Through-bond effects

The first IPs of these diamines are exceptionally low. In solution too these compounds are easily oxidized giving long-lived radical cations, which are presumed to contain 3-electron a bonds. This is in accordance with IP(nN+) > IP( n ), i.e. through-space interaction dominates any through-bond effects. 

Using databases or tables of SCS to predict proton chemical shifts, only through-bond effects are effectively considered, and a typical r.m.s. difference between calculated and experimental shifts is 0.3 ppm. This is a lower value than for shifts, but this is a much higher proportion of the chemical shift range (3 vs. 0.75%). 

Both the face-to-face dimers and the T- and L-shaped dimers have relevance to biomimetic systems, especially in light of the work on the x-ray structure of the R. Viridis reaction center ill). As will be discussed in the following, the results obtained for the face-to-face dimer with an interplanar spacing of 5.35 A is prototypical of the results obtained for almost all the dimers investigated, and thus it provides a reasonable basis for illustrating a number of important features that arise from the calculations. However, it should be noted that the lack oP covalent links in our model dimers precludes the possibility of "through-bond" effects. 

Whether the omission of such effects would significantly alter the conclusions arrived at as a consequence of our calculations is difficult to say, but we expect that major through-bond effects in photo-induced ET would only become important as the distance between the two porphyrin macrocycles becomes very large. Hoffman and co-workers ( ) have discussed several aspects of through-bond effects, and Beratan and Hopfield (32) have approximated them in a recent study of ET in a rigid system. 

G. D. Scholes, K. P. Ghiggino, A. M. Oliver and M. N. Paddon-Row, Through-space and through-bond effects on exciton interactions in rigidly linked dinaphthyl molecules, J. Am. Chem. Soc., 115 (1993) 4345 1349. 

The 13C NMR chemical shifts and one-bond coupling constants 7(13C-H) have been obtained for (1), (2) and (5) in carbon disulfide (Table 8) (74OMR(6)430) (see also Section 4.01.3.4). The chemical shifts for the 3a-position in the title compounds are the reverse of what would be expected on the basis of a purely inductive effect of the Group VIA heteroatoms. If the chemical shifts are controlled by through-bond effects, then mesomeric interaction of (2) is of considerable importance in these systems. Thus, the 13C NMR... 

We have investigated peptides whose structures were known beforehand from NMR or x-ray spectroscopy and related these structures to 2D-IR spectroscopy. Ultimately, one would like to deduce the structure of an unknown sample from a 2D-IR spectrum. In the case of 2D NMR spectroscopy, two different phenomena are actually needed to determine peptide structures. Essentially, correlation spectroscopy (COSY) is utilized in a first step to assign protons that are adjacent in the chemical structure of the peptide so that J coupling gives rise to cross peaks in these 2D spectra. However, this through-bond effect cannot be directly related to the three-dimensional structure of the sample, since that would require quantum chemistry calculations, which presently cannot be performed with sufficient accuracy. The nuclear Overhauser effect (NOE), which is an incoherent population transfer process and has a simple distance dependence, is used as an additional piece of information in order to measure the distance in... 

Many other electron-transfer reactions may be influenced by through-bond effects. Indeed the possibility of either an electron or a positive hole being transmitted through bonds, over large distances within a molecule, is intriguing and is being actively investigated by ourselves and, I hope, others 37... 

As mentioned earlier, there are two electron-nuclear mechanisms that result in relaxation of the carbon nucleus dipolar and scalar. A dipolar mechanism can be thought of as a through-space effect, whereas a scalar mechanism is a through-bond effect. Therefore, it might be envisaged that the scalar mechanism is limited, because the transmission of electron density is rather restricted through sigma bonds. However, it does occur, and quantitative distance information is difficult to obtain from this mechanism. 

The Taft adaptation of equation 1.9 to aliphatic systems (with substituent parameters cj) does not distinguish between through-space and through-bond effects. Very recently it has been convincingly shown that this can mislead effects of substituents in carbohydrate frameworks depend on their orientation. Many supposed inductive effects in aliphatic systems are in fact the simple effects of electrostatic dipoles, as discussed in detail in the carbohydrate context in Chapters 3 and 6. 

FIGURE 5. The ns bands in the PE spectra of the dithianes illustrating through-space and through-bond effects. Reproduced by permission of Springer-Verlag from Reference 6... 

Longer range ns-ns interactions, where the S atoms are separated by one or more carbons, are expected to be much weaker. However, if through-bond effects occur they can completely obscure the smaller through-space effect. One of the simplest molecules which... 

Through-bond effects on the ns IEs in 1,2,4-trithiolane (15) are shown to be subtly affected by methyl substitution as observed in the PE spectra obtained by Bock and coworkers23 for 3,5-dimethyl-1,2,4-trithiolane and tetramethyl-1,2,4-trithiolane. 

---