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Chemical Physics Letters 531 (2012) 80–85

Contents lists available at SciVerse ScienceDirect

Chemical Physics Lettersjournal homepage: www.elsevier.com/locate/cplett

Multinuclear NMR relaxometry studies in singly ﬂuorinated liquid crystal
M. Rajeswari a,⇑, Trivikram R. Molugu a,b, S. Dhara a, K. Venu c, V.S.S. Sastry a, R. Dabrowski daSchool of Physics, University of Hyderabad, Hyderabad 500 046, India
Department of Chemistry, University of Arizona, Tucson, AZ 85721, USAc
Soctronics Technologies Pvt. Ltd., Banjara Hills, Hyderabad 500 034, IndiadInstitute of Chemistry, Military University of Technology, 00-908 Warsaw, Polandb

a r t i c l e

i n f oArticle history:
Received 26 September 2011
In ﬁnal form 6 February 2012
Available online 15 February 2012a b s t r a c t
Nuclear spin–lattice relaxation rate dispersion study of 1H and 19F in the isotropic phase of a singly ﬂuorinated liquid crystal 40 -butoxy-3-ﬂuoro-4-isothiocyantotolane (4OTOLFo) points to their differing relaxation pathways and hence sensitivity to qualitatively different time modulations. In particular ﬂuorine
nuclear spins, with strong lattice coupling (larger by two orders) extending to very low frequencies, detectslowly relaxing local structures via the spin–rotation interaction. The ﬁeld-cycling technique used to carryout these very low frequency measurements, provides for level crossing of the two nuclear species at low
enough jump ﬁelds, facilitating an additional mechanism of cross-relaxation in the strong coupling limit.
Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction
NMR spectroscopy and relaxometry are powerful and wellestablished experimental techniques to study orientational orderand molecular dynamic processes in liquid crystalline systems[1–3]. In particular, relaxometry is rather unique to investigate
slow collective motions that usually occur in such soft systems
over a dynamic range spanning from tens of kHz to hundreds ofMHz [4,5]. In the present study we have chosen a ﬂuorinated liquid
crystal (with a lone ﬂuorine on its aromatic core), which has two
spin 12 nuclei (1H and 19F) having only slightly differing gyromagnetic ratios, with the objective of comparing the underlying
processes reported by these two species through their spin–lattice
relaxation proﬁles. In general the proton spin–lattice relaxation (inthe wide-line NMR regime of interest here) is essentially mediatedby inter-nuclear dipole interaction, and hence sensitive to reorientational time correlations. Its interpretation has been well established in terms of various dynamic processes in the liquid crystal
systems. However relaxation pathways of lone ﬂuorine could bein principle several: e.g., inter-molecular dipolar interaction withother ﬂuorine nuclei; hetero-nuclear dipolar interaction with
protons; effects due to chemical shift anisotropy (CSA) and spin–rotation interaction (SR). The ﬁrst mechanism, mediated by translational diffusion, is too fast to display dispersion in the frequency
range of our study, while the second should lead to dispersions
similar to protons. The experimental data indicating strong coupling of ﬂuorines to the bath, relative to protons however suggestthat there are other more effective pathways of relaxation. CSA⇑ Corresponding author.
E-mail address: raji.hcu@gmail.com (M. Rajeswari).
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2012.02.010

mechanism, important at the usual laboratory frequencies(102 MHz), is negligible in a ﬁeld-cycling experiment focussed
on sub-MHz regime, owing to its ﬁeld dependence. We ﬁnd thatthe main contribution to ﬂuorine spin–lattice relaxation rate (R1)here is from spin–rotation interaction modulated by torques on
the molecule and hence sensitive to angular momentum timecorrelations. In contrast, the protons are sensitive, via dipolarinteractions, to molecular reorientational time correlations. Suchﬂuorinated compounds have attracted recently signiﬁcant attention because of their applications in liquid crystal display technology and telecommunications [6–14]. Extensive optical anddielectric measurements were carried out on these systems tostudy their electro-optical and physical properties [11,12]. The
present study reports contrasting relaxation rate dispersions ofproton and ﬂuorine nuclei as a function of Larmor frequency inthe liquid crystal 40 -butoxy-3-ﬂuoro-4-isothiocyantotolane (4OTOLFo) in its isotropic phase at different temperatures.
The present Letter is organized as follows. In Section 2 wediscuss the experimental details and present results on R1 as afunction of frequency and temperature. In the next section, we discuss relaxation mechanisms for proton and ﬂuorine, and present
models which provide quantitative estimates of their contributions. Section 4 discuss the analysis of our ﬁndings in terms of relevant dynamic models, while salient features are summarized in
the concluding section.
2. Experimental details
This compound was synthesized and its phase sequence wasdetermined in the laboratories of Warsaw [15,16], and these arepresented in Figure 1. For our experiment, the compound wasAuthor's personal copy

M. Rajeswari et al. / Chemical Physics Letters 531 (2012) 80–85

81a4'-butoxy-3-fluoro-4-isothiocyantotolane (4OTOLFo)
F

C4H9O

C

C

NCS

Cr 96.7oC N 89.9oC Iso

4'-butoxy-3'-fluoro-4-isothiocyantotolane (4OFTOL)

b

F

C4H9O

C

C

NCS

Cr 64.2oC N 71.6oC Iso
4'-butoxy-2-fluoro-4-isothiocyantotolane (4OTOLFm)
F

C4H9O

C

C

NCS

Cr 70.3oC N 94.4oC Iso
Figure 1. Molecular structure and phase sequence of a liquid crystal 4OTOLFo(molecular structures of other related ﬂuorine compounds are also given forcomparison).

sealed in an NMR tube under vacuum after removing the dissolved
oxygen by freeze–pump–thaw technique. Frequency dispersions of
R1 were recorded with two spectrometers. A Fast Field Cycling
NMR relaxometer (SPINMASTER FFC-2000, Stelar) was used tocover the low frequency range (10 kHz to 10 MHz), and the higher
frequency data were collected with a home-built variable-ﬁeld
pulsed NMR spectrometer (extending to 50 MHz). A prepolarized
pulse sequence was used for all ﬁeld-cycling measurements below
4 MHz, while a non-prepolarized pulse sequence was used above
4 MHz. The prepolarized and acquisition frequencies were 10 and9.25 MHz, respectively. Typical switching time during the ﬁeld
cycle is 3 ms, and the duration of the p/2 pulse is 6 ls. Other
parameters were optimized catering speciﬁcally to each spin
system. An inversion recovery pulse sequence with a p/2 pulse
width of 4.5 ls was used at higher frequencies (20–50 MHz) on
the home-built spectrometer. The temperature (T) of the sample
was controlled (to within 0.1 °C) by passing thermostated dry air
over the sample. The frequency dispersion proﬁles of proton andﬂuorine thus obtained at different temperatures in the isotropic
phase are shown in Figures 2–4. The temperature dependences of
the relaxation rates were also measured at chosen Larmor frequencies mL (x = 2pmL), and are shown in Figure 5. The experimental
errors involved in the measurements on proton were estimatedto be less than 2%, while those on ﬂuorine were about 5% due topoor signal strength. The R1 dispersion proﬁles of proton show frequency independence in the sub-MHz region, and onset of moderate dispersions at higher frequencies as the sample is cooled

Figure 2. Proton (a) and ﬂuorine (b) spin–lattice relaxation rate dispersions ﬁtted
to Eqs. (3) and (8) at DTNI = 11 °C. The color dotted lines represents the contribution
from different molecular processes. (For interpretation of the references to color inthis ﬁgure legend, the reader is referred to the web version of this article.)

towards the isotropic-to-nematic transition temperature (TNI).
The ﬂuorine dispersion data on the other hand show a contrasting
behavior. They all exhibit a signiﬁcant dispersion even at the lowest of the frequencies irrespective of the temperature of thesample, signaling the presence of strong and highly dispersive ﬂuorine coupling to the lattice. The temperature dependent ﬂuorine
relaxation rate measurements at different Larmor frequencies(Figure 5) also support this observation. The interpretation of ﬂuorine data thus should focus on identifying plausible mechanism(s)exclusively sensitive to low frequency microscopic dynamics, but
could not be reported by protons.
3. Relaxation models
In this section, we present dynamic models relevant for the
analysis of the observed proton and ﬂuorine dispersions. Underlying molecular mechanisms mediating nuclear spin relaxation
processes in liquid crystals have been comprehensively reviewed
earlier [1–3,17], and interpretation of dispersions of proton data(wide-line NMR regime) is based on them [1]. The stronger lattice-coupling of ﬂuorine (a magnetically comparable spin 12 nucleus
residing on the same molecule) extending to even very low ﬁelds
points to the important role of its relatively larger spin–rotationinteraction in providing an efﬁcient relaxation pathway, and analysis of its dispersion explores possible slow dynamic processes
modulating SR interaction.
3.1. Proton relaxation
Proton spin–lattice relaxation, via the dipole–dipole interactionsof spin pairs, is inﬂuenced by time modulations due to:Author's personal copy82

M. Rajeswari et al. / Chemical Physics Letters 531 (2012) 80–85

a

a

b

b

Figure 3. Proton (a) and ﬂuorine (b) spin–lattice relaxation rate dispersions ﬁtted
to Eqs. (3) and (8) at DTNI = 4 °C. The color dotted lines represents the contribution
from different molecular processes. (For interpretation of the references to color inthis ﬁgure legend, the reader is referred to the web version of this article.)a

T (ºC)
Figure 5. Temperature dependence of spin–lattice relaxation rates plotted against
temperature at different Larmor frequencies (a) 1H nuclei and (b) 19F nuclei. The
dotted line in the ﬁgures indicate the isotropic-nematic transition temperature.

reorientations of the molecules around short and long axes, translational diffusion, and orientational order (critical) ﬂuctuations near
the transition temperature (TNI). In view of the presence of substantial dipole–dipole interaction among the several protons on the molecule their hetero-nuclear coupling with the lone ﬂuorine isneglected, and the proton data are analyzed based on homo-nuclear
dipolar interactions among them [1].
3.1.1. Molecular reorientations
Molecular reorientations about the long axis and dynamics ofend-chains are too fast of the present experiment, providing at besta constant background. Random reorientations about the shortaxes, characterized by the correlation time sR, lead to frequencydependent relaxation rate [3,18]b

R1HR ¼ A1HR

2
X

p2 sR

xsR Þ2p¼1 1 þ ðp

ð1Þ:Here A1HR represents the coupling strength of this mechanism to thelattice modes. This contribution could also become frequency independent for experimental condition satisfy xsR  1, depending
then only on T.
3.1.2. Critical ﬂuctuations
Near the transition, ﬂuctuations of the orientational orderparameter manifest as slow modulations, and their characteristic
time (sCF) increases critically as TNI is approached [1,19]. This
contribution is given by
Figure 4. Proton (a) and ﬂuorine (b) spin–lattice relaxation rate dispersions ﬁtted
to Eqs. (3) and (8) at DTNI = 1 °C. The color dotted lines represents the contribution
from different molecular processes. (For interpretation of the references to color inthis ﬁgure legend, the reader is referred to the web version of this article.)

2

312

sCF
6
7
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ5 :R1HCF ¼ AHCF 4
1 þ 1 þ x2 s2CF

ð2ÞAuthor's personal copy

M. Rajeswari et al. / Chemical Physics Letters 531 (2012) 80–85

Here AHCF is a temperature dependent parameter quantify the effective spin coupling to order ﬂuctuations. The total proton spin–lattice relaxation rate of protons is thus expressed, including a constant background (R1H0) due to very fast processes, as

R1H ¼ R1H0 þ R1HR þ R1HCF :ð3Þ

3.2. Fluorine relaxation
The ﬂuorine spin–lattice relaxation in the present system could
be due to several mechanisms: homo-nuclear dipole interaction
with ﬂuorines present on different molecules; hetero-nuclear
dipole interaction with protons; chemical shift anisotropy (CSA);
and spin–rotation interaction with the molecular angular momentum components. The contribution from ﬁrst interaction between
the two ﬂuorines (located on each of the molecules) is expected
to be very small, and dispersionless in the present experiment.
And CSA mechanism is unimportant in the present range of Larmor
frequency.
It is convenient to discuss the effect of hetero-nuclear coupling
between 1H and 19F in the high ﬁeld (WC: weak-coupling limit)
and low-ﬁeld (SC: strong-coupling limit) regimes separately. In
the former case, when the difference in the resonance frequenciesof the two participating nuclear species (say I for ﬂuorine and S for
proton) is much larger than their line-widths, the dipolar perturbation leads to a coupling between the two Zeeman reservoirs. Thiscan in principle result in a longitudinal magnetization recovery
on two time scales. For the I-spin system, say, the magnetization
decay to equilibrium is described by [3]dhIZ i1
1¼  II ðhIZ i  I0 Þ  IS ðhSZ i  S0 Þ;
dt
T1
T1

ð4Þ

where

1

2

¼ c2I c2S h SðSþ1Þ
II



T1


1 ð0Þ
3
3
J ðxI  xS Þþ Jð1Þ ðxI Þþ Jð2Þ ðxI þ xS Þ ;
12
2
4



1 ð0Þ
3 ð2Þ
2 2 2
¼
c
c
h

IðI
þ
1Þ

ð
x

x
Þ
þ
ð
x
þ
x
Þ
:
J
J
I
S
I
S
I S
12
4
T IS
1
1

ð5aÞ

ð5bÞ

hIZi, hI0i, hSZi and hS0i stand for the instantaneous and equilibrium
longitudinal magnetizations of the two systems, respectively. However, experimental observation of only a single time constant forrecoveries of the magnetizations of both the spin systems (overthree decades of time) indicates that the ﬁrst term in Eq. (4) isadequate for our analysis. Thus the effect of the coupling between
1
H and 19F manifests as a modiﬁed expression for the spin–lattice
relaxation rate of ﬂuorine Eq. (5a), unlike the case of protons. Contributions to the ﬂuorine relaxation rate due to its dipolar coupling
to protons are computed accordingly Eq. (5a), taking into accountthe time modulations due to short axes reorientations (R1FR) andcritical ﬂuctuations of the order parameter (R1FCF), besides including
a constant background term R1F0. The values of sR and sCF obtained
from the proton data provide useful inputs for the ﬂuorine analysis.
The stronger ﬂuorine spin–rotation interaction relative to protons ( 5 times larger [20]), seems to be crucial in providing the observed strong lattice coupling, and its contribution is sensitive to thecorrelation times of the molecular angular momentum components[20–23]. Contribution from SR mechanism is expressed as [22]R1FSR ¼ ASR T

sSR
1 þ x2 s2SR;

ð6Þ

where ASR quantiﬁes the degree of ﬂuorine coupling to the lattice,and depends on the values of the ﬂuorine spin–rotation interaction
terms. In the case of molecules constituting simple liquids, sSR is thecorrelation time associated with the molecular angular momentum

83

ﬂuctuations (typically in the range of 1012–1013 s). While suchtime scales can at best provide a negligible constant backgroundcontribution to R1 of ﬂuorine, the experimental data strongly suggests the presence of very slow dynamic processes, exclusively
probed by the ﬂuorine nuclei.
In this context, it may be noted that earlier line width measurements on the ESR spectra of several free radicals dissolved in liquid
crystals (e.g., Freed et al. [24,25]), required for their consistentinterpretation of the presence of slower processes modulating
the rotational diffusion of the probe molecules. These experiments,
supported by subsequent 2-D ESR methods [26,27], suggest thatthe free radicals not only undergo such diffusion in the presenceof an orienting potential (due to mean torque), but also experience
the effects of sufﬁciently slow ﬂuctuations of the potential itself[26]. This necessitated dealing with the problem of diffusing molecules in the presence of a slowly relaxing local structures (SRLS).
These were modeled as orienting cages undergoing slow rotational
diffusion (on a time scale, sC), and the corresponding relaxation
theory included these degrees of freedom as relevant random variables constituting a composite stochastic model [25]. Further,
these experiments also indicated that the degree of coupling of
the probes to the dynamics of the liquid crystal environment depended strongly on their size and anisotropy relative to the hostmolecules. With this perspective, the observed strong ﬂuorine-lattice coupling seems to be reﬂective of the presence of slow torques(alluded to by the earlier ESR studies) appropriately modulating
the SR interaction via molecular angular momentum. While suchcomposite processes require in principle solving a Fokker–
Planck–Kramer equation for joint probability density function of
the composite Markov process, we take here a more simplistic
view by accounting for the effect of such modes through a Lorentzian spectral density. We represent this contribution to R1 of ﬂuorine as

R1FC ¼ AFCsC
1 þ x2 s2C:

ð7Þ

Here AFC is a temperature dependent parameter quantifying thestrength of this mechanism. Thus, we model the observed relaxation rate in the weak-coupling limit as

R1F ¼ R1F0 þ R1FR þ R1FCF þ R1FC :ð8Þ

4. Analysis and discussion
The proton relaxation dispersions at three temperatures in theisotropic phase were analyzed based on Eq. (3) using the nonlinear least square method [28], and the best ﬁt data are shownin Figures 2–4 and the results are summarized in Table 1. The proton spin–lattice relaxation at 11 °C away from TNI is well describedby individual molecular reorientations (R) and constant background coming from other fast processes. At DTNI = 1 and 4 °C,
the dispersions need, besides R, critical ﬂuctuations of the orientational order (CF). The correlation times sR in 4OTOLFo are small
compared to 4OFTOL and 4OTOLFm. For example, away from thetransition (DTNI = 11 °C), sR is 1 ns in the present system while itis 1.39 ns in 4OTOLFm [30] and 2.4 ns in 4OFTOL [29] (Figure 1).
Typical correlation times of critical ﬂuctuations sCF in 4OTOLFo
are comparable with those of 4OTOLFm (67 ns at DTNI = 11 °C),
but are short compared to 4OFTOL (363 ns at DTNI = 11 °C). Also
such critical ﬂuctuations in the present system disappear quickly
on heating in this system. The activation energy associated withthe rotational diffusion is small in 4OTOLFo compared to the othertwo compounds (2.5, 7.5 and 20.8 kJ/mol for 4OTOLFo, 4OTOLFm
and 4OFTOL, respectively). The only structural difference among
these three compounds is the location of ﬂuorine atom on theAuthor's personal copy84

M. Rajeswari et al. / Chemical Physics Letters 531 (2012) 80–85

Table 1
Parameters extracted by ﬁtting the 1H dispersions to Eq. (3) and the parameters in thetable are explained in the text.

Table 2
Parameters extracted by ﬁtting the 19F dispersions to Eq. (8) and the parameters in
the table are explained in the text.

DT
(°C)

R1H0
(s1)

A1HR
(109 s2)

sR

AHCF(103 s3/2)

sCF

A1FR
(109 s1)

(ns)

AFC
(108 s2)

sC

(ns)

AFCF
(103 s3/2)

sCF

(ns)

DT
(°C)

sR

(ns)

1
4
11

–
–
0.12
± 0.051.24 ± 0.11
1.52 ± 0.1
1.85 ± 0.04

0.98 ± 0.11
0.75 ± 0.15
1.02 ± 0.02

13.35 ± 2.16
11.62 ± 2.79–

49.40 ± 7.17
34.03 ± 6.03
–

1
4
11

0.4 ± 0.02
0.5 ± 0.01
1.30 ± 0.01

11
113.53 ± 0.13
9.57 ± 0.13
–

49.5
34
–

0.35 ± 0.02
0.35 ± 0.02
0.36 ± 0.02

275.15 ± 28.74
233.5 ± 22.07
263.65 ± 24.97

aromatic cores (see Figure 1), and further the present system isreported to be monotropic [11]. These results indicate that the activation energy for the reorientations is higher when the ﬂuorine islocated on the aromatic core nearer to the alkyl chain, anddecreases considerably when it is located on the other aromaticring. The temperature dependent data at different Larmor frequencies (shown in Figure 5a) support the predominance of CF contribution as the transition is approached from above.
Analysis of ﬂuorine data in the WC limit based on Eq. (8), withthe above values for sR and sCF, leads to the determination of sC
characterizing the slow torques experienced by the probe. We
demonstrated clearly the necessity to include spin–rotation contribution to account for the experimental data: for 400 kHz and above(Figure 6). We ﬁnd that sC is in the range of 230–280 ns (Table 2),
and is practically the same over the temperature range within theerrors of estimation. It is interesting to note that similar time scales
were obtained from the ﬂuorine data in 4OFTOL (290 ns) [29] and4OTOLFm (270 ns) [30] as well. The insensitivity of protons to thisdynamics is understood as due to its relatively smaller spin–rotation constant. A1FR and AFCF (Table 2) represent the couplingstrengths of the ﬂuorine system to the lattice via the heteronuclear dipolar interaction with protons, arising from different
time modulations (R and CF).
The analysis of the data in the SC limit (typically below 400 kHz
in the present sample) differs from the above. We ﬁnd that theextrapolated relaxation rates based on the above ﬁt parametersdeviate substantially, and systematically, from the experimental
observations (Figure 6). The additional contribution in this regime
arises from a qualitatively different relaxation path made available,
once the ﬂuorine and proton resonance lines start overlapping signiﬁcantly. In terms of nuclear spin Hamiltonian comprising of the
Zeeman term and a perturbing nuclear dipole–dipole interaction,
and with the valid assumption of conservation of the total energy

Figure 6. Fluorine spin–lattice relaxation rate from dynamical models and crossrelaxation (overlap integral) at DTNI = 1 °C.

(ns)of the Hamiltonian, the generalized question of spin relaxation
phenomenon is connected essentially with ﬁnding out the probability of resonant absorption at a given frequency. This in turninquires, particularly in the SC regime, into the rate at which the
Zeeman and dipolar energies could come into mutual equilibrium[31]. In the presence of signiﬁcant overlap of lines, the resulting
transition probability for the Zeeman energy of a spin to be converted into dipolar energy leads to a characteristic time (T21),
which is intermediate in nature between the spin–spin relaxation
time and the spin–lattice relaxation time. This process will become
rapidly ineffective if the difference in the Zeeman splittings of thetwo spins becomes larger than the spin–spin (dipolar) coupling
among them. The simpliﬁed expression for T 1
21 , assuming speciﬁedline shapes g(m) for the two NMR lines (of proton and ﬂuorine) is
given by [31]T 1
21 ¼ AOLZZ

g H ðm0 Þg F ðm00 Þdðm0  m00 Þdm0 dm00

ð9Þ

with

AOL ¼ jhEH ; EF jHdHF jEH  hm; EF  hmij2 :Here AOL is related to the square of the matrix element ofHamiltonian involving simultaneous energy-compensating ﬂipﬂops of the two spin species mediated by appropriate terms inthe dipolar interaction. To proceed further, we assume Lorentzian
shapes for both the resonance lines with widths DH and DF, forproton and ﬂuorine respectively. The resultant contribution to theﬂuorine relaxation rate, denoted by R1FOL ¼ T 1
21 , is clearly dependent
on the extent of overlap of the two resonance lines. With thisadditional mechanism, we express the total computed contribution
to the ﬂuorine relaxation as a sum of the dynamic contribution R1F
from WC regime and cross relaxation rate R1FOL with proton:

RT1F ¼ R1F þ R1FOL :We now account for the differential contribution in the SC regime(Figure 6), (R1EXPT  R1F), as arising from R1FOL, and ﬁt the corresponding data to Eq. (9) to obtain the three variable parameters(AOL, DH and DF). In Figure 6, we also include R1FOL, along with contributions from dynamic models (R1F) from Eq. (8) to compare thetotal computed relaxation rate with the experimental data. In viewof the limited number of data points pertaining to this region of
overlap (due to instrument limitations) the data ﬁt to R1FOL in termsof Eq. (9) is to be viewed more as an interesting demonstration ofthis mechanism that could be detected in the FCNMR experiment,
and the values of the ﬁt parameters are thus seen as providing reasonable order-of-magnitude estimates. We observe that the widthof the Lorentzian line of the protons is about 4 kHz while that of
the ﬂuorine is about 40 kHz. It may be further pointed out thatthe line widths obtained here are intrinsic and are free from the
instrument artifacts, since the observations pertain to dipolar interactions between nuclei residing on the same molecule. The larger
ﬂuorine line width is consistent with our interpretation of the SC
regime relaxation data based on much slower local modes: they
contribute more to the spectral density J(0) as probed by ﬂuorine,and hence to its line width. While the ﬂuorine relaxation is seen

Author's personal copy

M. Rajeswari et al. / Chemical Physics Letters 531 (2012) 80–85

to be profoundly inﬂuenced in the SC regime by T21 process, such is
not the case with protons. There are several protons residing on themolecule having homo-nuclear dipolar interaction all through thedispersion regime (and thus having appreciable overlap integral
values always), and addition of one more such coupling with ﬂuorine at the onset of the SC regime is obviously not making an observable difference.
Figure 5b shows the temperature dependent spin–lattice relaxation data of ﬂuorine at different Larmor frequencies. At 10 MHz
the relaxation rate is independent of temperature, and at 1 and
0.2 MHz it shows two maxima within the temperature range of
our study. These maxima in the low frequency data seems to betray the presence of possibly several slow processes in the system,
but their quantitative interpretation however is difﬁcult, complicated by the additional cross-relaxation process triggered in thisfrequency range.
5. Conclusions
In this Letter we presented results of our measurements of proton and ﬂuorine spin–lattice relaxation rates in the isotropic phaseof a liquid crystal 4OTOLFo. We ﬁnd that the major contribution to
the ﬂuorine relaxation arises from spin–rotation interaction, while
proton relaxation could be accounted from the known dipolar
relaxation pathways. Proton dispersions provide an insight into
the individual reorientational dynamics and collective orderparameter ﬂuctuations near the isotropic-nematic phase transition. The ﬂuorine dispersion proﬁles, showing strong coupling to
the lattice and strong dispersion extending to very low frequencies,
required invoking of other slower processes, as has been alreadysuggested by earlier ESR studies. This is traced to the presence ofslowly relaxing local structures, coupling to the ﬂuorine systemrather exclusively through spin–rotation interaction. We also observed that at low enough frequencies the dipolar interaction between proton and ﬂuorine transits from weak to strong coupling
limit, and acquires a degree of homo-nuclear character. This in turnprovides an additional relaxation path to ﬂuorine via cross-relaxation. Protons are not particularly sensitive in reporting on thismechanism in our experiment because of their abundant homonuclear dipolar coupling within the molecule. The present FCNMR
experiment demonstrated clearly the effects of level crossing between the two spin systems through low frequency enhancementof the lattice coupling of the rare spin species (19F). The differing
temperature dependences of the relaxation rates also point to the85

presence of qualitatively different pathways to the spin–lattice
relaxation of the two spin systems.
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