Electron-nuclear double-resonance study of hydrogen bonding in cadmium acetate dihydrate crystals containing Hg(I) centers

Abstract Electron-nuclear double-resonance (ENDOR) studies of a stable Hg(I) center in X- and γ-irradiated single crystals of Cd(CH 3 CO 2 ) 2 ·2H 2 0 at 4.2 K are reported. These studies were undertaken to explore the feasibility of using the ENDOR technique to investigate hydrogen bonding in a normally diamagnetic organometallic solid for the first time. The ENDOR transitions were observed by saturating the EPR transitions from mercury isotopes with I = 0 and analyzed using a numerical diagonalization program coupled with a least-squares fitting procedure. Ten proton superhyperfine tensors are analyzed and shown to originate from four distinct groups of protons within the crystal lattice. All the proton tensors are found to be of orthorhombic symmetry and have principal directions dominated either by the mercury nucleus or the symmetry of the water molecules. The results indicate that one of the protons surrounding the paramagnetic site has tunnelled towards the paramagnetic site from the position indicated by the X-ray structural data at 300 K. Hyperfine couplings have been analyzed from every proton within 5 A of the Hg(I) nucleus, except for the methyl protons on one of the acetate groups. A coupling was assigned to the protons of the other methyl group, which was found to be rotating. All the protons on the directly bonded water ligands have hyperfine couplings dominated by the water molecule symmetry and possess a positive unpaired electron spin density. Protons hydrogen bonded to the paramagnetic center are found to possess a nonzero isotropic coupling with negative sign. The negative spin density at these protons is attributed to σ-π spin polarization effects from orbitals on the oxygen involved in the O⋯H bond. Covalency effects are found even in long hydrogen bonds.