Chem: 1.8 angstroms! The longest C-C key record was refreshed by Japanese researchers

The bond properties are very important in chemistry A deeper understanding of covalent bonds also helps promote green chemistry, allowing chemists to design new chemicals and processes to improve sustainability A team led by Yusuke Ishigaki and Takanori Suzuki from Hokkaido University successfully synthesized highly strained "core-shell" hydrocarbons, which proved that the C-C covalent bond can expand to more than 1.80 angstroms, 1.17 times of the standard length (1.54 angstroms) (DOI: 10.1021 / ja00036a057) It is found that the covalent and non bond states are connected seamlessly in the distance between atoms Compounds with hypercovalent bonds are potential candidates for the preparation of new materials, whose crystals, films or polymers can react to external mechanical stimuli by anisotropic shrinkage or expansion of the material The length of the C SP 3-C SP 3 bond of the compounds mentioned in this paper (source: Chem) is generally 1.54 angstroms, such as in the case of ethane Although this standard bond length has been observed in almost all compounds, in order to understand the limit of covalent bond, some previous researchers have synthesized hexaphenylethane (HPE) and its derivatives As the parent HPE is unstable and can form dimer, more stable derivatives such as herges and toda have been developed The length of C 1-C 2 bond (d 1) is 1.713 angstrom and 1.734 Angstrom, respectively After theoretical calculation, it was thought that D 1 had a limit of about 1.8 angstroms, because when the two carbon nuclei separated by more than this distance, the bond splitting energy (BDE) changed to 0 Therefore, it seems impossible to synthesize stable covalent bonds with D 1 over 1.8 angstroms Theoretical calculation of new compounds (source: Chem) in order to create a new type of organic compounds with unprecedented C-C bond length, researchers designed a series of key points to break the existing record and obtain hypercovalent bond compounds that can maintain chemical stability In order to improve chemical stability, all possible degradation processes, such as isomerization, bond dissociation ionization, bond dissociation addition and dimer formation, must be excluded Only when the degradation products have higher energy than the original compounds, the degradation reaction will not occur By using the core-shell strategy, we can achieve the goal in some HPE derivatives By increasing the spatial repulsion of the shell, the C-C bond core can get a higher expansion In addition, the scissor effect can be applied by the end part of the shell is attached to the fusion ring to further strengthen the covalent bond expansion The synthesis route of the new compound (source: Chem) according to the proposed molecular core-shell strategy, researchers designed a series of two helix (dibenzo-cycloheptane) HPE derivatives 10a-10c The weak bonds in the prepared hydrocarbons are enlarged by forcing the use of overlapping conformations The maximum value of D 1 determined by X-ray analysis is 1.806 angstroms, which is determined by 10C at 127 ℃ Although the C 1-C 2 bond of 10C and other Di helix (dibenzo-cycloheptane) is very weak, it is a solid body with stable thermodynamics and kinetics, which can exist stably even in solution This is because chemically inert aromatic rings protect long bonds from breaking X-ray analysis data (source: Chem) because the observed value is greater than the shortest non bonding contact, covalent bonding state and non bonding state can be seamlessly connected according to the distance between atoms The next step is to summarize the existing experience and design and synthesize a longer hypercovalent bond with a bond length of 1.8-2.0 angstroms Corresponding author: Yuke Ishigaki, Takanori Suzuki