Dynamics of Polar Solvation (in Dipolar & Ionic Solvents)

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1 Dynamics of Polar Solvation (in Dipolar & Ionic Solvents) P L1 M4 Φ=2 KDP S L2 G2 S S 1 t Spectral Response S (t) N dipolar CF 3 ionic M3 wg M2 M Time / s Sp15 ACS - Hildebrand

2 Solvation Dynamics? Solute Perturbation Solute Charge -1 Time Solvation Response Solvation Energy Time 2

3 Why Study? Cl - + CH [ClCH 3 Cl] - ClCH 3 + Cl - 3 Cl R R Solvation Coordinate P Solution Phase Rxn R + P Gas-Phase Reaction Coordinate P Gas Phase Rxn Like it or not, solvent is part of the reaction coordinate! 3

4 The Dynamic Stokes Shift Energy S S 1 t hexane ~ gas t em abs DMS Solvation Coordinate Frequency / 1 3 cm -1 4

5 Pioneering Experiments 4-AP in 23 K TNS Adsorbed to BSA t/ns t t t=2,6,2 ns Ware & Co: Chem. Phys. Lett. 2, 356 (1968); J. Chem. Phys. 54, 4729 (1971). Brand & Gohlke, J. Biol. Chem. 246, 2317 (1971). 5

6 Dipolar Solvents (1995) An Ideal Probe of Polar Solvation simple S 1 state only non-specific solvation () C153 TR Emission: C153 in DMS t =7 D ss em abs 6

7 Spectral Response S (t) The Solvation Response S(t) S ( t) { ( t) } { } Representative Response Fns. C K CH 3 CN THF C 5 H C 3 H HMPA C 1 H Time / ps CH 3 CN C 5 H Time / ps Times 1e t /ps

8 Molecular Dynamics Simulations simulations provided many insights into solvation mechanism S (t) (C153) + CH 3 Cl CH 3 CN prominent inertial or Gaussian decay response often linear mainly solvent rotational dynamics speed closely linked to solvent polarity Time / ps H 2 8

9 Solvation & Solvent Rotation S(t), C rot (t), C rot (t) 1 1 C rot S solv C rot =2 D (=8) CH 3 H (=8) H 2 (=19) CH 3 CN (=2) A Simple Conection S ( t) { C ( t)} solv rot (1 1/ ) 3k B T = cooperativity low polarity =1 + high polarity >> Time / ps Time / ps + 9

10 Solvation & Dielectric Dynamics Solvation Dynamics Dielectric Response h solute perturb time solvation energy observe S(t) time E field perturb time Polarization observe time 1

11 Dielectric Predictions Spectral Response Function S (t) Spectral Response Fns. acetone DMS HMPA 1-PrH solid = obs dashed = calc Time / ps bserved Time / ps Response Times in 22 Solvents polar aprotic H-bonding Calculated Time / ps simple dielectric continuum predictions surprisingly accurate Horng et al., J. Phys. Chem. 99, (1995). 11

12 (Room T) Ionic Liquids = molten salts liquid at (near) room temperature Cation Families imidazolium R m N N Im mn + R n Properties pyrrolidinium ammonium R m N R n R j R i N R k R l Pr mn + N ijkl + phosphonium R j R i P R R l k P + ijkl Common Anions Cl -, CH 3 C - N 3-, CH 3 S 3 - BF 4-, PF 6 - F 3 C F 3 C S S C S CF 3 S CF 3 Applications N S CF 3 Tf 2 N - Tf 3 C - ion conductive materials for electrochemical devices solvents for chemical reaction solvents for bioscience Armand et al., Nature Materials 8, 621 (29) 12

13 Solvation Energies Unremarkable Emission Intensity C153 Solvatochromism Em. dmso Frequency / 1 3 cm -1 2mb polarity comparable to DMS, CH 3 H, CH 3 CN solvation free energies can be remarkably similar to those of conventional solvents 13 N CF 3 U El = solute-solvent interaction energy log 1 K [Im 61 + ][Tf2 N - ] Experimental Free Energies vs. nc 5 nc 6 C 2 THF C 1 H DEE cc 6 CH 3 CCl 4 cc 6 H pxyl C 3 H nc 12 N=28 r 2 = log 1 K CH 3 CN Abraham & Acree, Green Chem. 8, 96 (26).

14 Charge Structuring Effect of Charge 2.5 RDF about a Central Anion 2. g com (r) r / Å 14

15 Solvation Dynamics experiments with Niko Ernsting capture the full response 8 fs 4 ns Relative Intensity TR Emission of C153/[Im 41 ][PF 6 ] TCSPC 2 ns FLUPS 1 fs Frequency / 1 3 cm -1 t Spectral Response S (t) ~1-fold slower than typical dipolar solvents strongly bimodal & broadly distributed in time S(t) in CH 3 CN & IL IL 3x Time / s N CH 3 CN CF 3 15

16 Experimental Survey of 21 ILs S(t) Gaussian Frequency G / ps Time / s #1 [Im n1 ][Tf 2 N] [Pr n1 ][Tf 2 N] IRF S ( t) Zhang et al., JPCB 117, 4291 (213) Inverse Reduced Mass m u / f G #2 #1 21 ( M1 ) N=21 R=.92 exp{ t } (1 f )exp{ ( t / ) G G Slow Time str / ps 1 1 # M1 1 1 Viscosity / cp 42 } 43 [Im n1 ][Tf 2 N] [Pr ][Tf n1 2 N] Mechanism (from MD Simulations): primarily ion translation #1: 3-4% inertial motion of 1 st shell ions #2: overdamped, highly coupled ion motions subtle displacements of ions over <1 16

17 Relation to Dielectric Dynamics Solvation Response S(t) Some S(t) Comparisons Pred. #1 #2 [Im 41 ][DCA] 2x B1 B2 [Im 41 ][PF 6 ] bs. 5x B N* Time / ps [N 3 ][N 3 ] B* W* [S 333 ][Tf 2 N] dielectric continuum predictions qualitatively correct but too fast by factors of 3-4 solv 3x 4x B W Solvation Time solv / ps solv S( t) dt 4 solv and Conductivity continuum prediction ( Im + Pr + N + P + misc Resistivity -1 / m ) Tf 2 N - c misc ~const. 17

18 Summary solvation dynamics key component of solutionphase reactions dynamics in familiar dipolar solvents (reasonably) well understood dynamics in neoteric solvents like ionic liquids still being investigated dynamics in structured, mostly biological, also of current interest 18

19 Acknowledgements Min Liang Durba Roy Xin-Xing Zhang Collaborators: Graham Fleming, Ed Castner, Biman Bagchi, Rich Stratt, Branka Ladanyi, Gary Baker, Niko Ernsting, Richard Buchner 19