There is actually a statement, specifically the IRS 2014-21 where all virtual currencies are subjected to tax still. Therefore, stating that buying using BTC or any other crypto, is in fact, still liable to tax. As for the specifics, I don't really know but the 2014-21 already states that existing tax principles apply to transactions using virtual currency. The taxable events are Cashing out, Paying using crypto, Exchanging one crypto to another or Receiving mined crypto. These are some I just looked up in google and you can search for more information if you want specifics.