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

Publisher Website
Google Scholar
Abstract
The 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 Å 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 ge have been obtained with standard deviations ranging from 0.002 to 0.015 Å, with an average of 0.006 Å. The average standard deviation for the coordinates of the 4 H2O protons nearest to the Nd+3 ion, three of which form hydrogen bonds with nicotinate nitrogen atoms, is 0.004 Å. The average standard deviation for their distances from the Nd+3 is 0.003 Å. It is clear that by means of suggested improvements of the reported techniques it should be possible to measure proton coordinates to 0.001 Å. 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.