Delocalization cyclic

K Relaxation mechanism was also proposed [130-133], The % relaxation originates from cyclic delocalization of % electrons in the double bond through the hyperconjugation with a bonds on the saturated ring atoms under control of the orbital phase property [134, 135],... 

Scheme 6 Phase continuity of the n, a, and O orbitals for cyclic delocalization of a lone pair...

Analysis suggested that cyclic delocalization could, however, occur in 17 and 18 to a lesser extent than in pyrrole and benzene, respectively [97], This suggests low aromaticity of 17 and 18. Donation from sp lone pairs (Scheme 9) weakens the N-N sp -sp ) single bonds in the cyclic conjugated hydronitrogens and polynitrogens. 

The kinetic stability of 17 increases on deprotonation. The half-life times of 17 and its anion N 19 have been estimated [104] from the observed [105, 106] and computed free energy to be only 10 min and 2.2 days, respectively. The high kinetic stability of the anion 19 can be understood in terms of enhanced pentgon stability and aromaticity. The deprotonation raises the energy of lone pair orbitals and promotes cyclic delocalization of o- and rr-electrons. 

Acyclic Tricoordinate Radicals As one can easily note, the above examples of the cyclic radicals of Si and Ge atoms have rather particular structures featuring the cyclic delocalization of the odd electrons. The simple tricoordinate acyclic radicals of the type RsE (E = Si, Ge, Sn, Pb), lacking the stabilization effects of the cyclic Jt-delocalization, constitute another, more general and even more challenging, class of stable organometallic radicals. " Consequently, the search for such highly symmetrical species appeared to be of primary importance for synthetic chemists. 

Reindl, B. Clark, T. Scheleyer, P. v. R. Modern molecular mechanics and ab initio calculations on benzylic and cyclic delocalized cations. J. Phys. Chem. A. 1998, 102, 8953-8963 and references therein. 

In a review, Gorelik51 has shown that magnetic, structural, and energetic properties are determined by the electronic structure of cyclic conjugated systems, which are stabilized by a cyclic delocalization of electrons. Chemical reactivity cannot serve satisfactorily as a general criterion of aromaticity. 

The multicenter bonding interaction in 2A is classic in a sense that it results in a well known Hiickel aromatic, which is planar with cyclic delocalized An + 2 n electrons (no hyper-coordinate atoms). The most favorable site for a proton to bind to 2A is not the carbon atom, but the B-B edge. The non-dassical 1C is 47.6 kcal mol-1 more stable than the classical structure 1A [5],... 

Diels—Alder reactivity of the PC-bonded compounds can rather be attributed to the diminished strength of the PC bond as compared to the hydrocarbon bonding than to the decrease of the cyclic delocalization by a P=C vs C=C replacement. 

U, 3A, 5A -Triphosphinines were also investigated. In the homodesmotic reaction in eq 10 (X = PH2), 21.8 kcal/mol stabilization has been obtained at the MP2/ 6-31G(d) level (cf. with the 26.0 kcal/mol value for 12. 32. 5A -triphosphinine at the same level of the theory). This stabilization is in agreement with the cyclic delocalization stabilization resulting from donor—acceptor-type interactions discussed in section... 

Cation-radicals containing three electrons in the field of four atoms or anion-radicals with five electrons retained by four atoms represent a special group of multicentered ion-radicals. Thus, nonclassical, cyclically delocalized 3e/4C cation-radicals and 2e/4C (dication)-radicals of substituted cyclobutadienes are known (Allen and Tidwell 2001). The 3e/4N cation-radical and the 5e/4N anion-radical have also been discovered (Exner et al. 1998, 1999, 2000). The reactions in Scheme 3.31 illustrate structures of the 5e/4N anion-radical as well as the corresponding dianion and acety-lated products of the latter. 

The cyclic delocalization of jt-electrons in diazadiboretidines (XBNRjj, or borazines, (XBNRjj, etc., will be indicated by dotted lines over bonds, again favored over mesomeric structures. 

The connection of the principal criteria of aromaticity considered above with the main factor of aromatic stabilization, i.e., electron cyclic delocal-... 

Since the aromaticity is defined as the stabilization due to cyclic electron (bond) delocalization, the data on thermodynamics and kinetics of various reactions leading to removal of cyclic delocalization system (or, conversely, to its formation) may in principle be used for assessing aromatic stabilization or antiaromatic destabilization. 

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