Multi-Lorentzian representation of deuterium spectrum to study water spin magnetization exchange in MCM-41

Water behavior on the pore surface of nano-silica MCM-41, at a hydration level corresponding to one water molecule per OH group, is studied using ²H NMR spectra in the temperature range 213 to 313 K. In an earlier study [J. Hassan, E. Reardon, H. Peemoeller, Microporous Mesoporous Materials, 122 (2009) 121-127] it was shown that at this hydration level, deuterons of water at single OH sites exhibit a Lorentzian line shape and deuterons of water at hydrogen-bonded OH sites exhibit a powder pattern. Here it is shown that magnetization exchange occurs between these two deuteron spin groups. This exchange cannot be described using the common, two-site exchange model, involving two Lorentzians. We successfully apply a multi-Lorentzian exchange model, prompted by Woessners work [D. E. Woessner, Mol. Phys. 34, 4, (1977) 899-920] on the effects of motion on the shape of water spin resonance lines exhibiting doublet splitting. For this low hydration sample the rate of magnetization exchange out of the hydration site, where the water deuterons exhibit a Lorentzian line in the ²H spectra, is 1.3 ms ^-1 and the activation energy for the exchange is found to be 3.4±0.1 kcal/mole.