The combined effect of pressure and molecular vacancies on the atomic structure and electronic properties of solid nitromethane, a prototypical energetic material, is studied at zero temperature. The self-consistent charge density-functional tight-binding method is applied in order to investigate changes induced in the band gap of this system by uniform and uniaxial strain of up to 70%, corresponding to static pressure in the range of up to 200 GPa. The effects of molecular vacancies with densities ranging from 3% to 25% have also been considered. A surprising finding is that uniaxial compression of about 25-40 GPa along the b lattice vector causes the C-H bond to be highly stretched and leads to proton dissociation. This event also occurs under isotropic compression but at much higher pressure, being indicative of a detonation chemistry which is preferential to the pressure anisotropy. We also find that the band gap, although evidently dependent on the applied strain, crystal anisotropy and vacancy density, is not reduced considerably for electronic excitations to be dominant, in agreement with other recent first-principles studies.