Ring current contribution

This is not strictly true if the ring current contribution to orA,deloc. includes atom A in the delocalized ring. However, the perturbation of an asymmetric charge distribution on the ring current generally seems to be ignored. 

The H-NMR spectrum of the parent compound (4) 9,20,30.59 analyzed as an AA BB CC system the ratioJ j/Jj g = 0.73 is as expected when compared both with hetero analogs and with benzene and 1,3-cyclohexadiene. Calculations on the ring current contributions to diamagnetic anisotropy and chemical shifts have been reported. H-NMR data for other benzo[c]furans have been published.66.70,71.109,111.114,132... 

The conformation-dependent contribution to chemical shifts that is best understood is the ring-current contribution associated with conjugated groups. Various functional forms have been tested and calibrated against observed data (41-43). The basic ideas may be found in textbooks (8) and reviews (41). The general form of ring current contributions is ... 

As we discussed in the introduction, there are many potential contributions to chemical shift dispersion in nucleic acids. Here we have considered two that are likely to be the most important ring current contributions from the aromatic bases,... 

We have noted that results for sugar proton shifts are substantially poorer than those for the bases. There are several possible reasons for this that point the way to further research. First, ring current contributions are probably the best-understood... 

Figure 3). These upfield shifts reflect the base pair overlap geometries in the duplex state and result predominantly from ring current contributions due to nearest and next-nearest neighbor base pairs (33,34) These contributions can be computed for the B-DNA overlap geometry and are compared with the experimental upfield chemical shifts on duplex formation of poly(dA-dT) and the related synthetic DNA pol.y(dA-dU) in Table II.

The computed upfield ring current shifts are based on nearest neighbor, next-nearest neighbor and cross-strand ring current contributions tabulated by Arter and Schmidt.(34)... 

The large upfield shifts of 1.6 ppm for both phenolic protons which are located between rings B and C of the anthra-cycline ring chromophore demonstrates that the anthracycline ring intercalates into the nucleic acid duplex and experiences ring current contributions from adjacent base pairs. This in turn requires that anthracycline rings B and C overlap with adjacent base pairs at the intercalation site. 

A strong hydrogen bond of sodium 4,5-dihydroxynaphthalene-2,7-disulphonate (39) has been observed by NMR at a low temperature (5(OH) 17.72 ppm) . This value has been related to the O- O distance in relation to other measurements, in order to use OH chemical shifts to obtain oxygen-oxygen distances. One possible drawback of using values from the mono-ionized 1,8-dihydroxy naphthalene directly is that the chemical shift of the OH proton is likely to have a sizeable ring current contribution. Phenols are also involved in catalytic triads, e.g. in ketosteroid isomerase , leading to OH chemical shifts of 18.2 ppm. 

In addition to the through-space influence, a ring current contribution is... 

The n ring current contribution to the out-of-plane component of the magnetic susceptibility, Xj. <87ZN(A)49). 

The magnetic dipole approach used earlier gives a ring current contribution calculated as 1.46 ppm. Local contributions can be estimated from the chemical shift of the protons of a nonaromatic reference such as 1,3-cydohexadiene, which resonates at 5.8 ppm (protons in 2- and 3-position). With 5.8 +1.46 = 7.26 ppm, the calculated shift is quite close to the experimental value of 7.3 ppm. Waugh and Fessenden [42] and Johnson and Bovey [43] improved the dipole model by assuming a pair of current loops above and underneath the benzene ring separated by a distance of 0.9 or 1.28 A (Figure 13.5). 

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