Linear absorption and two-dimensional infrared spectra of N-methylacetamide in chloroform revisited: polarizability and multipole effects.

The effect of solvent polarizability and multipole effects on the amide I vibrational spectra of a peptide unit is investigated. Four molecular dynamics force fields of increasing complexity for the solvent are used to model both the linear absorption and two-dimensional infrared spectra. It is observed that, at least in chloroform solution, the predicted solvent shift is considerably improved when accounting for the polarizabiltiy and multipole effects. The latter are typically connected with halogen bonding. Significant deviations are still observed for more sensitive line shape parameters such as the spectral width and line skewness. However, the findings demonstrate that previously observed deviations have an origin in the force field treatment rather than in the electrostatic mapping procedure frequently employed to simulate linear absorption and two-dimensional infrared spectroscopy.