Jimmy S. Hwang

Electron spin resonance for quantitative assay of chlorpromazine in drug formulations by oxidation with cerium(IV) in sulfuric acid media.

For the first time electron spin resonance (ESR) spectroscopic technique has been used for a highly selective determination of chlorpromazine. The method is based on the oxidation reaction of chlorpromazine with cerium(IV) in sulfuric acid media. In this method 3.8x10(-3) mol dm(-3) cerium(IV) was used in 0.035 mol dm(-3) sulfuric acid with the ESR spectra recorded at room temperature. A calibration equation of the following form was obtained over the linear concentration range of 10-100 ppm with a correlation coefficient (r) of 0.999: A=1.355+0.0382C. The results obtained by the ESR method were found to be comparable with those obtained by the British Pharmacopoeia (BP) method. The method suffers no interferences excipients rendering the method suitable for determination of this drug in pharmaceutical preparations.

Catalytic activity and selectivity of group VIB metal carbonyl complexes in dehydrohalogenation reactions

Abstract The homogeneous catalytic activities and selectivities of a series of complexes ArM(CO) 3 , where M = Cr,Mo,W, and their derivatives in dehydrohalogenation reactions have been investigated. In addition to dehydrohalogenation, double-bond migrations also occurred. For Mo-based catalysts, their activity decreased and the induction time increased in the order: (Tol)Mo(CO) 3 , (Mes)Mo(CO) 3 , Mo(CO) 6 and (CH 3 CN) 3 Mo(CO) 3 . Phosphine ligands attached to the metal poisoned the catalytic activity of the system. For W derivatives, (Mes)W(CO) 3 > W(CO) 6 . The ArCr(CO) 3 catalysts were mild and their catalytic activity was not greatly affected by the nature of the arene ligand attached on the metal. Their approximate catalytic activity decreased in the order: anisole, methyl benzoate, benzene, toluene, p -xylene, mesitylene and tetralin. The catalytic activity with respect to the metal was: Mo > W > Cr. A coordinative unsaturation mechanism, probably of the redox type, is proposed. Preliminary ESR study shows that free radicals formed in a homogeneous solution of 2-chloro-2-methylbutane and Mo(CO) 6 can be trapped using a phenyl N -butylnitrone spin trap, and the resulting six-line ESR spectrum has a g -factor of 2.0045.

ESR studies of the interaction of propagating radicals with diacetylenes

ESR studies were carried out for the polymerization systems of some vinyl monomers in the presence of diacetylenic compounds in order to determine the nature of interaction between the propagating radicals with the diacetylenes. The systems used in this study were methylmethacrylate, styrene and methylacrylate with diphenylbutadiyne and phenyl-4′-vinylphenylbutadiyne using 2,2′-azobisisobutyronitrile as an initiator. ESR spectra showed a broad triplet for the styrene system, which became exchange narrowed for the methylmethacrylate system. This is explained in terms of increased interaction between the propagating radicals and the diacetylene for methylmethacrylate than styrene. The propagating radicals are considered to be trapped temporalily by the diacetylenes during polymerization, but they do not react until the polymerization system becomes extreemly viscous.

Evidence of tilt angle in an 8CB-10CB mixture

Abstract EPR spectra using perdeuterated 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxy as spin probe on a mixture of p - n -octyl- p ′ -cyanobiphenyl and p - n -decyl- p ′ -cyanobiphenyl at a 0.30106 mole fraction for p - n -decyl- p ′ -cyanobiphenyl revealed the existence of a tilt angle that has been attributed to the longer p - n -decyl- p ′ -cyanobiphenyl molecules. Temperature control to within ±0.01°C revealed the existence of two-phase regions across the smectic A-to-nematic and the nematic-to-isotropic transitions which point to first-order transitions.

Frequency dependent study of the correlation functions in EPR spectroscopy — the Cole–Davidson approach: 1. Perdeuterated 2,2,6,6-tetramethyl-4-piperidone N-oxide in toluene

EPR linewidth measurements of PD-Tempone in toluene at 1 (L-Band), 4 (S-Band), 9 (X-Band) and 34 GHz (Q-Band) microwave frequencies indicate the presence of a distribution of relaxation times. The empirical response parameter introduced by Cole-Davidson for the analysis of dielectric relaxation in liquids has been used for the analysis of EPR relaxation data in the low frequency region. The Cole-Davidson parameter can assume values in the range 0 < beta < or = 1. When beta = 1, one obtains the Debye-type spectral density. The calculated linewidth data at 1 GHz agrees with a Cole-Davidson distribution function with a width parameter 0.83 +/- 0.04 for a spherical solute. Beta < 1 at L-band suggests the presence of an asymmetrical distribution of relaxation times associated with different modes of relaxation mechanisms or internal molecular motions. This study shows EPR experiments at low microwave frequencies are more sensitive to the shape of the correlation function.

Intramolecular and reorientation dynamics of bis(triphenylphosphine)-3,6-di-tert-butyl-4,5-dimethoxy-o-semiquinone complex of copper(I) in viscous media

Abstract Two simultaneous dynamical processes are studied in solutions of varying viscosity by using the EPR linewidth data of a Cu(I) o-semiquinone complex with two triphenylphosphine ligands (PPh 3 ), i.e. the overall reorientation (tumbling) of the complex and intramolecular exchange of the PPh 3 ligands in the coordination sphere of the complex. It is shown that for fast motion the line broadening of the hyperfine components is due to both modulation of the anisotropic magnetic parameters by rotational diffusion and to modulation of the isotropic part of the 31 P hyperfine interaction by intramolecular exchange. The intramolecular dynamics of the coordination sphere appears to be dependent on the solvent dynamics.

Electron paramagnetic resonance application to polymers: 4. The segmental volume of poly(phenylacetylene) using different nitroxide spin probes

Electron paramagnetic resonance (e.p.r.) studies of T50G of poly(phenylacetylene) have been carried out at X-band as a function of spin probe size over a wide range of temperatures. It was found that T50G increased with increasing probe size. The polymer glass transition temperature Tg, determined from the break on the plot of ln TRversus1T for tempone in poly(phenylacetylene), was 296 K. The apparent activation enthalpy for the Tg relaxation process increased with increasing probe size, while the segmental size of the polymer appeared to be independent of the nitroxide used, and has a range of 898–922 A3. The segmental volume measured by the spin probe technique is in excellent agreement with the volume of 970 A3 of 10 repeat units on an average chain based on number-average molecular weight of 960. Thus our results showed that there are 17–20 backbone atoms in the segment of poly(phenylacetylene). Assuming the polymer segment to be of spherical shape, the diameter of the sphere was found to have a range of 11.7–12.4 A.

Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R∥/R⊥) whereR∥ andR⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10−3 in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.

An EPR line shape study of anisotropic rotational reorientation and slow tumbling in liquid and frozen jojoba oil

Abstract Spin probe investigation of jojoba oil was carried out by electron paramagnetic rresonance (EPR) spectroscopy. The spin probe used was 2,2,6,6-tetramethyl-4-piperidone- N -oxide. The EPR line shape studies were carried out in the lower temperature range of 192 to 275 K to test the applicability of the stochastic Liouville theory in the simulation of EPR line shapes where earlier relaxation theories do not apply. In an earlier study, this system was analysed by employing rotational diffusion at the fast-motional region. The results show that PD-Tempone exhibits asymmetric rotational diffusion with N = 3.3 at an axis z′= Y in the plane of the molecule and perpendicular to the NO bond direction. In this investigation we have extended the temperature range to lower temperatures and observed slow tumbling EPR spectra. It is shown that the stochastic Liouville method can be used to simulate all but two of the experimentally observed EPR spectra in the slow-motional region and details of the slow-motional line shape are sensitive to the anisotropy of rotation and showed good agreement for a moderate jump model. From the computer simulation of EPR line shapes it is found that the information obtained on τ R , and N in the motional-narrowing region can be extrapolated into the slow-tumbling region. It is also found that ln (τ R ) is linear in 1/ T in the temperature range studied and the resulting activation energy for rotation is 51 kJ/mol. The two EPR spectra at 240 and 231 K were found to exhibit the effects of anisotropic viscosity observed by B irell for nitroxides oriented in tubular cavities in inclusion crystals in which the molecule is free to rotate about the long axis but with its rotation hindered about the other two axes because of the cavity geometry. These results proved that the slow-tumbling spectra were very sensitive to the effects of anisotropy in the viscosity.

Magnetism and electronic structure of triplet binuclear niobium complexes in inorganic glasses, organic ligand environment, and polymers

We investigated paramagnetic properties of binuclear niobium complexes Nb–O–Nb with two nonequivalent Nb4+ ions in lithium–niobium phosphate glasses (LNPG), in the environment of catechol/ortho-quinone ligands and in polyethylene. Experimental electron paramagnetic resonance spectrum analysis revealed nonequivalent distribution of the charge and electron spin density between two Nb atoms. Mechanochemical interaction of LNPG with an organic donor–acceptor mixture catechol/ortho-quinone followed by organic solvent extraction leads to the formation of a new binuclear complex with catechol/ortho-quinone ligands. This complex can be further incorporated into polyethylene matrix to form the complex with properties close to the complex in LNPG.