Forced Bonding and QTAIM Deficiencies: A Case Study of the Nature of Interactions in Hedamantane and the Origin of the High Metastability

• Published in: Chemistry : a European journal 2013-08, Vol.19 (33), p.10945-10957
• Main Authors: Demy'anov, Piotr I, Polestshuk, Pavel M
• Format: Article
• Language: English
• Subjects: Bonding; Cage; Carbon; Chemical bonds; Destabilization; Helium; Helium atoms; Metastability
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201300317

Calculations within the framework of the interacting quantum atoms (IQA) approach have shown that the interactions of the helium atom with both tertiary, tC, and secondary, sC, carbon atoms in the metastable Hedamantane (Hedam) endohedral complex are bonding in nature, whereas the earlier study performed within the framework of Bader's quantum theory of atoms in molecules (QTAIM) revealed that only HetC interactions are bonding. The HetC and HesC bonding interactions are shown to be forced by the high pressure that the helium and carbon atoms exert upon each other in Hedam. The occurrence of a bonding interaction between the helium and sC atoms, which are not linked by a bond path, clearly shows that the lack of a bond path between two atoms does not necessarily indicate the lack of a bonding interaction, as is asserted by QTAIM. IQA calculations showed that not only the destabilization of the adamantane cage, but also a huge internal destabilization of the helium atom, contribute to the metastability of Hedam, these contributions being roughly equal. This result disproves previous opinions based on QTAIM analysis that only the destabilization of the adamantane cage accounts for the endothermicity of Hedam. Also, it was found that there is no homeomorphism of the rho (r) and -v(r) fields of Hedam. Comparison of the IQA and QTAIM results on the interactions in Hedam exposes other deficiencies of the QTAIM approach. The reasons for the deficiencies in the QTAIM approach are analyzed. Being forced to bond! The interactions of a helium atom with both tertiary (tC) and secondary (sC) carbon atoms in Hedamantane are demonstrated to be bonding in nature (see figure). These interactions are forced by the high pressure that the helium and carbon atoms exert upon each other. Destabilization of both the adamantane cage and the helium atom contribute to the metastability of Hedamantane.