Orientation Analysis

Sum Frequency Generation Orientation Analysis of Molecular Ammonia on the Surface of Concentrated Solutions

Danielle Simonelli and Mary Shultz Department of Chemistry, Tufts University, Medford, Massachusetts 02155 Journal of Chemical Physics, 112, 2000.
Ammonia-water complexes have been detected with sum frequency generation (SFG) at the liquid/vapor interface of concentrated ammonia solutions (0.3x NH₃, x = bulk mole fraction). SFG spectra taken with the ssp polarization combination (s-polarized sum frequency signal, s-polaried visible light, p-polarized infrared beam) are dominated by the N-H symmetric stretch (n₁) at 3312 cm^-1 and a weaker deformation mode (2n₄) at 3200 cm^-1. The dangling (free) OH peak due to water at 3700 cm^-1 is suppressed at this concentration, indicating that water molecules are complexed through hydrogen bonds to ammonia at the interface. The polarization dependence of n₁ indicates that the NH₃ C₃ molecular axis is tilted with respect to the surface normal by q<38^o. Variation of the antisymmetric stretch, n₃, sum frequency signal intensity with polarization restricts the lower limit for surface ammonia tilt angles such that 25^o<q<38^o. Based on this orientation, the structure of the ammonia-water complex is discussed.

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