Band-target entropy minimization (BTEM): An advanced method for recovering unknown pure component spectra. application to the FTIR spectra of unstable organometallic mixtures

A newly developed band-target entropy minimization (BTEM) algorithm was tested on experimental FTIR data of Rh-4(CO)(12)/Rh-6(CO)(16) Mixtures in order to recover the pure component spectra of the constituent complexes. Bands in the nonoverlapping bridging carbonyl region as well as bands in the highly overlapping terminal carbonyl region were targeted for retention. The bands are identified in the vector-space decomposition of the observations, a crucial first step in untangling the superposition of the pure component spectra. In both cases, the targeted band was retained, and exceptionally accurate whole spectral estimates of Rh-4(CO)(12) and Rh-6(CO)(16) were obtained. Due to the constructs used in BTEM, enhanced signal-to-noise characteristics result, and spectral nonlinearities arising from changing band positions and changing band shapes are essentially eliminated. For the experimentalist, the utility of BTEM arises from its direct one-spectrum-at-a-time spectral reconstruction approach-which is guided by the choice of the targeted region. As such, BTEM appears particularly applicable to spectroscopy possessing highly localized features: i.e., FTIR, Raman, etc. The BTEM algorithm is so useful that the spectral pattern from the minute presence of suspended particles of Rh6(CO)16 could be reconstructed. Indeed, the integrated absorbance of Rh-4(CO)(12), Rh-6(CO)(16), and Rh-6(CO)(16) solids account for only ca. 0.3, 0.09, and 0.04% of the experimental observations. The new BTEM algorithm was compared to other algorithms such as SIMPLISMA, IPCA, and OPA-ALS. The latter either fail with the present data set or are unable to produce reconstructed spectra of similar quality to BTEM. This new algorithm holds considerable promise for the analysis of in-situ spectroscopic reaction data such as those arising in, complex organometallic an organic syntheses, where absolutely no a priori information about the constituents/intermediates is available.