Photoelectron Spectra of Azabenzenes and Azanaphthalenes: I. Pyridine, diazines, s‐triazine and s‐tetrazine

Abstract

The experimental and theoretical basis of a recently proposed reassignment of the bands in the PE. spectra of pyridine, pyridazine, pyrimidine and pyrazine is discussed in detail. A characteristic feature of the derived orbital sequence is that it takes the ‘through‐space’ and ‘through‐bond’ interaction between the ‘lone pair’ basis orbitals explicitly into account. A simple parametrization of the orbital energies, based on HMO‐type models for the π‐orbitals and for the ‘lone pair’ linear combinations, yields excellent agreement with the observed band positions in the PE. spectra of s‐triazine and s‐tetrazine. Our new assignment is compared to those proposed previously.

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

“…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

“…Unlike benzene, the frontier orbitals of pyridine and the diazines include n-orbitals, i.e., orbitals with contributions from the carbon and nitrogen σ-system as well as from the nitrogen lone-pair. 97 These orbitals are more strongly affected by the SIE and, as shown in Figures 5-8, they tend to drift to lower energies with respect to the π-orbitals as the fraction of EXX is gradually increased. The ordering of the frontier orbitals, obtained with different methods, is summarized in Table 2.…”

“…The n-π-π ordering is in agreement with high level wave-function and Green's function based calculations [113][114][115]122 and PES experiments. 97,[100][101][102] The PBE spectrum appears compressed with respect to experiment, yet the spacing between the n HOMO and the π HOMO-1 is too large. This may be explained by a shift of the n-orbital to higher energies as a result of the SIE associated with the nitrogen lone-pair.…”

“…For pyridazine (Figure 6), the assignment of the n-π-π-n character to the HOMO to HOMO-3, respectively, is motivated by PES experiments 97,100 and Green function based calculations. 115,116 PBE predicts a wrong orbital ordering of n-n-π-π and the spectral shape is distorted with the HOMO-2 being very close to the HOMO-1 instead of to the HOMO-3.…”

“…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.…”