Refinement of the structure of orthorhombic sulfur, α-S8

Testing the sensitivity limits of ³³S NMR: an ultra-wideline study of elemental sulfur

Preliminary DFT investigations into the feasibility of using (33)S solid-state NMR to study organic and biological molecules suggest that very large (33)S quadrupolar coupling constants (>40MHz) are not uncommon. We have therefore investigated the possibility of using recently developed ultra-wideline techniques to record such (33)S powder patterns at a high magnetic field (21.1T). A WURST-echo sequence was used to record the spectrum from a>99.9% enriched sample of elemental sulfur, resulting in the largest (33)S quadrupolar coupling constant yet measured by solid-state NMR (C(Q)=43.3 MHz). Implications of this experiment are briefly discussed.

The Chemical Imagination at Work inVery Tight Places

Diamond-anvil-cell and shock-wave technologies now permit the study of matter under multimegabar pressure (that is, of several hundred GPa). The properties of matter in this pressure regime differ drastically from those known at 1 atm (about 10(5) Pa). Just how different chemistry is at high pressure and what role chemical intuition for bonding and structure can have in understanding matter at high pressure will be explored in this account. We will discuss in detail an overlapping hierarchy of responses to increased density: a) squeezing out van der Waals space (for molecular crystals); b) increasing coordination; c) decreasing the length of covalent bonds and the size of anions; and d) in an extreme regime, moving electrons off atoms and generating new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.

High-pressure structures and phase transformations in elemental metals

At ambient conditions the great majority of the metallic elements have simple crystal structures, such as face-centred or body-centred cubic, or hexagonal close-packed. However, when subjected to very high pressures, many of the same elements undergo phase transitions to low-symmetry and surprisingly complex structures, an increasing number of which are being found to be incommensurate. The present critical review describes the high-pressure behaviour of each of the group 1 to 16 metallic elements in detail, summarising previous work and giving the best present understanding of the structures and transitions at ambient temperature. The principal results and emerging systematics are then summarised and discussed.

Controlled aggregation of ME8(n-) binary anions (M = Cr, Mo; E = As, Sb) into one-dimensional arrays: structures, magnetism and spectroscopy

The [ME(8)](n)()(-) ions where M = Cr, Mo; E = As, Sb; n = 2, 3 have been prepared from the corresponding E(7)(3)(-) Zintl ions and M(naphthalene)(2) precursors. The complexes and their [A(crypt)](+) salts (A = Na, K) are formed in 20-45% crystalline yields and have been characterized by UV-vis spectroscopy, EPR, cyclic voltammetry, magnetic susceptibility, electrospray mass spectrometry (ESI-MS) and single-crystal X-ray diffraction. The structures are defined by crown-like cyclo-E(8) rings that are centered by transition metals. MoAs(8)(2)(-) (2) is a 16 e(-) diamagnetic complex whereas MoSb(8)(3)(-) (5) and the CrAs(8)(3)(-) salts (3 and 4) are 17 e(-) paramagnetic complexes. The ESI-MS spectra show free and alkali-complexed ME(8)(n)()(-) ions. The K(+) salt of CrAs(8)(3)(-) (4) crystallizes in a one-dimensional chain structure of [KCrAs(8)](2)(-) repeat units whereas the Na(+) salt (3) as well as 2 and 5 crystallize in "free ion" structures. The Cr atoms in 3 and 4 are formally d(1) Cr(5+) centers that show EPR signals at g = 2.001 with small As hyperfine interactions of 3.6 G. The susceptibility of the [KCrAs(8)](2)(-) salt 4 was modeled as a 1D Heisenberg antiferromagnet with a small -J/k(B) of 3K arising from antiferromagnetic couplings of the d(1) centers whereas 3 shows Curie-Weiss behavior. The electrochemical studies show metal-based oxidations for 3-5 but a ligand based oxidation for 2. The electronic spectra are interpreted in terms of the molecular orbital analysis of Li and Wu. The differences in formal oxidation states of the metals is described in terms of a Zintl-Klemm formalism involving E(8)(8)(-) rings that are isoelectronic to S(8). The factors governing the formation of 1D chains versus free ions are presented.

Preparation, crystal structure, and spectroscopic characterization of [(Se(2)SN(2)Cl](2)

The reaction of [(Me(3)Si)(2)N](2)S with an equimolar amount of SeCl(4) in dioxane at 50 degrees C affords [(Se(2)SN(2))Cl](2) (1) in excellent yield. Crystals of 1 are orthorhombic, space group Pbca, with a = 8.5721(7) A, b = 7.8336(6) A, c = 15.228(1) A, and Z = 8. The crystal structure contains two planar Se(2)SN(2)(*)(+) rings which are linked by intermolecular Se.Se interactions [d(Se-Se) = 3.0690(7) A]. The EI mass spectrum shows Se(2)SN(2)(*)(+) as the fragment of highest mass. Both the (14)N and (77)Se NMR spectra show a single resonance (-52 and 1394 ppm, respectively). The solid [(Se(2)SN(2))Cl](2) gives a strong ESR signal indicating the presence of a Se(2)SN(2)(*)(+) radical. The Raman spectrum was assigned through normal coordinate treatment involving a general valence force field. The vibrational analysis yielded a good agreement between the observed and calculated wavenumbers.

Theoretical and Experimental Studies of Six-Membered Selenium-Sulfur Nitrides Se(x)()S(4)(-)(x)()N(2) (x = 0-4). Preparation of S(4)N(2) and SeS(3)N(2) by the Reaction of Bis[bis(trimethylsilyl)amino]sulfane with Chalcogen Chlorides

The reaction of [(Me(3)Si)(2)N](2)S with equimolar amounts of SCl(2) and S(2)Cl(2) produces S(4)N(2) in a good yield. The reaction of [(Me(3)Si)(2)N](2)S with a 3:1:1 mixture of S(2)Cl(2), Se(2)Cl(2), and SeCl(4) yields a dark brown-red insoluble material that was inferred to be mainly SSeSNSN on the basis of the elemental analysis, mass spectroscopy, vibrational analysis, and NMR spectroscopy. Attempts to prepare selenium-rich species resulted in the formation of elemental selenium or Se(3)N(2)Cl(2). The experimental work was supported by ab initio MO calculations which establish the structural and stability relationships of the different members of the series 1,3-Se(x)()S(4)(-)(x)()N(2) (x = 0-4). Full geometry optimization was carried out for each molecular species using the polarized split-valence MIDI-4 basis sets. The effects of electron correlation were taken into account involving the second-order Møler-Plessett perturbation theory. Each molecule was found to lie in an approximate half-chair conformation that is well established for 1,3-S(4)N(2) (i.e., interacting planar NEN and EEE fragments; E = S, Se). The bond parameters agree well with experimental information where available. Whereas the lengths of the bonds in the NEEEN fragment approach those of the single bonds, the bonds in the NEN fragment show marked double bond character. The stabilities of the molecules decrease expectedly with increasing selenium content as judged by the total binding energy at the MP2 level of theory. Within a given chemical composition, isomers containing a N=Se=N unit lie higher in energy than those containing a N=S=N unit. These results may explain why selenium-rich Se(x)()S(4)(-)(x)()N(2) molecules have not been isolated.