6942
J. Phys. Chem. 1996, 100, 6942-6949
Comparison of ab Initio and Density Functional Theory for Alkali Peroxynitrite: A Highly Correlated System with Hartree-Fock Instability Hui-Hsu Tsai,† Tracy P. Hamilton,*,† Jyh-Hsin M. Tsai,‡ Joseph G. Harrison,‡ and Joseph S. Beckman§ Department of Chemistry, Physics, and Anesthesiology, UniVersity of Alabama at Birmingham, Birmingham, Alabama 35294 ReceiVed: August 7, 1995; In Final Form: January 3, 1996X
Ab initio methods including density functional theory are used to study alkali peroxynitrite (ONOOM, M ) Li, Na, and K). The energy, optimized geometries, and vibration frequencies are presented for different conformations of alkali peroxynitrite. The cis-cis (a pseudo-five-membered ring) and trans-cis conformers are minima from a search over the dihedral angles ONOO and NOOM, respectively. Other minima that appear on the potential surfaces depend on the level of theory and the alkali atom studied. All levels of theory predict the cis-cis structures to have the lowest energy, and the stability relative to the trans-cis structures decreases from Li to Na to K. The rotational barrier of cis-cis to trans-cis isomerization is about 20 kcal/mol in ONOOLi, 27 kcal/mol in ONOONa, and 22 kcal/mol in ONOOK. The barriers for the reverse process are 12, 17, and 13 kcal/mol, respectively. Singlet-triplet UHF Hartree-Fock wave function instabilities are found in some of the ONOOM molecules, which lead to poor values for ab initio rotational barriers, vibrational frequencies, and intensities. This leads to a survey of 22 density functional methods, with the computed Becke3-LYP vibration frequencies of cis-cis ONOOLi agreeing best with the experimental IR and Raman spectra. Density functional theory exhibits no instabilities for this class of problem molecules.
Introduction Peroxynitrite anion (ONOO-) plays important roles in biological and physical chemistry. In the nervous system, nitric oxide reacts with superoxide (O2-) to form the peroxynitrite anion.1 An unusually strong oxidant, peroxynitrite anion is a major cytotoxic agent produced by inflammatory cells of the immune system. It may be responsible for inherited forms of amylotrophic lateral sclerosis (Lou Gehrig’s disease) which result from mutant forms of superoxide dismutase (SOD) enzyme.2 It can nitrate phenols in the presence of catalytic Fe3+.3 It decomposes at physiological pH but is stable in base solution for months.4 Peroxynitrite anion is 36 kcal/mol higher in energy than its nitrate isomer and therefore has potential applications in explosives and rocket fuels. The alkali nitrate can rearrange to the yellow peroxynitrite by exposure to 254 nm ultraviolet light for several hours,5 but the yield is low since peroxynitrite also absorbs UV. A pure solid salt has been recently prepared.6 Formation of peroxynitrite by exposure of Martian soil to ultraviolet light from the sun may have caused the evolution of oxygen and amino acid decarboxylation observed during the Viking Mars mission.7 Recently, this surprisingly widespread and unusual molecule has received significant attention in both experimental and theoretical studies. In 1990, Shen et al. used Hartree-Fock self-consistent-field (HF) calculations with different basis sets to study ONOO-‚(H2O) complexes.8 Two stable ONOOisomers, cis and trans, formed hydrogen bonds with H2O molecules at different positions. A correlated method, second order Møller-Plesset perturbation theory (MP2), with a 6-311+G(d,p) basis set has been used to study the ONOO-‚(H2O)n (n ) 1 or 2) complexes.9 Koppenol et al. studied the cis and trans †
Department of Chemistry. Department of Physics. § Department of Anesthesiology. X Abstract published in AdVance ACS Abstracts, April 1, 1996. ‡
0022-3654/96/20100-6942$12.00/0
conformers as well as the transition state for torsional motion of ONOO- at the HF/6-31(d) level.10 In their calculations, the trans conformer is slightly more stable than the cis form, and the rotation barrier was high. However, correlated methods were also used to study this molecule, and they predict that the cis conformer is more stable than the trans isomer.11,12 Bohle et al. obtained 97% pure [Me4N]+[OONO]- by fractional crystallization from liquid ammonia and assigned the trans conformations of ONOO- to the IR and Raman spectra.6 In 1994, Tsai et al. reported the aqueous Raman spectra and concluded that only one isomer in solution was observed.11,13 Coupled cluster single and double excitation frequencies (CCSD) for cis ONOO- agree well with the aqueous Raman spectra except for the broad torsion band. The torsional band discrepancy may be caused by anharmonicity, solvent effect, or the need for an extremely accurate wave function. Simple anharmonicity14 and solvent effects (based on self-consistent polarizable continuum models or by computations using explicitly hydrogen bonded water molecules)9,14 were found to be unlikely sources of the disagreement. Computational and experimental studies on ONOOM (M ) Li, Na, and K) were undertaken to clarify the assignments. Plumb and Edwards reported that ONOOK is produced by irradiating solid KNO3 with 254 nm UV light.15 Three vibrational bands, 721, 815, and 943 cm-1, were observed and assigned as possible O-O and N-O stretching bands of ONOOK. The Raman spectra of alkali peroxynitrite (ONOOM; M ) K, Ru, and Cs) in crystal have also been studied, and only one isomer was apparently observed.13 Lo et al. used shortwave UV lasers to create the various 15N and 18O isotopomers of MNO3 (M ) Li, Na, and K) and isolated ONOOM in frozen Ar matrices.16 The NO2 and OM formed by irradiation recombined to form two different trapped conformers with different photochemical behavior. Based on the isotope shifts, the normal mode assignments were made for different ONOOM isomers. © 1996 American Chemical Society
Alkali Peroxynitrite
J. Phys. Chem., Vol. 100, No. 17, 1996 6943
Figure 1. Optimized geometries of cis-cis, cis-perp, trans-cis, and trans-perp ONOOLi in the HF, MP2, and Becke3-LYP with 6-311+G(d) basis set calculations.
This study uses ab initio and density functional theory (DFT) to study different conformers of ONOOM (M ) Li, Na, and K), with the primary emphasis on a comparsion of methods in their ability to accurately predict rotational barriers and vibrational frequencies for molecules that have Hartree-Fock instabilities. Appropriate CASSCF reference wave functions are able to surmount the instability problem, but at an increased cost in complexity and computer resources. Searches over the torsional angles were performed at different levels of theory, and the relative energies, optimized geometries, and harmonic vibration frequencies are presented. The harmonic vibration frequencies and isotopic frequency shifts were compared with the frozen argon matrix isolation IR spectra reported by Lo et al.16 The optimized geometries and harmonic frequencies of cis-cis ONOOLi were also used as the basis of comparison for 22 density functionals. Theoretical Methods Standard Pople 6-311+G(d) group basis sets17,18 were employed in the ONOOLi and ONOONa study, and triple ζ plus two sets of polarization (TZ2P) basis sets19,20 were used for the ONOOK calculations. The slightly smaller 6-31+G(d) basis set was used for potential energy surface scans.21 The wave functions computed were HF, MP2, and MP4(SDTQ). Density functional theory (DFT) was also employed in this study. The ab initio and DFT calculations were done with the Gaussian 92/DFT22 and Gaussian 9423 suites of computer programs. All possible combinations of DFT exchange and correlation functionals available in G92/DFT were tested for cis-cis and trans-cis ONOOLi, with the results given in the supporting information. The notation for density functionals is to give the
exchange functional first, followed by the correlation functional separated by a dash. The exchange functionals used are from Becke 1988 (symbolized by B),24 Slater exchange (labeled as S),25-27 and the X-alpha (given by Xa to make comparisons with G92/DFT keywords easier).25-27 The correlation functionals used were the Lee-Yang-Parr (LYP),28 the Perdew 1986 functional (P86),29 the Perdew-Local (PL),30 the VoskoWilk-Nusair (VWN, also known as the local spin density correlation functional when used with Slater exchange),31 and another correlation functional from Vosko-Wilk-Nusair (VWN5).31 Hybrid methods that mix in Hartree-Fock exchange were utilized also: the Becke half and half method (denoted BHandH)32 and the three-parameter combination of Hartree-Fock, Becke 1988, and Slater exchange (Becke3)33 combined with the LYP or the P86 correlation functionals as described in the G92/DFT manual. The latter two hybrid functionals are therefore labeled Becke3-LYP and Becke3-P86. The DFT method utilized most in this study is Becke3-LYP. Results and Discussion Relative Stability and Internal Rotation of ONOOLi. For convenience, we use the same nomenclature as McGrath and Rowland’s paper on ONOOH,34 which distinguishes different conformers by the dihedral angles. For example, the structure with a cis ONOO arrangement and the alkali atom bonded to the terminal oxygen with a perpendicular orientation is called “cis-perp”. Figure 1 presents the conformers studied. The first part of the structure’s name refers to the ONOO dihedral angle, whereas the second part of the name labels the NOOM dihedral angle. Figure 2 displays the ONOO torsion angle potential curve of ONOOLi (with NOOLi kept cis) from Becke3-LYP/6-31+G-
6944 J. Phys. Chem., Vol. 100, No. 17, 1996
Figure 2. The -*- potential energy curve is calculated with the Becke3-LYP/6-31+G* method by rotating the ONOO torsion angle and keeping the NOOLi dihedral angle at 0°. The - - - curve is fitted by a Fourier series expansion, where u1 ) 16.42 kcal/mol, u2 ) 20.06 kcal/mol, u3 ) 4.64 kcal/mol, and u4 ) -0.06 kcal/mol.
Tsai et al.
Figure 3. The -*- potential energy curve is calculated with the Becke3-LYP/6-31+G* method by rotating the NOOLi torsion angle of cis-cis ONOOLi. The - - - curve is fitted by a Fourier series expansion, where u1 ) 14.81kcal/mol, u2 ) 9.22 kcal/mol, u3 ) 3.34 kcal/mol, and u4 ) -1.03 kcal/mol.
(d) calculations. The torsion potential is fitted by a Fourier series expansion: ∞
∞
un [1 - cos(nω)] n-1 2
V(ω) ) ∑ Vn ) ∑ n-1
where ω is the torsion angle and n is truncated at 4. The u1, u2, and u3 Fourier coefficients have significant physical meanings in force field models. The 1-fold term V1 arises from charge-charge or dipole-dipole interaction. The 2-fold term V2 arises from the conjugation of the p orbitals. The 3-fold term V3 is interpreted as a steric effect in molecular mechanics (the 3-fold torsion potential in ethane is an example) and is insignificant in ONOO-. In Figure 2, there are only two stationary points at ω(ONOO) ) 0°and 180°, and a transition state is located at ω(ONOO) ) 78°, where ω is the dihedral angle. A dominant V2 term arises from the N-O partial doublebond character since π bonds have a periodicity of 360°/2. In the Becke3-LYP/6-31+G(d) calculations, the central N-O bond length (1.320 Å) of cis-cis ONOOLi has multiple bond character. It is longer than the standard OdN double bond (1.21 Å) and is shorter than the standard N-O single bond (1.42 Å). In the Becke3-LYP/6-31+G(d) transition state, the central N-O bond length (1.503 Å) is actually longer than the standard N-O single bond, with the p orbitals in the N-O bond being orthogonal. There is also a significant contribution from V1; cis-cis ONOOLi is most stabilized by electrostatics because the approach of the Li and the terminal oxygen is closest. The torsion potential of ONOOH is not as deep around ω ) 0 because the electrostatic attraction is missing in ONOOH. The cis-cis, cis-perp, and trans-perp forms of ONOOH are all within 4 kcal/mol at high levels of theory.14,34 Figures 3 and 4 show the Becke3-LYP/6-31+G(d) NOOLi torsional potential curves in cis and trans ONOO configurations, decomposed into a Fourier series expansion. These curves are quite different from ONOOH,34 which has low barriers (
