Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unpremeditated; he lists it under extemporaneous.
Where he writes about it:
Unpremeditated is graver and more formal, denoting absolute want of preparation, but is rather too heavy a word to be applied to such apt, ready utterance as is generally designated by impromptu. — from extemporaneous