national capital vs Port-of-Spain

national capital and Port-of-Spain both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Port-of-Spain	Yes	No	No	No

national capital (noun)	Port-of-Spain (noun)
the capital city of a nation	the capital and largest city of Trinidad and Tobago on the west coast of the island of Trinidad