Factors that Influence Carbon Shifts

A large ASIS appears on the relative chemical shifts of the nonequivalent methyl groups in A,A -dimethylformamide (3-26). The distance between the two methyl peaks increases 

In undeuterated dioxane, Hgq and coincidentally have the same chemical shift (at the field studied), so they cannot be differentiated at low temperatures. (See Sections 1-8 and 5-2.) In l,4-dioxane- 7 (an impurity in commercial l,4-dioxane- 5 g), both and Hgq exhibit isotope shifts to a lower frequency, but H x is shifted somewhat farther. As a result, the axial and equatorial protons give separate resonances at low temperatures, in contrast to the undeuterated material. Because of a chlorine isotope effect, chloroform is a poor substance for an internal lock or a resolution standard at fields above about 9.4 T. At high resolution, the chloroform proton resonance shows up as several closely spaced peaks, due to CH( 5c1)( C1)2, CH( C1)2( C1), CH( C1)3, and CH( C1)3. 

Carbon is the defining element in organic compounds, but its major nuclide ( C) has a spin of zero. The advent of pulsed Fourier transform methods in the late 1960s made the examination of the low-abundance nuclide (1.11%) apractical spectroscopic technique. The low probability [(0.0111) = 0.0001, or 0.01%] of having two adjacent nuclei in a single molecule removes complications from carbon-carbon couplings. When 

The quantity AE is the average energy of excitation required to reach certain excited states. The radial term r includes the average distance r of the 2p electrons from the nucleus and serves as a measure of electron density. Finally, Qij is a measure of tt bonding to carbon. The negative sign in the equation indicates that shielding is in the opposite direction from cr.  

---

Section 3-4 Factors that Influence Carbon Shifts 79... 

The factors that influence the shift of v(C=N) stretches of the cyanide-bridged complexes relative to those of the free CN ligand with v(C=N) = 2080 cm (234), can be understood on the basis of the molecular orbital (MO) diagram of the CN anion (Scheme 2). Upon coordination to a metal ion, the carbon end of the... 

A second factor that influences chemical shifts is the presence of pi electrons. Hydrogens attached to a carbon that is part of a multiple bond or aromatic ring usually appear downfield from hydrogens attached to saturated carbons. Compare these values from Table 12.2 ... 

Other chapters in this volume have explored carbon cycles within and among ecosystems, especially their response to the global changes that are occurring on earth today. In this chapter, the focus shifts from factors that inlluence carbon flux dynamics to the ways in which the composition of the atmosphere and thermal environment influence the type of photosynthetic system that predominates within a terrestrial ecosystem. In turn, the kind of photo.synthetic. system present has signihcant impacts on the distribution of the grazing animals that are dependent on primary productivity generated acro.ss the land.scape, both in the. shortterm and over evolutionary time periods. 

Another case where resonance influences carbon acidities is the comparison of toluene to diphenylmethane and lastly triphenylmethane (pK s = 41.2,33.0, and 31.5, respectively). The large shift between toluene and diphenylmethane is due to additional resonance stabilization of the conjugate base. However, the third additional phenyl ring has little effect. Several factors are involved to account for this small change. One is that the phenyl rings cannot all be planar with the anionic carbon, which is required for full resonance stabilization (examine the trityl radical discussed in Chapter 2), and instead a propeller twist develops in the anion. This is an example of a steric inhibition of resonance. A second factor is called a resonance saturation effect. Once the charge on the conjugate base is stabilized via resonance, the additional resonance is not as effective at stabilization. 

The particular array of chemical shifts found for the nuclei of a given polymer depends, of course, on such factors as bond orientation, substituent effects, the nature of nearby functional groups, solvation influences, etc. As a specific example, derivatives of the carbohydrate hydroxyl moieties may give rise to chemical shifts widely different from those of the unmodified compound, a fact that has been utilized, e.g., in studies (l ) on commercially-important ethers of cellulose. Hence, as illustrated in Fig, 2, the introduction of an 0-methyl function causes (lU,15) a large downfield displacement for the substituted carbon. This change allows for a convenient, direct, analysis of the distribution of ether groups in the polymer. Analogously, carboxymethyl, hydroxyethyl and other derivatives may be characterized as well... 

---