Correlation Effects in the Two-Dimensional Vibrational Spectroscopy of Coupled Vibrations

Correlated energy shifts in the spectral broadening of coupled vibrational transitions are shown to have clear signatures in infrared two-dimensional dispersed vibrational echo (DVE) spectroscopy. A model that includes correlation effects through a correlation coefficient (ρ) for a bivariate distribution is used to describe two-dimensional experiments on the coupled carbonyl stretches of Rh(CO)₂(C₅H₇O₂) (or RDC) in chloroform. Signatures of correlated (0 < ρ < +1) and anticorrelated (−1 < ρ < 0) broadening in DVE experiments manifest themselves in the depth of modulation of the signal and the magnitude of the echo peak shift. For the case of RDC, the broadening is highly correlated (ρ = 0.9) and can be explained in terms of the solvent-induced modulation of the Rh electron density.