Ammonia-Water Complexes on the Surface of Aqueous Solutions Observed with Sum Frequency Generation

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Danielle Simonelli, and Mary Shultz Department of Chemistry, Tufts University, Medford, Massachusetts 02155 Steve Baldelli,
Department of Chemistry, University of California, Berkeley, California 94720 Chemical Physics Letters.
Sum frequency generation (SFG) is used to obtain the vibrational spectrum of aqueous ammonia solutions at the liquid/vapor interface. The sum frequency spectrum of concentrated x (x = mole fraction) NH₃, is dominated by the N-H symmetric stretch ( n₁) at 3312 cm ^-1 and a weaker deformation mode (2n₄) at about 3200 cm . The free OH peak of water in this mixture is suppressed, compared to pure water, and the hydrogen-bonded region (3000 - 3450 cm^-1) has interference in the presence of ammonia molecules at the interface. In dilute solutions of 0.14 x NH₃, n₁ is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia-water complex at the liquid/vapor interface.


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