The direct one-phonon processes of spin-lattice relaxation (SLR) in the excited triplet states of organic molecular crystals are investigated at low temperatures and high magnetic fields. The mixing of the translational and rotational motions of the molecule (TRM relaxation mechanism) is chosen as the major mechanism, leading to the coupling of the spin system with acoustic vibrations. The angular, field and temperature dependences of SLR rate are calculated for an isotopic impurity in deuterobenzophenone crystal using the atom-atom potentials method of the intermolecular interaction. The anisotropy of the electronic (g) over cap-tensor is analyzed. The obtained results are in accordance with available experimental data.