Near-infrared spectroscopy: I. Spectral identification and analytical applications

Abstract

The near-infrared region of the spectrum contains a great wealth of spectral data primarily concerned with hydrogenic (CH, NH, OH) stretching vibrations. instruments which are now available permit the rapid scanning of this region with sufficient signal-to-noise ratio and resolution for many analytical and molecular structure problems. In order to draw generalized conclusions regarding the origin of near-infrared absorption bands, assignments are made for nearly all of the overtone and combination bands of chloroform, bromoform, methylene chloride, and methylene bromide in the region from 0.7 to 3.5μ The spectra of benzene, methanol, and m-toluidine are also examined. in the case of the haloforms, all possible combinations can be accounted for as resolved bands or as background absorption. The selection rules and anharmonicity corrections are readily applicable to the near-infrared region. Vibrations predominantly localized in two molecular groups within a given molecule do not couple to produce intense combination bands unless they share a common atom or are connected through multiple bonds. A Colthup-type chart summarizing a study of the position, range, and intensity of characteristic absorption bands is presented along with pertinent references to the literature. Factors perturbing the position and intensity of bands are discussed. A formula is developed for the prediction of the lower limit of detectability of a compound in a mixture as a function of component absorptivities and instrument noise. This limit of detectability for a number of typical mixtures is tabulated. Precision in the range from 0.004 to 0.5% is possible when OH, NH, or CH groups can characterize the unknown.