What is the difference between 「なく」 and 「なしに」?

Question

I know there is an adjective 「関係ない」 which is basically a noun + ない, but I recently came across 「なしに」 in this sentence :

準備なしに発表した。

And I noticed that the に in なしに was making it act like an adverb. So I just wanted to know if I can say :

準備なく発表した。

And if yes, what is the difference between なしに and なく?

Answer 1

準備なく発表した。
is equal to
"準備なしに発表した。"

The difference is so subtle that I can't explain it.

Examples

許可{きょか}なく出発{しゅっぱつ}した＝許可なしに出発した (without permission)
十分なデータなく判断{はんだん}した＝十分なデータなしに判断した (without adequate data)
誰にも知られることなく立ち去った＝誰にも知られることなしに立ち去った (without someone knowing about ...)

Thus "名詞+なく" is equal to "名詞+なしに," but there are exceptions.

There is an idiom "お構{かま}いなしに(without any regard to ...)," but we don't say "お構{かま}いなく" as the same meaning.

Actually "お構いなく" has a different meaning.

"How can I help you? (何かお困{こま}りですか？)"
"Thank you, but no thank you. (いえ、大丈夫です。お構{かま}いなく)"