NEAR-INFRARED SPECTROSCOPIC INVESTIGATION OF INCLUSION COMPLEXES BETWEEN CYCLODEXTRINS AND AROMATIC-COMPOUNDS

Near infrared (NIR) absorption in the 900-1800 nm region has been used to characterize and measure the association constants of the inclusion complexes between aromatic compounds (e.g., phenol, 4-chlorophenol, sodium 2-naphthalenesulfonate, and sodium 2-pyrenesulfonate) and alpha-, beta-, and gamma-cyclodextrin. It was found that the NIR method is particularly suited for this type of study because upon forming complexes, the oscillator strengths of the C-H stretching [Delta nu = 2 (at similar to 1650 nm) and 3 (at similar to 1120 nm)] overtone bands of the aromatic guest compounds were increased by at least 2-5-fold. The observed increases are rationalized in terms of the enhancement in the anharmonicity and the libration motions of the guest molecule in the CD cavity. This enhancement effect facilitates the calculation of the association constants of the inclusion complexes. As expected, the calculated values are in relatively good agreement with previously determined values.