J. Phys. Chem. 1994, 98, 12526-12529
12526
Electronic Structure of Arsinous Azides How Ghee Ang, Yiew Wang Lee, and Igor Novak* Department of Chemistry, National University of Singapore, Singapore 051I , Singapore
Anthony W. Potts Department of Physics, King's College London, Strand, London WC2R 2LS, U.K. Received: August 2, I994@
He and I and He I1 photoelectron spectra (UPS) of (CF&AsN3 and CF3As(N3)2 have been measured and assigned on the basis of Heme11 intensity variations, band shapes, comparison with UPS of HN3 amd (CF3)xA~H3-x(x = 1, 2), and semiempirical MO calculations. The results for CF3As(N3)2 represent the first UPS study of a covalent diazide. They indicate that in both azides, the As 4p orbital has lost its lone pair character, being heavily mixed with the azide group orbitals.
Introduction
He1
Covalent azides X N 3 (X = various monoatomic or polyatomic substituents) are an interesting class of molecules for the following reasons: (1) their electronic and molecular structures exhibit interesting variations as a result of interactions (or the lack of them) between X and N3 moieties,' ( 2 ) they show a wide range of thermodynamic and kinetic stabilities, depending on the nature of the substituent X, and (3) they can be used as gas phase pyrolytic precursors for the generation of short lived imines and azomethines2 The electronic and molecular structure of many monoazides has been studied by the~retical'.~ and spectroscopic methods. Photoelectron spectroscopy in combination with quantum chemical MNDO calculations has been successful in elucidating many aspects of the electronic structure of aliphati~,~ hal~gen,~ and other types of azides.2 The focus of interest in all these studies is the interactions taking place between the off-the-axis substituent and the linear (or nearly so) N3 group. The type of substituent strongly affects the geometry (e.g., nonlinear in halogen azides and linear in methyl azide) and electron distribution within the N3 group. Studies of the electronic and molecular structure of covalent diazides are very rare, due to the compounds' instability (explosiveness). The diazides are, however, of special interest since they may reveal the interactions between the two azide groups in the molecule. The purpose of the present work is to investigate the electronic structure of azides with a group 15 (arsenic) substituent, as well as possible N3-N3 group interactions. Experimental and Computational Details
(CF&AsN3 and CF3As(N3)2 have been prepared and characterized as described previously.6 The He I and He I1 photoelectron spectra were recorded on a modified Perkin Elmer PS 16/18 spectrometer enabling spectra to be scanned with fixed pass energies. He I spectra were recorded with a pass energy of 3 eV and with a resolution (measured on the Ar+ 2P3/2line) of ' 3 0 meV. He I1 spectra were recorded with an increased pass energy and with lower energy resolution in order to achieve an adequate signallnoise ratio. The accuracy of the measured ionization energies (E,) was n ~ ~The * . reason for the inversion is that the substituent orbital (As 4p) has a lower
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Electronic Structure of Arsinous Azides ionization energy than the N3 group orbital (ON,*) of the same symmetry (a’). The mixing between As 4p (n) nd ON,* will then push the resulting (CF&AsN3 orbital (also labeled ON,*) to lower ionization energies. The ZN,* orbital of the N3 group, on the other hand, has no suitable counte,Tart with which to interact leaving its energy little changed (Figure 3). CFAs(N3)z. A similar analysis can be applied to the UPS of CF3As(N3)2. There are more bands in the 10-15 eV region than in monoazide due to the presence of two N3 groups, which leads to some band overlap and difficulties in Ei measurements and assignment. A total of five ionizations in the 10-14 eV region were observed and assigned from the analysis of band intensities and the correlation diagram. One can also study HeV HeII intensity variations in order to ascertain which MOs have As 4p character. X and show a decrease of relative intensity which can be attributed to some As 4p character. An inspection of Table 1 and Figure 5 reveals that an empirical assignment does not quite agree with the PM3 results which assign the 0 band (instead of to the MO with As 4p character. In view of the approximations inherent in the PM3 method, we consider the empirical assignment to be more reliable.
c)
Conclusion
The most interesting question in the analysis is whether the N3 group’s interaction with the As atom is stronger in monoor diazide. One can first examine the As-N bond lengths. The diazide bond is only 1.3 pm longer than that in monoazide (ref 6) which hardly constitutes a reliable guide. An inspection of spectra, on the other hand, clearly shows that the 2-c bands in monoazide are broader than those in diazide. The larger width may signify greater delocalization and bonding interactions between N3 and the rest of the molecule. This implies that the N3 group’s participation in bonding is more pronounced in the monoazide. Since N3 group bands are sharper in diazide, it is an indication of weaker N3-N3 interactions. These comments on the electronic structure may also be relevant for
J. Phys. Chem., Vol. 98, No. 48, 1994 12529 the relative thermodynamic stabilities of the two compounds. It is well-known that diazides are usually much less stable than the corresponding mono derivatives. The relative “isolation” of azide groups in the diazide may be able to account for this experimental fact. Another interesting observation is that the Heme11 decrease of relative band intensity for As 4p containing ionizations, though observable, is not as pronounced as might be expected on the basis of the As 4p atomic photoionization cross section. The failure to observe intensity changes is evidence of orbital mixing between As 4p and N3 group orbitals in both molecules. Acknowledgment. The authors wish to thank the National University of Singapore for research funding (Grants Rp900624 and RP820035). One of us (Y.W.L.) thanks the University for a research scholarship. References and Notes (1) Otto, M.; Lotz, S. D.; Frenking, G. Inorg. Chem. 1992, 31, 3647 and references quoted therein. (2) Bock, H.; Dammel, R. Angew. Chem., Int. Ed. Engl. 1987,26,504. ( 3 ) von Niessen, W.; Tomasello, P.; J . Electron Spectrosc. Relat. Phenom. 1989, 48, 187. (4) (a) Costa, M. L. S.L.; Almoster Ferreira, M. A,, J. Mol. Strucr. 1988, 175, 417. (b) Costa, M. L.; Costa Cabral, B. J.; Almoster Ferreira, M. A., J . Mol. Struct. 1990, 220, 315. (c) Costa, M. L.; Costa Cabral, B. J.; Almoster Ferreira, M. A., J. Mol. Struct. 1991, 249, 181. ( 5 ) (a) Frost, D. C.; MacDonald, C. B.; McDowell, C. A.; Westwood, N. P. C. Chem. Phys. 1980, 47, 111. (b) Rademacher, P.; Bittner, A. J.; Schatte, G.; Willner, H. Chem. Ber. 1988, 121, 555. (6) (a) Ang, H. G.; Kwik, W. L.; Lee, Y. W.; Liedle, S.; Oberhammer, H. J . Mol. Struct. 1992, 268, 389. (b) Ang, H. G.; Kwik, W. L.; Lee, Y. W.; Oberhammer, H. Inorg. Chem. 1994, 33, 4425. (7) HyperChem Release 3, Autodesk Inc., Sausalito, CA 94965. (8) CvitaS, T.; Novak, I.; Klasinc, L. Int. J. Quantum Chem., Quantum Chem. Symp. 1987, 21, 737. (9) Elbel, S . ; Tom Dieck, H. J. Fluorine Chem. 1982, 19, 349. (10) CvitaS, T.; Klasinc, L. J . Chem. SOC.,Faraday Trans. 2 1976, 72, 1240. (11) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1.
