I am trying to explain bond length change through NBO analysis. I can not calculate bond order directly with the present version of software. In that case, what are the quantities I should look for? If a bond has ionic character, is there any way to say it change qualitatively?
May be you can look into the charge of the atoms that form the bond...if u are using Gaussian you can look into the keyword "Pop" for further details. Hope this helps you.
You can analyze the NBO orbital asociates to the bond and to determine ionic character in the bond:
for example: for N-C bond in methylamine, nbo analyisis show that 59.93% of the electrons in the bond are condensed in the nitrogen atom, the, covalent bond is not 100% covalent. In ionic bond, this difference would be larger.
1. (1.99858) BD ( 1) C 1- N 2
( 40.07%) 0.6330* C 1 s( 21.71%)p 3.61( 78.29%)
-0.0003 -0.4653 -0.0238 -0.8808 -0.0291
-0.0786 -0.0110 0.0000 0.0000
( 59.93%) 0.7742* N 2 s( 30.88%)p 2.24( 69.12%)
-0.0001 -0.5557 0.0011 0.8302 0.0004
0.0443 -0.0098 0.0000 0.0000
How large the difference that can be classified as ionic bond? If one atom has 25% and other atom has 75%, is it a covalent or ionic bond?
The ionic versus covalent nature of bonds can be quantified using the net atomic charges (to determine if the bond is ionic) and the bond orders (to determine if the bond is covalent). Ionic bonds will display large net atomic charges and small bond orders. Covalent bonds will appreciable bond orders and small net atomic charges. For polar-covalent bonds, both the net atomic charges and the bond orders will be substantial.
You can get these printed out in a nice format by using the free Chargemol program (http://ddec.sourceforge.net). This algorithm for computing bond orders is more widely applicable than alternative methods. See the following paper for details: T. A. Manz, "Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders," RSC Advances, 7 (2017) 45552-45581 (http://dx.doi.org/10.1039/C7RA07400J ).
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