Nitrogen-vacancy defects in Si are of interest due to their ability to suppress the formation of large vacancy cluster during growth but there are problems in their characterisation. We use local density functional theory to determine the local vibrational modes, electrical levels and stability of a number of nitrogen defects. A prominent nitrogen local vibrational mode at 663 cm-1 is attributed to a nitrogen-vacancy centre and tentative assignments of the ABC photoluminescence line and the trigonal SL6 BPR centre are made.