HeI photoelectron spectrum (PES) of fluorine nitrate, FONO2

29 November 1996

CHEMICAL PHYSICS LETTERS

II ELSEVIER

Chemical Physics Letters 262 (1996) 771-775

HeI photoelectron spectrum (PES) of fluorine nitrate, FONO 2 Wang Dianxun *, Jiang Peng, Zhang Qiyuan State Key Laboratoryfor Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100080, People's Republic of China

Received 10 May 1996; in final form 4 September 1996

Abstract The HeI photoelectron spectrum (PES) of the FONO 2 molecule is reported. The assignment of the PE spectral bands is also made. The first band centered near 13.21 eV with vibrational spacings 660 +_ 40 and 1090 _+ 40 cm -t is assigned as electron ionization of both the HOMO (4a"(20)) and the SHOMO (16a'(19)) orbitals which possess the character of the dominant NO 2 group. The secondary band centered near 13.65 eV comes from electron ionization of the 15a'(18) orbital which also possesses the character of the dominant NO 2 group. Comparing the PES results of both CIONO2 and BrONO 2, it is shown that FONO 2 is an inert molecule. As a reservoir species for ozone depletion of the atmosphere, FONO 2 may be the best in the YONO2 series.

1. Introduction Halogens play an important role in atmospheric chemistry. In most investigations, chlorine has been the center of attention since it is the catalyst of the well-known CIO ozone depletion cycle: CI + 0 3 ~ CIO + 0 2 0 + C I O ~ C1 + 0 2 O + 0 3 ----I.2 0 2 However, it is evident from atmospheric measurements [1] that fluorine, bromine and iodine may be important in atmospheric chemistry. Bromine is predicted to play an important role in the Antarctic ozone hole [1,2]. A primary constituent of the oceanic

* Corresponding author.

troposphere is CH3I, which yields the iodine atom upon photolysis by the sun [3]. It is well known that CI and CIO react with several nitrogen oxides to produce C1NO, CIONO, C1ONO 2 and isomers of these compounds. Similar chemical reactions and products have been proposed for fluorine, bromine and iodine [1-4]. YONO 2, where Y is F, CI, Br, I, H or CH 3, is considered as an important reservoir species [5]. That is to say, the ozone depletion is restrained, because the above-mentioned ozone depletion cycle is cut off due to the formation of the Y O N O 2 species in the atmosphere. HeI photoelectron spectroscopic (PES) studies on the electronic structure of these series YONO 2 can not only give further insight into the nature of the chemical bonding, but also predict their role in atmospheric chemistry. In a previous publication [6,7], we presented the studies of HeI photoelectron spectroscopy (PES) on the electronic structure of C1ONO 2 and BrONO 2

0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)01154-2

772

W. Dianxun et a l . / Chemical Physics Letters 262 (1996) 771-775

molecules. As a part of our systematic works, here we would like to report the HeI photoelectron spectroscopic study on the electronic structure of fluorine nitrate, FONO 2, and to discuss its action in the YONO 2 series in the ozone depletion.

FONO 2 is planar, similar to that of nitric acid [11] and that of the calculation reported [12].

3. Results and discussion

3.1. Assignment of spectral bands 2. Experimental

2.1. Synthesis of fluorine nitrate FONO 2 It should first be pointed out that fluorine nitrate is shock sensitive! FONO 2 is prepared according to the method of the literature [8,9] by the direct reaction of fluorine with KNO 3. Fluorine diluted with helium is added to an evacuated 1 I flask containing excess reagent-grade KNO 3. The flask is agitated for about 30 min, and then the gaseous mixture is pumped through a liquid nitrogen cooled trap. This procedure is repeated several times to obtain enough FONO 2. The FONO 2 sample which is collected in the trap is a colorless solid. Liquid and gaseous FONO 2 are also colorless. The sample gas was brought in contact with solutions of silver nitrate after its spectrum had been recorded, and a black precipitate (Ag203) formed at once. This reaction with silver ion is convenient for detecting FONO 2 [9].

The PE spectrum of FONO 2 is given in Fig. 1. The expanded PE spectra with fine vibrational structure in different ionization potential (1P) regions for the FONO 2 molecule are shown in Fig. 2. Although the vibrational frequencies appearing on the PES bands are characteristic of the resulting cation, the assignment of the bands associated with these vibrational frequencies can be done by comparing them with vibrational frequencies of the neutral molecule. Because the change in vibrational frequency at different ionic states is a reflection of the bonding character of the molecular orbital ionized, it is also a reflection of the vibrational model for the entire molecule or of a special atomic group in the molecule. It is well known that ionization of lone-pair electrons leads to a sharp peak and results in lower ionization potentials (IPs). The band with longer vibrational progressions should be related to ionization of antibonding or bonding orbital electrons, and the broad band without vibrational structure should usually be ascribed to ionization of stronger antibonding or bonding orbital electrons.

2,2. PES measurement The HeI (21.22 eV) photoelectron spectrum of FONO 2 has been measured" on a double-chamber LIPS machine-II [10], which was built specifically to detect transient species. The operational resolution for the 2p3/2 peak of argon (Ar +) is around 30 meV. Experimental ionization potentials (IPs) are calibrated by the simultaneous addition of a small amount of methyl iodide sample. Digitized spectra, timeaveraged for periods of around 40 min, are also obtained. In order to assign the bands of the PE spectrum, ab initio GAUSSIAN 86 SCF MO calculation at the 6-31G* basis sets has been performed for the molecule studied. The geometry of the molecule is taken from the result of ab initio SCF MO GAUSSIAN 86 6-31G* optimization. The geometry of

FONOz

r..)

I000

12,00

14.00

16,00

1800

IPs(eV) Fig. I. Hel photoelectron spectrum of FONO 2.

20.00

773

W. Dian.run et a l . / Chemical Physics Letters 262 (1996) 771-775

1090 :t:40 ¢~m"

....

660:1:40 cm-D '

,

,--'"

FON02

O

i

i

12.50

13.00

i

13.~0

i

14.00

FONO2

d.00

d.00

~7'.00 ,~'.00 ws(ev)

~9.00'

2o'.00

Fig. 2. The expanded Hel photoelectron spectra with fine vibrational structure in different ionization potential regions (IPs) for FONO2.

From Figs. 1 and 2 it can be deduced that two bands in the low ionization potential region ( < 14.00 eV) should be the result of ionization of the antibonding or bonding orbital electrons, because some fine vibrational structures appear on the PE spectral band. The vibrational spacings 660 ___40 and 1090 ___ 40 c m - ' in the first band centered near 13.21 eV are much less than that of the symmetric NO stretch ( v 2 = 1301 cm - l ) and of the asymmetric NO stretch ( v 3 = 1759 cm - I ) in the neutral F O N O 2 molecule [8], respectively. This means that the first band of the PE spectrum of the F O N O 2 molecule should be assigned to electron ionization of a bonding orbital with different vibrational models or two bonding orbitals in which the vibrational structure is different,

because ionization o f the bonding orbital electron always leads to a reduction of the vibrational frequency in the resulting cation. The orbitals corresponding to the first band of the PE spectrum should mainly reflect the character of the dominant NO 2 group in the molecule. There is no obvious fine vibrational structure in the secondary band centered near 13.65 eV. This can be considered as the result of electron ionization of stronger antibonding or bonding orbitais. According to the results reported for simple nitroalkanes [13] this band can also be ascribed to electron ionization of the orbital character of the dominant NO 2 group in the F O N O 2 molecule. The PE spectral bands of the F O N O 2 molecule in the high ionization potential region ( > 14.00 eV) should be related to electron ionization of deeper shell orbitals. The low intensity of the PE spectral bands in the high ionization potential region ( > 14.00 eV) can be explained as the result of electron ionization of the orbital which is composed of the contribution of the dominant F atom in the molecule, because the PES study of alkyl fluorides shows that the PE spectral bands corresponding to electron ionization of the orbital associated with the contribution of the dominant F atom appear always in the high ionization potential region and have a lower intensity [13]. The above-mentioned assignment for the PE spectral bands of F O N O 2 is supported by ab initio SCF M O calculation at the G A U S S I A N 86 6-31G* basis sets. The experimentally determined ionization potentials (IPs in eV) and the ionization energies ( - el)

Table 1 Experimental ionization potentials (IPs in eV) and ionization energies ( - e i) computed using ab initio SCF MO GAUSSIAN 86 with 6-31 G * basis sets for the FONO2 molecule I, !2 13 14 I5 16 17 !8 19

-- e i (eV)

MO

Character

13.857 14.784 15.250 16.112 18.396 19.726 20.412 21.558 22.753

4a"(20) 16a'(19) 15a'(18) 3a"(17) 14a'(16) 13a'(15) 2a"(14) 12a'(13) la'(12)

I-Io2N tro2N Oo2N Hov O-voN O'FONO 2 IIFoNO2 O'02NO F FINo2O F

IPs (eV) 13.21 13.65 16.24 17.05 18.19 19.17

774

W. Dianxun et a l . / Chemical Physics Letters 262 (1996) 771-775

computed of ab initio GAUSSIAN 86 SCF MO calculation at the 6-31G" basis sets based on Koopmans' approximation [14] are given in Table 1. The molecular orbital (MO) associated with each ionization is assigned according to its atomic and bonding character. The experimental ionization potentials are given in Table 1 in the form of overlapping band maxima because the observed bands are associated with varying numbers of orbital ionizations. From Table 1 it is obvious that both the HOMO (4a"(20)) and the SHOMO (16a'(19)) are the bonding orbitals, and their characters mainly embody the contribution of the dominant NO 2 group in the FONO 2 molecule. The first band of the PE spectrum is assigned to electron ionization of both the HOMO (4a"(20)) and the SHOMO (16a'(19)) orbitals associated with different vibrational structures (660 + 40 and 1090 + 40 cm- J) for the FONO 2 molecule. The 15a'(18) orbital contains also the character of the dominant NO 2 group in the molecule. The electron ionization of the 15a'(18) orbital can be related to the secondary band of the PE spectral band for the FONO 2 molecule. The assignment of the PES bands in the high ionization potential region ( > 14.00 eV) is also shown in Table 1. The obvious contribution of the F atom in the FONO 2 molecule is also seen in the character of the deeper shell orbitals. In other words, the bands of the PE spectrum can reasonably be assigned with the aid of the band shapes, fine vibrational structure and the ab initio SCF MO calculation for the FONO 2 molecule. It is also seen from Table 1 that the agreement between the values of experimental ionization potentials (IPs) and orbital energies ( - e i ) computed is poor. The reason for this can be Koopmans' theorem itself. Because if Koopmans' theorem is applied to the photoionization of the closed-shell molecule, there are still three approximations for the difference between experimental ionization potentials (IPs) and ionization energies ( - 6 i) computed. These three approximations are: (1) frozen orbital approximation, (2) correlation energy approximation and (3) relativistic energy approximation. It should be said from the comparison of the results of both CIONO 2 and BrONO 2 that the agreement between experimental IPs and ionization energies ( - ¢ i ) computed for the FONO2 molecule is better than that of CIONO 2 and

BrONO 2 [6,7]. This is attributed to smaller relativistic effects in the lighter atom. 3.2. Role of FONO 2 as a reservoir species in the atmosphere

In PES studies of some biological molecules, it was pointed out that there is a linear relation between the ionization potential (IP) of the HOMO and the biological activity of the molecules studied [15]. The IP of the HOMO is a reflection of the electrondonating ability of the molecule studied, i.e. if the IP of the HOMO of the molecule studied is lower, then the electron-donating ability of the molecule will be stronger and the activity of the molecule will also be stronger. In the YONO2 series compounds studied by PES, the IP (13.21 eV) of the HOMO of the FONO 2 molecule is largest. Therefore, although the FONO 2 molecule is an inert molecule, as a reservoir species in the process of ozone depletion FONO 2 may be the best, because it is difficult to give an FO radical which will re-participate in the ozone depletion cycle.

Acknowledgement The authors are grateful for financial support from the National Science Foundation of China. Jiang Peng would like to thank the Academia Sinica for receipt of a Scholarship during the period of this work.

References [I] M.E. McEIroy, R.J. Salawitch, S.C. Wofsy and J.A. Logan, Nature (London), 321 (1986) 755. [2] K.K. Tunk, M.K.W. Ko, J.M. Rodriguez and N.D. Sze, Nature (London) 332 (1986) 811. [3] W. Chameides and D.D. Davis, J. Geophys. Res. 85 (1980) 7383. [4] S.P. Sanders and R.T. Watson, J. Phys. Chem. 85 (1981) 400. [5] F.S. Rowland, Ann. Rev. Phys. Chem. 42 0991) 731. [6] Wang Dianxun, Li Ying, Jiang Peng, Wang Xiaohui and Chen Benming, Chem. Phys. Lett., in press. [7] Wang Dianxun and Jiang Peng, J. Phys. Chem. 100 (1996) 4382.

W. Dianxun et al. / Chemical Physics Letters 262 (1996) 771-775

[8] R.H. Miller, D.L. Bemitt and 1.C. Hisatusne, Spectrochim. Acta A 23 (1967) 223. [9] D.M. Yost and A. Beerbower, J. Am. Chem. Soc. 57 (1953) 782. [10] Zhao Hengqi, Wang Dianxun and Xu Gangzhi, J. Anal. Instrum. 4 (1992) 23. [11] A.P. Cox and J.M. Riveros, J. Chem. Phys. 42 (1965) 3106. [12] B.J. Smith and C.T. Marsden, J. Comput. Chem. 5 (1991) 565.

775

[13] K, Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. lwata, eds., Handbook of Hel photoelectron spectra of fundamental organic molecules (Japan Scientific Societies Press, Tokyo, 1981). [14] T. Koopmans, Physica 1 (1934) 104. [15] D. Dougherty, E.S. Younathan, R. Voll, S. Abdulnur and S.P. McGlynn, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 379.