J . Phys. Chem. 1990,94, 7414-7418
7414
blue-shifted in the N, matrix and in the crystal. It seems that the spectral shifts discussed above support the proposed assignment of the mentioned bands. 4. Conclusions The main conclusions of this work are as follows: (1) Theoretical calculations performed at the SCF(DZP) MBPT(2) level predict the thiol (pyrSH) form of 2( 1ti)-pyridinethione to be 27.3 kJ/mol more stable than the thione (pyrS) form, while the experimental estimation of the stabilization energy is 9.9 0.5 kJ/mol. (2) Experimental geometry of pyridinethiolato ligands resembles the calculated geometry of the thiol (pyrSH) tautomeric form. (3) In inert gas low-temperature matrices the pyrSH form predominates, but a small percent (3%) of molecules exist in the thione pyrS tautomeric form. (4) UV-vis irradiation of the 2(114)-pyridinethione sample isolated in the matrix leads to the conversion of the thione tautomeric form into the thiol tautomer pyrSH. (5) The comparison of calculated and experimental matrix IR spectra permits an assignment of the absorption bands in the spectrum of pyrSH and pyrS tautomeric forms.
+
*
5. Discussion The discrepancy between the theoretical and experimental estimations of the energy difference between the pyrSH and pyrS tautomeric forms is not fully understood at this point. There are several approximations in the theoretical approach corresponding to the level of theory used to evaluate the electronic energy, the incompleteness of the basis set, and the approximate treatment of the energy of the internal nuclear motion, etc. For example, the higher order electron correlation effects may contribute to the stabilization effect of the pyrS form by a few kJ/mol. A calculation of these effects for the tautomeric pair of 2-pyridonel2-
hyroxypyridine resulted in a 3 kJ/mol contribution in favor of the oxo form.27 It seems, however, unlikely that with all approximations removed the theoretical result will equal the experimental value. A possible explanation of this discrepancy may emerge from reexamination of the experimental data. A point, which one may raise, is whether the relative concentration of tautomers in the matrix deposit really correspond to the thermodynamical equilibrium in the gas phase. If one examines the matrix deposition process, one sees that the sublimated sample remains for a short period of time in the chamber before it is deposited. The assumption is that the time is sufficient for the transformation of some molecules from the pyrS form to the pyrSH form so their concentrations reflect the thermodynamical equilibrium. If the time is not long enough, then the pyrS form will be more abundant than is theoretically predicted. There is another fact that seems to support this hypothesis. When the matrix-deposited sample is exposed to the UV-vis radiation, the relative concentration of the pyrS form significantly decreases. We think that this can happen because one of the bonding electrons in the N H bond is promoted to an antibonding orbital. As a result of this, the hydrogen atom becomes much more loosely bound and can easily migrate toward the sulfur atom, to whom it forms a stronger bond. At present, we are designing experiments to verify the above hypothesis. Acknowledgment. This study was supported by an institutional grant from the National Cancer Institute and by a Biomedical Research Support grant provided by The University of Arizona. A.L. has been partly supported by the Polish Academy of Sciences within the project CPBP 01.12. The experimental part of this work was financed by the grants C.P.B.P. 0 I . 12 and C.P.B.R. 1 1.5 provided by the Polish Academy of Sciences.
Ab Initlo Self-Consistent Field Study of the Vibrational Spectra for NO3 Geometric Isomers Vernon R. Morris,+*$Subhash C. Bhatia,*.tg*and John H. Hall, Jret** Dolphus E . Milligan Science Research Institute, Atlanta University Center, Inc., 440 Westview Drive, SW , Atlanta, Georgia 3031 0,School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Chemistry, Spelman College, Atlanta, Georgia 303 I4 (Received: February 6,1990)
Ab initio self-consistentfield results at the UHF/6-31G* and UHF-DZP levels for harmonic vibrational frequenciesof symmetric NO3 with C, C,, and D3h symmetry; cis and trans forms of OONO are reported. At both levels of calculations (6-31G* and DZP), the theoretical vibrational frequencies for C, sym-NOSare in agreement with the recent experimental results. Our calculations for cis and trans isomers of OONO show large deviations from the only observed vibrational frequency (1838 cm-I) for trans OONO. The trans isomer of OONO is predicted to be more stable than the cis isomer by approximately 2 kcal/mol and in each case, the ground state for OONO is predicted to be 2A”.
Introduction
The nitrate free radical (symmetrical NO,) is an intermediate in reactions involving nitrogen oxides, oxygen, and ozone. Symmetric nitrate radical ( s y m - N 0 3 )has been observed in the lower troposphere under night-time conditions.’ However, the specific role of this molecule in atmospheric chemistry is not well understood.l” Another isomer, asymmetric NO3 or peroxynitrate, was identified as a structural isomer of sym-N03in 1947 and was also observed in the isotopic exchange (I8O)reaction of enriched N205.798 Peroxynitrate (OONO) has also been proposed as an ‘Atlanta University Center, Inc. ‘Georgia Institute of Technology. ispelman College.
intermediate in thermal oxidation of nitrogen oxide”l and in the reaction of nitrogen dioxide with ozone.i2 The electronic and (1) Russell, A. G.; Cass, G. R.; Seinfeld, J. H. Enuiron. Sci. Technol. 1986, 20. 1167. (2) Dlugokencky, E. J.; Howard, C. J. J . Phys. Chem. 1989, 93, 1091. (3) WMO Global Ozone Research and Monitoring Project Report 1985, l6(11), 600. (4) Wassell, P. T.; Wayne, R. P.; Ballard, J.; Johnston, W. B. J . Amos. Chem. 1989, 63. ( 5 ) Cantrell, C. A.; Stockwell, W. R.; Anderson, L. G.; Busarow, K. L.; Perner, D.; Schmeltekopf, A.; Calvert, J.; Johnston, H. S. J . Phys. Chem. 1985, 89, 139. (6) Graham, R. A.; Johnston, H. S. J . Phys. Chem. 1978, 82, 254. (7) Ogg, R. A., Jr. J. Chem. Phys. 1947, 15, 613. (8) Ogg, R. A., Jr. J. Chem. Phys. 1953, 21, 2071.
0022-3654/90/2094-7414%02.50/0 0 1990 American Chemical Society
Vibrational Spectra for N O , Geometric Isomers molecular structures of sym-NO, and OONO are not well characterized. Several experimental and theoretical studies have supported both the C, and Djh symmetries for the equilibrium geometries of ~ y m - N O ~ . l ~ - ~ ~ The laser-induced fluorescence spectrum of sym-NO, by Nelson et al.13 and Ishiwata et aLi4showed the presence of 10 vibrational absorptions. Ishiwata et assumed the D3hsymmetry for NO3 to explain the vibrational bands, while Nelson et al." explained their observed spectra by assuming C? symmetry. The analysis of high resolution Fourier transform infrared spectra by Friedl and SanderI5 also suggested the D3r symmetry for sym-N03.,In addition, they suggested the possible existence of a low lying excited state (- 100 cm-I above ground state). Theoretical studies'622 have predicted both C, and D3h symmetries for sym-NO, molecules, but complete geometry optimization was not performed in these investigations except in one case.18 A multiconfiguration (CASSCF) study by Siegbahn,20using a basis set, has predicted *B2symmetry for a C, ground state near D,h symmetry with three ( 3 ) equal bonds but distorted angles. In D3hsymmetry, the ground state is 2E" with two Jahn-Teller components of 2Azand 2Bl symmetry.20 More recently, Boehm and Lohr2' have reported HF/DZP and MP2/DZP results, indicating a 2A'2 ground state of D3h symmetry. The equilibrium bond length was calculated to be 1.222 A and bond angles of 121' and 118.5'. A recent laser-induced fluorescence spectral study by Kim et al." suggests that the equilibrium geometry of sym-NO, should be with C2, symmetry. Kim et a1.26 have also recently calculated vibrational frequencies for sym-NO, with complete geometry optimization. Guillory and Johnston observed OONO in their infrared study of the low pressure gas-phase reaction of NO and 02.9J0 Also, they l o proposed the geometric parameters for OONO and performed normal coordinate analysis to calculate vibrational frequencies. More recently, Bhatia and Hall" have assigned the absorption at 1837.5 cm-' to the N=O stretch of OONO, but there is uncertainty in the assignment, and only one mode of vibration has been observed in the infrared spectra. The only theoretical study for OONO is by Boehm and Lohr2' where they only report the optimized geometries and electronic energies. This study reports the optimized geometries and harmonic vibrational frequencies for sym-NO, (D3h,C, C,) and cis and trans isomers of OONO. The theoretical vibrational frequencies may assist in clarifying the equilibrium geometry of sym-NO,, and will assist in the identification of OONO by spectroscopic methods. Computational Procedure
The a b initio unrestricted Hartree-Fock calculations were performed by using the GAUSSIAN 86 Revision C Program.27 The basis sets28*29employed were the 6-31GS and the Dunning po(9) Guillory, w. A.; Johnston, H. S. J. Am. Chem. Soc. 1963,85, 1695. (IO) Guillory, W. A.; Johnston, H. S. J . Chem. Phys. 1965, 42, 2457. (11) Bhatia, S.C.; Hall, J. H., Jr. J . Phys. Chem. 1980, 84, 3255. (12) Morris, V. R.; Bhatia, S. C.; Hall, J. H., Jr. J. Phys. Chem. 1987, 91, 3359. (13) Nelson, H. H.; Pastemack, L.; McDonald, J. R. J . Phys. Chem. 1983, 79. 4279. (14) Ishiwata, T.; Tanaka, I.; Kawaguchi, K.; Hirota, E. J . Chem. Phys. 1985,82, 2196. (IS) Friedl, R. R.; Sander, S. P. J . Phys. Chem. 1987, 91, 2721. (16) Olsen, J. F.;Burnelle, L. J . Am. Chem. SOC.1970, 92, 3659. (17) Walker, T. E. H.; Horseley, J. A. Mol. Phys. 1971, 21, 939. (18) Lund, A.; Thuomas, K. Chem. Phys. Lett. 1976, 44, 569. (19) Kim, B.; Johnston, H. S.; Clabo, D. A., Jr.; Schaefer, H. F., 111. J . Chem. Phys. 1988,88, 3204 (20) Siegbahn, P. E. J . Comput. Chem. 1985, 6, 182. (21) Boehm, R. C.; Lohr, L. L. J . Phys. Chem. 1989,93, 3430. (22) Davy, R. D.; Schaefer, H. F., 111. J . Chem. Phys. 1989, 91, 4410. (23) Wood,D. E.; Lozos, G.P. J. Chem. Phys. 1976, 64, 546. (24) Chantry, G. W.; Horsefield, A.; Morton, J. R.; Whiffen, D. H. Mol. Phys. 1962, 5, 589. (25) Kim, B.; Hunter, P.; Johnston, H. S., manuscript in preparation. (26) Kim, B.; Hammond, B.; Lester, W. A., Jr.; Johnston, H. S. Chem. Phys. Lett. 1990, 168, 131. (27) Frisch, M. Gaussian 86 User's Guide & Programmer's Reference Revision C Version; Carnegie-Mellon University, 1987.
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7415
TABLE I: SCF and Relative Energies for NO, Structures -EsCF (au) and relative energy (kcal/mol) structure
6-31G* 278.8 1447 (0.0)ll 278.80870 (3.6) 278.81066 (2.4) 278.74903 (41.1)
Cd2A')
Cd2Bdb I
_
C1"(2B2)C D3h(2A'2) C2A2A2)d
DZP 278.87210 (9.2) 278.86978 (10.7) 278.88679 (0.0) 278.8j599 (31.9) 278.86726 (1 2.3)
'Energy relative to C, NO, optimized geometry. short bond, two long bonds. long bond, two short bonds. dSingle-point calculation with optimized geometry of ref 21.
0
0 1.175
120.0° 1.255E
0
i
2
120.00
5
5
0 2B2
=A,
115.3" 1.203 e x
2
0
3 1.223
0
120.0
0 2A'
2K2 'D3h )
(C2")
(CJ
B Figure 1. Optimized geometries for sym-NO, at (A) 6-31G* and (B) DZP. Bond lengths in angstroms and angles in degrees.
larized full double-{ basis set [9s5pld/2s]/ [4s2pld/2s]. Standard d orbital exponents of 0.80 for nitrogen and 0.85 for oxygen were employed in both polarized basis sets. All geometry optimizations employed Schlegel's analytical gradient method^.^^^'^ Bond lengths and bond angles were optimized with precision better than 0.001 A and Oslo,respectively. Required convergence on the density matrix was 1 X for all energy calculations. The harmonic vibrational frequencies are determined from the analytically computed force constants at the equilibrium geometries determined at each level of approximation. Results and Discussion
Symmetrical Nitrate (NO,). The SCF energies and the relative stability for sym-NO, isomers are listed in Table I. All calculations were started with initial geometry of D3h symmetry and the wave functions converged to the proper ground state. If all parameters were allowed to vary independently, the resultant optimized geometry was of C,symmetry, but near C, symmetry. In subsequent calculations in which all bond lengths were equivalent, we observed similar behavior, a C,nuclear geometry with two bond angle values remaining within 0.06% of each other. Kim et al. have observed similar behavior with CASSCF DZP wave functions.26 Our HF/6-31G* results predict a 2A' ground (28) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J . Chem. Phys. 1972, 56,2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973,28,213. (29) (a) Dunning, T. H., Jr. J. Chem. Phys. 1970,53,2823. (b) Schaefer, H. F., 111. Ed. Applications of Electronic Structure Theory; Plenum Press: New York, 1977; Chapter 1. (30) Schlegel, H. B. J. Chem. Phys. 1984, 84, 4530.
7416 The Journal of Physical Chemistry, Vol. 94, No. 19, 1990
Morris et al.
TABLE 11: Vibrational Frequencies"and Relative Intensities for D3h-N03
mode sym N - 0 str (AI')
6-31G* 1062.1 (0.0)C 964.2 (0.0) 1686.2 (100.0) 672.8 (0.0)
u,
u2
out-of-plane
bend (A2") u3 deg N - 0 asym str (E,') u5 deg O N 0 def
(E,')
"Frequencies reported in cm-I. bAu = infrared intensities in KM/mol.
hub 2
1325.4 (0.0) 917.7 (2.0) 2177.3 ( 100.0) 772.7 (0.0)
202 I94 293
[uOb
(A,) -
NO, out-of-Dlane &nd (Bl) u5 asym N - 0 stretch u1
(Bl)
u6asym NOz bend (Bl)
158 685 393
6vC
(2B,)d
IO
1063.8 (13.3) 1503.8 (9.9) 715.3 (0.2) 852.8 (4.3) 1866.2 ( I 00.0) 546.8 (1 .O)
313 56 348 334 246
hd
Au
1057.4 (0.0) 949.3 (2.0) 1665.9 ( 1 00.0) 662.8 (0.0)
3
1060
187
762
174
1492
283
380
TABLE IV: Vibrational Frequencies" and Relative Intensities for Csand Cb-N03 DZP Basis
c,
CS
(2A,)b 1060.1 (54.5)f 1185.0 (1 00.0) 692.9 (8.1) 713.6 (33.3) 1673.8 (84.6) 528.9 ( 1 6.3)
DZPE
265
- ualC]. CGeometryreported in ref 21 for 2Azstate in D3* symmetry. dReference 14. #Relative
TABLE III: Vibrational Frequencies" and Relative IR Intensities for C, and Ca,-NOawith 6316* Basis mode Y, sym N-0 stretch (AI) u1 sym N-0 stretch (AI) u1 NO, bend
observed frequencies
predicted frequencies DZP Au
c*
6u
14
exptlC I050
mode Y,
sym N-O
6
1498
stretch (A,) uZ sym N-0 stretch (A,)
34
749
u3 NO2 bend
487
366
y4
142
2008
228
775
state of C, nuclear geometry with three equivalent N - O bonds and three angles (1 15.54', 122.25', 122.22'), see Figure 1. This state (2A') is predicted to be only 2.4 kcal/mol more stable than the 2B2 state of C2, symmetry. The harmonic vibrational frequencies with 6-31G* and DZP basis sets for the 2A'2 ground state with D3hsymmetry are listed in Table 11. The experimentally observed frequencies, assuming D3h symmetry, and calculated vibrational frequencies exhibit poor agreement. A deviation of 120% is observed for the degenerate O N 0 deformation mode. The average absolute deviation in the theoretical and experimental frequencies at the 6-31G* level is 30%. Even worse agreement with the experimental value is observed for DZP basis set. The absolute average deviation is 58%. The disagreement between our calculated values and experimental values suggest that the symmetry for the sym-NO, molecule may not be D3h. Vibrational frequencies calculated with 6-3 1G* basis set for sym-NO, with C,and C2, symmetry are listed in Table 111. At this level of calculation, the best agreement between this study and experimental values is observed for the 2Bzstate with C2, symmetry. A decent agreement is also observed with the ZA' state in C, symmetry. The calculated frequencies at the UHF/DZP level for C, and C,,structures are closer to experimental values-especially the v3 and u5 modes (Table IV). The vI mode for each structure exhibits a larger deviation from the experimental value than observed with 6-31G* basis set (Table 111). The magnitude of the absolute average deviations are largely influenced by the deviation for the u4 and v6 modes. If these vibrational frequencies (775 and 366 cm-I) are excluded, the average deviation is in the 4-1 2% range. The calculated harmonic vibrational frequency by Kim et alaz6with ROHF/TZP and CASSCF/DZP for C2, are in good agreement with the results of this study. The predicted infrared intensities shown in Table IV suggest that the us asym-NO stretch should be the strongest absorption in infrared for the C , and D,,, structures. The v2 mode is predicted to be the most intense for the C, structure in accordance with the
Cb 6u
(lA',)< 6u exptlf 910.8 139 I050 (13.3) 1408.8 89 1498 (9.9) 687.7 61 749 (0.2) 366 881.3 515
out-of-plane bond (B,) (33.3)
"Frequencies reported in cm-I. bThree equal bonds, three unique short bond, two long bonds. angles. c 6 u = [yo& - UHF].
