Raman and infrared spectra of solid nitrogen have been collected between 25 K and room temperature up to 41 GPa. A careful analysis of the spectral band transformations occurring across the high pressure transitions among the delta, delta(loc), epsilon, and zeta phases allowed to define the phase diagram in the whole P-T region investigated. In particular, the transition between the epsilon and zeta phases has been observed in the range 30-230 K and the corresponding phase-boundary drawn. A significant metastability region (spanning about 10 GPa in pressure) hinders the transformation between the epsilon and zeta phases when pressure is varied at low temperature. Group theory arguments suggest a centrosymmetric structure for the zeta phase and the number of Raman and infrared nu(1) and nu(2) components can be reproduced both with cubic and tetragonal structures. An appreciable coupling among neighboring molecules is observed, at room temperature, only in the epsilon phase where the relative orientations of the molecules are fixed.