San Marino vs national capital

San Marino and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
San Marino	Yes	No	No	No
national capital	Yes	No	No	No

San Marino (noun)	national capital (noun)
the smallest republic in the world; the oldest independent country in Europe (achieved independence in 301); located in the Apennines and completely surrounded by Italy	the capital city of a nation
the capital and only city of San Marino