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J. Phys. Chem. 1996, 100, 6076-6080
Can Oxywater Be Made? Hsing Hua Huang,† Yaoming Xie, and Henry F. Schaefer III* Center for Computational Quantum Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: October 6, 1995X
Ab initio quantum mechanical methods have been applied to an investigation of the oxywater (H2OO)hydrogen peroxide (HOOH) isomerization at high levels of theory. The investigation employed basis sets up to triple-ζ plus double polarization plus f functions (TZ2P+f) and levels of correlation up to coupled cluster including single, double, and perturbatively treated connected triple excitations [CCSD(T)]. Harmonic vibrational frequencies are obtained via finite differences of analytic gradients. Their infrared intensities are also reported. The classical barrier for isomerization of oxywater is substantial (5.7 kcal/mol) at the highest level of theory. Correction for zero-point vibrational energies yields a predicted activation energy of 3.3 kcal/mol. The equilibrium dipole moment of oxywater is predicted to be substantial, namely 4.5 D. Thus, oxywater indeed awaits synthesis.
Introduction Oxywater (H2OO) has been proposed as a structural isomer of hydrogen peroxide that may occur as a transient intermediate in oxidation reactions initiated by the latter species. Oxywater, if it exists, is likely to be connected by a (1,2)-hydrogen shift to hydrogen peroxide. In contrast to hydrogen peroxide which has been well studied both experimentally1-9 and theoretically,10-23 oxywater has never been observed and has seldom been considered in the literature. The first reference to oxywater appears to have been made by Bain and Giguere24 in 1955 when they analyzed the infrared (IR) spectrum of H2O2 using isotopically substituted species. These investigators concluded that the existence of “any tautomeric form of the molecule such as H2O-O” was unlikely because they failed to observe an O-O stretching fundamental other than that which is characteristic of hydrogen peroxide. The first theoretical study of oxywater was the classic 1966 paper by Kalder and Shavitt25 in which they carried out SCFLCAO wave function calculations with a minimal basis set for both H2O2 at a series of seven different dihedral angles as well as for a modified form, H2OO. In 1983, Pople, Raghavachari, Frisch, Binkley, and Schleyer26 published a paper in which they examined several simple (1,2)-hydrogen shifts and concluded that singlet isomers resulting from rearrangement to an “abnormal valence coordination” either have shallow potential minima or do not correspond to minima at all. Oxywater was found to exist as a stable minimum at the HF/6-31G* level of theory with a rearrangement barrier (∆E) of 21.2 kcal mol-1. However, when correlation corrections were applied at the MP4SDQ/6-31G** level, the barrier was reduced to 2.0 kcal mol-1, and subsequent application of a triple substitution correction eliminated the barrier entirely. Since Pople et al. did not optimize the geometries of oxywater, hydrogen peroxide, and the transition state connecting them at high levels of theory but merely evaluated MP4 single-point energies at the HF/631G* geometries, their conclusion that oxywater does not exist as a stable minimum is not beyond question. In the same year, Cremer27 reported the results of his RSMP/ SVdp [Rayleigh-Schro¨dinger Møller-Plesset perturbation theory * To whom correspondence should be addressed. † Permanent address: Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 0511. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6076$12.00/0
in conjunction with split-valence basis augmented by d (for heavy atom) and p (for H atom) functions] calculations on oxywater in a discussion on the various structures that are open to hydrogen peroxide. The parameters reported by him for a pyramidal structure are r(O-O) ) 1.521 Å, r(O-H) ) 0.969 Å, and OOH angle ) 108.7°, with an energy of 53 kcal mol-1 relative to the “bent-bent” form of hydrogen peroxide. More recently, Bach et al.28,29 have studied a variety of reactions involving hydrogen peroxide and oxywater. Of particular interest is their study on the oxidation of ammonia by hydrogen peroxide. Using methods similar to those of Pople et al. but optimizing structures at these higher levels of theory, Bach, McDouall, Owensby, and Schlegel28 reached a dramatically different conclusionsthat oxywater does indeed exist as a local minimum lying 6.3 kcal mol-1 below the energy of the transition structure for the (1,2)-hydrogen shift (although with the zero-point vibrational energy correction, this barrier drops to 3.1 kcal mol-1). In a later paper (1991) Bach, Owensby, Gonzalez, Schlegel, and McDouall29 confirmed the previous findings of Bach et al. concerning the gas phase mechanism of the oxidation of ammonia by hydrogen peroxide. However, they also noted that the high barrier for the (1,2)-hydrogen shift can be lowered by adding one or two molecules of solvent water and on this basis suggested that oxywater, although it may have only a transient existence in the gas phase, is likely to be longer lived in protic solvents. Furthermore, they noted that oxywater can serve as a model for theoretical studies of oxygen transfer from alkyl peroxides. In 1992, Meredith, Hamilton, and Schaefer30 published a theoretical study of the oxywater-hydrogen peroxide isomerization. Oxywater, hydrogen peroxide, and the transition state connecting them were located using the self-consistent field (SCF), configuration interaction including all single and double excitation (CISD), and coupled cluster with single and double excitations (CCSD) methods with several basis sets, the largest being of triple-ζ plus double polarization (including f functions on the oxygen atoms) quality (TZ2P+f). Harmonic vibrational frequencies were evaluated at the SCF, CISD, and CCSD levels of theory with the DZP basis set and the stationary points characterized as minima or transition states. The CCSD method with connected triple excitations [CCSD(T)] was also used in conjunction with the smallest set of DZP quality to obtain oxywater’s equilibrium geometry and frequencies as well as to compute single-point energies of all CCSD-optimized structures. © 1996 American Chemical Society
Can Oxywater Be Made?
J. Phys. Chem., Vol. 100, No. 15, 1996 6077
A classical barrier to isomerization of 5.7 kcal mol-1 was predicted at the highest level of theory. After correction for zero-point vibrational energies, the comparable ground state activation energy is 3.2 kcal mol-1. Thus, they concluded that although these ab initio predictions could be decreased by 1 or 2 kcal mol-1 at yet higher levels of theory, there can be little doubt that oxywater is a genuine minimum on the H2O2 potential energy hypersurface. In the present study, the ab initio methods applied to this problem have been expanded to include basis sets up to triple-ζ plus double polarization plus f functions (TZ2P+f) and levels of correlation up to coupled cluster including single double and perturbatively treated connected triple excitations [CCSD(T)] for the calculation of harmonic vibrational frequencies. The [CCSD(T)] method was also applied to obtain optimized geometries and energy with the (TZ2P+f) basis set. Methods We fully optimized the geometrical structures of oxywater, hydrogen peroxide, and the transition state between them using basis sets of double-ζ plus polarization (DZP) and triple-ζ plus double polarization augmented by f functions on the O atoms and d functions on the H atoms (TZ2P+f). The technical description of the DZP basis set is O(9s5p1d/ 4s2p1d), H(4s1p/2s1p).31,32 The Gaussian orbital exponents of the primitive s functions on the hydrogen atoms in the DZP set are scaled by (1.2)2 ) 1.44. The exponents of the DZP polarization functions are Rd(O) ) 0.85 and Rp(H) ) 0.75. The technical description of the TZ2P basis set is O(10s6p2d/ 5s3p2d), H(5s2p/3s2p).31,33 The orbital exponents for the TZ2P+f polarization functions are Rd(O) ) 1.70, 0.425 and Rp(H) ) 1.50, 0.375, and a set of f-like functions with Rf(O) ) 1.40 was added to the oxygen atoms; a set of d-like functions with Rd(H) ) 1.00 was added to each hydrogen atom. Five pure d-like functions and seven pure f-like functions were used in all polarized basis sets. The self-consistent field (SCF), the single and double excitation configuration interaction (CISD), the single and double excitation couple-clustered (CCSD), and the CCSD approach with connected triple excitations appended perturbatively (CCSD[T]) were used for optimization of the structures in conjunction with analytic gradient methods.34-37 Harmonic vibrational frequencies at the SCF level of theory were determined via analytic second-derivative methods.38 Their corresponding infrared intensities were determined via analytical first derivative of dipole moment components. At the CISD, CCSD, and CCSD(T) levels of theory, frequencies were obtained via finite differences of analytic gradients. Dipole moment derivatives and infrared intensities were evaluated via finite differences of analytic dipole moment components. The computations of all the correlation methods were performed with the two lowest-lying SCF MO’s (oxygen 1s-like) doubly occupied in all configurations and the two core counterpart virtual orbitals deleted. All the procedures were carried out using the program system PSI 2.0.8.39 Results and Discussion The equilibrium geometries for oxywater, hydrogen peroxide, and the transition state for the (1,2)-hydrogen shift were predicted at the CCSD(T) level of theory using both DZP and TZ2P+f basis sets. Figure 1 shows the geometrical parameters obtained at the TZ2P+f CCSD(T) level of theory for both the minima and the transition state, as well as the atomic numbering scheme employed here. The predicted geometries for the stationary points are given in Table 1. The harmonic vibrational
Figure 1. Structural parameters (bond lengths in angstroms) for oxywater, the transition state for rearrangement, and hydrogen peroxide at the TZ2P+f CCSD(T) level of theory. The torsional angles are defined as H4-O2-O1-H3.
TABLE 1: Structural Parameters (Bond Lengths in angstroms and Angles in degrees) for Oxywater, the Transition State, and Hydrogen Peroxidea structural parameters
DZP CCSD(T)
TZ2P+f CCSD(T)
r(OO) r(OH) ∠HOH ∠oop
Oxywater 1.578 0.974 105.8 74.4
1.549 0.967 106.4 73.4
r(O1O2) r(O2H4) r(O1H3) r(O2H3) ∠O1O2H4 τ
Transition State 1.656 0.975 1.428 1.026 96.8 100.8
1.634 0.968 1.398 1.031 97.4 103.8
Hydrogen Peroxide 1.475 0.973 99.2 114.3
1.461 0.964 99.7 111.9
r(OO) r(OH) ∠OOH τ
a For oxywater the out-of-plane (oop) angle is defined as the angle between the O-O vector and the HOH plane. The torsion angle for the transition state is defined as H4-O2-O1-H3.
frequencies reported in Table 2 lend further support to our characterization of the stationary points as minima and transition states on the potential energy surface. Hydrogen peroxide unscaled and scaled TZ2P+f CCSD(T) frequencies are compared with the experimentally obtained fundamentals in Table 3. Absolute energies at the TZ2P+f CCSD(T) level of theory and their relative energies before and after ZPVE corrections are shown in Table 4. The present work is mainly concerned with oxywater and the transition state for the (1,2)-hydrogen shift. Nevertheless, hydrogen peroxide was also included in our study because this species has been well studied both experimentally and theoretically, whereas oxywater has never been observed. It was felt that the extent to which our predictions for hydrogen peroxide agree with the experimental results would provide a good indication of the likely success of our methods for characterizing oxywater and the elusive transition state. The most commonly quoted equilibrium geometry for hydrogen peroxide is that obtained from the infrared data of Redington, Olson, and Cross.40 On the assumption that
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TABLE 2: Harmonic Vibrational Frequencies (in cm-1) for Oxywater, the Transition State for Rearrangement, and Hydrogen Peroxidea TZ2P+f sym
DZP CCSD(T)
SCF
CISD
CCSD
CCSD(T)
assignmt
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