A proclitic sometimes takes an accent, thus:
a. οὐ at the end of a sentence: φῄς, ἢ οὔ;
b. ἐξ, ἐν, and εἰς receive an acute in poetry when they follow the word to which they belong and stand at the end of the verse:
d. When the proclitic precedes an enclitic ( cross183 e): ἔν τισι.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].