I know that one can determine which type of bond is present between different atoms in a molecule by analyzing the output of an NBO analysis. Furthermore the second order perturbation theory analysis can provide information about the presence interactions like hyperconjugation. But is there also a way to determine whether there are three-center four-electron bonds, i.e. hypervalency in a molecule by a NBO analysis ?
It is possible to determine such 3-centre bonds using an NBO analysis.
Here is a sample output of an NBO analysis obtained from this website:
The number of 3C (3-centre) bonds can be seen in the Lewis Structure column.
A really good resource is the NBO 6.0 Manual to read more about such 3-centre bonds.
There is a chapter in the manual called: B.16 3-CENTER, 4-ELECTRON HYPERBOND SEARCH
Additionally for some theory on a quantitative definition of hypervalency, this paper discusses a method to determine hypervalency using various charge models, one of which is NBO analysis.
So I have found out that by default the NBO version implemented in GAUSSIAN 16 searches for 1c and 2c bonds but not for 3c bonds. One has to activate it by adding an NBO input at the end of the GAUSSIAN input file. An example looks like this:
%Chk=chk_file.chk # B3LYP/def2tzvp pop=nboread nbo 3c bond analysis 0 1 A x1 y1 z1 B x2 y2 z2 C x3 y3 z3 $nbo 3CBOND $end
Using this input causes the program to also search for three-center bonds.