Solid-State⁶⁷Zn NMR Spectroscopy in Bioinorganic Chemistry. Spectra of Four- and Six-Coordinate Zinc Pyrazolylborate Complexes Obtained by Management of Proton Relaxation Rates with a Paramagnetic Dopant

Abstract

Solid-state (67)Zn NMR spectra of model compounds for metalloproteins, such as [H(2)B(3,5-Me(2)pz)(2)](2)Zn (pz denotes pyrazolyl ring), have been obtained using low temperatures (10 K) to enhance the Boltzmann factor in combination with cross polarization (CP) from (1)H to (67)Zn. Attempts to observe spectra of other model compounds, such as [H(2)B(pz)(2)](2)Zn, were hindered by long relaxation times of the protons. To decrease the proton relaxation times, the high-spin six-coordinate complex [HB(3,4,5-Me(3)pz)(3)](2)Fe has been investigated as a dopant. NMR and EPR measurements have shown that this Fe(II) dopant effectively reduces the (1)H spin lattice relaxation time, T(1), of the zinc samples in the temperature range 5-10 K with minimal perturbations of the (1)H spin lattice relaxation time in the rotating frame, T(1)(rho). Using this methodology, we have determined the (67)Zn NMR parameters of four- and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful models for systems of biological importance. The (67)Zn NMR parameters are contrasted to the corresponding changes in the (113)Cd NMR parameters for the analogous compounds. Further, these investigations have demonstrated that a temperature-dependent phase transition occurs in the neighborhood of 185 K for [HB(3,5-Me(2)pz)(3)](2)Zn; the other poly(pyrazolyl)borate complexes we investigated did not show this temperature-dependent behavior. This conclusion is confirmed by a combination of room-temperature high-field (18.8 T) solid-state (67)Zn NMR spectroscopy and low-temperature X-ray methods. The utilization of paramagnetic dopants should enable low-temperature cross polarization experiments to be performed on a wide variety of nuclides that are important in bioinorganic chemistry, for example, (25)Mg, (43)Ca, and (67)Zn.

Ultra‐wideline 27Al NMR Investigation of Three‐ and Five‐Coordinate Aluminum Environments

“…Gaussian 98 [41] or Gaussian 03 [42] were used for the calculations employing the restricted Hartree-Fock (RHF), density functional (BLYP), [43,44] and the hybrid density functional (B3LYP) methodologies. [45,46] These methods were used in conjunction with the 6-31G**, 6-311G**, 6-31++ G**, 6-311++ G**, and 6-311 + G(2df,2p) basis sets. The quadrupole coupling constants of 27 Al were calculated with the formula C Q ( 27 Al) = (eQV 33 was applied to convert the atomic units into Hertz.…”

“…[34,35,45] The latter technique is substantially faster than the former, owing to the reduction in the number of subspectra that need to be acquired to form the wideline spectra. More recently, QCPMG [23] wideline techniques have been implemented to acquire megahertzwide NMR spectra of both halfinteger quadrupolar [36,46] and spin-1/2 nuclei. [48,49] The application of the ultra-wideline QCPMG and Hahn-echo methodologies to the study of threeand five-coordinate aluminum sites with extremely broad powder patterns is discussed below.…”

Signal-to-noise enhancement of NMR spectra of solids using multiple-pulse spin-echo experiments

“…For Hg(CH 3 CO 2 ) 2 , a total of nine subspectra were necessary to obtain the full 199 Hg NMR spectrum (47). Two methods for constructing a single spectrum with an accurate lineshape from a series of subspectra are: variable offset cumulative spectra (VOCS) (59) or skyline projection (60). VOCS consists of adding the subspectra, whereas the latter method involves combining the subspectra to form a "skyline projection," as seen in Fig.…”

“…The S/N gain achieved in the QCPMG experiment has allowed the exploration of important quadrupolar nuclei such as 35 Cl (66), 39 K (67, 68), 25 Mg (67), 67 Zn (60,(67)(68)(69)(70), and 99 Ru (71). The advantage of the QCPMG experiment is that it is easy to implement in conjunction with signal enhancement techniques such as CP (72,73), rotor-assisted population transfer H to two lowgamma nuclei, 67 Zn and 25 Mg, followed by the QCPMG pulse sequence.…”

On the practical aspects of recording wideline QCPMG NMR spectra

“…Therefore, for patterns with breadths greater than the excitation bandwidth, the full spectrum must be reconstructed from segments acquired at different offset frequencies [4,5,28,49,[76][77][78]. Attempts at overcoming excitation bandwidth limitations for quadrupolar nuclei have recently been reported which include the use of microcoils [79][80][81] or frequency-swept pulses [81][82][83][84].…”