Two-dimensional correlation spectroscopy: Effect of normalization of the dynamic spectra

Simulation studies have demonstrated that linear nonselective intensity variations coupled with nonlinear selective intensity changes may develop new features in two-dimensional (2D) correlation spectra. Some types of linear nonselective intensity changes are hardly seen in the normal and synchronous 2D correlation spectrum. In contrast, they may develop quite strong features in the companion asynchronous spectrum, especially if the selective intensity variations for different bands have similar response functions. The simplest way of removing this effect is normalization of the dynamic spectra prior to 2D correlation analysis. This operation can be easily performed if we know the relationship between the perturbation and the nonspecific intensity variations. Otherwise, one has to employ an "internal reference" for normalization of the experimental spectra. The "internal reference" means the band that does not selectively change in its intensity under given perturbation. Fourier transform near-infrared (FT-NIR) measurements of octan-1-ol in CCl4 revealed a strong correlation between the concentration and the integrated intensity of the second overtone of the v(C-H) band. Also a strong correlation was found between the integrated intensity of the same band and the temperature-induced density changes of pure octan-1-ol. Thus, in the NIR region the second overtone of the v(C-H) band can be successfully applied for normalization of both the concentration and temperature-perturbed spectra of numerous organic samples. The most complicated situation appears For a rheo-optical experiment involving pronounced deformation, where any simple normalization of the experimental spectra cannot be applied. In this instance, knowledge of the exact relationship between the strain and the sample thickness is required.