International Symposium on Molecular Spectroscopy FTIR AND ULTRAFAST INFRARED SPECTROSCOPY OF THE DICYANAMIDE ANION IN SOLUTION Kevin Dahl, Gerald M. Sando, and Jeffrey C. Owrutsky Chemistry Division U.S. Naval Research Laboratory International Symposium on Molecular Spectroscopy N C - [N(CN)2]-

Steady State/Dynamics Link Solvent dependence of vibrational spectroscopy and dynamics “measures” interactions Steady-state spectral shifts (N3-, NCO-) and width changes (NCS-) Fast Vibrational Energy Relaxation (VER) dynamics Population dynamics and steady-state spectroscopy – good Population dynamics not linked to width

N(CN)2- Introduction Differences from triatomics makes N(CN)2- interesting Can be used as anion in ionic liquids IR-active C≡N antisymmetric stretch at ~2130 cm-1 General solvent dependence? υas(C≡N) υas(C-N)+υs(C-N) υs(C≡N) More flexible anion than N3-

Experimental Concept IR Spectra Energy Levels Transient Experiment Static (FTIR) Transient DA(n) at Dt Excited state absorption (+DA) Ground state bleach (- DA) IR Spectra Energy Levels V=0 V=1 V=2 Vibrational Energy Relaxation (VER) DA(Dt) at n k1=1/T1 Transient Experiment Experiment pump probe Dt DA(n,Dt) Probe population dynamics in the ground or excited state.

Experimental Considerations Time-resolved IR pump – IR probe spectrometer <200 fs, >4 μJ pump at ~ 5 μm

N(CN)2- Solution-Phase Spectroscopy Strong solvent dependence for each band Similar spectral properties to N3-, NCO- εas ~ 3000 M-1cm-1 – similar to N3- 3 bands have strong solvent dependence  blue shift No ion-pairing

N(CN)2- Gas-Phase Frequency Estimate Shift from gas-phase relative measure of solvent interaction Need gas-phase frequency – use N3- to estimate Method works “well” for NCO-, < 5 cm-1 error N3-: M. Polak, M. Gruebele, and R. J. Saykally, J. Am. Chem. Soc. 109, 2884 (1987). *From: NCO-: M. Gruebele, M. Polak, and R. J. Saykally, J. Chem. Phys. 86, 6631 (1987).

Solution-Phase Dynamics Fast Vibrational Energy Relaxation (VER) on order of 1 to 15 ps! Single exponential dynamics of N(CN)2- in polar solvents Transient bleach recovery matches transient absorption decay Two-level dynamics in highly-polar solvents

Anomalous Solution-Phase Dynamics Evidence for intermediate or “bottleneck” state Less polar solvents have slower bleach recovery Multiple band excitation? Energy Levels V=0 V=1 V=2 Bottleneck Energy Levels V=0 V=1 V=2 Less polar solvents have more complex than two-level dynamics

Rate-Shift Correlation N(CN)2-: strong correlation between vibrational shift and VER dynamics Solvent dependence is the same as N3- and NCO- N(CN)2- complementary to triatomics

Rotational Dynamics f T k V h t = Rotation time used to identify ion pairing in solution Stokes-Einstein-Debye (SED) prediction: Lack of data  conclusions? f T k V B rot h t =

Conclusions Acknowledgements N(CN)2- is a sensitive probe of solvent environment Dynamics are more complex than for triatomic systems Complementary solute to N3-, with different solubility Acknowledgements $ – Office of Naval Research NRC Postdoctoral Associateship – KD ASEE Postdoctoral Fellowship – GMS Doug Fox, Tom Sutto, Andrew Purdy (NRL)