The phrase is not p whenever q.
I take this to mean the same thing as not p if q.
When p is false, q can be true or false.
When p is true, q is false.
When q is true, p is false.
When q is false, p can be true or false.
Stated logically, I could write this as: $q\to \lnot p$. Would that be sufficient or is there more to it?