A computational approach for the calculation of two-dimensional infrared spectra: Application to the amide I band

Two-dimensional (2D) vibrational spectroscopy provides a powerful means to probe molecular interactions, structure, and dynamics in condensed phases with high temporal and spectral resolution. Theoretical and computational studies have played a crucial and complementary role in its advancement through the development of mixed quantum-classical models that connect structural information with the vibrational spectral response. Here, we present an approach for calculating 2D infrared spectra based on the perturbed matrix method. Within this approach, the spectral signal is reconstructed by explicitly calculating the excitonic vibrational states, including the effects of the fluctuating environment without requiring any empirical parameterization. The methodology is validated through the computation of the amide I 2D spectrum of a dipeptide and benchmarked against experimental data. The excellent agreement with the experimental signal and, in particular, the accurate reproduction of the spectral variations experimentally observed upon pH change, allows us to clarify the structural origin of these variations in terms of hydrogen-bonding patterns and the mutual orientation of the side chain and backbone.