The determination of hydrogen coordinates in lanthanum nicotinate dihydrate crystals by Nd+3-proton double resonance

Abstract

Nitrogen and deuterium nuclear quadrupole resonance in lanthanum nicotinate dihydrate Deuteron, proton, and sodium magnetic resonance study of sodium bromide dihydrate and sodium chloride dihydrateThe EPR (electron paramagnetic resonance) and proton ENDOR (electron nuclear double resonance) spectra of single crystals of lanthanum nicotinate dihydrate with Nd+ 3 dilutely substituted for La+ 3 have been obtained and described by appropriate spin Hamiltonian operators. The ENDOR signals of protons lying within 5 A of the Nd+ 3 have been resolved and assigned to specific protons. The physical interpretation of the measured anisotropic rare earth ion-proton hyperfine interactions has been given. The spatial coordinates of the protons in the principal axis system of the electron Zeeman interaction matrix g.have been obtained with standard deviations ranging from 0.002 to 0.015 A, with an average of 0.006 A.The average standard deviation for the coordinates of the 4 H 2 0 protons nearest to the Nd + 3 ion, three of which form hydrogen bonds with nicotinate nitrogen atoms, is 0.004 A. The average standard deviation for their distances from the Nd+ 3 is 0.003 A. It is clear that by means of suggested improvements of the reported techniques it should be possible to measure proton coordinates to 0.001 A. The methods developed show promise for applications to problems concerning the structure of organic molecules, in particular organo-metallic complexes and large molecules of biological interest, such as proteins, to which paramagnetic ions may be attached.It is well-known that in ions of the lanthanide elements the I-electrons are highly localized in the vicinity of the nucleus. 8 -10 Consequently, it may be anticipated that in the case of a complex of an organic molecule and a rare earth ion, the electronic magnetic mo-

Rare-earth nicotinates: structures of tetra-μ-nicotinato-O,O'-bis[diaqua(nicotinato-O,O')holmium(III)], [Ho(C5H4NCO2)3(H2O)2]2, and tetra-μ-nicotinato-O,O'-bis[diaqua(nicotinato-O,O')thulium(III)], [Tm(C5H4NCO2)3(H2O)2]2

“…In view of (a) the necessity of precise structural data for the interpretation of such magnetic and optical measurements, and (b) the interest in the rare-earth nicotinates with respect to the determination of proton coordinates by magnetic resonance methods (Hutchison & Mckay, 1977;Hutchison & Orlowski, 1980), the structures of the Ho and Tm complexes have been determined.…”

Magnetic resonance studies of triplet carbonyls

“…These are called "distant ENDORs" as they originate from lattice protons close enough to interact with the BP triplet spin. Such signals are sometimes useful for mapping the proton environment around a probe spin [47]. In this example, however, they merely obscure the signal from one pair of rneta-protons.…”

Electron paramagnetic resonance spectroscopy of Cu2+ in hen egg-white lysozyme.

“…It has been shown by Hutchison et al (26,27) that the coordinates of hydrogen atoms may be determined by electronnuclear double resonance spectroscopy of protons. Studies that employ the EPR absorptions of the Cu2+ ions in the HEW lysozyme crystal for this purpose should elucidate the details of the chemical nature and geometrical arrangements of the Cu2+ ligands.…”