I have a molecule bearing a quaternary ammonium and I optimized it using M062X (also Hartree-Fock) using basis sets 6-31G(d,p) and 6-31G+(d,p) respectively. After optimization, I expect that the charge on the quaternary nitrogen would be positive. However, it's negative.

I ran the Gaussian to compute NBO charges and analysed the checkpoint files as well as log files. Nowhere could I see a positive charge on any nitrogen atom.

Why is this happening? And how can I obtain a positive charge on the quaternary nitrogen?

Many thanks in advance.

  • $\begingroup$ Where is the positive charge, then? $\endgroup$ – Ivan Neretin May 26 '19 at 10:06
  • $\begingroup$ What charge to the hydrogen atoms carry? How does the charge distribution differ from a neutral ammonia molecule? $\endgroup$ – The_Vinz May 26 '19 at 10:12
  • $\begingroup$ Run a calculation with neutral ammonia, and you will see it, indeed. I just verified by running the same calculation. $\endgroup$ – The_Vinz May 26 '19 at 10:16
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    $\begingroup$ Also, to understand the reason behind this behaviour, it could be a wise suggestion that of considering the concept of electronegativity $\endgroup$ – The_Vinz May 26 '19 at 10:18
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    $\begingroup$ Indeed, the electron pair is shared with a proton (charge +1). But nitrogen is still more electronegative than hydrogen, so two effects occur: 1.the proton sucks electrons from N's pair, and its charge tends to decrease 2.Nitrogen, electronegative, "wants" back its electrons: so its charge increases, but to a limited extent. As you can see, in neutral ammonia, N is around -1. When ammonia is protonated, charge delocalizes over all its atoms because of the two effects described. $\endgroup$ – The_Vinz May 26 '19 at 11:04

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