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 approachswhich 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 Rh 6 (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 and organic syntheses, where absolutely no a priori information about the constituents/intermediates is available.
A newly developed self-modeling curve resolution method, band-target entropy minimization (BTEM), is described. This method starts with the data decomposition of a set of spectroscopic mixture data using singular value decomposition. It is followed by the transformation of the orthonormal basis vectors/loading vectors into individual pure component spectra one at a time. The transformation is based in part on some seminal ideas borrowed from information entropy theory with the desire to maximize the simplicity of the recovered pure component spectrum. Thus, the proper estimate is obtained via minimization of the proposed information entropy function or via minimization of derivative and area of the spectral estimate. Nonnegativity constraints are also imposed on the recovered pure component spectral estimate and its corresponding concentrations. As its name suggests, in this method, one targets a spectral feature readily observed in loading vectors to retain, and then combinations of the loading vectors are searched to achieve the global minimum value of an appropriate objective function. The major advantage of this method is its one spectrum at a time approach and its capability of recovering minor components having low spectroscopic signals. To illustrate the application of BTEM, spectral resolution was performed on FT-IR measurements of very highly overlapping mixture spectra containing six organic species with a two-component background interference (air). BTEM estimates were also compared with the estimates obtained using other self-modeling curve resolution techniques, i.e., SIMPLISMA, IPCA, OPA-ALS, and SIMPLISMA-ALS.
In the preceding paper in this issue, the concept of band-target entropy minimization
(BTEM) was introduced, and it was successfully applied to spectral reconstruction from a
stoichiometric organometallic reaction system after spectral preconditioning. In this contribution, the BTEM algorithm is reapplied to semi-batch homogeneous catalytic reactions without
spectral preconditioning. The homogeneous catalytic hydroformylation of 3,3-dimethylbut-1-ene, starting with Rh4(σ-CO)9(μ-CO)3 as catalyst precursor in n-hexane as solvent, was
studied at 298 K and variable total pressure, using high pressure in situ infrared spectroscopy
as the analytical tool. The non-preconditioned data were then subjected to BTEM in order
to recover the pure component spectra of the species present. The pure component spectra
of background moisture and carbon dioxide, hexane, dissolved CO in hexane, and the
dissolved species present, namely the organic reactant 3,3-dimethylbut-1-ene, the organic
product 4,4-dimethylpentanal, the catalyst precursor Rh4(σ-CO)9(μ-CO)3, the observable
organometallic intermediate RCORh(CO)4, and Rh6(CO)16, were all readily recovered. An
unexpected finding was a very well resolved spectrum with two terminal CO vibrations
centered at 2068 and 2076 cm-1 (almost exactly overlapping with Rh4(σ-CO)9(μ-CO)3, but
without bridging carbonyls). With reasonable certainty we are assigning this new spectrum
to the previously unknown complex Rh4(σ-CO)12. The results indicate that spectral
reconstruction, using no libraries and no a
priori information, is indeed possible from semi-batch runs. This finding holds promise for rapid and cost-effective spectroscopic system
identification of reactive organometallic and homogeneous catalytic systems.
Keywords: Phosphorus / Tautomerism / NMR spectroscopy / IR spectroscopy The tautomeric behaviour of five secondary phosphane oxides (SPOs) with different electronic properties has been investigated by NMR and IR spectroscopy, density functional theory calculations and X-ray structural analysis. Proof is given that only with strong electron-withdrawing groups on the phosphorus atom can the relevant trivalent phosphinous
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