, 115.2; H,CCO, 60.6, 179.8,299.2; H,CCH,, 56.6, 300.9; COOC, 111.8 H,C, 1_113;CC, 1.512; HC, 1.137; CO, 1.386;00,1.389; H&C, 113.7; CCC, 116.8; HCO, 113.5; CCO, 108.7; COO, 115.4; CCOO, 113.3,241.4; HCOO, -2.5; COOC, 123.6 H,C, 1.112; CC (x 3), 1.536; CO, 1.396; 00,1.403; OC, 1.361; CH, 1.122;CC (x2), 1.504;CC (xl), 1.499;CH,, 1.107; CC0 (X 3), 108.5;CCC (X 3), 112.7;COO, 115_O;OOC, 109.6; OCH, 116.9; OCC (x 2), 118.5;CCOO, 59.3,177-g, 296.6; COOC, 154_4;OOCH, 0.9;OOCC, 145.9,215.0 HC, l.lll;CC, 1.497; CO!, 1.212; COU, 1.340; 01011, 1.395; HCC, 113.7; CCOI, 124.4; CCOII, 102.5; OfiOn, 133.1; CO,,OI,, 121.5; HCCO,, 59.3,179.3,299.2; O$OnOII, 0.0; co,o,c, 107.7 HC, 1.110; CC, 1_492;CO,, 1.219; CO*, 1.345;0,,0,1, 1.398; HCC, 113.9; CCOI, 121.9; CCOII, 122.6; O&On, 115.5; COIIOI,, 119.7; HCCO,, 60.2,180.3, 300.4; O,CO,O,, 179.1; COnO&, 180.0 HC, 1.110; CC, 1.493; COI, 1.219; COII, 1.343; OnOn, 1.394; Ofi, 1_346;CO,, 1.211;CC, 1.494;CH, l.lll;HCC, 114.0; CCO,, 121.4; CCOII, 122.5; O&OII, 116.1; COIIOn, 123.0; OIIO&, 120.9; O&O,, 132.8; O&C, 102.6; O&C, 124.6; CCH, 113_7;HCCO,, 60.3, 180.5,300.5; OfiOIIOII, 174.8; COIIOI$, 117.4; 01101~01, -10.9; OjXH, 60.5, 180.5, 300.5 o,o,, 1.400; o,o,, 1.453;0,0,0,, 117.1;o,o,030,, 120.2 O,C, 1.273; CO,*, 1.411: 0,,011, 1.402: O$O,, 116.4: O$O,, 127.2;COnOU, 122.4; OICOIIOn, -0.8,177.4; COIIO,C, 139.4 OIN, 1.217; NOII, 1.441; OuOII, 1.363; OINOm, 114.5; O,NO,, 131.0; NOIIOII, 119.8; O,NO,,O,, -8.3,171.8; NOIIOnN, 99.1
38 TABLE
3
Torsional
barriers A V( trclns) (kJ mol-’ )
Molecule
Molecule
P V(d) (kJ mol-I)
H:O,
10.38
13.89
W-I&O,
HOOF F:O, CH,OOH
42.86 64.18 10.09
28.37 88.3 5 12.34
(C,W,O,
CH,OOF
40.85
25.12
(CH,CO)zO,
CF,OOH CF,OOF
6.99 26.96
=Constrained
[(CJ%),CHl A=
(CH,)&OOC,H,=
10.32 -
optimizations;
(I)
A v( tmns)
A v(h)
(kJ mol-‘)
(kJ mol-’ )
3.76 3.54 0.85 0.86 3.80 3.11 12.06 22.06
22.88 20.18 25.63 41.23 24.78 38.64 36.36 41.91
see text.
For each peroxide the dissociation energy D(RIO--OR*) describing the reaction RlOORz + RI0 + OR* was calculated from the AHFvahles of the fragments RI0 and R20 and that of the parent peroxide: the energies and structures calculated for these fragments are given in SUP 26163 (7 pages), and the derived values of D(R,O-0R2) are in Table 1. RESULTS
Molecular
AND
DISCUSSION
structures
and energies
Free optimizations of the simpler peroxides RIOOR in which R, = Rz yielded structures of exact C2 symmetry: this was so for R, = H, F, CH3, and C2HS, and also for O,‘-; polyoxo species H20, and FzO, (n = 3,4) also optimized to C1 symmetry. As a consequence of this, the larger peroxides and I(CH3)zCHlzOz, (CH3COM’z ( isomers I and II; see below), (OzN)ZOz (O,C),O,*- were constrained to C2 in order to economize on computer time. The only other constraint imposed was that in iso-propyl and t-butyl groups, the CH3 fragments were constrained to be equivalent_ Satisfactory optimization was then achieved for all peroxides investigated with the exception of (CF3)*02: several different starting geometries were tried, but the optimization always failed to achieve self-consistence, despite use of the Pople bondorder matrix. For (CH3C0)202; three conformers are possible, I-III, shown for convenience in the tram, (COOC) = 180” form: CH,-C
(1)
,o \
o=c O-O
\ ,pCH, 0
/ \
CH, o=c O--O
\
cH/c=o 3 (11)
/ \
CH, O-O
\ 0”
(III)
C-CH,
39
At equilibrium, I is the most stable and II the least stable. In general, equilibrium dihedral angles are in good agreement with those observed experimentally in the cases where the molecular structure has been determined [l] ; in particular, as aIky1 groups became larger, the dihedral angles also became bigger. The largest system which the present version of MIND0 can handle is limited to 50 atomic orbitals: it was therefore not possible to investigate [(CH,),C] 202 which contains 58 valence shell orbitals. The largest symmetrical peroxide investigated was [ (CH,),CH] 20, (46 orbitals), and the largest unsymmetrical peroxide was t-butylcyclopropyl peroxide (CH3)&!OOCJH5 (50 orbitals). In dialkyl peroxides, the dihedral angle increases from 110.7” in (CH&02 through 111.8” in (C2H5)202 and 123.6” in [ (CH&CH] 20, to 154.4” in (CH,),COOC,H,. Peroxides in which one of the substituents is either H or F are calculated to have dihedral angles below loo”, as observed experimentally. The two anionic species considered, 0e2- and (O2C),O,2-, both have dihedral angles substantially below the value of 180” observed for (OjS)2022- in its ammonium and caesium salts [ 51. The equilibrium structures of (O,(Z), 022- and (02 N)202 (Table 2) may be compared with those of (0,C!),02(O,C, 1.276 BL;Con, 1.371 a ; OICOn, 116.9”; OICOI, 126.2” ; CO1,C, 171.9”; OICOIIC, 90.5”, 269.6”) and of (O,N),O (OIN, 1.210 a; NOI1, 1.410 a; O,NOII, 114.1”; OiNOr, 131.8O ; NOII N, 149.1O; OINOIIN, 90.7”, 269.3”). The MOIi distances are appreciably longer in the peroxo compounds, which are both markedly non-planar, having dihedral angles of 139-4” and 99.1” : in contrast, the 0x0 compounds are quite near planarity, the carbonate especially. Oxygen-oxygen distances are in general systematically underestimated in the peroxides considered here: this reflects the known underestimate in MIND0/3 of repulsions between adjacent atoms bearing non-bonding electrons [6]. However the ordering of the O-O distances is certainly correct: of those which have been determined experimentally that in FOOF is the shortest (1.217 A, talc. 1.253 A) and that in CF300F the next smallest (1.366 BL, talc. 1.322 a ), while those in diaikyl or disilyl peroxides are the longest (1.480 a, talc. 1.39 a). Noteworthy amongst the calculated values is the very long (1.453 A ) central O-O distance in Of-: this molecule is in fact thermodynamically unstable with respect to 20; (see below). In order to determine the bond dissociation energies D(R,O-OR,), the energies of the fragments RO were required: unconstrained optimization converged satisfactorily for all fragments except CF30 and OF. For CF30, free optimization gave a C,, structure with FC, 1.356 A ; CO, 1.184 a; FCO, 141.7”; FCF, 64.9”; aH7, - 886.23 kJ mol-‘. In this structure the FCF angle is far too small, presumably as a consequence of the underestimate in MIND0/3 of F - - - F repulsions [7], and the AH? value is probably also far too negative. Consequently an optimization was carried out in which all the FCO angles were fixed at 115.9”) the mean of the equilibrium values in CF300H and CF,OOF, when MF was calculated as -674.48 kJ mol-‘.
40
All attempts to calculate AHI * for the OF radical were unsuccessful, regardless of the starting bond length chosen. Attempts to calculate bond dissociation energies in F202 and HOOF directly by increasing the O-O distances progressively were also unsuccessful: at O-O distances greater than ca. 3.5 a, the optimizations failed to achieve self-consistence, but the AH”f values were still rising rapidly as a function of d(O-0). Consequently, a value of + 38 kJ mol-’ was adopted for AH?, based on the extensive ab initio calculations of O’Hare and Wahl [8] _ These authors computed a value of 1.321 a for the equilibrium bond length and a dissociation energy of 3.0 eV, associated with the rather wide limits of uncertainty of +0.3 eV, -0.8 eV. Their central value yields a AH? value of + 38 kJ mol-‘, but the extreme values yield a AH? value of + 10 kJ mol-* or + 106 kJ mol-I. Taking the value of 38 kJ mol-‘, D(RIO-OR2) for HOOF, F202, CHsOOF and CFsOOF to be 216, 167, 188, and 192 kJ mol-‘, respectively, these are rather low values, certainly lower than the 273 kJ mol-’ in HzOz, and do not seem consistent with the short O--O bond distances in these molecules. Values calculated taking A HF(OF) as + 106 kJ mol-’ are 284,303,256, and 260 kJ mol-‘, respectively, which correlate rather better with the bond lengths. For comparison the dissociation energy of 0, is 497.3 kJ mol-*, and in terms of the three-centre bond description of Fz02 [9] this compound would be expected to have both a high torsional barrier, as calculated {see below), and a high FC-OF dissociation energy. The anionic species 04’- and (O2C)2O22- are unstable with respect to 20; amd 2Ca, respectively, and consequently the calculated RIO-OR2 bond dissociation energies are negative: the ions are presumably prevented from optimizing to the dissociated radical-ions by a potential barrier, although this was not investigated. In contrast with (OZC)202- and (02C)2022-, the orUzo-dicarbonate (OsC)206- when freely optimized dissociates: (o&)*0+ Torsional
-+ 2co32- + 02potentials
Complete torsional potential functions were calculated for most of the peroxides considered in this paper. For the Iarger molecules, the computation was extremely time-consuming when the geometry was fully optimized at each point on the potential curve, and consequently only one conformer of ( CH3C0)202 was investigated : for the two largest molecules, [( CH3 )2 CH] 2 O2 and (CH3)&OOC3H5, only the COOC geometry was optimized, all other parameters being fixed at their equilibrium values. Since the potential minima in these examples correspond to full optimization, and the AH? value at 0” and 180” to constrained optimization, the torsional barriers (see Table 3) are somewhat too high for these molecules. For CF,OOF, the calculations at values of 6 (COOF) below 60” were seriously perturbed by the usual MIND0 underestimate of F - - - F rep& sions: as the dihedral angle was reduced below 60”) the OOF angle became
41
progressively smaller, and at very small dihedral angles the AH? value was calculated to be more negative that the equilibrium value. It should be noted that the calculated equilibrium geometry of CF,OOF, when compared with those of CH300F and CF300H, shows no anomalies: these only appear at small dihedral angles when the unique fluorine atom and the CF3 group are close to one another. On the other hand, no geometrical anomalies were observed at low dihedral angles for any other fluoro species although it is perhaps significant in this respect that HOOF and CH300F are the only two species in which A V(cis) is calculated to be less than A V( tram). With the exception of HOOF and CH300F all peroxides have A V(b) values greater than A V(trans); the difference in barrier heights calculated for H202 is smaller than the experimental value [lo], although the observed barriers for HzOz, A V(trans), 4.62 kJ mol-‘, and A V(cis), 29.4 kJ mol-‘, are very similar to those calculated for both dialkyl peroxides, and for diacetyl peroxide. As expected, the barriers.in FzOz are substantially greater than for any other peroxide. The absolute necessity for full optimization of the geometry at each point if meaningful results are to be obtained (cf. ref. 3) is apparent from SUP 26163 where molecular geometries at S = 0” and S = 180” are recorded for comparison with equilibrium geometry (Table 2). Even in the simplest peroxides, HzOz, HOOF, and F202, major changes in geometry are apparent between cis and trans conformers. In species containing CH, or CF3 groups, although C-X distances and OCX angles (X = H, F) are essentially unchanged, the torsional conformation of the CX3 group with respect to the COO plane differs between cis and conformers. In (0,C),0Z2- and (02N)202, the whole molecule is planar when 6 = 180”, but when 6 = 0” the two OzMO planes are twisted away from the MOOM plane by ca. 53” when M = C and by ca. 43” when M = N, although no such twist is observed at 6 = 0” for the conformer I of (CH,CO),O, which is formally analogous to the (01M)202 species. If the total energy is partitioned between electronic energy (attractive) and coulombic energy (repulsive) then the variation of these energies show three distinct patterns for molecular peroxides and representative examples are given in SUP 26163. For Fz02, the negative (attractive) energy shows a maximum at 6 = ca. 40”, while the positive (repulsive) energy shows a minimum also at 6 = ca. 40”; the total energy shows a minimum well removed from the component energy extrema at 6 = 89.7”. For (C2H5)202, the extrema are the opposite of those for F,O,; the attractive energy shows a minimum, and the repulsive energy a maximum, again both near 6 = 40”, again well removed from the minimum in total energy at 6 = 111.8”. For (CH3)202 both component energies vary monotonically with 6, the attractive and repulsive energy both decreasing in magnitude as S varies from 0” to 180”) although the total energy exhibits a clear minimum at 6 = 110.7°. In each case cited, (and most others also), the overall variation in total energy is very much
42
smakr than the variations in the two component energies, which generally vary in contrary senses: the resultant dihedral angIe is consequently dependent on a rather small variation in total energy with 6 which is itself the difference between two large contrary variations; given the disparate patterns
of the variation of attractive and repulsive energies shown in SUP 26163, simple rationalization of the equilibrium dihedral angles can be made.
no
Energies and equilibrium structures of the species HZO,, HO,F and FZO, (n = 1-4) are given in Table 4, and of the analogous radicals HO, and FO, in Table 5. It is known [7f that free optimization of F20 leads, because of the underestimated F . . . F repulsion, to an absurdly low FOF angle and to a LWF value which is too low: however if the FOF angle is fixed at the experimental value and only the FO distances are optimized the restitiig AH? value is very close to the experimental value 1111. AU the diamagnetic TABLE
4
Energies and structures of polyoxo Molecule
HO&f
Total energy (eV)
aHfG (kJ mol-‘)
Geometry (bond lengths in A; bond angles and dihedral angles in o )
fall haue c z symmefryf
WA HzO,
-650.2043 -959.2900
-132.22 -77.51
H*O,
-1268.3192
-17.36
FOs F,O F,O, F,O,
species XO,U
(ait fiave C, symmetry) -1261.5921 - 29.31 - 157 1.8824 -90.84 - 1880,7lao -11.23
FzO.,
-2189.8708
-i-36.24
HOrP HOF HO,F
-801.4594 -1110.9976
-118.l6 -107.12
HO,F
--1420,0629
-50.44
HO,F
- 1729.0943
t-9.50
HO, 0.965; 00,1.380; HOO, 106.9; HOOH, 84.6 HO, 0.964; 00, 1.384; HOO, 109.3;000,116.5; HOOO, 77.5 HO, 0.964; O,O,, 1.384; O,O,, 1.391; HOO, 109.2; 000,118.2; HOOO, 74.1; 0000, ‘77.0
FO, 1.447; FOF, 103.3 (fixed) FO, 1.492; 00,1.253; FOO, 113.3; FOOF, 89.7 FO, 1.481;00,1.331; FOO, 112.7;000,120.2; FOOO, 71.1 FO, 1.487; O,O,, 1.312; O,O,, 1.421; FOO, 112.0; 000,121 .O; FOOO, 79.6; 0000, 53.3
HO, 0.949; OF, 1.471; HOF, 95.5 HO, 0.970; 00, 1.319; OF, l-490; HOO, 109.9; OOF, 109.9; HOOF, 81.3 HO,, 0.961; O,O,, 1.389; O,O,, 1.316; O,F, 1.492; HO,O,, 110.9; O,O,O,, 119.2; O,O,F, 112.9; HO,O,O,, 61.6; O,O,O,F, 80.6 HO,, 0.965; O,O,, 1.374; O,O,, 1.406; O,O,, 1.316; O,F, 1.490; HO,O,, 109.2; O,O,O,, 118.8; O,O,O,, 120.4; O,O,F, 112.0; HO,O,O,, 78.4; O,O,O,O,, 66.5; O,O,O,F, 77.6
43 TABLE
5
Energies and structures of radicals HO, and FO, Radical
Total energy
(eV HO(’ n) HO,
Aff?
(kJ mol-‘)
HO,
-323.6856 -634.1906 -943.4075
+ 70.57 -11.67 + 30.37
HO,
-1252.2732
+ 106.31 a
FO FO, FO,
--1095_4976 ..-1403.6597
-36.13 + 107.69
F0.I
.-X713.0396
i- 134.01
aSee
Geometry (bond lengths in A ; bond angles and dihedral angles in ” )
HO, 0.948 HO, 0.977; 00,1.275; HOO, 111.7 HO,, 0.960;0,0,, 1.389; O,O,, 1.261; HO,O,, 114.3; O,O,O,, 122.7; HO,O,O,, 0.7 HO,, 0.966; O,O,, 1.376; O,O,, 1.431; 0,0,, 1.261; HO,O,, 108.7; 0,020,, 118.0;010,04, 120.3; HO,O,O,, 78.8; O,O,O,O,, 74.0 FO, 1.496; 00,1.2X.1; FOO, 117.1 FO,, 1.489; O,O,, 1.314; O,O,, 1.257; FO,O,, 116.2; O,O,Os, 124.2; F0,0,09, 53.1 FO,, 1.488: O,O,, 1.312; OzO,, 1.439; O,O,, 1.253; FO,O,, 110.1; O,O,O,, 122.0;0,0,0,, 123.3; FO,O,O,, 80.4; O,O,O,O,, 47.8
text.
polyoxo species except H&F and F,O, are endothermic compounds. In contrast all the HO, and FO, radicals except HOn and FOz are exothermic: the problem of the FO radical has been discussed above. As usual, O-O bonds adjacent to O-F bonds are shorter than those adjacent to O-H bonds, or those in the middle of an 0, chain: in diamagnetic species having n = 3 or 4, the 000 angles range from 116.5” in H203 via 119.2” in H03F to 120.2” in F203, and from 1182” in Hz04 via 118.8” (closest to H) and 120.4” (remote from H) in HOgF to 121.0” in dihedral angle in partiF204. Dihedral angles are all below 90“) the 0000 cular becoming smaller with increasing fluorine substitution in both X,0, and X0+ HO3 is Wlculated to be almost planar in the cis conformation_ No exploration of torsional barriers in these species was undertaken. By use of the AH? values for X20, and X0,, bond dissociation energies for each fra~enta~ion of X20, can be derived, and these are set out in SUP 26163. When n = 4, the cleavage of the central O-O bond is always exothermic (cf. 0,2-). Similarly the bond dissociation energy D( FO-02F) is extremely small; on the other hand the bond dissociation energies D( F--O,F) are all substantial. REFERENCES 1 C. B, 2 D. 3 0. 4 R.
Glidewell, D. C. Liles, D. J. Walton and G. M. Sheldrick, Acta Crystallogr., Sect. 35 (1979) 500. Kiss, H. Oberhammer, D. Brandes and A. Blaschette, 3. Mol. Struct., 40 (1977) 65. Kikuchi, A. Hiyama, H. Yoshida and K. Suzuki, Bull. Chem. Sot. Jpn., 51 (1978) 11. C. Bingham, M. 3. S. Dewar and D. H. Lo, QCPE, XI, ( 1975) 309.
44 5 6 7 8 9 10
W. H. Zachariasen and R. C. L. Mooney, 2. Krist., 88 (1934) 63. R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1302. R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1307. P. A. G. O’Hare and A. C. WahI, J. Chem. Phys., 53 (1970) 2469, R. H. Jackson, J. Chem. Sot., (1962) 4585. R. H:Hunt, R. A. Leacock, C. W. Peters and K. T. Hecht, J. Chem. Phys., 42 (1965) 1931. 11 J. L. Franklin, J. G. Diilard, H.M. Rosenstock, Y.T. Hemon, K. Draxl and F. H. Field, Nat. Stand. Ref. Data Ser., Nat. Bur- Stand., No. 26 (1969). 12 H. A. Skinner and L. Pilcher, Q. Rev. Chem. Sot., 17 (1963) 264.