Raman spectroscopy of n-pentyl methyl ether and deuterium labelledanalogues

Article abstract of DOI:10.1002/jrs.2595

The Raman spectra of n-pentyl methyl ether, C₅H ₁₁OCH₃, and six selectively deuteriated analogues arereported and discussed. Correlations between the observed ν(sp ³CH)stretching and bending bands and the position of the deuterium atoms in thealkyl chain are developed and refined. Similar progress is possible inassociating specific skeletal vibrations with bands in the Raman spectra. Therelevance of this study to improving the assignment of bands in the Ramanspectra of larger systems of biological interest is highlighted. Copyright

Full text of DOI:10.1002/jrs.2595

Research Article
Received: 15 July 2009
Accepted: 16 November 2009
Published online in Wiley Online Library: 3 February 2010
(wileyonlinelibrary.com) DOI 10.1002/jrs.2595
Raman spectroscopy of n-pentyl methyl ether
and deuterium labelled analogues
∗
Richard D. Bowen, Howell G. M. Edwards, Dennis W. Farwell
and Sara E Morgan
The Raman spectra of n-pentyl methyl ether, C5H11OCH3, and six selectively deuteriated analogues are reported and discussed.
3
Correlations between the observed ν(sp CH) stretching and bending bands and the position of the deuterium atoms in the alkyl
chain are developed and refined. Similar progress is possible in associating specific skeletal vibrations with bands in the Raman
spectra. The relevance of this study to improving the assignment of bands in the Raman spectra of larger systems of biological
interest is highlighted. Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Keywords: pentyl; ether; deuterium labelling; vibrational assignments
Introduction
An illustration of the value of more precise correlation of
individual Raman bands and the structural features to which
Raman spectroscopy[1] has been applied less extensively for
molecular characterization than infrared spectroscopy by organic
chemists for various reasons, not least because many of the
bond vibrations that are of greatest interest in organic chemistry
tend to be stronger in the infrared than in the Raman spectrum.
However, certain vibrations, especially those associated with C–H
and C C functional groups, often show Raman bands that are
at least as strong as those observed in the infrared. Furthermore,
the advent of longer wavelength excitation and CCD detection
[5]
they may be ascribed is provided by a study of a simple
2
13
model compound, allyl methyl ether, and four H and C-labelled
analogues. It was found that several tentative assignments of
2
3
bands in the spectra of materials containing sp and sp C–H, C–C,
and C C bonds were imprecise. In order to develop this work and
expand the analytical utility of Raman spectroscopy, the spectra
of 29 alkenyl methyl ethers containing up to seven carbon atoms
in which the position, stereochemistry, and substitution pattern
of the double bond were systematically varied were obtained
[
2,3]
[6]
or Fourier-transform laser Raman instruments,
which permits
and investigated. This work not only led to the development
the analysis of a microgram or less of material in a few minutes,
has opened the way for the application of Raman spectroscopy
as a non-destructive, non-invasive technique for in vivo studies
of skin, nail, and other tissue samples particularly where the
desiccation of samples is not favoured. These advantages of
Raman spectroscopy are potentially very important in forensic
contexts, particularly in establishing the identity of fragile samples
of more accurate correlations between the presence of bands in
the Raman spectra and structural features associated with the
double bond, but also revealed an error in previous assignments
of bands associated with the C–H stretching vibrations of the
bonds in a terminal CH CH2 entity. Thus, ethers containing either
a 1,1-disubstituted or a monosubstituted C C bond showed
[
4]
−
1
−1
a band in the ranges 3087–3075 cm and 2986–2979 cm ;
(
e.g. ancient documents), which are suspected of being fakes, or
these bands may be attributed to the asymmetric and symmetric
C–H stretching vibrations, respectively, of the terminal C CH2
group. In addition, ethers with a monosubstituted C C bond
showed an additional band in an intermediate range (usually
in determining whether materials (e.g. ivory) being imported or
exported are controlled or prohibited.
Unfortunately, however, the application of Raman spectroscopy
is often hindered by the imprecise correlation between the
structureof thesubstrateandtheassociatedbands. Consequently,
previous Raman and infrared studies of complex biological
molecules have tended to concentrate on well-defined skeletal
modes, rather than specific bands that may be associated with
particular ν(C–H), δ(C–H), and ρ(C–H) vibrations. This pragmatic
means of dealing with the complexity of the regions of the
spectrum containing bands arising from specific C–H vibrations
canresultinimportantinformationbeingoverlooked.Thus,several
applications of vibrational spectroscopy at the chemical and
biological interface indicate that significant changes in the ν(C–H)
and δ(C–H) bands are induced by a range of biological, clinical,
and environmental factors, including the degeneration of healthy
tissue, the effects of chemical enhancing agents on the delivery
of drugs across the human stratum corneum, and the degradative
decomposition of archaeological samples.
−
1
3008–2998 cm ). This band may be attributed to the C–H
stretching vibration of the bond connecting the solitary hydrogen
atom to the penterminal carbon atom. Formerly, this band was
ascribed to the symmetric vibration of the terminal methylene
group; the revised assignments were confirmed by investigating
the Raman spectra of selected deuterium labelled analogues.
All these ethers showed a strong Raman band in the
−
1
1640–1680 cm region associated with the stretching vibration
of the C C bond. Moreover, clear correlations were developed
∗
Correspondence to: Richard D. Bowen, Chemical and Forensic Sciences, School
of Life Sciences, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK.
E-mail: r.d.bowen@bradford.ac.uk
Chemical and Forensic Sciences, School of Life Sciences, University of Bradford,
Bradford, West Yorkshire BD7 1DP, UK
J. Raman Spectrosc. 2010, 41, 1725–1734
Copyright ꢀc 2010 John Wiley & Sons, Ltd.

R. D. Bowen et al.
2
between the stereochemistry, position, and substitution pattern
of the C C bond and the position of the associated band in
the Raman spectrum of the ether concerned. In contrast, the
corresponding absorptions in the infrared spectra of at least some
of these ethers, particularly those with a trans-1,2-disubstituted
double bond, were found to be extremely weak in intensity or not
observable at all.
Following on from the success of these labelling and model
studies in refining the use of Raman spectroscopy in analytical
work on compounds containing C–H bonds, the approach has
been extended; pentyl methyl ether (PME) was chosen as a model
system in order to determine whether it is possible to distinguish
between closely similar C–H bonds such as those in adjacent
methylene groups in a moderately long alkyl chain with an
electronegative heteroatom at one end. This will provide a critical
evaluation of the diagnostic capabilities of Raman spectroscopy
for closely similar vibrational modes that will be transferrable to
more complex biochemical systems.
Methyl n-pentyl-1,1- H2 ether
2
n-Pentanol-1,1- H2. Asolutionofredistilledmethyln-pentanoate
5.1 g, 44 mmol) in dry ether (30 ml) was added dropwise during
0 min to a magnetically stirred suspension of lithium aluminium
(
3
deuteriide (1.1 g, 26 mmol) in dry ether (80 ml) under a nitrogen
atmosphere at a rate sufficient to promote a gentle reflux, which
was maintained by external heat for a further 30 min after the ad-
dition was complete. After the mixture had been allowed to cool
to room temperature, water (1 ml), followed by aqueous sodium
hydroxide solution (3 ml, 15% w/v) and water (1 ml) was added
dropwise with caution to give a white granular precipitate. The
solution was filtered through a Buchner funnel and the inorganic
salts were extracted with ether (2 × 10 ml). The combined ethereal
phases were dried, filtered, and evaporated at reduced pres-
sure to give crude product (3.9 g), which was distilled to yield
n-pentanol-1,1- H2 (3.20 g, 82%; bp 136–139 C) with
characteristic odour.
2
◦
a
2
Methyl n-pentyl-1,1- H2 ether
2
Experimental
n-Pentanol-1,1- H2 (1.05 g, 12 mmol) was converted into its alkox-
ideanionbytreatmentwithsodiumhydride(0.9 g, 60%dispersion,
22 mmol) in triglyme (20 ml) and methylated with methyl io-
Synthesis
2
dide (1.60 g, 11 mmol) in the manner described for methyl- H3
n-pentyl ether to give methyl n-pentyl-1,1- H2 ether (0.51 g, 50%;
Methyl n-pentyl ether was prepared by condensation of a slight
deficiency of iodomethane with sodium alkoxide prepared in
triglyme suspension from a 25% excess of sodium hydride and
n-pentanol. After stirring the reaction mixture overnight, the
crude ether was distilled directly from the reaction flask, washed
with water, dried with magnesium sulfate, filtered and redistilled
through a 10-cm helix-packed column from a small portion of
sodium hydride. The deuterium labelled analogues were prepared
by the corresponding condensation of the alkoxide derived from
suitably labelled analogues of n-pentanol or iodomethane. These
synthetic procedures, Scheme 1, are illustrated by the following
detailed descriptions. Ether (diethyl ether) was dried by distillation
from sodium hydride under nitrogen; unless otherwise specified,
ethereal extracts and other organic solutions and liquids were
dried with magnesium sulfate. Deuteriated starting materials
2
◦
bp 95–97 C).
2
Methyl n-pentyl-2,2- H2 ether
2
Methyl n-pentanoate-2,2- H2. Sodium metal (0.23 g, 10 mmol)
was added under a nitrogen atmosphere to methanol-O- H
25 g, 758 mmol) in a 100 round-bottomed flask equipped with
a condenser. When the reaction was complete, redistilled methyl
n-pentanoate (10.5 g, 90 mmol) was added and the solution was
refluxed for 48 h. The bulk of the methanol-O- H was recovered
by distillation through a 10-cm helix-packed column; the residue
2
(
2
was diluted with water (30 ml) and extracted with ether (3 ×
2
5 ml); the combined ethereal extracts were dried, filtered, and
carefully evaporated at reduced pressure (cold water bath) to give
partially deuteriated methyl n-pentanoate (7.8 g). This material
was subjected to a second exchange in the same manner with a
2
2
(deuterium oxide, methanol-O- H1, acetic- H4 acid, and lithium
aluminium deuteriide) were obtained from Aldrich and used as
received.
2
fresh portion of methanol-O- H (20 g, 606 mmol) in which sodium
metal (0.19 g, 8.3 mmol) had been dissolved to give methyl
n-pentanoate (5.9 g) with a higher level of deuterium incorpo-
2
Methyl- H3 n-pentyl ether
ration. A second batch of redistilled methyl n-pentanoate (5.2 g,
2
4
5 mmol) was treated with the methanol-O- H (16.5 g, 500 mmol
Redistilled n-pentanol (3.3 g, 37 mmol) was added from a
syringe during 10 min to a magnetically stirred suspension of
sodium hydride (2.0 g, 60% dispersion in mineral oil, 50 mmol)
in triglyme (triethylene glycol dimethyl ether, 20 ml) under a
nitrogen atmosphere in a 100 ml two-necked round-bottomed
flask equipped with a condenser and a septum seal. There was
a significant evolution of heat and hydrogen gas; stirring was
continued for a further 30 min. A solution of trideuteriomethyl
iodide (4.1 g, 28 mmol) in triglyme (3 ml) was added from a
syringe to the stirred suspension during 20 min. After stirring
overnight, the apparatus was rearranged for distillation and the
and 9.9 g, 300 mmol, respectively) recovered from the exchanges
on the first batch and sodium metal (0.12 g, 5.2 mmol and 0.08 g,
3.5 mmol, respectively) to give a further portion of deuteriated
methyl n-pentanoate (2.6 g). These two batches of deuteriated
methyl n-pentanoate (8.5 g) were then combined and treated in
2
the same manner with a final portion of methanol-O- H (25 g,
7
58 mmol) and sodium metal (0.23 g, 10 mmol); distillation of the
2
crude product (5.6 g) gave methyl n-pentanoate-2,2- H2 (4.3 g,
2
◦
7% overall; bp 126–129 C) with a very high level of deuterium
incorporation.
crude product was collected as a fraction boiling between 95 and
◦
2
2
1
10 C. This material (2.2 g) was washed with water (2 × 2 ml),
n-Pentanol-2,2- H . Methyl
n-pentanoate-2,2- H
(3.32 g,
2
2
dried, filtered, and distilled from a small portion of sodium hydride
to give methyl- H3 n-pentyl ether (1.45 g, 49%; bp 96–97 C) as a
28 mmol) was reduced in dry ether (20 ml) with lithium aluminium
hydride (1.8 g, 47 mmol) in dry ether (40 ml) in the manner
described for the preparation of n-pentanol-1,1- H2 to give after
2
◦
2
colourless liquid with a pleasant aroma.
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 1725–1734

Raman spectroscopy of n-C5H11OCH3 and deuterium labelled analogues
i, NaH in
triglyme
1
2
3
n-C H11OH
n-C H11OCD3
5
5
ii, CD3I
i, LiAlD4
in ether
i, NaH in
triglyme
n-C H CO CH
n-C H CD OH
n-C H CD OCH
4 9 2 3
4
9
2
3
4
9
2
ii, NaOH
in water
ii, CH3I
-
+
i, CH O Na
3
in CH OD
3
n-C H CH CO CH
n-C H CD CO CH
3 7 2 2 3
3
7
2
2
3
ii, Repeat
i, LiAlH4
in ether
i, NaH in
triglyme
n-C H CD CH OH
n-C H CD CH OCH
3 7 2 2 3
3
7
2
2
ii, NaOH
in water
ii, CH3I
i, Reflux in
i, LiAlH in
4
D O/pyridine
triglyme
2
4
C H CH CHO
C H CD CHO
C H CD CH OH
2 5 2 2
2
5
2
2
5
2
ii, Repeat
ii, tetragol
i, Mg/ether
ii, CO2
I2/Red P
C H CD CH I
C H CD CH CO H
2 5 2 2 2
2
5
2
2
iii, H O/HCl
2
i, LiAlH4
in ether
CH OH/HCl
3
C H CD CH CO CH
C H CD CH CH OH
2 5 2 2 2
2
5
2
2
2
3
ii, NaOH
in water
i, NaH in
triglyme
C H CD CH CH OCH
3
2
5
2
2
2
ii, CH3I
i, Mg/ether
i, LiAlD in
triglyme
ii, oxetane
in benzene
4
HBr/H SO
4
2
5
CH COCl
CH CD OH
CH CD Br
3 2
3
3
2
iii, NH4+Cl-
ii, tetragol
i, NaH in
triglyme
CH CD (CH ) OH
CH CD (CH ) OCH
3 2 2 3 3
3
2
2 3
ii, CH3I
i, LiAlH in
tryglyme
4
HBr/H2SO4
PhCOCl
6
CD CO D
CD COCl
CD CH OH
3 2
3
2
3
ii, tetragol
i, Mg/ether
ii, oxetane
in benzene
i, NaH in
triglyme
CD CH Br
CD (CH ) OH
CD (CH ) OCH
3
3
2
3
2 4
3
2 4
iii, NH4+Cl-
ii, CH3I
Scheme 1. Synthetic routes to deuterium labelled methyl n-pentyl ethers.
2
workup with water (2 ml), aqueous sodium hydroxide solution
Methyl n-pentyl-3,3- H2 ether
2
(
5.5 ml, 15% w/v) and water (2 ml), n-pentanol-2,2- H2 (1.75 g,
2
n-Butyl-2,2- H2 iodide. Redistilled n-butyraldehyde (29 g,
03 mmol) was converted into n-butyl-2,2- H2 iodide (17.3 g, 23%
overall; bp 128–131 C) in five steps as described elsewhere.
◦
6
9%; bp 136–139 C).
2
4
◦
[7]
2
2
Methyl n-pentyl-2,2- H3 ether. n-Pentanol-2,2- H2 (0.75 g,
3 mmol) was converted into its alkoxide anion by treatment
with sodium hydride (0.6 g, 60% dispersion, 15 mmol) in triglyme
12 ml) and methylated with methyl iodide (1.02 g, 7.2 mmol) in
2
8
n-Pentanoic-3,3- H acid. Magnesium turnings (1.8 g, 75 mmol)
2
were placed in a 500 ml three-necked round-bottomed flask
(
equipped with a pressure-equalizing dropping funnel and an
efficient condenser. The apparatus was flushed with nitrogen
and sufficient dry ether to cover the metal was added, followed
2
themannerdescribedformethyl- H3 n-pentylethertogivemethyl
2
◦
n-pentyl-2,2- H2 ether (0.35 g, 48%; bp 96–98 C).
J. Raman Spectrosc. 2010, 41, 1725–1734
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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R. D. Bowen et al.
2
2
by a small portion of n-butyl-2,2- H2 iodide (0.3 g) and a single
Methyl n-pentyl-4,4- H2 ether
crystal of iodine. The flask was warmed gently to initiate the
formation of the Grignard reagent, after which a solution of
n-butyl-2,2- H2 iodide (9.8 g, total 10.1 g, 54 mmol) in dry ether
2
Ethanol-2,2- H2. Redistilled acetyl chloride (16.5 g, 210 mmol)
in triglyme (50 ml) was added dropwise during 3 h to a
magnetically stirred suspension of lithium aluminium deuteriide
2
(50 ml) was added dropwise during 15 min at such a rate as
(
5.0 g,119 mmol)intriglyme(150 ml)underanitrogenatmosphere
to maintain a gentle reflux. The mixture was warmed gently to
maintain reflux for a further 30 min, allowed to cool to ambient
temperature and diluted with dry ether (30 ml). Solid carbon
dioxide (ca 15 g, 340 mmol) was quickly crushed in a stout
envelope and transferred to a dry 500 ml beaker. The ethereal
solution was immediately poured in a continuous stream onto the
crushed solid which was stirred vigorously with a glass rod. The
resultant pasty mass was allowed to attain ambient temperature
and was then acidified with cold moderately concentrated
hydrochloric acid (40 ml, 50% w/w). The ethereal phase was
separated and the residual aqueous phase was extracted with
ether (3 × 25 ml). The combined ethereal phases were dried,
filtered, and evaporated at reduced pressure to give crude n-
ina1-lthree-neckedround-bottomedflaskimmersedinanicebath
and equipped with a condenser and pressure-equalizing dropping
funnel. After stirring over the weekend, tetragol (tetraethylene
glygol, 80 ml) was cautiously added dropwise; the crude product
◦
(
8.9 g, 88%; bp 76–79 C) was isolated by careful distillation.
Redistillation through a 5-cm helix-packed column gave ethanol-
2
◦
2
,2- H2 (8.3 g, 82%; bp 76–77 C).
2
Ethyl-2,2- H2 bromide. Cold concentrated sulfuric acid (15 g)
was added cautiously with stirring and cooling to concentrated
hydrobromic acid (54 g) in a 100 ml round-bottomed flask
equipped with a condenser. The flask was cooled in an ice bath
2
and ethanol-2,2- H2 (12.3 g, 256 mmol) was added cautiously
2
pentanoic-3,3- H2 acid (4.8 g, 85%). This material was dissolved
down the condenser from a pressure-equalizing dropping funnel.
The mixture was swirled and cooled as cold concentrated sulfuric
acid (25 g) was added dropwise down the condenser from a
clean pressure-equalizing dropping funnel. The apparatus was
rearranged for distillation with the tip of the receiver adaptor
dipping just below the surface of ice-cold water (20 ml) in a 100 ml
conical flask, which was further cooled in ice water. The reaction
mixture was slowly distilled during a period of 2 h; the crude
in ether (30 ml) and extracted with saturated aqueous sodium
hydrogen carbonate solution (3 × 25 ml); the combined extracts
were washed with diethyl ether (10 ml) and cautiously acidified
under a layer of diethyl ether (20 ml) with ice-cold moderately
concentrated hydrochloric acid (25 ml, 50% w/w). The ethereal
phase was separated and the aqueous phase was extracted
with ether (3 × 20 ml). The combined ethereal phases were
dried, filtered, and evaporated at reduced pressure to give
◦
◦
product (21.5 g, 76%; bp 32–50 C, bulk 35–7 C) was separated,
washed sequentially with concentrated hydrochloric acid (5 ml)
and saturated sodium hydrogen carbonate solution (10 ml), dried
2
n-pentanoic-3,3- H2 acid (3.9 g, 69%) as a colourless liquid with a
very disagreeable odour.
2
with calcium chloride, filtered and distilled to give ethyl-2,2- H2
◦
bromide (19.5 g, 69%; bp 36–38 C).
2
Methyl n-pentanoate-3,3- H2. Redistilled acetyl chloride (2.2 g,
2
8 mmol) was added dropwise to redistilled methanol (30 ml)
2
n-Pentanol-4,4- H2. Magnesium turnings (2.9 g, 120 mmol) were
in a 100 ml two-necked round-bottomed flask equipped with a
reflux condenser. n-Pentanoic-3,3- H2 acid (3.7 g, 36 mmol) was
added and the mixture was refluxed overnight. The bulk of the
methanolandmethylacetate(20 ml,bp60–66 C)wasremovedby
distillation;theresiduewaspouredintosaturatedaqueoussodium
hydrogen carbonate solution (40 ml) and the ester was isolated
by extraction with dichloromethane (4 × 15 ml). The combined
organic extracts were dried, filtered, and carefully evaporated at
reduced pressure (cold water bath) to leave a colourless residue
placed in a 500 ml three-necked round-bottomed flask equipped
with a pressure-equalizing dropping funnel and an efficient
condenser. The apparatus was flushed with nitrogen and sufficient
dry ether to cover the metal was added, followed by a small
portion of ethyl-2,2- H2 bromide (0.4 g). The flask was warmed
gently to initiate the formation of the Grignard reagent, after
2
◦
2
2
which a solution of ethyl-2,2- H2 bromide (10.0 g, total 10.4 g,
4 mmol) in dry ether (50 ml) was added dropwise during 20 min
9
at such a rate as to maintain a gentle reflux. The mixture was
warmed gently to maintain reflux for a further 30 min, allowed to
cool to ambient temperature, and filtered through a dry all glass
apparatus into a dry 250-ml three-necked round-bottomed flask,
equipped with a magnetic stirring bead, an efficient condenser
and a pressure-equalizing dropping funnel. The grey solution was
stirredmagneticallyunderanitrogenatmosphereandasolutionof
oxetane (4.3 g, 74 mmol) in dry ether (12 ml) was added dropwise
during 15 min. A white colouration appeared as each drop of
oxetane solution was added and there was a slight evolution of
heat, but not enough to cause the ethereal solution to reflux. The
solutionwasthenrefluxedfor1 h, dilutedwithdrybenzene(80 ml)
and allowed to cool. A 10-cm helix-packed column was attached to
2
which was distilled to give methyl n-pentanoate-3,3- H2 (2.9 g,
6
◦
9%; bp 125–128 C).
2
2
n-Pentanol-3,3- H2. Methyl
n-pentanoate-3,3- H2
(2.75 g,
23 mmol) in dry ether (20 ml) was reduced with lithium aluminium
hydride (1.8 g, 43 mmol) in dry ether (40 ml) in the manner
described for the preparation of n-pentanol-1,1- H2 to give after
workup with water (2 ml), aqueous sodium hydroxide solution
2
2
(5.5 ml, 15% w/v) and water (2 ml), n-pentanol-3,3- H2 (1.65 g,
◦
7
9%; bp 136–139 C).
◦
the flask and the ether (bp 35–75 C) was removed by distillation.
2
2
Methyl n-pentyl-3,3- H2 ether. n-Pentanol-3,3- H2 (0.65 g,
.2 mmol) was converted into its alkoxide anion by treatment
with sodium hydride (0.6 g, 60% dispersion, 15 mmol) in triglyme
12 ml) and methylated with methyl iodide (0.95 g, 6.7 mmol) in
More benzene (30 ml) was added to the residue and the reaction
mixture was refluxed for a further 3.5 h before being allowed to
stand overnight. Saturated aqueous ammonium chloride solution
(50 ml) was added dropwise to the stirred reaction mixture, the
organic phase was separated and the inorganic salts and aqueous
phase were extracted with ether (3 × 50 ml). The combined
7
(
2
themannerdescribedformethyl- H2 n-pentylethertogivemethyl
n-pentyl-3,3- H2 ether (0.31 g, 51%; bp 96–98 C).
2
◦
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 1725–1734

Raman spectroscopy of n-C5H11OCH3 and deuterium labelled analogues
organic phases were dried, filtered, and distilled through a 10-
H
C
H
1C
H
C
H
C
H
H 5
cm helix-packed column to remove the bulk of the solvent (ca
3
◦
2
00 ml, bp 35–80 C). The residue was transferred to a 10-ml pear-
H
O
2C
4C
H
shaped flask and carefully distilled through a 5-cm helix-packed
column (severe problems were encountered with foaming) to give
H
H
H
H
H
H
2
◦
n-pentanol-4,4- H2 (3.6 g, 54%; bp 135–138 C). This method is a
Figure 1. Structure and nomenclature of n-pentyl methyl ether (PME),
[
8]
modification of that previously reported.
CH OCH CH CH CH CH .
3
2
2
2
2
3
2
2
Methyl n-pentyl-4,4- H2 ether. n-Pentanol-4,4- H2 (1.01 g,
1
1 mmol) was converted into its alkoxide anion by treatment
with sodium hydride (0.9 g, 60% dispersion, 22 mmol) in triglyme
25 ml)andmethylatedwithmethyliodide(1.42 g, 10 mmol)inthe
(a)
.
.
.
.
.
6
5
4
3
2
(
2
manner described for methyl- H2 n-pentyl ether to give methyl
n-pentyl-4,4- H2 ether (0.62 g, 60%; bp 98–100 C).
2
◦
2
Methyl n-pentyl-5,5,5- H3 ether
2
2
Ethanol-3,3,3- H3. Acetic- H4 acid (24.8 g, 387 mmol) was con-
verted in two steps to ethanol-3,3,3- H3 (14.8 g, 80% overall; bp
8
2
.1
0
◦
[9]
0–82 C) as described elsewhere.
2
2
3400 3300 3200 3100 3000 2900 2800 2700 2600 2500
Wavenumber/cm-1
Ethyl-3,3,3- H3 bromide. Ethanol-3,3,3- H3 (25.2 g, 514 mmol)
2
was converted to ethyl-3,3,3- H3 bromide (43.8 g, 76%; bp
7–38 C) as described for ethyl-2,2- H2 bromide by treatment
with a brominating mixture prepared from cold concentrated
sulfuric acid (30 g) and concentrated hydrobromic acid (105 g),
with the subsequent addition of more cold concentrated sulfuric
acid (50 g).
◦
2
3
(
b)
.1
.
.
08
06
2
2
n-Pentanol-5,5,5- H3. Ethyl-3,3,3- H3
19 mmol) was converted to n-pentanol-5,5,5- H3 (3.75 g,
5%; bp 136–139 C) by treatment with magnesium metal (3.5 g,
5 mmol) in dry ether (70 ml) followed by adding oxetane (5.3 g,
1 mmol) in ether (15 ml) and benzene (100 ml) as described for
n-pentanol-4,4- H2.
bromide
(13.3 g,
2
1
4
1
9
◦
.04
.
02
2
0
1
800 1600 1400 1200 1000 800 600 400 200
Wavenumber/cm-1
2
2
Methyl n-pentyl-5,5,5- H3 ether. n-Pentanol-5,5,5- H3 (0.99 g,
1 mmol) was converted into its alkoxide anion by treatment
with sodium hydride (0.85 g, 60% dispersion, 21 mmol) in triglyme
20 ml) and methylated with methyl iodide (1.25 g, 8.8 mmol) in
triglyme (2 ml) in the manner described for methyl- H2 n-pentyl
ether to give methyl n-pentyl-5,5,5- H3 ether (0.64 g, 80%; bp
1
Figure 2. FT-Raman spectra of n-PME; 1064 nm excitation, 2000 scans,
−1
−1
−1
(
4 cm spectral resolution; (a) 3400–2500 cm ; (b) 1800–1850 cm .
2
2
◦
9
7–99 C).
Vibrational Spectroscopic Theory
For Cs symmetry, as indicated diagrammatically in Fig. 1, the
vibrational modes of PME may be classified as follows:
Raman Spectroscopy
Raman spectra of methyl pentyl ether and its labelled analogues
were obtained using a Bruker FRA106 Raman module on a Bruker
ꢁ
ꢁꢁ
ꢀ
vib = 33A + 24A
3
+
IFS66 infrared optics system. A Nd : YAG laser operating at
064 nm was used as an excitation source; the laser beam was
1
All of these vibrations are Raman active; 33 of the associated
bands are polarized in the Raman spectrum.
For the hepta-atomic skeletal modes, the following vibrational
classification applies:
focused to a spot of 100 µm diameter at the sample. Samples of
PME and its labelled analogues were contained in sealed glass
capillary tubes mounted in a specially constructed holder. Laser
powers of up to 500 mW were used; typically, 2000 scans were
−
1
collected at 4 cm spectral resolution. A liquid–nitrogen cooled
germanium detector with extended bandwidth was used over
skeletal
ꢁ
ꢁꢁ
ꢀ
vib = 11A + 4A
−
1
the scan range of 50–3500 cm ; the spectra were corrected
for instrument response. The observed band wavenumbers,
calibrated against the internal laser frequency, are correct to
Of these 15 bands, 11 are polarized in the Raman spectrum.
The remaining 42 vibrational modes, 22 of which are polarized,
belong to CH(CD) stretching, CH2 and CH3 bending, and CH2 and
CH3 rocking vibrations.
−
1
better than ± 1 cm . Polarization measurements were made with
an optical vector rotator in the laser beam.
J. Raman Spectrosc. 2010, 41, 1725–1734
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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R. D. Bowen et al.
.
.
.
.
.
5
4
3
2
1
0
.
.
.
4
3
2
.1
0
3
000
2500
2000
Wavenumber/cm-1
Figure 3. FT-Raman spectrum of CH3(CH2)4OCD3 (PMOCD3); 1064 nm
1500
1000
500
3
000
2500
2000
Wavenumber/cm-1
1500
1000
500
Figure 5. FT-Raman spectrum of CH3(CH2)2CD2CH2OCH3 (PM2D2); condi-
tions and wavenumber range as for Fig. 3.
−
1
−1
excitation, 2000 scans, 4 cm spectral resolution; 3400–50 cm .
.
.
.
.
.
.
.
7
6
5
4
3
2
1
0
.
.
.
.
.
5
4
3
2
1
0
3000
2500
2000
Wavenumber/cm-1
1500
1000
500
3000
2500
2000
Wavenumber/cm-1
1500
1000
500
Figure 4. FT-Raman spectrum of CH3(CH2)3CD2OCH3 (PM1D2); conditions
and wavenumber range as for Fig. 3.
Figure 6. FT-Raman spectrum of CH3CH2CD2(CH2)2OCH3 (PM3D2); condi-
tions and wavenumber range as for Fig. 3.
Results and Discussion
polarized bands are accidentally degenerate with other polarized
or depolarized features of the spectra.
The complexity of the ν(CH) and δ(CH) regions of the FT-Raman
spectrum (Fig. 2) of the parent (unlabelled) PME illustrates the
problems faced in assigning the vibrations of a molecule in which
there is only one functional group and relatively little variation
in the nature of the C–H bonds. However, the introduction of
deuterium atoms instead of protium atoms in the methoxy
group or any one of the five distinct positions of the pentyl
chain permits the identification of the vibrations associated
with the methyl and methylene groups. The isotopically labelled
ethers used in this approach were CH3(CH2)4OCD3 (PMOCD3),
CH3(CH2)3CD2OCH3 (PM1D2), CH3(CH2)2CD2CH2OCH3 (PM2D2),
CH3CH2CD2(CH2)2OCH3 (PM3D2), CH3CD2(CH2)3OCH3 (PM4D2),
and CD3(CH2)4OCH3 (PM5D3). In the abbreviation used to define
each ether, a letter (‘O’) or number (‘1’–‘5’) and a subscript denotes
the position and number, respectively, of the deuterium atoms.
The FT-Raman spectra of PMOCD3, PM1D2, PM2D2, PM3D2,
PM4D2, and PM5D3 are shown in Figs 3–8, respectively. The
wavenumbers, assignments, and relative intensities of the bands
are summarized in Table 1. Only 18 of the 34 polarized bands
expected in the spectra of this series of compounds are observed.
This discrepancy probably indicates that a significant number of
To avoid undue complexity, the polarized and depolarized
components are not shown separately in Figs 1 and 3–8. However,
a stack plot of the polarized and depolarized Raman spectra
−
1
of PMOCD3 over the range 3400–500 cm is shown in Fig. 9
for illustrative purposes. Similarly, for the sake of simplicity,
information on which bands are polarized or depolarized are
showninTable 1onlyforPMEandPMOCD3.Ingeneral,thein-plane
modes are polarized in the Raman spectrum of a molecule with Cs
symmetry, whereas the out-of-plane modes are depolarized.
Even a cursory inspection of the spectra reveals that the ethers
with deuterium atoms in quite similar positions can be distin-
guished by Raman spectroscopy. The value of the bands that may
be attributed to ν(CD) stretching vibrations is particularly evident
in this context. Although only two such bands would be expected
from theoretical considerations, at least three bands appear in this
region of the spectra of the dideuteriated compounds; a fourth
band is found in the spectra of the trideuteriated analogues.
This increased multiplicity possibly reflects Fermi resonance or a
similar effect. Both the overall range and the position of most of
the individual bands are at lower wavenumber in the spectrum
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 1725–1734

Raman spectroscopy of n-C5H11OCH3 and deuterium labelled analogues
A more detailed discussion of the spectra is conveniently made
by focusing on three important regions spanning the ranges
−
1
.
.
.
.
4
3
2
1
0
3400–2600,2400–1800,and1800–250 cm .Thefirstandsecond
of these regions contain, respectively, the bands associated with
the ν(CH) and ν(CD) stretching vibrations, whereas the third
includes bands that may be ascribed to the ν(CC) and ν(CO)
stretching vibrations, and δ(CH2) skeletal and deformation modes.
The 3400–2600 cm 1 region
−
As expected on the basis of the generalization that the strongest
ν(CH) bands in the spectra of simple aliphatic molecules are found
between 2800 and 3000 cm , the ν(CH) stretching vibrations
dominatethisregion.Thecharacteristicshifttolowerwavenumber
induced by the increase in reduced mass when protium (H) is
replaced by deuterium (D) lowers the corresponding ν(CD) bands
−
1
3
000
2500
2000
Wavenumber/cm-1
1500
1000
500
−1
to between 2000 and 2200 cm , thus permitting many bands to
be ascribed to particular vibrations.
Figure 7. FT-Raman spectrum of CH3CD2(CH2)3OCH3 (PM4D2); conditions
and wavenumber range as for Fig. 3.
Group theoretical considerations indicate that 12 ν(CH) bands
ought to appear, eight of which should be polarized. For Cs molec-
ular symmetry, both the symmetric and asymmetric stretching
ꢁ
modes of each of the two methyl groups are of A symmetry;
.
.
.
.
.
5
4
3
2
1
0
consequently, all four of these ν(CH) bands are polarized. On the
other hand, the symmetric and asymmetric ν(CH) vibrations of
each methylene group give rise to a polarized and a depolarized
band. As a result, eight of the total of 12 ν(CH) bands are polarized;
the remaining four are depolarized. All 12 of the bands associated
with these ν(CH) modes are moved by deuteriation from their
−
1
usual positions in the 3000–2800 cm range to the 2200–2000
region. Consideration of which bands are present or absent in
the spectra of the selectively labelled ethers permits the bands
corresponding to the ν(CD2) modes of the four methylene groups
and the ν(CD3) modes of the two methyl groups to be identified.
Theassignmentoftheasymmetricν(CH3)vibrationofthemethoxy
−
1
group illustrates this approach. This band is present at 2987 cm
3
000
2500
2000
1500
1000
500
in the spectrum of PME; but it is not seen in the spectrum of
Wavenumber/cm-1
−1
PMOCD3, which instead contains a new band at 2236 cm
,
which may therefore be attributed to the corresponding ν(CD3)
Figure 8. FT-Raman spectrum of CD3(CH2)4OCH3 (PM5D3); conditions and
wavenumber range as for Fig. 3.
−
1
vibration. The original band at 2987 cm ascribed to the ν(CH3)
vibration is, however, present in the spectra of PM1D2, PM2D2,
PM3D2, PM4D2, and PM5D3, as would be expected if it was
specifically associated with the unlabelled methoxy group.
One valuable application of the spectra of the deuteriated
ethers is the confirmation or correction of the assignment of
of PMOCD3 than in the spectrum of PM5D3. Thus, the bands at
highest and lowest wavenumber in this part of the spectrum of
−
1
PM5D3 are at 2240 and 2057 cm , respectively, whereas the
corresponding bands in the spectrum of PMOCD3 appear at 2236
−
1
bands to specific structural features. Thus, a band at 2860 cm
had previously been attributed on the basis of related studies
on deuteriated analogues of n-propanol to a ν(CH2) symmetric
stretch,[ ratherthanaν(CH3)mode.Thisassignmentisconfirmed
by the spectra of the labelled PMEs. In contrast, the band at
−
1
and 2076 cm . These shifts to lower wavenumber are consistent
with the influence of the more massive oxygen atom to which
the methyl group of the methoxy group is attached. These trends
suggest that Raman spectroscopy can be applied to distinguish
a methoxy group from a C-terminal methyl group, even when the
heavy atom skeleton of the molecule is unbranched.
10]
−
1
2
874 cm clearly is a ν(CH3) asymmetric stretch of the terminal
methyl group of the pentyl chain. It is not present in the spectrum
of PM5D3. Consequently, the previous assignment[ of this band
toaν(CH2)symmetricstretchingmodemustbeincorrect.Similarly,
1,8]
Similar effects are found for labelled methylene groups: the
strong band at lowest wavenumber is found at 2061, 2120,
an assignment[ of a band at 2735 cm to a ν(C CH) aliphatic
mode cannot be correct. This band is observed in the spectra
of PME and its deuteriated analogues, none of which contains a
double bond. Therefore, this band can confidently be assigned to
a ν(CH2CH2) or ν(CH2CD2) stretching vibration.
3]
−1
−
1
2
094, and 2109 cm , respectively, in the spectra of PM1D2,
PM2D2, PM3D2, and PM4D2. The range of values found for the
position of this band associated with stretching vibrations of a
methylene group which is not directly flanked by an oxygen atom
−
1
(
2108 ± 12 cm ) in the spectra of PM2D2, PM3D2, and PM4D2
−
1
clearly lies above that of the band at 2061 cm in the spectrum
of PM1D2. Parallel trends are found in the corresponding ν(CH)
bands, but the effects are less obvious because of overlap with the
large number of other bands in this part of the spectrum.
The 2400–1800 cm 1 region
−
The assignment of the bands in this region concurs with the
literature[
11]
for alkanes, halogenoalkanes, and alcohols. On
J. Raman Spectrosc. 2010, 41, 1725–1734
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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R. D. Bowen et al.
Table 1. Vibrational wavenumbers (cm 1) and assignments of bands in the Raman spectra of n-pentyl methyl ether and six deuterium labelled
−
analogues
a
Compound PME
PMOCD3
PM1D2
PM2D2
PM3D2
PM4D2
PM5D3
Probable Assignment
d
2
2
2
2
2
2
2
2
2
2
2
987 m, sh
2987 m,sh
2961 m
2987 m,sh
2961 m
2935 s
2914 s
2989 s
2874 s
2861 s
2823 s
2814 m
2987 m,sh
2961 m
2935 s
2982 m
ν(CH3) asym, CH3O
ν(CH3-CH2) sym
ν(CH3) asym
d
d
961 m
2960 m,sh
p
p
937 ms
2937 ms
2936 s
2930 s
2929 s
p
p
916 s
2915 s
2917 s
2914 s
2907 s
ν(CH2-CH3) asym
ν(CH3) sym
900 mw,sh
2900 mw,sh
2900 mw,sh
2874 s
2895 mw,sh
p
p
874 s
2874 s
2873 s
2873 s
ν(CH3-C) sym
d
d
861 ms,sh
2960 ms,sh
2960 ms,sh
2861 ms
2827 s
2858 ms,sh
2827 s
2860 s
ν(CH2) asym
p
827 ms
2827 s
ν(CH3-O) sym
d
810 m
2814 ms
2733 mw
2814 ms
2755 w
2735 w
2812 m,sh
2755 w
2810 m,sh
2755 w
2737 mw
ν(CH3-O) asym
ν(CH2-CH2) stretch
ν(CH2-CH2) sym
ν(CH2-CH2) asym
p
p
755 w
2755 w
p
p
735 mw
2735 w
2733 mw
2734 mw
2705 w
2669 w
2669 w
2671 w
2672 w
2650 w
d
2
265 w,sh
d
2
236 m
2240 m
ν(CD3)
2220 w
2219 ms
ν(CD2)
2
195 mw
158 m
2196 ms
ν(CD2)
d
2
182 m
2182 ms
2158 ms
2138 m
2183 ms,br
2137 m
ν(CD2) sym
ν(CD2)
2
2140 m
p
2
129 m
2121 m
2120 s
2125 s
2176 s
d
2
110 mw,sh
2110 w
2109 s
2094 s
2070 ms
p
2
054 s
2061 s
2063 m,br
ν(CD3); ν(CD2) sym
1
929 m,br
d
d
1
470 m,sh
1470 m,sh
1470 m,sh
1453 s
1470 mw,sh
1453 s
1470 mw,sh
1453 s
1470 mw,sh
1448 s
δ(CH2) scissors
δ(CH2) scissors
d
d
1
446 s
1444 s
1449 s
1368 w
1
310 w,sh
d
d
1
302 ms
1302 ms
1303 ms
1258 w
1301 m
1280 m
1255 w
1292 m,br
1257 w
1300 s
δ(CH2) wag
δ(OC–H)
δ(OC–H)
δ(OC–H)
d
d
1
280 mw
1280 mw
d
1
250 w
1238 w
d
1
210 w
1220 mw
1210 w
1
184 w
d
d
1
1
1
159 w
1150 w
1159 w
1150 w
1130 m
ν(C–C)
ν(C–C)
ν(C–C)
ν(C–C)
ν(C–C)
ν(CCO) oop
ν(CCO)
p
p
137 w,sh
1137 ms
1130 ms
1105 mw
1064 ms
1130 ms
1131 m
1116 m
p
119 w,br
1122 s
091 s
d
1
103 m
1105 w
1
p
d
1
068 m
1083 ms
1070 w
1067 m
1035 w
1058 s
1041 ms
1035 w
p
p
1
030 mw
1031 w
1024 w
982 mw
950 w
p
9
89 w
994 m
953 w
993 mw
964 w
992 w
955 w
δ(CD2)
p
9
61 m,br
952 m
930 w
ρ(CH2)
9
42 w
ρ(CH2)
p
9
30 w
930 w
931 s
ρ(CH3)
9
15 w
ρ(CH3)
d
8
99 m
900 mw
884 mw
899 w
884 w
893 w
892 w
ν(COC) asym
ν(COC) sym
p
p
8
71 ms
863 mw
874 m
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 1725–1734

Raman spectroscopy of n-C5H11OCH3 and deuterium labelled analogues
Table 1. (Continued)
a
Compound PME
PMOCD3
PM1D2
845 m
PM2D2
27 mw
PM3D2
PM4D2
PM5D3
Probable Assignment
p
p
8
46 ms
837 ms
851 ms
859 mw
832 mw
854 mw
838 s
ν(CCO); ν(CO)
ν(CCC)
8
7
99 mw
ν(CCC)
p
p
7
73 m
769 m
775 mw
783 m
760 w
783 m
783 mw
750 w
ν(CCC)
7
60 w
30 w
755 mw
750 ms
729 ms
537 mw
503 w
442 w
392 mw
330 m
309 s
ν(CCC)
7
ν(CCC)
5
5
4
4
3
3
3
30 w
05 w
60 w
510 w,br
508 w,br
401 mw
505 w,br
ν(CCC)
δ(CCO)
δ(CCC)
δ(CCC)
δ(CCC)
δ(CCC)
460 w
p
p
10 mw
405 mw
408 w
394 w
404 w
p
30 w,sh
330 w,sh
317 ms
300 w
330 w,sh
317 ms
300 w
330 w,sh
312 ms
p
p
18 ms
311 ms
317 ms
300 w
00 w
300 w
300 w
a
See text for explanation of the meaning of the abbreviations PME, etc; the entries s, ms, m, mw, w, br and sh denote, respectively, strong, medium
strong, medium, medium weak, weak, broad and shoulder.
d
Denotes a depolarized band.
Denotes a polarized band.
p
− −1
1
2
987–2810 cm to 2265–2054 cm , respectively. The ‘isotopic
(a)
ratio’ of the wavenumbers of the ν(CH2) and ν(CH3) bands to
those of the corresponding ν(CD2) and ν(CD3) bands range from
.
.
.
3
2
1
0
−
1
1
.34 to 1.37 : 1. Thus, the bands at 2937 and 2916 cm associated
with the ν(CH2) stretching modes move to 2182/2183 (isotopic
−
1
ratio of 1.34) and 2158 cm (isotopic ratio of 1.35), respectively.
−
1
Similarly, the ν(CH3-CH2) combination band at 2735 cm moves
to ca 1936 cm (isotopic ratio of 1.38).
−1
Two polarized CD stretching bands would be anticipated in
the spectrum of a molecule of Cs symmetry containing one CD3
group; in contrast, only one such band is expected for the ethers
with a single CD2 group. These expectations are confirmed by the
assignments given in Table 1.
-.1
-.2
-.3
3
500
3000
Wavenumber/cm-1
2500
2000
The 1800–250 cm 1 region
−
A total of 15 skeletal modes, namely the ν(CC), ν(CO), and δ(CCC)
and δ(CCO) bands, are expected to appear in this wavenumber
region. Only 9 of the 12 expected bands of A symmetry can
(
b)
.
.
.
.
.
.
06
05
04
03
02
01
ꢁ
actually be identified in the spectrum of PME, including ν(CC)
−1
−1
−1
at ca 1150 cm , ν(CCO) at 1050 cm , ν(COC) at ca 880 cm ,
δ(CCC) at 770 cm 1, and δ(CCO)/δ(CCC) at ca 400 cm . Small, but
significant,variationsintheskeletalstretchingwavenumbersinthe
spectra of the labelled analogues may be ascribed to the changes
in reduced mass of the skeletal mode caused by substitution of a
CD3 or CD2 entity in place of CH3 or CH2, respectively. In theory,
the wavenumber of the CC and CO skeletal modes should not
be influenced by replacement of protium by deuterium, because
the skeletal vibrations are independent of those associated with
C–H or C–D bonds. In practice, however, significant shifts are
observed. This deviation from theory may be attributed to the
influence of mode mixing, primarily of skeletal vibrations with CH2
and CH3 deformation modes of the same symmetry class. Mode
mixing of this sort was observed in an earlier study in which the
−
−1
1
800 1600 1400 1200 1000 800 600 400 200
Wavenumber/cm-1
Figure 9. FT-Raman spectra of CH3(CH2)4OCD3 (PMOCD3); conditions as
for Fig. 3; 1064 nm excitation, 2000 scans, 4 cm spectral resolution;
a) 3800–1800 cm ; (b) 1800–100 cm ; upper spectrum ‘depolarized’,
−1
1
3
−1 −1
introduction of a C atom in the methoxy group of allyl methyl
(
lower spectrum ‘polarized’.
ether caused minor shifts in the wavenumbers of the ν(CCC),
[
5]
ν(CCO), and δ(CCC) and δ(CCO) bands. In the present case, the
−
1
δ(CCC) band at 410 cm in the spectrum of PME is moved to
−
1
replacement of protium with deuterium in PME, the ν(CH2) and
ν(CH3) asymmetric and symmetric stretching bands move from
392 cm in the spectrum of PM5D3. Similarly, the δ(COC) band at
846 cm is shifted to 837 cm in the spectrum of PMOCD3.
−
1
−1
J. Raman Spectrosc. 2010, 41, 1725–1734
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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R. D. Bowen et al.
In addition to the 15 skeletal vibrations expected for the C5OC
backbone, modes with CH2 and CH3 bending motion should occur
in this region. Both deformations [δ(CH2), δ(CCH), and δ(CH3)] and
rocking or wagging [ρ(CH2) and ρ(CH3)] modes are observed.
The band corresponding to the δ(OCH) mode generally cannot
be identified with confidence in most Raman spectra: it is usually
the spectra of both PMOCD3 and PM5D3, it cannot be associated
with the vibration of a methyl group. Therefore, it is better to
ascribe this band to a ρ(CH2) vibration, which fits the evidence
better than an alternative tentative assignment to a ν(CC) mode.
Similarly, the data in Table 1 clearly establish that the assignment
of the band observed at 883/891 cm in the Raman spectra of
biological specimens to a ρ(CH2) rocking mode is incorrect. It is
better to associate it with a ν(CCC) or ν(CCO) mode.
−
1
−
1
a medium intensity feature in the 1420–1300 cm region, in
which the stronger δ(CH2) and δ(CH3) bands are found. In the
−
1
PME series, however, a band in the 1280–1238 cm range can be
unambiguouslyassignedtothismode;theCHbandsthatnormally
obscureitcanbedifferentiatedfromitbyconsideringtheinfluence
of deuteriation on their wavenumber. This band appears at similar
On the other hand, the spectra of the deuteriated ethers
sometimes confirm previous tentative assignments. A band at
−
1
[1]
1450 cm in the spectrum of keratotic tissue had previously
[
12]
been attributed on the basis of literature precedent to a δ(CH2)
−
1
wavenumber to the band at 1274 cm assigned to the analogous
scissors mode. This assignment is correct because a band appears
[
10]
−1
δ(OCH) mode in the spectrum of n-propanol.
between 1446 and 1453 cm in the spectra of all the ethers
There is little observed difference between the wavenumbers
of the bands ascribed to skeletal C–C and C–O stretching modes
in the Raman spectra of PME and unbranched alkanes. This result
is, perhaps, hardly surprising: the masses of carbon and oxygen
atoms do not differ greatly; moreover, a strong coupling between
thesetwomodeswouldbeexpected.Ontheotherhand,variations
studied in this work. Similarly, the presence of a band in the
−
1
spectra of the PMEs in the 1302–1292 cm
range indicates
that the assignment[ of a band at 1290 cm in the spectra of
3]
−1
biological tissues to a δ(CH2) mode is accurate.
in the intensity of the bands in this region of the corresponding Conclusions
Raman spectra are more significant, not least because of the
This study of labelled PMEs permits useful corrections and
marked difference in polarity of the C–O and C–C bonds.
The δ(CH2) and δ(CH3) modes expected in the 1300–1470 cm
−
1
refinementstobemadeintheassignmentofnumerousubiquitous
features of the Raman spectra of simple aliphatic compounds. The
closer association of specific structural features with particular
bands is of obvious value in enhancing the analytical value
of Raman spectroscopy in a variety of biological and forensic
contexts.
range should be converted by deuteriation to δ(CD2) and δ(CD3)
−
1
features at lower wavenumber (960–1080 cm ). Unfortunately,
however, these CD deformation modes occur at similar wavenum-
ber to the ρ(CH2) and ρ(CH3) vibrations, thus further complicating
the assignment. The analogous ρ(CD2) and ρ(CD3) modes should
−
1
appear at even lower wavenumber (640–680 cm ). Nevertheless,
the CD2 and CD3 deformation modes may be assigned by careful
comparison of the spectra of the deuteriated PMEs.
Acknowledgements
The award of research grants from the Royal Society of Chemistry
and the British Mass Spectrometry Society for the purchase of
isotopically labelled starting materials to permit the synthesis
of deuteriated alkenyl methyl ethers and related compounds is
gratefully acknowledged.
In this low wavenumber region, the skeletal modes are best
described as combinations of vibrations of groups of three ‘heavy’
atoms, especially δ(CCC) and δ(CCO). Consequently, the details
of modes listed in Table 1 are again affected by mode mixing.
Notwithstanding this limitation, several interesting features may
be identified including a δCH deformation band that cannot be
associatedsimplywiththeatomsofparticularmethylormethylene
−
1
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−
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J. Raman Spectrosc. 2010, 41, 1725–1734