Synthesis of Alkylcyclophosphanes from Red Phosphorus – molecular structure of i-Pr4P4 by gas electron diffraction

A polyphosphide of the type (NaP)_X 1 was obtained from red phosphorus and sodium in presence of small amounts of naphthalene in 1,2-dimethoxyethane as solvent. 1 reacts with alkyl halides in dependence on the kind of alkyl group with formation of different alkylcyclophosphanes. As definite compounds Me₅P₅ 2 , Et₅P₅ 3 (with not definite constitution), i-Pr₄P₄ 4 and t-Bu₄P₄ 5 could be isolated. 1 was identified by X-ray diffraction, 2 – 5 by means of their ¹H, ¹³C, ³¹P n.m.r. and mass spectra. The molecular structure of 4 was identified by electron diffraction.     

The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction

The electron scattering pattern of gaseous dicyclopentadienylberyllium, Cp₂Be, has been recorded from s = 2.00 to 39.00 Å⁻¹ with a nozzle temperature of about 120°C. Molecular models of D_5d symmetry or models containing one π-bonded and one σ-bonded Cp ring are not compatible with the data. The possibility the gaseous Cp₂Be consists fo a mixture D_5d and π-Cp, σ-Cp conformers is considered and rejected. A model of C_5v symmetry can be brought into satisfactory agreement with the data. It is also found that a slip sandwich model obtained from the C_5v model by moving sideways the ring which is at the greatest distance from Be, while keeping the two rings essentially parallel is compatible with the electron diffraction data. The best fit between experimental and calculated intensity curves is obtained with a model with a sideways slip of 0.8(1) Å. This model is similar to that indicated by the X-ray diffraction investigations by Wong and coworkers [4,5]. It is suggested that the potential energy of the molecule does not change much as the magnitude of the slip changes and that the molecule thus undergoes large amplitude vibration.     

Studies on Alkyl Metal Alkoxides of Aluminium,Gallium, and Indium,II. A Gas Phase Electron Diffraction Investigation of the Dimethylaluminium Methoxide Trimer, [(CH3)2AIOCH3]3

[(CH₃)₂AIOCH₃]₃ has been studied by gas phase electron diffraction. The mainmolecular parameters are R(A1? C) = 1.957(3) Å, R(A1? 0) = 1.861(3) 8, R(0? C)= 1.436(3) Å, <C? AI? C = ll7.3(0.8)°, <0? A1? 0 = 103.2(1.1)° and (AI? O? Al = 125.8(0.4)°. The three valences of the 0 atoms appear to be lying in one plane. The AL₃O₃ ring is nonplanar.     

Infrared spectra of edge-shared silicate tetrahedra

Dehydroxylated silica exhibits two well-defined infrared bands at 888 and 907 cm⁻¹ previously assigned to a highly reactive strained surface defect. Comparisons of the spectra of dehydroxylated silica to frequencies and intensities calculated using molecular orbital (MO) calculations and to spectra obtained for cyclodisiloxanes suggest that the strained surface defect consists of an edge-shared silicate tetrahedral ring. Changes in vibrational frequencies and peak intensities with ¹⁸O labeling for both the surface defect and the cyclodisiloxanes are consistent with those expected for ring vibrational modes. The IR bands associated with the strained edge-shared ring disappear when either the surface defects or the cyclodisiloxanes react with water. Reactions of the surface defect can be used to study how strain enhances the reactivity of Si-O-Si bonds for modeling phenomena such as stress corrosion cracking.     

Molecular structures of tris(dipivaloylmethanato) complexes of the lanthanide metals, Ln(dpm)3, studied by gas electron diffraction and density functional theory calculations: a comparison of the Ln-O Bond distances and enthalpies in Ln(dpm)3 complexes and the cubic sesquioxides, Ln2O3

The molecular structures of tris(dipivaloylmethanato)neodymium(III), Nd(dpm)3, and tris(dipivaloylmethanato)ytterbium(III), Yb(dpm)3, have been determined by gas electron diffraction (GED) and structure optimizations through density functional theory (DFT) calculations. Both molecules were found to have D3 molecular symmetry. The most important structure parameters (r(a) structure) are as follows (GED/DFT): Nd-O = 2.322(5)/2.383 A, Yb-O = 2.208(5)/2.243 A, O-Nb-O = 72.1(3)/71.3 degrees , and O-Yb-O = 75.3(2)/75.8 degrees . The twist angles of the LnO6 coordination polyhedron, defined as zero for prismatic and 30 degrees for antiprismatic coordination, were theta = 19.1(3)/14.2 degrees for Nd and 20.4(2)/19.2 degrees for Yb. Structure optimizations of La(dpm)3, Gd(dpm)3 Er(dpm)3, and Lu(dpm)3 by DFT also yielded equilibrium structures of D3 symmetry with bond distances of La-O = 2.438 A, Gd-O = 2.322 A, Er-O = 2.267 A, and Lu-O = 2.232 A. The Ln-O bond distances in 12 Ln(dpm)3 complexes studied by GED decrease in a nearly linear manner with the increasing atomic number (Z) of the metal atom, as do the Ln-O bond distances in the cubic modifications of 14 sesquioxides, Ln2O3. The bond distances in the dpm complexes are, however, about 2% shorter. The mean Ln-O bond rupture enthalpies of the cubic sesquioxides calculated from thermodynamic data in the literature vary in an irregular manner with the atomic number; the La-O, Gd-O, Tb-O, and Lu-O bonds are nearly equally strong, and the remaining bonds are significantly weaker. The Ln-O bond rupture enthalpies previously reported for 11 Ln(dpm)3 complexes are on the average 13 kJ mol(-1) or about 5% smaller than in the sesquioxides, but they vary in a similar manner along the series: it is suggested that the pattern reflects variations in the absolute enthalpies of the gaseous Ln atoms.     

The energetics of ions,atoms, and salts: Sodium and its chloride

In this study we reconcile three seemingly contradictory assertions regarding sodium chloride. First, gaseous sodium chloride is Coulombically bound and highly ionic. Second, upon pulling the molecule apart atomic sodium and chlorine are produced. This is somewhat surprising; despite the high ionicity of NaCl, since IP(Na) > EA(Cl). Third, heterolytically dissociating NaCl(g) into Na⁺+Cl⁺ costs more energy than ionizing gaseous Na into Na⁺+e^?. Does this violate Coulomb's law since the Na-Cl bond distance in NaCl(g) is greater than the average Na nucleus-valance electron distance?     

Syntheses, photophysical properties, and application of through-bond energy-transfer cassettes for biotechnology

We have designed fluorescent "through-bond energy-transfer cassettes" that can harvest energy of a relatively short wavelength (e.g., 490 nm), and emit it at appreciably longer wavelengths without significant loss of intensity. Probes of this type could be particularly useful in biotechnology for multiplexing experiments in which several different outputs are to be observed from a single excitation source. Cassettes 1-4 were designed, prepared, and studied as model systems to achieve this end. They were synthesized through convergent routes that feature coupling of specially prepared fluorescein- and rhodamine-derived fragments. The four cassettes were shown to emit strongly, with highly efficient energy transfer. Their emission maxima cover a broad range of wavelengths (broader than the four dye cassettes currently used for most high-throughput DNA sequencing), and they exhibit faster energy-transfer rates than a similar through-space energy-transfer cassette. Specifically, energy-transfer rates in these cassettes is around 6-7 ps, in contrast to a similar through-space energy-transfer system shown to have a decay time of around 35 ps. Moreover, the cassettes are considerably more stable to photobleaching than fluorescein, even though they each contain fluorescein-derived donors. This was confirmed by bulk fluorescent measurements, and in single-molecule-detection studies. Modification of a commercial automated DNA-sequencing apparatus to detect the emissions of these four energy-transfer cassettes enabled single-color dye-primer sequencing.     

Covalent versus Dative Bonds to Main Group Metals,a Useful Distinction

Textbooks of inorganic chemistry describe the formation of adducts by coordination of an electron donor to an electron acceptor, often using the amine-boranes, X₃N → BY₃, as examples. In the Lewis (electron dot) formulas of the compounds, the dative bond in H 3 N → BH₃ and the covalent bond in H₃C?CH₃ are both represented by a shared electron pair. In the simple molecular orbital or valence bond models the wave functions of both electron pairs would be constructed in the same manner from the appropriate sp³ type atomic orbitals on the bonded atoms; the difference between the covalent and the dative bond becomes apparent only after the orbital coefficients have been analyzed. This may be the reason why many structural chemists seem reluctant to distinguish between the two types of bonds. The object of this article is to remind the reader that the physiocochemical properties of covalent and dative bonds may be – and often are – quite different, and to show that a distinction between the two provides a basis for understanding the structures of a wide range of main group metal compounds.