Ultrafast dynamics of pyrrolidinium cation ionic liquids

Hideaki Shirota, Alison M. Funston, James F. Wishart, and Edward W. Castner

Citation: J. Chem. Phys. 122, 184512 (2005); doi: 10.1063/1.1893797

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THE JOURNAL OF CHEMICAL PHYSICS 122, 184512 ͑2005͒

Ultrafast dynamics of pyrrolidinium cation ionic liquids

Hideaki Shirota

Department of Chemistry & Chemical Biology, Rutgers, The State University of New Jersey, Piscataway,

New Jersey 08854-8087

Alison M. Funston^a͒and James F. Wishart

Brookhaven National Laboratory, Chemistry Department, Upton, New York 11973-5000

Edward W. Castner, Jr.

Department of Chemistry & Chemical Biology, Rutgers, The State University of New Jersey, Piscataway,

New Jersey 08854-8087

͑Received 3 January 2005; accepted 23 February 2005; published online 10 May 2005͒

We have investigated the ultrafast molecular dynamics of ﬁve pyrrolidinium cation room

temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced

Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrro-

−

lidinium

bis͑triﬂuoromethylsulfonyl͒imide

P₁₄⁺/NTf₂͒,

N-methoxyethyl-N-methylpyrro-

lidinium bis͑triﬂuoromethylsulfonyl͒imide P_1E₋_OE⁺/NTf₂͒, N-ethoxyethyl-N-methylpyrrolidinium

−

bis͑triﬂuoromethylsulfonyl͒imide P_1EOE⁺/NTf₂͒, N-ethoxyethyl-N-methylpyrrolidinium bromide

P_1EOE⁺, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P_1EOE/DCA⁻͒. For comparing

+

dynamics among the ﬁve ionic liquids, we categorize the ionic liquids into two groups. One group

of liquids comprises the three pyrrolidinium cations P₁₄₊⁺, P_1EOM⁺, and P_1EOE⁺paired with the NTf₂

−

anion. The other group of liquids consists of the P_1EOEcation paired with each of the three anions

−

NTf₂, Br⁻, and DCA⁻. The overdamped relaxation for time scales longer than 2 ps has been ﬁt by

a triexponential function for each of the ﬁve pyrrolidinium ionic liquids. The fast ͑ϳ2 ps͒ and

intermediate ͑ϳ20 ps͒ relaxation time constants vary little among these ﬁve ionic liquids. However,

the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range

from 0 to 750 cm⁻¹are quite similar for the group of three pyrrolidinium ionic liquids paired with

−

the NTf₂anion. The intermolecular vibrational line shapes between 0 and 150 cm⁻¹are ﬁt to a

multimode Brownian oscillator model; adequate ﬁts required at least three modes to be included in

enzymatic catalysis,¹⁷and enantiomeric synthesis.^18,19Ana-

I. INTRODUCTION

lytical applications of ionic liquids include uses such as sta-

tionary phases for chiral separations,²⁰and as nonvolatile

solvents for matrix assisted laser desorption ionization time-

of-ﬂight mass spectroscopy.^21–23Each of these applications

can beneﬁt from a detailed physical understanding of the

dynamics and interactions in ionic liquids. Thus, we have

undertaken a spectroscopic study of ﬁve pyrrolidinium-

cation ionic liquids.

Femtosecond optical heterodyne-detected Raman-

induced Kerr effect spectroscopy ͑OHD-RIKES͒ is a nonlin-

ear optical polarization technique that measures the depolar-

ized Raman signal from a transparent condensed phase

sample in the time domain.^24,25Using OHD-RIKES, we can

measure the dynamics in condensed phases on time scales

ranging from 20 fs to 1 ns. The time domain data are con-

verted to the low-frequency depolarized Raman spectrum by

direct Fourier transform deconvolution.^25–27Spectral sensi-

tivity in the range from 0.1 to 750 cm⁻¹makes OHD-RIKES

a powerful tool to investigate the intermolecular dynamics

and interactions in condensed phases. Recently, femtosecond

OHD-RIKES has been used for simple molecular

liquids,^{24,25,28–30}and for more complex condensed phases

Room temperature ionic liquids ͑RTILs͒ are molten salts

for which the low temperature limit of the broad liquidus is

at or below ambient temperature. The strong electrical inter-

actions in these liquids cause them to have very low vapor

pressures, which result in solvent properties that are favor-

able for “green” chemistry.¹RTIL vapor pressures are orders

of magnitude lower than for neutral organic solvents. RTILs

show high solubility for both inorganic and organic com-

pounds, including peptides, proteins, and enzymes. As re-

placements for volatile organic solvents, the low vapor pres-

sures of RTILs provide opportunities for reducing emissions

of volatile organic compounds to the atmosphere.^2–10In par-

ticular the low vapor pressures of RTILs also provide oppor-

tunities for reducing the risk of uncontrolled gas phase chain

reactions, ﬁres, and explosions. Many important chemical

reactions, such as hydrogenations,^11,12oxidations,¹³and cou-

pling reactions^14,15have been investigated in RTILs to

test their use as organic solvent replacements. RTILs have

also proven useful as solvents for materials synthesis,¹⁶

a͒

Current address: School of Chemistry, The University of Melbourne, Vic-

toria, 3010 Australia.

0021-9606/2005/122͑18͒/184512/12/$22.50

122, 184512-1

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184512-2

Shirota et al.

J. Chem. Phys. 122, 184512 ͑2005͒

In a number of cases, cyclic aromatic molecules are more

toxic than molecules with the corresponding aliphatic ring

systems. If this proves true for RTILs, then the aliphatic cat-

ion RTILs might be better choices from an environmental

perspective. We have studied the following ﬁve different

pyrrolidinium ionic liquids shown in Fig. 1: N-butyl-

N-methylpyrrolidinium

bis͑triﬂuoromethylsulfonyl͒imide

−

͑P₁₄⁺/NTf₂͒,

bis͑triﬂuoromethylsulfonyl͒imide

N-ethoxyethyl-N-methylpyrrolidinium bis͑triﬂuoromethyl-

N-methoxyethyl-N-methylpyrrolidinium

−

͑P_1EOM⁺/NTf₂͒,

−

sulfonyl͒imide

N-methylpyrrolidinium

͑P_1EOE⁺/NTf₂͒,

N-ethoxyethyl-

+

bromide

͑P_1EOE/Br⁻͒,

and

N-ethoxy-ethyl-N-methyl-pyrrolidinium

dicyanoamide

+

͑P_1EOE/DCA⁻͒. We will compare the dynamical aspects

of the cations using measurements from the series of

−

NTf₂anion RTILs of P₁₄⁺/NTf₂, P_1EOM⁺/NTf₂, and

P_1EOE⁺/NTf₂. The effects on liquid dynamics from anion

−

motions are investigated by comparing dynamics from

−

+

P_1EOE⁺/NTf₂, P_1EOE/Br⁻, and P_1EOE/DCA⁻. For the case

+

of P_1EOE/Br⁻, we can measure cation dynamics directly be-

cause the spherical bromide anion cannot directly contribute

to the OHD-RIKES signal, since this signal depends on ﬂuc-

tuations of the polarizability anisotropy.

FIG. 1. Chemical structures of the pyrrolidinium cation ionic liquids used in

+

this study. P₁₄is N-butyl-N-methylpyrrolidinium cation, P_1EOMis

+

N-methoxyethyl-N-methylpyrrolidinium cation, P_1EOEis N-ethoxyethyl-

−

N-methyl-pyrrolidinium cation, NTf₂is bis͑triﬂuoromethylsulfonyl͒imide

anion, DCA⁻is dicyanoamide anion, and Br⁻is bromide anion.

such as nanoporous glasses,^31,32microemulsions,^33,34hy-

drated peptide and protein solutions,^35–37and polymer

solutions.^34,38

II. EXPERIMENT

A. Sample preparation

The synthesis of the ionic liquids used in this study will

be reported in detail elsewhere,⁴³so only a brief description

of the procedures used are presented here. The pyrrolidinium

cations were synthesized via standard methods, viz. quater-

nization of N-methylpyrrolidine with the appropriate alkyl

halide ͑in this case bromide͒ to produce the pyrrolidinium

bromide salts.^44–46These were metathesized to the bis͑trif-

luoromethylsulfonyl͒imide salt using equimolar quantities of

the pyrrolidinium bromide and lithium bis͑triﬂuoromethyl-

sulfonyl͒imide in aqueous solution.^45,47Metathesis of the

pyrrolidinium bromide to the dicyanoamide salt was done

according to Refs. 47 and 48 by reacting equimolar quanti-

ties of silver dicyanoamide⁴⁸with the appropriate pyrroli-

dinium bromide in water, ﬁltering off the silver bromide pre-

cipitate, and evaporating off the aqueous phase. The

pyrrolidinium dicyanoamide was extracted from the residue

into dichloromethane, which was ﬁltered and evaporated to

yield the pure ionic liquid.^48,49The resultant liquids were

decolorized by stirring with activated charcoal overnight and

then passing a solution of the liquid in acetonitrile through

an activated alumina column. All ionic liquids were dried at

reduced pressure at 318 K for more than 24 h prior to char-

acterization and use. All starting materials were purchased

from Aldrich in the highest available purity grade and used

as received. Viscosity measurements were made using a

temperature-controlled Cambridge Applied Systems recipro-

cating electromagnetic piston viscometer ͑ViscoLab 4100͒.

Several groups have studied the ultrafast dynamics of

aromatic imidazolium ionic liquids using OHD-RIKES.

Quitevis and co-workers characterized how changes of the

alkyl substituent affected the intermolecular vibrational dy-

namics for a series of 1-alkyl-3-methylimidazolium cations

paired with bis͑triﬂuoromethylsulfonyl͒imide anion.³⁹

Recently they also reported the temperature dependence of

the low-frequency Kerr spectrum for the RTIL 1-pentyl-3-

methylimidazolium

bis͑triﬂuoromethylsulfonyl͒imide.⁴⁰

Giraud et al. studied ﬁve typical imidazolium ionic liquids.⁴¹

They analyzed the low-frequency Kerr spectra of the ionic

liquids using a multimode Brownian oscillator model and

concluded that the signals from each of the ﬁve ionic liquids

are dominated by the imidazolium cation dynamics. Cang,

Li, and Fayer studied the dynamics of the RTIL 1-ethyl-3-

methylimidazolium nitrate and concentrated on diffusive re-

laxation processes with the time range from 1 ps to 2 ns.⁴²

They described the temperature dependence of the observed

diffusive relaxation decays using a mode-coupling theory for

the dynamics, in which the data are ﬁt to a function where a

sum of two power-law decays describes the faster relaxation

processes, and this sum is multiplied by a slower exponential

decay assigned to the ␣ relaxation.

In this paper, we present high quality Kerr transient data

for each of the ﬁve pyrrolidinium RTILs shown in Fig. 1.

There are several reasons for investigating nonaromatic ionic

liquids. The electronic structure of the aromatic molecular

cations is more delocalized than for the aliphatic molecular

cations leading to ion-ion interactions of a different nature.

1

No impurity signals were detected in the H-NMR spectra of

the neat ionic liquids.

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184512-3

Ultrafast dynamics of ionic liquids

J. Chem. Phys. 122, 184512 ͑2005͒

−

dinium cations P_1EOE⁺, P_1EOM⁺, and P₁₄⁺, and for the NTf₂

B. OHD-RIKES measurement

and DCA⁻anions of the ionic liquids were performed by

using the GAUSSIAN 03 program package.⁵¹Becke’s three-

parameter gradient-corrected exchange⁵²and Lee–Yang–Parr

gradient-corrected correlation functions⁵³͑B3LYP͒ were

used with the 6-31+G͑d,p͒ basis set.⁵¹Calculations were

done for the isolated molecular ion and again for the ion in a

dielectric continuum model using a dielectric constant of

36.64 for acetonitrile. The value for the acetonitrile dielectric

constant was chosen because of the similar solvatochromic

effects observed in ionic liquids.⁵⁴The integral equation for-

malism, polarized continuum model ͑IEF-PCM͒ of Men-

nucci and Tomasi^55,56was used to mimic a solvent environ-

ment similar to that observed for solvatochromic shifts in

several ionic liquids. Molecular ion geometries were opti-

mized with no constraints. The XYZ Cartesian coordinates

for cation and anion optimized by B3LYP/6-31+G͑d,p͒

calculations are summarized in the EPAPS supporting infor-

mation document.⁵⁷

Details of the OHD-RIKES apparatus were reported

elsewhere.^38,50Brieﬂy, a lab-built femtosecond titanium sap-

phire laser operating at 800 nm center wavelength was used

as a light source. The output power was about 320 mW when

the laser was pumped by 3.2 W of 532 nm light from a

frequency-doubled diode-pumped continuous Nd:VO₄laser

͑Spectra Physics, Millennia V͒. To compensate for the group

velocity dispersion introduced by the optics in the spectrom-

eter, the laser output was ﬁrst routed through a retroreﬂecting

dispersive delay line consisting of a pair of fused silica

Brewster prisms separated by Ϸ120 cm. After the delay line

the laser pulses passed thorough a half-wave plate to rotate

their polarization to vertical, and were incident on an un-

coated fused silica interferometric wedge to produce pump

and probe beams with an intensity ratio of about 90:5. The

pump beam is routed though a half-wave plate and a Glan–

Thompson polarizer transmitting vertically polarized light

prior to the focusing lens. The pump beam intensity was

modulated at 1 kHz by a mechanical chopper ͑Stanford Re-

search Systems, SR540͒. The probe beam is routed through

an antireﬂection coated interferometer wedge, a variable op-

tical delay line ͑Paciﬁc Precision Labs, 0.1 micron accuracy͒,

a half-wave plate and a Glan–Thompson polarizer to set the

polarization 45° from vertical, and then through a quarter-

wave plate before the fused silica focusing lens. The pump

and probe beams were focused on the sample and recolli-

mated with a matched pair of 100 mm focal length fused

silica lenses. The pump beam was blocked while the probe

was sent though a second Glan–Thompson polarizer and a

spatial ﬁlter consisting of two 150 mm lenses and a 0.15 mm

pinhole. To achieve heterodyne detection, an out of phase

local oscillator was introduced by rotating the input polarizer

by ±1° away from the homodyne orientation ͑deﬁned at the

polarizer extinction angle͒. The OHD-RIKES probe beam

was detected by a large area ampliﬁed p-i-n photodiode

͑New Focus, 2032͒ and recorded as a function of pump-

probe delay by a digital lock-in ampliﬁer ͑Stanford Research

Systems, SR830͒ referenced to the chopper frequency. The

laser pulse cross correlation measured with a 200 ␮m potas-

sium dihydrogen phosphate crystal was 36±3 fs FWHM

͑equivalent to a 25±2 fs pulse, assuming a Gaussian line

shape͒. The measured spectral bandwidth of the laser pulses

was about 55 nm. All the measurements were made at

295±2 K. Scans with high time resolution of 4096 points at

0.5 ␮m/step were recorded for a time window of 13.66 ps.

The slower orientational dynamics were captured in 115 ps

scans with 20 ␮m per data point and in 730 ps scans with

50 ␮m per data point. Ten scans were averaged for the short

time window transients ͑5 scans each for +1° and −1° rota-

tions of the probe input polarizer͒ and 20 scans were aver-

aged for the intermediate and long time windows ͑10 scans

each for both the +1° and −1° rotations of the probe input

polarizer͒.

III. RESULTS

A. Picosecond reorientational dynamics

Figure 2 shows the log-log plots of the Kerr transients of

the ﬁve pyrrolidinium ionic liquids. As shown in Fig. 2, the

−

Kerr transients of P_1EOE⁺/NTf₂, P_1EOM⁺/NTf₂, and

P₁₄⁺/NTf₂are quite similar. On the other hand, the Kerr

−

+

transients for P_1EOE/Br⁻and P_1EOE/DCA⁻are clearly dis-

+

tinguishable. The Kerr transient of P_1EOE/Br⁻shows that the

amplitude of the picosecond reorientational relaxation is

much smaller than that of the other ionic liquids.

In Fig. 2, one ﬁnds that for time delays from 10 fs to 1

ps, a strong beating pattern is observed in the Kerr transients.

The beats arise from multiple intramolecular normal modes,

and are most complex for the RTILs with the NTf₂⁻anion. A

different beating pattern with a smaller modulation depth is

+

observed over the same time range for the P_1EOE/Br⁻liquid.

+

For the P_1EOE/DCA⁻liquid, the beating pattern is a more

simple sinusoidal decay over this time range, indicating the

presence of a single dominant low-frequency mode. In the 1

to 10 ps time window, the intramolecular modes are damped.

For longer time scales beyond 10 ps, the intramolecular vi-

brational damping is complete and a nonexponential decay is

observed for each of the ﬁve liquids.

The overdamped nonexponential decay part of the Kerr

transients from 2 to 740 ps is ﬁt to several model functions.

Exponential, stretched exponential, biexponential, and a sum

of exponential plus stretched exponential functions do not

adequately ﬁt the data. Both triexponential and mode cou-

pling theory model functions⁴²provide improved quality ﬁts,

but a triexponential model is generally superior. We note that

the long time noise ﬂoor for our Kerr transients ranges be-

tween 2ϫ10⁻⁵to 5ϫ10⁻⁵on an intensity scale normalized

to the peak of the electronic response. To obtain a reduced

noise ﬂoor, it would be necessary to either use a scheme with

multiple laser pulses of different durations and intensities as

done by the Fayer group,⁴²or to employ a multiple modula-

C. Density functional theory calculations

Optimized geometries, intramolecular vibrational modes,

and effective atom-centered charges for the three pyrroli-

tion method as demonstrated by Fourkas and co-workers.⁵⁸

A

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184512-4

Shirota et al.

J. Chem. Phys. 122, 184512 ͑2005͒

notable points from Table I are the following: ͑1͒ the ampli-

+

tude of the diffusive relaxation for P_1EOE/Br⁻relative to the

electronic contribution is only about 10% of that for the

other ionic liquids, ͑2͒ the fast and intermediate relaxation

time constants ␶₁and ␶₂do not vary much between these ﬁve

ionic liquids, and ͑3͒ the slow relaxation time constants ␶

3

for the ﬁve pyrrolidinium RTILs range from 160 ps to 3.2 ns,

and correlate with the values of the shear viscosity.

B. Femtosecond dynamics: Low-frequency Kerr

spectrum

The Kerr transients are analyzed by means of the Fourier

transform deconvolution procedure introduced by

McMorrow and Lotshaw.^26,27To obtain a sufﬁcient number

of data points for a spectrum of reasonable accuracy, it is

necessary to extend the high time resolution data from Fig. 2

͑with 4096 data points over the time window from −1.5 to 12

ps͒ over a longer time range than it is practical to record. To

achieve this, the longer time scale Kerr transients ͑Fig. 2͒ in

the range from 2 to 740 ps were recorded with lower time

resolution, and are ﬁt to

a triexponential function

͑a₁exp͑−t/␶ ͒+a₂exp͑−t/␶ ͒+a₃exp͑−t/␶ ͒͒. Prior to

1

2

3

Fourier-transform analysis, the 4096 point data set from −1.5

to 12 ps is extended to 131 072 points using the triexponen-

tial ﬁt function with a time step of 3.3355 fs, yielding spectra

with about 0.0763 cm⁻¹/point spacing. All the spectra ob-

tained in this study are well resolved up to 750 cm⁻¹.

Figure 4 shows the Fourier-transform Kerr spectra of the

ﬁve ionic liquids. These spectra have had the contribution

from the diffusive components subtracted prior to Fourier

−

deconvolution. As shown in Fig. 4, P_1EOE⁺/NTf₂,

−

P_1EOM⁺/NTf₂, and P₁₄⁺/NTf₂show rich intramolecular vi-

+

brational bands spectra in comparison with P_1EOE/Br⁻and

P_1EOE/DCA⁻. The spectra of P_1EOE⁺/NTf₂, P_1EOM⁺/NTf₂,

+

−

and P₁₄⁺/NTf₂are quite similar. The frequencies of the in-

−

tramolecular vibrational modes observed in the Kerr spectra

+

of the ionic liquids are summarized in Table II. P_1EOEin-

tramolecular vibrational bands are observed at band centers

+

of 283, 380, 527, and 642 cm⁻¹in P_1EOE/Br⁻and

+

P_1EOE/DCA⁻, though they are of much lower intensity com-

−

pared to the strong intramolecular bands of the NTf₂anion.

C. Density functional theory calculations

The electrostatic properties for each of the ﬁve molecu-

lar ions are obtained following geometry optimizations.

Atom-centered charges from the ChelpG model are obtained

for the gas-phase ion, for the PCM model using the gas-

phase geometry, and again for a geometry reoptimization for

the stable ground state with PCM polarization. The magni-

tudes of the atom-centered charges are enhanced relative to

the gas phase when the IEF-PCM model is applied, and are

further enhanced as the geometry is reoptimized from the gas

phase. The electric multipole moments obtained are for the

standard orientation, center-of-mass coordinate system. It is

important to note that dipole moments for ions depend on the

choice of origin for the atomic coordinates. Optimized ge-

−

FIG. 2. Log-log plots of Kerr transients for ͑a͒ P_1EOE⁺/NTf₂

, ͑b͒

P_1EOM⁺/NTf₂⁻, ͑c͒ P₁₄⁺/NTf₂⁻, ͑d͒ P_1EOE/Br⁻, and ͑e͒ P_1EOE/DCA⁻.

+

Triexponential ﬁt functions are also shown in the ﬁgure.

reduced noise ﬂoor will be required to differentiate between

the triexponential decay and mode-coupling theory⁴²models.

The triexponential ﬁt curves are also shown in Fig. 2.

The ﬁt parameters are summarized in Table I and plotted in

Fig. 3. The sum of the amplitudes is normalized to unity. The

amplitudes of the diffusive relaxation components relative to

the instantaneous electronic responses are also listed. The

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184512-5

Ultrafast dynamics of ionic liquids

J. Chem. Phys. 122, 184512 ͑2005͒

TABLE I. Triexponential ﬁt parameters for diffusive Kerr transients of pyrrolidinium ionic liquids.

a

b

c

d

e

Ionic liquid

P_1EOE⁺/NTf₂

a_od

a₁

␶ ͑ps͒

a₂

␶

͑ps͒

a₃

␶ ͑ps͒

1

2

3

−

3.52ϫ10⁻³

3.63ϫ10⁻³

3.41ϫ10⁻³

0.47ϫ10⁻³

5.66ϫ10⁻³

0.71

0.68

0.72

0.78

2.31

2.27

2.43

1.89

2.02

0.23

0.26

0.22

0.17

0.20

15.4

14.6

18.3

21.2

16.2

0.06

0.05^f

0.02

165

161

253

P_1EOM⁺/NTf₂

−

P₁₄⁺/NTf₂

+

P_1EOE/Br⁻

3200^g

686

+

P_1EOE/DCA⁻

^aSum of amplitudes of triexponential function normalized by the instantaneous electronic Kerr response.

^bError is ±10%.

^cError is ±20%.

^dError is ±30%.

^eError is ±35%.

^fError is ±50%.

^gError is ±100%.

ometry coordinates, rotational constants, CHelpG charges,

and electrostatic properties are reported in the supporting

information.⁵⁷

g₂

j₂

␶

=

␶_rot,

͑1͒

col

Harmonic frequencies for each of the optimized geom-

etries were calculated, both for the isolated gas-phase mo-

lecular ion, and with the polarized continuum model using

the value for the room temperature dielectric constant of ac-

etonitrile. The complex vibrational spectroscopy resulting

from the ﬂexibility of each ion makes most comparisons of

observed versus calculated modes difﬁcult. An exception is

the pentaatomic dicyanoamide anion, which has a strong

bending mode observed at 180 cm⁻¹for which calculated

values are 160 cm⁻¹͑gas phase͒ and 174 cm⁻¹͑PCM opti-

mized͒; and the symmetric in-plane cyano wagging mode

observed experimentally at 663 cm⁻¹and calculated to be at

650 cm⁻¹for the PCM model.

where g₂is the static pair orientational correlation parameter

and j₂is the dynamical orientational correlation pair

parameter.⁵⁹Although it is difﬁcult to estimate g₂and j₂for

the RTIL molecular ions, the collective ͑structural͒ reorien-

tational time is proportional to the single molecule reorien-

tational time if the ratio of g₂to j₂does not vary between the

ﬁve pyrrolidinium RTILs. Comparisons between NMR and

neutron scattering experiments and molecular dynamics

simulations will be required to address the characteristics of

g₂and j₂for ionic liquids.

Results from the triexponential analysis of the longer

time Kerr transients are summarized in Table I. Only the

longest time constant ␶ varies with viscosity in a manner

3

consistent with diffusive relaxation. For diffusive orienta-

tional relaxation, the reorientation time constant ␶_rotof a sol-

ute in many simple organic molecular liquids is well de-

scribed by the Stokes–Einstein–Debye hydrodynamic

model,⁶⁰

IV. DISCUSSION

Reorientational relaxation signals from optical Kerr ef-

fect experiments provide a measure of the collective re-

sponse ␶_col, unlike single particle experiments, such as NMR

6V␩f

L

rot

␶

=

,

͑2͒

L͑L + 1͒k_BT

relaxation. The collective reorientational relaxation time ␶

col

is correlated with the single molecule reorientation time

␶ ,

rot

59

where V is the volume of the rotating species, ␩ is the shear

viscosity of the liquid, f is a shape factor, k_Bis the Boltz-

mann constant, T is the absolute temperature, and the rank L

equals 2 for depolarized Kerr reorientation signals. Figure 3

shows the plots of the longest reorientation time ␶ versus V

3

␩

for the pyrrolidinium ionic liquids. Because the ﬁtted time

constant ␶₃for P_1EOE⁺/Br⁻is 3.2 ns, which is more than four

times longer than the time window for the present OHD-

−

RIKES measurements, only the ␶ values for P ⁺/NTf₂,

3

14

P_1EOM⁺/NTf₂, P_1EOE⁺/NTf₂, and P_1EOE/DCA⁻are plotted

in Fig. 3. The values for several physical properties including

formula weight M_W, density d, formula volume V, viscosity

␩ of the pyrrolidinium RTILs are summarized in Table III.

The values of V for the ionic liquid cations and anions are

estimated by the method of van der Waals increments,^61,62

with the value of V for the Br⁻ion taken from Israelachvili’s

book.⁶³

−

+

FIG. 3. Plots of the slow relaxation time constant ␶ vs ␩V for the ﬁve

3

pyrrolidinium ionic liquids. Squares, triangles, and circles represent points

calculated using the molecular volumes of the anion, cation, and the sum of

cation and anion, respectively.

Figure 3 plots the longest Kerr relaxation time constant

␶₃versus the product of the shear viscosity times the volume

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184512-6

Shirota et al.

J. Chem. Phys. 122, 184512 ͑2005͒

of the viscosity-volume products V_anion␩ or V_cation␩. In a re-

cent molecular dynamics study of solvation in the RTIL

1-butyl-3-methylimidazolium hexaﬂuorophosphate, Kobrak

and Znamenskiy found that the dynamics of anions and cat-

ions are strongly coupled.⁶⁴Margulis also showed the strong

correlation between cation and anion diffusions in 1-alkyl-3-

methylimidazolium hexaﬂuorophosphate ionic liquids by

molecular dynamics simulation.⁶⁵Thus, we believe it is plau-

sible that the Kerr orientational signal is comprised of a sum

of orientational correlations of individual cations and anions,

as well as cation-anion pairs. We cannot exclude the possi-

bility that there is a contribution to the overall diffusive re-

sponse from more complex transient aggregates, such as

cation-anion-cation or anion-cation-anion trimers. To con-

tribute to the Kerr diffusive reorientation signal, such aggre-

gates would have to have a cluster lifetime greater than the

jump diffusion time.

Cang, Li, and Fayer measured the Kerr relaxation dy-

namics of 1-ethyl-3-methylimidazolium nitrate at the tem-

perature range of 295–410 K.⁴²They found that the relax-

ation was best ﬁt to a model from mode coupling theory.

This model is the product of the sum of two power-law de-

cays multiplied by a slower exponential decay, which is as-

signed to the overall ␣ relaxation of the glassy liquid.⁴²Their

results showed that the longest ␣-relaxation time constant

follows the trend predicted by the Stokes–Einstein–Debye

hydrodynamic model if the rotating solute volume is taken as

the volume of only the cation. In the pyrrolidinium RTILs, it

is possible that the appropriate volume of a rotor may be that

of the sum of the cation and anion volumes. Temperature

dependent Kerr dynamics experiments of both nonspherical

anion and cation ionic liquids will be needed to obtain fur-

ther understanding of reorientation dynamics in room tem-

perature ionic liquids. To study whether clustering affects the

measured reorientation, it will also be helpful to have inde-

pendent experimental data from NMR and quasielastic neu-

tron scattering. In particular, temperature-dependent NMR

experiments will elucidate to what degree internal torsions

occur on competing time scales with overall diffusive tum-

bling motions of the molecular ions.

The values of the fast and intermediate relaxation time

constants ␶ and ␶ do not vary much between the ﬁve pyr-

1

2

rolidinium ionic liquids and have values of about 2 ps and 17

ps, respectively. The corresponding normalized amplitudes

for a₁and a₂are about 0.73 and 0.22, respectively. Several

groups have characterized the solvation dynamics in ionic

liquids by time dependent ﬂuorescence Stokes shift measure-

ments of solvatochromic probes such as coumarin 153 and

4-aminopthalimide.^54,66–71The solvation dynamics show

strongly nonexponential relaxation, ﬁtting well to a stretched

exponential function with a ␤ value of 0.4–0.5, for both aro-

matic and nonaromatic RTILs. The average solvation time

constants observed in these experiments are substantially

longer than the observed Kerr reorientation dynamics, typi-

cally occurring on subnanosecond to nanosecond time scales.

We have measured solvation dynamics for the pyrrolidinium-

FIG. 4. Fourier-transform frequency power spectra of ͑a͒ P_1EOE⁺/NTf₂⁻, ͑b͒

+

P_1EOM⁺/NTf₂⁻, ͑c͒ P₁₄⁺/NTf₂⁻, ͑d͒ P_1EOE/Br⁻, and ͑e͒ P_1EOE/DCA⁻. The

contribution of the picosecond relaxation was subtracted.

+

for each of the pyrrolidinium RTILs, excepting P_1EOE/Br⁻.

Using either the cation or anion volume to calculate the

graph abscissa, a rough correlation is observed. However,

noticeably better agreement and a better statistical linear cor-

relation are found when the volume is taken to be the sum of

the cation plus anion molecular volumes, V_total=V_cation

+V_anion. If the slowest orientational relaxation process ob-

served in the Kerr signal ͑characterized by the time constant

−

cation RTILs P₁₄⁺/NTf₂and P_1EOE⁺/NTf₂. Preliminary

␶ ͒ arises from predominantly cation or anion reorientation,

3

one expects to ﬁnd a linear relationship between ␶₃and either

analysis shows that the average relaxation time constant

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184512-7

Ultrafast dynamics of ionic liquids

J. Chem. Phys. 122, 184512 ͑2005͒

TABLE II. Intramolecular vibrational band frequencies observed in Fourier-transform Kerr spectra of ionic liquids. In the Assignment column, A denotes an

anion vibration and C a cation vibration.

−

+

P₁₄⁺/NTf₂͑cm⁻¹

͒

P_1EOM⁺/NTf₂͑cm⁻¹

͒

P_1EOE⁺/NTf₂͑cm⁻¹

͒

P_1EOE/Br⁻͑cm⁻¹

͒

P_1EOE/DCA⁻͑cm⁻¹

͒

Assignment

121

168

121

168

121

168

A

C

A

C

A

C

A

C

A

C

A

180

282

203

284

208

278

208

278

208

278

288

297

313

326

331

340

350

371

288

297

313

326

331

341

350

288

297

313

326

332

341

350

381

380

399

407

398

408

399

408

410

440

411

439

440

475

435

433

526

528

534

551

557

571

588

630

535

551

557

571

589

535

551

557

571

587

641

587

643

663

740

741

740

TABLE III. Formula weight M_W, density d, formula volume V, and viscosity ␩ of the ﬁve pyrrolidinium ionic

liquids.

c

Ionic liquid

P_1EOE⁺/NTf₂

Constituent

M_W͑g/mol͒

d^a͑g/cm³͒

V^b͑Å³͒

␩ ͑cP͒

−

438.4

158.3

280.1

1.41

378

175

203

60

+

P_1EOE

−

NTf₂

−

P_1EOM⁺/NTf₂

424.4

144.3

280.1

1.46

1.40

1.31

1.10

361

158

203

59

85

+

P_1EOM

−

NTf₂

−

P₁₄⁺/NTf₂

422.4

142.3

280.1

372

169

203

+

P₁₄

−

NTf₂

+

P_1EOE/Br⁻

238.2

158.3

79.9

206

175

31

11 940

480

+

P_1EOE

Br⁻

+

P_1EOE/DCA⁻

219.8

158.3

66.0

231

175

56

+

P_1EOE

DCA⁻

^aValues at 295 K.

^bEstimated by van der Waals increments ͑Refs. 61 and 62͒.

^cValues at 296 K.

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184512-8

Shirota et al.

J. Chem. Phys. 122, 184512 ͑2005͒

from ﬂuorescence Stokes shift measurements seems to be

substantially slower than the slowest observed Kerr relax-

ation time constant ␶₃.

+

The bands observed in the P_1EOE/Br⁻spectrum are as-

+

signed to the intramolecular vibrational bands of the P_1EOE

cation because the monatomic Br⁻ion has no vibrational

+

modes. Comparing the spectra of P_1EOE/Br⁻and

+

P_1EOE/DCA⁻, we ﬁnd that P_1EOE/DCA⁻shows a strong

vibrational mode at 180 cm⁻¹. Lower intensity intramolecu-

+

lar modes for P_1EOE/DCA⁻are observed in the

200–750 cm⁻¹range and are quite similar to those of

+

P_1EOE/Br⁻, except for the 663 cm⁻¹band which is present

+

only in P_1EOE/DCA⁻. The normal modes calculated at the

B3LYP/6-31+G͑d,p͒ level, listed in Table II, show that the

180 cm⁻¹DCA⁻band is the cyano-N-cyano bending mode

and the 663 cm⁻¹band is the N-cyano bending mode.

−

The ionic liquids having the NTf₂anion display a rich

vibrational structure, as seen in Fig. 4. The intramolecular

−

vibrational bands of the NTf₂₋anion dominate the spectra of

−

P_1EOE⁺/NTf₂, P_1EOM⁺/NTf₂, and P₁₄⁺/NTf₂because the

−

NTf₂anion contribution to the Raman spectral density is

more than an order of magnitude greater than from the pyr-

rolidinium cations. For each of the pyrrolidinium NTf₂⁻ionic

liquid spectra, there are ten intense vibrational transitions in

the 250–420 cm⁻¹frequency range ͑Table II͒. An electronic

structure calculation at the B3LYP/6-31G+͑d,p͒ level for a

gas-phase C₂͑trans͒ symmetry NTf₂⁻anion shows only eight

intramolecular vibrational bands in the corresponding fre-

quency range. The inconsistency between the number of

bands observed in the experimental liquid spectra and the

density functional theory normal mode calculations likely

FIG. 5. Multimode Brownian oscillator ﬁts to the intermolecular spectrum

+

of P_1EOE/Br⁻from 0 to 200 cm⁻¹. The bottom ﬁgure presents the best ﬁt

using three Brownian oscillators, the middle ﬁgure the best ﬁt with two

Brownian oscillators, and the residuals for both ﬁts are shown at top.

One of the primary challenges in the interpretation of

low-frequency Kerr spectra is to fully understand the inter-

mediate time scale dynamics bridging the intermolecular vi-

brational to diffusive time scales. In a pure liquid, the pro-

gression of time scales in the observed dynamics begins at

the shortest time scales with the intramolecular vibrations,

followed by underdamped intermolecular vibrations ͑includ-

ing librational motions͒, with the longer time scales com-

prised of a quasiexponential decay for the intermediate time

dynamics, with the longest time scale relaxation being rep-

resented in the simplest case by a single exponential decay

that is correlated with diffusive reorientational motions. De-

tailed tests of the models have been made for two different

cases. Loughnane et al. showed that the intermediate time

scale exponential decay can be described as resulting from

motional narrowing of intermolecular oscillators.⁷⁵They

showed that this explanation was consistent for six symmet-

ric top liquids for a wide variety of temperatures. On the

other hand, McMorrow et al. showed that a single “quan-

tum” harmonic oscillator reproduces the dynamics of CS₂

over a range of dilution concentrations for which the chemi-

cal environment changes substantially ͑as evidenced by the

change in observed linewidth͒.^76,77The quantum harmonic

oscillator has the advantage that the dephasing rate is not

coupled to the frequency of the oscillator. A limitation of the

“classical” harmonic oscillator model is that it exhibits fre-

quency pulling, and hence distortions of the lineshapes be-

cause of the strong overlap between inhomogeneous spectral

widths and dynamical time scales. For the room temperature

ionic liquids discussed here, the low-frequency spectra mea-

−

arises because the ﬂexible NTf₂anion has multiple confor-

mations that lead to shifts in some of the vibrational bands.

We note that the Kerr spectra of imidazolium ionic liquids

with the NTf₂⁻anion also show ten strong vibrational modes

in the same frequency region,⁴¹in common with the pyrro-

−

lidinium NTf₂ionic liquid spectra presented here. Holbrey,

Reichert, and Rogers have reported crystal structures of alky-

−

limidazolium salts, with the NTf₂anion having the cis

conformation.⁷²We ﬁnd overall agreement with the anion

and cation vibrational spectra with the Raman spectra of

73

+

P₁₃⁺/NTf₂⁻and P₁₃/I⁻reported recently by Castriota et al.

and with the IR and Raman spectra of NTf₂⁻reported by Rey

et al.⁷⁴

In order to emphasize the intermolecular vibrational dy-

namics, Fig. 6 shows the Kerr spectra from Fig. 4 plotted

over a narrower spectral window of 0–200 cm⁻¹. The low-

frequency dynamics arise from several types of intermolecu-

lar interactions including collision-induced and dipole-

induced motions with the dominant contributions likely

arising from intermolecular librational motions. As shown in

Figs. 5 and 6, the broad low-frequency spectra of the

−

P_1EOE⁺/NTf₂, P_1EOM⁺/NTf₂, and P₁₄⁺/NTf₂liquids are

quite similar. However, the low-frequency spectra of

+

P_1EOE/Br⁻and P_1EOE/DCA⁻are quite distinct. The inter-

−

molecular dynamics and interactions between NTf₂and

the three pyrrolidinium cations lead to similar spectra be-

cause of the structural and dynamical similarities of the

+

P_1EOE⁺, P_1EOM⁺, and P₁₄cations.

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184512-9

Ultrafast dynamics of ionic liquids

J. Chem. Phys. 122, 184512 ͑2005͒

The low-frequency intermolecular dynamics in the pyr-

rolidinium RTILs are analyzed by line shape ﬁtting of the

broad spectral band to a multimode Brownian oscillator

model,^78,79

3

a_BOi

␻

I_BOi͑␻͒ =

,

2

͑3͒

͚

2

2 2

2␲͓͑␻

− ␻ ͒ + ␥_BOi␻ ͔

i=1

BOi

where a_BOi, ␻_BOi, and ␥_BOiare the amplitude, the frequency,

and the damping parameters for the ith Brownian oscillator,

respectively. Though the linewidth coefﬁcient ␥_BOiwas origi-

nally assumed to be frequency dependent,^78,79here it is as-

sumed to be constant for simplicity. Fits to the intermolecular

spectral bands using two-mode versus three-mode Brownian

+

oscillators are shown in Fig. 5 for P_1EOE/Br⁻. For the two-

mode model, the ﬁt curve deviates somewhat from the data

and the ﬁt residuals are noticeably worse than for the three-

mode model for which we determined that adequate ﬁts were

obtained. We note that even small molecular liquids normally

show at least two features in ﬁts to the intermolecular Kerr

spectrum. For the more complex ionic liquid case, it is plau-

sible that even more than three modes could be present,

likely with substantial overlap between modes. At least three

Brownian oscillators are needed to obtain good ﬁts to the

intermolecular dynamics of the pyrrolidinium RTILs. We

note that Giraud et al. used three Brownian oscillators to

describe the intermolecular Kerr spectra of ﬁve imidazolium

cation ionic liquids.⁴¹A Lorentzian function is added to the

model to account for the intramolecular bands observed at

+

180 cm⁻¹for P_1EOE/DCA⁻and at 122 cm⁻¹for

−

P_1EOE⁺/NTf₂, P_1EOM⁺/NTf₂, and P₁₄⁺/NTf₂. Figures 5

and 6 show the low-frequency spectra and Brownian oscilla-

tor ﬁts. The ﬁt parameters are summarized in Table IV. The

sum of the spectral area A_BOi=͐I_BOi͑␻͒d␻/͚ ͐I_BOi͑␻͒d␻ for

i

the Brownian oscillators is normalized to unity.

−

From the line shape analysis of the three NTf₂anion

RTILs shown in Fig. 6, one ﬁnds that the band center fre-

quencies ␻ of about 28, 45, and 72 cm⁻¹are very similar

BOi

−

for P₁₄⁺/ NTf₂, P_1EOM⁺/NTf₂and P_1EOE⁺/NTf₂. The

+

P_1EOE/Br⁻intermolecular spectrum arises predominantly

+

from P_1EOEcation dynamics, because the spherical bromide

ion has no reorientational contribution to the depolarized

OHD-RIKES signal. Three Brownian oscillator components

+

centered at 43, 66, and 91 cm⁻¹are found in the P_1EOE/Br⁻

spectrum, with the band center for the intermolecular spec-

trum shifted to higher frequency, as observed in Fig. 6. In-

−

FIG. 6. Low-frequency power spectra of ͑a͒ P_1EOE⁺/NTf₂

, ͑b͒

P_1EOM⁺/NTf₂⁻, ͑c͒ P₁₄⁺/NTf₂⁻, ͑d͒ P_1EOE/Br⁻, and ͑e͒ P_1EOE/DCA⁻͑bold

dotted lines͒. Entire ﬁt curves ͑solid lines͒ and each Brownian oscillator ﬁts

͑small dotted, short dashed, and long dashed lines͒ are also shown. Intramo-

lecular modes are also ﬁtted by a Lorentzian function ͑dot-dash lines͒.

+

termolecular dynamics for P_1EOE/DCA⁻liquids also show a

higher frequency ﬁrst moment of the spectrum. Brownian

oscillator line shape components are ﬁt to the center frequen-

cies of 34, 64, and 97 cm⁻¹. We note that the reduced mass

−

between cation and anion for the NTf₂liquids is approxi-

+

mately two times greater than for either of the P_1EOE/Br⁻or

+

P_1EOE/DCA⁻liquids. Thus a spectral component arising

sured in the 0–150 cm⁻¹range arise from intermolecular dy-

namics from both the cations and anions. The complexity of

the observed ionic liquid spectra does not permit us to dis-

tinguish between the motional narrowing or quantum har-

monic oscillator models in the Kerr dynamics observed for

the pyrrolidinium RTILs.

from a vibrational coordinate between cation and anion

could account for the observed higher band center frequen-

cies for the latter two liquids.

For the ﬁve pyrrolidinium ionic liquids considered here,

the case of the intermolecular vibrations is simplest to con-

+

sider for P_1EOE/Br⁻because of the spherical bromide anion.

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184512-10

Shirota et al.

J. Chem. Phys. 122, 184512 ͑2005͒

TABLE IV. Brownian oscillator ﬁt parameters for the low-frequency power spectra of the pyrrolidinium ionic liquids.

a

b

c

b

d

b

d

Ionic liquid

A_BO1

␻

͑cm⁻¹

͒

␥

͑cm⁻¹

͒

A_BO2

␻

͑cm⁻¹

͒

␥

͑cm⁻¹

͒

A_BO3

␻

͑cm⁻¹

͒

␥

BO3

͑cm⁻¹

͒

BO1

BO2

BO3

−

P_1EOE⁺/NTf₂

0.23

0.17

0.21

0.27

0.17

28

29

28

43

34

61

71

68

72

76

0.44

0.45

0.55

0.40

0.48

47

41

45

66

64

76

81

86

62

79

0.33

0.38

0.24

0.32

0.35

74

73

69

91

97

65

73

61

57

72

−

P_1EOM⁺/NTf₂

P₁₄⁺/NTf₂

−

P_1EOE⁺/Br⁻

P_1EOE⁺/DCA⁻

^aError is ±30%.

^bError is ±15%.

^cError is ±20%.

^dError is ±10%.

The only way in which the Br⁻motions can couple to the

vibrational coordinates that should result in more clearly de-

ﬁned modes than are observed for the intermolecular inter-

actions found between aprotic organic liquid molecules.

These proposed vibrational coordinates between cations and

anions would likely overlap with both the translational

͑interaction-induced͒ and orientational ͑librational͒ compo-

nents of the Kerr spectrum. It seems clear that careful mo-

lecular dynamics simulations followed by calculation of the

Kerr spectra will be required to obtain any more deﬁnitive

assignments of the low-frequency spectra of ionic liquids.

Our conclusion is that the clearest contributions from a line

shape analysis are that one obtains an accurate parametriza-

tion of the line shape and a reasonably good picture about the

minimum number of different modes that contribute to the

intermolecular line shape.

The intermolecular dynamics for the ﬁve pyrrolidinium

RTILs studied here are qualitatively similar to those ob-

served for the imidazolium cation based RTILs studied by

Giraud et al.⁴¹and Rajian et al.⁴⁰Despite the qualitative

similarity in intermolecular dynamics for the two classes of

RTILs, the solvent reorganization dynamics probed using

solvatochromic ﬂuorescent dyes very different. For each of

the imidazolium-cation RTILs investigated thus far, the sol-

vation dynamics present an ultrafast decay component that is

qualitatively assigned to librational motions of the aromatic

imidazolium cation. Solvation dynamics of RTILs having the

same anions but with the aromatic cation replaced by an

aliphatic tetraalkylammonium, dialkylpyrrolidinium, or tet-

raalkylphosphonium cation show slower, glassylike relax-

ation with long time scale solvation time constants in the

range from 50 ps to 50 ns.^{54,66–71,97}

+

observed Kerr signal is if P_1EOE/Br⁻interactions lead to

+

substantial changes to the P_1EOEangular momentum. The

complex nature of ion-ion structure and interactions in ionic

liquids leads us to consider the possibility that low-frequency

cation-anion modes may contribute to the intermolecular

spectrum along with librations and interaction-induced mo-

tions. Visualization of the normal modes for AM1 gas-phase

+

semiempirical cluster calculations on the P_1EOE/Br⁻dimer

+

and the Br⁻/P_1EOE/Br⁻trimer gave preliminary evidence

that several of the numerous low-frequency torsional modes

of the cation are strongly coupled to bromide translations,

leading to effective cation-anion stretching coordinates.

Several line shape models have been used to analyze the

low-frequency intermolecular part of OHD-RIKES spectrum

for molecular liquids.^{33,34,38,50,80–91}The low-frequency inter-

molecular spectrum of liquids contains the interaction-

induced ͑translational͒ and librational ͑reorientational͒

motions.^92–96The recent study by Ryu and Stratt on the in-

termolecular dynamics and calculated Kerr spectra of ben-

zene and CS₂liquids provides insight for how we may con-

sider ionic liquids in a way that goes beyond line shape

analyses.⁹⁶They demonstrated that signiﬁcant overlap occurs

between translational and orientational components of the

calculated Kerr low-frequency spectrum in the range from 0

to 200 cm⁻¹. We believe that similar overlap and coupling

between translational and orientational modes will also occur

for ionic liquids in general. Our geometry optimized gas-

phase structures for the three pyrrolidinium cations and two

molecular anions show that each ion is an asymmetric rotor.

Structures of these ions are actually surprisingly close to

symmetric rotors, showing a crude similarity to the rotational

symmetry of benzene in the sense that they have two very

similar inertial axes for the prolate rotors. In order to unravel

the complex intermolecular spectrum in a binary system such

as an ionic liquid, we must recognize that there will be low-

frequency interaction-induced contributions from both cation

and anion, as well as cross terms. Because of the near sym-

metric top nature for both the cation and anion geometries,

there will exist librational motions on two different time

scales. Because of the asymmetric nature of the ions, we

expect depolarized contributions from all modes. We believe

that the strong electrostatic interactions between ions will

lead to a substantially higher degree of clustering for ionic

liquids compared to neutral organic liquids. The speciﬁc at-

tractive associations between cations and anions will lead to

It is well known that the dipole moment for an ion is not

a very meaningful quantity, since its magnitude depends on

the choice of origin for the atomic coordinates. Wynne and

co-workers avoid this difﬁculty by estimating cation effec-

tive dipole moments from chemically and structurally similar

neutrals.⁴¹Kobrak has described the how the concept of a

“charge arm” can be used in place of the dipole moment for

describing polarity in ionic liquids.⁹⁸The charge arm is de-

ﬁned as the charge times distance separating the center of

charge and the center of mass. This picture may qualitatively

explain the similarity of low-frequency dynamics between

−

+

NTf₂ionic liquids with P₁₄and the aromatic analog

1-butyl-3-methylimidazolium cations because the charge-

+

arms for P₁₄and 1-butyl-3-methylimidazolium cation have

similar values of 0.9 and 1.0 e Å, respectively.⁹⁸

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184512-11

Ultrafast dynamics of ionic liquids

J. Chem. Phys. 122, 184512 ͑2005͒

Much progress has been made in understanding the

properties of room temperature ionic liquids using molecular

dynamics simulations. To date, only dialkylimidazolium-

cation liquids have been studied. Nonpolarizable potential

force ﬁelds have been developed using both CHARMM⁹⁹

and AMBER^100,101models, with one work reporting en-

hanced accuracy by use of a polarizable potential.¹⁰²Funda-

mental physical properties can be effectively studied with

molecular dynamics simulations,^{99–101,103,104}as well as solva-

tion energies^105–108and dynamics.^64,65,109It will be quite ben-

eﬁcial to compare experiments and theory for optical Kerr

dynamics in ionic liquids, as well as to parametrize standard

potentials for both nonpolarizable and polarizable potentials

for nonaromatic cations and anions such as pyrrolidiniums

partment of Energy and supported by its Division of Chemi-

cal Sciences, Ofﬁce of Basic Energy Sciences. Support from

the BNL LDRD Program ͑Project No. 03-118͒ is acknowl-

edged. The authors thank Mark Kobrak, Elina Troﬁmovsky,

and Kimberly Odynocki for helpful discussions and assis-

tance in the preparation of the ionic liquids.

¹S. A. Forsyth, J. M. Pringle, and D. R. MacFarlane, Aust. J. Chem. 57,

113 ͑2004͒.

²T. Welton, Chem. Rev. ͑Washington, D.C.͒ 99, 2071 ͑1999͒.

³M. J. Earle and K. R. Seddon, Pure Appl. Chem. 72, 1398 ͑2000͒.

⁴P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed. 39, 3772 ͑2000͒.

⁵T. Kitazume, J. Fluorine Chem. 105, 265 ͑2000͒.

⁶G. W. V. Cave, C. L. Raston, and J. L. Scott, Chem. Commun. ͑Cam-

bridge͒ 2001, 2159.

⁷J. Dupont, R. F. de Souza, and P. A. Z. Suarez, Chem. Rev. ͑Washington,

D.C.͒ 102, 3667 ͑2002͒.

−

and NTf₂.

⁸H. Zhao and S. V. Malhotra, Aldrichimica Acta 35, 75 ͑2002͒.

⁹J. S. Willkes, Green Chem. 4, 73 ͑2002͒.

¹⁰J. H. Davis, Jr., Chem. Lett. 33, 1072 ͑2004͒.

V. CONCLUSIONS

¹¹Y. Chauvin, L. Mußmann, and H. Olivier, Angew. Chem., Int. Ed. Engl.

34, 2698 ͑1995͒.

The ultrafast dynamical aspects of the ﬁve pyrrolidinium

ionic liquids have been investigated using femtosecond

optical-heterodyne-detected Kerr effect spectroscopy. The pi-

cosecond relaxation processes of the ﬁve pyrrolidinium ionic

liquids show nonexponential behavior, with the best ﬁts ob-

tained using a triexponential model. The fast and intermedi-

ate relaxation time constants are about 2 and 20 ps and are

almost same for each of the ﬁve ionic liquids. The slow

relaxation time constant for the ionic liquids correlates with

the product of the shear viscosity times the sum of anion and

cation volumes, so is roughly consistent with the Stokes–

Einstein–Debye hydrodynamic model for orientational fric-

¹²P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza, and J. Dupont,

Polyhedron 15, 1217 ͑1996͒.

¹³C. E. Song and E. J. Roh, Chem. Commun. ͑Cambridge͒ 2000, 837.

¹⁴W. Chen, L. Xu, C. Chatterton, and J. Xiao, Chem. Commun. ͑Cambridge͒

1999, 1247.

¹⁵C. de Bellefon, E. Pollet, and P. Grenouillet, J. Mol. Catal. A: Chem. 145,

121 ͑1999͒.

¹⁶K. Jin, X. Y. Huang, L. Pang, J. Li, A. Appel, and S. Wherland, Chem.

Commun. ͑Cambridge͒ 23, 2872 ͑2002͒.

¹⁷M. Eckstein, P. Wasserscheid, and U. Kragl, Biotechnol. Lett. 24, 763

͑2002͒.

¹⁸A. L. Monteiro, F. K. Zinn, R. F. de Souza, and J. Dupont, Tetrahedron:

Asymmetry 8, 177 ͑1997͒.

¹⁹A. Hu, H. L. Ngo, and W. Lin, Angew. Chem., Int. Ed. 43, 2501 ͑2004͒.

²⁰A. Berthod, L. He, and D. W. Armstrong, Chromatographia 53, 63 ͑2001͒.

²¹D. W. Armstrong, L. K. Zhang, L. F. He, and M. L. Gross, Anal. Chem.

73, 3679 ͑2001͒.

−

tion. The observed viscosities of the pyrrolidinium NTf₂

and DCA⁻ionic liquids indicate that these ﬂuids are well

above the glass transition at ambient temperatures. It appears

that simple hydrodynamic scaling may be sufﬁcient to ex-

²²S. Carda-Broch, A. Berthod, and D. W. Armstrong, Rapid Commun. Mass

Spectrom. 17, 553 ͑2003͒.

plain the faster reorientation of P_1EOE⁺/NTf₂and

−

²³Z. B. Alfassi, R. E. Huie, B. L. Milman, and P. Neta, Anal. Bioanal. Chem.

377, 159 ͑2003͒.

P_1EOM⁺/NTf₂relative to P₁₄⁺/NTf₂because of the relative

−

²⁴D. McMorrow, W. T. Lotshaw, and G. A. Kenney-Wallace, IEEE J. Quan-

tum Electron. 24, 443 ͑1988͒.

differences in viscosity.

The Fourier-transform Kerr spectra of the ﬁve pyrroli-

dinium ionic liquids have also been obtained. The spectra of

²⁵W. T. Lotshaw, D. McMorrow, N. Thantu, J. S. Melinger, and R. Kitch-

enham, J. Raman Spectrosc. 26, 571 ͑1995͒.

²⁶D. McMorrow and W. T. Lotshaw, Chem. Phys. Lett. 174, 85 ͑1990͒.

²⁷D. McMorrow and W. T. Lotshaw, J. Phys. Chem. 95, 10395 ͑1991͒.

²⁸S. Kinoshita, Y. Kai, T. Ariyoshi, and Y. Shimada, Int. J. Mod. Phys. B 10,

1229 ͑1996͒.

−

the ionic liquids with the NTf₂anion ͑P_1EOE⁺/NTf₂,

P_1EOM⁺/NTf₂, and P₁₄⁺/NTf₂͒ are quite similar. The in-

−

+

tramolecular vibrational bands of P_1EOE/Br⁻and

+

P_1EOE/DCA⁻are similar except for the addition of the

²⁹E. W. Castner, Jr. and M. Maroncelli, J. Mol. Liq. 77, 1 ͑1998͒.

³⁰N. A. Smith and S. R. Meech, Int. Rev. Phys. Chem. 21, 75 ͑2002͒.

³¹B. J. Loughnane, R. A. Farrer, A. Scodinu, T. Reilly, and J. T. Fourkas, J.

Phys. Chem. B 104, 5421 ͑2000͒.

DCA⁻bending modes. The broad intermolecular spectra of

the ionic liquids have been analyzed by the multimode

Brownian oscillator model. The intermolecular spectral den-

³²R. A. Farrer and J. T. Fourkas, Acc. Chem. Res. 36, 605 ͑2003͒.

³³N. T. Hunt, A. A. Jaye, and S. R. Meech, J. Phys. Chem. B 107, 3405

͑2003͒.

sities of P_1EOE⁺/NTf₂, P_1EOM⁺/NTf₂, and P₁₄⁺/NTf₂

−

+

show maxima at lower frequencies than those of P_1EOE/Br⁻

³⁴N. T. Hunt, A. A. Jaye, A. Hellman, and S. R. Meech, J. Phys. Chem. B

108, 100 ͑2004͒.

+

and P_1EOE/DCA⁻. A sum of three Brownian oscillators pro-

³⁵G. Giraud and K. Wynne, J. Am. Chem. Soc. 124, 12110 ͑2002͒.

³⁶G. Giraud, J. Karolin, and K. Wynne, Biophys. J. 85, 1903 ͑2003͒.

³⁷J. D. Eaves, C. J. Fecko, A. L. Stevens, P. Peng, and A. Tokmakoff, Chem.

Phys. Lett. 376, 20 ͑2003͒.

vides a good ﬁt to the intermolecular line shapes for each of

the ﬁve ionic liquids studied, though we note that this model

may underestimate the complexity of the interactions occur-

ring in such a binary ionic system.

³⁸H. Shirota and E. W. Castner, Jr., J. Am. Chem. Soc. 123, 12877 ͑2001͒.

³⁹B. R. Hyun, S. V. Dzyuba, R. A. Bartsch, and E. L. Quitevis, J. Phys.

Chem. A 106, 7579 ͑2002͒.

ACKNOWLEDGMENTS

⁴⁰J. Rajian, S. Li, R. Bartsch, and E. Quitevis, Chem. Phys. Lett. 393, 372

͑2004͒.

Work done at Rutgers was supported by the National

Science Foundation ͑Grant No. CHE-0239390͒. Part of this

work was carried out at Brookhaven National Laboratory

under Contract No. DE-AC02-98CH10886 with the U.S. De-

⁴¹G. Giraud, C. M. Gordon, I. R. Dunkin, and K. Wynne, J. Chem. Phys.

119, 464 ͑2003͒.

⁴²H. Cang, J. Li, and M. D. Fayer, J. Chem. Phys. 119, 13017 ͑2003͒.

⁴³A. M. Funston, E. Troﬁmovsky, K. Odynocki, M. N. Kobrak, and J. F.

Downloaded 16 Dec 2012 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

184512-12

Shirota et al.

J. Chem. Phys. 122, 184512 ͑2005͒

Wishart ͑unpublished͒.

⁷⁵B. J. Loughnane, A. Scodinu, R. A. Farrer, J. T. Fourkas, and U. Mohanty,

J. Chem. Phys. 111, 2686 ͑1999͒.

⁴⁴P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis ͑Wiley-VCH,

Weinheim, 2003͒.

⁷⁶D. McMorrow, N. Thantu, V. Kleiman, J. S. Melinger, and W. T. Lotshaw,

J. Phys. Chem. A 105, 7960 ͑2001͒.

⁴⁵D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, and M. Forsyth, J. Phys.

Chem. B 103, 4164 ͑1999͒.

⁷⁷D. McMorrow, W. T. Lotshaw, J. S. Melinger, and V. Kleiman, ACS

Symp. Ser. 820, 14 ͑2002͒.

⁴⁶N. L. Lancaster, P. A. Salter, T. Welton, and G. B. Young, J. Org. Chem.

67, 8855 ͑2002͒.

⁷⁸Y. Tanimura and S. Mukamel, J. Chem. Phys. 99, 9496 ͑1993͒.

⁷⁹S. Mukamel, Principles of Nonlinear Optical Spectroscopy ͑Oxford Uni-

versity Press, New York, 1995͒.

⁴⁷D. R. MacFarlane, J. Sun, J. Golding, P. Meakin, and M. Forsyth, Elec-

trochim. Acta 45, 1271 ͑2000͒.

⁴⁸D. R. MacFarlane, S. A. Forsyth, J. Golding, and G. B. Deacon, Green

Chem. 4, 444 ͑2002͒.

⁸⁰Y. J. Chang and E. W. Castner, J. Phys. Chem. 98, 9712 ͑1994͒.

⁸¹Y. J. Chang and E. W. Castner, Jr., J. Phys. Chem. 100, 3330 ͑1996͒.

⁸²N. A. Smith, S. Lin, S. R. Meech, and K. Yoshihara, J. Phys. Chem. A

101, 3641 ͑1997͒.

⁴⁹D. R. MacFarlane, J. Golding, S. Forsyth, M. Forsyth, and G. B. Deacon,

Chem. Commun. ͑Cambridge͒ 2001, 1430.

⁵⁰P. P. Wiewior, H. Shirota, and E. W. Castner, Jr., J. Chem. Phys. 116, 4643

͑2002͒.

⁸³N. A. Smith, S. Lin, S. R. Meech, H. Shirota, and K. Yoshihara, J. Phys.

Chem. A 101, 9578 ͑1997͒.

⁵¹M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., GAUSSIAN 03, Gaussian,

Inc., Pittsburgh, PA, 2003.

⁸⁴H. Shirota, K. Yoshihara, N. A. Smith, S. Lin, and S. R. Meech, Chem.

Phys. Lett. 281, 27 ͑1997͒.

⁵²A. D. Becke, J. Chem. Phys. 98, 5648 ͑1993͒.

⁵³C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 ͑1988͒.

⁵⁴N. Ito, S. Arzhantsev, M. Heitz, and M. Maroncelli, J. Phys. Chem. B

108, 5771 ͑2004͒.

⁸⁵N. A. Smith and S. R. Meech, J. Phys. Chem. A 104, 4223 ͑2000͒.

⁸⁶N. T. Hunt and S. R. Meech, J. Chem. Phys. 120, 10828 ͑2004͒.

⁸⁷S. Palese, J. T. Buontempo, L. Schlling, W. T. Lotshaw, Y. Tamimura, S.

Mukamel, and R. J. Dwayne Miller, J. Phys. Chem. 98, 12466 ͑1994͒.

⁸⁸S. Palese, L. Schilling, R. J. D. Miller, P. R. Staver, and W. T. Lotshaw, J.

Phys. Chem. 98, 6308 ͑1994͒.

⁵⁵B. Mennucci and J. Tomasi, J. Chem. Phys. 106, 5151 ͑1997͒.

⁵⁶B. Mennucci, E. Cances, and J. Tomasi, J. Phys. Chem. B 101, 10506

͑1997͒.

⁸⁹M. Cho, M. Du, N. F. Scherer, G. R. Fleming, and S. Mukamel, J. Chem.

Phys. 99, 2410 ͑1993͒.

⁵⁷See EPAPS Document No. E-JCPSA6-122-506519 for details of the

GAUSSIAN 03 calculation results. Optimized geometries, electrostatic param-

eters, and rotational constants, and CHelpG charges are tabulated. This

document can be reached via a direct link in the online article’s HTML

reference section or via the EPAPS homepage ͑http://www.aip.org/

pubservs/epaps.html͒.

⁹⁰P. Bartolini, M. Ricci, R. Trroe, R. Righini, and I. Santa, J. Chem. Phys.

110, 8653 ͑1999͒.

⁹¹H. Shirota, J. Chem. Phys. 122, 044514 ͑2005͒.

⁹²P. A. Madden and T. I. Cox, Mol. Phys. 43, 287 ͑1981͒.

⁹³P. A. Madden, in Ultrafast Phenomena IV, edited by A. H. Auston and K.

B. Eisenthal ͑Springer, Berlin, 1984͒, Vol. 38, p. 244.

⁹⁴L. C. Geiger and B. M. Ladanyi, J. Chem. Phys. 89, 6588 ͑1988͒.

⁹⁵L. C. Geiger and B. M. Ladanyi, Chem. Phys. Lett. 159, 413 ͑1989͒.

⁹⁶S. Ryu and R. M. Stratt, J. Phys. Chem. B 108, 6782 ͑2004͒.

⁹⁷P. K. Chowdhury, M. Halder, L. Sanders, T. Calhoun, J. L. Anderson, D.

W. Armstrong, X. Song, and J. W. Petrich, J. Phys. Chem. B 108, 10245

͑2004͒.

⁵⁸B. J. Loughnane and J. T. Fourkas, J. Phys. Chem. B 102, 10288 ͑1998͒.

⁵⁹D. Kivelson and P. A. Madden, Annu. Rev. Phys. Chem. 31, 523 ͑1980͒.

⁶⁰G. R. Fleming, Chemical Applications of Ultrafast Spectroscopy ͑Oxford

University Press, New York, 1986͒.

⁶¹J. T. Edwards, J. Chem. Educ. 47, 261 ͑1970͒.

⁶²Because the values for S of NTf₂and N- for NTf₂and DCA⁻are not

listed, the value for S is used as 2ϫSϭand the value for N is tentatively

used as ϾN-.

−

⁹⁸M. N. Kobrak and N. Sandalow, in Molten Salts XIV, edited by R. A.

Mantz ͑The Electrochemical Society, Pennington, NJ, 2005͒.

⁹⁹T. I. Morrow and E. J. Maginn, J. Phys. Chem. B 106, 12807 ͑2002͒.

¹⁰⁰J. de Andrade, E. S. Boes, and H. Stassen, J. Phys. Chem. B 106, 13344

͑2002͒.

⁶³J. N. Israelachvili, Intermolecular and Surface Forces, 2nd ed. ͑Academic,

London, 1992͒.

⁶⁴M. N. Kobrak and V. Znamenskiy, Chem. Phys. Lett. 395, 127 ͑2004͒.

⁶⁵C. J. Margulis, Mol. Phys. 102, 829 ͑2004͒.

⁶⁶R. Karmakar and A. Samanta, J. Phys. Chem. A 106, 4447 ͑2002͒.

⁶⁷R. Karmakar and A. Samanta, J. Phys. Chem. A 107, 7340 ͑2003͒.

⁶⁸D. Chakravarty, P. Harza, A. Chakravarty, D. Seth, and N. Sarkar, Chem.

Phys. Lett. 381, 697 ͑2003͒.

¹⁰¹C. G. Hanke, S. L. Price, and R. M. Lynden-Bell, Mol. Phys. 99, 801

͑2001͒.

¹⁰²T. Y. Yan, C. J. Burnham, M. G. Del Popolo, and G. A. Voth, J. Phys.

Chem. B 108, 11877 ͑2004͒.

⁶⁹S. Arzhantsev, N. Ito, M. Heitz, and M. Maroncelli, Chem. Phys. Lett.

381, 278 ͑2003͒.

¹⁰³M. G. Del Popolo and G. A. Voth, J. Phys. Chem. B 108, 1744 ͑2004͒.

¹⁰⁴C. J. Margulis, H. A. Stern, and B. J. Berne, J. Phys. Chem. B 106,

12017 ͑2002͒.

⁷⁰J. A. Ingram, R. S. Moog, N. Ito, R. Biswas, and M. Maroncelli, J. Phys.

Chem. B 107, 5926 ͑2003͒.

¹⁰⁵V. Znamenskiy and M. N. Kobrak, J. Phys. Chem. B 108, 1072 ͑2004͒.

¹⁰⁶C. G. Hanke, A. Johansson, J. B. Harper, and R. M. Lynden-Bell, Chem.

Phys. Lett. 374, 85 ͑2003͒.

⁷¹N. Ito, S. Arzhantsev, and M. Maroncelli, Chem. Phys. Lett. 396, 83

͑2004͒.

⁷²J. D. Holbrey, W. M. Reichert, and R. D. Rogers, J. Chem. Soc. Dalton

Trans. 2004, 2267.

¹⁰⁷C. G. Hanke and R. M. Lynden-Bell, J. Phys. Chem. B 107, 10873

͑2003͒.

⁷³M. Castriota, T. Caruso, R. G. Agostino, E. Cazzanelli, W. A. Henderson,

and S. Passerini, J. Phys. Chem. A 109, 92 ͑2005͒.

⁷⁴I. Rey, P. Johansson, J. Lindgren, J. C. Lassegues, J. Grondin, and L.

Servant, J. Phys. Chem. A 102, 3249 ͑1998͒.

¹⁰⁸J. B. Harper and R. M. Lynden-Bell, Mol. Phys. 102, 85 ͑2004͒.

¹⁰⁹Y. Shim, J. S. Duan, M. Y. Choi, and H. J. Kim, J. Chem. Phys. 119,

6411 ͑2003͒.

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Ultrafast dynamics of pyrrolidinium cation ionic liquids

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