Dilution shifts

Dilution shifts of 13C signals may reach a magnitude of several ppm. The 13C resonance of methyl iodide dissolved in cyclohexane [91 a] or tetramethylsilane [91b] shifts upheld by about 7 ppm upon dilution. A much smaller upheld dilution shift (0.5 ppm) is observed for the 13C signal of chloroform in cyclohexane [92]. A constant shift independent of further dilution may be reached at lower concentrations. In this case, the solution behaves as if it were infinitely diluted in terms of chemical shifts. This was observed for substituted benzenes in trifluoroacetic acid at a solute concentration in moles of 10 to 15% [93]. 

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Bond alternation A-= SAJV Fm s.-s/ Dilution shift, g Solvent shift is B A Aat"... 

Figure 2. Infinite dilution shifts for 87Rb derived from Figure 1...

Bond alternationa A A° 2ANd e Vm 8a-8bl Dilution shift, g Solvent shift(s)h Bl Aj A k... 

Protonation shifts of pyridine (Table 4.67) are much stronger but follow the trend of dilution shifts portrayed in Fig. 4.12. Shielding in the a position is attributed to a change of the N —C-a bond order. Deshieldings at C-3 and particularly at C-4 arise from an increased electron withdrawal of the positively charged nitrogen. Shift changes induced... 

In 1992, Zimmerman and Duerr reported the use of the hydrogen bond-mediated cyclic aggregation strategy in the construction of cyclic trimer 1 from monomer 2 as opposed to dimer 3 or the open trimer 4 (Scheme 3.1) [6]. The cyclic structure of the aggregate and its corresponding trimerization constant (Ktrimer = 20000 M-2) were estimated from the Saunders-Hyne analysis of the NMR dilution shifts of 1 in 10% (CD3)2S0/CDC13. 

An excellent linear correlation between the dilution shift parameters (A) for thiophene and the aromaticity index /a... 

The aromaticity index A = is based on the dilution shift method A5i is the 14 nuclear magnetic... 

One obtains a relation between AG and the infinite dilution shift 6, if the first term on the right hand side of Eq. (37) is simplified under the assumption that the relative concentrations of all individual ionic species correspond to the statistical distribution,... 

Excessive dilution, however, should be avoided for three reasons. Since dilution shifts all the peaks to higher concentrations, an increasing propor-... 

There are two other noteworthy features of Fig. 4-5. First the change of 6 is much larger for acetic acid than for the phenols, despite the fact that at zero concentration the acetic acid is in part H bonded to acetone. Huggins et aL estimate (5d — Sm) to be 9 db 2, at least five times as big as this difference for phenol (981). The other interesting facet of the curve for CCI4 is that the dilution shift at concentrations... 

HCNfag) -> H+(aq) + CN-(agJ the solution is diluted Shift to right the product [H+] [CN ] diminishes more rapidly than does [HCN],... 

The dilution shift A i defined as the difference between the chemical shift of aromatic protons in the pure liquid and that in an infinitely dilute solution in nonpolar solvents, is inversely proportional to the molar volume (Vm) of the solute and directly proportional to the difference in the magnetic susceptibilities parallel and perpendicular to the axis of the ring.las, 134 On this basis Khetrapal and co-workers have argued134 135 that the percentage aromaticity (A) of a system relative to benzene (100%) can be calculated from the expression ... 

The diamagnetic susceptibility exaltation of pyridine,8413.4 X 10-8 cm3 mole-1, is fractionally lower than that for benzene (13.7 X 10-6) estimation of aromaticity from proton chemical shift does not appear to have been attempted. By the dilution shift criterion (Section II,D, 4) a particularly low estimate of the aromaticity has been obtained, viz. 61% that of benzene.137 On the other hand, the ratio of the ir-ring current in pyridine to that in benzene has been calculated to be 1.06 from molecular susceptibility data,270 while from studies of the Faraday effect the order of decreasing aromaticity, pyridine AT-oxide, pyridine, benzene, has been drawn.162... 

From the structural and electronic point of view, pyridine is the heteroaromatic that is most similar to benzene and, as such, examination of the extent of agreement between the various estimates of aromaticity is of interest. By the stability criteria pyridine appears to have about the same as or somewhat less than the aromatic character of benzene. Of the other methods, with the exception of the dilution shift result, there is also an encouraging similarity of results and they reinforce the view that the two rings have comparable aromatic character. It is of interest that the ratio of the ring currents of pyridine and of benzene is also reported to be close to unity.270... 

All the aromaticity indices that have been calculated are summarized in Table XV. The first two indices are derived from the NMR spectra the A parameter is based on the dilution shift method 17,1 the B parameter is based on the uniformity of the methyl effects on aromatic proton chemical shifts.19 The next two indices, EAN and Julg s (J)... 

The agreement among the other six empirical aromaticity parameters is not only qualitative, but even quantitative, as evidenced by the linearity of the plots of Fig. 3, in which the indices B, LAN, J, and fim are diagrammed against the dilution shift parameter, A. 

The dilution shift parameter (A F ) is another spectroscopic parameter used to determine aromaticity, where A(5j is the difference between the chemical shifts of aromatic protons in a pure liquid and in an infinitely dilute solution in a nonpolar solvent, and the molar volume of the compound <74JCS(P2)332>. Based on this concept, the aromaticity of a compound is defined as A = A(5i <72BSF1022>. The values correlate well with the Pauling resonance energies <33JCP(l)606>... 

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