capital of Iceland vs national capital

capital of Iceland and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
capital of Iceland	Yes	No	No	No
national capital	Yes	No	No	No

capital of Iceland (noun)	national capital (noun)
the capital and chief port of Iceland on the southwestern coast of Iceland; buildings are heated by natural hot water	the capital city of a nation