FTIR AND ULTRAFAST INFRARED SPECTROSCOPY OF THE DICYANAMIDE ANION IN SOLUTION Kevin Dahl, Gerald M. Sando, and Jeffrey C. Owrutsky Chemistry Division U.S.

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