Monitoring structural changes of liquids frozen in nanopores

Less dense packing of molecules in frozen liquids confined to cylindrical glass pores was observed to depend on pore size. This conclusion was derived by monitoring the rotational tunneling of methyl protons, which reside on studied molecules, with nuclear magnetic resonance. For example, the tunneling frequency of dimethyl sulfide and propionic acid at 10 K was observed to be larger in pores than in bulk. This is interpreted as being due to a decrease in the hindering potential. In another type of tunneling spectrum which is due to methyl-methyl interaction, observed in acetyl acetone at 10 K, the splitting decreases as the pores become smaller. It is demonstrated that in both types of materials the shifts of the methyl tunneling splittings in pores are the result of the reduced intermolecular interaction in the pore core region. This in turn indicates that the unit cell size of liquids frozen in nanopores is slightly increased. The increase is largest in smallest pores.