Upload
victor-daniels
View
218
Download
4
Tags:
Embed Size (px)
Citation preview
FTIR AND ULTRAFAST INFRARED FTIR AND ULTRAFAST INFRARED SPECTROSCOPY OF THE SPECTROSCOPY OF THE
DICYANAMIDE ANION IN SOLUTIONDICYANAMIDE ANION IN SOLUTIONKevin Dahl, Gerald M. Sando, and Jeffrey C. Owrutsky
Chemistry DivisionU.S. Naval Research Laboratory
International Symposium on Molecular Spectroscopy
N
C
N
C
N
-
[N(CN)2]-
Steady State/Dynamics Link• Solvent dependence of vibrational spectroscopyand dynamics “measures” interactions
• Steady-state spectral shifts (N3
-, NCO-) and width changes (NCS-)
• Fast Vibrational Energy Relaxation (VER) dynamics
N3-: Rate-Shift Correlation
Vibrational Band Shift / cm-1
0 10 20 30 40 50 60 70
VE
R R
ate
/ ps-1
0.1
1
H2O
ProticAproticIonic Liquid
Frequency / cm-1
21002150220022502300N
orm
aliz
ed I
nten
sity N(CN)2
- in DMSO
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)
υs(C≡N)
υas(C-N)+υs(C-N)
Experimental Concept
Frequency / cm-1
1970198019902000201020202030
Static (FTIR)
TransientA() at t
Excited state absorption
(+A)
Ground state bleach (- A)
IR Spectra
Experiment
pump
probet A(t)
Energy Levels
V=0
V=1
V=2
Vibrational Energy
Relaxation(VER)
Time / ps-20 0 20 40 60 80 100 120
Abs
orba
nce
Cha
nge
A(t) at
k1=1/T1
Transient Experiment
Experimental Considerations
• Time-resolved IR pump – IR probe spectrometer
• <200 fs, >4 μJ pump at ~ 5 μm
AgGaS22
T i:Sapphire Regen Amp
Diode-pumped Nd:VO 4
T i:Sapph fs O scillator
Nd:YLF
O PA
532nm3.5 W
~ 800nm100fs10nJ 1kHz, 527nm , 10m J
~ 800nm , 150fs, 1m J , 1kHz
Signal, 1 .1-1.6m
f/ 2 chopper
Lock- inAmplifier
I dler, 1 .6 -2.9m
Sam ple cell pum p
M onochrometer
Boxcars
- 4
- m
Lock- inAmplifierLock- inAmplifier
-2 .9 m
probe
M onochrometer
Hg
Cd
Te
de
tecto
rH
gC
dT
ed
ete
ctor
BoxcarsBoxcars
polarizer
polarizer < 7 m< 200 fs
m
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
-
N(CN)2- Bulk Spectra
Frequency / cm-1
21002150220022502300
Nor
mal
ized
Int
ensi
ty
21102160
H2O
NMFDMSO
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
*From:N3
-: M. Polak, M. Gruebele, and R. J. Saykally, J. Am. Chem. Soc. 109, 2884 (1987).NCO-: M. Gruebele, M. Polak, and R. J. Saykally, J. Chem. Phys. 86, 6631 (1987).
Gas-Phase Extrapolations
Vib
rati
onal
Ban
d M
axim
um /
cm-1
2140
2150
2160
2170NCO-
Extrapolated - 2129.1 cm-1
Experimental - 2124.3 cm-1
N3- Vib. Band Shift / cm-1
10 20 30 40 50 60
2130
2140
2150 N(CN)2-
Extrapolated - 2121.6 cm-1
Experimental - ? cm-1
H2O
H-BondingAprotic
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
N(CN)2- in MeOH
Time / ps
-10 0 10 20 30
Nor
mal
ized
|A
| / m
OD
2153 cm-1 -- 5.0 ps2130 cm-1 -- 5.0 ps
Anomalous Solution-Phase Dynamics• Evidence for intermediate or
“bottleneck” state
• Less polar solvents have slower bleach recovery
• Multiple band excitation?
N(CN)2- in DMSO
Time / ps
0 20 40 60 80
Nor
mal
ized
|A
| / m
OD
2126 cm-1 -- 16.8 ps2103 cm-1 -- 11.0 psEnergy Levels
V=0
V=1
V=2
Energy Levels
V=0
V=1
V=2Bottleneck
Rate-Shift Correlation
• N(CN)2-: strong correlation
between vibrational shift and VER dynamics
• Solvent dependence is thesame as N3
- and NCO-
• N(CN)2- complementary to
triatomics
N(CN)2-: Rate-Shift Correlation
Vibrational Band Shift / cm-1
10 20 30
VE
R /
ps-1
0.1
1H2O
ProticAproticIonic Liquid
Rotational Dynamics
• Rotation time used to identify ion pairing in solution
• Stokes-Einstein-Debye (SED) prediction:
• Lack of data conclusions?
fTk
V
Brot
N(CN)2- Rotations
Viscosity / cP
1 2 3
< R
ot>
/ ps
5
10
15
H2O
ProticAprotic
Conclusions• 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
• $ – Office of Naval Research
• NRC Postdoctoral Associateship – KD
• ASEE Postdoctoral Fellowship – GMS
• Doug Fox, Tom Sutto, Andrew Purdy (NRL)
Acknowledgements