Experimental NMR and DFT Studies of Persistent Carbocations Derived from Hetero-Polycyclic Aromatic Hydrocarbons Containing Oxygen Atom: Dibenzo[ b , d ]furan, Benzo[ b ]naphtho[1,2- d ]furan, Benzo[ b ]naphtho[2,3- d ]furan, Benzo[ b ]naphtho[2,1- d ]furan, and Dinaphtho[2,1- b :1′,2′- d ]furan