Hyperfine structure in EPR spectra of the catalytic system TiCl4 + Al(i-C4H9)3

Abstract

Key words: catalytic system TiCl4 + Al(i-C4H9)3, electron paramagnetic resonance, hyperfine structure.Introduction. The catalytic system TiCl 4 + Al(i-C 4 H 9 ) 3 (with an equimolar ratio of the components Ti/Al, solution in an inert solvent), widely used for synthesis of polyisoprene rubber, has been vigorously studied by the EPR spectroscopy method [1][2][3]. Observations of the signal from the unpaired electron in the 3d 1 configuration of the titanium ions have made it possible to evaluate both the ligand environment of the ions and the nature of their interaction. In this case, primary attention has been focused on the values of the g factors; discussion of data on the hyperfine structure (hfs) in the EPR signals has generally been sporadic. The use of such an approach can be explained by the complexity of the processes occurring in the considered catalytic system and the difficulties involved in analysis of the hyperfine structure in these systems. At the same time, hfs data make it possible to considerably add to and refine information obtained on the values of the g factors.In this paper, we attempt to interpret hfs data for the EPR spectra of the catalytic system TiCl 4 + Al(i-C 4 H 9 ) 3 , prepared in different solvents, with an equimolar ratio of the components Ti/Al. Experimental Section. The catalytic system TiCl 4 + Al(i-C 4 H 9 ) 3 was prepared in different solvents by gradual addition of Al(i-C 4 H 9 ) 3 to a solution of TiCl 4 under an atmosphere of purified nitrogen at a temperature of 203 K. In this case, we followed the recommendations in [4,5]. The molar concentration in the solutions was maintained equal to 0.58 M for TiCl 4 and 0.53 M for Al(i-C 4 H 9 ) 3 . Preliminary preparation and analysis of the components and analysis of the components of the catalytic system were done by the procedures in [4,6]. Oligomerization of piperylene in the presence of TiCl 4 was carried out by reaction of the components of the catalytic system under an atmosphere of purified nitrogen at 373 K for 30 min according to the procedure in [7]. The EPR spectra were recorded on a Bruker ER 220D spectrometer with working wavelength 3 cm. The values of the g-factors were determined relative to the position of the EPR signal from the standard material 1,1-diphenyl-2-picrylhydrazyl (DPPH) with g st = 2.0036.Discussion of Results. Upon formation of the catalytic system TiCl 4 + Al(i-C 4 H 9 ) 3 , an EPR spectrum was expected in the form of a superposition of signals from the Ti(III) ions as the stable isotopes of titanium 47 Ti (content 7.3%, nuclear spin I = 5/2 [8]), 48 Ti (73.8%, I = 0), and 49 Ti (5.5%, I = 7/2). Interaction of the unpaired electron with the nuclei of the odd isotopes 47 Ti and 49 Ti is responsible for additional splitting of the EPR spectrum in the form of the hyperfine interaction lines. Since the nuclear g factors of the isotopes 47 Ti and 49 Ti are the same, the hfs constants from these isotopes coincide. The 48 Ti isotopes do not cause hyperfine interaction; their signal should be observed in th...

Sol-Gel Synthesis and Characterization of Nanoparticles

“…Once Al 3+ ions are incorporated into the sample B2-B5, the crystal field is distracted, as the effect or the spin-lattice relaxation time becomes longer. It results the possibility of hyperfine splitting at higher temperature [33,34] as in the case of B2-B3. The hyperfine splitting of the sample B2-B5 shows six parallel and perpendicular lines with ‖ ∼2.0233 and ⊥ ∼2.0233, which are not well-resolved lineshapes due to the small distraction.…”