CNDO/2 localized molecular orbital studies on disulfur dinitride and tetrasulfur dinitride

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low‐Lying Electronic States of S2N2

“…This is in line with two of the electronic structure models discussed in the Introduction, 1 and the resonance between structures 4 – 4′′′ , according to which the nitrogen and sulfur atoms are negatively and positively charged, respectively. The charge distribution in S 2 N 2 was discussed by Head‐Gordon et al., who mentioned that the formal charges N(−) and S(+) suggested by structures 4 – 4′′′ are in line with the differences between the electronegativities of nitrogen and sulfur (quoted as 3.0 and 2.8, respectively), and who also reported negative natural and Mulliken charges for nitrogen and positive natural and Mulliken charges for sulfur, in agreement with earlier findings …”

“…Calculations with a π‐space spin‐coupled (SC) wave function, which is the most general wave function utilizing a single product of non‐orthogonal MOs, lend support to a singlet diradical version of 1 , similar to the Adkins and Turner model; the important new element is that two of the six singly occupied active π orbitals are found to closely resemble S(3p π ) atomic orbitals (AOs), with spins coupled to a singlet. On the other hand, a series of VB studies performed by Harcourt and co‐workers (see ref.…”

“…Calculationsw ith a p-space spin-coupled (SC) wave function, which is the most general wave functionu tilizing as ingle product of non-orthogonal MOs, lend support to as inglet diradical version of 1, [12] similart ot he Adkins and Turner model; [7] the important new elementi st hat two of the six singly occupied active p orbitals are found to closely resemble S(3p p ) atomic orbitals (AOs), with spins coupled to as inglet.O nt he other hand, as eries of VB studies performed by Harcourt and co-workers( see ref. [13] and references therein)p rovide backing for the alternative singlet diradical structure 3.T horsteinsson and Cooper obtained three individually optimized p-space CASSCF(6,6)( "6 electrons in 6o rbitals" complete-active-space self-consistent field) wave functions corresponding to structures 1, 2 and 3.…”

“…S 2 N 2 is the first example of an inorganic ring for which theory predicts substantialc hanges in aromaticity upon vertical transition from the ground state to the first singlet excited or lowest triplet electronic states.the bonding in this four-membered inorganic ring with six p electrons and its level of aromaticity have been the subjecto f numeroustheoretical studies.One of the popularm odelsf or the electronic structure of S 2 N 2 ,s tructure 1 shown in Scheme1,c an be traced back to an Edmiston-Ruedenberg [6] localization of the molecular orbitals (MOs)f rom as emi-empirical CNDO/2 spin-restricted Hartree-Fock (RHF)c alculation carried out by Adkins and Turner. [7] They obtained localized MOs (LMOs) correspondingt ot wo threecentre p orbitals localized mostly on the nitrogen atoms, a sulfur-sulfur non-bonding p orbital, four SÀN s bonds and four s lone pairs (one on each atom).Another model, featuring on the pages of aw ell-known inorganic chemistry textbook, [8] was suggestedb yF indlay et al [9] on the basis of aF oster-Boys [10] localization of the MOs from Scheme1.[a] Dr.…”

Electronic Structures and Molecular Properties of Chalcogen Nitrides Se2N2 and SeSN2

“…18,19 As 5 opposed to this view, Skrezenek and Harcourt have shown that the primary Lewis-type valence bond (VB) structure for S2N2 resembles the spin-paired singlet diradical structure 2 with a long NN bond across the ring. [20][21][22][23] This opinion, however, has been questioned by Gerratt et al 24 who used spin-coupled VB theory calculations to show that while the structure is a singlet diradical in nature, the diradical character is solely assigned to the sulfur atoms, 3.…”