ENDOR study of oxovanadium(IV) tetraphenylporphyrin randomly oriented in rigid solution

Abstract

An ENDOR study has been made of oxovanadium(IV) tetraphenylporphyrin randomly oriented in rigid solution.The ENDOR spectra give information on the 14N hyperfine and quadrupole tensor components as well as the hyperfine tensor components of the pyrrole protons. A comparison of the 14N data obtained here with those previously reported for silver and copper tetraphenylporphyrin clearly establishes the difference in orbital ground state of the d1 2and d9 systems. Using chloroform-toluene as a solvent matrix, we could measure all three hyperfine tensor components of the pyrrole protons. As expected for a system in which the unpaired electron is confined almost completely to the 3d^a tomic orbital of the metal ion, the pyrrole proton hyperfine tensor is axial with values of 1.38, -0.26, and -0.26 MHz. Assuming that the traceless component of the proton hyperfine is due to point dipole-dipole interaction between the unpaired electron at the vanadium nucleus and the proton spin, one calculates that the distance between the two spins is 5.24 A. This value is in good agreement with crystallographic data. Adduct-forming solvents, such as pyridine, have little effect on the 14N parameters but affect the proton ENDOR strongly.

EPR, ENDOR, and HYSCORE Study of the Structure and the Stability of Vanadyl−Porphyrin Complexes Encapsulated in Silica: Potential Paramagnetic Biomarkers for the Origin of Life

“…In addition, we observed no EPR spectrum when only BSA was encapsulated (BSA-SiO 2 sample), showing that the intense line in the middle of the spectrum is not due to organic radicals or defects of the SiO 2 matrix but represents the orientation-independent m I ) -1/2 vanadium hf line. The shape of the spectrum, characterized by narrow hf lines at room temperature, is identical to that of VOTPP in frozen solution at low temperature, 54 which shows that VOTTP is immobilized in silica cages, avoiding tumbling motion. The magnetic parameters deduced from simulation of the spectrum ( Figure 2 and Table 1) are very close to those found for VOTPP in frozen solution, which confirms that the four-fold symmetry of the complex is conserved in SiO 2 and that despite its immobilization, the vanadyl-porphyrin complex is not perturbed by the mineral matrix.…”

“…In their ENDOR study of VOTPP in frozen solution, Mulks and Van Willigen observed this doublet in pyridine but not in CHCl 3 . 54 This variability indicates that it is most probably due to an axial water molecule impurity present in the solvent, forming a H 2 O · · · VdO bond parallel to the C 4 axis. Our DFT calculations predict proton hf parameters a iso ) +0.1 MHz and T ) +4.5 MHz for this axial water molecule (Table 2).…”

Right-angle wiggling electron paramagnetic resonance spectroscopy

“…Oxovanadium͑IV͒ tetraphenylporphyrin, VO͑TPP͒, complexes are extensively used in EPR studies to probe biomolecular structures, [19][20][21] where the VO 2ϩ ion substitutes for a variety of divalent metal ions in metal-activated enzymes and metalloproteins, 18,22 or as models for acid demetallization in crude oils. 23 Here, we illustrate the versatility of RAW-EPR on VO͑TPP͒ in frozen CHCl 3 -toluene solution.…”

Thirty years through vanadium chemistry

“…One of the most active was that of Selbin, the author of two of the few reviews published concerning V IV O-compounds [2,3]. Some other relevant groups were those of Gillard [4,5], Kustin [6][7][8], Chasteen [9][10][11], Rieger [12][13][14], Tapscott [15,16], McCormic [17], Syamal [18][19][20][21][22], Wasson and Stoklosa [23,24], Saito [25][26][27], VanWilligen [28,29], Nicholls [30][31][32], Fraústo da Silva [5,33], Theriot [21,[34][35][36], Riechel [37], Floriani [38] and some others.…”