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A number of published reports are available regarding 2D correlation spectroscopy studies of basic molecules.1-17 Noda et al.6- reported 2D correlation spectroscopy studies of temperature-dependent NIR spectral variations of oleyl alcohol, and IR, NIR, and Raman spectral variations of N-methylamide (NMA). They also reported IR-Raman heterospectral analysis of NMA.10 Czarnecki etal.11-14 studied 2D NIR correlation spectra of various alcohols including deuterated compounds. They investigated detailed band assignments of NIR spectra, thermal dynamics of hydrogen bondings, and rotational isomers of alcohols. IR-NIR heterospectral analysis of polyamide was reported by Czarnecki et al.15 Sasic et al.16 used sample-sample 2D NIR correlation spectroscopy to investigate phase transitions of oleic acid in the pure liquid. Segtnan et al}1 applied both variable-variable and sample-sample 2D NIR correlation spectroscopy to explore the structure of water.
Of note is that besides IR and Raman spectroscopy, NIR spectroscopy has been used extensively. NIR spectroscopy has the following characteristics in studying the hydrogen bonds and molecular interactions of self-associated molecules, such as alcohols, carboxylic acids and amides.5 918 (i) OH and NH stretching bands due to monomeric and polymeric species are better separated in the NIR region than in the IR region. Even bands ascribed to free-terminal OH and NH groups of the polymeric species can be clearly identified. (ii) Because of their larger anharmonicity, bands ascribed to the first overtones of OH and NH stretching modes of monomeric species appear much more strongly than the corresponding
Two-Dimensional Correlation Spectroscopy-Applications in Vibrational and Optical Spectroscopy
I. Noda and Y. Ozaki © 2004 John Wiley & Sons, Ltd ISBN: 0-471-62391-1
Applications of 2D Correlation Spectroscopy to Basic Molecules
bands arising from hydrogen-bonded OH and NH group. Therefore, it may be easier in the NIR region than in the IR region to monitor the dissociation process from the polymeric species into monomeric ones by means of the first overtone of the OH or NH stretching mode of the monomeric species. (iii) NIR bands have much weaker absorption intensities than IR bands, so that a more convenient pathlength of a cell can be used and the exact volume of the sample can be evaluated. In IR spectroscopy, one must use a very thin cell or an attenuated total reflection (ATR) prism, and therefore one often encounters problem of adsorption.
While NIR spectroscopy has several advantages over IR and Raman spectroscopy for the studies of self-associated molecules, NIR region contains more bands due to overtones and combination modes that are often heavily overlapped. For this reason, it is not easy to make unambiguous assignments for NIR bands and to extract the spectral parameters of the individual bands. The high resolution aspect of 2D NIR correlation spectroscopy should be a powerful tool for unraveling complicated NIR spectra. It has been repeatedly shown that 2D NIR spectra can accentuate useful information obscured in the original spectra in a surprising way. In this chapter, four illustrative examples concerning 2D correlation spectra of simple molecules will be discussed. One of them deals with a 2D fluorescence correlation spectroscopy study.
9.1 2D IR STUDY OF THE DISSOCIATION OF HYDROGEN-BONDED N -METHYLACETAMIDE
N-Methylacetamide (NMA) is a simple but very important model for amide groups of peptides, proteins, and polyamides. A 2D IR study of NMA in the pure liquid state provides new insight into the mechanism of the dissociation of hydrogen-bonded NMA. Figure 9.1 shows IR spectra of NMA in the pure liquid measured at 30, 45, and 65 °C.6 Hydrogen-bonded associated species of various sizes are expected for NMA in the pure liquid and solutions, so that all the amide bands should consist of several component bands assignable to each species. Nevertheless, no definitive study has been undertaken until lately to fully unravel the overlapping bands.
The synchronous 2D spectrum of the Amide I and II regions of NMA, generated from the IR spectra observed over a temperature range of 30-65 °C, is shown in Figure 9.2.6 The synchronous spectrum is dominated by two autopeaks at 1650 and 1565 cm-1. While the synchronous spectra do not seem to provide immediately useful information, the corresponding asynchronous spectrum gives rise to valuable information about the band assignments and the mechanism of the dissociation of the hydrogen-bonded associated species of NMA. Figure 9.3(A) and (B), respectively, shows the asynchronous spectra of the Amide I and Amide II regions of NMA. It is easily observed that at least four distinct IR bands at 1685, 1665, 1650, and 1635 cm-1 are located in the Amide I region (Figure 9.3(A)), and that at least two bands at 1570 and 1545 cm-1 are
2D IR Study of the Dissociation of Hydrogen-bonded N-Methylacetamide