1. 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]-

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

3. 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-

4. 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.

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

6. 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

7. 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).

8. 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

9. 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

10. 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

11. 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 =

12. 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)