Flow birefringence and conformational characteristics of molecules of para-and meta-isomers of polyoxyphenyl-benzoxazoleterephthalamides

Flow birefringence (FB) has been used to study sulphuric solutions of two homologous series of polyoxyphenylbenzoxazoleterephthalamides (POPhBT) differing in the position (para- or meta-) of phenyl ring in the chain. In the framework of the FB method alone by using the theory of flow birefringence for kinetically rigid wormlike chains, it was possible to determine quantitatively the optical anisotropy of the monomer unit Δa = (330 ± 30) 10^?25 cm^?3 and the length of the Kuhn segment $\text{A = (330 ± 30)}\text{A}\text{?}$ and $\text{A = (115 ± 20)}\text{A}\text{?}$ for para- and meta-isomers, respectively. Analysis of possible mechanisms of flexibility in the chains of both polymers gives theoretical values of the rigidity parameter A in good agreement with experimental values of A. confirming the validity of the molecular models used.     

Hydrodynamic and dynamo-optical properties and conformation of cyanoethyl cellulose molecules in solution

Translational diffusion, velocity sedimentation and viscosity in acetone as well as flow birefringence (FB) and viscosity in cyclohexanone have been investigated for cyanoethyl cellulose (CEC) with degree of substitution 2.6 in the range of M = (24.5?317) × 10³. The dependences of [ν], S_o and D_o on M were obtained. The value of the hydrodynamic constant is $\text{A}{}_0\text{= 3.27 × 10}{}^{\mathrm{?10}}\text{erg deg}{}^{\mathrm{?1}}\text{mol}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$. According to hydrodynamic data, the equilibrium rigidity of CEC molecules is characterized by the length of the Kuhn segment $\text{A = 240 ? 350}\text{A}\text{?}$ and the coefficient of hindrance to intramolecular motion σ = 4.5-5.4. The hydrodynamic diameter of the chain is 8–14 Å. According to the FB data, the value of A is 260 Å. This value is in agreement with hydrodynamic data. The high value of optical anisotropy of the monomer unit, a_| - a_⊥ = 17.8 × 10^?25 cm³, is in agreement with the structure and anisotropy of the substituting groups, and the investigation of orientation angles of FB leads to the conclusion that, apart from high equilibrium rigidity, CEC in solution is characterized by considerable kinetic chain flexibility. The data for CEC are compared with the characteristics of other cellulose esters and ethers.     

Hydrodynamic properties and equilibrium rigidity of polyamidobenzimidazole molecules in dimethylacetamide and sulphuric acid

Translational diffusion, velocity sedimentation and viscometry of polyamidobenzimidazole (PABI) solutions in the range of M = (1–61) · 10³ have been investigated in N,N-dimethylacetamide (DMA) and 98% H₂SO₄. The dependences of D₀, S₀ and [η] on M were obtained. Tsvetkov-Klenin's hydrodynamic invariant was found to be $\text{A}{}_0\text{= 3.55 · 10}{}^{\mathrm{?10}}\text{erg deg}{}^{\mathrm{?1}}\text{mol}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$. The equilibrium rigidity of PABI molecules was characterized by the length of the Kuhn segment $\text{A = 250 ± 100}\text{A}\text{?}$. The chain diameter was $\text{7 ± 4}\text{A}\text{?}$. The values of A in 98% H₂SO₄ and in an aprotic solvent, DMA, were virtually identical, implying that the rigid-chain conformation of PABI molecules in 98% H₂SO₄ is due to their geometrical structure rather than to the protonization of amide bonds. The significance of the latter evidently increases in PABI solutions in 100% H₂SO₄ in which A is 1.5 times as high. The decrease in rigidity of PABI as compared to that of poly-p-phenylene terephthalamide ($\text{A = 400 ± 100}\text{A}\text{?}$) is in reasonable agreement with the presence of imidazole rings in PABI molecules. The presence of these rings results in kinks in the PABI chain with angles of about 30° and hence, in the depature from parallelism of rotating bonds.     

Light-scattering of polyamide hydrazide solutions and conformational characteristics of its molecules

Light-scattering measurements have been carried out for 18 samples of polyamide hydrazide (PAH) in dimethylsulphoxide (DMS) with an over 10-fold variation in molecular weights. It was shown that the excluded volume effect in DMS does not appreciably affect the size of PAH molecules. The length of the Kuhn segment (A ≈ 240 · 10^?8 cm) was estimated from the experimental determinations of molecular weights $\text{M}{}_{\mathrm{w}}$ and the radii of gyration 〈R²〉_z. Viscometric determination of the rigidity of PAH chains yields the value of A ≈ 200 · 10^?8 cm and the hydrodynamic diameter d ≈ 5 · 10^?8 cm. The whole collection of data shows that PAH is a typical rigid-chain polymer; the extended conformation is due to the backbone rigidity tather than to the excluded volume effects.     

Conformational properties of aromatic polyesters of terephthalic acid

Hydrodynamic (sedimentation, diffusion and viscometry) and optical [flow birefringence (FB)] properties of solutions of p-aromatic copolyesters obtained by the polycondensation of three components

with various molar ratios of N, D and T, have been investigated. Independent methods of quantitative evaluation of the parameter of equilibrium rigidity, the length of the statistical Kuhn segment, for the most rigid of copolymers soluble in tetrachloroethane with the molar ratio of components N:D:T = 0.7:0.3:1 gave the values of A = 160·10^?10 m (translational friction) and 180·10^?10 m (FB). The equilibrium chain rigidity of an insoluble p-aromatic polyester containing no D component, ensuring chain flexibility and polymer solubility, was evaluated from the dependence of birefringence on copolymer composition. The value of the statistical Kuhn segment obtained in this case A = (500–800)·10^?10m is close to that for the most rigid aromatic polyamide-poly-p-benzamide.     

Investigation of the B2Σ and test of the A2II interaction potential for Na(32P)-Ar from high-resolution differential scattering measurements

We have measured differential cross sections for scattering of laser-excited Na(3²P_{$\text{3}\text{2}$}) by Ar(¹S₀) at thermal collision energies with high angular resolution (0.1°). In the investigated range of scattering angles (1°–15°) the cross sections contain contributions from scattering along the excited state B²Σ potential (rainbow scattering) and the A²II potential (supernumerary rainbows). By performing fit calculations in which the spectroscopically determined A²II potential was adopted we were able to obtain information about the B²Σ potential. With the assumption of a Lennard-Jones (12.6) potential shape we obtain a well depth ?^BΣ = (0.14±0.02)×10^?3 au and an equilibrium distance r_m^BΣ = 10.4±1.0 au. This work presents the first experimental determination of the B²Σ potential well parameters.     

The crystal structure of polyacrylonitrile

Polyacrylonitrile (PAN) has been prepared by a free-radical mechanism using azobisisobutyronitrile as the initiator. Fibres were produced by extruding a solution of this polymer into dilute sodium thiocyanate. Wide-angle X-ray analysis showed that the resulting fibres exhibited a moderate degree of crystalline order. An orthorhombic unit-cell has been proposed to account for the observed reflections, the cell constants are: $\text{a = 21·48 ± 0·02}\text{A}\text{?}\text{; b = 11·55 ± 0·03}\text{A}\text{?}\text{; c = 7·096 ± 0·003}\text{A}\text{?}$. A helical structure has been proposed for PAN with four monomer units per crystallographic repeat. The probability of a syndiotactic configuration has been suggested to account for the meridional reflections; the calculated density of the proposed cell (1·199 g cm^?3) was found to agree very well with values quoted in the literature.     

Measurement of molecular dimensions of polystyrene chains in the bulk polymer by low angle neutron diffraction

The radius of gyration of polystyrene molecules in the bulk polymer has been measured by low angle neutron scattering from a dilute solid solution of poly proto-styrene in a matrix of polydeutero-styrene. The radius of gyration is $\text{90}\text{A}\text{?}\text{± 5}$ percent ($\text{M}{}_{\mathrm{<mtext>a</mtext>}}\text{= 97,200}$), which agrees within experimental error with the unperturbed dimensions of the polymer chain (? 84 Å) as determined from the solution properties of the polymer.     

Microwave-optical double resonance in molecular ions. Co+

Microwave—optical double resonance signals have been detected in a mass-selected ion-beam spectrometer for ¹²C¹⁶O⁺. With the optical excitation of fluorescence from the R₁ ($\text{1}\text{2}$) line of the (0,0) band of the A²Π_{$\text{3}\text{2}$} ← X²∑ transition of 20350.6 cm^?1, the microwave resonances occurred at 118101.8 ± 0.2 MHz and at 117694 ± 2 MHz corresponding to the N= 1, J = $\text{3}\text{2}$ ← N = 0, J = $\text{1}\text{2}$ and the N = 1, J = $\text{1}\text{2}$ ← N = 0, J = $\text{1}\text{2}$ transitions.     

X-ray study of Hg2Cl2Br2 and HgCl2HgBr2 reactions in solid state

The reactions (I) Hg₂Cl₂(s) + Br₂(g) and (II) HgCl₂(s) + HgBr₂(s) have been investigated by an X-ray method. Both the reactions yield two forms of the mixed halide HgClBr, designated as α-HgClBr and β-HgClBr. The cell parameters of the two are as follows:α-HgClBr: $\text{a = 6.196}\text{A}\text{?}$, $\text{b = 13.12}\text{A}\text{?}$, $\text{c = 4.37}\text{A}\text{?}$, z = 4, ? = 5.91 g/cm³. The powder pattern and cell parameters are similar to that of HgCl₂. Therefore it is probable that the chlorine atoms, in the linear halogenHghalogen molecules of HgCl₂ structure have been replaced by bromines, and since the radius of the bromine atom is larger than that of chlorine, the lattice is larger in this case.β-HgClBr: $\text{a = 6.78}\text{A}\text{?}$, $\text{b = 13.175}\text{A}\text{?}$, $\text{c = 4.17}\text{A}\text{?}$, z = 4, ? = 5.40. These parameters are the same as those reported in the literature for β-Hg(ClBr)₂, and its X-ray powder pattern is similar to HgCl₂. Therefore this phase also has linear halogenHghalogen molecules but the distribution of Cl and Br atoms is perhaps random.Heating the products (I) and (II) up to the melting point increases the amount of α phase and decreases the β phase, whereas crystallization increases the β phase. DTA study has supported the X-ray findings.     

Molecular structures of acetyl fluoride and acetyl iodide

The molecular structures of acetyl fluoride and acetyl iodide have been determined by making use of the average distances obtained in the present study together with the moments of inertia reported in the literature. The large amplitude theory for a molecule with an internal top was used in the joint analysis. The thermal-average values of internuclear distances r_g and the bond angles in the zero-point average structure Φ_z are as follows: r_g(C-O) = 1.185 ±0.002 $\text{}\text{?}$rA, r_g(C-F) = 1.362± 0.002 Å, r_g(C-C) = 1.505±0.002 Å, r_g(C-H) = 1.101 ±0.004 Å, Φ_z(OCF) = 120.7°±0.4°,Φ_z(CCF) = 110.5° ± 0.5°, Φ_z(HCH) = 109.3°±0.6° tilt(CH₃) = 0.1°±1°, for acetyl fluoride; r_g(C=O) = 1.198±0.013 $\text{}\text{?}$rA, r_g(C-I) = 2.217±0.009 Å, r_g(C-C) = 1.492±0.015 $\text{}\text{?}$rA, r_g(C-H) = 1.101 ± 0.004 Å, Φ_z(OCI) = 119.5°± 0.8°,Φ_z(CCI) = 111.7°±0.9°, Φ_z(HCH) = 110.8°±0.8° and tilt(CH₃) = 1.7°+5.4° for acetyl iodide. The uncertainties represent the estimated limits of error. The barriers V₃ to internal rotation have been reanalyzed making use of the effective moments of inertia of the methyl top estimated on the basis of the large amplitude theory and resulted in 1039 and 1176 cal mol^?1 for acetyl fluoride and acetyl iodide, respectively. The structure parameters have been compared with those of other CH₃COX (X = Cl, Br, H, CH₃) type molecules.     

The influence of the orientation of the NO molecule upon the chemiluminescent reaction NO+O3→NO2*+O2

A beam of state-selected NO molecules (J = Ω = $\text{3}\text{2}$) has been produced by an electrostatic hexapole and has been collided with O₃ molecules in a scattering chamber. The E-field dependence of the chemiluminescent cross section, σ_hr, has been investigated and resulted in the determination of the M-dependence of σ_hr: σ_hr (M)/σ₀ = 1.192±0.009, 0.0848±0.015, 1.177±0.015, 0.783±0.009 for M = $\text{3}\text{2}$, $\text{1}\text{2}$, $\text{?1}\text{2}$ and $\text{?3}\text{2}$, respectively. Application of the Legendre expansion technique and the density matrix formalism provided a deconvoluted σ_hr(γ), for a single angle of attack γ of the NO axis, expressed in simple model functions with adjustable parameters. From this analysis it is concluded that chemiluminescence only occurs when cos γ ≈ 1, the “end-on-head” orientation of NO yielding ≈ 30% of all collected light, and when cos γ ≈ ?0.275, the “broad-side-tail” orientation of NO yielding the remaining 70%. The steric factors belonging to these reactive orientations have been estimated and are S₁ = 0.25±0.07 and S₂ = 0.40±0.09, respectively. The observed dependence of σ_hr has been confronted with the rules of Woodward and Hoffman. Although there are indeed two symmetries (bpl and cpl) correlating the electron orbitals of the reactants and the products, these rules do not lead to an explanation of the steric effects of the NO+O₃ reaction.     

Hydrodynamic properties and conformation of molecules of para- and meta-isomers of polyoxyphenylbenzoxazoleterephthalamide

Translational diffusion and intrinsic viscosity in 96% H₂SO₄ have been investigated for 20 samples of the para-isomer of polyoxyphenylbenzoxazoleterephthalamide (PpOPhBT) and 17 samples of its meta-isomer (PmOPhBT). The Mark-Kuhn equations have been obtained. The equilibrium rigidity of macromolecules in solution, calculated by using the wormlike chain theory, was characterized for PpOPhBT by the length of the Kuhn segment $\text{A = 320}\text{A}\text{?}$, the number of monomer units in a segment s = 17 and the coefficient of hindrance to intramolecular rotation in the chain σ = 1.5. For PmOPhBT the corresponding values are: $\text{A = 96}\text{A}\text{?}$, s = 5.9 and σ = 1.6. Analysis of flexibility mechanisms was carried out for PpOPhBT and PmOPhBT chains in solution.     

Fluorescence lifetime studies of no a 2Σ+(ν = 5, N = 9), B2Π32(ν = 8, J = 8.5), CwΠ32(ν = 1, J = 8.5), D2Σ+ (ν = 0, N = 5) and D2Σ+(ν = 1, N = 9)

Fluorescence decay profiles of NO excited levels slightly above the dissociation limit have been measured by a single-photon counting technique with nanosecond pulse excitation using an iodine flash lamp. Three iodine atomic lines in the vicinity of 180 nm are found to bring NO molecules into the levels A²Σ⁺(ν = 5, N = 9), B²²Π_{$\text{3}\text{2}$}(ν = 8, J = 8.5), C²Π_{$\text{3}\text{2}$}(ν = 1, J = 8.5), D²Σ⁺(ν = 0, N = 5) and D²Σ⁺(ν = 1, N = 9). Extrapolated zero-pressure lifetimes for single rotational levels are obtained, except for the C state where only a lifetime of ⩽0.4 ns was obtained. Self-quenching rate constants are also determined under higher-pressure conditions. Helium was found to quench the NO A²Σ⁺(ν′ = 5) fluorescence very efficiently.     

Spectrophotometric and potentiometric study of the cobalt(II)-thiocyanate system in aqueous medium

The cobalt(II)—thiocyanate system was spectrophotometrically studied at 2.0 M ionic strength (NaClO₄) and 25°C. The following formation constants were obtained: β₁ = 6.9 M^?, β₂ = 28.9 M^?2, β₃ = 12.1 M^?3 and β₄ = 1.30 M^?4. Three wavelengths were considered, 515, 590 and 615 nm, and the molar absorptivities of each species were calculated. Linear relationships were obtained for $\text{ε}$ vs $\text{n}$ and α_i. There is strong evidence that the tetrahedral [Co(SCN)₄]^2? is virtually the only species absorbing at 590 and 615 nm. An indirect potentiometric method led to comparable equilibrium constants. The cadmium(II)—thiocyanate formation constants used in the indirect method, under the same conditions, were found to be β₁ = 21.51 ± 0.09 M^?1, β₂ = 123 ± 1 M^?2, β₃ = 130 ± 3 M^?3 and β₄ = 173 ± 1.2 M^?4, in good agreement with earlier literature data.     

The enthalpies of formation of bis-(benzene)molybdenum and of bis-(toluene)tungsten

Microcalorimetric measurements at 520–523 K of the heats of thermal decomposition and of iodination of bis-(benzene)molybdenum and of bis-(toluene)tungsten have led to the values (kJ mol^?): ΔH^o_f[Mo(η-C₆H₆)₂, c] = (235.3 ± 8) and ΔH^o_f[W(η⁶-C₇H₈)₂, c] = (242.2 ± 8) for the standard enthalpies of formation at 25°C. The corresponding ΔH^o_f(g) values, using available and estimated enthalpies of sublimation, are (329.9 ± 11) and 352.2 ± 11) respectively, from which the metalligand mean bond-dissociation enthalpies, $\text{D}$(Mo—benzene) = (247.0 ± 6) and $\text{D}$(W—toluene) = (304.0 ± 6) kJ mol^?1, are derived.     

Experimental and theoretical investigations of the nuclear quadrupole resonance of 127I in orthoperiodic acid,H5IO6

Pure nuclear quadrupole resonance of ¹²⁷I in H₅IO₆ has been reported to occur at 44.976 MHz (m = ±$\text{3}\text{2}$?m = ±$\text{1}\text{2}$ transition) and 83.960 MHz (m = ±$\text{5}\text{2}$? m = ±$\text{3}\text{2}$ transition) at 296 K (ref. 1). Using an externally quenched frequency modulated super regenerative spectrometer Zeeman studies have been performed at 296 K on single crystals of H₅IO₆ growm from aqueous solution by observation of the ±$\text{sol3}\text{2}$?±$\text{1}\text{2}$ transition. The zero-splitting locus method has been employed for the determination of the EFG parameters. The data show that there are two physically inequivalent sites, having average asymmetry parameters, η, of 0.25, located in the unit cell. The maximum field gradient is found to lie along the short bond I-O(2) as suggested by Rama Rao and Weiss (ref. 1). The EFG parameters obtained by detailed molecular orbital calculations (CNDO/2 and INDO method) are compared with the experimental results.     

Electric birefringence of solutions of aromatic poly(amide hydrazide) in dimethyl sulphoxide

Birefringence in a pulsed electric field has been investigated for solutions of para-aromatic polyamid hydrazide (PAH) in dimethyl sulphoxide. The values of the specific Kerr constant K extrapolated to zero concentration have been determined. The experimental data are adequately described by the theoretical dependence of K on the contour length of the PAH molecules for kinetically rigid wormlike chains. The angle formed by the dipole moment of the monomer unit and the direction of the PAH chain was found to be 61.5 ± 1.5°. Comparison between relaxation times obtained from the curves of the decay of birefringence after the end of the electric pulse and the intrinsic viscosities of some PAH samples confirm the conclusion about high kinetic rigidity of PAH macromolecules.     

The importance of spin relaxation in the delayed fluorescence of molecular crystals

Deactivation rate constants of spin-orbital excited Br atoms in the reactions Br(²P_{$\text{1}\text{2}$}) + O₂ → Br(²P_{$\text{3}\text{2}$}) + O₂ (k₁), and Br(²P_{$\text{1}\text{2}$}) + NO → Br(²P_{$\text{3}\text{2}$}) + NO (k₄) have been measured with a photodissociative IBr laser on the electronic transition ²P_{$\text{1}\text{2}$}?²P_{$\text{3}\text{2}$} in the Br atom (λ = 2.7 μm). The values obtained are (6.4 ± 1.8) × 10^?14 cm³ s^?1 and (1.9 ± 0.6) × 10^?12 cm³ s^?1, respectively. Comparison with published data leads to the conclusion that, contrary to a widely accepted point of view, the high rate constants for the quenching of excited halogen atoms are due to resonant energy transfer processes and not to the paramagnetic nature of the quencher.