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Binding Energy Shifts for Carbon, Nitrogen, Oxygen, and Sulfur Core Electrons from Extended Hiickel Theory Valence Molecular Orbital Potentials at the Nuclei Maurice E. Schwartz” and Jurgen D. Switalski Contribution from the Department of Chemistry and Radiation Laboratory,’ University oj’Notre Dame, Notre Dame, Indiana 46556. Received February 4, 1972 Abstract: The potential at a nucleus has been calculated as the quantum mechanical average from extended Huckel wave functions for a large number of C, N, 0, and S containing molecules. Experimental core level binding energy shifts have been correlated against the calculated potentials. Good fits were found for 1s in C and 0, and for 2p in S, while N 1s shifts were poorly described. Discussion of the results and comparison to
other approaches and their applicability, as well as suggestions for further experimental and theoretical studies, are given.
S
ince the demonstrations2 that shifts in core-electron binding energies (which we denote by ABE) are essentially the same as shifts in the quantum mechanical average of the potential at the nucleus on which the core orbital is located, there has been increasing interest in the use of simple electronic structure models to calculate potentials and, hence, the binding energy shifts. 3--5 In the present paper we extend our previous work3 in which C 1s ABE’s were successfully fit to the potential at the nuclei (@“,I), calculated as the quantum mechanical average from extended Hiickel theory (EHT) molecular orbital (MO) wave functions for valence electrons. That paper3 should be consulted for specific remarks about the method, while the general background can be found in ref 2b, 4, and 5. As we have emphasized previously, 3 ’ j we do not generally expect that ABE = AQval will be an equality for from approximate valence MO theories (especially those as crude as EHT). Rather, we hope that will be a useful parameter against which to correlate measured ABE’s (and subsequently predict and interpret other ABE’s). It is certainly more reliable than the simple “atomic charge” of the atom whose core levels are considered,3,5and it is much easier to obtain (especially for large molecules) than, say, core orbital energies from ab initio all-electron SCF-MO calculations, which in themselves do not give quantitative measures of ABE’s unless a large, accurate basis set is used.”5 The study here follows the same pattern as before,3 with the following exceptions. (a) Instead of standard Slater orbital exponents we used the best-atom exponents from atomic calculations of Clementi and Raimondi.6 H l s exponents were 1.25. (b) In the Gaus(1) The Radiation Laboratory is operated by Notre Dame under contract with the U. S. Atomic Energy Commission. This is AEC Document No. COO-38-829. Partial support for this work has come from the Petroleum Research Fund, administered by the American Chemical Society, Grant No. 5122-AC6. (2) (a) H. Basch, Chem. Phys. Lett., 5,337 (1970); (b) M. E. Schwartz, ibid., 6, 631 (1970). (3) M. E. Schwartz, ibid., 7, 78 (1970). (4) D. W. Davis, D. A. Shirley, and T. D. Thomas, J . Chem. Phs’s., 56, 671 (1972). ( 5 ) Cf. also pauers by the authors of ref 3 and 4 in the “Proceedings of the I n t e r n a t i o h Conference on Electron Spectroscopy, Asilomar, Calif., Sept 7-10, 1971,” D. A. Shirley, Ed., North-Holland Publishing Co., Amsterdam, in press. (6) E. Clementi and D. L. Raimondi, J . Chern. Phqs., 38, 2686 (1963).
sian fits to the Slater-type orbitals,3 3G STO fits were used instead of 5G fits. This simplifies the calculation of nuclear attraction integrals needed for @\,,I, with no essential change in accuracy, as determined in numerous test calculations by us. (c) Standard molecular geometries’ were used except where obviously inappropriate, in which case experimental values8 were used. We also required the additional diagonal matrix elements of -20.08 and -13.32 eV for S 3s and 3p, respectively. g Using a linear least-squares fit of the form ABE
=
+b
(1)
we examined 1s ABE’s for C, N, and 0; and 2p ABE’s for S. Note that eq 1 does not explicitly contain A@.,,, from a reference molecule; the parameter b can absorb that. All discussion is relevant to isolated gaseous molecules, so no experimental or theoretical problems of solid-state effects and work functions need concern us. This will, however, restrict us to a limited set of data, Results for the four atoms are tabulated in Tables I-IV and plotted in Figures 1-4. References for the experimental ABE’s are given in footnotes to the tables, while the least-squares parameters a and b of eq 1, along with the root-mean-square errors of the fits, are given in the figure captions. We shall discuss the results atom by atom. The C 1s BE’S studied here are for a larger, more varied set than in our previous study,3 and this variety shows up in the ABE - @>.,I fit of Figure 1. However, except for CO, which has generally given trouble before,3.10 the fit is systematic; systems with C bonded to second row atoms (C1 and S) seem to have their own essentially linear relation and should be treated separately, as we have done in Figure 1 and Table I. Indeed, in future work one might do well to look for correlations within families of similar molecular structure, (7) J. A. Pople and M. S, Gordon, J . Amer. Chenz. Soc., 89, 4253 (1967). (8) “Tables of Interatomic Distances and Configurations in MoleculesandIons,” Chem. Soc., Spec,, Publ., No. 11 (1958); No. 18 (1965). (9) J. Hinze and H. H. Jaffi, J. Amer. Chem. Soc., 84, 540 (1962). (IO) I
