FT-Raman spectra of n-propanol and selected partially 2H-labelled analogues

Article abstract of DOI:10.1016/j.molstruc.2006.08.018

Fourier-transform Raman spectra of CH₃CH₂CH₂OH and some of its selectively deuteriated analogues have been obtained. Comparisons of the Raman spectra of the protiated and partially deuteriated species, in conjunction with polarization data, has enabled improved vibrational assignments to be made for the C-H modes. As a result, confirmation of some literature assignments of stretching and bending modes and revision of other tentative assignments for large biopolymer molecules have been proposed.

Full text of DOI:10.1016/j.molstruc.2006.08.018

Journal of Molecular Structure 832 (2007) 184–190
www.elsevier.com/locate/molstruc
FT-Raman spectra of n-propanol and selected
2
partially H-labelled analogues
*
H.G.M. Edwards , D.W. Farwell, R.D. Bowen
Chemical and Forensic Sciences, School of Life Sciences, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK
Received 9 November 2005; received in revised form 18 August 2006; accepted 22 August 2006
Available online 5 October 2006
Abstract
Fourier-transform Raman spectra of CH3CH2CH2OH and some of its selectively deuteriated analogues have been obtained. Com-
parisons of the Raman spectra of the protiated and partially deuteriated species, in conjunction with polarization data, has enabled
improved vibrational assignments to be made for the C–H modes. As a result, confirmation of some literature assignments of stretching
and bending modes and revision of other tentative assignments for large biopolymer molecules have been proposed.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Raman spectra; n-Propanol; Deuteriated analogues; Assignments
1. Introduction
anism of transdermal drug delivery is little understood [5,6]
but is believed to involve a lipid transmission mechanism in
which the role of water is crucial; this interpretation has yet
to be related to the spectroscopic changes in the m(CH)
region of the Raman spectrum.
Further problems can arise in the vibrational assignment
of large biological molecules when small molecular species
are used as model compounds; in some cases, the rigidity
imposed by tertiary skeletal structures can substantially
alter the appearance of the spectra. This effect has been
noted in previous work from our laboratories on cellulose
[3] and bacterial cell walls [7], in which the importance of
the b-glycosidic bonds on the biopolymeric structures have
a controlling influence on the vibrational skeletal modes
and their observed spectra.
We have initiated a programme designed particularly to
provide initial identification of m(CH) and d(CH) modes of
simple organic molecules, in order to assist in the assign-
ment of vibrational bands in more complex biological
materials. Improved assignments for the m(CH) and
d(CH2) regions will assist the identification of these fea-
tures in biopolymers, lipids and proteins, thus improving
the understanding of the vibrational modes of which the
skeletal structures are composed. In this paper we have
applied selective deuterium labelling in specially synthesised
In the assignment of the vibrational spectra of large
complex molecules of biological interest including wool
and cotton [1,2] and biopolymers, such as the human ker-
atotic species skin, hair and nail [3,4], the unequivocal iden-
tification of important modes is often difficult because of
overlapping spectroscopic features which arise from
m(CH), m(CO) and m(CN) bands which occur in similar
wavenumber ranges. It is increasingly important for bio-
medical diagnostic applications of vibrational spectroscopy
to be able to assign properly the vibrational bands in mate-
rials which have been modified by biological processes.
Thus, recent Raman spectroscopic studies of diseased and
healthy human tissues [4] revealed that some critical spec-
troscopic differences have been noted in the m(CH) and
d(CH2) regions. In some cases, it has been possible to relate
the observed vibrational band changes to real biological
effects such as the de-lipidization of human stratum corne-
um in the epidermis of diseased tissue samples. Likewise,
the molecular basis for the observed role of chemical
enhancing agents such as dimethyl sulphoxide on the mech-
*
Corresponding author. Tel.: +44 1274 233787; fax: +44 1274 235350.
E-mail address: h.g.m.edwards@bradford.ac.uk (H.G.M. Edwards).
0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.08.018

H.G.M. Edwards et al. / Journal of Molecular Structure 832 (2007) 184–190
185
compounds to address the problems of assigning the m(CH)
2.1.2. CH3CD2CH2 OH
and d(CH2) vibrational bands in a compound (n-propanol)
containing the CH3CH2CH2-moiety, in order to derive
improved molecular vibrational assignments that can be
assimilated into more complex species.
CH3CD2CH@O. A mixture of redistilled n-propionalde-
hyde (32 g, 0.55 mol), deuterium oxide (48 g, 2.4 mol) and
pyridine (4 g, 0.05 mol) was refluxed in a nitrogen atmo-
sphere for 72–96 h. The isotopically enriched aldehyde
was removed by distillation through a 10 cm helix-packed
column (b.p. 47–48 °C) and subjected to two further
exchanges with fresh portions of deuterium oxide (35 g,
1.75 mol) and pyridine (4 g, 0.05 mol). The final yield of
n-propionaldehyde-2,2-d2 having b.p. 47–48 °C was 13.5 g
(41%), but additional batches of aldehyde could be
exchanged in somewhat higher yields using the deuterium
oxide employed for the first batch of aldehyde.
2. Experimental
2.1. Origin of specimens
The compounds investigated in this study comprised
unlabelled n-propanol, CH3CH2CH2OH, together with
three specifically deuteriated analogues, CH3CH2CD2OH,
CH3CD2CH2OH and CD3CH2CH2OH.
All the unlabelled materials (including n-propanol)
required for this work were high purity samples obtained
from Aldrich or BDH. Isotopically labelled starting mate-
rials [D2O, LiAlD4 and CD3CO2D] were obtained from
Aldrich and were used without further purification. The
labelled alcohols were synthesised by the routes summa-
rised in Scheme 1; details of the procedure for the synthesis
of CH3CH2CD2OH and CH3CD2CH2OH are given below;
the preparation of CD3CH2CH2OH has been described
previously [8]. The isotopic purity of the partially labelled
propanols was estimated from their mass spectra to be
96–99%.
2.1.3. CH3CD2CH2OH
A solution of n-propionaldehyde-2,2-d2 (18 g, 0.30 mol)
in triglyme (50 cm3) was added dropwise during a period of
1.3 h to a magnetically stirred suspension of lithium alu-
minium hydride (8.7 g, 0.23 mol) in triglyme (300 cm3)
under a nitrogen atmosphere. The temperature was kept
below 40 °C by means of external cooling (cold water bath)
during the early stages of the addition when the reaction
was moderately vigorous. Stirring was continued over-
night, after which tetragol (50 cm3) was added dropwise
over 1 h. Careful distillation gave an almost colourless
liquid (16.6 g, 89%) having b.p. 95–101 °C (bulk 98–
99 °C); redistillation through a 5 cm helix packed column
gave n-propanol-2,2-d2 (16.8 g, 89%) having b.p. 98–99 °C.
2.1.1. CH3CH2CD2 OH
A suspension of lithium aluminium deuteriide (5.0 g,
0.119 mol) in triglyme [triethylene glycol, dimethyl ether,
CH3O(CH2CH2O)3OCH3, (150 cm3)] was stirred magneti-
cally under a nitrogen atmosphere. The temperature was
maintained between 30 and 50 °C by means of external
cooling (ice/salt bath) during the dropwise addition over
a period of 2 h of a solution of propionyl chloride (19 g,
0.205 mol) in triglyme (50 cm3). Stirring was continued
overnight, after which tetragol [tetraethylene glycol,
HO(CH2CH2O)3OH, (80 cm3)] was added dropwise over
1 h. Careful distillation gave a colourless liquid product
(12.1 g, 95%) having b.p. 96–99 °C; redistillation through
a 5 cm helically packed column gave n-propanol-1,1-d2
(10.4 g, 84%) having b.p. 98–99 °C.
2.2. Raman spectroscopic instrumentation
Fourier-transform Raman spectra were recorded using a
Bruker FRA106 Raman module attachment on an IFS66
infrared optics system. A Nd3+/YAG laser operating at
1064 nm was used as an excitation source and the laser
beam focussed to a 100 lm diameter spot at the sample.
Samples of the propanols were contained in sealed capil-
lary tubes mounted in a specially constructed holder. Laser
powers of up to 500 mW were used with typically 2000
scans at 4 cmꢀ1 spectral resolution being collected; the
average spectral accumulation time was estimated as
60 min per specimen under these conditions. A liquid-nitro-
gen cooled germanium detector with extended spectral
bandwidth was used over the normal scan range of 50–
3500 cmꢀ1; spectra were corrected for the instrument
response and the observed band wavenumbers, calibrated
against the internal laser frequency, are correct to better
than 1 cmꢀ1. The depolarization ratio measurements were
made with an optical vector rotator (90°) in the incident
laser beam.
iv
i
ii,iii
1
CD3CO2D
CD3CH2I
CD3COCl
CD3CH2OH
v, vi, vii
ii,iii
i
CD3CH2CO2H
CD3CH2COCl
CD3CH2CH2OH
CH3CH2COCl
viii,iii
ix
2
3
CH3CH2CD2OH
CH3CD2CHO
ii,iii
CH3CD2CH2OH
CH3CH2CHO
3. Vibrational theory
Scheme 1. Reagents and conditions: (i) excess PhCOCl, distil., (ii) excess
LiAlH4, triglyme, stir 24 h, (iii) excess tetragol, (iv) I2, red P, (v) Mg,
(C2H5)2O, (vi) excess CO2, ꢀ78 °C, (vii) HCl/H2O, (viii) excess LiAlD4,
triglyme, stir 24 h, (ix) excess D2O, pyridine, reflux 72–96 h; repeat twice.
For CS molecular symmetry as indicated diagrammati-
cally in Fig. 1, the vibrational modes may be classified as
follows:

186
H.G.M. Edwards et al. / Journal of Molecular Structure 832 (2007) 184–190
comparison of these spectra reveals the influence of
deuterium substitution on the a-, b- or c-carbon atoms in
the carbon backbone on the molecular vibrations.
The wavenumbers of the bands in the Raman spectra of
the protiated and deuteriated compounds are given in
Table 1 with the proposed vibrational assignments.
Generally, the latter have been deduced from the literature
[9] with specific reference to earlier studies [10] made in our
laboratories for 3-methoxy-1-propene (allyl methyl ether),
CH2@CHCH2OCH3, and deuteriated analogues.
Broadly, the molecular vibrational assignments may be
classified as modes involving CC, CO, OH stretching and
CCC, CCO and COH bending modes (in-plane and out-
of-plane); the in-plane modes will be polarized in the
Raman spectrum for CS molecular symmetry whereas the
out-of-plane modes will be depolarized. For simplicity,
the polarized and depolarized components of the individual
spectra are not shown in the stackplots in Fig. 3, but the
polarization properties of the modes can be seen in
Fig. 4a, b and c for the deuteriated compounds and the
details have been summarised in Table 1. It is appropriate
to consider each vibrational wavenumber region
separately:
Fig. 1. Molecular structure of propanol, CH3CH2CH2OH, on the basis of
CS molecular symmetry; molecular plane of symmetry in plane of paper.
Cvib ¼ 18A0 þ 12A00
all of which are Raman-active and 18 bands are expected to
be polarized in the Raman spectrum.
For the tetra-atomic skeletal modes only, the following
vibrational classification applies:
skeletal
vib
C
¼ 5A0 þ 1A00
for which five bands will be polarized in the Raman spec-
trum, comprising m(CO) and m(CC) stretching (of which
there are two, one involving the terminal carbon atom
and the other the carbon atom bound to the OH group),
and the d(CCO) and d(CCC) in-plane bending vibrations.
All three deuteriated species substituted at the a-, b- and
c-carbon atoms, like the protiated parent molecule, belong
to the CS molecular point group.
4.1. 3400–1900 cmꢀ1 region
4. Results and discussion
The FT-Raman spectrum of protiated CH3CH2CH2OH
is shown in Fig. 2; even for such an apparently simple
molecule, the complexity of the m(CH) and d(CH2) wave-
number regions indicates the problems faced in molecular
assignments. However, investigation of partially deuteriat-
ed propanols in which specific CH2 or CH3 groups have
been replaced by CD2 or CD3 groups should facilitate
the identification of the CH modes which arise from
these units in the molecule; the assumption is made that
there is little or no vibrational coupling between adjacent
CH3 and CH2 groups in the molecules. Consequently, the
isotopically labelled materials chosen for this study were
CH3CH2CD2OH, CH3CD2OH and CD3CH2CH2OH.
The Raman spectra of the three partially deuteriated
analogues of propanol are shown as a stackplot in Fig. 3;
This is the region where m(CH), m(OH) and m(CD)
stretching are expected [9], the strongest m(CH) bands are
found between 2800 and 3000 cmꢀ1 for saturated aliphatic
molecules, whereas the m(CD) modes are expected to lie in
the region of 2000–2200 cmꢀ1. The m(OH) stretching vibra-
tions of medium–weak intensity expected around 3238–
3250 cmꢀ1, occur in the spectra of all samples since the
AOH group is present in each compound. However, deute-
riation at the a-, b- and c-carbon atoms produces a shift of
ꢀ12, ꢀ7 and ꢀ5 cmꢀ1, respectively, from the protiated
molecular m(OH) stretching wavenumber at 3250 cmꢀ1
.
We considered the preparation of a deuteriated analogue
of the n-propanol substituted on the OH group but this
proved rather difficult to purify; since the assignments of
the hydroxyl group are not primarily in question here,
Fig. 2. FT-Raman spectrum of CH3CH2CH2OH. (a) 2500–3400 cmꢀ1 and (b) 300–1700 cmꢀ1; wavelength of excitation 1064 nm; 2000 accumulated scans
at 4 cmꢀ1 resolution.

H.G.M. Edwards et al. / Journal of Molecular Structure 832 (2007) 184–190
187
One CH3 asymmetric stretching band will be depolarized
and each CH2 group will contribute one polarized and
one depolarized stretching band in this wavenumber
region. Hence, for this molecular symmetry, there should
be a total of 4A0 and 3A00 CH stretching modes observable
in the Raman spectrum.
Clearly, examination of the Raman spectra and tabulat-
ed data shown in Table 1 reveals that the medium intensity,
depolarized band at 2970 cmꢀ1 in CH3CH2CH2OH which
is present at 2965 and 2958 cmꢀ1, respectively, for the
CH3CH2CD2OH and CH3CD2CH2OH species and is
absent from the spectrum of the CD3CH2CH2OH species
must therefore be a CH3 mode and should be assigned to
the CH3 asymmetric stretching mode on the basis of the
polarization data. The progressive change in the observed
wavenumber of this CH3 band with CD substitution at
the CH2 group in the alkyl chain is strongly suggestive of
the presence of some degree of vibrational coupling
through the CCC skeleton. Likewise, the medium strong
intensity polarized band in the range 2937–2926 cmꢀ1 can
be assigned to the CH2 symmetric stretching mode as it is
present in all four species studied here. In contrast, the
medium strong intensity depolarized band occurring in
the range 2912–2917 cmꢀ1 is seen to be absent from the
spectrum of the CH3CH2CD2OH molecule and this can
be therefore confidently assigned to the A00 symmetry class
CH2 stretching mode at the a-carbon atom. The CH
stretching band in the range 2878–2874 cmꢀ1 appears as
a polarized band in all four molecules and this therefore
places it firmly as an A0 class CH2 symmetric stretching
mode, located at either the b- or c-carbon atoms. The band
in the range 2860–2830 cmꢀ1 is again seen to be present for
the CH3 molecules but absent from the CD3 substituted
species, so identifying it as another CH3 stretching mode
belonging to the A0 symmetry class. Hence we have identi-
fied two of the three expected CH3 modes and three of the
expected four CH2 modes predicted on the basis of the CS
molecular symmetry. We can only conclude that the miss-
ing CH3 A00 symmetry class mode and the CH2 A00 symme-
try class mode are accidentally degenerate with other
observed bands in the Raman spectrum of the suite of mol-
ecules studied.
The four identified polarized m(CH) modes in the region
of 3000–2800 cmꢀ1 move to the 2200–2000 cmꢀ1 region on
deuteriation, where three polarized bands may be identi-
fied; these correspond to the m(CD2) or m(CD3) modes of
the a-, b- and c-carbon atom substituents.
Fig. 3. FT-Raman spectra of partially deuteriated analogues of propanol.
(a) 2500–3400 cmꢀ1, (b) 1700–2500 cmꢀ1 and (c) 300–1700 cmꢀ1; condi-
tions as for Fig. 2; samples: (i) CH3CH2CD2OH, (ii) CH3CD2CH2OH and
(iii) CD3CH2CH2OH.
the only advantage in the current work in having such a
deuterium substituted compound would be in an assign-
ment verification of the skeletal CACAOH bending mode
which is expected near 1000 cmꢀ1. The m(CH3) and
m(CH2) assignments follow generally from the deuteriation
pattern and theses are tabulated in Table 1.
It should be noted, however, that for CS molecular sym-
metry one CH3 symmetric and one CH3 asymmetric
stretching mode are both of A0 symmetry species and
should hence appear polarized in the Raman spectrum.
Generally, the assignment of the bands in this region
concurs with the literature [9] for alkane, haloalkane and
alcohol molecules. On substitution of D for H in
CH3CH2CH2OH the m(CH3) asymmetric and symmetric
stretching bands move from 2970, 2878 and 2860 cmꢀ1 to
2227, 2127 and 2076 cmꢀ1, respectively, for CD3CH2
CH2OH; the isotopic ratios of mH to mD for the m(CH3)
and m(CH2) bands observed here range from 1.35 to 1.38.
The CH3 asymmetric stretching band of A0 symmetry class,
identified as the 2860 cmꢀ1 band, and similarly the CH2

188
H.G.M. Edwards et al. / Journal of Molecular Structure 832 (2007) 184–190
Table 1
Wavenumbers and vibrational assignments for CH3CH2CH2OH and deuteriated analogues
CH3CH2CH2OH
CH3CH2CD2OH
CH3CD2CH2OH
CD3CH2CH2OH
3245 mw, br, pol
Approximate assignment of vibrational mode
3250 mw, br, pol
2970 m, sh, depol
2937 ms, pol
2912 ms, depol
2878 s, pol
3238 mw, br, pol
2965 m, depol
2938 s, pol
2917 ms, depol
2877 s, pol
3243 mw, br, pol
2958 m, depol
2935 s, pol
m(OH) stretching; free and hydrogen-bonded
m(CH3) asymmetric A00
2926 s, br, pol
2915 br, sh
2874 s, pol
m(CH2) symmetric A0
m(CH2) asymmetric A00
2878 s, pol
2830 m, sh, pol
m(CH2) symmetric A0
2860 m, sh, pol
2860 m, sh, pol
m(CH3) symmetric A0
2799 mw, pol
2738 mw, pol
2720 mw, pol
m(CD3-CH2) stretching
2737 mw, pol
2676 w, pol
2735 mw, pol
2733 mw, pol
m(CH2-CH) stretching combination
m(CH2-CD3) stretching combination
m(CH3-CH2) stretching combination
m(CD3) asymmetric; m(CD2) symmetric
m(CD2) asymmetric
2648 w, pol
2210 mw,pol
2222 mw, pol
2197 mw, depol
2184 mw, pol
2157 w
2227 mw, pol
2187 mw, pol
2157 mw
m(CD2) symmetric
m(CD2) asymmetric
2127 s, pol
2110 w, sh
2090 mw, pol
2078 w
2127 s, pol
2076 s, pol
m(CD3); m(CD2) symmetric
2110 w sh
2089 s, pol
m(CD2) symmetric
m(CD3); m(CD2) asymmetric
2070 mw, depol
1936 mw, br, depol
1932 mw, br, depol
1931 mw, br, depol
1485 mw, depol
d(CH2)
1470 m, sh, pol
1456 ms, depol
d(CH2) scissors
d(CH2) scissors
d(CH2) scissors
d(CH2) wagging
d(CH2) wagging
d(CHO)
1456 ms,depol
1440 m, depol
1350 w, depol
1460 ms, br, depol
1452 ms, br, depol
1350 w, depol
1298 m, depol
1274 m, depol
1350 w, depol
1298 mw, depol
1272 mw, depol
1260 w
1258 m, br, depol
1197 mw, depol
1111 s, pol
1260 mw, depol
1230 w, depol
d(CHO)
1230 w, depol
1132 mw, pol
1140 w, sh
1118 ms, pol
1097 mw, pol
m(CC)
m(CO)
m(CC)
m(CCO) o-o-p
d(CD2) scissors
m(CO)
d(COH)
d(CD2)
1120 w, pol
1075 m, pol
1097 mw, depol
1070 mw, sh
1061 ms, pol
1050 sh, pol
1000 mw, depol
1056 ms, pol
1015 vw
1056 br, pol
1005 w, depol
980 m, pol
968 s, pol
1054 ms, br, pol
1000 w
969 m, depol
q(CH3)
933 m, pol
d(CD2) wagging
910 mw, depol
905 w
888 ms, pol
859 s, pol
882 ms, pol
m(CCC)
m(CCC); m(CCO) ip
m(CCC) ip
857 s, pol
847 ms, pol
865 s, pol
834 ms, pol
789 s, pol
837 ms, pol
m(CCC) symmetric
800 vw
780 m, pol
774 s, pol
d(CCC)
750 w, depol
755 m, pol
737 m, pol
d(CCC)
d(CCC),q(CD3)
q(CD2)
680 mw, pol
452 ms, pol
464 ms, pol
459 ms, pol
445 m, pol
d(CCO)
322 mw, br, depol
d(CCC)
320 mw, br, depol
308 mw, br, depol
297 mw, br, depol
d(CCC)
stretching band of A0 symmetry class at 2937 cmꢀ1 could
each now comprise a CH2 stretching mode, one of which
is part of the terminal carbon group and the other attached
to the OH group, which were not positively identified in the
analysis earlier. Selective deuteriation of the CH2 attached
to the terminal carbon group shows that this band is
responsible for the observed feature at 2938 cmꢀ1, whereas
these move to 2187 and 2184 cmꢀ1, respectively, upon deu-
teriation, with an isotopic ratio of 1.35. The deuteriation
pattern seems to confirm the degeneracy of the CH2 mode
pattern alluded to here.
The weaker features in the Raman spectra in the
2600–2750 cmꢀ1 region are assignable to combination
modes of the fundamentals of the terminal CH3ACH2
moiety [11]; selective deuteriation has permitted their
the other CH2 group gives rise to the band at 2935 cmꢀ1
;

H.G.M. Edwards et al. / Journal of Molecular Structure 832 (2007) 184–190
189
numbers from the values observed in CH3CH2CH2OH
are ascribed to mass-effects of CD3 and CD2 substitution
on adjacent carbon atoms of the skeletal mode concerned.
Thus, the d(CCO)/d(CCC) vibrational mode at 464 cmꢀ1
for CH3CH2CH2OH shifts to 445 cmꢀ1 for CD3CH2
CH2OH and the m(CCC)/m(CCO) band at 859 cmꢀ1 moves
to 837 cmꢀ1 for the same two molecular species. Theoreti-
cally, the skeletal modes should be independent of m(CH)
substitution by m(CD). In practice, however, significant
wavenumber shifts may be observed; these are ascribed to
the influence of mode mixing, primarily with CH2 and
CH3 deformation modes in the same symmetry classes.
Even the O13-substitution of the oxygen atom, undertaken
in a recent investigation of allyl methyl ether in an attempt
to derive unambiguous information about the m(CCO),
m(CCC) and d(CCO) and d(CCC) vibrational modes, was
determined to be not without interference from mode mix-
ing of this sort [10]. In this region, therefore, for the
CH3CH2CH2OH and its deuteriated analogues, in addition
to the six modes expected for the CCCO skeleton, we
would expect a further 16 modes (7A0 + 9A00) arising from
CH2 and CH3 motion. These will consist of deformations
of the type d(CH2), d(CCH), d(CH3) and rocking or wag-
ging modes of the type q(CH2) and q(CH3).
It is hardly surprising, perhaps, that the d(COH) mode is
not uniquely identifiable from the Raman spectra, since it
occurs as a medium–weak feature near the 1000 cmꢀ1
region, where the d(CO, CHO), and d(CD3,CD2) modes
also occur. It is possible that the Raman spectrum of oD
substituted propanol would assist in the clarification of this
assignment, but as mentioned earlier the preparation of a
sufficiently pure sample of this material was not possible
in this study. The skeletal CAC and CAO stretching modes
show little difference from their wavenumbers for the n-al-
kanes; the masses of C and O do not differ greatly and a
strong interaction is to be expected here; the infrared spec-
tra exhibit some differences in observed band intensity
between alkanes and alcohols in this region because of
the CAO band polarity. However, in several cases in Table
1 the deuteriation assists materially in the identification of
a specific CH mode (e.g., the 1274 cmꢀ1 band assigned to a
d(CHO) mode).
The d(CH2) and d(CH3) modes expected around 1300–
1470 cmꢀ1 will occur around 960–1080 cmꢀ1 for the analo-
gous CD2 and CD3 species. Unfortunately, the deforma-
tion modes of the CD2 and CD3 moieties occur in a
similar wavenumber region as the rocking vibrations for
CH2 and CH3 groups, whose deuteriated modes are them-
selves expected around 640–680 cmꢀ1 and this complicates
the observed spectra in this wavenumber region.
Fig. 4. FT-Raman spectra of partially deuteriated analogues of propanol.
(a) CD3CH2 CH2OH, (b) CH3CD2CH2OH and (c) CH3CH2CD2OH; 100–
3400 cmꢀ1 wavenumber range, 2000 scans accumulated at 4 cmꢀ1 spectral
resolution, 1064 nm excitation; upper spectrum—depolarized, lower
spectrum—polarized.
assignment as shown in Table 1. The combination
modes at 2680, 2648 cmꢀ1, move to ꢁ1936 cmꢀ1 upon
deuteriation with an isotopic ratio of 1.38. Other weak
features in this region are assignable to combination
bands of fundamentals involving CC and CO stretching
modes.
4.2. 1800–250 cmꢀ1 region
In this wavenumber region we expect to observe the six
skeletal modes predicted, including the m(CC), m(CO) and
d(CCO), d(CCC) modes; the five skeletal polarized bands
of A0 symmetry can be identified as m(CO)/m(CC) at
ꢁ1130 cmꢀ1, m(CO) at 1056 cmꢀ1, m(CCO) in-plane at
ꢁ850 cmꢀ1, d(CCC) at ꢁ770 cmꢀ1 and d(CCO)/d(CCC)
at ꢁ460 cmꢀ1. Variations in the skeletal stretching wave-
In the low wavenumber region, the modes are best
described as mixed modes, particularly of d(CCO) and
d(CCC) vibrations. Hence, the listing of modes in Table 1
is again subject to mode mixing; nevertheless, several fea-
tures of interest are apparent. For example, the band at
1298 cmꢀ1 in the spectrum of CH3CH2CH2OH also occurs
at 1298 cmꢀ1 in the spectrum of CD3CH2CH2OH but not

190
H.G.M. Edwards et al. / Journal of Molecular Structure 832 (2007) 184–190
in those of CH3CD2CH2OH and CH3CH2CD2OH. This
band is assigned here to a d(CH2) wagging mode, because
it must be ascribed to a ACH2CH2A grouping, rather than
a ACD2CH2A or ACH2CD2A moiety.
891 cmꢀ1 observed in biological specimens and ascribed
to a q(CH2) rocking mode is shown to be mis-assigned
and more accurately described as a m(CCC)/m(CCO) mode.
We propose to extend this study to other series of specif-
ically labelled analogues of simple organic compounds
which serve as models for larger biological molecules, so
as to refine the tentative assignment of bands in several
regions of interest, thereby enhancing the value of Raman
spectroscopy for biological molecular applications.
The conclusions of this study demonstrate the value of
isotopic substitution (particularly D for H) especially in
the CH3 and CH2 stretching, bending and rocking regions
of the spectrum. For example, the combined polarization
and deuteriation data clearly confirm the assignment of
the 2860 cmꢀ1 band to a m(CH3) symmetric stretch, which
had been tentatively proposed in the literature to hitherto
correspond to a m(CH2) mode [1,10]. However, the
2878 cmꢀ1 mode is clearly a m(CH3) asymmetric stretch
and this has formerly been ascribed only tentatively [1,10]
to a m(CH2) symmetric stretching mode. Likewise, the
2730 cmꢀ1 band, previously assigned to a m(C@CH) ali-
phatic mode [3,9] cannot be of this description as it is pres-
ent in the protiated and deuteriated saturated alcohols
studied here; it is clearly assignable on the basis of the pres-
ent work to m(CH2CH2) or m(CH2CD2) stretching. On the
other hand, the earlier tentative assignment on a literature
basis [9] of a band at 1450 cmꢀ1 in keratotic tissue [1] as a
d(CH2) scissors mode is reaffirmed as such by this deuteri-
um-labelling study. Similarly, a band at 1298 cmꢀ1 which
has also been ascribed in biological tissues to a d(CH2)
mode, is also confirmed in assignment on the basis of this
work [3].
References
[1] L.J. Hogg, H.G.M. Edwards, D.W. Farwell, A.T. Peters, J. Soc.Dyers
Col. 110 (1994) 196.
[2] H.G.M. Edwards, D.W. Farwell, A.C. Williams, Spectrochim. Acta
Part A 50 (1994) 807.
[3] A.C. Williams, H.G.M. Edwards, B.W. Barry, J. Raman Spectrosc.
25 (1994) 95.
[4] H.G.M. Edwards, A.C. Williams, B.W. Barry, J. Mol. Struct. 347
(1995) 379.
[5] A.N.C. Anigbogu, A.C. Williams, B.W. Barry, H.G.M. Edwards,
K.R. Brain, J. Hadgraft, V.J. James, K.A. Walters, Prediction of
Percutaneous Penetration, 3B 1993 STS Publishing, Cardiff, 27.
[6] A.N.C. Anigbogu, A.C. Williams, B.W. Barry, H.G.M. Edwards,
Intl. J. Pharm. 125 (1995) 265.
[7] A.C. Williams, H.G.M. Edwards, J. Raman Spectrosc. 25 (1994) 673.
[8] (a) R.D. Bowen, D. Suh, J.K. Terlouw, J. Chem. Soc. Perkin Trans. 2
(1995) 119;
(b) , see alsoR.D. Bowen, A.W. Colburn, P.J. Derrick, J. Am. Chem.
Soc. 113 (1990) 1132.
The band at 968 and 969 cmꢀ1 observed here in both the
parent protiated and CH2 deuteriated molecules is assigned
to a q(CH3) terminal methyl mode; it is absent from the
spectrum of CD3CH2CH2OH, and a new band appears
at 737 cmꢀ1 so this assignment better fits the evidence than
does the current literature [9], where it is tentatively
ascribed to a m(CC) stretching mode. A band at 883 and
[9] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The
Handbook of Infrared and Raman Characteristic Frequencies of
Organic Molecules, Academic Press Inc., San Diego and London, 1991.
[10] R.D. Bowen, H.G.M. Edwards, D.W. Farwell, J. Mol. Struct. 351
(1995) 77.
[11] H.G.M. Edwards, E.E. Lawson, A.F. Johnson, Spectrochim. Acta
part A 51 (1995) 2057.