The rootinng hormone will be diluted to a point where it will not help you are starting with a dilution 1 part hormone to 1 milllion dilution.
the hormone would cause the death because it was a stronger dilution OR you did not mist the cuttings OR the leaf area was to great OR the soil dried out OR ...OR...OR

I've used it before in water and nothing ever happened except it made the water yucky and the plants slimy. You would probably be better off using the rooting hormone with the potting soil. Just dip the ends in the rooting hormone, shake off the excess and and stick the cutting in the soil. I've had more luck that way but patience is required as it takes awhile to take effect. I've noticed that only really fleshy cuttings have worked in water (without rooting hormone) Like impatiens and begonias. Also I have a type of succulent (NOID) that I propagated in water on accident. Which got me started on rooting cuttings. :)

I have never tried this yet, but if you have access to w willow tree, cut up some pieces and add them, thie may release a chemical(s) which aid in rooting

as yiorges-z5il suggested, the actual compound that aids rooting needs to be used in such minute quantities (in pats per million) that dissolving the hormone in water might make the concentration too strong along the stem under water and you'd get more in the soil than on the base of the cutting (if you are using soil).

The rooting compounds usually contain some auxin (usually IBA) - auxins are herbicides at high concentrations (hence the low concentrations in rooting hormones). That being said, because the concentration is so low, the majority of the rooting powders are fillers with the IBA mixed in. If you've ever gotten baby powder or flour wet, imagine using a thin/watery solution of flour and water on your plant...

greenpassion -

You should check out the Brugmansia Forum for info on 'propagating' Datura. Some say Datura cuttings can be rooted, though I've tried several times w/o luck. Most say the seeds are plentiful (the prickly pockets that form on the plant are full of them), thus rooting cuttings isn't necessary.

Best of luck -

D.