Electronic Structures and Spectroscopic Properties of 6π-Electron Ring Molecules and Ions E₂N₂ and E₄²⁺ (E = S, Se, Te)

Abstract

The electronic structures and molecular properties of square-planar 6-electron ring molecules and ions E2N2 and E4 2+ (E = S, Se, Te) were studied using various ab initio methods and density functionals. All species were found to contain singlet diradical character in their electronic structures. Detailed analysis of the CAS wave function of S2N2 in terms of different valence bond structures gives largest weight for a Lewis-type singlet diradical VB structure in which the two unpaired electrons reside on nitrogen atoms, though the relative importance of the different VB structures is highly dependent on the level of theory. The diradical character in both E2N2 and E4 2+ was found to increase in the series S < Se < Te. The diradical nature of the chemical species is manifested in the prediction of molecular properties, in which the coupled cluster and multiconfigurational approaches, as well as the BPW91 functional show consistent performance. 77 Se NMR chemical shifts of chalcogen cations SxSe4-x 2+ (x = 0-3) were calculated with CAS, BPW91 and B3PW91 methods using the GIAO formalism. The hybrid functional B3PW91 shows inferior performance, but both CAS and BPW91 unquestionably confirm the experimental assignment and are able to predict the NMR chemical shifts of these computationally difficult cases with excellent accuracy.

Electronic Structures and Molecular Properties of Chalcogen Nitrides Se2N2 and SeSN2

“…We have recently shown that even at the equilibrium geometry, the RHF wave function of S2N2 displays a singlet instability. 16 Because a symmetry-broken UHF solution can be found, it is perfectly valid to state that the strong electron correlations described by Jung et al 25 Before turning attention to the molecular properties of S2N2 and the related heavier chalcogen containing species SeSN2 and Se2N2, we seek to answer another open question, whether the diradical character in S2N2 should be attributed to nitrogen or to sulfur atoms. Previous VB calculations have come to two different conclusions, [20][21][22][23][24] whereas our recent MO-theory based wave function analyses indicated that the diradical character of S2N2 in its equilibrium geometry can be unquestionably addressed to the nitrogen atoms.…”

“…However, this lack of symmetry-broken spin-unrestricted DFT solution cannot be held as a definite proof of a non-existent diradical character because DFT solutions are found to be stable even for some of the obvious diradicals such as ozone whose diradical character has been found to be 26% by ab initio methods. 16 In ab initio theory, the instability of the spinrestricted SCF solution is a direct indication of the quality of the wave function, whereas within the DFT framework of electronic structure theory it mirrors the capability of the exchange-correlation functional to describe the electron density with a single Slater determinant comprised of Kohn-Sham (KS) orbitals. If the exact exchange-correlation functional were available, a single Slater determinant of KS orbitals would be adequate to describe even pure diradical systems and, hence, no lower-energy spin-unrestricted solutions would therefore be found.…”

“…The conclusions of Jung et al 25 are for the most part in agreement with our recent high level ab initio analysis of the electronic structure of S2N2, as we also found it to be a primarily aromatic system. 16 Our detailed wave function analysis showed that S2N2 possesses 6% diradical character which could be attributed to the nitrogen atoms. This value is in good agreement with the antibonding effect discussed by Jung et al 25 However, they do not directly relate the antibonding effect to the diradical character in their conclusions.…”

“…In CAS optimizations, the active space consisted of full valence space and 7 included all possible configurations that arise from the distribution of 22 valence electrons in 11 highest occupied and 5 lowest virtual orbitals i.e. [22,16] Vibrational frequencies were calculated utilizing all aforementioned methods within the harmonic approximation. For methods for which analytical second derivatives were not implemented, frequencies were calculated using finite differences of energy.…”

“…In CAS and CASPT2 calculations the active space size was [22,16]. First, a state averaged CAS calculation was performed (with equal weights for all states) for the ground state and the lowest eight excited singlet (triplet)…”