An experimental and theoretical study of the valence shell photoelectron spectra of purine and pyrimidine molecules

Abstract

The valence shell photoelectron spectrum of purine has been studied experimentally and theoretically. Synchrotron radiation has been used to record spectra at photon energies of 45 and 85 eV. Photoelectron angular distributions have been determined and these provide an experimental means of distinguishing between σ-and π-type orbitals. Vertical ionization energies and photoelectron spectral intensities have been evaluated using the many-body Green function method. The calculated spectra agree well with the experimental results in the outer valence region and have proved to be indispensable for interpreting the structure at higher binding energies where the single particle model of ionization breaks down. The photoelectron spectrum of pyrimidine has also been studied and is compared to that of purine.

Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of pyrimidine molecules

“…Photoelectron spectra of pyrimidine have been recorded using synchrotron radiation, 34 34 It should also be noted that although the C 2v point group was adopted, a different notation has been used to label the orbitals in some of the previous pyrimidine PES studies. 22,36 The highest occupied molecular orbital (HOMO) is the 7b 2 orbital which has a binding energy of 9.8 eV.…”

“…A double peak type structure in the binary peak of atomic orbitals is characteristic of a p-type orbital and reflects the momentum probability density distribution of electrons in these orbitals. 41 The 7b 2 orbital is of N 2p character, 34 thus the observed structure most likely reflects the 2p nature of the molecular orbital. The M3DW predicts a double binary peak as well but the peak positions are shifted to larger scattering angles by about 20 • and the second peak has a much lower intensity than the experimental data.…”

“…The binding energy spectrum has been fitted with a sum of eight Gaussian functions of a fixed width, which corresponds to the experimental coincidence energy resolution. Note that as our coincidence energy resolution is much larger than the natural widths of the various orbitals, [34][35][36][37][38] this is a reasonable approximation in this case.…”

Benchmark ofGWmethods for azabenzenes

“…98,100,101,103,104,108,111,112,115,116 For both molecules, as for pyridazine, PBE underbinds the n-orbitals with respect to the π-orbitals, whereas HF overbinds the norbitals with respect to the π-orbitals. This leads to an incorrect orbital ordering and a distorted spectral shape.…”

“…They embody the basic physics of such systems including the strong correlation effects in aromatic π-systems 84,89,92 and the self-interaction effects introduced by the nitrogen lone pairs. 51,59 Another advantage of these systems is that they are well-characterized experimentally [93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108] and well-studied theoretically by high-level wavefunction and Green's function methods. 93,104,[109][110][111][112][113][114][115][116][117][118][119][120][121][122] We calculate the electronic structure of benzene, pyridine, and the diazines using: (i) semi-local and hybrid DFT (ii) G 0 W 0 , (iii) ev-scGW, (iv) scGW 0 , (v) scGW, and (vi) G 0 W 0 combined with the second-order exchange self-energy (2OX), as an attempt to go beyond the GW approximation.…”