Vibrational Spectrum and Gas-Phase Structure of Disulfur Dinitride (S2N2)

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.

Are Multicentre Bond Indices and Related Quantities Reliable Predictors of Excited-State Aromaticity?

Systematic scrutiny is carried out of the ability of multicentre bond indices and the NOEL-based similarity index d_AB to serve as excited-state aromaticity criteria. These indices were calculated using state-optimized complete active-space self-consistent field wavefunctions for several low-lying singlet and triplet states of the paradigmatic molecules of benzene and square cyclobutadiene and the inorganic ring S₂N₂. The comparison of the excited-state indices with aromaticity trends for individual excited states suggested by the values of magnetic aromaticity criteria show that whereas the indices work well for aromaticity reversals between the ground singlet and first triplet electronic states, addressed by Baird’s rule, there are no straightforward parallels between the two sets of data for singlet excited states. The problems experienced while applying multicentre bond indices and d_AB to singlet excited states are explained by the loss of the information inherently present in wavefunctions and/or pair densities when calculating the first-order density matrix.

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.

Cluster-model DFT simulations of the infrared spectra of triazine-based molecular crystals

A cluster-model approach is developed to simulate the IR spectra of triazine-based molecular crystals, and the distinct hydrogen-bonding environments of the crystallographically independent molecules can be clearly revealed.

Ln3I5(S2N2)(S2)(THF)10 - a new type of molecular compounds

Unprecedented complexes of the composition Ln3I5(S2N2)(S2)(THF)10 were obtained in the reactions of neodymium and dysprosium iodide-nitrides with sulfur. The inorganic core of the molecules contains the cyclic fragments Ln(μ-S2)Ln, LnSNSN and LnSN. Ten of the fourteen atoms of the core are coplanar, the remaining four S2 and I2 atoms lie in the other two orthogonal planes. The dysprosium complex upon excitation with UV light exhibits the metal-centered luminescence characteristic of the Dy(3+) ion. Geometric parameters of the molecules, computational data, electron spectroscopy and fluorescence suggest the existence of some conjugation in the mentioned heterocycles.

SN2P2: a neutral five-membered sulfur-pnictogen(III) ring

The novel aromatic ring compound 2,4-diphospha-3,5-diaza-thiole (cyclo-SNPNP) was synthesized via flash pyrolysis of SP(N3)3 and characterized by IR spectroscopy and (15)N isotope labeling. Quantum chemical computations indicate its formation by head-to-tail dimerization of SNP and subsequent elimination of a sulfur atom from the highly unstable boatlike six-membered-ring compound cyclo-SNPSNP.