In this paper, the NMOB molecule is studied in terms of DFT B3LYP method using 4-31G^* basis set. The geometry of NMOB was obtained by replacing the ethyl in 2-nitro-5-(N-methyl-N-ethyl) amino benzoic acid (NMEB) whose geometry was optimized by B3LYP/3-21G^* with octadecyl from dimethyloctadecylamine (DMOA) optimized by PM3. We defined the dihedral angle as D, which was composed of the amino nitrogen atom, the carbon atom in benzene ring connecting directly to the amino nitrogen atom and the two carbon atoms in octadecyl group nearest to the nitrogen atom. It was found that the system potential curve had minimums when the D of NMOB was equal to about 90ⅲ or 270ⅲ . There was a maximum at 180ⅲ whose system potential was 31kJ/mol higher than that of 90ⅲ and 28kJ/mol than that of 270ⅲ (shown in Tab. 2 and Fig. 2). The most stable conformation of NMOB (D ≈90ⅲ or 270ⅲ) exists in its self-alternated multilayer. In NMOB/Cadmium arachidate (CdA) alternated film, NMOB exists in the form of vertical conformation (D=180ⅲ). We suggested the unstable vertical conformation of NMOB come from the inducement of CdA, which stood, vertically on the substrate.

Key words: L-B MEMBRANE, MOLECULAR CONFORMATION, BENZENECARBOXYLIC ACID P, BENZAMINE P, AB INITIO CALCULATION