4382
J. Phys. Chem. 1996, 100, 4382-4384
He I Photoelectron Spectroscopy Study on the Electronic Structure of Bromine Nitrate, BrONO2 Wang Dianxun* and Jiang Peng State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Academia Sinica, Beijing 100080, People’s Republic of China ReceiVed: August 4, 1995; In Final Form: December 11, 1995X
The gas phase He I photoelectron (PE) spectrum of BrONO2, an important atmospheric trace species, is reported for the first time. The assignment for the PE spectral bands of BrONO2 is made on the basis of band shape, fine vibrational structures, and ab initio calculation using the Gaussian 86 321G basis set level. From the expanded PE spectra between 10.50 and 12.00 eV and between 12.00 and 14.50 eV, it is seen that the spin-orbital splitting reflected on the HOMO 8a′′ (33) orbital is 0.31 eV and comes from ionization of lone pair electrons of bromine atoms in the molecule. The vibrational spacings 1050 ( 60 cm-1 and 1130 ( 60 cm-1 on the band centered near 12.50 eV are ascribed to ionization of electrons of both 22a′ and 21a′ orbitals which have the dominant contribution of an -NO2 atomic group in the molecule. The band centered near 13.12 eV with vibrational spacing 670 ( 60 cm-1 is designated to ionization of electrons of 6a′′ (28) orbital which has a dominant contribution of a -BrO atomic group and belongs to a weak Π bond in the molecule.
Introduction It is well-known that the chlorine atoms released in the stratosphere by solar UV photolysis of chlorofluorocarbon compounds can remove ozone by the ClOx catalytic chain (reactions 1 and 2).
Cl + O3 f ClO + O2
(1)
ClO + O f Cl + O2
(2)
Bromine compounds released at the earth’s surface have also been linked to ozone destruction in various regions of the earth’s atmospheresfor example, in the lower stratosphere,1-3 in the Antarctic during an ozone hole evene,4 and in the lower Arctic troposphere in Spring.5-8 In the stratospheric reactions of bromine atoms, the BrOx catalytic chain of 3 and 4 is also very effective in removing ozone.
Br + O3 f BrO + O2
(3)
BrO + O f Br + O2
(4)
Chlorine nitrate, ClONO2, which can be formed in the stratosphere by the reaction of ClO with NO2, can be an important temporary reservoir species of Cl,9 and it is also involved in the chemical reactions on the surface of polar stratospheric clouds (PSCs) for ozone depletion.10-12 Bromine nitrate, BrONO2, can also be formed by the analogous reaction 5.
BrO + NO2 + M f BrONO2 + M
(5)
As a part of our systematic works, He I photoelectron spectroscopic (PES) studies on the electronic structure of YONO2, where Y is F, Cl, Br, I, H, and CH3, should give further insight in the nature of chemical bonding in these compounds as well as their role in atmospheric chemistry. In the previous publications,13,14 we presented the PES studies of FONO2 and ClONO2. Here we would like to report the He I photoelectron * To whom all correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4382$12.00/0
spectroscopic (PES) study on the electronic structure of BrONO2 for the first time. The expanded PES associated with fine vibrational structures have also been recorded. To assign the PES bands, an ab initio Gaussian 86 SCF MO calculation at the 321G atomic basis set level is also performed for BrONO2. Experimental Section 1. Synthesis of Bromine Nitrate, BrONO2. The BrONO2 sample was synthesized by the reaction of Br2 with ClONO2.15 The preparation and PES spectrum of chlorine nitrate have been discussed earlier.14 Dry Br2 (10.01 mmol) and ClONO2 (30.54 mmol) were successively condensed into a prepassivated (with ClONO2) 75-mL stainless steel cylinder. The cylinder was then warmed to 25 °C and was kept at this temperature for 1.5 h with frequent agitation. The volatile material was separated by fractional condensation under an oil pump vacuum through a series of traps kept at -31 °C (bromobenzene slush), -45 °C (chlorobenzene slush), and -196 °C. The trap at -45 °C contained a yellow solid which melts at 245 K.15 It can decompose to Br2, N2O5, and O2.15,16 Its reaction with HCl and quantitative conversion to HONO2 can also characterize it as BrONO2.17 2. PES Measurement. The He I (21.22 eV) photoelectron spectrum of bromine nitrate BrONO2 has been measured on the double-chamber UPS machine-II which was built specifically to detect transient species.18 The operational resolution for the 2P + 3/2 peak of Argon (Ar ) is around 25 meV. Experimental ionization potentials (IPs in eV) are calibrated by simultaneous addition of a small amount of methyl iodide to the sample. Digitized spectra, time-averaged for periods of around 40 min, are also obtained. In order to assign the bands of the PES, ab initio Gaussian 86 SCF MO calculation at the 321G atomic basis set level is performed. The geometry of the molecule studied is taken from the result of ab initio SCF MO Gaussian 86 321G optimization. This is a planar molecule. Results and Discussion 1. General Features of the PES. The He I photoelectron spectrum of bromine nitrate BrONO2 is given in Figure 1. From this figure, it is seen that the separate bands associated with © 1996 American Chemical Society
He I PES Study of BrONO2
Figure 1. He I photoelectron spectrum of BrONO2.
Figure 2. Expanded He I photoelectron spectra with fine vibrational structure in different ionization potential (IP) regions for BrONO2.
fine vibrational structures and sharp peaks appear in the low ionization potential region (14.00 eV) there are very broad bands associated with low intensity. The expanded PE spectra which are scanned respectively from 10.50 to 12.00 eV and from 12.00 to 14.50 eV are shown in Figure 2. The fine vibrational structure is also seen from the expanded PE spectrum of BrONO2. 2. Assignment of Spectral Bands. It is well-known that
J. Phys. Chem., Vol. 100, No. 11, 1996 4383 the ionization of lone pair electrons or degenerate Π orbital electrons leads to a sharp peak and lower ionization potential in the PE spectrum of the molecule studied. The band associated with fine vibrational structure and longer vibrational progressions should be related to ionization of antibonding or bonding orbital electrons. The broad band in which there is no vibrational structure is usually attributed to the result of ionization of stronger antibonding or bonding orbital electrons.19 It is also known from the PES studies of alkyl bromides that the double-peaks with lowest ionization potential and about 0.31 eV interval were designated as the spin-orbital splitting peaks.19-21 The sharp double-peaks at 10.60 and 10.91 eV in the PE spectrum of BrONO2 should be considered as the spin-orbital splitting peaks which result from ionization of lone pair electrons of bromine atoms in the molecule studied because both the 0.31 eV interval of the double-peaks and the fine vibrational structures (400 ( 60 cm-1) on the double-peaks are the same as that of the first band with lowest ionization potential for alkyl bromides.19-21 The high intensity of secondary peak (10.91 eV) can be ascribed to its overlapping on the second band (11.25 eV). The bands centered near 11.25 and 11.78 eV in which there is no vibrational structure should be the result of ionization of stronger antibonding or bonding orbital electrons. The vibrational spacings (1050 ( 60 cm-1) and (1130 ( 60 cm-1) in the band centered near 12.50 eV are designated as the result of ionization of electrons of two different orbitals. The ionized orbitals that corresponded to the band centered near 12.50 eV are the bonding orbitals because the vibrational spacings (1050 ( 60 cm-1) and (1130 ( 60 cm-1) are much smaller than the vibrational spacings of 1285 cm-1 (symmetric NO stretch) and 1711 cm-1 (asymmetric NO stretch) in the neutral molecule,17 respectively. These orbitals should mainly reflect the character of -NO2 atomic group in the molecule. The band at 13.12 eV associated with vibrational spacing (670 ( 60 cm-1) and shorter vibrational progressions seems to be the result of ionization of electrons of the orbital which should be the weak bonding orbital because the adiabatic ionization potential (13.12 eV) which is given by this band is equivalent to vertical ionization potential (13.12 eV). The ionized orbital that corresponded to the band at 13.12 eV should mainly embody the character of -BrO atomic group in the molecule because this assignment coincides with the character embodying the vibrational spacing 690 cm-1 (BrO stretch) in the neutral molecule.22 The broad bands in the high ionization potential region (>14.00 eV) for the PE spectrum of BrONO2 should be related to ionization of electrons for bonding orbitals with different atomic and bonding characters. The above-mentioned assignment for the PE spectral bands of BrONO2 is supported by ab initio SCF MO calculation at the Gaussian 86 321G basis set level. The ab initio calculated ionization energies (-i) based on the Koopman’s approximation23 and the experimentally determined ionization potentials (IPs) are listed in Table 1. The molecular orbitals (MO) associated with each ionization band are designated according to their atomic and bonding characters. Experimental ionization potentials are given in Table 1 in the form of overlapping band maxima since the observed bands are associated with varying orbital ionization numbers. It is known that the vibrational spectra of gaseous, matrixisolated (N2) and solid BrONO2 were recorded and analyzed, which supported a planar structure of the molecule (Cs symmetry). Eight of the nine fundaments (seven of the species A′, two of the species A′′) and their approximate description were presented.24,25 The N-Br torsional vibration (υ9) could not be
4384 J. Phys. Chem., Vol. 100, No. 11, 1996
Dianxun and Peng
TABLE 1: He I Experimental Ionization Potentials and Calculated Eigenvalues (-Ei) Using the ab Initio SCF MO Gaussian 86 321G Basis Set Level for the BrONO2 Molecule I
-i (eV)
MO
I1
12.239
8a′′ (33)
nBrO
I2 I3 I4 I5 I6 I7 I8
12.523 13.104 14.229 14.544 16.567 16.671 20.652
23a′ (32) 7a′′ (31) 22a′ (30) 21a′ (29) 6a′′ (28) 20a′ (27) 19a′ (26)
σBrO ΘO2N σO2N σO2N ΘOBr σOBr,NO2 σO2N
character
IPs (eV)
}
} }
10.60 S.O. 10.91 11.25 11.78 12.50 13.12 13.66
identified in the spectrum,24 but an estimated value υ9 ) 90 cm-1 was also reported.26 According to our result of the ab initio SCF MO Gaussian 86 321G optimization, a planar structure is a stable geometry with Cs symmetry which is the same with other members of YONO2, where Y is F, Cl, I, H, and CH3. The HOMO 8a′′ (33) is a lone pair orbital of the bromine atom and belongs to Π character in the BrONO2 molecule. The two ionic states 2Π3/2 and 2Π1/2 which are the result of spin-orbital coupling in the photoionization process of lone pair electrons should result in two sharp peaks. This is just right, with the double-peaks of the PE spectrum with lowest ionization potentials at the 10.60 and 10.91 eV for the BrONO2 molecule. The secondary HOMO 23a′ (32) is a stronger antibonding orbital with the dominant contribution from the BrO atomic group in the molecule. The ionization of electrons of secondary HOMO 23a′ (32) should lead to the band centered near 11.25 eV on which there is no vibrational structure. The band centered near 11.78 eV is a sharper band with high intensity and could be designated as the result of ionization of electrons of the 7a′′ (31) orbital (the Π bond) with dominant contribution of two oxygen atoms of -NO2 atomic group in the molecule. Because the ionization of orbital electrons in the Π bond could lead to sharper band with high intensity.19 Both 22a′ and 21a′ orbitals belong to σ bonds, and the ionization of electrons of these orbitals result in the band centered near 12.50 eV. Because the approximate description of vibrational structures in this band reflects mainly the character of -NO2 atomic group in the molecule, i.e., symmetric NO stretch and asymmetric NO stretch.17 This result is the same with the result of ab initio calculation, because both 22a′ and 21a′ orbitals have the character of the -NO2 atomic group. The sudden increase of the intensity from the first to the second observed member on the band with vibrational spacing 1050 ( 60 cm-1 could be ascribed to overlapping with the band with vibrational spacing 1130 ( 60 cm-1. The 6a′′ (28) orbital is the Π bond with dominant contribution of BrO atomic group in the molecule, and the ionization of electrons of 6a′ (28) orbital should also lead to a sharper band. The ionization of 6a′′ (28) orbital electrons leads to the band centered near 13.12 eV. The vibrational spacing (670 ( 60 cm-1) associated with the band centered near 13.12 eV is closer with vibrational spacing (690 cm-1) in the neutral molecule. This means that the ionized orbital is a weak Π bonding orbital. The bands in the high ionization potential region (>13.50 eV) are broad bands on which there are no vibrational structures
and should be related to ionization of the electrons of deeper shell orbitals. This could be seen from the result of ab initio calculation in Table 1. It is also seen from Table 1 that the difference in the values between experimental ionization potentials and orbital energies calculated (-i) for inner shell orbitals is large. The reason could be ascribed to the calculation in which both the correlation of electrons and the relativistic effect have not been considered. The calculation considering both the configuration interaction (CI) levels and the relativistic effects should give the better result because the relativistic effect in heavy atom (such as bromine atom) is large. The coincidence between experiment and calculation for FONO2 and ClONO2 molecules is better.13,14 In a word, the correct assignment of the PE spectral bands is made on the basis of band shape, fine vibrational structure, and ab initio calculation for the BrONO2 molecule. Acknowledgment. The authors are grateful for financial support from the Natural Science Foundation of China. J.P. thanks the Academia Sinica for receipt of a scholarship during the period of this work. References and Notes (1) Yung, Y. L.; Pinto, J. P.; Watson, R. T.; Sander, S. P. J. Atmos. Sci. 1980, 37, 339. (2) Poulet, G.; Pirre, M.; Maguin, F.; Ramaroson, R.; Lebras, G. Geophys. Res. Lett. 1992, 19, 2305. (3) Garcia, R. R.; Solomon, S. J. Geophys. Res. 1994, 99, 12937. (4) McElroy, M. B.; Salawiteh, R. J.; Wofsy, S. C.; Logan, J. A. Nature 1986, 321, 759. (5) Barrie, L. A.; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmussen, R. A. Nature 1988, 334, 138. (6) Bottenheim, J. W.; Barrie, L. A.; Atlas, E.; Heidt, L. E.; Niki, H.; Rasmussen, R. A.; Shepson, P. B. J. Geophys. Res. 1990, 95, 18555. (7) McConnell, J. C.; Henderson, G. S.; Barrie, L.; Bottenheim, J.; Niki, H.; Langford, C. H.; Templeton, E. M. J. Nature 1992, 355, 150. (8) Fan, S. M.; Jacob, D. J. Nature 1992, 359, 522. (9) Rowland, F. S. Annu. ReV. Phys. Chem. 1991, 42, 731. (10) Prather, M. J. Nature 1992, 355, 534. (11) Hanson, D. R.; Ravishankara, A. R. J. Phys. Chem. 1992, 96, 7674. (12) Abbatt, J. P. D.; Molina, M. J. J. J. Phys. Chem. 1992, 96, 7674. (13) Dianxun, W.; Peng, J.; Qiyuan, Z. He I Photoelectron Spectra (PES) of Fluorine Nitrate FONO2. J. Am. Chem. Soc., submitted. (14) Dianxun, W.; Ying, L.; Peng, J.; Xiaohui, W.; Benming, C. The Study of He I Photoelectron Spectroscopy (PES) on the Electronic Structure of Chlorine Nitrate ClONO2. Chem. Phys. Lett., in press. (15) Wilson, W. W.; Christe, K. O. Inorg. Chem. 1987, 26, 1573. (16) Schmeisser, M.; Talinger, L. Chem. Ber. 1961, 94, 1533. (17) Spencer, J. E.; Rowland, F. S. J. Phys. Chem. 1978, 82, 7. (18) Dianxun, W.; Dong, W.; Sheng, L.; Ying, L. J. Electron Spectrosc. Relat. Phenom. 1994, 70, 167. (19) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. In Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981. (20) Kimura, K.; Katsumata, S.; Achiba, Y.; Matsumoto, H.; Nagakura, S. Bull. Chem. Soc. Jpn. 1973, 46, 373. (21) Brolgi, F.; Heilbronner, E. HelV. Chim. Acta 1971, 54, 1424. (22) Schack, C. J.; Christie, K. O.; Pilipovich, D.; Wilson, R. D. Inorg. Chem. 1971, 10, 1078. (23) Koopmans, T. Physica 1934, 1, 104. (24) Wilson, W. W.; Christe, K. O. Inorg. Chem. 1987, 26, 1573. (25) Jacox, M. E. J. Phys. Chem. Ref. Data 1990, 19, 1478. (26) Sander, S. P.; Ray, G. W.; Watson, R. T. J. Phys. Chem. 1981, 85, 199.
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