Translational and Rotational Dynamics of Molecules Confined in Zeolite Nanocages by Means of Deuteron NMR

• Published in: Journal of physical chemistry. C 2017-11, Vol.121 (47), p.26472-26482
• Main Authors: Lalowicz, Z. T, Birczyński, A, Krzyżak, A
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b06894

The main aim of our approach is to gain a comprehensive view of mobility of small molecules in confinement as reported by 2H NMR spectroscopy. The spectra and spin–lattice relaxation were measured in a wide range of temperature. A set of molecules, D2,CD4,D2O,ND3,CD3OD and (CD3)2CO, was chosen and introduced into NaX and NaY zeolites. A wide range of loadings provides another dimension in studies of molecular mobility in confinement. Observed features reflect evolution on decreasing temperature of molecular dynamics from gaseous state over liquid-like rotational phase to immobilized molecules. Molecules become immobilized below the temperature T S , which appears to be an important parameter related to the strength of interactions with zeolite framework. For chosen zeolites, NaX and NaY, hydrogen bonding, and electrostatic interaction dominate, respectively. We restrict ourselves in reporting results above T S, as below molecular mobility is reduced to internal rotations when possible. The existence of D2O clusters and trimers of CD3OD gave particularly significant evidence for importance of their mutual interactions. A transition from translational to rotational mobility on decreasing temperature was a common observation, with transition temperature T TR as a significant parameter. Fast magnetization exchange between these two mobilities was considered as a model in analysis of the relaxation temperature dependence. We point out the effective value of the quadrupole coupling constant as justification for using the exchange model. A wealth of observed features proves particular sensitivity of 2H NMR spectroscopy in studies of molecular dynamics.