4-PULSE ELECTRON-SPIN ECHO ENVELOPE MODULATION STUDIES OF AXIAL WATERLIGATION TO DIAQUATETRACYANONICKELATE(III)

Four-pulse electron spin echo envelope modulation (ESEEM) studies aime d at enhancing our previous characterization of the hyperfine interact ions between the protons of axially bound water molecules and the nick el ion of Ni-III(CN)(4)(H2O)(2)(-) were carried out. Because the ligan d hyperfine coupling of the strongly bound water protons is characteri zed by a large anisotropic interaction, the v(alpha) + v(beta) sum com bination peak, resolved in two- and four-pulse ESEEM measurements, sho ws a pronounced shift from twice the proton Larmor frequency. In contr ast to our previous study where a two-pulse (pi/2-tau-pi) microwave pu lse sequence was used, four-pulse data show deep modulation with reduc ed damping for the sum combination feature that results in a 10-fold i ncrease in spectral resolution. The corresponding ESEEM spectra provid e line shape and frequency constraints that allow for a more accurate and complete characterization of the bound water proton hyperfine coup ling tenser. Theoretical simulation of the magnetic field dependence o f the v(alpha) + v(beta) line shapes and frequency shifts from the twi ce the Larmor frequency gave an effective Ni-H dipole-dipole distance of 2.33 +/- 0.03 Angstrom and a theta(n), the orientation for the prin cipal axis system of the H-1 hyperfine coupling tenser with respect to the g(3) axis of the Ni(III) g tensor, of 18 +/- 3 degrees. The tau-s uppression behavior of the v(alpha) + v(beta) line shape at a fixed ma gnetic field position was used to place more exact constraints on the isotropic hyperfine coupling constant than possible with a simple two- pulse approach. An isotropic hyperfine coupling constant of \2.5\ +/- 0.5 MHz was found for the bound axial water protons.