Sum frequency generation study of Langmuir Blodgett film architecture

We present sum frequency generation vibrational spectroscopy data on monolayer films of pure stearonitrile and mixed films of stearonitrile and ferric stearate deposited by the Langmuir Blodgett technique. Films ranging in composition from pure stearonitrile to 65% stearonitrile/35% ferric stearate deposit in a head-to-tail, Z-type architecture while films of composition <65% stearonitrile deposit in a head-to-head, tail-to-tail, Y-type structure. The vibrational spectra of the monolayer films corresponding to these two different architectures show distinct and characteristic features in their CH stretching regions. Spectral analysis indicates little difference in the modes associated with the terminal methyl groups but large differences in the methylene CH(2) vibrational modes. These spectral differences are consistent with alkyl chains that possess uncompensated methylene oscillator strength arising from portions of the chain that are bent. This behavior is consistent with the energetics expected from a close-packed monolayer of stearonitrile molecules whose CN headgroups possess sizable dipole moments and are expected to adopt a large center-to-center angle to minimize electrostatic, dipole-dipole repulsive interactions.