 2D IR cross peak

A 2D IR cross peak is an off-diagonal spectral feature correlating excitation and detection frequencies that differ in a two-dimensional infrared spectroscopy experiment. Its location is a relation between frequency coordinates, not a second physical location in the sample. Such features can reveal interactions between vibrations, but their interpretation depends on the measured response and timing.

For a concrete primary example, phase-controlled peptide spectra showed cross peaks interpreted as coupling between amide-I modes. The experiment measured the phase as well as amplitude of vibrational photon echoes, supplying information beyond an unsigned intensity map.^[ref]

An original positive-envelope model illustrates 2D IR diagonal-peak overlap. Let $g(x)=e^{-x^2/(2{\sigma }^2)}$ and put two purely diagonal features at frequencies $a$ and $b$:At the off-diagonal coordinate $(a,b)$, this model givesFor $a=1600$, $b=1650$ and $\sigma =20$, all in $\mathrm{c}{\mathrm{m}}^{-1}$, the value is approximately0.0879. The diagonal value $S(a,a)$ is approximately1.0019. Thus a nonzero off-diagonal pixel about8.8% of the diagonal height can arise solely from broad diagonal tails, with no separate cross feature in the stipulated model.

A real response can have positive and negative components, so this envelope is not a universal line-shape formula. It demonstrates why a peak assignment needs a fitted background/overlap model and appropriate spectral resolution, rather than the observation of any off-diagonal intensity.

Changing 2D IR waiting time, polarization or sample conditions can test proposed mechanisms. Growth or decay of a 2D IR cross peak alone does not uniquely establish chemical conversion: coupling, relaxation and changes in response factors can contribute. Interpret the feature jointly with the rest of the spectrum and the experimental pulse sequence.