Calculations of deuterium NMR spectra were performed using a model of slow motions based on the collective modes present in liquid crystalline systems and evaluated within the Landau de Gennes free energy expansion on the order parameter tensor. Simulations obtained with this model are applied to the case of deuterium NMR spectra collected in static and rotating samples of organosiloxane tetrapodes exhibiting uniaxial and biaxial nematic phases. The analysis of the slow motions influence on deuterium NMR spectra shows that molecular motions within a time-scale of the order of magnitude of NMR observation times are particularly effective on modulating the NMR line-shape in the case of the liquid crystalline system investigated.