3c/2e bonds for nonclassic

3c/2e bonds for nonclassic

Postby vitaliy » Fri May 25, 2018 9:29 am

Dear All,

Please, help us to make conclusions for 3c/2e bonds in nonclassical compound Al4H4:

Al 0.000000000 1.499805000 -0.530233000
Al 0.000000000 0.000000000 1.590663000
Al -1.298869000 -0.749902000 -0.530233000
Al 1.298869000 -0.749902000 -0.530233000
H 0.000000000 0.000000000 3.186219000
H 0.000000000 3.004139000 -1.061915000
H -2.601661000 -1.502069000 -1.061915000
H 2.601661000 -1.502069000 -1.061915000

We can see 3c bonds in output file of Gaussian NBO Version 3.1.

But if we use NBO 6.0, we can see several donor-acceptor interactions between pairs of Al atoms, but 3c bonds have not been detected.

Which variant is more correct in terms of current NBO theory?
And is it possible to find 3c/2e bonds in our compound with NBO 6.0 help?

Kind regards,
Vitaliy.
vitaliy
 
Posts: 4
Joined: Sun Jul 03, 2016 1:17 am

Re: 3c/2e bonds for nonclassic

Postby ericg » Sat May 26, 2018 8:12 am

Vitaliy,

The 3c/2e solution is preferred over 2c/2e solution. One can judge this based on non-Lewis density (rhoNL), the number of electrons not described by the orbitals of the natural Lewis structure (NLS). For the 2c/2e NLS, rhoNL is about 4.2 electrons, whereas for the 3c/2e NLS, rhoNL is about 1.3 electrons.

We modified NBO 6.0 a few years ago to allow it to search automatically for 3c/2e bonds (without 3CBOND keyword input). The search does consider 3c/2e, but finds none. I'll have to examine the search more closely to determine why.

You can recover the 3c/2e-based NLS with NBO 6.0 using $CHOOSE input. The following worked for me, using your geometry:

Code: Select all
$nbo $end
$choose
  bond s 1 6 s 2 5 s 3 7 s 4 8 end
  3c s 1 2 3 s 1 2 4 s 1 3 4 s 2 3 4 end
$end


Eric
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Re: 3c/2e bonds for nonclassic

Postby vitaliy » Mon May 28, 2018 3:03 am

Eric,

Thank you for advice.

With best wishes,
Vitaliy.
vitaliy
 
Posts: 4
Joined: Sun Jul 03, 2016 1:17 am


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