J. Phys. Chem. 1985, 89, 1227-1232 the large difference in ET is expected to enhance (km)DApcompared with (km)p,,r,If the energy gap law found for a series of aromatic hydrocarbon2' is also applicable to the present system, we obtain [Fpo(ET)]DAP/[FPO(ET)]pyr 100. In this case, we obtain kgAp 3 X lo4 s-l. This value is close to the lower limit of k"' estimated above. Therefore, the direct mechanism can predict a large value for k"'. However, if the decay rates of the TREPR signals are determined by k"', an unusually large [ F p o ( & ) ] ~ ~ p is still needed to explain the large k"'. The present result shows that the T1state of DAP in biphenyl host is considerably separated from the Tz(7r7r*) state and is of nearly pure 3n7r3 character. Though it is possible that the proximity effect can enhance the decay rate in a system with a relatively large at present it is not certain whether this effect is significant in enhancing the k"' of 'nn* DAP.
-
5. Summary and Concluding Remarks We have clarified the nature of the TI state of DAP in a variety of environments by the time-resolved EPR technique with laser excitation. It is unambiguously demonstrated that the TI state (27) W. Siebrand, J. Chem. Phys., 44, 4055 (1966). (28) W. A. Wassam and E. C. Lim, J. Chem. Phys., 68, 433 (1978).
1227
of DAP is 3n7r* in character except in a highly polar solvent such as TFE. The magnitude of the vibronic interaction between %7r* and %7r* is large, producing temperature and solvent dependences of the zfs of DAP. It is also shown that the spin distribution of jn,* DAP is similar to that of pyridazine. It is suggested that the mechanism based on the direct SOC can provide a fast radiationless decay. The decay rate constants of the transient EPR signals of DAP in biphenyl were found to be on the order of lo6 s-l at 4.2 K. Further investigations of the dynamic behaviors of the transient EPR signals as well as direct measurements of the triplet decay rate constants are needed in order to fully understand the dynamic properties of the TI state of DAP. Work in this direction is currently in progress. Acknowledgment. We are indebted to Mr. Fuji of JEOL Co. who provides us with a preamplifier with a fast time resolution. This research was supported by a Grants-In-Aid from Mitubishi Foundation. The work was also supported by a Grant-In-Aid for Scientific Research (No. 58430003) from the Ministry of Education, Science and Culture. Registry No. DAP, 230-17-1; biphenyl, 92-52-4; fluorene, 86-73-7; dibenzofuran, 132-64-9.
Theoretical Study of the Ground-State Geometry and Excited-State Spectrum of H02N02 Roberta P. Saxon* Molecular Physics Department, SRI International, Menlo Park, California 94025
and B. Liu IBM Research Laboratory, San Jose, California 951 93 (Received: September 4, 1984; In Final Form: November 30, 1984)
SCF and perturbation theory calculations indicate the ground-state equilibrium geometry of H02N02has no symmetry and the plane of the HOz group is roughly perpendicular to the plane of the NO2 group. CI calculations have been performed for the first five states at the ground-state geometry. These states may be characterized as single excitations on either the NO2 fragment or the HOz fragment. Comparison with the observed UV photolysis is presented.
Introduction Significant effort has been devoted to the study of the formation, thermal decomposition, and photolysis of pernitric acid, HO2NO2, in the past eight years, following a suggestion by Simonaitis and Heicklenl that a long-lived complex between H 0 2 and NO2could have an effect on atmospheric chemistry. The compound itself was identified soon thereafter in the gas phase by Niki et aL2 by Fourier transform infrared spectroscopy. HOz and NOz both play a crucial role in the chemistry of the upper and lower atmosphere. Thus,reaction 1 and its reverse are potentially of great importance in the HO, and NO, cycles. HO2 + NO2 M F! HOzN02 + M (1, -1) Available kinetic data include measurements of the rate of the forward reaction by Sander and Peterson3 and by Howard4 and a measurement of the decomposition reaction at 1 atm by Graham et aL5 Baldwin and Golden6 have applied RRKM theory to
reaction -1 to enable specific measured rates to be extended to the range of temperatures and pressures required by kinetic models of atmospheric chemistry. Parameters which are needed for application of this theory include the frequencies of the HO2NO2 molecule at equilibrium, some of which were based on observations and the remainder of which were estimated from analogous molecules. There have been a number7-" of studies of the ultraviolet absorption spectrum of HO2NOZover some portion of the range between 190 and 330 nm. While the qualitative shape of the spectrum in all studies is similar, a decreasing function of increasing wavelength, there is some discrepancy in the magnitude of the cross section. N o data on photolysis products or quantum yields are available. To our knowledge there is no theoretical work on pernitric acid in the literature and when this work was undertaken, the equilibrium geometry had not been determined theoretically or experimentally. Furthermore, the nature of the electronically excited
(1) R. Simonaitis and J. Heicklen, J. Phys. Chem., 80, 1 (1976). (2) H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, Chem. Phys. Lett., 45, 564 (1977). (3) S.P. Sander and M. E. Peterson, J. Phys. Chem., 88, 1566 (1984). (4) C. J. Howard, J. Chem. Phys., 67, 5258 (1977). (5) R. A. Graham, A. M. Winer, and J. N. Pitts, Chem. Phys. Lett., 51,
(7) J. P. Jesson, L. P. Glasgow, D. L. Filkin, and C. Miller, Geophys. Res. Lerr., 4, 513 (1977). ( 8 ) R. A. Cox and K.Patrick, Int. J . Chem. Kiner., 11, 635 (1979). (9) 0. Morel, R. Simonaitis, and J. Heicklen, Chem. Phys. Lett., 73, 38
+
215 (1977). (6) A. C. Baldwin and D. M. Golden, J. Phys. Chem., 82, 644 (1978).
(1980). (10) R. A. Graham, A. M. Winer, and J. N. Pitts, Geophys. Res. Lett., 5, 909 (1978). (11) L. T. Molina and M. J. Molina, J. Photochem., 15, 97 (1978).
0022-365418512089-1227$01.50/00 1985 American Chemical Society
Saxon and Liu
1228 The Journal of Physical Chemistry, Vol. 89, No. 7 , 1985
. -
TABLE I: HO,NO, H 0 2 N 0 2 Geometrv' Geometry' SCF 6-31G N-0,
1.199
N-0, LO,-N-03
1.206 130.0 1.426
SCF 6-31G**
MP2 6-31G 1.238
1.173
1.232 136.9 1.501
1.175 130.5 1.366
0.989 102.1 1.741
0.950 104.5 1.370
(1. 190)b
05-04
N-O4
0.961 104.8 1.418
dihedral angleh LO,-N-O, LO,-N-O, LN-O,-O, dihedral angle'
102.9 1 1 2.9 117.5 110.3 23.9
H-OS LH-OS-O~
(136.6)b (1,382)' (0.960)c (106.9)'
exP NO2: 1.193"
(1 .165)b ( 136.0)b
(1.309)c (0.950)c (105.8)c
HNOB: 1.199, 1.211' NO2: 134" H02: 1.335' H202: 1.475' HO2: 0.997' HO2: 104' HNO,: 1.406'f N 2 0 4 : 1.7828 H202: 111.5'
'Distances in angstroms, angles in degrees. bUHF optimized geometry for NO2 with same basis set. CUHFoptimized geometry for HO2 with same basis set. "Reference 20. 'Reference 21. f H 0 - N 0 2 bond distance, ref 22. 8N-N bond distance. *Angle between H 0 2 and NO2 plane. 'Reference 19. jAngle between planes 02-N-03 and N-04-OS.
state(s) participating in the UV absorption is completely unknown. In this theoretical study, therefore, we have calculated the H02N02geometry and vibrational frequencies, which may be compared with those used by Baldwin and Golden in the RRKM treatment. We have also characterized the four lowest excited states in the vertical exciation spectrum by a configuration interaction treatment. Excitation energies for the first two of these states have been obtained by more detailed calculations. The calculations are briefly described and results and discussion presented in the following sections.
Calculations The equilibrium geometry of the ground state, which has a closed-shell singlet electronic structure, was determined by analytical gradient techniques using the GAUSSIAN82 prOgri3"2 Optimizations were carried out at the S C F level for the 6-31GI3 (split-valence) and 6-31G**14(split-valence plus polarization) basis sets and with second-order Moiler-Plesset perturbation theory (denoted MP2) for the 6-31G basis set. Analytic force constant calculations were performed a t the S C F level for both basis sets. C I calculations designed to characterize the vertical excitation spectrum were carried out with a double { (DZ) basis set,15with a double f basis augmented with 3s and 3p Rydberg functions16 on the heavy centers (DZR), and with a double {plus polarization16 (DZP) basis set. All calculations were done a t the SCF/6-3 1G optimized geometry, which has no symmetry, using SCF orbitals. For the DZ and DZR calculations, the configuration list was generated, keeping the nine lowest energy SCF orbitals doubly occupied, by distributing 22 electrons in 11 valence orbitals, i.e., the S C F configuration, and by distributing 21 electrons in the valence orbitals and one electron in the remaining external orbitals. This small calculation, termed a singles CI, totals 353 CSF's for the D Z basis and 573 for the DZR basis. The singles CI with the DZP basis was constructed analogously, keeping 14 orbitals doubly occupied, which leaves six orbitals in the valence space, resulting in 361 configurations. Examination of the D Z and DZR singles C I results indicated that no configuration with orbitals 11 though 14 singly occupied contributed to the excited states of interest here. A singles and doubles CI(SDC1) calculation was designed to accurately determine the excitation energy of the first two excited states, using the DZP basis set. As explained below, both of these states may be characterized loosely as n r* excitations on the
-
(12) J. S. Binkley, M. J. Frisch, D. J. DeFrees, K. Raghavachari, R. A. Whiteside, H.B. Schlegel, E. M. Fluder, and J. A. Pople, GAUSSIAN 82, Carnegie-Mellon University, Pittsburgh, PA. (13) W. J. Hehre, R. Ditchfield, and J. A. Pople, J . Chem. Phys.. 56, 2257 (197 1). (14) P. C. Hariharan and J. A. Pople, Theor. Chim. Acfu, 328, 213 (1973). (15) D. McLean and G. Chandler, unpublished. (16) T. H. Dunning and P. J. Hay in "Methods of Electronic Structure Theory", H. F. Schaefer, Ed., Plenum, New York, 1977, p 1.
W
Figure 1. Schmatic representation of H 0 2 N 0 2ground-state geometry. Atom O4is nearly coplanar with 02-N-03, atom Osis below the plane, and H is above the plane in this drawing.
NO2 fragment, Le., both states have the same excited r* orbital, but the singly occupied nonbonding orbital in each case is a different linear combination of the S C F orbitals. For SDCI calculations, the S C F orbitals were transformed by the corresponding orbital procedure1' to enable a compact configuration list to be constructed. In this procedure, the two linear combinations of the six valence S C F orbitals (orbitals 15 through 20) were constructed which most nearly resembled the nonbonding orbital from the second and third state natural orbitals of the singles CI. In this orbital set, the ground state and the first two excited states may each be described by a single configuration: 19a2 20a2 19a2 20a 21a 19a 20a2 21a where orbitals 1 through 18 are doubly occupied. Correlating four electrons, and excluding the five heavy-particle core-complement orbitals, single and double excitations with respect to these three reference configurations totaled 14 461 configurations. Orbitals 15-18, which are kept doubly occupied in the CI are not significantly affected by the corresponding orbital transformation. All C I calculations used the ALCHEMYII symbolic matrix direct C I program.'* Results and Discussion Ground-State Geometry. The ground-state equilibrium geometries for H02N02determined by SCF calculations with 6-31G (17) A. T. Amos and G.G. Hall, Proc. R. SOC.London, Ser. A , 263,483 ( 1961).
(18) B. Liu and M. Yoshimine, J. Chem. Phys., 74, 612 (1981).
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Theoretical Study of HOzNOZ TABLE 11: H02N02 Ground-State Extrema SCF/6-3 1G
TABLE 111 Frequencies (em-') SCF/6-31G**
energy above equilibrium (no symmetry) plana? cis trans two perpendicular planes" cis trans a See
1229
total equil, total energy, au kcaI/mol energy, au -353.994 077 -354.199 650
energy above equil, kcal/mol
-353.990022 -353.990 112
2.5 2.5
-354.196615 -354.193 907
1.9 3.6
-353.970096 -353.984629
15.0 5.9
-354.171 009 -354.181 655
18.0 11.3
SCF/ 6-31G
SCF/ 6-31G**
117 285 39 1 592 727 810 922 1050 1405 1540 1705 3910
145 302 436 652 821 896 1046 1215 1576 1615 1960 4100
description in text.
and 6-31G** basis sets and by MP2/6-31G calculations are all in qualitative agreement. The SCF geometries were verified to be true minima; all calculated frequencies were real. As illustrated in Figure 1, the molecule has no symmetry and the plane of the HOz group is roughly perpendicular to the plane of the NOz group (dihedral angle of 103O). This result may be explained qualitatively by noting that the singly occupied orbit 1 in ground-state HOz is out of plane. Thus a perpendicular approach by the NOz group would be energetically preferred. For HzOzthe comparable dihedral angle is 11 1.50.19 Optimized bond lengths and bond angles are compared in detail in Table I, where analogous results for HOz and NOz are noted as well as some experimental values. The 0 - H bond length and H - 0 4 bond angle are very similar to those calculated for the isolated hydroperoxyl radical, while the 00bond length is somewhat longer. The N-OZbond lengths on the NOz group become slightly assymmetric and slightly longer than in NOz while the 0 - N - O bond angle is -6O smaller. Comparing the 6-31G and 6-31G** results, we found that addition of polarization functions leads to shorter bond lengths, by an average of 3%, but has relatively little effect on bond angles. Comparing the MP2 and SCF calculations, including correlation, leads to longer bond lengths, by an average of -3% excluding the N-O, bond, which is lengthened by -22%. The dramatic effect of correlation on the determination of the N-N bond length in N z 0 4 has been analyzed by Bauschlicher et al.zl The comparison between our calculated bond lengths for the isolated radicals and experimental results is consistent with the observations of DeFrees et al?3 who noted that for bonds between non-hydrogen species SCF/6-31G* lengths are all too short. (The 6-31G** basis set differs from the 6-31G* set only by addition of a polarization function on H and thus should provide very similar results.) They also noted that, when electron correlation is included at the MP2/6-31G* level, all bond lengths increase, as we have noted above for the 6-31G basis set, and become mostly longer than experiment. Thus, our MP2/6-31(3 bond lengths are almost certainly too long. Therefore, the calculations of the vertical excitation spectrum have all been performed at the SCF/6-31G optimized geometry. We may note that from this calculation the N-04 bond length (1.41 8 8,)is similar to the experimental H 0 NOz bond length of 1.406 8,in H N 0 3 . We may also note that a microwave spectrum of HOzNOZhas recently been obtained by Suenram and L o v a ~ .They ~ ~ report that preliminary fits to the data with internal bond lengths and bond angles for the NOz and HOz fragments assumed from analogous species give (1 9) R. L. Redington, W. B. Olson, and P. C . Cross, J . Chem. Phys., 36, 1311 (1962). (20) G. Herzberg, 'Molecular Spectra and Molecular Structure", Vol. 111, Van Nostrand, New York, 1966. (21) M. D. Harmony, V. W. Laurie, R. L. Kuezkowski, R. H. Schwendeman, D. A. Ramaay, F. J. Lovas, W. J. Lafferty, and A. G. Maki, J . Phys. Chem. Ref.Data, 8, 619 (1979). (22) C. W. Bauschlicher, A. Komomicki, and B. Roos, J. Am. Chem. Soc., 105, 745 (1983). (23) D. J. DeFrees, K. Raghavachari, H. B. Schlegel, and J. A. Pople, J . Am. Chem. Soc., 104, 5576 (1982). (24) R. D. Suenram and F. J. Lovas, private communication.
description from calcn H02N02 . . OH bend
NOz out of plane NO3 in-plane 0-0 stretch NOz (H) OH bend
NOz in plane OH stretch
ref 6 125 200 400 500 633 735 803" 880 1304" 1396" 1728" 3540"
&NOz torsion HO-0 torsion OOH bend NO2 rock NO stretch NO2 wag
NO3
0-0 stretch NO3 OH bend NO3
OH stretch
Experiment NO2
774 1451 1591
833 1612 1880
bend sym stretch asym stretch
897 1521 3938
1253 1602 4078
0-0 stretch bend 0-H stretch
750b 1320b 1618b
HOZ 1098c 1392' 3436c
"Observed experimentally (ref 2). 'Reference 20. CReference25. agreement with the data for N - 0 4 bond lengths between 1.50 and 1.53 8,for a range of assumed parameters. The SCF/6-31G bond length is much closer to these values than the MP2/6-31G result. The dependence of the vertical excitation spectrum on the N - 0 4 bond length will be discussed below. We have also optimized the ground-state geometry within certain symmetry restrictions at the SCF level for both basis sets. The resulting energies are compared in Table 11. If the molecule is confined to a plane, one extremum of the potential energy surface is obtained for the H cis to the 05-04 bond and one for the H trans to that bond (using the numbering system of Figure 1). In both cases, the 05-04-N angle is 1l o o , as it is in the equilibrium geometry, and the H-05-04 angle is similar to that in free HOZ. The cis planar geometry has one imaginary frequency, corresponding to out-of-plane motion of the HOz group. The energy, however, is not very sensitive to these conformational changes; the planar geometries lie only 2-4 kcal/mol (depending on basis set) above the equilibrium geometry. In the other restricted symmetry, the HOz plane was required to be perpendicular to the NOz plane and to bisect the Oz-N-03 angle. Again, two extrema were found corresponding to the H cis and trans to the 05-04 bond. For the cis configuration, two imaginary frequencies were obtained. These geometries are considerably higher in energy, as shown in Table 11, with the cis geometry higher in energy than the trans for both basis sets. Frequencies. Frequencies calculated at the SCF level with the 6-31G and 6-316** basis sets at the optimized geometries for HOZNOzand the triatomic radicals separately are listed in Table 111, where they are compared with the values used by Baldwin and Golden6 for the RRKM treatment of pernitric acid and with experimental determinations for the radicals. A description of the motion from the calculation is provided for those frequencies for which a simple interpretation is possible. The qualitative description of the frequencies is in excellent agreement with that of ref 6. For the 6-31G frequencies of the radicals, there are discrepancies in both directions, while the 6-3 1G** radical frequencies are all too large by 11-22%. This is consistent with the observed tendency for SCF/6-31G* frequencies to be too large.z6 Similarly, the 6-3 1 G** calculated frequencies are larger than the five known HOzNOZfrequencies by 13-30%. A similar comparison holds for the unobserved frequencies of HOzNOZ,provided
-
(25) M. Jacox, compilation, to be published. (26) J. A. Pople, R. Krishnan, H. B. Schlegel, and J. S. Binkley, Int. J. Quantum Chem. Symp., 13, 225 (1979).
1230 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Saxon and Liu
TABLE I V Vertical Excitation Spectrum (eV) H02N02 state 2 3 4 5
DZ 5.168 5.971 7.169 7.848
8
singles CI DZR 5.170 5.958 7.155 7.719
DZP 5.823 6.703 7.739 8.593
SDCI DZP 5.610 5.483
9.768
DZ/SDCI. *Minimum basis/SDCI. tally at'4.97 eV (ref 30). (I
DZP/MCSCF.
---
--
character NO2 n ?r* NO2 n T* H02n pu* NO2 nI ?r*
NO, state 2A24b2 2b, 2B1 6al 2bld
ref 27' 5.04 2.79
BZB2*la2
5.51
NO2 Rydberg
6al
-+
3s, 3p
2b,
1.7
-
calcn ref 2Sb 4.82 2.45
ref 29E 2.8
5.18 8.8
6a, is N-O antibonding, not directly comparable to H02N02. Cobservedexperimen-
by ref 6, as well, the largest discreppcy being found for the G O stretch. Thus the calculated results support the estimated frequencies used in the RRKM treatment. While these calculations were not designed to obtain an accurate bond dissociation energy, we have obtained an estimate of the binding with respect to HOZand NO, from the MP2 energy of the S C F optimized geometry of pernitric acid, HOz and NO,. With the 6-31G basis, the dissociation energy is 12.9 kcal/mol while the 6-31G** calculations predict a dissociation energy of 29.8 kcal/mol. These values bracket the 23 kcal/mol deduced by Baldwin and Goldwin from kinetic data. Vertical Excitation Spectrum. Vertical excitation energies for the first four excited states of HOzNOZhave been obtained with the DZ, DZR, and DZP basis sets by singles C I calculations. Results are compared in Table IV, where the excitation energies for the first two states from the DZP/SDCI calculation are also listed. Inclusion of correlation lowers the excitation energy by approximately 0.2 eV to 5.610 and 6.483 eV for the second and third states. The nature of the excited states has been characterized by examining the natural orbitals. Calculations with all three basis sets led to the same description. The second, third, and fifth states may be described as excitations on the NO, fragment and the fourth state as an excitation on the HOz fragment. Contour plots of the singly occupied natural orbital 20 of the second and third states are shown in Figure 2. Each of these orbitals consists primarily of nonbonding p orbitals on 0,and O3atoms in the plane of the NOz and perpendicular to the N - 0 bonds. For the second state, the linear combination is such that the nodal plane bisects the Oz-N-03 angle and, for the third state, the nodal plane is perpendicular to the bonds. In the fifth state, the singly occupied natural orbital 20 is composed of nonbonding out-of-plane p orbitals on Oz and O3atoms. For all three states the singly occupied natural orbital 21 is a r* orbital on NOz. Thus the second, third, and fifth states may be charr* excitations on NOz. For the fourth state, acterized as n as shown in Figure 3, orbital 20 is a nonbonding p orbital on the O5position perpendicular to the HO, plane and orbital 21 is an antibonding pa orbital on the 05-O4axis. Calculations with the DZR basis set predicted the lowest Rydberg state of H 0 2 N O zto be the eighth state with an energy of 9.768 eV, which may be compared with the S C F ionization potential of 11.5 14 eV. The Rydberg orbital is centered on the NOz portion of the molecule. A limited study of the dependence of the qualitative description of the vertical excitation spectrum on geometry was performed with the double [ basis set. The order of the NO2 and HOz fragment excitations was found to depend most sensitively on the N-O4 bond distance, Le., varying the NOz and HO, fragment internal geometry and the dihedral angle between fragments with the N-04 bond length fixed did not change the order. For N-O, distances between 1.41 and 1.62 A, the same qualitative description was obtained for the first four states. The fifth and sixth states, which are very close in energy, were interchanged in some geometries and the separation in energy between the lowest HOz excitation and the first two NOz excitations became smaller with increasing N-O4 distance. The differences in excitation energy were on the same order as the geometry differences between different basis sets tabulated in Table IV. At 1.74 A, the lowest H 0 2 excitation became lower in energy than the two lowest NOz n r* transitions.
\
,
-
-
Figure 2. Electron density contour plots of the excited-state natural orbitals drawn in the NO2 plane: (a) state 2 singly occupied orbital 20; (b) state 3 singly occupied orbital 20.
Vertical excitation energies for the roughly comparable states of NOz are also listed in Table IV. NO, is, of course, an oddelectron system and the comparison is only approximate. The second state of HOzNOz corresponds quite well in energy to the zAz state of NOz.279z8 The comparison with the ,B1state may not be very meaningful since the nonbonding orbital of HO2NO2, while of approximately a] symmetry on the NOz group, contains no N character, unlike the radical orbital. For the fifth state, the pernitric acid orbital is of the same character as for the radical, but the excitation energy is greater by several volts. No comparable comparison is possible for the fourth state since the vertical (27) S.-K. Shih, S. D. Peyerimhoff, and R. J. Buenker, Chem. Phys. Lett., 46, 201 (1977). (28) P. J. Hay, J . Chem. Phys., 58, 4706 (1973).
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1231
Theoretical Study of H 0 2 N 0 2
HO,NO,
Dissociation Products
8.0 NO, n -* n"
7.0 - HO, n 6.0
'..
-+
po"
NO, n -+ n"
5.0 - NO, n + n * I
-h> 4.0 0
HO + NO, t 0
w
HNO t 0,
a b$ 3.0
HO + NO + 0,
2.0 HNO, + 0-
1 .o
HO, + NO,
(a)
0
HO + NO,
Ground State
HNO, + 0, -1 .o Figure 4. Vertical excitation energies of HOzNOzand the dissociation products, drawn by assuming the ground state is bound by 23 kcal/mol
with respect to HOz + NO2.
(6.2 eV) is also obtained. Although there is some uncertainty due to the dependence of the vertical excitation spectrum on the ground-state geometry, our calculated results suggest these features may be identified with the first two states of H 0 2 N 0 2 ,which are n r* transitions on NO2. In contrast, Morel et aL9 attribute the known UV peaks of nitro compounds33at 280 and
