IR extinction coefficient

Extinction coefficients for the various nitrates and nitrites are collected in Table I. Our experimentally determined IR extinction coefficients for other species are collected in Table II. It must be emphasized that the methods discussed above do not quantify all possible oxidation products. An obvious omission is the absence of a reliable method for dialkyl peroxides. An HI based method has been suggested in the literature, but proves to be extremely imprecise on film samples in our laboratories because of large reagent blanks (27). 

IR extinction coefficients as a criterion for chemical activation upon adsorption propene interaction with cationic forms of y zeolite... 

Table 4.5 Selected experimentally determined IR extinction coefficients for surface species and adsorbates on zeolites. (Adapted from Karge and Geidel [87]).

Table 2. Experimentally derived IR extinction coefficients of surface species of zeolites and adsorbates on zeolites... 

TABLE 11 IR Absorptions and Extinction Coefficients (c) for Oxidation Product... 

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

One cm3 of the reactant/product/catalyst mixture was sampled periodically during the reaction for the transmission infrared analysis (Nicolet Magna 550 Series II infrared spectrometer with a MCT detector). The concentrations of reactants and products were obtained by multiplying integrated absorbance of each species by its molar extinction coefficient. The molar extinction coefficient was determined from the slope of a calibration curve, a plot of the peak area versus the number of moles of the reagent in the IR cell. The reaction on each catalyst was repeated and the relative error for the carbamate yield measured by IR is within 5%. 

The use of electrochemical transmittance spectroscopy in both the UV-visible and IR regions of the spectrum is elegantly shown by the work of Ranjith et al. (1990) who employed an OTTLE cell to study the reduction of benzoquinone, BQ. The authors were the first to report the UV-visible spectrum of BQ2- and to demonstrate the quantitative aspects of the technique by reporting extinction coefficients for the major bands of BQT and BQ2- in both the UV-visible and IR. 

Before in situ external reflectance FTIR can be employed quantitatively to the study of near-electrode processes, one final experimental problem must be overcome the determination of the thickness of the thin layer between electrode and window. This is a fundamental aspect of the application of this increasingly important technique, marking an obstacle that must be overcome if it is to attain its true potential, due to the dearth of extinction coefficients in the IR available in the literature. In the study of adsorbed species this determination is unimportant, as the extinction coefficients of the absorption bands of the surface species can be determined via coulometry. 

Keywords IR spectroscopy, integral molar extinction coefficients, propene, adsorption, zeolites. 

The density of Bronstcd and Lewis acid sites was determined by IR spectroscopy (Nicolet 710) of adsorbed pyridine, after desorption at 250°C, using the molar extinction coefficients previously obtained by Emeis [11]. The acid strength distribution of selected zeolites was studied by NH3-TPD in an Autochem 2910 Equipment (Micromeritics) coupled to a quadrupole mass spectrometer. First, NH3 was adsorbed at 175°C until saturation and then desorbed by increasing the temperature up to 800°C at a heating rate of 10°C/min. 

T-O-T stretch measured by framework IR, as discussed in Section 4.5.3.2. The comparison of areas as described above does provide quantitative information about the relative changes in acidity between the samples since the area is direction proportional to the concentration (Beer-Lambert law, discussed in Section 4.5.2.) It most cases, this relative, but quantitative comparison between samples is sufficient to provide information about how various treatments or modifications have altered acid site distributions. Since extinction coefficients can change with zeolite type (Table 4.5), these comparisons are best for samples of the same zeolite type. Therefore, caution should be used when comparing data from samples with different zeolite structures. 

Fortunately, the mid-IR stretches of the terminal carbonyls of simple carbonyl iodide complexes of Rh and Ir occur in the region 1950 to 2150 cm Their extinction coefficients are sufficiently strong that even in aqueous AcOH, which would not be a solvent of choice for mid-IR spectroscopy, at concentrations in the range of 100s of ppm w/w, the simple Rh or Ir carbonyl iodides can be detected by FTIR with a modest acquisition time. Indeed much of the original IR work to study both Rh and Ir catalysed carbonylation by workers at Monsanto [11, 25] and by Schrod et al. [26] appears to have been carried out using continuous wavelength machines. 

When Ru carbonyl iodides are used as promoters for Ir catalysed carbonylation of MeOH to AcOH, the accurate quantification of Ir species becomes more difficult because the bands of the Ru species overlap those of the Ir species and they have larger extinction coefficients, so dominating the spectra. In batch reactions followed by HP IR, the Ru bands present initially before Ir is added and carbonylation commences have been assigned to neutral Ru carbonyl iodides and [Ru(CO)3l3] ... 

Perhaps the most widely studied chemical class of near-IR absorbers are the cyanines. The chemistry and synthetic routes to cyanines are discussed in other parts of this book (Chapter 2, section 2.3.1.4 and Chapter 3, section 3.5.1.7). The factors influencing the position of their absorption maxima and molar absorption (extinction) coefficients areC ... 

---