Notes on Valence Bond Structures for S2N2and Related Systems

FP(μ-N)2 S: A Sulfur-Pnictogen Four-Membered Ring with 6π Electrons

The new 6π-electron four-membered ring compound 3-fluoro-1λ2 ,2,4,3λ3 -thiadiazaphosphetidine, FP(μ-N)2 S, has been generated in the gas phase through high-vacuum flash pyrolysis (HVFP) of thiophosphoryl diazide, FP(S)(N3 )2 , at 1000 K. Subsequent isolation of FP(μ-N)2 S in cryogenic matrices (Ar, Ne, and N2 ) allows its characterization with matrix-isolation IR and UV-vis spectroscopy by combination with 15 N-isotope labeling and computations at the CCSD(T)-F12a/VTZ-F12 level of theory. Upon visible-light irradiation at 550 nm, this cyclic compound undergoes ring-opening to the thiazyl isomer FPNSN, followed by dissociation to FP and SN2 under subsequent UV-irradiation at 365 nm. In sharp contrast to the square planar structure for the isolobal four-membered ring S2 N2 , a puckered structure with significant biradical character has been found for FP(μ-N)2 S.

Isomerism and Biradical Character of Tetrapnictide Dianions: A Computational Study

We present a computational study on tetrapnictide dianions Pn₄^2– (Pn = P, As, Sb, Bi), using density functional theory (DFT), coupled‐cluster [DLPNO‐CCSD(T)] and complete active space self‐consistent field (CASSCF) methods. Environmental effects such as solvation and coordination of counterions are included. The calculations reveal that out of three isomers (square‐planar, butterfly and capped‐triangle), the square planar isomers are generally the most stable. The counterion (Li⁺ and Mg²⁺) used in the calculations have a substantial effect on the relative stabilities. The square planar isomers show considerable biradical character. Calculated reactions toward alkenes indicate that this unusual electronic structure has significant implications on the reactivity of the Pn₄^2– dianions.

Neutral binary chalcogen-nitrogen and ternary S,N,P molecules: new structures, bonding insights and potential applications

Early theoretical and experimental investigations of inorganic sulfur-nitrogen compounds were dominated by (a) assessments of the purported aromatic character of cyclic, binary S,N molecules and ions, (b) the unpredictable reactions of the fascinating cage compound S4N4, and (c) the unique structure and properties of the conducting polymer (SN)x. In the last few years, in addition to unexpected developments in the chemistry of well-known sulfur nitrides, the emphasis of these studies has changed to include nitrogen-rich species formed under high pressures, as well as the selenium analogues of well-known S,N compounds. Novel applications have been established or predicted for many binary S/Se,N molecules, including their use for fingerprint detection, in optoelectronic devices, as high energy-density compounds or as hydrogen-storage materials. The purpose of this perspective is to evaluate critically these new aspects of the chemistry of neutral, binary chalcogen-nitrogen molecules and to suggest experimental approaches to the synthesis of target compounds. Recently identified ternary S,N,P compounds will also be considered in light of their isoelectronic relationship with binary S,N cations.

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of S2 N2

Aromaticity, antiaromaticity and chemical bonding in the ground (S₀ ), first singlet excited (S₁ ) and lowest triplet (T₁ ) electronic states of disulfur dinitride, S₂ N₂ , were investigated by analysing the isotropic magnetic shielding, σ_iso (r), in the space surrounding the molecule for each electronic state. The σ_iso (r) values were calculated by state-optimized CASSCF/cc-pVTZ wave functions with 22 electrons in 16 orbitals constructed from gauge-including atomic orbitals (GIAOs). The S₁ and T₁ electronic states were confirmed as 1¹ A_u and 1³ B_3u , respectively, through linear response CC3/aug-cc-pVTZ calculations of the vertical excitation energies for eight singlet (S₁ -S₈ ) and eight triplet (T₁ -T₈ ) electronic states. The aromaticities of S₀ , S₁ and T₁ were also assessed using additional magnetic criteria including nucleus-independent chemical shifts (NICS) and magnetic susceptibilities calculated at several levels of theory, the highest of which were CCSDT-GIAO/cc-pVTZ for S₀ and CASSCF(22,16)-GIAO/aug-cc-pVQZ for S₁ and T₁ . The results strongly suggest that: 1) the S₀ electronic ground state of S₂ N₂ is aromatic but less so than the electronic ground state of benzene; 2) S₁ is profoundly antiaromatic, to an extent that removes any bonding interactions that would keep the atoms together; and 3) T₁ is also antiaromatic, but its antiaromaticity is more moderate and similar to that observed in the electronic ground state of square cyclobutadiene. S₂ N₂ is the first example of an inorganic ring for which theory predicts substantial changes in aromaticity upon vertical transition from the ground state to the first singlet excited or lowest triplet electronic states.