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32144458569
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note
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At the time, the variation in C-H bond strengths from 1° to 3° was believed to be much larger. Presumably, Mulliken's model could have been successfully parametrized to fit the modern bond strength values.
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24
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32144449718
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NIST computational chemistry comparison and benchmark database
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NISTStandard Reference Database No. 101 Johnson, R. D., Ed.; National Institute of Standards and Technology: Gaithersburg, MD
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Richter, U. NIST Computational Chemistry Comparison and Benchmark Database. In NIST Chemistry WebBook; NISTStandard Reference Database No. 101 (http://srdata.nist.gov/cccbdb); Johnson, R. D., Ed.; National Institute of Standards and Technology: Gaithersburg, MD, 2005.
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Richter, U.1
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25
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32144448863
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note
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One might argue that the major hyperconjugative interaction is with the C-H σ* orbital and that weakening of the bond could increase the H-C-C angle. This would be energetically unfavorable (trading a strong C-H bond for a weaker partial π-bond) and is not supported by computational evidence. If an adjacent C-H bond in the ethyl radical is weakened by stretching it to 1.35 Å (i.e., this type of hyperconjugation is facilitated), the H-C-C angle contracts in response (110.5° vs 111.5° at the MP2/6-31+G** level).
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26
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0005075059
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Raghavachari, K.; Whiteside, R. A.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1981, 103, 5649.
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28
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0001637204
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The tert-butyl radical prefers a very slightly pyramidalized geometry with internal C-C-C angles of about 118°: Griller, D.; Ingold, K. U.; Krusic, P. J. J. Am. Chem. Soc. 1978, 100, 6750.
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Griller, D.1
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Avakyan, V. G.; Guselnikov, S. L.; Gusel'nikov, L. E. J. Organomet. Chem. 2003, 686, 257.
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4 is 91.8 kcal/mol: Jasinski, J. M.; Becerra, R.; Walsh, R. Chem. Rev. 1995, 95, 1203.
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32144464725
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note
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In an acyclic system, an extra 1,3 interaction is developed as the nucleophile approaches the reaction center. In the cyclization system, the price of this added 1,3 interaction has already been paid in the form of steric strain (1,3 repulsion) in the substrate.
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41
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0041689437
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Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744.
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32144454857
-
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note
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c that was appropriate for methylene and methyne.
-
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44
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0004305726
-
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Standard Reference Database No.78, Version 3.0 National Institute of Standards and Technology: Gaithersburg, MD
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Ralchenko, Y.; Kramida, A. E.; Reader, J. NIST Atomic Spectra Database; NIST Standard Reference Database No.78, Version 3.0 (http://physics.nist.gov/ PhysRefData/ASD/index.htm); National Institute of Standards and Technology: Gaithersburg, MD, 2005.
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Ralchenko, Y.1
Kramida, A.E.2
Reader, J.3
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45
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32144432090
-
-
note
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Undoubtedly the terms for the 1,3 interactions also contain some energetic contribution from changes in geometry that occur with bond cleavage.
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-
-
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46
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32144436563
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note
-
A referee was interested in whether the model would fit diamond accurately. For a diamond structure, each carbon experiences 4 C-C bonds and 12 C-C-C geminal interactions. However, each of these terms are shared with another carbon, so per carbon, there are 2 C-C interactions and 6 C-C-C interactions. Using the parameters in Table 3, eq 3 gives a heat of formation of 0.6 kcal/mol for diamond. This is very close to the experimental value (0.5 kcal/mol). However, the model does not include corrections for gauche interactions and therefore a close fit to experiment suggests some error in the value from eq 3.
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47
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0035810505
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Schleyer has shown that bond electron density measures indicate surprisingly similar intrinsic bond energies for most C-H bonds. However, the absolute values from the study are much different than those reported here because they were parametrized in a conventional way without including terms for 1,3 repulsions: Exner, K.; Schleyer, P. v. R. J. Phys. Chem. A 2001, 105, 3407.
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0345988060
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53
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32144459650
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note
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Unfortunately, Laidig's level of theory did not reproduce the experimental preference for branching. As a result, no quantitative interpretations of the data are possible.
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54
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21844460398
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60
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32144459480
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note
-
sp-H bond strengths, such as in ethyne. If it is assumed that all repulsive 1,3 interactions are missing in ethyne due to the linear geometry, eq 3 predicts a C-H bond strength of 124.2 kcal/mol. This is well below the experimental bond strength (133.3 kcal/mol). A good fit is not expected because the model contains no terms to account for the changes that occur in the transition to sp-hybridization, but it does indicate that the majority of the bond strengthening found in ethyne might be the result of its lack of geminal repulsion.
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61
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32144450433
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32144444857
-
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note
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+. The failing of the model is a lack of terms to account for electron-electron repulsion as the carbon takes on additional electron density. It is true that atomic charges from MO calculations indicate that the carbon charge linearly increases with each hydrogen, but this analysis involves partitioning shared electron density and it is not appropriate to use these values as if they were point charges centered at nuclei.
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70
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0141704726
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Frisch, M.J.1
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