[Rex]

Above gauche form is stable when Z is:

I was thinking about reducing steric hindrance to stabilize the gauche, thus answer should be (a). But answer is (c). Any hits are appreciated.

[Babcock_Hall]

Besides Van der Waals repulsion, what other intermolecular forces do you know about?

[Rex]

Besides Van der Waals repulsion, what other intermolecular forces do you know about?

Hydrogen bonding and London Dispersion Forces.  I also know about back bonding.

[Babcock_Hall]

What about dipole-dipole forces or ion-dipole forces?

[Rex]

What about dipole-dipole forces or ion-dipole forces?

Yes, I know about them. Yeah there will be interaction between positive charge of nitrogen and partial negative charge of oxygen. But how do I know which will dominate? Steric hinderence or ion-dipole interaction?

[Babcock_Hall]

Another factor to consider is that sp² carbons such as carbonyl groups will probably have different steric behavior from sp³ carbons.  Speaking only for myself, I would find it more difficult to predict relative stabilities in advance than I would to explain it.

[Rex]

Thanks a lot Babcock_Hall. I really appreciate it.