Odd-memberedC2? ?-cyclic mono- and dications with internal cross-linking. A H�ckel approach for the description of antiaromaticity

Abstract

ABSTRACT:With the Hückel method using a bond-length-dependent coupling (β), a description is given for the various C 2ν geometries for odd-membered cyclic 4n π electrons under cross-linking, so we arrive at a more general conclusion with respect to their antiaromatic behavior. Model systems are introduced to illustrate this quantum-chemical approach. As an interesting spin-off of these model systems, we may conclude that the tetrahedral configuration for saturated carbon is not the absolute geometry.

Small‐scale and automatable high‐throughput compositional analysis of biomass

“…There is sin θ for the equatorial bond length, besides the parameter 1 − cos θ, in the trigonal bipyramidal to complete our model description2–5. With the ab initio values of θ as shown in Table VI, we calculated the CCN bond length in the corresponding trigonal bipyramid.…”

“…We focused our attention on a model derived from the dynamics of a regular tetrahedron as originated by van't Hoff (vtH) in which the interstitial carbon changes its position from tetrahedral into a trigonal (bi)pyramidal configuration as proposed by Hughes and Ingold for S N 2 reactions1. This model based on the geometry of the corresponding transition state has been used for the introduction of dimensionless ratio numbers derived from the ab initio values in comparison with the vtH model as given by Buck2–5. As an illustration of this reaction type, we demonstrate the reaction progress for the identity reaction as shown in Figure 1.…”

Elementary addition-substitution reactions. H�ckel approach for the description of aromatic and antiaromatic arylmethyl cations

“…Since we are dealing with antiaromaticity in a subsystem, the HOMO-LUMO gap is not a correct criterion [27]. As was shown, this interaction leads to a pentadi- enyl π conjugation with a fixed geometry.…”

“…Recently we published a study on intermediates in the reaction of 1,1-diphenylethylenes with halogens (Cl 2 , Br 2 , and I 2 ) [27]. This study was initiated in the past by Pfeiffer and Wizinger [28].…”

Charge at the migrating hydrogen in the transition state of hydride transfer reactions from CH groups to hydride acceptors. Dynamics of the redox‐couple NADH‐NAD+

“…For the dynamics of this redox‐couple we consider that the barrier for a linear stereospecific hydride transfer under biochemical conditions indicated as A‐ and B‐specificity to divergent carbonyl and imino substrates takes place with an out‐of‐plane orientation of the carboxamide group [2, 3]. We suggest that the hydride transfer is effectuated by a trigonal pyramidal (TP) geometry in which the transferred hydrogen is in the axial location 4. Combination of the TP with the out‐of‐plane orientation of the carboxamide group of the pyridinium moiety will be of importance for the high stereo‐ and regiospecificity of the hydride transfer 5.…”