General Appearance of the Spectrum

A brief inspection of the bar-graph spectral presentation can tell the experienced interpreter a substantial amount about the unknown molecule. You have already seen how the mass and relative abundance of the molecular ion indicate the size and general stability of the molecule (Section 3.6). In addition, the number of abundant ions in the spectrum and their distribution in the mass scale are indicative of the type of molecule and the functional groups present. For example, a glance at the spectrum of Unknown 5.1 should tell you that this is a highly stable molecule. Deduce its elemental composition this should correspond to a value for rings plus double bonds which is consistent with this stability. Not only are there no weak bonds in the molecule, but apparently the main fragmentation pathways are of approximately the same low probability. What is a possible structure  

In the spectrum of Unknown 5.2 the m/z 43 peak is by far the most prominent in the spectrum. The bond that is cleaved in the formation of this ion would be expected to be the weakest in the molecule from its known chemical reactivity. 

Unknowns 5.3 and 5.4 are the spectra of larger molecules producing only a few prominent peaks. In evaluating the m/z 50 and 51 peaks of Unknown 5.3, don t forget that the importance of a peak decreases with decreasing mass, as well as with decreasing abundance. 

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Further, one should pay attention to the general appearance of the spectrum (parameters of the mass spectrometer, the most intense peaks, and characteristic groups of peaks). Thus, if there are many peaks of fragment ions, while their intensity increases toward the low m/z values, the sample most probably belongs to the aliphatic compounds. On the contrary, rare intense peaks indicate the aromatic nature of the sample. You should start the detailed interpretation with identification of the molecular ion peak. 

Make any deductions which are possible from the general appearance of the spectrum identify any ion series and characteristic ions. 

The class of compounds to which the unknown belongs can be deduced from M + I abundance and the general appearance of the spectrum. 

Conpound 28 was isolated as colorless crystals. The spectral data of conpound 28 closely resembled those of conpound 27. The FD-MS of conpound 28 showed an at m/z 704 and the EI-MS spectrum furnished a significant ion at m/z 370. These ion peak were 2 amu more than the corresponding peaks in the mass spectrum of compound 27. H-NMR spectral details for the acetylated compound 28a are given in Table 3.6. The general appearance of the spectrum of... 

Pay attention to the general appearance of the spectrum long-chain aliphatic compounds show a characteristic hydrocarbon pattern, whereas aromatic compounds produce a few but highly abundant ions. 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

Fox et al. (9 ) also investigated the nuclear magnetic resonance spectrum of NOFg which indicated that the molecule contained three equivalent fluorine atoms. This result, along with the general appearance of the infrared spectrum, clearly establishes that the molecule has Cg symmetry. The N-0 bond distance and bond angles are estimated quantities from the work of Curtis et al. (7 ). 

The recent development of a gradient high-resolution MAS probe will extend the utility of 2D experiments by removing artifacts that generally accompany MAS 2D NMR data on resin samples (46). The lack of artifacts is illustrated by the high quality SECSY spectrum shown in Fig. 16. SECSY data contain the same information as a COSY spectrum, but the appearance of the spectrum is different. The diagonal lies along the FI =0 and the off-... 

What we see is that in general the appearance of the spectrum depends on the position of the signal at time zero, that is on the phase of the signal at time zero. Mathematically, inclusion of this phase shift means that the (complex) signal becomes ... 

The general aspect of the spectrum of CO2 species adsorbed on Ce02 is practically not modified by evacuation at room temperature (Figure 3b), excepted the small shifts of the bands at 1730, 1354 and 1140 cm"T Ml the bands at 1730, 1570, 1505, 1470 (shoulder), 1400, 1355, 1290, 1218, 1140, 1050 and 1040 (shoulder) cm-1 are attributed to carbonates and carboxylates species [19-23], It is important to mention that the bands present at 1648 and 1235 cm-i in the spectrum of CO2 irreversibly adsorbed on alumina, appear at positions allowing their discrimination from the contribution of ceria. Therefore, they can be tentatively used to characterize free alumina smface in ceria-alumina samples. 

The bar graph is preferred when the general appearance of a spectrum is an important factor in interpretation. 

The solvent may affect the appearance of the spectrum, sometimes dramatically. Polar solvents generally wipe out the vibrational fine structure in a spectrum. Solvents may also shift the position of the absorption band, as will be discussed. For the visible region of the spectrum, any colorless solvent can be used in which the sample is soluble. 

The general appearance of the mass spectrum depends on the type of compound analyzed. Seeing the patterns that distinguish, say, a normal alkane from an aromatic hydrocarbon requires practice and lots of it. The following examples include simple gas molecules and simple compounds representative of a variety of organic chemicals. 

Analytical features. As a typical example of the form in which experimental data appear during PIXE analysis, Fig. 33.4 presents an X-ray spectrum obtained on a sample of coarse fraction atmospheric aerosol deposited on a polycarbonate membrane filter of 7 pm thickness. The spectrum was taken with protons of 2 MeV bombarding energy in a run of 1,200 s and beam intensity of 30 nA. The physical conditions in this case correspond to the approximation of the thin uniform homogeneous sample. Full and dashed lines show curves fitted by the spectrum evaluation code to the total spectrum and to its continuous background, respectively. The general features of the spectrum can be well explained on the basis of the simplified diagrams in Fig. 33.1. 

Comparison of the 720-730 cm" (13.89-13.70 pm) doublet shows that only the 720 cm (13.89 pm) component in the air-irradiated sample remains in the spectrum of the vacuum samples, whereas there is only a slight decrease of the 730 cm i (13.69 pm) component in the air-irradiated samples. Additional evidence of structural changes is shown in the spectra of the air-irradiated sample. Here -OH and C=0 hands appear, and there is a general depression of the spectrum background from 1300 cm to 900 cm" (7.69-11.11 pm). 

Diastereotopic groups are inequivalent and generally resonate at different chemical shifts. We will therefore see a /(P,H) doublet for 10.1 and a pair of /(P,H) doublets for 10.2. The appearance of the spectrum changes on moving to a higher field spectrometer (Fig. 10.3) because the diastereotopic resonances differ by a certain chemical shift in parts... 

Compound 51 (Table I) is a biradical. In this case exchange interaction between the radical centers is manifest in the ESR spectra Contrary to the case of the ESR spectra of monoradicals, which are triplets characteristic for nitroxides with splitting 14.5 Oe, in the spectrum of the biradical a quintet is observed with the ratio of the amplitudes of the first derivative lines 1 1.7 3 1.7 1. This ratio is rather close to that expected for the case of strong exchange between the radical centers (J %). Direct measurement of the exchange integral value, carried out by the method of electron spin echo (Sadykov et al, 191 A) showed that J = 3 X 10 Hz, i.e., J a 8. The general appearance of the quintet and... 

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