Some thermodynamic aspects of the thermal degradation of wholly aromatic polyamides

The mechanism of thermal degradation of wholly aromatic polyamides has been investigated in the light of several thermodynamic parameters, such as resonance stabilization of the free radicals formed, bond enthalpy changes and entropy changes. It has been shown that the scission of the NH bond in the beginning is both thermodynamically and kinetically favoured over the scission of the aromatic ring-carbonyl carbon bond. CN bond and the aromatic ringNH bond in the polymer chain backbone. Formation of the major products and some minor products has been explained with the help of the proposed mechanism.     

Variation of bond polarizabilities of adsorbed pyrazine molecules on the silver electrode as a function of applied voltage from surface enhanced Raman scattering

An algorithm using the Wolkenstein approximation to calculate the bond polarizabilities of adsorbed pyrazine molecules on the Ag electrode from the surface Raman intensities has been employed. The result shows that the pyrazine molecule is end-on adsorbed on the electrode surface instead of the face-on configuration. The large variation of the CN bond polarizability with applied voltage shows that there is strong perturbation on the ring charge from the electrode and electron sharing between them is expected. The variation of the CC and CH bond polarizabilities is however relatively small. As the applied voltage is shifted toward −0.8 V (with respect to a saturated calomel electrode) the bond polarizabilities decrease, in general, while in the piperidine case the situation is opposite. This may be due to the different behaviour of the aromatic and saturated ring in surface enhanced Raman scattering (SERS).     

Gas-phase electron diffraction determination of the molecular structure of perfluoro(methyloxirane)

The molecular geometry of perfluoro(methyloxirane) has been studied using gas-phase electron diffraction data, effective least-squares refinement of the structure being achieved with the aid of constraints to limit the number of variable parameters. With the CCF₃ bond constrained to be 0.078 Å longer than the ring CC, the refined bond- length values CF (av.) = 1.323(2), CO (av.) = 1.410(8), and CC (ring) = 1.467(7) Å ($\text{r}$_g values, with e.s.d. in parentheses) were obtained; the angles between ring bonds and substituent CF bonds were CCF (av.) 121(1) and OCF (av.) 114(1)^o, the corresponding parameters involving the bulkier CCF₃ fragment being larger by 3^o in each case [∠CCCF₃ 124(1)^o∠OCCF₃ 117(2)^o]. The remaining refined parameters were ∠CCF(of CF₃) = 110.6(4)^o and τ , a torsion angle defining the orientation of the CF bonds of the CF₃ group with respect to ring bonds, = 29(2)^o. Dependent bond angles possessed the values 62.7 (COC), 58.7 [OCC (ring)], 108.3 [FCF (CF₃ group)], 114 [FCF (ring CF₂)], and 111^o (FCCF₃).     

Preparation and properties of metallacyclobutanes of nickel and palladium

Bis(phosphine)-3,3-dimethylnickela- and palladacyclobutanes have been prepared by intramolecular C-H insertion reaction of the corresponding dineopentyl metal complexes. Nickelacyclobutane complexes decompose when heated thereby undergoing competitive carbon-carbon bond cleavage to give isobutene and ethylene, with reductive elimination affording 1,1-dimethylcyclopropane and skeletal isomeri-zation of the metallacyclic ring yielding 3-methyl-1-butene, whereas the palladium analog gave no significant amounts of CC bond cleavage products.Added phos-phine was seen to have an effect on CC bond scission of nickelacyclobutane complexes. Nickelacyclobutane complexes in solution are thought to be in equilibrium with olefin-coordinated nickel-carbene complex on the basis of available experimental evidence from hydrogenolysis, carbene-trap reactions with olefins and reaction with carbon monoxide     

Structure and chemistry of 1-silafluorenyl dianion, its derivatives, and an organosilicon diradical dianion.

1-Silafluorene dianion was synthesized by potassium reduction of 1,1-dichloro-1-silafluorene in refluxing THF. The X-ray structure of 1,1-dipotassio-1-silafluorene (3b) shows C-C bond length equalization in the five-membered silole ring and C-C bond length alternation in the six-membered benzene rings, indicating aromatic delocalization of electrons in the silole ring. The downfield (29)Si chemical shift at 29.0 ppm and theoretical calculations also support electron delocalization in the silole ring of 3b. Dianion salt 3b underwent nucleophilic reactions with Me(3)SiCl and EtBr to form the corresponding disubstituted products. Benzaldehyde underwent reductive coupling in the presence of 3b. Slow oxidation of 3b yielded 1,1'-dipotassio-1,1'-bis(silafluorene) (16). The X-ray structure and (29)Si chemical shift (-38.0 ppm) of 16 indicate localized negative charges at the silicon atoms and no aromatic character. Heating a DME/hexane solution of 3b in the presence of 18-crown-6 led to a novel diradical dianion salt.     

Synthetic and structural studies on some 1,3-diselena- and 1,3-dithia-[3]ferrocenophanes of germanium and tin. crystal and molecular structure of 1,3-diselena-2,2- dichlorogermyl-[3]ferrocenophane

A series of [3]ferrocenophanes of general formula Fe(C₅H₄X)₂YCl₂ and the spiro compounds [Fe(C₅H₄X)₂]₂Ge (X = S, Se; Y = Ge, Sn) have been prepared by the reaction of ferrocene 1,1′-dithiol and ferrocene 1,1′-diselenol with tetrahalides of germanium and tin. Spectroscopic properties of the compounds are reported. In solution, the compounds are fluxional by a bridge reversal process. The crystal structure of 1,3-diselena-2,2-dichlorogermyl-[3]ferrocenophane at 163 K. has been determined by X-ray diffraction methods. At that temperature, crystals have space group P2₁/n with a 6.222(3), b 16.156(13), c 12.968(4) Å, β 97.53(1)° and Z = 4. Least-squares refinement gave R = 0.033 for 2834 unique significant reflections whose intensities were measured by counter diffractometry. The two SeGe bond lengths are 2.323 and 2.325(1) Å, with GeCl 2.148 and 2.161(1) Å. The SeGeSe bond angle is 118.2(1)°, ClGeCl 104.7(1)°, and SeGeCl angles range from 106.2 to 109.8(1)°. The SeC bond lengths are 1.901 and 1.904(5) Å, with CSeGe angles of 95.8 and 96.5(2)°. The cyclopentadienyl rings are in an eclipsed conformation with a mean twist angle of 2.7°, and are inclined to one another at 6.1°. The Se atoms are displaced from the ring planes by 0.17 and 0.20 Å yielding a non-bonded intramolecular Se…Se contact of 3.99 Å.     

Microwave spectrum,structure and dipole moment of cyclopentadienylberyllium hydride

Microwave spectra of C₅ H₅ BeH, C₅ H₅ BeD, ¹³CC₄ H₅ BeH, and ¹³CC₄ H₅ BeD are reported. The molecule is a C_5v symmetrical top. The BeH bond length was found to be 1.32 Å with an error limit of 0.01 Å and the CC bond length was determined as 1.423 Å with one standard deviation of 0.001 Å. The distance from the beryllium atom to the centre of the cyclopentadienyl ring, h, and the CH bond length were assumed to be 1.49 Å and 1.09 Å, respectively. The dipole moment was determined through the Stark effect to be 2.08 D with one standard deviation of 0.01 D. Four different vibrationally excited normal modes were identified and their frequencies determined by relative intensity measurements.     

The PP bond length in 1,2-diphenyl-1,2-diphospholane-1,2-disulphide compared with that in other compounds

The x-ray structure of 1,2-diphenyl-1,2-diphospholane-1,2-disulphide has been determined (PP 2.253 Å and PS 1.944 and 1.943 Å). In this compound the two phosphorus atoms are linked by three methylene groups into a five-membered heterocyclic ring, which prevents free rotation about the PP bond, and results in a non planar configuration for the SPPS part of the molecule. The PP bond length is very similar to that in the diphosphine disulphides which all have a trans-planar arrangement of phosphorus and sulphur atoms, and the phosphorus sulphides which are based on a tetrahedron of phosphorus atoms. This indicates that the bond length is unaffected by rotation, and hence does not involve any π interaction between phosphorus atoms. This is confirmed by the similarity of the PS bond lengths with those in a number of compounds, showing that the π electron on phosphorus is localised in the PS bond.     

The structure of 1-methyl-1-silaadamantane as determined by gas phase electron diffraction

The structure of 1-methyl-1-silaadamantane (MSA) has been determined by gas phase electron diffraction. There appears to be somewhat less ring strain at the silicon bridgehead of MSA than in the previously studied 1-methyl-1-silabicyclo[2.2.1]heptane (MSBH). The average SiC bond length [1.879(3) Å is comparable to those found in acyclic organosilicon systems. Also, the average CC bond length (1.547(2) Å) is only slightly longer than that observed for adamantane (1.540(2) Å). Valence angles at the silicon bridgehead experience only a moderate perturbation away from their unstrained tetrahedral values. On this basis it is expected that MSA should be somewhat less reactive than MSBH under S_N2 conditions according to the reaction mechanism suggested by L.H. Sommer.     

Electron diffraction investigation of the molecular structure of cyclopropyl silane

The molecular structure of cyclopropyl silane (CPS) has been determined by gas phase electron diffraction. Among other parameters the bond distances (r_a) are: C₁C₂ = 1.528(2) Å, C₂C₃ = 1.490(4) Å, SiC = 1.840(2) Å, CH = 1.095(3) Å. The angle between the ring plane and the bond SiC is 55.9(3)°. The introduction of a tilt of the silyl group is in agreement with the secondary effect of substituents. The role of the silicon 3d orbitals in the interpretation of the structural data of CPS is discussed. Our results support the interpretation of the SiC bond to silicon in terms of dπ conjugation. This causes an asymmetry in the structure of the ring. The (pd)_π bonding concept is considered to be the sum of three different contributions: ionic, steric and dπ conjugation.     

The molecular structure of p-bis(trimethylsilyl)-benzene from gas phase electron diffraction

Nearly regular tetrahedral silicon bond configuration and a considerably distorted ring characterize the p-bis(trimethylsilyl)benzene molecular geometry according to an electron diffraction study. The SiC_methyl bond is longer than the SiC_phenyl bond, in agreement with expectation but contrary to an X-ray diffraction determination. The extent of ring deformation is consistent with the electropositive character of the trimethylsilyl substituent and with the structural variations in other para-disubstituted benzene derivatives. The electron diffraction data are consistent with either free rotation around the SiC_phenyl bonds or with a rotamer deviating by about 15° from the eclipsed form. The following bond lengths (r_g, pm) and bond angles (°) have been determined with parenthesized estimated total errors: (CC)_mean 140.8(3), (C_ipso)(C_orthoC_meta) 1.6(7), (SiC)_mean 188.0(4), (SiC_methyl)(SiC_phenyl) 3.3(7), (CH)_methyl 111.3(3), CC_ipsoC 115.7(6), and C_phenylSiC_methyl 109.2(4).     

Cyclic polysilanes : VIII. The crystal structure of 1,2,3,4-tetra-tert-butyl-tetramethylcyclotetrasilane

The crystal structure of [Si(CH₃)(t-C₄H₉)]₄ has been determined by single crystal X-ray diffraction. The crystals are tetragonal, P4₂/n; a = b = 13.069(4), c = 7.880(2) Å, Z = 2. The structure was determined using 745 independent data and refined with anisotropic least-squares to a final unweighted R-value of 3.5%. Each tetrameric molecule was found to be arranged about a $\text{4}$ axis, with the independent crystallographic unit comprising one silicon atom, one methyl and one tert-butyl group. The four-membered ring of silicon atoms is nonplanar with an unusually large dihedral angle of 36.8°. The principal mean bond lengths are SiSi 2.377(1), SiC(methyl) 1.893(4), SiC(tert-butyl) 1.918(3) Å, and the SiSiSi bond angle is 86.99°. The SiSi bond length is somewhat longer than in other polysilanes.     

Die kristall- und molekúlstruktur der dimeren dimethylaluminium- und dimethylgallium-N,N′-dimethylacetamidine [(CH3)2M(NCH3)2CCH3]2 (M = Al,Ga)

Dimethylaluminium- and dimethylgallium-N,N′-dimethylacetamidine (I and II) are doubly associated forming a puckered eight-membered ring. They crystallize isostructurally in the monoclinic space group P2₁/c with two dimers per unit cell. The lattice constants of I are a 8.187, b 7.266, c 14.778 Å, β 103.58° and those of II a 8.163, b 7.277, c 14.835 Å, β 103.46°. The MN and the NC bond lengths within the rings are nearly equal, their mean values are for I: AlN 1.925 Å, CN 1.330 Å and for II: GaN 1.979 Å, CN 1.335 Å. This is also true for the exocyclic bond lengths with average values AlC 1.975 Å, NC 1.474 Å, CC 1.509 Å (for I) and GaC 1.998 Å, NC 1.484 Å and CC 1.507 Å (for II). The metal atoms are tetrahedrally coordinated, and the distortion is only slight. The final R-values are 0.034 and 0.056, respectively.     

Studies of alkylmetal alkoxides of aluminium,gallium and indium : IV. The molecular structure of dimethylaluminium t-butoxide dimer by gas phase electron diffraction

The electron diffraction data for gaseous dimethylaluminium t-butoxide dimer are consistent with a molecular model of effective D_2h symmetry. The Al₂O₂ ring is planar and the three valencies of the O atoms are lying in a plane. The t-butyl groups undergo nonhindered or slightly hindered internal rotation. The most important bond distances and valence angles are: AlO = 1.864(6), AlC = 1.962(15), OC = 1.419(12), CC = 1.533(5) Å, ∠AlOAl = 98.1(0.7), ∠CAlC = 121.7(1.7) and ∠OCC = 110.4(0.5)°.     

An examination of some effects of OH rotation in phenol and p-nitrophenol

A computational analysis of phenol and p-nitrophenol has been carried out with the objective of determining the extent to which the conjugation between the -OH group and the aromatic ring is affected by a 90° rotation of the former. The ab initio SCF-MO GAUSSIAN 82 procedure was used to compute optimized geometries, electronic densities and electrostatic potentials. The effect of rotation upon the structures and distributions of π electronic charge are rather small. However, the electrostatic potential reveals that as a consequence of rotation around the COH bond, there should be an overall preference for the ortho positions as the initial sites for electrophilic attack. The -NO₂ group deactivates the aromatic ring toward electrophiles and increases the positive character of the hydroxyl hydrogen.     

The structure of the metallated iridium complex [(C8H12)Ir{P(OC6H3Me)(OC6H4Me)2} {P(OCH2)3CMe}]

The crystal structure of [(C₈H₁₂)$\text{Ir{P(OC}$₆H₃Me)(OC₆H₄Me)₂} {P(OCH₂)₃CMe}] has been determined. a 18.32, b 18.98, c 9.35 Å, U 3251 ų, Pn2_1^a, Z = 4, R = 0.048, 2541 observed data.The coordination about the iridium atom is distorted trigonal bipyramidal; the two phosphorus atoms are equatorial, the σ-bonded carbon is axial, and the bidentate cyclooctadiene is bonded axialequatorial. The IrC(axial) bonds are longer than the IrC(equatorial) bonds: 2.22, 2.26; 2.17, 2.19 Å. The IrC(σ) bond length is 2.19 Å, not significantly different from the formally π-bonded C to Ir distances. The IrP lengths of 2.201 and 2.240 Å and the PIrP angle of 108.7° are normal. The longer IrP bond is in the five-membered chelate ring. The inertness to substitution is discussed.     

Palladium-catalyzed C-C bond formation involving aromatic CH activation. X-ray structure of 5-nitro-8-(2[2.2.1]bicycloheptyl)-1,2,3,4,4a,8b-hexahydro-1,4-methanobiphenylene

A new CC bond formation reaction involving activation of two aromatic CH bonds is reported. The X-ray structure of the title methanobiphenylene derivative is described.     

Application of Fourier transform Raman spectroscopy to a range of compounds of pharmaceutical interest

Fourier transform (FT) Raman spectra have been obtained for a range of synthetic and semisynthetic samples to evaluate the utility of the technique for the characterization of compounds of pharmaceutical interest. These spectra are compared with the IR KBr disc spectra. Examples of the additional, important information which may be extracted from Raman data in comparison with IR spectroscopy are reported for the range of chemical structures studied. The superior ability of Raman to characterize the stretching modes of the CH bond compared with IR spectroscopy is demonstrated for several compounds.The characterization of highly symmetric vibrational modes is one of the most prominent and useful advantages of Raman spectroscopy and several examples of this primary application are also cited. These include the assignment of the ring breathing frequency of monosubstituted benzene rings, the CCC symmetric skeletal mode of a tricyclic fused ring system, the NO symmetric stretch of aromatic nitro groups and the stretching vibrations of various double bonds, several of which possess a high degree of local symmetry.     

Crystal and molecular structure of 9-methyl-9-phenyl-9,10-dihydro-9-sila-3-azaanthrone

The crystal and molecular structure of 9-methyl-9-phenyl-9,10-dihydro-9-sila-3-azaanthrone has been determined from three-dimensional X-ray diffraction data. The title compound crystals are monoclinic, space group P2₁/b, a = 12.818(2), b = 16.508(2), c = 7.694(1) Å,γ = 105°, 34′(2), Z = 4 and D_cal = 1.278 g cm^?3. The structure was determined by direct methods and refined by full-matrix least-squares calculations in the block-diagonal anisotropic approximation for non-hydrogen atoms to R = 0.043 for 2190 independent reflections, registered at room temperature. This is the first crystal structure determination of a Si-dihydroanthracene derivative with two heterocycles and a planar carbon atom in the C10-position. The tricyclic fragment takes up a planar configuration, the silicon atom having a tetrahedral surrounding, with an endocyclic angle of 103.7(1)° and average bond length SiC, 1.862(1) Å. The CO, 1.220(2) Å, bond length in the carbonylic group exactly corresponds with the double bond length. Average distance NC is 1.335(3) Å, angle CNC, 116.5(2)°.     

Spectres de vibration et champs de force de valence de quelques composes π benzoate de methyle Cr(CO)3-nLn: II. Benzoate de methyle chrome tricarbonyle

A valence force field of methyl benzoate tricarbonylchromium is determined from the force field of methyl benzoate and that of hexacarbonylchromium. The results show the perturbation made both to the aromatic ring and to the carbonyl bonds by the chromiumring bond. This work is the first complete analysis of the vibrational spectra of methyl benzoate tricarbonylchromium.