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Structure and Bonding Nature of the Strained Lewis Acid 3‐Methyl‐1‐boraadamantane: A Case Study Employing a New Data‐Analysis Procedure in Gas Electron Diffraction
Author(s) -
Vishnevskiy Yury V.,
Abaev Maxim A.,
Rykov Anatolii N.,
Gurskii Mikhail E.,
Belyakov Pavel A.,
Erdyakov Sergey Yu.,
Bubnov Yuri N.,
Mitzel Norbert W.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201200264
Subject(s) - electron diffraction , electron , diffraction , lewis acids and bases , gas electron diffraction , chemistry , materials science , crystallography , computational chemistry , reflection high energy electron diffraction , physics , optics , organic chemistry , nuclear physics , catalysis
Base‐free 3‐methyl‐1‐boraadamantane was synthesized by starting from its known THF adduct, transforming it to a butylate‐complex with n ‐butyllithium, cleaving the cage with acetyl chloride to give 3‐ n ‐butyl‐5‐methyl‐7‐methylene‐3‐borabicyclo[3.3.1]nonane and closing the cage again by reacting the latter with dicyclohexylborane. The identity of 3‐methyl‐1‐boraadamantane was proven by 1 H, 11 B and 13 C NMR spectroscopy and elemental analysis. The experimental equilibrium structure of the free 3‐methyl‐1‐boraadamantane molecules has been determined at 100 °C by using gas‐phase electron diffraction. For this structure determination, an improved method for data analysis has been introduced and tested: the structural refinement versus gas‐phase electron diffraction data (in terms of Cartesian coordinates) with a set of quantum‐chemically derived regularization constraints for the complete structure under optimization of a regularization constant, which maximizes the contribution of experimental data while retaining a stable refinement. The detailed analysis of parameter errors shows that the new approach allows obtaining more reliable results. The most important structural parameters are: r e (B‐C) av =1.556(5) Å, ${\angle }$ e (C‐B‐C) av =116.5(2)°. The configuration of the boron atom is pyramidal with ${\sum \angle }$ (C‐B‐C)=349.4(4)°. The nature of bonding was analyzed further by applying the natural bond orbital (NBO) and atoms in molecules (AIM) approaches. The experimentally observed shortening of the BC bonds and elongation of the adjacent CC bonds can be explained by the σ(C‐C)→p(B) hyperconjugation model. Both NBO and AIM analyses predict that the BC bonds are significantly bent in the direction out of the cage.