Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Record number 319227
Title Delocalization does not always stabilize : a quantum chemical analysis of -substituent effects on 54 alkyl and vinyl cations
Author(s) Alem, K. van; Lodder, G.; Zuilhof, H.
Source The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 106 (2002)44. - ISSN 1089-5639 - p. 10681 - 10690.
Department(s) Laboratory for Organic Chemistry
VLAG
Publication type Refereed Article in a scientific journal
Publication year 2002
Abstract The effects of -substituents on alkyl and vinyl cations are studied using high-level ab initio calculations. The geometries, stabilities, and electronic properties of 27 alkyl cations and 27 vinyl cations with -substituents are computed at the B3LYP/6-311 G(d,p), MP2/6-311 G(d,p), and CBS-Q levels. The substituents studied vary from strongly destabilizing (e.g., -CN and -CF3) to strongly stabilizing (e.g., -OSi(CH3)3 and -NH2). The calculations show that in the case of vinyl cations the stabilization provided by the -substituents is larger by an average value of 4 kcal/mol than for the alkyl ones. This is the result of the intrinsically lower stability of vinyl cations (on average 17 kcal/mol). However, strong inductively donating or withdrawing -substituents show different behavior. Because of the high amount of s character in the carbon--substituent bond in the vinyl cations (sp hybridized), more pronounced effects are found than in the corresponding alkyl cations, leading to lower stabilization for inductively withdrawing -substituents and higher stabilization for inductively donating ones. Thus, distinct effects of -substituents on the stabilization of the cations are observed. However, no correlation is found between NBO-computed charge increases or bond-order increases at either the carbocationic center or at the -substituent of the molecule and the stability provided by an -substituent. This demonstrates the conceptual difference between stabilizing and electron-donating effects. Only for the C-H hyperconjugative effect in the vinyl systems is a correlation with the computed reaction enthalpies observed. Finally, the effect of leaving-group variation is studied. Changing the leaving group from H to Cl yields geminal effects ranging from 7 kcal/mol destabilization to 9 kcal/mol stabilization of the neutral precursor.
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