STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2001
EFFECT OF DEUTRATION ON THE VIBRATIONAL SPECTRA
OF ORGANIC MOLECULES
FERENC BILLES, HENRIETTA ENDRÉDI and BALÁZS VÁRADY
Department of Physical Chemistry, Budapest University of Technology and Economics,
H-1025 Budapest, Budafoki út 8, Hungary. E-mail: billes.fkt@chem.bme.hu
ABSTRACT. The deuteration of organic molecules is a very important tool for the
assignment of their vibrational spectra. This article deals with the theoretical and
experimental problems of the effect of deuteration on the vibrational spectra. The
authors introduce several examples, the results of their own measurements and
calculations and interpret the effects observable these vibrational spectra.
1. Introduction
The deuteration of organic compounds means the change of 1H atom for 2H
ones. From the point of view of deuteration of the H atoms of organic molecules can be
divided into two groups.
The hydrogens belonging to the first group are easily changed. Simply, one have
to shake the solution of the compound in heavy water for a longer time or boiling the
solution with reflux for a longer time. These hydrogen atoms participate in very polar
bonds and therefore they are „mobile". On can find such kind hydrogens e.g. in OH,
NH or COOH groups.
The deuteration with heavy water has a great drawback: the result is an
equilibrium between the hydrogenated and the deuterated compounds.
The mentioned process does not act on the second type hydrogens that are
„stable" in comparison of the first type ones. The hydrogen atoms of these type take
part in slightly polar bonds. The hydrogen atoms in the CH groups are representatives
of these group. The deuteration of these H atoms is more complicate. One have to
synthesize the deuterated compound starting the preparation with simpler deuterated
compounds. This method is more complicate than that with heavy water, however, it
yields a compound deuterated theoretically perfectly, practically near to 100 %.
The hydrogens are very stable in the C-H bonds. However, sometimes the
electronic effect of the neighbouring N atoms act on these bonds and the C-H hydrogen
can be changed for deuterium through shaking with heavy water.
This article deals with our own results in this field.
2. Vibrational spectroscopy and deuteration
The deuteration redouble the atomic mass of 1H and this has a very large effect
on the vibrational spectra of the molecule.
According to the theory of the vibrational spectra [1] the calculation of the
fundamental frequencies of a molecule can carried out according to the equation for the
matrix eigenvalues
GF  E  0
EFFECT OF DEUTRATION ON THE VIBRATIONAL SPECTRA OF ORGANIC MOLECULES
where G is the so-called inverse kinetic energy matrix, depending on the atomic masses
and the geometric parameters of the molecule. F denotes the force constant matrix
representing the intramolecular forces. Its estimation is difficult. The best way for the
determination of F is the calculation applying quantum chemical methods [2]. Besides,
the equilibrium geometry of the molecules is also calculable in this way. E is a unit
matrix. : containing the vibrational frequencies,
  4 2 c 2~ 2
where c stands for velocity of light in vacuum and the vibrational wavenumbers are
denoted with ~ .
In the case of isotopomers very good approximations are widely applied: the
intramolecular forces (matrix F) and the molecular geometry are independent of the
isotopic change. Of course, this is applicable also for the deuteration, too. That means,
during the calculation of the vibrational frequencies of isotopomers only the changes in
the atomic masses in the matrix G are considered.
The most important methods of the experimental vibrational spectroscopy are
the infrared and the Raman ones.
How does act the deuteration on the experimental spectra?
Compounds with „mobile" hydrogens build often intermolecular hydrogen
bonds. This self-association is reflected in the energy levels of the molecule. The
hydrogen bond building causes electronic shifts in the molecules. This effect decreases
the energy levels of the molecule and in this way weakens the XH bonds.
The environments of the XH (X=O,N) bonds are very different, i.e. the energy
levels of molecules in the molecular ensemble differ slightly each from the other. The
result is a nearly normal distribution of the molecular energy levels.
The effect of association on the infrared and Raman spectra is different.
The characteristic XH stretching bands shift to lower frequencies in both
infrared and Raman spectra.
The intensities of infrared spectral bands depend on the changes in dipole
moments during the vibration of the vibrational mode where the XH stretching motion
dominates. The association increases the dipole moments. As a result of the hydrogen
bond building the XH stretching bands shift to lower frequencies and broaden. Very
strong, sometimes very broad association bands are observed, sometimes they cover
other weaker bands.
The intensities of the Raman bands depend on the changes in the molecular
polarizability during the vibration of the vibrational mode where the XH stretching
motion dominates. The XH stretching bands shifted and broaden similarly like in the
infrared spectrum but he bands are less intense. The association structure is hardly
observable, the bands covered in the infrared spectrum by association bands are here
quite well observable.
The deuteration changes the G matrix , some of its diagonal elements decrease.
Therefore the characteristic wavenumbers of the XD stretching bands are in good
approximation 1/ 2 fold the wavenumbers of the XH stretching bands.
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FERENC BILLES, HENRIETTA ENDRÉDI and BALÁZS VÁRADY
The deuteration of „mobile" hydrogen atoms decreases the self-association.
Since, however, the deuteration results in an equilibrium this is useful only in the cases
if the equilibrium is shifted to a large manner to the deuterated compound and/or the
association is not very strong. Otherwise, this method is not very effective.
The deuteration of the stable hydrogen atoms can help in the assignment of the
spectral bands, with its shift and intensity changes. The deuteration ceases also Fermi
resonance and so the Fermi bands disappear and therefore it helps in the elucidation of
the Fermi resonance.
3. Examples
The next figures illustrate the mentioned effects of the deuteration. The effects
are introduced on the vibrational spectra of five molecules. The structure of furan,
phenol, pyrrole and tetrazole are presented in Fig. 1, while the same of bilirubin is
shown in Fig. 2.
Furan
Phenol
Tetrazole
Pyrrole
138
EFFECT OF DEUTRATION ON THE VIBRATIONAL SPECTRA OF ORGANIC MOLECULES
Fig. 1. The molecular structure of furan, phenol, pyrrole and tetrazole.
60
O
64
H
H
62
C
54
59
H
O
61
56
H66
63
C
65
H
H57
53
C
C
H55
C
58
H
C13
12
C
C
77
H
O
H31
C
29
C
1
H
N 21
33
H
C
H
30
C
H
28
22
C 15
N 23
78
H
C
16
C
C9
H 25
H 47
68
67
H
C
N 24
32
C
3
C
H
C 36
H 38
C
C6
5
C
H 35
C 34
37
8
45
H
46
43
C
7
C
H
H 76
40
C 39
H H 42
O 50
C C
18
H
72
C 71
C
19
2
4
69
C 17
H26
27
N
70
H
14
11
10
H
H79
C
O 20
49
H48
51
O
H
52
73
75
H
H 74
44
H 41
BILIRUBIN
Fig. 2. The molecular structure of bilirubin.
Fig 3 gives the vapour infrared spectra of phenol, phenol-d5 and phenol-d6 in
the 3700 to 2100 cm-1 region [3]. The spectrum of phenol-d6 shows the mixture of
phenol-d5 and phenol-d6, the OH stretching band about 3650 cm-1 is more intense than
the OD stretching band at 2700 cm-1: the equilibrium is shifted to the 1H isotopomer.
However, the deuteration of the CH hydrogens is perfect: the broad band of the CH
stretching shifts from about 3150 to about 2250 cm-1.
Fig. 4 contains the neat infrared spectra of pyrrole, pyrrole-ND and pyrrole-D5
[4]. The N-deuteration shifts the NH stretch band from 3400 cm-1 to 2520 cm-1. The
equilibrium is shifted to the 2H isotopomer. It is interesting, the last band in the spectrum of
the D5 compound shifts to 2560 cm-1. At the same time the decrease of the intensity of
the NH stretch band indicates the shift of the equilibrium to ND. This fact proofs that
the deuteration of the CH hydrogens affect on the equilibrium of the isotopomers.
Fig. 5 introduces the infrared and Raman spectra of tetrazole-d0 and tetrazoleND [5]. The ND deuteration was carried out with heavy water. The infrared spectrum
of the non-deuterated tetrazole reflects the very strong intermolecular interactions. The
deuteration of the „mobile" hydrogens decreases a little the intensity of the association
band but shows that the equilibrium is shifted to tetrazole-NH isotopomer. However,
with this deuteration the CH stretching band is shifted from the position about 3150 cm-1 to
about 2300 cm-1 indicating the perfect deuteration of the CH group of tetrazole. The
deuteration of the CH group is observable similarly in the Raman spectrum. Comparing the
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FERENC BILLES, HENRIETTA ENDRÉDI and BALÁZS VÁRADY
infrared and the Raman spectra one can conclude that the Raman spectrum is hardly
sensitive to the association.
Vapour infrared spectra
Absorbance
0.06
0.04
0.02
0.00
Phenol-D0
3600
0.010
3200
2800
2400
2800
2400
2800
2400
Phenol-D5
0.005
0.000
-0.005
0.025
0.020
0.015
0.010
0.005
0.000
3600
3200
Phenol-D6
3600
3200
Wavenumber / cm -1
Fig. 3. Infrared spectra of vapour phase phenol, phenol-D5 and phenol-D6 in
the 3700 - 2100 cm-1 region.
Liquid infrared spectra
0.6
0.4
Pyrrole-D0
0.2
Absorbance
0.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
3600
3200
2800
2400
Pyrrole-ND
3600
3200
2800
2400
Pyrrole-D5
3600
3200
2800
2400
2000
Wavenumber / cm -1
Fig. 4. Infrared spectra of liquid pyrrole, pyrrole-ND and pyrrole-D5 in the 3650 - 2000 cm-1 region.
140
EFFECT OF DEUTRATION ON THE VIBRATIONAL SPECTRA OF ORGANIC MOLECULES
0.20
4
Tetrazole IR
0.15
Tetrazole RA
3
0.10
2
1
0.00
4000
0.4
3000
2000
0
4000
2.5
1000
ND-Tetrazole IR
2.0
0.3
1.5
0.2
1.0
0.1
0.5
2000
1000
ND-Tetrazole RA
0.0
0.0
4000
3000
Intensity / a.u.
Absorbance
0.05
3000
2000
-0.5
4000
1000
3000
2000
1000
Wavenumber /cm -1
Fig. 5. Infrared (4000 - 400 cm-1) and Raman spectra (4000 - 200 cm-1) spectra
of tetrazole and deuterated tetrazole.
Fig. 6 presents the infrared and Raman spectra of a large molecule, the bilirubin
[6], see Fig. 1. The molecule contains NH and COOH groups as proton donors and
acceptors and C=O groups as proton acceptors. The deuteration of the „mobile"
hydrogens decreases the intensity of the stretching band of the non-associated NH
groups (about 3400 cm-1) and a new band appears about 2500 cm-1 belonging to the
stretching of the non-associated ND groups. The building of intermolecular hydrogen
bonds by the NH and COOH groups is spatially hindered. The NH and OH stretching
band region of the Raman spectrum is practically insensitive to the deuteration.
0.8
Bilirubin
MIR FIR
6
Bilirubin
RAMAN
0.6
4
0.4
2
0
0.0
4000
0.8
0.6
3000
2000
1000
0
Deuterated
MIR FIR
4000
6
4
3000
2000
1000
0
2000
1000
0
Deuterated
RAMAN
Intensity /a.u.
Absorbance
0.2
0.4
2
0.2
0
0.0
4000
3000
2000
1000
0
4000
3000
Wavenumber /cm-1
Fig. 6. Infrared (4000 - 100 cm-1) and Raman spectra (4000 - 200 cm-1) of bilirubin.
141
FERENC BILLES, HENRIETTA ENDRÉDI and BALÁZS VÁRADY
Figs. 7 and 8 show the infrared and Raman spectra of furan, respectively [7]. The
effect of deuteration is introduced on both experimental and calculated (simulated)
spectra. Since the molecule contains only „stable" hydrogen atoms the efficiency of the
deuteration is practically perfect, as it was awaited. The calculated band positions are
very close to the measured ones both in infrared and Raman spectra but the calculated
intensities differ sometimes from the measured ones. Namely, the approximation used
for the band intensities by the quantum chemical methods is weak. However, better
method does not exist so far.
F u r a n in f r a r e d s p e c t r a
0 .6
1 .0
0 .5
0 .8
0 .4
D 0 c a lc u la te d
D 0 m easu red
0 .6
0 .3
0 .4
0 .2
A b so rb a n c e
0 .0
0 .0
4000
3000
2000
1000
1 .0
4000
3000
2000
1000
1 .0
0 .8
0 .8
D 4 m easu red
D 4 c a lc u la te d
0 .6
0 .6
0 .4
0 .4
0 .2
0 .2
0 .0
I n te n s ity / k m m o l - 1
0 .2
0 .1
0 .0
4000
3000
2000
1000
4000
3000
W a v e n u m b e r /c m
2000
1000
-1
Fig. 7. Experimental and calculated infrared spectra of furan and furan-D4
in the 4000 - 400 cm-1 region.
F u r a n R a m a n sp e c tr a
2 .5
16
2 .0
12
8
1 .0
4
0 .5
0
0 .0
3000
2000
1000
3000
2000
1000
2 .0
1 .5
D 4 m easu red
D 4 c a lc u la te d
a m u -1
30
4
40
I n te n s ity / A
I n te n s ity / a .u .
D 0 c a lc u la te d
1 .5
D 0 m easu red
1 .0
20
0 .5
10
0 .0
0
3000
2000
1000
3000
W avenum ber / cm
2000
1000
-1
Fig. 8. Experimental and calculated Raman spectra of furan and furan-D4
142
EFFECT OF DEUTRATION ON THE VIBRATIONAL SPECTRA OF ORGANIC MOLECULES
in the 4000 - 200 cm-1 region.
With Fig. 9 we return to the spectra of phenol [3]. The infrared spectra of the
CCl4 solutions of the parent molecule and its full deuterated derivative were recorded
and the corresponding spectra of the isolated molecule were calculated. The
calculations cannot consider the existence of the equilibrium and the association. Of
course, these are also absent in the spectra calculated for isolated molecules. The
diluted solutions show very weak association bands. The equilibrium is shifted to the
direction of the deuterated isotopomer, in contrary to the vapour phase (see Fig. 3).
Infrared spectra /CCl solutions
4
0.4
0.3
0.3
Penol-D6, measured
Phenol, measured
0.2
0.2
0.1
0.0
0.0
4000
3.5
3000
2000
1000
3.0
2.5
4000
3.5
2000
1000
3.0
Phenol, calculated
2.5
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
4000
3000
Phenol-D6, calculated
Intensity / km mol-1
Absorbance
0.1
0.0
3000
2000
1000
4000
3000
2000
1000
Wavenumber /cm-1
Fig. 9. Experimental ( in CCl4 solution) and calculated infrared spectra of phenol
in the 4000 - 400 cm-1 region.
4. Conclusions
The examples tried to support the effects explained in the theoretical part. The
deuteration of „mobile" hydrogens are in close connection with the investigation of
hydrogen bonds. The deuteration of the „stable" hydrogens can help in the assignment
of the vibrational spectra. Calculated (simulated) infrared and Raman spectra show the
spectrum of the isolated molecule. The calculated frequencies are close to the
experimental ones, while the intensities are sometimes far from the measured ones. The
calculations cannot reflect the association bands and the isotopic equilibria but can
help in the assignment of the spectra.
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FERENC BILLES, HENRIETTA ENDRÉDI and BALÁZS VÁRADY
REFERENCES
1. Wilson, E.B., Decius, J.C., Cross, P.C.:Molecular vibrations (McGraw-Hill, New York, 1953).
2. Gribov, L.A., Orville-Thomas, J.W.: Theory and methods of calculation of molecular spectra
(J. Wiley and Sons, Chichester, 1988).
3. Keresztury, G., Billes, F., Kubinyi, M., Sundius, T.: J. Phys. Chem., A102, 1371-1380 (1998).
4. Geidel, E., Billes, F.: J. Mol. Structure THEOCHEM 507, 75-87 (2000).
5. Billes, F., Endrédi, H., Keresztury, G.: J. Mol. Structure (THEOCHEM), 530, 183-200 (2000).
6. Billes, F,. Várady, B.: article in preparation.
7. Billes, F., Böhlig, H., Ackerman, M.: article in preparation.
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