national capital vs Saint John's

national capital and Saint John's both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Saint John's	Yes	No	No	No

national capital (noun)	Saint John's (noun)
the capital city of a nation	a port and provincial capital of Newfoundland
	the capital and largest city of Antigua and Barbuda; located on the island of Antigua