Rational canonical form

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

Bu3Si(OH2)] [H-CBuMesBBrg] 106, which can be rationalized in two canonical forms, namely a Lewis acid-Lewis base adduct of the silylium ion and water, and, alternatively, a protonated silanol (Scheme 18).264... 

The barriers to rotation of esters deserve mention here, especially in comparison to amide barriers. The H NMR spectra of some nitrites (45) were measured in 1957 (83). The temperature had to be lowered to - 58°C at 30 MHz to see the separate signals of propyl nitrite. The barriers to rotation were ca. 10 kcal/mol. This result may be rationalized by considering the lesser electron-donating ability of the alkoxy relative to the dialkylamino group. The dipolar canonical form (46) of nitrite esters is not as stable as that of nitrosamines. 

Now when MACSYMA is asked to differentiate (Dl) with respect to X, it does so in a straightforward manner and simplifies the result using the rational canonical simplifier RATSIMP. This command puts the expression in a numerator-over-denominator form canceling any common divisors. In (D2) the symbols %E and %P1 are MACSYMA s representations for the base of the natural logarithms and pi, respectively. 

The weak and highly polar Cl—F bond in FCIO can be rationalized in terms of either a (p—7T )a bond (see Section II, C) or a simple valence bond model (66) resulting in a resonance hybrid of the following canonical forms FCIO2 F + C102. It has been discussed in detail by Parent and Gerry (220), by Carter et al. (43), and in Section II, C of this review. 

The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel s review (6), and has been used to rationalize (i) variations in IR stretching frequencies between bonded and nonbonded isocyanides, and (ii) the better Tt-acceptor qualities of aryl versus alkyl isocyanide groups (53,54). In valence bond theory the canonical forms involved in metal isocyanide bonds are... 

The experimentally observed orientations can be rationalized by considering the stability of the a-complexes (Wheland intermediate) involved. For instance, when comparing the a-complexes resulting from the addition of electrophiles to the 2-, 3- and 4-positions in pyridine, it is found that only electrophilic attack on the 3-position avoids the energy-rich nitrenium canonical form. Dications are postulated as intermediates for reactions involving pyridinium ions. Among these, the product resulting from attack on the 3-position has the most favourable electronic stability. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

A molecule whose bonding can be readily rationalized by the sp2 hybridization scheme is BF3. As indicated by the Lewis formulas shown below, there are three canonical strructures, each with three a bonds, one n bond, and eight lone pairs on the F atoms. The three a bonds are formed by the overlap of the sp2 hybrids (formed by 2s, 2px, and 2py) on B with the 2pz orbital on each F atom, while the n bond is formed by the 2pz orbital on B and the 2p orbital on one of the F atoms. This description is in accord with the experimental FBF bond angles of 120°. Also, it may be concluded that the bond order for the B-F bonds in BF3 is IV3. 

The anomeric effect is readily rationalized in PMO terms, using the hybrid model for the lone pairs (see p. 27 and Deslongchamps, 1983). In the g g form of dimethoxymethane [21], one lone pair on each oxygen atom is anti-periplanar to the cr c o orbital involving the other oxygen atom. Thus two favourable n-o interactions exist in this conformation. The generalized anomeric effect was rationalized in similar terms by David et al. (1973) who used the canonical model for the lone pairs. Calculations for the CHCl—O—C system predicted stabilization of conformer [23a] due to the n-cf c-ci interaction by 3.3kcalmol with respect to conformer [23b], a... 

The different acceptor strengths were rationalized on the basis of the resonance stabilities of the radical anions 45and 48 . Thus, the reduction of 45 to form the radical anion leads to an aromatic sextet. In contrast, the canonical structure of the radical anion of 48 involves tetravalent sulphur. These non-classical Kekule thiophenes are unstable compared with classical Kekule thiophenes [118]. 

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