X-ray spectral determination of the sequence of the bands in the photoelectron spectra of carbonyl compounds

Abstract

UDC 539.26 The electronic structure of complex organic molecules is widely and effectively studied by physical methods making use of the phenomenon of the photo-effect. The analysis of ~he spectra of the electrons emitted by a substance when it is irradiated with ultraviolet radiation (photoelectron spectroscopy, PES) or with x-rays (x-ray electron spectroscopy, XES) makes it possible to obtain information on the binding energies of the core and valence electrons of a molecule, to establish the symmetry and nature of the bonding of the molecular orbitals (MO), and to study the influence of different substituent groups on the structure of the MO [i]. The characteristic features of the PES method, however, do not make it possible to estimate the contributions of individual atoms to MO of a definite symmetry, and this does not always make it possible to carry out a correct identification of the lines in the spectrum, using for these purposes quantum-chemical calculations of the ground state of the molecule. The development of experimental technique has made it possible to apply to the solution of a number of problems in organic chemistry another physical method, namely x-ray spectroscopy (XS), the characteristic features of which make it possible to estimate the contributions of individual atomic orbitals of definite symmetry of different atoms to the MO. The combination of all three methods by a comparison of the PE and x-ray spectra on a single energy scale using data on the binding energies~of the core electrons makes it possible to obtain exhaustive information on the symmetry, composition, and nature of the bonding of the highest occupied orbitals (HO0) of organic molecules [2]. In addition, in a number of cases it is possible to give a correct interpretation of the lines of the photoelectron spectrum. The problem of studying the electronic structure and interpreting the PES of aromatic carbonyl compounds was solved in the present work by x-ray spectroscopy. The PES of compounds 1-4 (Table i) were studied earlier [3], and the energies of the ls levels of these molecules are also known [4]. In an analysis of the PES of compounds 1-4 [3], it was concluded that there is a decrease in the negative contribution of the oxygen atom from fluorenone (FN) to benzophenone (BP), and this was attributed to improvement, in this order, in the overlap of the interacting ~-orbitals of the fragment ~ and the aromatic rings. In fact, according to x-ray structural data, compounds 1-4 differ in the extent to which the ~-orbitals of the bond and the aromatic rings are parallel. A change in the negative charge of the oxygen in the series of compounds 1-4, however, should produce a corresponding trend in the order of the binding energies of the is electrons of oxygen, and this is not observed, as can be seen from the data given in Table i. These facts were analyzed by comparing on a single energy scale the published PE and XE data with the x-ray fluorescence K~ spectra of the oxygen