Bond orders, i.e., single bonds, double bonds, etc., are a simple concept originally based on Lewis structures. However, they can be tough to determine exactly. In 2017, Thomas Manz, New Mexico State University, Las Cruces, USA, developed a general method for the calculation of bond orders, based on a Density Derived Electrostatic and Chemical (DDEC6) approach [1]. This method assigns a number of electrons to each atom. The bond order is then calculated based on the delocalization of electrons between the atoms.

Taoyi Chen and Thomas A. Manz, New Mexico State University, have used this approach to calculate the bond orders of 288 diatomic molecules and ions. According to the team, there had been no systematic studies of accurate quantum-mechanically calculated bond orders across a large number of diatomic compounds so far. The results showed that the calculated bond orders for homodiatomic molecules (e.g., H₂, O₂, etc.) correlate with the measured bond energies. Within structurally different groups of molecules, however, this correlation does not necessarily exist.

The team also introduced a method to assign components of the bond order to individual molecular orbitals, called bond order component analysis (BOCA). The method provides insight into the contributions of different orbitals to the bond order. According to the researchers, this study could be useful both in chemical research and education.

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• Bond orders of the diatomic molecules,
  Taoyi Chen, Thomas A. Manz,
  RSC Adv. 2019, 9, 17072–17092.
  https://doi.org/10.1039/c9ra00974d

Reference

• [1] Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders,
  Thomas A. Manz,
  RSC Adv. 2017, 7, 45552–45581.
  DOI: 10.1039/c7ra07400j

 

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