The spin-lattice relaxation mechanism based on the energy modulation of spin-orbital interaction has been investigated in the triplet excited states of impurity molecular crystals. This modulation emerges as a result of the admixture of the rotational motions of a molecule and the vibrational motions of nuclei to the translational molecular motions at the acoustic lattice vibrations. In the framework of this spin-lattice relaxation mechanism the direct one-phonon processes are studied at low temperatures and in high magnetic fields. The calculations of angular, field and temperature dependences of the spin-lattice relaxation rates for a real deuterobenzophenone crystal with an isotopic impurity were carried out using the molecular oribital and atom-atom potential methods.