Origin of superhyperfine interactions in the antiferromagnetic ring Cr7Ni

W-band pulsed ESR measurements on the antiferromagnetic ring Cr7Ni reveal that the spin-lattice relaxation time is T-1 = 5.3 +/- 0.2 mu s at T = 4.0 K, and decreases strongly with increasing temperature. The phase coherence time is determined to be T-M = 357 +/- 10 ns. Pulsed W-band ENDOR measurements quantify the superhyperfine coupling of the electron spin to the proton nuclear spins. Calculating the dipolar contribution of all the protons to the hyperfine splitting, in the point-dipole approximation, gives a good simulation of the experimental data, demonstrating that magnetic dipole interactions between electron and nuclear spins are the origin of the superhyperfine interaction.