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Abstract

Finding optimal ion-solvent configurations using FTIR For Studying ion association dynamics of thiocyanate salt by 2DIR spectroscopy Maria Gonzalez Office of Science, Science Undergraduate Internship Program Loyola University, New Orleans, LA Stanford Linear Accelerator Center

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Abstract

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  1. Finding optimal ion-solvent configurations using FTIR For Studying ion association dynamics of thiocyanate salt by 2DIR spectroscopy Maria Gonzalez Office of Science, Science Undergraduate Internship Program Loyola University, New Orleans, LA Stanford Linear Accelerator Center Menlo Park, California August 14, 2008

  2. Abstract • The ion association and dissociation dynamics in thiocyanate salt solutions will be probed using 2D-IR spectroscopy allowing for the determination of equilibrium association (or dissociation) rate constants of the thyocyanate anion and its counter cation. Thiocyanate’s nitrile stretch, which is sensitive to ionic interactions, was used in revealing the interaction among the thiocyanate ion and the cation in solution. The optimal solution parameters for the thiocyanate salts was determined by a one to one area ratio of the nitrile vibrational frequency of free thiocyanate to the contact ion pair using one dimensional infrared spectroscopy.

  3. Infrared &Thiocyante Nitrile Stretch ~2100-2240 cm-1 Free thiocyanate ion ~ 2050 cm-1 Contact Ion Pair ~ 2070 cm-1

  4. S1 v` = 2 v` = 1 v` = 0 S0 v = 2 v = 1  Anharmonicity v = 2 v = 0 First overtone v = 1 Fundamental v = 0 Ultrafast Vibrational Spectroscopy • Understanding of the ultrafast structural dynamics of complex molecular systems in solution has been restricted by the fast time scale such processes take place on • Vibrational excitations, in contrast to electronic excitations, •  produce a negligible perturbation with less energetic • IR photons. •  don’t change chemical properties of molecules • under study. • Ultrafast Vibrational spectroscopy •  allows study molecular systems under thermal equilibrium • conditions. •  measures dynamics occurring on fs and ps time scales.

  5. beam combiner ksig= -k1+k2+k3 Esig k1 k3 k2 Monochromator k2 ksig k3 k1 Sample local oscillator k2 k3 MCT Array Detector k1 2DIR Experiments - 2DIR experiment is performed with multiple pulse sequences. - Time-delayed three IR pulses are focused onto the sample in a noncollinear geometry. - Emitted signal is overlapped with a local oscillator for heterodyne detection. - Heterodyned signal is dispersed through a monochromator and is frequency-resolved. - Dual scan method with two different pulse sequences is used to measure purely absorptive part of signal. Laser Phys. Lett., 4, 704 (2007)

  6. A B Chemical Exchange 0-1 region only AwAspecies A - frequencywA A BwBspecies B - frequencywB wm A and B givediagonal peaks B wt Consider one diagram for the 0-1 region AB BA AB A 1st interaction - wB 1st interaction - wA wm A B Last interaction - wA Last interaction - wB B wt Off-diagonal Off-diagonal wA wB t t Tw A A B wm B wt BA wB wA t t Tw

  7. A B Chemical Exchange - Combining ABand BA A B A A B wm B wt - Including the 1-2 pathways Off-diagonal peaks in each blockgrow in as Tw is increased. The Tw dependent growthof the off-diagonal peaks in each block gives thechemical exchange rate.

  8. kAB T1,A, or,A FFCFA T1,B, or,B FFCFB AB Decay Spectral diffusion Decay Spectral diffusion kBA Two-state chemical exchange dynamics

  9. Finding Optimal Solution Parameters

  10. Contact Ion Pair Free Ion Nitrile Stretch = (2100-2240 cm-1) Free thiocyanate ion ~ 2050 cm-1 Contact Ion Pair ~ 2070 cm-1

  11. Conclusion • Optimal solution parameters: • Lithium Thiocyante: • 0.040 ±0.002M lithium thiocyanate in diethyl ether with 0.118mols ± 0.001 of lithium chloride • Sodium Thiocyante: • 0.060±0.003M sodium thiocyanate in acetonitrile

  12. Special Thanks • Department of Energy • Dr. Steven Rock, Farah Rahbar, & Susan Schultz • Dr. Kelly Gaffney • Dr. Sugnam Park • Minbiao Ji

  13. References • Suydam, I. T.; Boxer, S. G. Biochemistry 2003, 42, 120 • Park, S.; Kwan, K. Laser Phys. Lett., 2007, 705, 710 • Zheng, J.: Kwan, K; Fayer, M.D. Acc. Chem. Res. 2007, 76, 78 • Marcus, Y., Ion Solvation, New York, Wiley, 1985, in Chapter 3 “Infra-red Spectra and Solvation of Ions in Dipolar Protic Solvents” • Butcher, P.N; Cotter, D. The elements of Nonlinear Optics, Cambridge University Press: Cambridge, U.K., 1990, 50-78 • Landolt-Börnstein, Group IV Physical Chemistry, Springer Berlin Heidelberg Press: Berlin, Germany, 2008, Volume 17, 269-270

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