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3
-
-
0342825589
-
-
note
-
A charge-separated resonance contributor, with no π bonds, can also be drawn (e.g., see refs 10 and 18). To a first approximation, however, this resonance structure is equally important for the neutral molecule and the anion. For the purposes of the present discussion, the resonance contribution to acidity will be understood as referring to the interplay between the more traditional resonance contributors shown in Figure 1.
-
-
-
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10
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0000167264
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(d) Wiberg, K. B.; Cheeseman, J. R.; Ochterski, J. W.; Frisch, M. J. J. Am. Chem. Soc. 1995, 117, 6535-6543.
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0001766959
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(e) Wiberg, K. B.; Hadad, C. M.; Rablen, P. R.; Cioslowski, J. J. Am. Chem. Soc. 1992, 114, 8644-8654.
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0031766531
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(f) Hadad, C. M.; Rablen, P. R.; Wiberg, K. B. J. Org. Chem. 1998, 63, 8668-8681.
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0001220452
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Siggel, M. R. F.; Streitwieser, A., Jr.; Thomas, T. D. J. Am. Chem. Soc. 1988, 110, 8022-8028.
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0343260466
-
-
note
-
If the coefficient of two for the electrostatic terms in eq 1 is omitted, the qualitative conclusions do not change substantially. However, inclusion of this coefficient provides consistency with the more detailed electrostatic model that is developed and justified later in the text.
-
-
-
-
30
-
-
0343696097
-
-
note
-
The simple resonance picture, in which delocalization is identical in the anionic and cationic cases, of course represents quite a crude approximation. It is well to bear in mind, however, that all models are simplifications, and in fact must be so to serve a useful purpose. Furthermore, this simple resonance picture, in which the energy of delocalization depends only on the number of contributing structures and their energetic similarity to each other, corresponds to both the description given in most textbooks and to the thinking of many practicing organic chemists. It also represents the point of view espoused by Wheland in his classic monograph (ref 1). A more sophisticated MO picture of delocalization would allow for differences between the anion and the cation, but presumably would still predict substantial stabilization in both cases.
-
-
-
-
31
-
-
0342825584
-
-
note
-
Reactions 1.1 and 1.2 yield estimates of the electrostatic and resonance contributions that differ by 11.4 kcal/mol - 5.2 kcal/mol = 6.2 kcal/mol. The close numerical agreement of this value with the difference in calculated proton affinity between acetic acid and carbonic acid, 4.5 kcal/ mol, is very encouraging, as it suggests internal consistency of the model.
-
-
-
-
32
-
-
0342390834
-
-
note
-
This set of four equations in two unknowns is of course overdetermined, even though two of the equations are in fact not linearly independent of each other, and two others are nearly so. The values for x and y reported in the text represent averages obtained by graphical solution of the system of equations. The graphical solutions are provided as Figures S2 and S3 in the Supporting Information.
-
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-
-
33
-
-
0343260465
-
-
note
-
The estimates of resonance and electrostatic contributions derived from eqs 3 and all four of the reactions in Scheme 2 differ somewhat from those obtained using eqs 1 and 2 and only reactions 2.1 and 2.2. In particular, including all four reactions reduces the resonance estimate from 14.5 kcal/ mol to 13.1 kcal/mol, although the electrostatic estimate remains essentially constant at 6.0 ± 0.2 kcal/mol per C-N bond. This difference probably arises because multiple resonance contributions do not accumulate in a strictly linear fashion. As a result, consideration of the doubly resonance stabilized guanidinium ion reduces the average stabilization attributed to a resonance contributor. The observation of nonadditivity is hardly surprising, since a saturation effect is expected as additional resonance structures are included, i.e., adding a third equivalent resonance structure when two are already present yields less of an advantage than adding a second when only one structure was available before. Alternatively, from a molecular orbital point of view, the two nitrogen lone pairs compete to donate into the protonated carbonyl π* orbital, and neither can do so as effectively as it could if it were the only lone pair present.
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Ph.D. Thesis, Department of Chemistry, Yale University
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0342390831
-
-
note
-
The plot in Figure 4 was generated using eq 4 with the value of C already set to 11.0 kcal/mol. However, the plot would of course look the same with C set to 1, except that the slope would be approximately 11.0 kcal/mol instead of very close to 1.0 kcal/mol.
-
-
-
-
60
-
-
0342825579
-
-
note
-
These rough estimates were obtained by averaging two sets of results: those obtained from the analyses of Schemes 1 and 2, using eqs 3, and those obtained from Table 4, using the electrostatic model represented by eq 4 and Figure 4.
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0342390827
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In cases where the HF/3-21G* energies of one or more rotamers were similar, HF/6-31G* optimizations were used to verify which conformer was of lowest energy.
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