Split infinitives are accepted by some and not by others. It is
almost a matter of personal preference. My opinion follows, if
you're interested.
Sometimes you have to split an infinitive because there is no other
logical place to put an adverb, and some adverbs sound wrong (to some
speakers) when placed before an infinitive. However,
not is such a short adverb that it can easily be placed before the infinitive without sounding awkward, so there's no reason
not to place it there except stubborn perversity.
CJ