Structure and Dynamics of Selectively Deuterated Self-Assembled Silver n-Octadecanethiolate Layered Materials

The dynamics of the alkyl chains below and above the chain-melting transition temperature of long-chain silver n -alkanethiolate layered compounds were studied using variable-temperature 2 H NMR and IR spectroscopies. Two specifically deuterated compounds, silver- d 35 -octadecanethiolate [AgSCH 2 (CD 2 ) 16 CD 3 ] and silver-1,1- d 2 -octadecanethiolate [AgSCD 2 (CH 2 ) 16 CH 3 ], were used to identify the behavior of different regions of the chains. The 2 H NMR studies provide details about the alkyl chain motion, whereas IR spectroscopy provides details of the alkyl chain conformation. A key structural component of these materials is a polymeric −Ag−S− backbone from which the alkyl chains extend outward from both sides. The result is a two-dimensional, ribbonlike structure. The alkyl chains are highly ordered and adopt an extended all-trans conformation, which is maintained up to a few degrees below the phase transition temperature. The occurrence of a significant gauche population is observed above the phase transition temperature. The thermally induced structural changes are reversible. Above the phase transition temperature, the alkanethiolate chains experience complex motions. The structure of the post-transition phase is discussed in the context of related systems.