Sum Frequency Generation and Raman Spectral Study of Nitrate-Water Systems Man Xu, Heather C. Allen The Department of Chemistry Environmental Science Graduate program The Ohio State University Nitrate ion one of the most abundant ionic species in atmospheric aerosols plays an important role in many atmospheric chemical processes Vibrational spectrum a function of the ionic environment serves as an indicator of ionic interactions Introduction free ions A C solvent separated (SSIP) A C solvent shared (SSHIP) A C contact (CIP) A C Ion Pairing CIP NO 3 - M 2+ NO 3 - SSHIP NO 3 - M 2+ NO 3 - H2OH2O SSIP NO 3 - M 2+ NO 3 - H2OH2O M 2+ NO 3 - Marcus and Hefter Chem. Rev. 2006 monochromator/CCDexcitation 785 nm sample collection Intensity N Raman Spectroscopy Stokes scattering Rayleigh scattering virtual excited state Vibrational energy levels v=0 v=1 IR absorption Media must lack inversion symmetry Like an Interface!! Like an Interface!! Vibrational mode must be Raman & IR active sample vis IR vis + IR = sum sum Sum Frequency Generation Spectroscopy NaNO m KNO 3 3.5m Mg(NO 3 ) 2 4.9m Ca(NO 3 ) 2 5.1m Sr(NO 3 ) 2 3.3m Ba(NO 3 ) 2 0.3m Pb(NO 3 ) 2 1.7m Symmetric Stretching Asymmetric Stretching In-Plane Deformation Raman Results Free vs Ion pairing Free NO 3 - (D 3h ) IR SS Ion paired (C 2v /C s )ASSS Out-of-plane deformation In-plane deformation ~1400 cm -1 ~1050 cm -1 ~830 cm -1 ~720 cm -1 3(E) (R, IR) 3(E) (R, IR) 1(A1) (R) 1(A1) (R) 2(A2) (IR) 2(A2) (IR) 4(E) (R, IR) 4(E) (R, IR) ~1450 cm -1 ~1360 cm -1 ~1050 cm -1 ~830 cm -1 ~718 cm -1 ~740 cm -1 4(B1) 4(B1) (R, IR) 1(A1) 1(A1) (R, IR) 2(A1) 2(A1) (R, IR) 6(B2) 6(B2) (R, IR) 5(B1) 5(B1) (R, IR) 3(A1) 3(A1) (R, IR) Hester and Plane JPC 1964, Irish and Walrafen JCP 1967 Raman Na + K + Mg 2+ Ca 2+ Sr 2+ Ba 2+ Pb 2+ Saturated 2m NO 3 - 1m NO 3 - Asymmetric Stretch Peak Splitting is attributed to perturbation of nitrate ion by water molecules. Separation of two maxima is increased by long-range electrostatic perturbation from cations. Irish JCP 1969, Peleg JPC 1972, Wahab JPC B 2005 In-Plane Deformation Peak Mg(NO 3 ) 2 Ca(NO 3 ) 2 Sr(NO 3 ) 2 Pb(NO 3 ) 2 Splitting of the in-plane deformation 4(E) band is a criterion for CIP formation Peak splitting of Ca, Sr, Pb nitrate indicates the presence of contact ion pairs Mg nitrate: no contact ion pairing Irish Inorg. Chem Fleissner JPC 1993 Tang JCP 1997 Wahab JPC B 2005 free+bound bound Ca 2+, Sr 2+ & Pb 2+ ? If component band at ~719 is free NO 3, ~740 is assigned to contact ion pairs If only look at the ~740 side of the bound NO 3 In-Plane Deformation Peak In-plane deformation ~720 cm -1 4(E) (R, IR) 4(E) (R, IR) ~718 cm -1 ~740 cm -1 5(B1) (R, IR) 5(B1) (R, IR) 3(A1) (R, IR) 3(A1) (R, IR) Contact ion pairing in nitrates: Mg 2+ < < Ca 2+