Arenes proton chemical shifts

Chapter 10 Table 10.4. Proton Chemical Shifts of Arene Complexes of Transition Metals ... 

Figure 6.31. The proton chemical shifts (relative to 5 = 0.00 ppm for TMS) for the arene naphthalene (CioHg) and its biue, nonbenzenoid-but-aromatic isomer azulene (CioHg).

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

The chemical shifts of arene protons (6.5 ppm to 8.0 ppm) characteristically are toward lower magnetic fields than those of protons attached to ordinary double bonds (4.6 ppm to 6.9 ppm). The difference of about 2 ppm cannot be easily explained because the hydrogens in both types of systems are bonded to carbon through sp 2-a bonds (Sections 6-4C and 6-5A). 

Chemical Shift Values of Oxirane Protons of Some Arene Oxides... 

Hybrid amide-thiourea hosts based on p-f-butylcalix[4]arene have been prepared with coloured p-nitrophenyl (11.37a) or fluorescent 1-napthyl substituents (11.37b). In DMSO the host binds strongly to dicarboxylates, particularly adipate (log/T= 2X10" M" ), accompanied by a colour change from yellow to red in the case of the nitrophenyl derivative. The colour change is reversible on addition of protic solvents such as methanol but is not observed for acetate or other basic, monovalent anions. While large chemical shift changes for the thiourea NH protons are observed for 11.37b (A5 up to ca. 3.5 ppm), the thiourea NH resonances for the more acidic 11.37a disappear due to deprotonation. The fluorescence of the napthyl substituent in 11.37b is significantly quenched in the absence of anions due to a PET process. Addition of dicarboxylates results in an increase of fluorescence emission intensity in a broad band from 410 - 600 nm. Addition of anion reduces the efficiency of the PET process. 

In NMR, alkene and benzene carbons came in the same region of the spectrum, but in the NMR spectrum the H atoms attached to arene C and alkene C atoms sort themselves into two groups. To illustrate this point, look at the i Cand H chemical shifts of cyclohexene and benzene, shown in the margin. The two carbon signals are almost the same (1.3 ppm difference, < 1% of the total 200 ppm scale) but the proton signals are very different (1.6 ppm difference = 16% of the 10 ppm scale). There must be a fundamental reason for this. 

Many similarities exist between metallabenzenes and conventional arenes. Among these similarities are structural features such as ring planarity and the absence of bond length alternation, spectroscopic features such as downfield chemical shifts for ring protons, and chemical reactions such as electrophilic aromatic substitution and arene displacement from (arene)Mo(CO)3. All of these features, taken together, strongly support the thesis that metallabenzenes represent a new class of aromatic compounds, one in which metal d orbitals participate fully with carbon p orbitals in the formation of ring 7r-bonds. 

In most cases the signals of bridging methylene protons of cafix[4]arenes 1 together with the signals of the lower or upper rim substituents in H-NMR spectra clearly identify the macrocycle conformation as well as the spatial arrangement of the substituents [2]. Fnrthermore, Mendoza et al [3] introduced an exclusively useful rule that correlates C-NMR spectra of cafix[n]arenes ( = 4 - 6) with their conformations. Chemical shifts of bridging carbon atoms are equal to 8 31 if two adjacent aromatic groups are in the. yyn-orientation (u, u or d, d) and to 8 37, if they have anti-orientation (u, d). 

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