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/CCD excitation 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+ <Sr 2+ <Pb 2+ Ionic Radii

SFG Results Raman SS ω SFG ω vis ω IR IR SS

 Mg 2+ Ca 2+ Sr 2+  Interface vs Bulk

Conclusion  In aqueous solutions:  more ion pairing at higher concentrations  CIP formation increases with increasing ionic radius, Mg 2+ <<Ca 2+ <Sr 2+ < Pb 2+  At air-water interfaces:  effects of Mg 2+ on NO 3 -

Acknowledgments  Dr. Heather C. Allen  Dr. Gang Ma  Cheng Y. Tang  Mazen Roshdy  All other Allen group members