Pure component reconstructions using entropy minimizations and variance-weighted piecewise-continuous spectral regions. Application to the unstable experimental system Co-2(CO)(8)/Co-4(CO)(12)

The organometallic complex Co-2(CO)(8) is an air-, light-, heat- and moistu re-sensitive species and rapidly decomposes in n-hexane solutions to give C o-4(CO)(12); Owing to the instability of Co-2(CO)(8), calibration is extrem ely difficult. Moreover, the published literature measurements of the mid-i nfrared spectrum of Co-2(CO)(8) are inconsistent. To solve this problem, a chemometric approach involving singular value decompositions and entropy mi nimizations was used. First, n-hexane solutions of mixtures of Co-2(CO)(8) and Co-4(CO)(12) were prepared and measured in the mid-infrared on the inte rval 1700-2200 cm(-1). After spectral subtractions of n-hexane and backgrou nd gases, two absorbance matrices were constructed, namely the unweighted m atrix A(8x2501) and the piecewise-continuous variance-weighted matrix A*(8x 2501). The piecewise-continuous variance-weighted matrix possesses a discon tinuity at 1925 cm(-1), between the region of strong terminal carbonyl vibr ations and the region of weak bridging carbonyl vibrations, where both regi ons have strongly overlapping characteristics. The significant right singul ar vectors V-2x2501(T) and V-2x2501(T*) were then subjected to both second- derivative and fourth-derivative entropy minimizations. In addition, two nu merical modifications were implemented, namely full-spectrum optimizations and half-spectrum optimizations. It was found that extremely good spectral estimates for Co-2(CO)(8) and Co-4(CO)(12) were obtained from (i) the globa l solutions for both the unweighted and variance-weighted second-derivative entropy minimizations when using the half-spectrum minimizations and (ii) the local solutions for both the unweighted and variance-weighted fourth-de rivative entropy minimizations when using both the half-spectrum and full-s pectrum minimizations. These results suggest that (a) half-spectrum optimiz ations and second-derivative entropy measures are potentially very useful f or particular types of real strongly overlapping spectra, (b) the failure o f the fourth-derivative entropy measures to produce the correct estimates i s probably associated with the sensitivity to experimental noise and (c) al though the variance-weighted approach did not help in the present case, var iance-weighted analyses still hold potential for spectral analyses involvin g both strong and weak absorbance characteristics. Copyright (C) 2000 John Wiley & Sons, Ltd.