Analysis of 2 H NMR spectra of water molecules on the surface of nano-silica material MCM-41: Deconvolution of the signal into a Lorentzian and a powder pattern line shapes J. Hassan n Applied Mathematics and Sciences, KUSTAR, UAE and Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 article info Article history: Received 1 March 2011 Received in revised form 24 August 2011 Accepted 10 October 2011 Available online 17 October 2011 Keywords: Water 2 H NMR Powder pattern line shape MCM-41 Chemical exchange abstract Water 2 H NMR signal on the surface of nano-silica material MCM-41 consists of two overlapping resonances. The 2 H water spectrum shows a superposition of a Lorentzian line shape and the familiar NMR powder pattern line shape, indicating the existence of two spin components. Exchange occurs between these two groups. Decomposition of the two signals is a crucial starting point to study the exchange process. In this article we have determined these spin component populations along with other important parameters for the 2 H water NMR signal over a temperature range between 223 K and 343 K. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Important information regarding dynamics, structure and exchange rates between different spin groups that might exist in a system can be extracted from their NMR signals. To accom- plish this, one often needs to decompose different NMR signal components within the final spectrum. Often, this step is very straightforward as in the case of a spectrum consisting of multiple Lorentzian lines with different chemical shifts. In this case, one usually deals with NMR Free Induction Decay (FID) to decompose the two (or more) spin groups using an equation with bi- exponential or multi-exponential decay rates [1–5]. On the other hand, if the system contains two (or more) spin groups with restricted motions, each with an NMR powder pattern line shape [6], with different splittings [7,8], one usually deals with the frequency data to decompose individual signals. There are cases when the system is comprised of a spin group that is mobile and isotropic in motion in such a way that it exhibits a Lorentzian line shape along with another spin group that is more restricted and anisotropic in motion. The latter might produce an NMR powder pattern such that the final spectrum includes Lorentzian and powder pattern line shapes. Spin exchange can take place between the two groups. This exchange causes the individual NMR signal within the final spectrum to overlap more (with increasing thermal energy), so that the shape of the spectrum changes with temperature. This is the case for the 2 H water NMR signal on the surface of MCM-41. Recently, we have shown that water molecules on the surface of nano-silica material MCM-41 are attached to different distinct hydration sites (namely single and hydrogen-bonded silanol, Si–O 2 H, groups) [9]. In addition to that, we have determined that deuterons of water bound to hydrogen-bonded silanol groups produce the powder pattern in the 2 H spectrum, while deuterons of water bound to single silanol groups produce the single Lorentzian line shape. Water molecules bound to single silanol groups are expected to be more mobile (rotation of the water molecule about hydroxyl group bond and motion of the silanol group itself). The mobility of surface silanols has been reported in the literature for the MCM-41 pore surface [10] and similar surfaces [11]. These motions combine to average the quadrupolar Hamiltonian to a considerable degree, which results in a Lorentzian line shape. In this article we decomposed the 2 H water NMR spectra into individual signals from different spin components in the system over a temperature range of 223 K–343 K. This is a first step toward the study of the exchange process and to obtain dynamic parameters of different spin species within the sample. A successful simplified method of extracting exchange rates for 1 H data (comprising of a Lorentzian and powder pattern line shapes) in clays was previously used [12] by focusing on FID data. In this article we focused on the NMR frequency-domain data. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.10.023 n Corresponding author. Tel.: þ1 971 2 4018094; fax: þ1 971 2 4018099. E-mail address: jamal.hassan@kustar.ac.ae Physica B 407 (2012) 179–183