Para-substituted compounds, spectra

Chemical shift increments are the shifts from that of the protons of benzene (8 7.27). Thus for a CH C=0 substituent (line 26, Appendix Chart D.l), the ortho increment is +0.63, and the meta and para increments are both +0.28 (+ being at higher frequency than 8 7.27). The C-2 proton has two ortho substituents the C-4 and C-6 protons are equivalent and have ortho and para substituents the C-5 proton has two meta substituents. Thus the calculated increment for C-2 is + 1.26, for C-4 and C-6 is +0.91, and for C-5 is +0.56. The spectrum shows increments of +1.13, + 0.81, and +0.20, respectively. This agreement is adequate for determining the substitution pattern. Integration of the spectrum would show the expected ratio of 1 2 1. Furthermore, both the ortho- (with identical substituents) and para-substituted compounds would show the characteristic, symmetrical patterns of Figures... 

An example of an aromatic fluorine-containing compound can be found in Figure 7.7, where we have recorded the 19F spectra (both proton coupled and decoupled) of p-fluoroacetophenone. Once again we find a singlet for the fluorine atom in the proton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho and meta protons in this para-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 4. 

The ortho substituents used were supposed to have no influcence on the absorption spectrum and to possess the same circular dichroism as the parent compound. The ortho substituted compound can only photocyclize in its exo-conformations, but equilibration by a rotation over the phenylethylene bond converts an M-exo into a P-endo form (or P-exo into M-endo) which cannot cyclize. Indeed, the optical yields of the 4-substituted [6]hclicenes from the ortho substituted (1+4) precursors are not lowered in comparison with the unsubstituted (2 + 3) precursor. On the other hand (4+1) precursors having a para substituent give again a low optical yield. The values found suggest that racemization in the excited state is slightly hindered, what may be due to steric hindrance. 

The N=N stretching vibration of a symmetrical trans-azo compound is forbidden in the IR but absorbs in the 1576 cm-1 region of the Raman spectrum. Unsymmet-rical para-substituted azobenzenes in which the substituent is an electron-donating group absorb near 1429 cm1. The bands are weak because of the nonpolar nature of the bond. 

These compounds are suggested if sulphur is present. If nitrogen is also present the compound may be an aminosulphonic acid. The infrared spectrum will show absorption at 3400-3200 cm -1 (OH str.) and 1150 and 1050 cm-1 (S=0 str. in a sulphonic add) or at 1090cm-1 (S=0 str. in a sulphinic acid). For derivative preparations for sulphonic acids see Section 9.6.26, p. 1284. The presence of an aromatic sulphinic add may be further confirmed by dissolving in cold concentrated sulphuric add and adding one drop of phenetole or anisole when a blue colour is produced (Smiles s test), due to formation of a para-substituted aromatic sulphoxide. The reaction is ... 

Temperature dependent lineshape variation of the Pake doublet represents then an experimental evidence of a motion occurring in the solid state. 2H NMR investigations on selectively deuterated compounds, therefore, are important tools for the recognition of molecular motions in solids. This method has been applied to the characterization of molecular motions in molecular solids [8] and inclusion compounds [9]. As an example in Fig. 3.2.4 is reported the effect of the motion about the molecular symmetry axis, either by two-fold or n-fold (n>3) flips, on the deuterium spectrum of a deuterated para-substituted benzene [10]. 

The acidic product generated in the reaction is assumed to be 4-chlorobenzoic acid. This material also is a para-substituted aromatic compound thus, the spectrum of this compound (Fig. 6.17) possesses a macro frequency train (a), similar to those of the aldehyde and alcohol. In addition, this benzoic acid derivative exhibits an extended aromatic acid macro group frequency train (b).The macro frequencies are as follows ... 

At this point, it should be re-emphasized that, although it is intellectually pleasing to correlate the observed CID spectrum with the expected fragments of a known structure, it is not necessary to know the chemical structure of a compound in order for its CID spectrum to serve as a fingerprint. Observing two compounds (in this case dye anions) with identical ESI-MS peaks - which also produce identical CID spectra from those peaks - essentially ensures that the compounds are identical or at least very closely related in structure. Rare exceptions could arise e.g., in cases of positional isomerism such as the ortho or para substitution pattern alluded to above for the extra chlorine in dye 3. 

Cyclic ethers, cyclic acetals, and some vinyl compounds can be polymerized by cationic processes. Photoinitiation of these polymerizations by ultraviolet light are known (12,106). Some extension of direct photolytic sensitization of cationic processes to visible wavelengths is obtained by the use of colored diazonium salts as initiators. For example, Schlesinger (11a) used diazonium salts substituted in the para position with electron withdrawing groups, but sensitivity was limited to the blue to green regions of the spectrum. 

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