Daniel Weidinger1, Cassidy Houchins2 and Jeffrey C. Owrutsky3

(1) National Research Council Postdoctoral Researcher

(2) SRA International

(3) Chemistry Division, Naval Research Laboratory, Washington, DC

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OSU International Symposium on Molecular Spectroscopy June 23, 2011

Vibrational Dynamics of Tricyanomethanide

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Tricyanomethanide Infrared Spectroscopy

A. Why study tricyanomethanide (TCM)?• Ionic liquids for fuel cells, solar cells

• Similar to anion N(CN)2- (DCA)

low viscosity → high conductivity

B. Vibrational probes and solvation probes• Anion studies: NCO-, N3

-, N(CN)2-, NCS-

• TCM hydrophilic & strong IR absorber

C. Metal cyanides • Contrast with metal cyanides, e.g. Au(CN)2

-

• Prussian blue and CN adsorbates

D. New research• Steady state and dynamic spectra

• Ab initio calculations

Source: http://www.ensem.inpl-nancy.fr/

Source: http://www.chem.monash.edu.au

D. Weidinger et al., J. Chem. Phys. 134 (2011) 124510

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Tricyanomethanide Vibrations

• IR-active frequencies around 2170 cm-1

• Asymmetric CN-stretch (E’)• High Intensity (~50,000 M-1cm-2)

Raman Shift (cm-1)

2200 2220 2240 2260

Inte

nsity

(to

tal c

ount

s)

3.7e+5

3.8e+5

3.8e+5

3.9e+5

3.9e+5

4.0e+5

4.0e+5

TCM Raman Spectrum (solid KTCM)

TCM Vibrational Modes (D3h symmetry)

• Raman A’ band at 2222 cm-1; previous spectra:• Beaumont et al., Inorg. Chim. Acta 84 (1984) 141• Hipps et al., J. Phys. Chem. 89 (1985) 5459• Dixon et al., J. Am. Chem. Soc. 108 (1986) 2582

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Tricyanomethanide IR Spectra

Vibrational Band FrequenciesTCM Spectra

1 Dahl et al., J. Chem. Phys. 123 (2005) 0845042 Dixon et al., J. Am. Chem. Soc. 108 (1986) 2582

TCMCenter Freq.(cm-1)

DCA Center Freq.(cm-1)

NCS- Center Freq.(cm-1)

H2O 2172.0 2151.91 2164.01

D2O 2171.5 2149.41 2163.31

Methanol 2172.3 2147.81 2157.01

Formamide 2167.1 2141.91 2158.71

[BMIM][BF4] 2162.4 2131.01 2057.11

Solid 21622 -- --

DMSO 2161.7 2128.21 2055.81

Frequency (cm-1)

212021602200

No

rma

lize

d A

bso

rba

nce

(a

rb. u

nits

)

H2O

D2O

Methanol

DMSO

BMIM

Formamide

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IR Pump-Probe – Vibrational Relaxation

ν = 0

2

1

Transient Absorption

Transient Bleach

Frequency

Ab

sorp

tio

n

Pump-probe diagramUltrafast Pump-Probe IR setup:

•4 μJ, ~350 fs IR pulses

•5 cm-1 resolution

Ti:Sapphire Oscillator

Regenerative Amplifier

OPA

DFG Crystal

Sample

Monochromator

Delay Stage

IR Detector

To Lock-in,

Computer

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IR Pump-Probe – Vibrational Relaxation

Frequency (cm-1)

2120214021602180

Abs

orba

nce

Cha

nge

(mO

D)

-40

-20

0

20

40

• Strong transient absorption (as much as 40 mOD with 1 uJ pump)

• Concentrations of ~0.1 M

• Similar widths, decay times from adsorption, bleach features

• FWHM = 10 cm-1

• Anharmonicities of ~16 cm-1

Transient Spectrum:

TCM in DMSO, 1 ps Delay

Time (ps)

0 20 40 60 80

Ab

sorb

an

ce C

ha

nge

(m

OD

)

0.0

0.2

0.4

0.6

0.8

1.0

H2O FA BMIM

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IR Pump-Probe – Vibrational Relaxation

• Slower vibrational relaxation than DCA, N3

-

• Relaxation in MeOH slower than H2O, opposite of frequency trend

TCM TA Lifetime

(ps)

TCM TB Lifetime

(ps)

DCA T1

(ps)NCS- T1

(ps)

H2O 4.8 5.2 1.9 2.5

D2O 12.2 12.7 2.9 21.3

Methanol 12.0 17.8 4.7 22.6

Formamide 18.1 18.1 4.6 21.5

[BMIM][BF4] 27.5 33.0 8.4 63.1

DMSO 28.1 36.8 11.0 77.0

TCM TA Decay

• Decay lifetimes vary from 5 to 30 ps

• Solvent trend is similar to DCA & most small anions

Table of VER lifetimes

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Calculations of CN bands

Calculated TCM Frequencies

Model / Basis Set Symmetric Anti-Symmetric

νcnS

cm-1

IR Intensity

km / mole

νcnAS,1

cm-1

IR

Intensity

km / mole

MP2/aug-cc-pVDZ 2137.2 -- 2146.7 370

B3LYP/aug-cc-pVDZ 2282.5 -- 2230.9 481

B3LYP/aug-cc-pVTZ 2292.0 -- 2238.1 483

Experimental 2222 -- 2170 --

• Calculations performed using Gaussian 09

• Structures optimized within the C1 point group

• Frequencies calculated for all optimized structures to ensure minimum

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Calculations of CN bands

Experimental and Calculated TCM and DCA Frequencies

Model DCA1 TCM

νcnS

cm-1

νcnAS,1

cm-1

νcnS

cm-1

νcnAS,1

cm-1

MP2 2183 2199 2137 2147

B3LYP 2209 2186 2282 2238

Experiment 2232 2179 2292 2170

• MP2 calculations for DCA and TCM have same reversed energy order

• Order is correct in B3LYP calculations

1 Georgieva et al.., J. Mol. Struct. 752 (2005) 14

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Calculations of Thermochemistry

• Electron affinities:

• Structures optimized at

MP2/aug-cc-pVDZ and

B3LYP/aug-cc-pVXZ (x=2,3)

• Proton Affinity calculated from

MP2 and B3LYP optimized

structures

• Pertinent to transport properties &

electrolytic applications1,2

Model VDE

(eV)

ADE

(eV)

PA

(eV)

MP2/aug-cc-pVDZ 3.8 4.3 13.3

B3LYP/aug-cc-pVDZ 4.0 4.0 13.0

B3LYP/aug-cc-pVTZ 4.0 4.0 13.1

Calculated electron detachment energies and proton affinities (TCM)

1 S. Y. Kim et al., Nature Communications 1 (2010)2 Q. Dai et al., Comptes Rendus Chimie 9 (2006) 6013 B. Jagoda-Cwiklik et al., J. Phys. Chem. A 111 (2007) 7719

• Observed3 DCA electron affinity

(ADE) = 4.135 eV

• Calculated DCA ADE (by MP2) is

4.1 eV

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Conclusions

• IR Spectroscopy and IR Pump-Probe Studies of TCM

• “New” IR solvent probe

• High frequency

• Strong CN stretch

• Good solubility

• Shift-lifetime trend similar to DCA

• Long decay time for CN-containing anion

• MeOH is anomalous solvent

• Computations

• IR and Raman frequencies

• Electron Affinities

• Proton Affinities and transport properties

Acknowledgements

• Scientists• Jeffrey C. Owrutsky (NRL)• Cassidy Houchins (SRA International)• Code 6111

• Funding:

Office of Naval Research

• Sponsorship:

National Research Council

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Extras

TCM TA Lifetime

(ps)

TCM TB Lifetime

(ps)

DCA T1

(ps)NCS- T1

(ps)

H2O 4.8 ± 0.4

5.2 ± 0.4

D2O 12.2 ± 0.6

12.7 ± 0.4

Methanol 12.0 ± 1.9

17.8 ± 1.7 4.7 ± 0.5

22.6 ± 2.1

Formamide

18.1 ± 2.9

18.1 ± 0.4

[BMIM][BF4]

27.5 ± 4.0

33.0 ± 9.5 8.4 ± 0.7

63.1 ± 3.1

DMSO 28.1 ± 2.9

36.8 ± 4.6 11.0 ± 4.9

77.0 ± 5.6

Model / Basis Set

Symmetric Anti-symmetric

νcnS

cm-1

IR Intensity

km / mole

νcnA

S,1

cm-1

IR

Intensity

km / mole

νcnAS,

2

cm-1

IR

Intensity

km / mole

MP2/aug-cc-pVDZ

2137.2

-- 2146.7

370 2146.8

370

B3LYP/aug-cc-pVDZ

2282.5

-- 2230.9

481 2231.0

481

B3LYP/aug-cc-pVTZ

2292.0

-- 2238.1

483 2238.1

483