χρή (χρῆν, ἐχρῆν) takes either μή or οὐ.
χρὴ μὴ καταφρονεῖν τοῦ πλήθους
χρῆν οὔ σ' ἁμαρτάνειν
εἰπεῖν οὐδέν' ὄλβιον βροτῶν
a. For original οὐ χρή was substituted (for emphasis) χρὴ οὐ, where the οὐ was still taken with χρή; ultimately οὐ was felt to belong with the infinitive and hence came to be separated from χρή.
b. δεῖ takes μή, as
μὴ ὀκνεῖν δεῖ αὐτούς
δεῖ οὐχ ἁπλῶς εἰπεῖν
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].