Study of NMR spin-lattice relaxation mechanism and mutual viscosity in some substituted alcohols
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Transcript of Study of NMR spin-lattice relaxation mechanism and mutual viscosity in some substituted alcohols
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www.elsevier.com/locate/molliq
Journal of Molecular Liqu
Study of NMR spin-lattice relaxation mechanism and
mutual viscosity in some substituted alcohols
Anupam Singh, A.K. Singh, N.K. MehrotraT
Department of Physics, Lucknow University, Lucknow-226 007, India
Received 16 April 2003; accepted 10 October 2004
Available online 17 May 2005
Abstract
The present communication reports the experimental values of NMR spin-lattice relaxation time (T1) and dielectric relaxation time (s) ofa-naphthol, h-naphthol, o-aminophenol, benzyl alcohol, phenol, pyrogallol, catechol, and the experimental values of mutual viscosity g12 of
o-aminophenol, m-aminophenol, and p-aminophenol. The correlation of mutual viscosity with dielectric relaxation time (s) of the
compounds investigated leads to the conclusion that mutual viscosity is a better representation of the resistance to the rotation of the
individual solute molecule. The experimental values of T1 have been correlated with calculated values of T1 obtained using various equations
of dielectric relaxation time. It has been concluded that Murty’s equation is a better substitute of dielectric relaxation phenomenon.
D 2005 Elsevier B.V. All rights reserved.
Keywords: NMR spin-lattice relaxation time (T1); Mutual viscosity (g12); Dielectric relaxation time (s)
1. Introduction
The dielectric investigation of relaxation and nuclear
magnetic resonance studies of organic polar complexes
having different dipole bearing groups, provide useful
information about the structure of the molecules. The
dielectric relaxation time is very intimately connected with
the molecular motion and intramolecular interaction in
molecular species.
NMR relaxation time T1 has been used to investigate the
rotational and translational motions and their relations to
molecular structure, size, shape, and intramolecular forces
causing internal friction. The chemical shift of the proton
depends on the various substituent groups at different
positions and is affected when positions of the substituents
are interchanged or when one polar group is replaced by
another one. Therefore, the measurements of chemical shift,
spin-lattice relaxation time, dielectric relaxation time, and
mutual viscosity are important for the study of molecular
structure and intramolecular forces.
0167-7322/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2004.10.041
T Corresponding author. Tel.: +91 522 2376230.
E-mail address: [email protected] (N.K. Mehrotra).
Many workers [1–3] calculated the values of T1 from
BPP theory [4] and found that the calculated values ranged
from 1/2 to 1/10 of the experimental values. The possibility
of narrowing the gap between the experimental and the
calculated values stimulated the work reported in this paper.
We also wish to find out for these molecular systems that the
dipole orientation process is due either to molecular or to
both molecular as well as intramolecular rotations [5,6]. The
values of dielectric relaxation time (s) calculated using
Debye equation [7] were found to be nearly 5 to 10 times
the experimental values. This difference between the
calculated and observed values of the relaxation time was
explained by Hill [8], who suggested that the microscopic
viscosity of the solvent (g1) should be replaced by the
mutual viscosity of solute and solvent (g12) which is a
measure of solute–solvent interaction.
2. Materials and methods
The dielectric relaxation times have been determined
using the fixed frequency method of Gopal Krishna for dilute
solutions, using a microwave bench of 3.13-cm wavelength.
ids 121 (2005) 110–114
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A. Singh et al. / Journal of Molecular Liquids 121 (2005) 110–114 111
The microwaves were generated by a 723 A/B Klystron and
dielectric constant eV and losses e Wwere determined using the
standing wave technique of Roberts and Von Hippel [9] and
its subsequent simplification by Dakin and workers [10]. The
accuracy in the measurement of dielectric constants and loss
tangents was F5% to F2%, respectively.
Viscosities were measured with an accuracy of F2%
with a Hoppler precision viscometer. This method requires
the determination of time of fall of a glass or a metal ball
between two marks in a glass tube filled with a liquid with
known density. Viscosity of the liquid (gm) is calculated
using the relation gm=F(SK�SF)K, where F is the time of
the fall of the ball in seconds. SK is the specific gravity of
the ball, SF is the specific gravity of the liquid, and K is the
ball constant (=0.009495).
NMR experiments were performed with a Bruker Avance
DRX 200 M Hz FT-NMR spectrometer, equipped with a 5-
mm multinuclear inverse probe head with z-shielded
gradient. For normal proton experiments, typical exper-
imental conditions are as follows.
Flip angle 908, spectral width 4139.073 Hz, data size 32
K; relaxation delay 5 sec; number of transients 8. The FIDs
were line broadened by 0.3 Hz prior to Fourier trans-
formation. The sample concentrations were kept in the range
of 32 to 50 mM.
For T1 experiments the inversion recovery method
(1808–s–908) of Freeman and Hill [11] was used in each
system for the evaluation of nuclear spin-lattice relaxation
time. The time chosen initially was 10 sec which varied in
graduated manner in order to obtain correct phase modu-
lation of the series of NMR spectra in each system so as to
calculate accurately the spin-lattice relaxation time T1
values. The experiments were performed in automation
mode using a standard pulse programme from the Bruker
software library.
All the compounds used were of pure quality obtained
from M/s British Drug House Ltd., England. The percentage
purity of the compounds investigated ranged from 99.8% to
99.9%. Purest quality of deuterated benzene, dioxane, and
heptane obtained from M/s British Drug House were
distilled before use.
3. Theory
Earlier investigations by Meakins [12] and Roberti et al.
[13] on the dielectric dispersion in a number of organic
polar compounds show that there is a big discrepancy
between observed values of relaxation time and those
calculated using Debye’s equation. This discrepancy can
be explained if we use mutual viscosity g12 representing the
interaction between both solute and solvent molecules
instead of solvent viscosity g1. The expression of g12 as
proposed by Hill is given by
gmrm ¼ X 21 r1g1 þ X 2
2 r2g2 þ 2X1X2g12r12 ð1Þ
where gm, g1 and g2 are the coefficients of viscosity of
solution, solvent, and solute, respectively. X1 and X2 are the
mole fractions of the solvent and the solute and the
quantities r represent the average intermolecular distances.
On rearrangement, the Eq. (1) yields
gmrm � X 21 g1r1
X 21 r2
¼ g2 þ 2X1
X2
�r12
r2
�g12
��ð2Þ
This equation represents a straight line, the slope of
which gives the mutual viscosity g12.Bloemberger et al. [4] have derived an expression for the
magnetic relaxation in terms of correlation time (sc) whichis closely related to Debye’s theory of the dielectric
dispersion in polar liquids as discussed in our earlier paper
[14].
NMR spin-lattice relaxation of a single nuclear spin in
a liquid is induced by the fluctuating local magnetic field
of neighbouring spins. If the spin which induces the
relaxation is attached to the same molecule as the relaxing
spin, the fluctuating field is produced by the molecular
reorientational motion. The contribution of this mecha-
nism to the overall T1 is denoted by (T1)rot. If the
relaxation which occurs when relaxing spin and spin
which induces relaxation are attached to different mole-
cules the contribution of this mechanism to overall T1 is
denoted by (T1)trans. Bloemberger et al. have calculated
the probability of induced transition and the equations
thus obtained for (T1)rot and (T1)trans are given in our
earlier paper [15].
4. Results
The chemical shift positions and NMR spin-lattice
relaxation times of various protons of pyrogallol, catechol,
a-naphthol, h-naphthol, benzyl alcohol, and phenol are
given in Table 1. Table 2 shows the values of mutual
viscosity (g12) and the ratios (s/g1) and (s/g12) of ortho-,
meta-, and para-aminophenols. The experimental and
calculated values of dielectric relaxation time s and NMR
spin-lattice relaxation (T1) of these compounds at 293 K are
given in Tables 3 and 4.
5. Discussion
5.1. Chemical shift
In the 1H spectrum of a-naphthol, the peak multiplicity
of the Ha proton is a triplet at lower field region, due to the
presence of the ortho-directing –OH group. The doublet of
the Hb proton is resonating at higher field region, due to
shielding effect. The multiplet of the Hc protons is
resonating at 7.08 ppm. The triplet of the Hd proton is
resonating at slightly lower field region, due to ortho
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Table 1
Chemical shift position (d) and NMR spin-lattice relaxation time (T1) of various protons
Polar compound
β-Naphthol
O-Aminophenol
Hd
NH2
OH
Ha
Hb
HcBenzyl alcohol
Ha
C
Hb
Ha
Ha
Ha
Phenol
Hb
Hb
Pyrogallol
Ha
Hb
OH
Hb
Ha
Ha
OH
OH
OH
Hb
Ha
Hb
OH
Hc
Hc
HbHd
Hd
Ha
Hb
Hc
OH
Hc
Hc Hb
Hd
Hb
Ha
Proton Statistical average of NMRspin-lattice relaxationtime T1 (Sec)
Chemical shift(δ) ppm
HaHbHcHd
4.31
8.13 6.09 7.12 7.44
HaHbHcHd
3.79
6.87 7.61 7.35 7.01
HaHbHcHd
4.94
6.75 6.62 6.42 6.30
2.94 HaHb
6.907.15
4.79 HaHb
2.404.50
α-Naphthol
OH
3.59 HaHb
5.206.81
A. Singh et al. / Journal of Molecular Liquids 121 (2005) 110–114112
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Table 2
Values of dielectric relaxation time (s), mutual viscosity (g12), and the ratio
(s/g1), (s/g12) for the investigated compounds (viscosity of dioxane at 293
K, g1=1.42 cps)
Polar compounds sExpa (s/g1)�1010 (g12) cps (s/g12)�1010
o-Aminophenol 7.47 5.270 1.469 5.98
m-Aminophenol 7.05 4.980 1.450 4.85
p-Aminophenol 6.22 4.390 1.440 4.30
a Ref. [17].
A. Singh et al. / Journal of Molecular Liquids 121 (2005) 110–114 113
coupling with neighbouring proton. So, we obtain a double
doublet at lower field region due to the increase of electron
density at orthoposition. The peak multiplicity of the Ha
proton of h-naphthol is doublet at higher field region due
to ortho coupling with neighboring proton. The triplet of
the Hd proton resonates at slightly lower field due to the
presence of the neighboring phenolic ring. Similarly, the
triplet of the Hc proton also resonates at lower field region,
which is ortho-coupled with the Hb and Hd protons. The
PMR spectrum of o-aminophenol have complex spectrum
pattern. The Ha proton of o-aminophenol resonates at lower
field region, due to the presence of the ortho-directing –
OH group. The Ha proton is ortho-coupled with neighbor-
ing protons. So, we obtain a triplet at 6.75 ppm. The triplet
of the Hb proton resonates at slightly lower field region,
due to ortho coupling with neighboring protons. The peak
multiplicity of Hd is doublet at higher field region, due to
the presence of the –NH2 group. In the 1H spectrum of
benzyl alcohol Hb protons of –CH2 are slightly shielded.
Hence, we obtain the singlet at 4.50 ppm. The protons of
the aromatic ring resonating at lower field region, due to
deshielding effect. The phenolic proton peak is usually a
sharp singlet, there is no coupling, and its range depends
on concentration dependence of the –OH peak. In the
NMR spectrum, the orthoposition Hb protons of phenol are
resonating at slightly lower field region due to the presence
of the ortho-directing –OH group. The remaining protons
of the phenol ring are resonating at 6.90 ppm. The
statistical average of the overall relaxation time for the
obtained NMR relaxation time for pyrogallol and catechol
can be explained similarly.
Table 3
Values of dielectric relaxation time (s) in 1012 sec at 293 K for compound
studied
Polar compound (s)
Exp Debye Perrin Writz Murty
Pyrogallol 18.30a 91.30 32.86 15.05 18.05
Catechol 16.40a 83.80 30.17 9.81 16.06
a-Naphthol 8.30a 59.18 21.31 8.54 8.42
h-Naphthol 6.64a 56.35 20.28 7.39 6.22
Benzyl alcohol 6.70b 46.68 16.80 7.26 5.44
o-Aminophenol 7.47a 81.24 29.24 10.38 8.12
Phenol 6.17c 25.15 9.05 8.80 7.34
a Ref. [17].b Ref. [18].c Ref. [19].
5.2. Mutual viscosity
The result tabulated in Table 2 shows that the dielectric
relaxation time (s) of aminophenols decreases from ortho
via meta to para compounds. The ratio s/g1 for these
compounds also decreases in the same order as s. But
according to Debye’s theory this ratio s/g1 should remain
constant for molecules of similar size. This anomaly can be
explained if g12, mutual viscosity of the solute and solvent,
is used in place of solvent viscosity (g?). As g12 also
decreases from ortho via meta to para compounds, the ratio
(s/g12) is almost constant. Mehrotra et al. [16] have also
found similar results in the case of some substituted
benzaldehydes.
5.3. Dielectric relaxation time
It is observed from Table 3 that the relaxation time of
phenol is found to be smaller than that of o-aminophenol.
This can be explained on the basis of the intramolecular
interaction to rotation of the –OH group by the neighbour-
ing –NH2 group in the latter molecule while in the case of
phenol the –OH group has greater freedom of rotation round
its bond with the benzene ring resulting in the decrease of its
relaxation time.
The higher value of relaxation time of pyrogallol as
compared to catechol can be attributed to the greater steric
hindrance to the intramolecular rotation of the three –OH
groups resulting in the increase of the relaxation time of the
former molecule.
The relaxation time of a-naphthol is slightly greater than
that of h-naphthol although both are of similar size. This is
probably due to the greater steric hindrance experienced by
the former molecule in molecular and intramolecular
motions.
5.4. NMR spin-lattice relaxation time
It is evident from Table 4 that the values of spin-lattice
relaxation time calculated using the BPP equation are
smaller than the experimental values. Moniz et al. [1] also
agree with the view that BPP treatment gives much smaller
values of (T1), but according to them the discrepancy in
results is due to the time dependence of the rotational
Table 4
Values of NMR spin-lattice relaxation time T1 (in sec.) of 293 K
Polar compound T1
Exp Debye Perrin Writz Murty
Pyrogallol 3.59 2.14 3.16 3.70 3.60
Catechol 3.89 2.32 3.41 4.15 3.88
a-Naphthol 4.31 3.21 4.67 5.46 4.47
h-Naphthol 3.79 3.00 4.15 4.82 3.88
Benzyl alcohol 4.79 3.22 4.19 4.76 4.87
o-Aminophenol 4.94 2.52 3.80 4.66 4.78
Phenol 2.94 2.74 3.16 3.17 2.90
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A. Singh et al. / Journal of Molecular Liquids 121 (2005) 110–114114
angular auto-correlation function of these molecules. They
suggested that this time dependence is dominated by
dynamical coherence rather than by the frictional forces
as used in the BPP theory. When Writz and Perrin equation
is used, a better correlation is obtained. This is probably due
to the introduction of the microfriction factor in the
equation.
However, the values of (T1) calculated using Murty’s
equation and the experimental values of (s) are in
quantitative agreement with the experimental values. This
is probably due to the polarizability of molecule used for the
calculation of the dielectric relaxation time s.
6. Conclusion
It is concluded that Murty’s equation is a better substitute
for the correlation time in BPP equation for T1 and mutual
viscosity (g12) is a better representation for inner friction
experienced by the rotating solute molecule. It has been
observed from the structural studies of these molecular
species that the process of dipole orientation is contributed
by both molecular as well as intramolecular rotations.
Acknowledgement
The authors are deeply indebted to Dr. G.P. Gupta,
Professor and Head of the Physics Department for the
encouragement and keen interest throughout the progress of
the work. Thanks are also due to Dr. Raja Roy, Scientist
Incharge, NMR Unit, CDRI, Lucknow, for providing the
experimental facility.
One of the authors (AKS) is thankful to the University
Grants Commission, New Delhi, for the award of a
Research Fellowship during this period of research.
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