no bonding orbital but strong interaction

no bonding orbital but strong interaction

Postby mnahali » Mon Aug 07, 2017 3:08 am

Hi

The adsorption of CN- on AgCu5 cluster is about -2.0 eV and CN- is adsorbed on Cu atom. In the following you can see bond order data between C(8) and Cu(2).
8 and 2 are seq. number of atoms in the system.
The issue is that there is not any BD (bonding orbital) between Cu and C and the bond order seems to be small. I am searching to find how does CN- bind Cu ?!
Is there an ionic bond ?
I would appreciate if anyone help me to understand the bonding character between Cu and CN- in the above system. Attached you can find the output of our calculation.

----
C(8)-Cu(2)
Wiberg bond index: 0.25
NBI: 0.4989
Net Linear NLMO/NPA bond order: 0.085
Natural bond order(total/covalent/ionic): 0.0607/0.0038/0.0572
----

Sincerely Yours
Masoud Nahali
NIT
Attachments
Cu5Ag-a-CN-1-B.zip
output-G09-NBO6
(100.54 KiB) Downloaded 11 times
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Re: no bonding orbital but strong interaction

Postby ericg » Mon Aug 07, 2017 9:39 am

A good way to address your question is to re-run your calculation using the $CHOOSE keylist to force the calculation of a Cu(2)-C(8) NBO.

The keylist printed in your output shows one lone pair on C(8) and no Cu(2)-C(8) bond:
Code: Select all
  $CHOOSE
    LONE 1 5 2 5 3 5 4 5 5 5 6 5 7 1 8 1 END
    BOND S 1 2 S 3 5 S 4 6 T 7 8 END
  $END

The lone pair can be replaced by a single bond:
Code: Select all
  $CHOOSE
    LONE 1 5 2 5 3 5 4 5 5 5 6 5 7 1 END
    BOND S 1 2 S 3 5 S 4 6 T 7 8 S 2 8 END
  $END

A quick b3lyp/sdd calculation of your geometry with this keylist resulted in a Cu-C bond that is 6%/94% polarized toward C. That is, the bond has an ionicity of about 88% (=94%-6%) and is best described as ionic.

Eric
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