Sum Combination Harmonics in Four-Pulse ESEEM Spectra. Study of the Ligand Geometry in Aqua-Vanadyl Complexes in Polycrystalline and Glass Matrices

Abstract

Structural differences in the vanadyl pentaaqua complex in two different matrices, a frozen glassy water solution and a polycrystalline Tutton salt, were investigated using the recently designed one dimensional four-pulse electron spin-echo envelope modulation (ESEEM) experiment. The four-pulse ESEEM spectrum consists of the basic nuclear frequencies, να and νβ, of the ligands′ protons along with their sum combination harmonics, (να + νβ). This particular example demonstrates the potential of this new experiment which lies in the generation of highly resolved combination harmonics from which information regarding the hyperfine and quadrupole interactions can be derived. While the combination lines in the two-pulse ESEEM spectra of the polycrystalline sample suffered from low resolution due to a relatively short T2, highly resolved peaks were observed in the four-pulse ESEEM spectra since in this experiment T1 dominates the echo decay rather than T2. Orientation-selective four-pulse ESEEM experiments were performed under conditions that optimize the intensities of the sum combination harmonics. Aqua complexes with H2O and D2O were studied and clear differences in the Fourier transform ESEEM spectra of the two matrices, not detected previously by electron-nuclear double-resonance spectroscopy or two-pulse ESEEM, were evident. The analysis of the spectra was done by simulating the FT-ESEEM spectra of both the H2O and the D2O complexes. For this purpose, an analytical expression describing the four-pulse ESEEM for I = 1 was derived, taking into account the 2H quadrupole interaction as a first-order correction to the nuclear frequencies. The simulations showed that the positions and orientations of the ligands in VO2+(H2O)5 and VO2+(D2O)5 are somewhat different in the frozen glassy solution and in the crystalline matrix of the Tutton salt.