Does chemistry change under high voltage?
Long story short, no, they don't.

Charging a vessel to 1 MV is not a big deal, if you look at it from the inside. To put things into perspective, imagine a vessel of about 10 cm across, which is kinda OK for a flask. Imagine it spherical to make the calculations easier.


Now, the capacitance of a sphere is $C=4\pi\varepsilon_0R$, which evaluates to 5.56 pF (yes, "p" stands for "pico").
So charging your vessel to 1 MV implies putting $5.56\;\mu\rm C$ of electric charge on it.
That's $3.48\cdot10^{13}$ extra electrons (or some negative or positive ions, that doesn't matter) ...
... which may seem big until you compare it to the amount of water that would fit into the vessel: $1.75\cdot10^{25}$ molecules.


So it is like 1 electron per quite a few billions of molecules. That would not make much of an influence. The molecules will just sit there as usual, feeling nothing; most of them have never seen an electron in their entire lives, and have no idea that their vessel is being charged.

Things will change a great deal if you touch the vessel from the outside, but that's just ordinary electrochemistry which you seem to put out of scope of your question.