Simulations of infrared spectra of nanoconfined liquids: acetonitrile confined in nanoscale, hydrophilic silica pores.

The infrared spectrum of acetonitrile confined in hydrophilic silica pores roughly cylindrical and 2.4 nm in diameter has been simulated using molecular dynamics. Hydrogen bonding interactions between acetonitrile and silanol groups on the pore wall involve charge transfer effects that have been incorporated through corrections based on electronic structure calculations on a dimer. The simulated spectrum of confined acetonitrile differs most prominently from that of the bulk liquid by the appearance of a blue-shifted shoulder, in agreement with previous experimental measurements. The dominant peak is little changed in position relative to the bulk liquid case, but broadened by approximately 40%. A detailed analysis of the structure and dynamics of the confined liquid acetonitrile is presented, and the spectral features are examined in this context. It is found that packing effects, hydrogen bonding, and electrostatic interactions all play important roles. Finally, the molecular-level information that can be obtained about the dynamics of the confined liquid from the infrared line shape is discussed.