Semiconductor surface-induced 1,3-hydrogen shift: the role of covalent vs zwitterionic character

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to compare the reaction of 1,2-cyclohexanedione (1,2-CHD) with Si(001) and diamond(001) surface dimers under ultra-high-vacuum conditions. 1,2-CHD is known to undergo a keto-enol tautomerization, with the monoenol being the primary equilibrium species in the solid and gas phases. XPS and FTIR data demonstrate that 1,2-CHD reacts with diamond(001) through the OH group of the monoenol, resulting in only one O atom being bonded to the surface. In contrast, XPS and FTIR data suggest that both oxygen atoms in the 1,2-CHD molecule bond via Si-O-C linkages to the Si(001) surface dimer, and that the molecule undergoes an intramolecular 1,3-H shift. While the Si(001) and diamond(001) surfaces are both comprised of surface dimers, the diamond(001) dimer is symmetric, with little charge separation, whereas the Si(001) dimer is tilted and exhibits zwitterionic character. The different reaction products that are observed when clean Si(001) and diamond(001) surfaces are exposed to 1,2-CHD demonstrate the importance of charge separation in promoting a 1,3-H shift and provide new mechanistic insights that may be applicable to a variety of organic reactions.     

Moleküle mit ungewöhnlich langen N(sp2)–Si(sp3)-Bindungen: Synthese und Kristallstrukturen von 1,2-Benzoldisulfonylaminosilanen

Polysulfonylamines. XCIV. Molecules with Unusually Long N(sp²)–Si(sp³) Bonds: Synthesis and Crystal Structures of 1,2-Benzenedisulfonylaminosilanes The following compounds were obtained by metathesis of silver 1,2-benzenedisulfonimide (AgZ) with the appropriate chlorosilanes: ZSiMe₃ ( 4 ), ZSiⁿPr₃, ZSiMe₂ⁿBu, ZSiMe₂(CMe₂–CHMe₂) ( 7 ), (Z)₂SiMe₂ ( 8 ). In the crystal structures of 4 (monoclinic, space group P2₁/n), 7 (monoclinic, P2₁/c) and 8 (monoclinic, P2₁/n), which were determined by low-temperature X-ray diffraction, the molecules adopt the N-silyl form and display unusually long bonds between the trigonal-planar N and the tetrahedrally coordinated Si atoms ( 4 : 182.6, 7 : 184.1, 8 : 177.8 and 180.5 pm). For 7 in CDCl₃ solution, ¹H and ¹³C NMR data indicate N,O-silylotropy. The solid state structures of molecules 4 and 7 strongly suggest that the N–Si bond lengthening in N,N-disulfonylated aminosilanes is mainly induced by the π-acceptor character of the SO₂ groups and not by the occasionally observed coordination expansion of the Si atom through short intramolecular O…Si contacts.     

Bifunctional hybrid mesoporous organoaluminosilicates with molecularly ordered ethylene groups

We report the synthesis and characterization of crystal-like structurally well-ordered ethylene-containing hybrid mesoporous organoaluminosilicate materials, which exhibit molecular-level periodicity in the pore walls and enhanced hydrothermal stability. Distilled 1,2-bis(triethoxysilyl)ethylene (BTEE) was used as the organosilica precursor, aluminum isopropoxide as aluminum source, and cetyltrimethylammonium bromide as template. The materials are structurally well-ordered and exhibit high surface area (>1300 m(2)/g) and pore volume (>1.10 cm(3)/g). The presence of molecularly ordered ethylene groups was confirmed by powder X-ray diffraction, (29)Si and (13)C MAS NMR, and Raman spectroscopy. The ethylene groups are thermally stable up to a temperature of 300 degrees C. The presence of ethylene groups enhances the hydrothermal stability in boiling water of both organosilica and organoaluminosilicate materials. The organoaluminosilicate materials possess a bifunctional character arising from the presence of both tetrahedrally coordinated Al and molecularly ordered ethylene groups in their frameworks.     

Stacking‐Induced Diamagnetic/Paramagnetic Conversion of Imidazo[1,2‐a]pyridin‐2(3 H)‐one Derivatives: Near‐Infrared Absorption and Magnetic Properties in the Solid State

Herein, we present three imidazo[1,2‐a]pyridin‐2(3 H)‐one derivatives that are diamagnetic in solution, but paramagnetic in the solid state, possibly owing to a stacking‐induced formation of phenoxide‐type radicals. Notably, a larger bathochromic shift of the absorption (even up to the near‐ infrared region) of these three compounds was observed in the solid state than in solution, which was attributable to the ordered columnar stacking arrangements or their single‐electron character as radicals in the solid state. Interestingly, compared to that in solution, (E)‐3‐(pyridin‐4′‐ylmethylene)imidazo[1,2‐a]pyridine 2(3 H)‐one displayed a largely red‐shifted emission (centered at 660 nm, with tailing above 800 nm) in the solid state. A larger bathochromic shift (260 nm) of the emission is an indication of better order and tight stacking in the solid state, which is brought about by the rigid and polar acceptor. These three compounds also reveal different magnetic susceptibilities at 300 K, thus implying that they possess various columnar stacking structures. Most interestingly, these three radicals exhibit unusual ferromagnetic‐to‐antiferromagnetic phase transitions, which can be attributed to anisotropic contraction and non‐uniform slippage of the columnar stacking chains.     

N‐Heterocyclic Carbene (NHC)‐Stabilized Silanechalcogenones: NHC→Si(R2)E (E=O,S, Se,Te)

A series of N‐heterocyclic carbene‐stabilized silanechalcogenones 2 a , b (Si?O), 3 a , b (Si?S), 4 a , b (Si?Se), and 5 a , b (Si?Te) are described. The silanone complexes 2 a , b were prepared by facile oxygenation of the carbene–silylene adducts 1 a , b with N₂O, whereas their heavier congeners were synthesized by gentle chalcogenation of 1 a , b with equimolar amounts of elemental sulfur, selenium, and tellurium, respectively. These novel compounds have been isolated in a crystalline form in high yields and have been fully characterized by a variety of techniques including IR spectroscopy, ESIMS, and multinuclear NMR spectroscopy. The structures of 2 b , 3 a , 4 a , 4 b , and 5 b have been confirmed by single‐crystal X‐ray crystallography. Due to the NHC→Si donor–acceptor electronic interaction, the Si?E (E=O, S, Se, Te) moieties within these compounds are well stabilized and thus the compounds possess several ylide‐like resonance structures. Nevertheless, these species also exhibit considerable Si?E double‐bond character, presumably through a nonclassical Si?E π‐bonding interaction between the chalcogen lone‐pair electrons and two antibonding Si? N σ* orbitals, as evidenced by their high stretching vibration modes and the shortening of the Si–E distances (between 5.4 and 6.3 %) compared with the corresponding Si? E single‐bond lengths.     

Insights into the making of a stable silylene

Reduction of Cl2Si[(NR)2C6H4-1,2] (R = CH2Bu(t)) with potassium is known to lead to the stable silylene Si[(NR)2C6H4-1,2] (1). However, silylene is now shown to react further with an alkali metal (Na or K) to yield the (1)(2)2-, c-(1)(3)-*, c-(1)(3)2- or c-(1)(4)2- derivatives. Reduction of Cl2Si[(NR)2C6H4-1,2] (R = CH2CH3 or CH2CHMe2) with potassium does not lead to an isolable silylene, but such a silylene is proposed to be an intermediate and, as for 1, reacts further to afford the potassium salts of c-[Si{(NR)2C6H4-1,2}]4-* and c-[Si{(NR)2C6H4-1,2}](4)2-. The pathways leading to the anionic cyclotri- and cyclotetrasilanes are discussed and supported experimentally; including by X-ray structures of relevant intermediates.     

A Persistent 1,2‐Dihydrophosphasilene Adduct

The reaction of the arylchlorosilylene–NHC adduct ArSi(NHC)Cl [Ar=2,6‐Trip₂‐C₆H₃; NHC=(MeC)₂(NMe)₂C] 1 with one molar equiv of LiPH₂^.dme (dme=1,2‐dimethoxyethane) affords the first 1,2‐dihydrophosphasilene adduct 2 (ArSi(NHC)(H)?PH). The latter is labile in solution and can undergo head‐to‐tail dimerization to give [ArSi(H)PH]₂ 3 and “free” NHC. Further stabilization of 2 by complexation with {W(CO)₅} affords the isolable 1,2‐dihydrophosphasilene–tungsten complex 4 [ArSi(NHC)(H)?P(H)W(CO)₅]. Additionally, the new 1‐silyl‐2‐hydrophosphasilene ArSi(NHC)(H)?PSiMe₃ 5 could be synthesized and structurally characterized. DFT studies confirmed that the Si?P bond in 2 and 4 is mostly zwitterionic with drastically decreased double‐bond character.     

Reversal of polarization in amidophosphines: neutral- and anionic-kappaP coordination vs. anionic-kappaP,N coordination and the formation of nickelaazaphosphiranes

Nickel(ii) chloride reacts with the bis(tert-butylamino)diazadiphosphetidine {Bu(t)(H)NP(micro-NBu(t))(2)PN(H)Bu(t)} to form trans-[{Bu(t)(H)NP(micro-NBu(t))(2)PN(H)Bu(t)}(2)NiCl(2)]. In solution and the solid-state each heterocyclic ligand coordinates nickel through one phosphorus atom only. For comparison the solid-state structure of the known trans-[NiCl(2)(PEt(3))(2)] was also determined and it was found that the two complexes have almost identical bond parameters about nickel. The nickel-amidophosphine complexes [{Bu(t)OP(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))], [(PBu(n)(3))ClNi{Bu(t)NP(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))], and [{Me(2)Si(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))] were synthesized and X-ray structurally characterized. In these mono- and di-nuclear nickel complexes the nickel ions are coordinated in pseudo square-planar fashions, by one trialkylphosphine ligand, one chloride ligand and one kappaP,N-coordinated amidophosphine moiety from tert-butylamido-substituted heterocycles. Attempts to create nickel complexes chelated in a kappa(2)P fashion by the o-phenylenediamine-tethered mono- and di-anionic 1-{Me(2)Si(micro-NBu(t))(2)PN} 2-{Me(2)Si(micro-NBu(t))(2)PNH}C(6)H(4) and 1,2-{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4), respectively, afforded instead [1,2-{Me(2)Si(micro-NBu(t))(2)PN}{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4)NiCl] and [1,2-{Me(2)Si(micro-NBu(t))(2)PN}{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4)Ni{PEt(3)}], each complex having kappaP,N and kappaP coordinated amidophosphine ligands.     

Structural trends and chemical bonding in Te-doped silicon clathrates

The recently discovered tellurium-doped silicon clathrates, Te7+xSi20-x and Te16Si38, both low- and high-temperature forms (cubic and rhombohedral, respectively), exhibit original structures that are all derived from the parent type I clathrate G8Si46 (G = guest atom). The similarities and differences between the structures of these compounds and that of the parent one are analyzed and discussed on the basis of charge distribution and chemical bonding considerations. Because of the particular character of the Te atom, these compounds appear to be at the border between the clathrate and polytelluride families.     

Self-assembled silane monolayers: fabrication with nanoscale uniformity

Illustrating direct connections between surface chemical events and mechanical and topological characteristics of self-assembled monolayers derived from octadecyltrichlorosilane (OTS) adsorption on Si(100), layers prepared in the presence and absence of moisture have been characterized. Uniform and robust self-assembled monolayers are demonstrated provided the Si(100) surface is fully hydroxylated by treatment in piranha solution and the dried surface is exposed to OTS under strict anhydrous conditions. With nanoscale resolution, the uniform mechanical properties are confirmed by interfacial force microscopy while the uniform topological properties are evident in atomic force microscopy images. The monolayer character of the OTS coverage is confirmed by X-ray photoelectron spectroscopy, ellipsometry, and patterning experiments. Analogous surfaces, prepared in the presence of moisture, exhibit nonuniform topological and mechanical properties.     

Control of stability through overlap matching: closo-carborynes and closo-silaborynes

The matching of ring and cap orbitals for overlap is used to arrive at the best carborynes among the many possibilities. Accordingly, 1,2-carboranes, 1,2-silaboranes (C2BnHn+2, and Si2BnHn+2, n = 4, 5, 8, and 10), and their dehydrogeno derivatives were studied with use of the Density Functional Theory (B3LYP/6-311+G*). The dehydrogenation of 2,3-C2B5H7 (6a) to 2,3-C2B5H5 (13a) is estimated to be even less endothermic than those of benzene and 1,2- C2B10H12 (1a) to benzyne and 1,2-C2B10H10 (8a) by more than 21 kcal/mol. This is due to the extra stabilization gained through better overlap of the C2B3H3 ring with the 2 BH caps. The relatively larger size of the Si atom leads to overlap requirements in silaboranes that are different from those in carboranes. The lower Si-Si single bond energy and the preference of Si for lower coordination result in unusual structures in dehydrogenosilaboranes. One of the Si atoms moves away from the surface in Si2B10H10 (15), Si2B8H8 (16, 17, and 18), and 1,2-Si2B5H5 (19). One Si atom forms a bridge to a trigonal surface in 2,3-Si2B5H5 (20) and 1,2-Si2B4H4 (21). Estimates of three-dimensional aromaticity with NICS calculations show that the exohedral double bond does not influence three-dimensional aromaticity.     

Isolable anion radical of blue disilene (tBu2MeSi)2Si=Si(SiMetBu2)2 formed upon one-electron reduction: synthesis and characterization

The highly twisted tetrakis(di-tert-butylmethylsilyl)disilene 4 was prepared and reacted with (t)BuLi in THF, producing disilene anion radical 5 upon one-electron reduction. The anion radical 5 was isolated in the form of its lithium salt as extremely air- and moisture-sensitive red crystals. The molecular structure of 5 was established by X-ray crystallography, which showed a nearly orthogonal structure (twisting angle of 88 degrees ) along the central Si-Si bond, with a length of 2.341(5) A, which is 3.6% elongated relative to that of 4. The interesting feature of 5 is that one of the central Si atoms has radical character, whereas the other Si atom has silyl anion character. An electron spin resonance (ESR) study of the hyperfine coupling constants of the (29)Si nuclei indicates that rapid spin exchange occurs between these central Si atoms on the ESR time scale.     

Surface chemistry of monochlorinated and dichlorinated benzenes on Si(100)2x1: comparison study of chlorine content and isomeric effects

Using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), the room temperature (RT) adsorption and thermal evolution of monochlorobenzene (MCB) and 1,3-dichlorobenzene (1,3-DCB) on Si(100)2x1 have been investigated and compared with that of 1,2-dichlorobenzene (1,2-DCB) reported previously. Like 1,2-DCB, the C 1s features observed at 284.6 (C(1)) and 286.0 eV (C(2)) for both MCB and 1,3-DCB could be attributed to the C-H and C-Cl bonds, respectively. The C(1)/C(2) intensity ratios for MCB (5.0) and 1,3-DCB (2.0) are found to follow the stoichiometric ratios of the C-H to C-Cl bonds for MCB and 1,3-DCB, respectively, indicating that both MCB and 1,3-DCB adsorb on Si(100)2x1 molecularly with negligible C-Cl dissociation at RT, in marked contrast to the partial C-Cl dissociation found for 1,2-DCB. Unlike 1,2-DCB with two discernible Cl 2s features at 270.3 and 271.2 eV, a single Cl 2s feature at 271.2 eV is observed for MCB and 1,3-DCB, in accord with the single local chemical environment for Cl. The TPD results show that MCB undergoes molecular desorption exclusively, similar to that found for benzene. Both molecular desorption and recombinative HCl desorption are found for 1,3-DCB, similar to that for 1,2-DCB. Despite the different Cl contents and relative Cl locations on the benzene ring, both MCB and 1,3-DCB exhibit RT adsorption behavior remarkably similar to that of benzene. To explain the C-Cl dissociation observed for 1,2-DCB, we propose a possible transition state involving the Cl atoms located at more physically compatible positions with the surface Si dimers in order to facilitate the conversion of 1,2-DCB (preferentially over 1,3-DCB) to dissociated products at RT. However, the thermal evolution of 1,3-DCB is closer to that of 1,2-DCB than that of MCB and benzene. The breakage of C-Cl bonds is found to occur at a relatively low temperature of 425 K, which suggests a relatively low activation barrier for the dechlorination of 1,3-DCB adspecies. Calculated energetics for 1,4-DCB on Si(100)2x1 shows that double dechlorination is not as favorable a process as those for 1,2-DCB and 1,3-DCB.     

Photooxidation of Diimine Dithiolate Platinium(II) Complexes Induced by Charge Transfer to Diimine Excitation

A photochemical and photophysical investigation was carried out on (tbubpy)Pt(II)(dpdt) and (tbubpy)Pt(II)(edt) (1 and 2, respectively, where tbubpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dpdt = meso-1,2-diphenyl-1,2-ethanedithiolate and edt = 1,2-ethanedithiolate). Luminescence and transient absorption studies reveal that these complexes feature a lowest excited state with Pt(S)(2) --> tbubpy charge transfer to diimine character. Both complexes are photostable in deoxygenated solution; however, photolysis into the visible charge transfer band in air-saturated solution induces moderately efficient photooxidation. Photooxidation of 1 produces the dehydrogenation product (tbubpy)Pt(II)(1,2-diphenyl-1,2-ethenedithiolate) (4). By contrast, photooxidation of 2 produces S-oxygenated complexes in which one or both thiolate ligands are converted to sulfinate (-SO(2)R) ligands. Mechanistic photochemical studies and transient absorption spectroscopy reveal that photooxidation occurs by (1) energy transfer from the charge transfer to diimine excited state of 1 to (3)O(2) to produce (1)O(2) and (2) reaction between (1)O(2) and the ground state 1. Kinetic data indicates that excited state 1 produces (1)O(2) efficiently and that reaction between ground state 1 and (1)O(2) occurs with k approximately 3 x 10(8) M(-)(1) s(-)(1).     

Trimethylsilylverbindungen der Vb-Elemente. V. Molekül- und Kristallstruktur des Lithium-bis(trimethylsilyl)-arsenids · DME

Trimethylsilyl Derivatives of Vb-Elements. V. Molecular and Crystal Structure of Lithium Bis(trimethylsilyl)arsenide · DME Lithium bis(trimethylsilyl)arsenide · DME 1 obtained from tris(trimethylsilyl)-arsine and n-butyl or methyl lithium in 1,2-dimethoxyethane crystallizes monoclinic with {a = 1813(3); b = 1327(3); c = 968(1) pm; β = 119.3(1)°; Z = 4} at +20°C. Experimental conditions unfavourable for an X-ray structure determination caused high standard deviations of all structural parameters. The refinements of these values calculated with respect to the centrosymmetric space group C2/m converged at a relatively high R-value of 0.090. In contrast to the homologous antimonide lithium bis(trimethylsilyl)arsenide · DME 1 is found to be dimeric in solution as well as in the solid state. The four-membered ring built up by bis(trimethylsilyl)arsino groups and DME-coordinated lithium atoms in alternating sequence is planar; the carbon atoms statistically occupy positions on both sides of a mirror plane. Characteristic bond lengths and angles are: As? Si 230.7(7); As? Li 259(2); Li? O 205(4) and 215(4) pm; Si? As? Si 103.2(4)°; Li? As? Li 81(1)°; As? Li? As 99(1)° and Li? As? Si 115(1)°.     

Crystalline Na-Si(NN) derivatives [Si(NN)= Si((NCH2tBu)2C6H4-1,2)]: the silylenoid [Si(NN)OMe]-, the dianion [(NN)Si-Si(NN)]2-, and the radical anion c-[Si(NN)]3-

Reactions of the silylene Si[(NCH2Bu(t))2C6H4-1,2], [Si(NN)], with NaOMe, excess Na or 1/3 Na yield the X-ray-characterised crystalline compounds [Na(micro-Si(NN)OMe)(THF)(OEt2)]2 (2b), [Na(THF)2Si(NN)]2 (3) and [Na(THF)4][(Si(NN))3-c] (4).     

1,2-Dihydrotriazinyl-N-oxy free radicals

Triazinyl-N-oxy free radicals, 2-methyl-2,4,6-triphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (6a), 2,2,4,6-tetraphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (6b), 2,2-dimethyl-4,6-diphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (13), and 2,6-dimethyl-2,4-diphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (14), in which the unpaired electron is delocalized over three nitrogen atoms, have been prepared and characterized. A method has been devised for introducing an N-oxide function into the triazinyl core. Then, by using a Grignard reagent, substitution α to the N-oxide group was achieved and the resulting 1,2-dihydrotriazine-N-oxide oxidized into the corresponding nitroxide. Solution EPR spectra exhibit hyperfine splitting that confirms spin delocalization over the three nitrogen atoms of the triazinyl ring. They also show that spin delocalization diminishes with increasing distance for the coupling and is largest for nitrogen N1 and weakest for N5. Free radicals 6a and 13 are stable in the solid state and have been characterized by X-ray diffraction, but they tend to gradually degrade in solution. In the solid state, these two free radicals are arranged into antiferromagnetically exchange-coupled pairs, J=-5.2(6) for 6a and -3.7(4) cm(-1) for 13 (H=-2JS(1)S(2)).     

Raman and 29Si MAS NMR spectroscopy of framework materials containing three-membered rings

Raman and ²⁹Si MAS NMR spectroscopies are evaluated for the identification of three-membered rings (3MR) in framework oxide materials. Raman and ²⁹Si MAS NMR spectra from the 3MR-containing materials euclase, phenakite, clinohedrite, willemite, lovdarite, VPI-7, ZSM-18 and dipotassium zinc tetrasilicate are presented. The Raman spectra from these materials do not exhibit common bands representing vibrational modes assignable to individual 3MR. The dense beryllosilicate and zincosilicate minerals exhibit ²⁹Si MAS NMR resonances indicative of silicon positioned in 3MR while the molecular sieves lovdarite and VPI-7 give ²⁹Si MAS NMR resonances that can be assigned to silicons located at the center of “spiro-5” units that are constructed from two 3MR. Silicon atoms located in isolated 3MR in the molecular sieves ZSM-18 and dipotassium zinc tetrasilicate do not exhibit ²⁹Si MAS NMR resonances that can be distinguished from those assigned to silicons residing in 4MR and larger.The ²⁹Si MAS NMR spectra from the new materials VPI-8, VPI-9 and VPI-10 do not exhibit ²⁹Si MAS NMR resonances indicative of “spiro-5” units. The presence of isolated 3MR in these materials cannot be ruled out from the ²⁹Si MAS NMR spectroscopic results.     

Identification of solvated species present in concentrated and dilute sodium silicate solutions by combined 29Si NMR and SAXS studies

Both concentrated and diluted sodium silicate solutions have been investigated by combining (29)Si NMR spectroscopy and SAXS experiments. The chemical nature of the entities responsible for the high siliceous species solubility observed in such alkaline concentrated sodium silicate solutions and their evolution according to dilution have been identified. For the most concentrated solution ([Si]=7 mol/l; pH=11.56; Si/Na atomic ratio=1.71), the results evidence the preponderant presence of neutral Si(7)O(18)H(4)Na(4) complexes, which behave like colloids of about 0.6-0.8 nm able to form very small aggregates with an average size lower than 3 nm. Addition of distilled water to this initial concentrated solution leads, on one hand, to a doubling of the colloid size, i.e. 1.2-1.5 nm, and, on the other hand, to a progressive decrease of the aggregate size until their total disappearance. Such a behavior could be explained by considering, first, the dissociation of the neutral Si(7)O(18)H(4)Na(4) complexes present in the concentrated solution into Na(+) ions and charged (Si(7)O(18)H(4)Na(4-n))(n-) complexes (with 1 ≤ n ≤ 4) and, second, the condensation of these siliceous charged species in order to form larger (Si(7y)O(18y-z)H(4y-2z)Na((4-n)y))(ny-) colloids. The mean size of these colloids suggests that the condensation occurs between 2 and 8 (Si(7)O(18)H(4)Na(4-n))(n-) groups.     

K x-ray emission spectra from natural silicates

Chemical shifts of Si Kα_1,2 and Al Kα_1,2 lines in seven natural silicates has been measured comparatively to the lines of the pure elements. An attempt is made to correlate these shifts to the surroundings of Si and Al in the minerals.