Cola is an option for an easily available acidic potable (barely!
) liquid and maybe the strong flavour helps conceal the less-than-palatable rue flavour. Caffeine and theobromine may play their own pharmacological (stimulant) role too, although one should perhaps be wary of the potential for tachycardia with that. There were also some intimations that phosphoric acid possibly enhances absorption of DMT in some way, perhaps because phosphate is an important component of the physiological energy transport chain.
Soda water and tonic water will be far less acidic than cola, so I'd suggest adding a tiny pinch of some kind of food acid. Acetic acid tends to be unpleasant because of its strong vinegary odour, hence the common suggestion of citric acid. Personally, I favour using a little ascorbic acid as it's good to keep your vitamin C levels up and it does the job of solubilising alkaloids quite nicely too.
I don't think there's any particular chemical reaction specific to cola in this context, outside of the possible absorption enhancing effect of the phosphoric acid. And even if you were to swallow freebase DMT, it would get converted to the protonated salt fairly rapidly on contact with your stomach acid.
“There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli