Electronic structures of monosubstituted benzenes and X-ray emission spectroscopy 5. Nitrobenzene

Abstract

The electronic structure of nitrobenzene was studied by X ray emission spectroscopy. The O Kα, N Kα, and C Kα spectra of the title compound in the gas and solid phases were obtained. Based on the results of quantum chemical MNDO calculations, theoretical spectra were constructed. An interpretation of the experimental spectra is given. π Interaction between phenyl fragment and nitro group in nitrobenzene is weak.UV Photoelectron spectroscopy (UPS) 2-12 and X ray photoelectron spectroscopy (XPS) 13 were repeatedly used in studies of the electronic structure of nitrobenzene molecule (C 2v point symmetry group). The UPS data were interpreted using semiempirical quantum chemical calculations 3-6,12 of C 6 H 5 NO 2 . The electronic struc ture of nitrobenzene molecule was also studied 14 by frag ment analysis in the MNDO and MINDO/3 approxi mations.Ab initio quantum chemical calculations of the ni trobenzene molecule in the DZ, MB, and STO 3G (see Ref. 8) and 6 31G* (see Ref. 9) basis sets were carried out. Based on the results of ab initio calculations with the 6 21G* basis set, theoretical spectra of nitrobenzene were constructed, 15 but no correspondence between particu lar MOs (and their symmetry) and spectral lines was re ported.In this study the X ray emission spectra (XES) of nitro benzene in the gas and crystalline phases were investi gated and interpreted using the results of MNDO calcu lations.The experimental XES are shown in Fig. 1. All of them were referenced to a uniform scale of ionization potentials (IP) using the energy positions of the core levels, 13 the C(1)1s level being used for the C Kα spec trum. The use of the C(2,6)1s, C(3,5)1s, or C(4)1s levels causes a slight (by at most ~1 eV) shift of the spectrum toward higher transition energies (E) (see Fig. 1). Since different authors report different experimental energies of the 1s levels (see Ref. 13), we used averaged values.When optimizing the geometry in the framework of the MNDO method, the angle between the planes in which the phenyl fragment and nitro group lie was set to zero in accordance with the data of gas phase electron difraction studies. 17 The theoretical spectra constructed based on the results of these calculations are shown in Fig. 2 and in the Table 1. They were referenced to the energy scale of X ray transitions using the average values of the experimental ionization potentials obtained by XPS. 13 The procedure for constructing theoretical spectra based on the results of semiempirical quantum chemical calculations was reported earlier (see, e.g., Ref. 18). The X ray photoelectron spectrum of nitrobenzene was inter preted by comparing it with the theoretical spectra and with the X ray spectra of nitrogen dioxide anion and benzene. These two compounds can be considered as reasonably correct models for the nitro substituent and phenyl fragment, respectively. The MO composition of NO 2 -anion (point symmetry group C 2v ) and its manifes tation in the X ray spectra was analyzed elsewhere. 19

Kohn-Sham density functional theory calculations of non-resonant and resonant x-ray emission spectroscopy

“…Also shown are the values observed in experiment. 15,[64][65][66][67][68] This provides a measure of the capability of the calculations to predict the spectra with the correct absolute energy scale. The transition energies computed with the B3LYP functional are too low, and the extent that the experimental values are underestimated increases with the nuclear charge of the absorbing atom.…”

Quantum Chemical Calculations of X-ray Emission Spectroscopy

“…Experimental data. [58][59][60][61] S indicates satellite lines due to multielectron processes and the theoretical spectrum for CF 3 Cl has been shifted by +7.9 eV to account for relativistic effects.…”

Using core-hole reference states for calculating X-ray photoelectron and emission spectra

“…Three different carbon, one nitrogen, four oxygen, and one fluorine K-edge are considered, including comparison to experimental measurements. [50][51][52][53] These results have been obtained using ECP:s for all non-hydrogen atoms except the probed one, and using a denominator threshold of 0.1 a.u. Only a limited number of virtuals are frozen in these calculations, excluding at most 4.7% of the total number of MOs.…”

“…FIG.4. X-ray emission spectra calculated using ADC(2), ADC(2)-x, and ADC(3/2), as compared to experiment [50][51][52][53]. Spectra of theK-edge of: a) C in fluorobenzene, b) C in methanol, c) C in ethanol, d) N in nitrobenzene, e) O in methanol, f) O in ethanol, g) O in acetone, h) O in DMSO,and i) F in fluorobenzene.…”