Spectrum An Example

The formulas in cells C10 (converting wavelength X to wavenumber v) and D10 (calculating the Gaussian band profile of band 1) are  

niax values for bands 2 and 4, from other experimental measurements, are 445 and 880 nm, respectively. 

Start with a table of wavelength, absorbance data pairs. 

Determine the number of bands necessary to describe the spectrum. This can usually be arrived at by inspection a strongly asymmetric band generally indicates one or more hidden bands a band with a flat maximum indicates two strongly overlapped bands, etc. Alternatively, you can use the first derivative of the spectrum (AA/Ax). Except for the most hidden shoulders, each A/I/Ajt = 0 value indicates a band maximum. 

Estimate the half-width of the bands by using one or more bands not overlapped by other bands. As first approximation, use this value for all bands in the spectrum 

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The particle size as well as the method and duration of grinding can have an influence on the spectrum. An example of these effects can be found in Ref. (247) and references contained therein. Considerable grinding is necessary to achieve reproducible data for Ba2Co04 (47) and various divanadates (242). Other general information about the pellet technique is given in (243). 

In the syndiotactic sequence the two methylene protons adjacent to an M—C—X group are in the same environment and are chemically equivalent, whereas in the isotactic sequence they are chemically nonequivalent and give rise to an AB spectrum. An example of the spectrum of a vinyl polymer, polymethyl methacrylate, is shown in Fig. 13.6. Note that the CH2 resonance around 2.1 ppm is a singlet in a but an AB quartet in b. 

Scanning the frequency of the dissociation laser and collecting the total OH fluorescence, while the state-selection and probe frequencies are kept fixed on specific transitions, produces a PHOFEX spectrum an example is displayed in the right-hand panel of Fig. 10. The lines correspond to specific resonance states with rotational quantum number J and projection quantum number if = 2 in vibrational state (6,0,0). If the individual lines are broader than the resolution of the laser system, one can determine the width from fitting the spectrum and thus determine the state-specific dissociation rate. If the true linewidth caused by dissociation is smaller than the resolution of the laser system, the rates can be extracted from time-resolved measurements. All three laser frequencies are fixed, and the OH probe laser used to detect a particular state of OH is delayed with respect to the dissociation laser. In this way one can monitor the appearance of the OH products as function of the delay time, in the same way as described above for N02- In contrast to NO2, however, the rate is a state-specific rate rather than an average rate, because of the high selectivity of the overtone... 

The concentration required for satisfactory analysis was between 0.1 and 0.5% by weight, assuming an injection volume of between 400 and 150 )iiL, respectively. These conditions are consistent with normal methods used in GPG analysis. At this concentration and assuming a distribution of the sample over a 20-min time period, a signal level of between 0.1 and 0.5 AU is observed on the FTIR spectrum. An example of a typical set of spectra is shown in Figure 2. The signal-to-noise ratio in all spectra are quite adequate for qualitative interpretation. 

Acetone, for example, exhibits a high-intensity tt tt transition and a low-intensity R — rr transition in its absorption spectrum. An example of r —> tt transition occurs in ethers (R—O—R ). Since this occurs below 200 nm, ethers as well as thioethers (R—S—R ), disulfides (R—S—S—R), alkyl amines (R—NH2), and alkyl halides (R—X) are transparent in the visible and UV regions that is, they have no absorption bands in these regions. 

The aromatic protons of symmetrically ortho-disubstituted benzenes also give a symmetrical AA XX spectrum. An example is <7-dichlorobenzene (Fig. 4.42). 

All the recorded spectra result in an overlay of spectra representing the progress of the photoreaction. This sequence of spectra is called a reaction spectrum. An example is given in Fig. 4.16 for the photoisomerisation of trans- to c. r-stilbene with consecutive rearrangement to phenanthrene. Since the dihydrophenanthrene is very unstable at the chosen conditions and time domain, it is not observable as an intermediate as in flash photolysis experiments [60,61]. In liquids and solids, absorption spectra do not permit the characterisation of the individual reactants. Therefore a reaction spectrum merely informs the photochemist about the complexity of a reaction at first glance. In addition, wavelengths can be selected which best represent the... 

The most common two-dimensional NMR experiment is COSY (pronounced cozy ), which is an abbreviation for correlated. spectroscopy. This experiment exploits internuclear coupling to establish relationships among peaks in the spectrum an example will be discussed in Sec. II.H.4. In homo-nuclear COSY, the pulse sequence is essentially the same as that depicted in Fig. 16 the heteronuclear version utilizes a somewhat more complex scheme. The whimsically named INADEQUATE (mcredible atural-abun-dance do xb e-quantam transfer experiment) reveals a molecule s carbon skeleton. Similarly, /zeteronuclear wultiple-ftond correlation (HMBC) estab-... 

A totally different approach has been reported by Steinhauer et al. The IR spectrum is used as input into a CPG neural network. The network supplies the corresponding radial code (see Section 4.3) as output. This simulated radial code can be iteratively decoded to the molecular 3D structure. This method allows one, for the first time, to obtain 3D information from an IR spectrum. An example, the prediction of the 3D structure of diphenyl ether, is shown in Figure 9. 

Typically, once the measurement of the background spectrum is complete, the sample is placed in the infrared beam. Interferograms are measured with the sample present, are added together, and then Fourier transformed to obtain the sample single beam spectrum, an example of which is shown in Figure 2.17. A sample single beam spectrum contains contributions from the environment, instrument, and sample. Note... 

If the spectral coverage of the spectrometer or multichaimel radiometer is wide enough, detailed information on surface temperatures can be obtained by analyzing the shape of the spectrum. An example provided by the Voyager infrared measurements of lo is discussed in Subsection 6.5.b. 

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