A joint analysis of electron diffraction and spectroscopic data is carried out for BF3, PBr3, AsBr3, SbCl3, SeO2 and ClO2 in terms of the harmonic force fields. The scheme of analysis was extended to include the following spectroscopic observables: vibrational frequencies, rotational, Coriolis coupling and centrifugal distortion constants and, whenever available, those for the isotopic species. For ClO2 a simplified anharmonic analysis was also performed, the anharmonic spectroscopic observables being involved in this case.
Compelling evidence has been presented that the conventional harmonic approximation for the force field in terms of rectilinear internal coordinates yields the simplest satisfactory representation of diffraction and spectroscopic observations. However, a consistently better fit to experimental data was found when natural curvilinear internal coordinates were used. It is shown here that for the systems considered the joint analysis of data from various sources ensures that reliable and accurate values for equilibrium distances and force field parameters are obtained. The optimized values of spectroscopic constants, structural and force field parameters obtained are presented and compared with those available from the literature. 相似文献
The molecular structure of 1,1,2,2-tetrabromodisilane has been investigated using gas-phase electron diffraction data obtained at 110°C. At this temperature the molecules exist as a mixture of about equal parts (X = 0.5 ±0.2) of the two conformers with the H---Si---Si---H torsion angle equal to 180° (anti) or 60° (gauche). Assuming that the two conformers differ in their geometries only in the torsion angle φ, some of the important distance (ra) and angle () parameters are: r(Si---Si) = 2.349(19) Å, r(Si---Br) = 2.205(5) Å, r(Si---H) = 1.485 Å (assumed), Br---Si---Br = 110.1(1.6)°, Si---Si---Br = 107.1(1.2)° Si---Si---H = 108.6° (assumed). The error limits are 2σ. The observed conformational composition (Xanti = 0.5(0.2)) corresponds to an energy difference between the conformers of ΔE = E(gauche) — E(anti) = 0.5 ± 0.6 kcal mol−1, assuming ΔS = Rln2. 相似文献
A measuring system is described which permits study of all stages of combustion processes as functions of carrier gas, temperature, residence time and tube filling. The organic sample is fed at constant speed into a stream of carrier gas. The mixture reaches the combustion chamber within a few milliseconds via a transfer capillary. With the help of a viscous inlet system, a sample of the resulting reaction products is taken and fed into a mass spectrometer. Reaction time and temperature can be adjusted within wide ranges or varied continuously. A plot of the extent of reaction of the various combustion products against temperature at a chosen reaction time yields an oxidation-thermogram which gives a clear picture of the combustion process. It is evident from thermograms of selected compounds that the samples decompose in the presence of oxygen at appreciably lower temperatures than in inert gas. The primary step of the decomposition is “oxidative pyrolysis” which often leads to other products than “inert pyrolysis”. The intermediate products found are partly structurally specific and, especially with nitrogen-containing samples, are numerous and long-lived (for example, carbon monoxide, nitric oxide, cyanogen, hydrocyanic acid, cyanic acid and methyl cyanate). The notorious “difficult combustibility” is largely due to the fact that carbon monoxide, cyanic and hydrocyanic acids undergo complete combustion only at very high temperature. The combustion properties of the “empty tube” can be improved noticeably by a filling of quartz wool and markedly by partly filling with platinum wool. 相似文献
A practical search system for proton n.m.r. spectra is reported. The coding rules and search algorithms are described in detail. Data for 8000 spectra have been converted into a computer-readable file from printed charts. Several search tests are used to evaluate the usefulness of the search system, and various effects of experimental conditions such as different instruments, frequencies and solvents on recall efficiency are described. The results presented indicate that the system should be applicable to routine analytical work. 相似文献
The azacyclopentadienyl compounds (2,5-C4tBu2RHN)MCl3 (M = Ti, Zr, Hf; R = H, SiMe3) have been prepared as stable solids from the lithiated pyrroles and MCl4. The π-coordination of the azacyclopentadienyl ligands, as suggested from 13 C NMR data, has been confirmed for (2,5-C4tBu2H2N)TiCl3 by an X-ray diffraction study. 相似文献
meso-Tetraphenylporphyrinatothallium(III) cyanide, Tl(tpp)(CN), was previously assumed to be monomeric and has been confirmed by X-ray analysis to exist as two independent molecules in one asymmetric unit. This unit displays two square-pyramidal coordination geometries for the thallium atoms with the cyano ligand coordinated to both Tl atoms. It crystallizes in the triclinic space group P
, with a 10.003(3), b 16.231(7), c 21.277(8) Å, 89.98(3), β 90.57(3), γ 90.31(3)°' and z = 4. The structure was solved by direct methods. A total of 7995 unique reflections having I > 3σ(I) was measured with an automated diffractometer and used to refine the crystal structure to a conventional R factor of 6.05 %. The thallium-cyanide distances are 2.140(14) Å (for thallium(I)) and 2.277(14) Å (for thallium(2)) respectively, with thallium(1) situated 0.908 Å above the porphyrin ring and thallium(2) located 1.027 Å below the ring. IR and NMR spectroscopy p rovide complementary methods for investigation of the CN ligand. The characteristic band observed at 2160 cm−1 in the FTIR spectrum is assigned to the CN stretching in the Tl(tpp)(CN) complex. The 13C resonance of axial cyano ligand is observed with a pulse delay of 3.5 s at 24°C at 139.2 ppm (with 1J(205Tl-13C) 5394 and 1J(203Tl-13C) 5344 Hz). This observation disagrees with the conclusion, drawn from previous work, in that an exchange process involving the apical ligand explains the invisibility due to line broadening at 35°C of the 13C signal. 相似文献
The oxidation of Cp2NbCl2 with pure WF6 in SO2 solution yielded the cationic metallocene species [Cp2NbCl2]+[WF6]− essentially in quantitative yield. The same reaction carried out in the presence of either equimolar amounts or a two-fold excess of HCN led to the preparation of the new niobocenium salt [Cp2NbCl2]4+[WF6]2− which was studied by single crystal X-ray diffraction. This compound represents the first example of a structurally characterized metallocene-WF6− complex, and crystallizes in the tetragonal system: space group, P41212(No. 92), a = 11.083(8) Å, c = 48.285 (9) Å; Z = 8; R = 0.0759, RW = 0.0841. ab]Die Oxidation von Cp2NbCl2 mit reinem WF6 führt in SO2-Lösung zur Synthese von [Cp2NbCl2 ]+[WF6]− in nahezu quantitativer Ausbeute. Die analoge Reaktion führt unter Anwesenheit der äquimolaren Menge oder eines zweifachen Überschusses an HCN zur Ausbildung des Niobocenium-Komplexsalzes [Cp2NbCl2]4+ [WF6]2−[WCl6]2−, von dem eine Röntgenstrukturanalyse angefertigt wurde. Diese Verbindung repräsentiert den ersten structurell charakterisierten Vertreter eines Metallocen-WF6−-Komplexes und kristallisiert im tetragonalen System: Raumgruppe P41212 (Nr. 92), a = 11.083(8) Å, c = 48.285(9) Å; Z = 8; R = 0.0759, RW = 0.0841. kw]Niobium; X-ray diffraction; Oxidation; Metallocenes 相似文献
The photochemistry of three novel t-butylperester derivatives of fluorenone was examined and compared with unsubstituted fluorenone and a mono-t-butylperester of benzophenone using both conventional microsecond and nanosecond laser flash photolysis. On conventional microsecond flash photolysis in 2-propanol, all four fluorenone compounds gave transient absorption in the region 300–400 nm due to a ketyl radical formed from the abstraction of a hydrogen atom from the solvent by the upper excited triplet n—π* state of the fluorenone chromophore. This assignment was confirmed by a pH-dependent study on the transient absorption spectra. The nitro-t-butylperester derivative of fluorenone gave additional absorption above 400 nm due to species associated with the nitro group. No evidence for benzoyloxy radical formation could be found in non-hydrogen-atom-donating solvents with microsecond flash photolysis which is associated with homolysis of the perester groups. On nanosecond laser flash photolysis of the fluorenone compounds at 355 nm excitation in acetonitrile and hexa-fluorobenzene, transient absorptions were observed in the region 320–640 nm due to the corresponding triplet states. All the t-butylperester derivatives showed residual absorbances at longer time delays which were tentatively assigned to the corresponding benzoyloxy radicals produced by homolysis of the perester groups. In contrast, the mono-t-butylperester of benzophenone, included for comparison only, showed very weak transient absorption in the region 320–640 nm compared with that of the strong triplet of benzophenone under the same excitation conditions. The triplet absorptions and lifetimes of the fluorenone compounds were correlated with their photopolymerization activities in bulk methylmethacrylate monomer. In oxygenated solutions, the triplet absorptions of fluorenone and benzophenone were effectively quenched; however, long-lived transient growths were observed for all the t-butylperester derivatives. The intensities of these novel transient absorptions appear to correlate with the total number of t-butylperester groups in the fluorenone molecule and tentative assignments are discussed. 相似文献
Crosslinking is one of the effective routes for improving the orientation stability of poledpolymer films. The derivative of polyvinyl alcohol containing 4-nitro-4'-alkoxystilbene andphoto-crosslinkable cinnamyl groups as side chains has been synthesized. The in-situ simul-taneous photo-crosslinking poling of synthesized polymer films has been performed. Thesecond order nonlinear optical coefficient d_(33) of poled film is 11 pm/V. The SHG mea-surements show that the break-over temperature of SHG signal is raised obviously afterirradiation, its orientation stability is doubled as compared with that of non-crosslinkingsamples. 相似文献