Nanoconfined liquid dynamics are often distinct from that of the bulk, and desirable differences can be harnessed to design novel processes and materials. Liquids confined in mesoporous silica (pore diameter 2-50 nm) have applications in hybrid electrolytes, heterogeneous catalysis, and drug delivery. The dynamics of a nonaqueous solvent, 1-methylimidazole, confined in mesoporous silica (2.8, 5.4, and 8.3 nm pore diameters) were examined using femtosecond infrared vibrational spectroscopy and molecular dynamics simulations of a dilute probe, the selenocyanate anion. Compared to water, a polar protic solvent, the polar aprotic solvent, 1-methylimidazole, experiences a much more dramatic slowdown of liquid dynamics upon confinement in mesoporous silica. The effects were quantified by modified two-state models used to fit three spatially averaged experimental observables: vibrational lifetime, orientational relaxation, and spectral diffusion. Modeling the subnanometer distance dependences of the observables with insights from simulations guided interpretation of the molecular roots of confinement effects, illustrating the importance of electrostatic effects and H-bonding interactions in the behavior of confined liquids.