Electronegativity effects

As noted above, electron ative atoms, such as chlorine, deshield the hydrogen atom of a Cl—C—H unit. The degree of deshielding, as measured by a shift to lower field (larger S value), increases with the electronegativity of the atom. The trend within the group of halogens is shown below. 

A similar effect of the electronegativity of the atom on the chemical shift occurs within a row of the periodic table. 

The effects of several groups bonded to a common atom are cumulative and often additive. For example, the chlorinated methanes show a nearly linear shift to lower field with increased numbers of chlorine atoms. 

The trend of chemical shifts that is easiest to explain is that involving electronegative elements substituted on the same carbon to which the protons of interest are attached. The chemical shift simply increases as the electronegativity of the attached element increases. Table 3.5 illustrates this relationship for several compounds of the type CH3X. 

Multiple substituents have a stronger effeet than a single substituent. The influenee of the substituent drops off rapidly with distance, an electronegative element having little effect on protons that are more than three earbons distant. Table 3.6 illustrates these effeets for the underlined protons. 

APPROXIMATE CHEMICAL SHIFT RANGES (PPM) FOR SELECTED TYPES OF PROTONS  

Compound CHsX CH3F CHjOH ClhCi ClLBr (Hd CM-  

Copyright 2013 Cengage Learning. AH Rights Reserved. May not he copied, scanned, or duplicated, in whole or in part. 

126 Nuclear Magnetic Resonance Spectroscopy Part One Basic Concepts 

Compound CH3X CH3F CH3OH CH3CI CHjBr CH3I CH4 (CH3)4Si 

Hydrogen bond Compounds involved Medium Strength (kcal/mol) 

---

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

Another type of correction, which is related to cross terms, is the modification of parameters based on atoms not directly involved in the interaction described by the parameter. Carbon-carbon bond lengths, for example, become shorter if there are electronegative atoms present at either end. Such electronegativity effects may be modelled by adding a correction to the natural bond length based on the atoms which are attached to the A-B bond. 

These trends are general ones, observed with other oxoadds of the nonmetals. Recall, for example, that nitric acid, HNO3 (oxid. no. N = +5), is a strong acid, completely ionized in water. In contrast, nitrous add, HN02 (oxid. no. N = +3), is a weak acid (Ka = 6.0 X 10-4). The electronegativity effect shows up with the strengths of the oxoadds of sulfur and selenium ... 

The vibrational frequencies vs and vas of the Ge—Y—Ge in hexasubstituted digermyl chalcogenides (12) were studied. The electronegative effect of CF3 groups displaces the bands to higher frequencies45. 

Thus, r° for an X—Y bond is shortened or elongated when electronegative or electropositive atoms (a, b, c, d,...) are connected to either X or Y, respectively. The amount of change in r° decreases with the substituent number (i.e. the first substituent has the largest effect, the second a smaller one and so on substituents are ordered according to their Ar values). A secondary electronegativity effect which changes r° of X—Y in X—Y—Z based on the substituent on Z, and which amounts to 0.4 times the primary effect, is also used in MM3. 

In amines, dialkoxyl subsfilufion resulls in much higher barriers to inversion fhan in alky famines a fad that has also been explained in terms of an electronegativity effect increased p-character in the a bonds results in more s-character for the electron pair on nitrogen. The planar-inversion transition state is therefore destabilized since, in it, the lone pair must develop pure p-character. This transition state is also destabilized by a six-electron anti-bonding interaction between heteroatom lone pairs and, additionally, the better anomeric overlap that is possible in sp rather than sp systems may also play a role (Figure 4) . These phenomena have also been rationalized theoretically ... 

Comparison of the intrinsic acidities and basicities of pyrrole (I), imidazole (4), and pyrazole (6), together with complementary information coming from the azine held, illustrate the main effects that control the acidity and the basicity of unsubstituted azoles (86JA3237). Particularly important are the role of electrostatic interactions between adjacent charged nitrogens (NH) and between adjacent lone pairs (N), as well as the aza electronegative effects. 

In such systems, electronegativity effects of X and Y will be minimized in the ratio... 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

---