Dipolar interactions table

For a rigidly held, three-spin system, or when existing internal motion is very slow compared to the overall molecular tumbling, all relaxation methods appear to be adequate for structure determination, provided that the following assumptions are valid (a) relaxation occurs mainly through intramolecular, dipolar interactions between protons (b) the motion is isotropic and (c) differences in the relaxation rates between lines of a multiplet are negligibly small, that is, spins are weakly coupled. This simple case is demonstrated in Table V, which gives the calculated interproton distances for the bicycloheptanol derivative (52) of which H-1, -2, and -3 represent a typical example of a weakly coupled, isolated three-spin... 

Table 2 lists a number of publications of recent years on investigations of spatial orderings, interactions, and local structures, most of which have employed methods based on heteronuclear dipolar interactions. 

In the case of pharmaceutical solids that are dominated by carbon and proton nuclei, the dipole-dipole interactions may be simplified. The carbon and proton nuclei may be perceived as dilute and abundant based upon then-isotopic natural abundance, respectively (Table 1). Homonuclear 13C—13C dipolar interactions essentially do not exist because of the low concentration of 13C nuclei (natural abundance of 1.1%). On the other hand, H—13C dipolar interactions contribute significantly to the broad resonances, but this heteronuclear interaction may be removed through simple high-power proton decoupling fields, similar to solution-phase techniques. 

As can be seen from Table I, in most cases the axial Si-Fl distances are significantly longer than the respective axial Si-F2 distances. This weakening of the Si-Fl bond can be interpreted in terms of dipolar interactions between the fluorine atom FI and the ammonium moiety. This suggestion is supported by the results of ab initio studies of 4, 6, 7,17,19, and some related model species.20 In general, intra- and/or intermolecular N-H - F hydrogen bonds play an important role in the structural chemistry of 4-6, 8, 13, 17-19, 21, and 22. 

Table II compares these empirical estimates with those obtained from the DSW calculation. Relativistic contributions have little effect on the spin-dipolar interactions, and both calculations are in reasonably good agreement with the empirical estimates. The spin-orbit contributions are also in moderately good agreement with the empirical estimates, showing that electron currents about the -axis are considerably more important than those about axes in the plane of the ligand. Indeed, in view of the approximations that enter into Equation 7, (, ), one might have as much confidence in the DSW result as in the empirical estimate given in the final column.

While the boiling points of chloro- and bromomethanes always increase according to the number of halogen atoms, this correlation does not exist in the case of fluoromethanes. The bp increases from CH4 to CH2F2 and then decreases until CF4 (Table 1.4). Indeed, a parallelism exists between boiling points and dipolar moments. A partially fluorinated compound will exhibit nonnegligible intermolecular interactions according to the importance of the dipolar moment (Table 1.5). ... 

A review of experimental work prompted the suggestion of the importance of dipolar interactions (Hammond and Hawthorne, 1956). de la Mare and Kidd (1959), observing a parallelism in the parajmeta and ortho/meta ratios, predicted the ortho effect to be primarily electronic in origin. Norman and Radda (1961) explored the general significance of this idea. They studied the orthojpara ratios for the substitution of a series of monosubstituted benzenes by two reagents with the same electrophilic properties but different steric requirements. The reactions, nitration by N02+ and chlorination by CI+, fulfill the requirements. The results are summarized in Table 3. 

The amylose-iodine interaction has a dipolar nature, as deduced from a distinct difference between the molecular coefficient of iodine in starch and in nonpolar solvents.123 The additional stabilization may result from the formation of resonating polyiodine chains at high dipolar interactions. The composition of the total energy of stabilization is shown5 in Table VI. 

Careful choice of solvent and dilution is particularly important for some samples. In general, the spectra of the sulfones show a marked solvent dependence. The line width is especially sensitive to the nature of the solvent. For example, the line widths for 5 mol dm solutions of sulfolane in acetone and water are 16 and 60 Hz, respectively. A shift difference of 6.5 ppm is observed between 5moldm solutions of sulfolane in water and dioxane. Table 50 shows how the chemical shift (quoted relative to that for neat sulfolane) and line width vary with concentration of sulfolane in acetone. No nuclear Overhauser effect is observed for sulfolane, which suggests that sulfur-hydrogen dipolar interactions are not significant as a relaxation mechanism. 

Interesting solvent scales based on NMR measurements have been proposed by Taft et al. [90] and by Gutmann, Mayer et al [91]. A solvent polarity parameter, designated as P, has been defined by Taft et al [90] as the F chemical shift (in ppm) of 4-fluoro-nitrosobenzene in a given solvent, relative to the same quantity in the reference solvent cyclohexane cf. Table 6-6 and the discussion in Section 6.5.1). These parameters define a scale ranging from P = 0.0 in cyclohexane to P = 2.7 in sulfolane, and can easily be measured in a wide variety of solvents. The P values appear to be related to the ability of the solvents to form specific 1 1 complexes with the nitroso group of the standard compound. A compilation of P values can be found in reference [92], In addition, chemical shifts of (trifiuoromethyl)benzene and phenylsulfur pentafiuoride have been used by Taft et al. to study nonspecific dipolar interactions with HBD solvents and utilized to define n values of solvent dipolarity/polarizability for protic solvents [249]. 

The primary acceptors of the two plant photosystems differ fundamentally from each other, no doubt because of their different redox midpoint potentials (about -100 to -200 mV for PS II, -705 to -730 mV for PS I [R3-R5]). In PS I two iron-sulfur (ferredoxin-type) proteins, and Fg, with characteristic EPR spectrum in the reduced state ( m between -450 and -550 mV), have been observed (Fig. 2) that function either parallel or in series (see Ref. R5 for a recent review). The shape of the spectra of the two ferredoxin-type acceptors and in particular their principal g values depend on whether one or both acceptors are reduced (Fig. 2). It is unlikely that this is due to a magnetic interaction, as the differences depend linearly on the microwave frequency, i.e. on the applied magnetic field (exchange and dipolar interactions are independent of field Table 3) [16,42], Possibly, Coulomb repulsion causes strain-induced g shifts. 

Substituent effects in the decomposition of benzoyl peroxide have been thoroughly investigated. Data in dioxane solvent with added 3,4-dichlorostyrene to prevent induced decomposition are presented in Table 72. A Hammett plot using the sum of the substituent constants gave p = —0.38 (ref. 339). The observation that electron-releasing substituents increased the rate of decomposition was explained in terms of a dipolar interaction between the two aroyloxy groups , viz. 

An important result of the relaxation studies on Li carried out by Wehrli [9,40] was the finding that an appreciable H, Li nuclear Overhauser effect (NOE) exists which amounts to ti = 2.61 and 1.19 for Li in aqueous LiCl and n-butyllithium in n-hexane, respectively. The theoretical value is Ti=y( H)/ 2y( Li) = 3.40 (Table 1). Due to the inefficient quadrupolar relaxation, the Li nucleus shows considerable dipolar interactions with neighbouring protons which can yield valuable structural information. Similar effects for Li are uncommon because of the stronger dominance of quadrupolar relaxation for this nucleus, but appreciable H, Li NOEs have been found for systems like 7 [37,38]. 

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