Critical evaluation of the bond order concept with application to thermal hydrogen shifts

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ABSTRACT:With semi-empirical MO and ab initio calculations at different levels, we investigated the conjugation of alternating XOY bonds with XOY for N/B and N/C combinations in an open and cyclic arrangement. Although the intrinsic symmetry is lost for the acyclic even-membered compounds, the alternation is still reflected in its geometry and electron-density transfer. For the cyclic compounds, we focused our attention on borazine N 3 B 3 H 6 (D 3h symmetry), which is isoelectronic with benzene (D 6h symmetry). Specific attention is given to the electrophilic behavior of borazine with respect to CH 3 ϩ and SiMe 3 ϩ . The dynamics based on the results of FT-ICR mass spectrometry was studied in more detail. In addition, the results of the cyclic systems with 4n and 4nϩ2 electrons concerning their geometries are compared with the corresponding carbon compounds. Attention is also given to the dication of borazine, because of the corresponding triplet ground state of the benzene dication.

Section: Methods Of Calculationmentioning

confidence: 99%

Quantum chemical study of alternating N/B and N/C π bonds in (un)charged even‐membered 4n and 4n+2 cyclic systems and their related open systems

Buck¹

2004

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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⁺

Buck¹

2004

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ABSTRACT:We consider the controversial conclusions of the charge at the migrating hydrogen in the transition state of hydride-transfer reactions from CH-groups to hydride acceptors. Quantum chemical calculations were performed on elementary organic reactions involving carbenium ions, which can be considered as hydride acceptors. We also discuss the biochemical hydride-transfer reactions in which the coenzyme NADH-NAD ϩ plays an important role. With the calculations and the experimental model systems, an answer is given for the stereospecificity of the hydride transfer. Generally, the hydride transfer occurs via a trigonal pyramidal geometry in which the transferred hydride of the CH-group is located in the axial position. In the case of the coenzyme NADH-NAD ϩ , the hydride transfer is coupled with an out-of-plane orientation of the carboxamide group of the pyridinium moiety, resulting in an increased stereospecificity.

A combined experimental, theoretical, and Van't Hoff model study for identity methyl, proton, hydrogen atom, and hydride exchange reactions. Correlation with three‐center four‐, three‐, and two‐electron systems

Buck¹

2008

ABSTRACT:We have studied carbon transfer reactions following an S N 2 reaction profile. With ab initio calculations and experimental geometries concerning the nature of the various complexes indicated as stable, intermediate, and transition state we were able to show the additional value of van't Hoff's tetrahedral configuration by changing its geometry via a trigonal pyramid into a trigonal bipyramid. The ratio of the apical bond and corresponding tetrahedral bond distances is then nearly 1.333. The relevance of this approach has also been shown for identity proton-(hydrogen atom-, and hydride-) in-line reactions. The use of this geometrical transmission will be demonstrated for the hydrogen bonding distances in e.g., DNA duplexes and other biological (supra) molecular systems.