2-DIMENSIONAL INFRARED AND NEAR-INFRARED CORRELATION SPECTROSCOPY - APPLICATIONS TO STUDIES OF TEMPERATURE-DEPENDENT SPECTRAL VARIATIONS OFSELF-ASSOCIATED MOLECULES

This paper demonstrates the potential of generalized two-dimensional ( 2D) Fourier transform (FT) infrared (IR) and near-infrared (NIR) corre lation spectroscopy in the studies of temperature-dependent spectral v ariations of self-associated molecules, Three examples of the 2D corre lation analysis are discussed in this paper. The first two are concern ed with the temperature-dependent IR and NIR spectral changes of N-met hylacetamide (NMA) in the pure liquid state. The 2D IR correlation app roach revealed that almost all the peaks of NMA in the 3400-1100-cm(-1 ) region consist of two to four separate bands. For example, the amide I band contains contributions from at least four distinct bands at 16 85, 1665, 1650, and 1635 cm(-1). The analysis of the asynchronous 2D I R spectrum in the amide I region showed that the sequence of spectral intensity change in the ascending order of temperature was given by 16 35<1650<1665<1685 cm(-1), These bands at 1635, 1650, 1665, and 1685 cm (-1) were assigned to the amide I modes of chain oligomers of various sizes and dimer of NMA; the longer the chain, the lower the frequency. The closeup view of 2D NIR correlation spectra of NMA obtained at nar row spectral and temperature windows enabled us to propose not only ba nd assignments in the 6800-6050-cm(-1) region but also a detailed mech anistic picture of the thermally induced dissociation of Nh IA for eac h temperature range, We also applied the generalized 2D correlation ap proach to the analysis of a set of FT NIR spectra of oleyl alcohol und er temperature variations. The 2D NIR analysis enhanced the spectral r esolution and simplified the spectra with overlapped bands, For exampl e, it was found that a hand at 7090 cm(-1) arising from the first over tone of an OH stretching mode of the monomeric alcohol consists of two bands due to the rotational isomerism of the free OH group. An intrig uing possibility of correlating various overtone and fundamental bands to establish unambiguous assignments was also suggested from the 2D N IR study on oleyl alcohol.