Spectra, interpretation exercises

As we will see in Chapter 4, g-matrices are often difficult to interpret reliably. The interpretation of isotropic g-values is even less useful and subject to misinterpretation. Thus isotropic ESR spectra should be used to characterize a radical by means of the hyperfine coupling pattern, to study its dynamical properties through line width effects, or to measure its concentration by integration of the spectrum and comparison with an appropriate standard but considerable caution should be exercised in interpreting the g-value or nuclear hyperfine coupling constants. 

In order to ameliorate the sharply sloping background obtained in an STS spectrum, the data are often presented as di,/dFh vs. Vb, i.e. the data are either numerically differentiated after collection or Vb has a small modulation applied on top of the ramp, and the differential di,/d Vb is measured directly as a function of Vb. The ripples due to the presence of LDOS are now manifest as clear peaks in the differential plot. dt,/dFb vs. Vb curves are often referred to as conductance plots and directly reflect the spatial distribution of the surface electronic states they may be used to identify the energy of a state and its associated width. If V is the bias potential at which the onset of a ripple in the ijV plot occurs, or the onset of the corresponding peak in the dt/dF plot, then the energy of the localised surface state is e0 x F. Some caution must be exercised in interpreting the differential plots, however, since... 

Exercise 9-43 Figure 9-50 shows the 1H and 13C nmr spectra of a compound C6H10O. With the aid of these spectra, deduce the structure of C6H10O. It will be seen that the 13C spectrum is quite simple, even though the proton spectrum is complex and difficult to interpret. 

Exercise 17-4 a. The proton nmr spectrum of 2,4-pentanedione is shown in Figure 17-1. Interpret this spectrum by assigning each resonance to a structurally different proton, and explain why the broad resonance at 15 ppm is at unusually low field strengths. 

Exercise 17-47 Interpret the proton nmr spectrum shown in Figure 17-6 in terms of possible structures of compounds with molecular formula C10C10O2 with one phenyl group, CgHg. ... 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

All evidence of isotope patterns in the product ion spectrum of the m/z 304 parent (Fig. 19, bottom panel) disappears because the selected parent is monoisotopic—CiqHisNOF Cli. Justification for the absence of isotope patterns is identical to that of interpretation of the m/z 300 product ion spectrum. The detailed explanation of this excellent example serves to illustrate the care one needs to exercise in interpreting isotope patterns (or the lack thereof) in MS-MS spectra. At the same time, it demonstrates the structure-indicating power of careful interpretation. 

The manual interpretation of a mass spectrum - nature and origin of the fragmentation peaks - is a difficult but interesting exercise. Organic chemists are usually familiar with these methods of interpretation since they retrieve different types of transient ions considered to explain reaction mechanisms in condensed phases. The difference here is that these ions move in a vacuum and do not collide. The very short period of time between ion formation and ion detection (a few pis), allows to observe the existence of very unstable species that are unstable under normal conditions. 

At this point, it may seem to the reader that the detailed consideration of quantum beat phase distributions is a somewhat abstract exercise bearing little relation to IVR. We would justify our attention to the problem of phases by noting that the proper interpretation of experimental results from picosecond-jet experiments on IVR relies on the ability to determine how closely one s experimental conditions correspond to one s theoretical model of the experiment. A particularly convenient way to do this is by comparing phase characteristics from experiment with those from theory. In addition, phase characteristics are useful in helping one assign the various bands in a fluorescence spectrum to band types. 

This exercise introduces you to the quadrupole mass filter. Briefly describe how the mass spectrum is obtained and if you so desire, attempt to provide a brief mass spectral interpretation. You may want to review a text that introduces GC-MS, such as that dted in Ref. 5. 

Despite the characteristic features in the spectra of furanoses, some caution (24) should be exercised to exclude the possibility of rearrangements when interpreting the mass spectrum of an unknown. It has been reported (25) that isomeric tetrahydrofuran and tetrahydropyran diacetates have virtually identical mass spectra due to rearrangement of the latter to give after appropriate eliminations, a common cyclic oxonium ion at m/e 71. 

Caution should be exercised in the interpretation of the PMR spectra of some bisbenzylisoquinolines. For example, the spectrum of thalsimine at room temperature indicated a 1 1 mixture of isomers, showing ten methoxyls and two A-methyls. The two isomers are stable conformers, but on heating to 95° the spectrum showed only the expected five methoxyl groups. A similar phenomenon had previously been observed in the case of a related synthetic imino base. ... 

The appearance of three peaks for the carbon atom of CDCI3 illustrates this (Figs. 8.46-8.48). Care should therefore be exercised when interpreting NMR spectra to avoid mistaking peaks due to solvent for those of the sample itself. If overlap of sample and solvent peaks is suspected, it may be necessary to select a different solvent for a NMR spectrum. 

It is important to recognise that the relaxation time spectrum and the retardation time spectrum are only mathematical descriptions of the macroscopic behaviour and do not necessarily have a simple interpretation in molecular terms. It is a quite separate exercise to correlate observed patterns in the relaxation behaviour, such as a predominant relaxation time, with a specific molecular process. It should also be emphasised, as will be apparent from the further detailed discussion, that qualitative interpretations in general molecular terms can often be obtained from the experimental data directly, without recourse to calculation of the relaxation time spectrum or the retardation time spectrum. 

Tetiyl, a nitramine with a trinitroaromatic nucleus, gives a discrete chromatographic peak when analyzed by GG-MS so it was only natural to assume that the mass spectrum of this peak represented the mass spectrum of tetryl [22,23]. ffowever, it was found [24,25] that the mass spectrum was not that of tetryl but of 7V-methylpicramide. The latter is the hydrolysis product of tetryl, as shown in Figure 2. This hydrolysis, which takes place in the gas chromatrograph, most probably in the injector, is an example of an artifact which, if overlooked, may lead to an erroneous attribution of the mass spectrum of one compound N-methylpicramide) to another compound (tetryl). It demonstrates the caution that should be exercised in interpreting GG-MS results, especially of thermally labile compounds. 

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