J. Phys. Chem. 1995,99, 3109-3113
3109
Structures on the Singlet and Triplet O3H2 Potential Energy Surfaces: Implications for Photonucleation of Water in the Presence of Molecular Oxygen Mark A. Vincent and Ian H. Hillier* Department of Chemistry, University of Manchester, Manchester, M13 9PL, U.K. Received: September 23, 1994; In Final Form: December 2, 1994@
All the minimum energy structures on the singlet and triplet potential energy surfaces of O3H2 have been calculated at a common level of theory, involving a high level of electron correlation (QCISD(T)). Of potential interest are the energies relative to the charge-transfer complex 02-*H20+, which has been suggested as being a participant in the photonucleation of water droplets. A number of species on the O3H2 surfaces, such as dioxywater and 2-hydrotrioxy radical, are studied for the first time at a high level of theory.
Introduction Byers Brown' has proposed that the mechanism for the photonucleation of water vapor by ultraviolet radiation in the presence of oxygen involves the initial formation of the chargetransfer complex H20+.02- from the OyH20 van der Waals complex. The former, because of its large dipole moment, subsequently attracts neighboring water molecules so as to form a cluster. However, the proposed nucleating species, H20+.02-, is an excited state and may subsequently decompose to form other species such as H202 that may themselves act as nucleating agents or be of importance in atmospheric chemistry phenomena for other reasons. In this paper, we examine theoretically all possible energy minima on the singlet and triplet potential energy surfaces of O3H2, in order to ascertain which decomposition products of H20+'02- might be favorable from an energetic viewpoint. A number of the species we investigate have not been studied before, while others have been previously studied, often extensively. The reason for our further studies of these species in this paper is to allow all the energy minima to be calculated both at the same level of basis set and including the same degree of electron correlation.
Theoretical Methods All calculations reported here were carried out with the Gaussian92 suite of programs.2 Initially, geometry optimization of a variety of singlet and triplet molecular species were performed at the SCF level, with the 6-31G**3,4basis set. The structures of those species that were shown to be minima on the relevant potential energy surface at the Hartree-Fock level were subsequently optimized using the correlated method, QCISD.5 This particular method of including electron correlation was chosen to obtain energies and geometries because energy gradients are available to enable efficient determination of structures, and it is size consistent and infinite order. Having obtained a geometry by the QCISD method, the QCISD(T)5 energy was determined with both the 6-31G** and 6-311++G( 2 d , ~ )basis ~ , ~ sets. It was hoped that these calculations at a high level of electron correlation and employing large basis sets would yield better relative energies when comparing molecular species. The QCISD(T) calculations will be referred to as Q631(T) and Q6311(T) for the smaller and larger basis sets, respectively. In a few cases some doubt remained as to the validity of the structures determined, so a CASSCF8 geometry
* Author to whom correspondence @
should be addressed. Abstract published in Advance ACS Abstracts, February 1, 1995.
0022-365419512099-3109$09.OO/O
TABLE 1: QCISD(T) Energies (au) of OJH2 Species 6-31G** 6-311++G(2d,~,) -0.49823 -0.49982 H('S) -74.89668 -74.95340 WP) -74.81275 -74.86970 O('D) -1.16516 -1.16836 H-H(Ix,+) O-H('lI) -75.54851 -75.6 1234 O-H('X) -75.38896 -75.46054 -149.96213 -150.07968 o-o(~$) O-O('Ag) - 149.91274 - 150.03111 H-0-H( 'AI) -76.30208 -76.23166 0-0-H('A") - 150.65987 -150.53776 - 150.62794 0-0-H('A') -150.50645 O-O-O( 'AI) -225.05863 -224.87819 0-0-0(3~") -224.99809 -224.83558 o-o-o(~B') -224.83958 -225.0157 1 -225.00710 -224.83243 0dD3h) H-0-0-H('A) -157.17414 -15 1.30148 - 151.09564 Hz-O-O(' A') -151.22766 H-O-O-O('A")(cis) -225.509 16 -225.690 15 H-O-O-O('A")(trans) -225.50733 -225.68920 -225.44591 H-O-O-O('A')(cis) -225.62569 H-O-O-O('A')(trans) -225.44805 -225.62750 0-O(H)-0( CJ(QC1SD) -225.40063 -225.5 8845 0-O(H)-0( CJ(QCISD(T)) -225.401 06 0-O(H)-O( Cl)(QCISD) -225.39275 -225.58300 0-O(H)-O( Ci)(QCISD(T)) -225.39781 O-O(H)'-O( 'Al)(CASSCF) -225.44348 O-O(H)z-O( 'Ai) -225.93948 -226.13630 H-0-0-0-H('A) -226.13736 -226.32267 -226.26501 H-O(0) -OH(' A) -226.07646 02-*H20+(3A1) -226.17446 -225.98594 optimizationgcalculation was carried out. Also the CASSCF force constants were determined2 to fully characterize some structures. Throughout this paper all geometries given have been determined by the QCISD method with the 6-31G** basis set, unless otherwise stated. Also bond lengths will be angstroms and angles in degrees.
Results The calculated energies and structures are shown in Table 1 and Figure 1, respectively. We first discuss the charge-transfer complex which provided the focus for the present study, followed by other O3H2 species. 02-*H20+. We have previously reportedlo preliminary calculations of the charge-transfer species 02-*H20+ (1) and the van der Waals complex of 0 2 and H20. Optimized structures of the 02-*H20+ complex were obtained at the QCISD 0 1995 American Chemical Society
3110 J. Phys. Chem., Vol. 99, No. IO, 1995
Vincent and Hillier 0 738
90.6' H H -
(11)
0.974 1.761
Dihedral sngleHOOH = 1478' (5)
0975 ('A')
owo9 0-H7 (7a) 4 (In) 1038
H
'Ig
0 0
'A IC,)
1221 (38)
0-0
1.233
b
'4
1.451/0\0
I124
-0
l3b)
Old) Dihedral angle HCQH = 118 5'
0971-,290
?eo
1 488
'A
18)
Dihedral angle HOOO = 80 0 (4)
H 98.4'
1106'
1066'
0
C, symmetry ' QClSD
C, symmetry.QCISD (T)
107.2'
19)
0979& H
(12b)
1.349
0 ....,,,,,,i.426
0
106.5'
1Z46'
1423
C, symmetry : QClSD
C, symmetry : QClSDiT) (13b)
Figure 1. Optimized structures (at the QCISD(6-31G**)level) on the 03Hz potential energy surfaces. For structure 1,values in parentheses are for the CASSCF structure. Values for structure 12c are from ref 44.
and CASSCF levels of theory. The latter calculation employed a valence triple zeta basis.'" In the charge-transfer complex, electron excitation from the lbl orbital of H20 to the lngorbitals of 0 2 occurs. These three orbitals and the 3a,, ln,,, and 30, of 0 2 form the active space of the CASSCF calculation. Most of the correlating orbitals reside on the 0 2 fragment. Thus it is expected that the 0 2 - part of the complex will be described better than the H20+ fragment. The QCISD method is expected, however, to give a more balanced description of the complex. It thus appears to be fortuitous that the 0-0 bond lengths given by both methods are very similar, in view of the different basis sets and correlation methods used. The corresponding differences in the structure of the H20+ part are not great, again given their different theoretical origins. These calculations were restricted to CzVsymmetry, since only at this structure is the lbl lng charge-transfer state orthogonal, by symmetry, to all the states of lower energy. This enables the SCF wave function to be easily calculated. Departure from CzVsymmetry results in SCF wave functions of the charge-transfer state collapsing to "nonzwitterionic" states of the complex. For this reason it was not possible to calculate the harmonic frequencies of this zwitterion by finite differences employing a QCISD wave function. However, a UHF SCF calculation of the frequencies of 02--HzO+ gave four imaginary values, 1077i, 636i, 605, and 376i cm-'. Other vibrational modes are at 452 cm-' associated with the stretching of the ionic bond 02-*H20+ and one at 1418 cm-' which is the 0-0 stretch, and the remaining ones at 1611, 3249, and 3468 cm-l are the three modes of H20+. The four complex frequencies represent movement of the 02-*H20+ fragments with respect to each other. Despite the fact that this charge-transfer complex is probably not a minimum on the triplet energy surface, it does yield a transition energy (5.9 eV) consistent with a maximum in the rate of photonucleation observed experimentally." Thus we believe that it provides a point on the energy surface to which other species, including possible rearrangement products, can be compared.
-
+
HzO 0 2 . The global minimum on the triplet potential energy surface of 03H2 is 02(3E,) and H20(lA1). Both 0 2 and H20 have been studied extensively in the past and the geometries calculated here (2,3a) are close to the experimental values. The 0-0 and 0 - H bond lengths are 0.014 and 0.003 8, too long12 and the H-0-H angle is 0.5" less than the experimental value. The energy difference between the chargetransfer complex and this global minimum is 546 kJ/mol at the Q631(T) level and 544 kJ/mol at the Q6311(T) level. Due to the possibility of the involvement of singlet species in droplet nucleation we report comparable calculations on 0 2 (lAg). Herzberg13 gives the energy of the transition X a of 0 2 as 95 kJ/mol, compared with our value of 128 H/mol at the 4631 1(T) level. Our bond length of 02(lAg) (3b) is predicted to be too long by 0.017 .-&.I3 Hydrogen Trioxide, H-0-0-0-H. Hydrogen trioxide has been the subject of several previous theoretical ~ t u d i e s , ' ~ - ~ l which have highlighted the need for including electron correlation to predict the correct 0-0 bond length. Our QCISD structure (4) does not significantly differ from the MP2 or CASSCF structures reported p r e v i o ~ s l y . ~At ~ ~the ~ ~4 6- 3~1~ (T) and Q6311(T) levels the energy difference between 0 2 and H20, and hydrogen trioxide, is 148 and 155 kJ/mol, respectively. However, hydrogen trioxide is still 389 kJ/mol(Q631 l(T)) more stable than the charge-transfer complex 02-"20+. H-O(0)-OH and OzH OH. No previous high-level calculations of H-O(0)-OH have been reported. Initial SCF calculations on singlet H-O(0)-OH, oxyhydrogen peroxide, indicated a stable molecular species, and triplet calculations gave the triplet coupled hydroperoxo and hydroxy radicals. QCISD structures for these three species (5, 6, 7) are shown. It is noteworthy that the length of one of the 0-0 bonds is particularly large in 5 (1.761 A). To study this effect further a two-electron two-orbital CASSCF geometry optimization was carried out using a 6-31G** basis, involving the 0-0 bonding and antibonding u orbitals. No stable species was found, the
-.
+
. I . Phys. Chem., Vol. 99,No.
Potential Energy Surfaces of O3H2 oxygen-oxygen bond lengthened considerably, and the wave function consisted of two configurations (a2,a*2)of equal weight. Our calculations thus indicate that a complex of HOz and HO exists, but there is no strong bond between them on the singlet surface. In fact the Q6311(T) energy difference between the isolated fragments (HOz and HO) and H-O(0)OH makes the molecule less stable by 19 kJ/mol, using the QCISD structures. The hydroxy radical and the hydroperoxo radical have been studied extensively in their own right and in the context of the H2O3 potential energy surface. We calculate bond lengths of 0.975 and 1.003 A for the and 2Zstates of OH. These values compare with the corresponding experimental values of 0.97113 and 1.012 A.13 Previous theoretical studies of OH are many, and give a bond length for the 211 state similar to ours.14,20,22 Werner et a1.22also studied the 2Zexcited state, giving a bond length closer to experiment. However, given the modest basis employed in our study for geometry determination, our error of 0.009 is acceptable. The experimental energy separation of the II and Z states is 391 k J / m ~ l .Werner ~ ~ et a1.22obtained a value of 393 kJ/mol, while our Q6311(T) energy separation is 399 kJ/mol. For the hydroperoxo radical (6), the experimental geometry of the ground state (2A") has been determined by Lubic et aLz3 and the excited-state geometry (2A') by Tuckett et aLZ4 Comparing our excited-state geometry with experiment, our 0-0 length is too long by 0.025 A and the bond angle too small by 1.8". As with the 0-0 bond, the 0-H bond is too long, but only by 0.006 A. Our bond lengths are close to those calculated by Langhoff and Jaffe,25though the angle differs by 3.9". 02H has a 2Aff ground state whose experimental geometry23is 0-0 = 1.331 A, 0-H = 0.971 8, and 0-0-H = 104.3'. Compared to our values (6a) the 0-0 bond is too long by 0.022 A, while the two other geometrical parameters are very close. Other theoretical studies have yielded 0-0 distances close to e ~ p e r i m e n t , while ~ ~ , ~ Sicilia ~ and RussoZ8 using density functional techniques obtained and oxygen-oxygen distance close to ours. MP2 studies with a 6-31G** basis set gave an 0-0 distance of 1.325 A,20 in close agreement with experiment, which is a little surprising given that our QCISD result is further from experiment. Finally, we report our calculated energy separation of the 2A" ground state and 2A' excited state as 84 kJ/mol, which is very close to the experimental value.24 Hydrogen Peroxide, Oxywater, and Dioxywater. Cremer29 has studied both hydrogen peroxide and oxywater. More recently, Meredith et al. have studied hydrogen peroxide, oxywater, and the transition state linking them.30 They find that oxywater lies in a high-energy shallow well which is 195 kJ/mol above the hydrogen peroxide minimum. They also discuss the problem of determining the structure of hydrogen peroxide from experimental data. The results of our study on hydrogen peroxide (8), oxywater (9), and dioxywater (10) are shown in Figure 1. Comparison with the results of Meredith et al. for the two H202 isomers shows no important differences. We do note, however, that the hydrogen peroxide dihedral angle differs by 7.3', which, in view of the shallowness of rotational potentials near stationary points, is not unexpected. The 4631 1(T) level of theory gives an energy difference between the two isomers of H202 as 194 kJ/mol, close to the previous value of 195 k J / m ~ l . Compared ~~ to the charge-transfer complex (02-"20+), H20-0 and O(3P)are 17 kJ/mol lower in energy. Thus, hydrogen peroxide and O(3P) lie 211 kJ/mol below the charge-transfer species. Another feature of oxywater that must be considered, given our interest in water cluster formation, is
IO, 1995 3111
TABLE 2: CASSCF Frequencies (cm-9 of Dioxywater frequency mode 189 sym 0-0 361 bend O-O-O 531 asym 0-0 602 deformation of O-O-O 959 twist of H-0-H 1011 deformation of H-0-H 1673 bend H-0-H 4001 sym 0-H 4124 asymO-H its dipole moment. The QCISD value is 4.08 D which is somewhat smaller than that of the charge-transfer complex (5.08 D). However, it is still large, suggesting that oxywater could act as an effective nucleating agent. Bach et al. have studied3132 a variety of reactions involving hydrogen peroxide and oxywater and find that in the presence of water, oxywater becomes much more stable when compared to hydrogen peroxide. This confirms our view that oxywater is an excellent species for droplet formation. We turn now to a consideration of dioxywater (lo), which has not been previously studied at a high level of theory. We find it to have a large dipole moment (4.41D, QCISD). However, we predict that H20(0)2 is high in energy which probably precludes its consideration as a nucleating agent. It lies 100 kJ/mol above the 02-*H20+ charge-transfer complex and 102 kJ/mol below O(lD) and oxywater. The QCISD structure of dioxywater (10) highlights that this species is somewhat unusual due to the four covalent coordination of oxygen in a neutral molecule. To study this species further we have carried out geometry optimization at the CASSCF level. Here the active space consisted of the four electrons involving the two 0-0 bonds and their a* counterparts. No symmetry was imposed on the system during optimization. After optimization the harmonic frequences were obtained and these are given in Table 2. They show conclusively that dioxywater is a minimum on the singlet O3Hz potential energy surface. The symmetric and asymmetric H-0 stretches occur at 4001 and 4124 cm-', while the H-0-H bend is found at 1673 cm-'. An indication of the weak nature of the 0-0 bonds is given by the low values of the vibrational frequencies at 537 and 189 cm-'. The O-O-O bend occurs at 361 cm-', while the O-O-O deformation is at 602 cm-'. A twist and a deformation of the H-0-H group occur at 959 and 1011 cm-', respectively. These results indicate that the high-energy modes are on the H20 fragment, while the lower energy modes involve the terminal oxygens. This is perhaps to be expected given the long 0-0 bonds. Comparing the QCISD geometry with the CASSCF geometry, the latter treatment lengthens the 0-0 distances by 0.04 A and reduces the O-O-O angle by 4". The H-0 bonds are shortened by 0.02 8, rather than lengthened since the CASSCF treatment does not correlate the bonding electrons involved. It also increases the H-0-H angle by 1.6". M c D o ~ a l has l~~ pointed out that singlet oxygen atom is very electrophilic and, given the lone pairs of electrons on water, this would account for the fact that our tetracoordinatedoxygen species is predicted to be a minimum energy structure. 0 3 H2. Ozone and its excited states have previously been studied e x t e n s i ~ e l y . ~ ~Most - ~ ~ studies have imposed CzV symmetry for the ground and excited states. However, we have removed this constraint. For the 'A1 and 3B2 states the CzV symmetry is maintained, within numerical tolerance, but for the 3A2 (C2J, C, symmetry results. For this latter state at C2, symmetry the Hartree-Fock solution is unstable and a C, symmetry solution is obtained. A set of QCISD(T) calculations
+
3112 J. Phys. Chem., Vol. 99, No. IO, 1995
Vincent and Hillier
at both C2” and C, symmetry using orbitals arising from the two solutions to the Fock equations could not resolve the problem of the symmetry of this state. The experimental structure of singlet ozone is of C2, symmetry with an angle of 116.8’ and a bond length of 1.272 Our calculated geometry ( l l a ) reproduces these values fairly well, the 0-0 distance being 1.275 8, and the angle being 117.4’. For our 3B2structure ( l l b ) the geometry is essentially the same as found p r e v i o ~ s l y the , ~ ~angle ~ ~ ~and bond length being close to 108’ and 1.375 A, respectively. Energetically, H2 O3(IA1)is below 02-*H20+ by 138 kJ/mol (Q6311(T)) and 406 kJ/mol (Q6311(T)) above 0 2 H2O. For the 3A”(3A2) state, due to the Hartree-Fock instability, the energy difference must have large errors, but it is calculated to lie 21 kJ/mol above 02-*H20+, while the 3B2 state lies 25 kJ/mol below the charge-transfer complex. The final O3 species to be considered is the D3h cyclic form (lA’1). This state has been previously studied by a number of g r o ~ p s . ~ OWith - ~ ~ a large basis set, Lee41 obtains an 0-0 distance of 1.444 A, which compares favorably with our value of 1.451 8,. He also calculates an energy difference between the open (C2,) and cyclic structures of 120 kJ/mol, which compares to our value of 135 kJ/mol (Q6311(T)). HOJ. There have been a few previous studies of Ho3.u-48 (12) Most have been of the 1-hydrotrioxy radical, H-0-0-0 and not of its isomer O-O(H)-0,2-hydrotrioxy (13). We first discuss the ground state (2A“). Although our calculated structure at the QCISD level for the 1-hydrotrioxy species (12) is not grossly different from the CASSCF structureu ((12c), there are some significant differences. Firstly, the QCISD cis and trans structures are planar. This was not imposed on our studies but resulted from the geometry optimization. Secondly, our structures have a shorter terminal 0-0 bond (1.290 8, compared to 1.355 8,). This bond length is much closer to that of 0 2 (1.221 8,) than to hydrogen peroxide (1.463 8,). These differences in structure are due to the correlation method employed and not the basis set differences, since a CASSCF calculation with our basis set yielded a structure close to that of Dupuis et al.44 Furthermore, this difference is not due to spin contamination, since the QCISD wave function projects out the first spin c0ntaminant4~leaving a very pure spin state with = 0.753. The most likely explanation for the geometric differences is that the QCISD method gives a better description of correlation and does not emphasize correlation effects of a small number of valence orbitals, as does the CASSCF calculation. A QCISD(T) calculation using the structure of Dupuis et al.“ confirms it to be higher in energy than our calculated QCISD structure. We calculate a cis/trans energy difference of the 1-hydrotrioxy radical of 2.5 kJ/mol. H HOs(cis) lies 504 kJ/mol above 0 2 H20, 41 kJ/mol below the 02-.H20+ complex and 348 kJ/mol above H-O-O-OH. The geometry of the first excited state (2A’) constrained to C, symmetry is given in Figure 1 (12d, 12e). The main difference between the geometries of the ground and excited state is the length of the terminal 0-0 bond, being 0.13 8, longer in the latter. In both states the unpaired electron resides on the terminal oxygen atom; the geometry of the remaining HO2 part of the molecule is thus very similar for the two states. The energy difference between the lowest energy conformers of these two states is 164 kJ/mol. The other isomer of HO3,2-hydrotrioxy radical, 0-O(H)-0 (13), proved a very difficult system to study. There are at least two solutions to the Hartree-Fock equations, one where the unpaired electron is delocalized equally on both terminal oxygen atoms (C, symmetry) (13a) and one where the unpaired electron
+
+
+
+
TABLE 3: 0-0 Bond Strengths (kJ/mol)” and Lengths
(AY
bond
bond
strength 454 (499) o,(’z-) 20(3~) 202 (215) H2028A) 2H0(211) 247 (268) HO~(~A”) HO(~II) o ( 3 ~ ) 67 (107) Os(’A1) o,(~x-) + o(3~) 147 HzO-O(’A’) &O(’AI) + O(’D) 102 O(lD) H20(0)2(’A1) HzO-O(’A’) H-O-O-O(2A”)H02(’A”) O(3P) 202 ( 179) H-0-0-0(2A”) -5 (-52)
--- -
H-O(TI)
+
+
- + -I.~O~(~AII)+
+ O,(~Z-)
o-o(H)-o(~A’) H-0-0-0-H(’A) HOz(,A”) + HO(’II)
length
1.221 (1.207) 1.463 (1.464) 1.353 (1.331) 1.275 (1.272) 1.575 1.550 1.290 1.488
o(3~) -65
1.374 132 (117) 1.438
Calculated at the Q631 l(T)//QCISD (6-31G**)level. The experimental value^^^^^^ are given in parentheses. Calculated at the QCISD (6-31G**) level. Experimental values are given in parentheses (see text).
TABLE 4: H-0 Bond Strengths (kJ/mol)”and Lengths
(AP
~
bond
---
H-O(TI) ~ ( 2 s+ ) o(3~) H-O-H(’A1) H-0(211) H(’S) H-O-O(ZA”) ~(2s) +~,(~z;) H-0-0-H(’A) H02(’A”) + H(’S) HzO-O(’A’)- H02(2A”) + H(2S) H-O-O-O(’A”) 03(’AI) H(’S) O-O(H)-O(’A’) Os(’A1) + H(’S) H-0-0-0-H(’A) H-0-0-O(’A”) + H(’S) HzO(OM’A1) O-O(H)-O(’A’) + H(’S)
-
-.-
+
+
strength
bond length
418 (428) 499 (499) 211 (197) 372 (375) 178 346 (269) 79 348 (366)
0.975 (0.971) 0.961 (0.958) 0.974 (0.971) 0.967 (0.965) 0.968 0.975 0.978 0.970
126
0.974
Calculated at the Q6311(T)//QCISD(6-31G**)level. The experimental value^^^,^^ are given in parentheses. Calculated at the QCISD (6-31G**) level. Experimental values are given in parentheses (see text). is localized on one terminal oxygen atom (13b). There is also a third SCF solution which, after geometry optimization at the QCISD level, resembled the localized structure. With these problems in mind it was decided to optimize this molecule at the QCISD(T) level. The resultant two QCISD(T) structures (Figure l), while still being different, are much closer to each other than are the QCISD structures. Energetically they differ by -3 x au. We thus conclude that the QCISD(T) wave function does not appear to be extensive enough to describe the 2-hydrotrioxy radical, though its failure is small in chemical terms. However, this error is greater than for most of the other systems studied here. Nevertheless, it is of interest to estimate the energy difference between 1-hydrotioxy and 2-hydrotrioxy, which turns out to be 267 kJ/mol in favor of 1-hydrotrioxy, using the QCISD structure (13a). 0-0 and 0 - H Bond Lengths. It is of interest to compare the 0-0 and 0-H bond lengths of the species we have studied with the corresponding bond strengths (Tables 3 and 4). As far as the 0-0 bond lengths are Concerned, it is not surprising that by far the shortest length is for 02(3X,) in line with its bond strength. However, for the remaining species no definite bond lengthbond strength relationship emerges, reflecting the complex bonding situation in a number of the species, which we have highlighted in our work. As far as the 0 - H bond lengths are concerned, although there are some inconsistencies, it can be seen that there is more correlation between the calculated bond lengths and bond strengths than for the 0-0 bonds.
J. Phys. Chem., Vol. 99, No. IO, 1995 3113
0,H (,A')
+ OH ('2)
0-0-0 ('Al)
+ H, ('2;)
K-0-0-H ('A) t 0 (9) DzH (,A') + OH (,n)
Figure 2. Energies ($226 au) of the minima on the H203 potential energy surface. Relative energies in kJ/mol are given in parentheses.
Comparison of our calculated bond strengths with those derived from the experimental data of Bens0n~O9~l (Tables 3 and 4)reveals general agreement to within 40 kJ/mol, although the OH bond in H 0 3 is an exception. Some discrepancy is to be expected as our data refer to 0 K with no zero-point corrections. The Global Potential Energy Surface. We have attempted to study all of the singlet and triplet energy minima on the potential surface 03H2, involving one or two distinct species. In Figure 2 we show the relative energies of the species studied. The global minimum (H20, 02(3Z,)) is followed by (H20,Oz(lAg)) and by H-O-O-O-H('A), the latter being some 155 kJ/mol above the global minimum. Dissociation of the 0-0 bond in this species, of calculated strength 132 kJ/mol, leads to OZH(~A")and OH(217). To higher energy come H-O(0)0-H( lA), (OZH(~A'),OH(21-l)), and hydrogen peroxide O(3P), the latter being 333 kJ/mol above the global energy minimum. There now follows ozone ('AI) and H2('Z:) and to higher energy a group of species that are close in energy and include the 02-"20+ charge-transfer state that may be implicated in water droplet nucleation.' From the other species in this group, of particular interest is oxywater which, from an energetic viewpoint, might arise from decomposition of 02-*H20+(3A1) and in view of its large dipole moment may act as a possible nucleating agent. Studies of this and other possible pathways for the decomposition of 02-*H20+ are underway. Such a study will require a fuller treatment of this charge-transfer state, relaxing the present restriction of CzV symmetry, using a method consistent with the multiconfigurational nature of the problem. It is of interest that both ozone in various states and HO3, of importance in atmospheric chemistry,
+
may result from decomposition of t h e charge-transfer state. The
remaining species studied to higher energy involve excited states, with the exception of O-O(H)z-O (dioxywater) and 0-O(H)-0 H.
+
Acknowledgment. We thank NERC for support of this research and Professor W. Byers Brown for discussions. References and Notes (1) Byers Brown, W. Chem. Phys. Lett., in press.
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