Direct one-phonon processes of spin-lattice relaxation are investigated in the excited tripler states of molecular crystals in high magnetic fields and at low temperatures. The mixing of the vibrational motions of the nuclei with the translational ones of the molecule is chosen as the major phenomenon, leading to the coupling between the spin system and acoustic vibrations. The angular, field and temperature dependences of the spin-lattice relaxation rate for an isotopic impurity in deuterobenzophenone crystal are calculated by the molecular orbitals and atom-atom potential methods.