grammar - Function of に in 怒るに怒れない

In the book I'm reading, I came across the following sentence:

あたしは[怒]{おこ}るに[怒]{おこ}れなくなった。 _{(furigana added by me)}

My translation is something like "I couldn't stay angry [at him]". ("My anger melted away.")

I don't actually understand the grammar, though. The phrase 「怒るに怒れない」 has what appears to be particle-に following the dictionary form of a verb. It sounds contrastive. I can't seem to find this pattern in my dictionaries, though, so I feel unsure.

I'm interpreting it as 「怒る(の)に怒れない」, with a zero-nominalizer after the verb. Is this correct?

Is this a set phrase preserving old grammar of some kind?

Answer

This is a common pattern that means "even if I wanted to V, I cannot V" or "no matter what, I cannot V". As such, in your sentence, it means " I could not get mad even if I wanted to.".

As for the grammar, this is a conjunctive particle (接続助詞). Rather than attaching to the "dictionary form" (終止形), though, it attaches to the attributive (連体形). That is why there is no need for a (zero-) nominalizer.