Monomers Fourier transform spectra

The initial observation is that PMMA is essentially completely degraded to monomer by heating to 375°C in a sealed tube while heating a mixture of red phosphorus and PMMA under identical conditions yields a solid, non-deqraded, product as well as a lower yield of monomer. One may observe, by 3C NMR spectroscopy, that the methoxy resonance is greatly decreased in intensity and methyl, methoxy phosphonium ions are observed by 31P NMR. Additional carbonyl resonances are also seen in the carbon spectrum, this correlates with a new carbonyl vibration near 1800 cm 1 in the infrared spectrum and may be assigned to the formation of anhydride. The formation of anhydride was also confirmed by assignment of mass spectra obtained by laser desorption Fourier transform mass spectroscopy, LD-FT-MS. 

It is well known that even modern Fourier-transform instruments require concentrations of at least 10 M to give a reasonably resolved NMR spectrum. Also, the minimum time necessary for a spectrum to be taken or for one given signal to be scanned is of the order of a minute. These constraints rule out the possibility of identifying short-lived transients or any compound present in low concentrations formed during a cationic polymerisation. Thus, while carbenium ions have been characterised by this technique under suitable conditions, their concentration in a polymerising solution is always too low to permit detection by NMR spectroscopy. On the other hand, one can easily follow the disappearance of the monomer and/or the formation of the polymer and of any other product (such as an ester) formed in appreciable quantities, an operation which is extremely convenient, not only from the point of view of the kinetics of pol)onerisation, Init also because it can lead to mechanistic information concerning the nature and the structure of various products. 

Previous work of one of the authors dealt with mid IR monitoring of IB polymerization using fiber-optic equipment. Fourier transform near infrared spectroscopy can be accomplished without expensive hardware. It therefore seemed to be a desirable goal to combine the advantage of fiber optics with low-cost fibers available for measurements in the NIR range. The NIR spectrum of IB obtained after solvent subtraction (Figure 2) reveals at least three signals, which should be suitable for the determination of monomer conversion. 

This method was in fact carried out around two decades ago [30, 31]. However, it was applied only in the fermentation of pure microbial cultures. In a recent report by Acros-Hernandez and coworkers [32], infrared spectroscopy was applied to quantify the PHA produced in microbial mixed cultures. Around 122 spectra from a wide range of production systems were collected and used for calibrating the partial least squares (PLS) model, which relates the spectra with the PHA content (0.03-0.58 w/w) and 3-hydroxyvalerate monomer (0-63 mol%). The calibration models were evaluated by the correlation between the predicted and measured PHA content (R ), root mean square error of calibration, root mean square error of cross validation and root mean square error of prediction (RMSEP). The results revealed that the robust PLS model, when coupled with the Fourier-Transform infrared spectrum, was found to be applicable to predict the PHA content in microbial mixed cultures, with a low RMSEP of 0.023 w/w. This is considered to be a reliable method and robust enough for use in the PHA biosynthesis process using mixed microbial cultures, which is far more complex. 

Figure 10.2 Absorption spectrum of adenine dimer (blue dashed line) and monomer (red solid line) obtained at pure electronic level (a) and at vibronic level (b) by adopting the vibronic Hamiltonian discussed in Section 10.3.1.3. It has been computed from the Fourier transform of the autocorrelation function obtained propagating a doorway state. The latter is a delocalized exciton state obtained mixing the two localized exciton states with equal weights.

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