The harmonic force field and the vibrational spectrum of N-methylnitramine have been calculated by TEXAS ab initio program using a 4-21G basis set. The directly computed theor. harmonic force field was scaled with empirical scale factors which were transferred from other mols. to provide a prediction of fundamental frequencies. The average deviations between prediction and experimental spectrum of N-methylnitramine in gas phase were 31 cm-1. A new set of scale factors was optimized to give better force field suitable for N-methylnitramine in gas phase and available for prediction of vibrational spectra of its isotopic derivatives These scale factors reduced the average deviations to 8.9 cm-1. The vibrational spectra of three isotopic derivatives of N-methylnitramine have been predicted with the force field resulting from the optimized set of scale factors, and are in good agreement with their experimental data in gas phase.

Key words: AB INITIO CALCULATION, VIBRATIONAL SPECTRUM, METHYL-NITRAMINE