Quantum mechanical and classical dynamics of double proton transfer in the hydrogen bond

Spin-lattice relaxation theories for classical and tunneling motions of protons in hydrogen bonds are discussed. It is shown that only one of these theories explains the experimentally determined frequency dependence of proton spin-lattice relaxation rates for benzoic acid and a few benzoic acid derivatives satisfactorily. In this theory classical and tunneling proton transfers are considered as a complex motion.