[Big-Daddy]

(b) Draw the species formed when the imine CH₃C(=N-H)H is protonated. How do the forms and energies of the pi and pi* orbitals of the neutral imine change upon protonation?

(c) How would you expect (i) the C=N IR stretching frequency and (ii) the 13C NMR spectrum to change upon protonation?

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I'm not sure how to approach b). I would think the nitrogen's lone pair (in an sp² orbital) interacts with the unfilled 1s orbital of the incoming H+ to form a new sigma bonding orbital, but why and how does this change the energies of the C=N pi and pi*?

As for part c), my thinking is this: the N will be less negative than before and thus the bond polarity will decrease, so the bond will weaken causing the IR peak to be at a lower wavenumber; the C will also be more deshielded (as will the methyl C, slightly) as the N draws more electron density from C to compensate for that moved towards the new proton, so both signals will appear at slightly higher shift. There will be no change in splitting (because the protons attached to the N are exchangeable via hydrogen-bonding, and in any case the spectrum is presumably proton-decoupled).

Am I right, and if not, why not?

[mjc123]

Draw the possible resonance structures for the unprotonated and protonated forms. How much would you expect each resonance structure to contribute? What are the implications for your questions?

[Big-Daddy]

Draw the possible resonance structures for the unprotonated and protonated forms. How much would you expect each resonance structure to contribute? What are the implications for your questions?

Really not sure how that helps for part b) on the orbital energies. I thought resonance forms were an approximate way of thinking about the results of MO theory, not the other way around. I was hoping the answer would come out of qualitative MO theory i.e. first principles rather than the resonance forms?

Having drawn the resonance forms with reasonably high contributions, I think my reasoning on the 13C NMR spectrum is right (because the protonated imine has a dominant resonance form where the +ve charge is on carbon, while in the unprotonated imine the positive C resonance form contributes much less to the overall). Not sure how to relate the IR shift (strength of C=N) to these resonance forms, except perhaps that my intuition seems right again, the N is more positive on protonation so the bond polarity and thus bond strength is lower on protonation.

The resonance forms cannot explain what happens to the methyl carbon which I also predict will be slightly more deshielded in the protonated imine.

[mjc123]

In the unprotonated form the main contributor is >C=N-. The other forms >C⁺-N^-- and >C^--N⁺- involve separation of charges, so are of higher energy (the latter more so as N is more electronegative than C) and contribute less. Thus the C-N bond is essentially a somewhat polarised double bond.
In the protonated form you have >C=NH⁺- and >C⁺-NH-, neither of which involves charge separation, so they are likely to be of comparable energy and both make a significant contribution. Thus the C-N bond order will be significantly less than 2, and both atoms have a significant δ+ charge.
I agree this is probably not that helpful for part b, as you would be mixing VB and MO approaches, but I think it helps qualitatively with part c.

[Big-Daddy]

Thus the C-N bond order will be significantly less than 2, and both atoms have a significant δ+ charge.

Yes, and the C is more positive than before protonation? Then you agree with my intuition.

I think we can add a further detail - that the C of the imine will have its shift increased (due to the deshielding effects) more than the methyl C, because the inductive effects play a more direct role for the imine C (they play the most direct role, and deshield the most, the N which is being protonated but of course this doesn't affect the 13C NMR directly).

Any way I can do this (part b and c) using molecular orbitals, though?