A NMR method for the analysis of mixtures of alkanes with different deuterium substitutions

Article abstract of DOI:10.1016/S1386-1425(97)00159-5

¹³C NMR at 125.76 MHz with ¹H and ²H decoupling, ²H NMR at 76.77 MHz with ¹H decoupling, and ¹H NMR at 500.14 MHz with ²H decoupling were employed as analytical tools to study the complex mixtures of deuterated ethanes resulting from the catalytic H-D exchange of normal ethane with gas-phase deuterium in the presence of a platinum foil. Reference samples consisting of 1:1 binary mixtures of pure normal ethane and ethane-d_n (n = 1-6) were used to identify the peak positions in the ¹³C, ²H, and ¹H NMR spectra due to each individual isotopomer, and the effect of isotopic substitution on the chemical shifts was determined in each case. While the NMR of all three nuclei worked well for the identification of the individual components of the 1:1 standard mixtures, both ¹H and ²H NMR suffered from inadequate resolution when studying complex reaction mixtures because of the broadening of the lines due to ¹H-¹H (¹H NMR) and ²H-²H (²H NMR) couplings. ¹³C NMR was therefore determined to be the method of choice for the quantitative analysis of the reaction mixtures. Using the ¹³C NMR results, a correlation that takes into account the primary and secondary isotope substitution effects on chemical shifts was deduced. This equation was used for the identification of the individual components of the mixtures, and integration of the individual observed resonances was then employed for quantification of their composition. This study shows that ¹³C NMR with ¹H and ²H decoupling is a viable procedure for studying mixtures of deuterated ethanes. Furthermore, the additivity of the isotopic effects on chemical shifts and the transferability of the values obtained with ethane to other molecules makes this approach general for the analysis of other isotopomer mixtures.

Full text of DOI:10.1016/S1386-1425(97)00159-5

Spectrochimica Acta Part A 53 (1997) 2481–2493
A NMR method for the analysis of mixtures of alkanes with
different deuterium substitutions
1
Alfonso Loaiza , Dan Borchardt, Francisco Zaera *
Department of Chemistry, Uni6ersity of California, Ri6erside, CA 92521, USA
Received 12 March 1997; accepted 19 June 1997
Abstract
1
3
1
2
2
1
1
C NMR at 125.76 MHz with H and H decoupling, H NMR at 76.77 MHz with H decoupling, and H NMR
2
at 500.14 MHz with H decoupling were employed as analytical tools to study the complex mixtures of deuterated
ethanes resulting from the catalytic H–D exchange of normal ethane with gas-phase deuterium in the presence of a
platinum foil. Reference samples consisting of 1:1 binary mixtures of pure normal ethane and ethane-d (n=1–6)
n
1
3
2
1
were used to identify the peak positions in the C, H, and H NMR spectra due to each individual isotopomer, and
the effect of isotopic substitution on the chemical shifts was determined in each case. While the NMR of all three
1
2
nuclei worked well for the identification of the individual components of the 1:1 standard mixtures, both H and H
NMR suffered from inadequate resolution when studying complex reaction mixtures because of the broadening of the
1
1
1
2
2
2
13
lines due to H– H ( H NMR) and H– H ( H NMR) couplings. C NMR was therefore determined to be the
13
method of choice for the quantitative analysis of the reaction mixtures. Using the C NMR results, a correlation that
takes into account the primary and secondary isotope substitution effects on chemical shifts was deduced. This
equation was used for the identification of the individual components of the mixtures, and integration of the
13
individual observed resonances was then employed for quantification of their composition. This study shows that
C
1
2
NMR with H and H decoupling is a viable procedure for studying mixtures of deuterated ethanes. Furthermore, the
additivity of the isotopic effects on chemical shifts and the transferability of the values obtained with ethane to other
molecules makes this approach general for the analysis of other isotopomer mixtures. © 1997 Elsevier Science B.V.
Keywords: NMR; Ethane mixtures; Isotopic substitution; H–D exchange; Catalysis
1
. Introduction
analytical technique to quantitatively analyze mix-
tures of deuterated alkanes was evaluated. The
analysis of such mixtures is in general quite
difficult, because it is not only important to con-
sider the number of hydrogen atoms being substi-
tuted by deuteriums, but also their specific
position within the molecules. This is particularly
relevant in catalysis, where detailed information
In this work the feasibility of using nuclear
magnetic resonance (NMR) spectroscopy as an
*
Corresponding author.
Permanent address: Laboratorio de Cin e´ tica y Cat a´ lisis,
1
Departamento de Qu ´ı mica, Facultad de Ciencias, Universidad
de Los Andes, M e´ rida 5101, Venezuela.
1
386-1425/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII S1386-1425(97)00159-5

2
482
A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
1
2
13
on the positions where H–D exchange takes place
can be directly related to the mechanism involved
in that process [1,2]. Here we use a prototypical
system, the reaction mixture obtained by the cata-
lytic exchange of normal ethane with deuterium
over a platinum foil [3–6], to test the usefulness of
NMR in this endeavor. The specific goal of this
research was to spectroscopically quantify the
amounts of symmetric and asymmetric deuterated
compound present in the final gas mixture, in
using the H- and H-decoupled C NMR spectra
of several mixtures of deuterium-substituted
ethanes of known compositions. Two types of
isotope effects on chemical shift were observed, as
expected: (1) a primary isotope effect for deu-
13
terium substitution on the C nucleus under ob-
servation ( D C); and (2) a secondary isotope
effect when the deuterium substitution takes place
1
13
13
on the carbon adjacent to the C nucleus under
2
13
observation ( D C). The additivity rules were
found to apply to this system not only when the
isotopic substitution occurs on only one of the
two ethane carbons (for instance, in the
CH3 CH3−nD molecules, where n=1–3), but
also in cases where substitutions were made on
a site different than that under measurement
(as in CH3−nDn CH D, CH3−nDn CHD , or
CH3−nDn CD ). The data obtained with these
controlled experiments were then successfully ap-
plied to the analysis of several complex mixtures
obtained from catalytic exchange studies. Finally,
primary isotope effects were also determined on
particular those for ethane-d . In a second paper
2
we will address the use of mass spectrometry for
the same purpose [7].
13
Little research has been reported so far on the
NMR of deuterated alkanes. Early NMR studies
on the effect of isotope substitutions on chemical
shift, reviewed by Batiz-Hernandez and Bernheim
in 1967 [8], indicated that the sign of the isotope
shift is almost always negative, that is, the reso-
nance frequencies decrease (move upfield) with
heavy isotope substitution (although there are a
few exceptions to this rule [9]). In addition, the
magnitude of the isotope shift has been found to
be a function of the resonant nucleus, being
higher for nuclei with larger chemical shift ranges
n
13
13
2
2
13
3
1
2
2
1
H-decoupled H NMR and H-decoupled
NMR spectra, and the results of the three meth-
ods were compared.
H
1
3
(
i.e. C has a larger chemical shift range than
1
2
either H or H, and therefore incurs greater
chemical shift differences upon isotopic substitu-
2. Experimental section
1
tion than those observed in the corresponding H
2
and H spectra). In addition, the magnitude of the
All NMR data were collected at 25°C on a
General Electric GN-500 NMR spectrometer
equipped with a home-built X-nucleus decoupler
and a Z-spec 5 mm inverse-detection broad-band
isotope-induced shift depends on: (1) the remote-
ness of the isotopic substitution site from the
observed nucleus; (2) the fractional change in
mass; and (3) the number of atoms substituted by
isotopes. With regard to the latter point, it ap-
pears that the intrinsic NMR isotope effects are
approximately additive, that is, that each isotopic
substitution acts almost independently of the oth-
ers, and that the overall shift observed in the
NMR spectra of a particular atomic center comes
out to be close to the sum of all possible contribu-
tions [10–12]. As an example of this, it has been
reported that CH4 D mixtures (n=0–4) yield
1
probe. H NMR spectra were collected as 16k
complex data points with an operating frequency
of 500.1357 MHz and a sweep width of 6536 Hz;
this sets the digital error for the chemical shifts at
90.0008 ppm. Typically sixteen transients were
acquired and averaged for each spectrum.
NMR spectra with H decoupling (CW) were
acquired in a similar manner except that the
X-nucleus decoupler was set to the H resonance
1
H
2
2
2
frequency of C H (76.7725 MHz) and gated on
−n
13
n
2
6
1
2
both H [13] and C NMR [14] spectra with
approximately equally-spaced peaks, and other
systems have shown similar behavior too [15–19].
Here, the correlation between NMR chemical
shifts and isotopic substitutions was studied by
during data acquisition. H NMR spectra were
collected as 16k complex data points with an
operating frequency of 76.7738 MHz and a sweep
width of 615 Hz (chemical shift digital preci-
sion= 90.0005 ppm). Typically 32 transients

A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
2483
were acquired and averaged for each spectrum.
H NMR spectra with H decoupling (500.1357
into a 5 mm o.d. Pyrex NMR tube filled with 0.5
ml of deuterated chloroform (Aldrich, 99.9%
atom D). The NMR tube was directly attached to
a larger glass tube equipped with a stopcock valve
and connected to the reaction system. The trap-
ping procedure for the reference binary mixtures
was as follows: (1) the whole reactor, including
the arm to which the NMR tube was connected,
was pumped to the mTorr range with a mechani-
cal pump; (2) 10 Torr of each of the gas samples
was introduced individually into the reaction sys-
tem while maintaining the stopcock of the NMR
tube closed; (3) that valve was opened to allow for
the gas mixture to condense into the liquid nitro-
gen-cooled NMR tube; and (4) the valve was then
closed and the tube flame-sealed. The same proce-
dure was employed for the multicomponent mix-
tures, except that 5 Torr of each of the gases was
used for mixtures containing more than two com-
ponents. The NMR signals of the deuterated chlo-
2
1
MHz, standard phase modulated) were acquired
1
in a similar manner except for the fact that the H
decoupler was gated on during the data acquisi-
1
3
tion. C NMR spectra were collected as above,
but with an operating frequency of 125.763 MHz
and a sweep width of 11364 Hz (chemical shift
digital precision= 90.006 ppm), and transients
were acquired and averaged until acceptable sig-
1
2
nal to noise levels were obtained. H- and H-de-
1
3
coupled C NMR spectra were obtained in the
1
2
same way, but the H and H decoupling channels
were again gated on during data acquisition. The
flip angle was set at 45°, and delay times of 60 s
were used for quantitative analysis and of 3 s
otherwise (see later). All H NMR spectra and
those requiring H decoupling were collected in
2
2
the unlocked mode. Spectral data were transferred
to a Silicon Graphics Personal Iris 4D35 com-
puter and processed using Felix (Biosym Tech-
nologies).
roform (CDCl ) were employed as reference for
3
the chemical shift scales by assuming values of
1
2
13
2
13
H, H, and C NMR spectra were recorded
for pure normal ethane (C H , Matheson, 99.9%)
7.26 and 77 ppm for the H and C peaks,
respectively. The small peak due to normal chlo-
2
6
1
and for each of the following deuterated ethanes
Merck-Canada and CIL, 99% atom purity):
CH CH D, CH CHD , CH DCHD , CH CD ,
roform (CHCl ) impurities seen in the H NMR
3
(
spectra was also set at 7.26 ppm.
Finally, several reaction mixtures of deuterated
ethanes were obtained by catalytic hydrogen-deu-
terium exchange between normal ethane and deu-
terium over a platinum foil (Aldrich, 99.99%
3
2
3
2
2
2
3
3
CH DCD , CHD CHD , CHD CD and C D . A
2
3
2
2
2
3
2
6
second set of spectra were taken for 1:1 mixtures
of C H +CH CH D, C H +CH CHD ,
C H +CH DCHD , C H +CH CD , C H +
2
6
3
2
2
6
3
2
2
purity) 0.1 mm thick and of 1.12 cm total area
2
6
2
2
2
6
3
3
2
6
1
3
CH DCD , C H +CHD CHD and C D +
[4]. 50 Torr of either normal C H or C H -1- C
2
3
2
6
2
2
2
6
2 6 2 6
CHD CD . Finally,
a
third set of NMR
and 500 Torr of D were introduced in the micro-
2
3
2
measurements were performed for the following
samples: (1) an equimolar mixture of all the
ethane molecules available to us (those listed
above, nine in total); and (2) several mixtures of
deuterated ethanes obtained via the catalytic hy-
drogen-deuterium exchange of normal ethane, as
batch reactor and allowed to mix for approxi-
mately 5 min with the recirculating pump, and the
foil was then heated to a constant temperature (to
625 K in the two cases reported here). The reac-
tions were followed mass spectrometrically by
continuously leaking the gas mixture into a com-
puter-interfaced Dycor MA-200M quadrupole
mass spectrometer linked to the circulating loop
system, and stopped after a preset overall conver-
sion was reached (approximately 20% for the
1
3
described below. In one instance CH CH (CIL,
3
3
9
9% isotopic purity) was used in the latter case in
order to increase the signal-to-noise ratio of the
1
1
3
3
C NMR spectrum. The spectrum of pure
CH CH was also taken for reference.
13
C-enriched mixture and 50% for the unenriched
3
3
All the NMR samples were prepared by first
one). The mixtures were then condensed into the
NMR tube, the excess deuterium gas was pumped
away, and the tubes were sealed as described
before.
premixing the gases in a stainless steel recirculat-
ing loop reactor system described in detail in a
previous publication [4] and then freezing them

2
484
A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
13
3
. Results
Fig.
2
shows the
C NMR spectra of
CH CHD , CH DCHD and CH DCD , each in
a 1:1 mixture with normal ethane. The chemical
3
2
2
2
2
3
1
3
3
.1. C Nuclear magnetic resonance
shifts for these compounds, two for each, were
1
3
13
The complete assignment of the C NMR reso-
found to be 6.499 and 6.119 ppm for CH CHD
3 2
1
3
nances from the mixtures of deuterated ethanes
was achieved by comparison with spectra from the
pure substances. The resonances for all of the
deuterated ethanes were measured with both H
and H decoupling to simplify the spectra, and lie
and CH
3
CHD
DCHD and CH
5.726 for CH DCD
order. From this, the isotope shift from substitut-
ing two h-deuteriums in CH CHD was deter-
2
, 6.210 and 6.020 ppm for
1
3
13
CH
D CHD
and CH D CD , in that
2 3
, and 6.106 and
2
2
2
2
1
3
13
2
3
1
2
3
2
in the range between 6.7 and 5.5 ppm. The NMR
shift for the majority of the deuterated pure stan-
mined to be −581 ppb, or −290.5 ppb per
deuterium, while the influence of two i-substi-
tuted deuteriums came out to be −201 ppb, or
dards
(CH CH D,
CH CHD ,
CH CD ,
3
2
3
2
3 3
−
−
100.5 ppb per deuterium. The isotope shifts of
680 and −490 ppb for ethane-1,2,2-d are the
CH DCHD , CHD CHD and CH DCD ) were
2
2
2
2
2
3
obtained in 1:1 mixtures with C H , while that for
3
2
6
result of the influence of either two h-substituted
plus one i-substituted deuterium (two times
CHD CD was measured in the presence of an
2
3
equal amount of C D . In order to simplify the
2
6
−
290 ppb plus one time −100 ppb) or one
h-substituted plus two i-substituted deuteriums
one time −290 ppb plus two times −100 ppb),
depending on the carbon atom being probed.
following discussion, substitutions of hydrogens by
deuteriums on the methyl moiety under measure-
ment, the one with the carbon atom which we will
call primary, will be named h-substitutions, while
isotopic substitutions on the neighboring methyl
fragment, that of the secondary carbon, will be
called i-substitutions. As examples of these deter-
(
Finally, the asymmetric ethane-1,2,2,2-d shows
4
isotope shifts of −974 and −594 ppb, which are
the result of the combined effect of three h-substi-
tuted plus one i-substituted deuterium for one
minations, Fig. 1 shows the spectra of CD CHD2
3
and CH CH D mixed with C D and C H respec-
3
2
2
6
2
6
13
tively. Under the C NMR measurement condi-
tions used here ( H and H decoupling), only one
and two resonance are observed for C H and
1
2
2
6
CH CH D, respectively: the resonance at 6.700
3
2
ppm corresponds to C H , and those at 6.410 and
2
6
13
13
2 3 2
6
.600 ppm to CH3 CH D and CH CH D, re-
spectively. From this, the measured isotope shift
due to one deuterium atom h-substituted in
1
3
1 13
ethane-d , CH CH D, was found to be D C=
1
3
2
−
290 ppb, while the isotope shift experienced by
the secondary carbon in CH CH D due to the
13
3
2
influence of the same deuterium atom (in this case
2
13
i-substituted) was D C= −100 ppb. The spec-
trum of CD CHD mixed with C D shows three
3
2
2
6
resonances as well: one at 5.530 ppm which corre-
sponds to C D , and two at 5.820 and 5.630 ppm
2
6
1
3
13
from CD3 CHD and CD CHD , respectively.
2
3
2
Fig. 1. Representative 1H- and 2H-decoupled 13C NMR spec-
tra from the 1:1 mixtures used in the calibration of the
chemical shifts induced by deuterium substitution. The left
Again, the isotope shifts resulting from substitut-
ing one normal hydrogen for a deuterium atom in
the h and i positions of ethane-d relative to the
5
panel shows the trace for an equimolar mixture of CD CD3
3
resonance frequency of C D were calculated to be
2
6
and CD CHD , while the right frame presents the data for the
3
2
−
290 and −100 ppb.
CH CH +CH CH D combination.
3 3 3 2

A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
2485
where nh-D and ni-D are the number of h- and
i-deuterium substitutions in the ethane isoto-
pomer being considered. Normally each com-
13
pound yields two peaks in the
C NMR
spectrum, one for each carbon (the definitions for
the h and i positions are relative to the carbon
atom being probed, and therefore reverse when
switching atoms), but in the symmetric molecules
(
CH CH , CH DCH D, CHD CHD , and
3 3 2 2 2 2
CD CD ) the two signals are identical and col-
3
3
lapse into one single feature. The accuracy of Eq.
1) is greater than 94 ppb, as estimated by
(
comparing the measured and calculated values for
the shifts in Table 1.
The reference data obtained with the experi-
ments reported above was then used to analyze
the composition of mixtures of deuterated ethanes
obtained after the catalytic H–D exchange reac-
tions described in the experimental section [4].
Fig. 3 displays the NMR spectra obtained for two
of those mixtures, one collected after a 20% con-
Fig. 2. Additional 1H- and 2H-decoupled 13C NMR for 1:1
mixtures of CH CH with one other pure ethane isotopomer:
3
3
CH CHD2 (left panel), CH DCHD (center panel), and
3
2
2
CH DCD (right panel). A summary of the data obtained with
2
3
mixtures such as these is given in Table 1.
carbon and of one h-substituted plus three i-sub-
13
version in the H–D exchange of C-enriched
stituted deuteriums for the other. This series of
13
perhydrido-ethane ( CH CH ) with deuterium
3
3
1
3
C NMR reference spectra was completed by
measuring the chemical shifts for CH CD and
(
left), and another prepared by 50% conversion of
3
3
normal (unenriched, CH CH ) ethane. It can be
3
3
CHD CHD (spectra not shown). The whole data
2
2
seen from that figure that the chemical shifts
observed for each of the components in the mix-
tures compare very well with those of the stan-
dards, never deviating by more than 93 ppb (see
Table 2, [4]). Notice in particular that the resolu-
tion of the resonances in the mixtures was suffi-
cient to allow for the identification of all the
species present. This is especially important in
catalytic experiments such as these, where the
most interesting species from the point of view of
set is summarized in Table 1.
The main conclusion from these experiments is
that the effect of each deuterium substitution on
13
the chemical shifts in the C NMR spectra of
deuterated ethanes is independent of any other
substitutions, and that therefore the additivity
rule applies well to this system. Indeed, the data
provided here show that the isotope shifts that
originate from any single h- or i-substitution
1
13
amount to about D C= −290 ppb and
mechanistic determinations (the ethane-d species
2
2
13
D C= −100 ppb, respectively, as measured di-
in this case) are only present in small amounts
(see below).
rectly on the ethane-d sample, and that all of
1
13
them add up to induce the overall observed dis-
placements. The general equation to calculate the
The quantitative analysis of the C NMR spec-
tra for the ethane mixtures is complicated by the
1
3
13
chemical shifts l C seen in the C NMR spectra
long T relaxation times of the carbon nucleus
1
of each deuterated ethane is:
[20]. As a consequence, long relaxation delay
times must be used during data collection in order
to prevent signal saturation and to insure that the
areas of the integrated resonances truly reflect the
concentration of the corresponding nuclei. With
the goal of evaluating the magnitude of this effect
1
3
l C(C H6−xDx)
2
1
3
1
13
2 13
=
l C(C H )+( D C·n + D C·ni-D)
2
6
h-D
1
3
13
13
D C=l C(C H
D )−l C(C H )
x 2 6
2
6−x
13
=
−(0.290·nh-D+0.100·ni-D (ppm)
(1)
in the analysis of our ethane mixtures, C NMR

2
486
A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
Table 1
1
3C NMR chemical shifts (l13C) for individual deuterated ethanes
l13C (ppm)
Isotope shift D C (ppb) (relative to C H )
13
Structure
2
6
Experimental
Calculated
Difference
1
1
3
3
CH CH3
6.700
6.600
6.410
6.499
6.119
—
0
−100
−290
−201
−581
—
0
−100
−290
−200
−580
−390
−300
−870
−490
−680
−590
−970
−780
−880
−1070
−1170
0
0
3
CH CH D
3
2
CH 13CH D
0
3
2
1
3
CH CHD2
−1
−1
—
3
3
CH 13CHD2
3
1
3
CH DCH D
2
2
1
3
CH CD3
6.403
5.828
6.210
6.020
6.106
5.726
5.919
5.820
5.630
5.530
−297
−872
−490
−680
−594
−974
−781
−880
−1070
−1170
3
CH 13CD3
−2
0
3
1
3
CH DCHD2
2
CH D13CHD2
0
2
1
3
CH DCD3
−4
−4
−1
0
2
CH D13CD3
2
1
3
CHD CHD2
2
1
3
CHD CD3
2
CHD 13CD3
0
2
1
3
CD CD3
0
3
The calculated isotope shifts were obtained by using Eq. (1). Digital precision in chemical shifts= 90.006 ppm.
1
3
spectra were acquired for the 1:1 C H +CH CD
3
C in normal samples to determine their compo-
2
6
3
13
mixture by using relaxation delays of 60 and 3 s
data not shown). The integrated relative areas for
sition by C NMR, this may sometimes not be
practical because of the lengthy data acquisition
times required to obtain reasonable signal-to-
(
the three peaks obtained in both cases are listed in
Table 3. It can be seen there that while the areas
in the spectrum recorded with the three second
delay deviate significantly from those expected
13
noise ratios. This is the reason why C-enriched
ethane was used in one of the H–D exchange
catalytic experiments (Fig. 3, left [4]): the delay
time between pulses could then be increased and
acceptable signal-to-noise levels could still be
reached without using prohibitively long data ac-
quisition times. Nevertheless, the data from the
experiments with normal (unenriched) samples in-
dicate that those are still quite feasible (see be-
low). The 60 s delay times used to acquire the
data in Table 1 were estimated to induce a sys-
tematic error in the concentration estimates of
about 10% towards lower values.
1
3
based on both the one-to-one ratio of C atoms
in CH CD (because of the expected equal con-
3
3
13
13
centrations of CH CD and CH CD ) and the
3
3
3
3
one-to-one ratio in the concentrations of C H6
2
and CH CD in the mixture, the data obtained
3
3
with sixty second delays approach the theoretical
values. Unfortunately, increasing the delay time
between pulses increases the data collection time
accordingly (the 60 s between-pulses spectrum
mentioned above required about six hours of data
acquisition time). As a consequence, while it is in
principle possible to use the natural abundance of
13
1
All C NMR data were collected with the H
2
and H decouplers off during the relaxation delay

A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
2487
Table 3
Comparison of signal intensities (peak areas) between 13C
NMR spectra of deuterated ethane mixtures taken with differ-
ent delay times between pulses. The sample probed consisted
of a 1:1 mixture of CH CH and CH CD
3
3
3
3
Compound l13
ppm)
C
Relative signal intensity
(
3
s delay
60 s delay Expected
1
1
3
3
CH CH 6.700
1.00
0.38
0.22
1.00
0.45
0.43
1.00
0.50
0.50
3
3
CH CD 6.403
3
3
1
3
CH3 CD3 5.828
tion decay (FID). The spectra in Fig. 3 were then
baseline corrected, and the peak signals were inte-
grated in order to evaluate the relative contribu-
tions of each component in the mixture. The
composition of the mixtures obtained this way
agree quite well with those estimated indepen-
dently by low-resolution mass spectrometry, even
for samples where unenriched ethane was used
Fig. 3. 1H- and 2H-decoupled 13C NMR spectra for two
mixtures of deuterium-substituted ethanes obtained by H–D
exchange reactions of normal (d ) ethane with deuterium over
0
a platinum catalyst. Left: mixture obtained with 13C-enriched
ethane after a 20% conversion. Right: mixture obtained with
1
3
natural abundance C-ethane after a 50% conversion. The
reaction conditions in both cases were initial C H6 and D2
2
pressures of 50 and 500 Torr, respectively, and a reaction
temperature of 625 K.
(
Table 2, [4]), but additional information (not
to eliminate nuclear Overhouser effects (NOE), a
common problem in C NMR; they were gated
on only during the acquisition of the free induc-
available from the mass spectrometric analysis)
was obtained with the NMR data regarding the
relative amounts of symmetrically- and asymmet-
rically-substituted isotopomers with the same
number of deuterium substitutions. There are
some differences between the results from both
techniques, but those may be not only due to the
underestimation of the concentration of the highly
deuterium-substituted species because of the short
delay times between pulses used, but also to the
deconvolution technique used in the analysis of
the low-resolution mass spectrometry data [4,7]. It
is important to highlight here that even though
13
Table 2
13C NMR and low resolution mass spectrometric analysis of a
sample consisting of a mixture of deuterated ethanes, obtained
after 10 min of reaction between 50 Torr of C H6 and 500
2
Torr of D2 over a platinum foil heated to 625 K. The NMR
spectra is shown in the right panel of Fig. 3.
Compound
l13C (ppm)
Concentration (% total)
13C NMR
Low Res.
MS
13
the use of C-enriched samples may make the
data analysis easier and more accurate, normal
unenriched samples do yield reasonable results as
well (see for instance the spectrum in the right
panel of Fig. 3). Moreover, several experimental
protocols can be used to improve the quantifica-
tion of the C NMR signals, including the use of
relaxation agents in the mixture or specific pulse
sequences during the NMR spectra acquisition
CH CH3
6.698
6.601, 6.410
6.501, 6.110
53.9
4.9
1.1
1.9
0
3.5
3.4
3.8
12.2
15.3
43.0
13.3
3
CH CH D
3
2
CH CHD2
3
5.3
3.4
5.2
CH DCH D 6.311
2
2
CH CD3
6.400, 5.828
3
CH DCHD2 6.207, 6.019
2
CH DCD3
6.107, 5.729
13
2
CHD CHD2 5.920
2
CHD CD3
5.818, 5.627
5.523
11.3
18.5
2
CD CD3
3
[20].

2
488
A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
was found to be −18 ppb, and a second h-substi-
tution on the resulting dideuteroethane-1,1-d
2
2
2
cause an additional isotope shift D H=l H
2
3 3 3 2
(
CH CD )-l H (CH CHD ) of about −20 ppb;
the average comes out to be an isotope shift of
1
2
D H= −19 ppb per h-substituted deuterium.
Next, the chemical shifts of CH DCHD2 and
2
CH CHD are also contrasted in Fig. 4. There are
3
2
two distinct types of deuteriums in CH DCHD ,
2
2
and consequently two (not completely resolved)
resonances are seen in that case at 0.836 and 0.845
ppm, with relative intensities of approximately
2
:1: the peak at 0.836 ppm belongs to the –CHD2
deuteriums and is the result of one h and one i
substitution, while the signal at 0.845 ppm be-
Fig. 4. 1H-decoupled 2H NMR of four pure ethane isoto-
pomers. From left to right, those compounds are: CH CH D,
longs to the –CH D deuterium and shifts because
2
of the influence of two i substitutions. Notice
3
2
CH CHD , CH DCHD , and CH CD . The chemical shifts
3
2
2
2
3
3
that the signal of the –CHD2 group in
obtained by experiments such as these with all the pure
isotopomers available to us are summarized in Table 4.
2
CH DCHD is shifted by about D H= −12 ppb
2
2
with respect to that for CH CHD , an effect
3
2
associated with one i-substitution; analysis of
2
3
.2. H Nuclear magnetic resonance
other samples provides a value for that effect
2
2
closer to D H= −9 ppb.
2
H NMR spectra for each deuterium-substi-
tuted ethane were recorded as well. All H NMR
measurements were done with H decoupling to
avoid the presence of multiplets due to H–D
couplings. The signals for the reference mixtures
Additional measurements were done with all
the other reference compounds available. Table 4
2
1
2
summarizes the H NMR parameters for each of
the individual deuterated ethanes. As mentioned
above, the individual shifts upon h- and i-isotope
2
appear in the range l H=0.866-0.800 ppm, the
1
2
substitutions were estimated to be about D H=
limits corresponding to monodeutero- and
perdeutero-ethane respectively, while those for the
H–D exchange reaction sample were slightly dis-
placed to the l H=0.870–0.806 ppm range.
A similar analysis to that used for the
2
2
−
19 ppb and D H= −9 ppb, respectively, and
were shown to also be additive. From this the
following equation was derived to determine the
chemical shifts for each species present in the
mixture of deuterated ethanes:
2
1
3
C
2
NMR spectra can be used here for the H NMR
data. To begin with, Fig. 4 compares the
2
2
H
l H(C H6−xDx)
2
NMR resonance frequencies for CH CH D,
3
2
2
1
2
2 2
=
l H(C H D)+( D H·n + D H·ni-D)
2
5
h-D
CH CHD and CH CD , the ethane isotopomers
3
2
3
3
2
2
2
that have deuteriums in only one of the carbon
atoms and therefore give rise to one single reso-
D H=l H(C H
2
6−x
D )−l H(C H D)
x
2
5
=
−(0.019·nh-D+0.009·ni-D) (ppm)
(2)
2
nance each. The observed chemical shifts, l H,
decrease as the number of deuteron substituents
on the ethane molecule increases, from 0.866 ppm
for CH CH D to 0.848 ppm for CH CHD and
The shifts calculated using this equation are
also reported in Table 4.
Inspection of Table 4 points to one of the
3
2
3
2
2
0
.828 ppm for CH CD . From this, the isotope
shortcomings of using H NMR for the analysis
3
3
2
2
2
shift D H=l H (CH CHD )−l H (CH CH D)
due to the h-substitution of one hydrogen by
of mixtures of deuterium isotopomers. It can be
seen there that the absolute errors incurred in
calculating the chemical shifts for unknown sam-
3
2
3
2
deuterium on the monodeuteroethane molecule

A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
2489
Table 4
2
2
H NMR chemical shifts (l H) for individual deuterated ethanes. The calculated isotope shifts were obtained by using Eq. (2).
Digital precision in chemical shifts= 90.0005 ppm.
l2H (ppm)
c D atoms
h-carbon
Isotope shift D H (ppb)
2
Structure
i-carbon
Experimental
Calculated
Difference
CH CH D
0.866
—
0.848
0.845
0
—
0
−9
0
—
1
−3
−2
0
−1
−3
0
3
2
CH DCH D
0
1
0
1
2
0
2
1
1
2
2
1
0
2
1
0
3
1
2
3
2
3
2
2
CH CHD2
−18
−21
−30
−38
−28
−50
−37
−50
−56
−66
−19
−18
−28
−38
−27
−47
−37
−46
−56
−65
3
CH DCHD2
2
0.836
CH CD3
0.828
0.838
3
CH DCD3
2
0.816
CHD CHD2
0.829
0.816
2
CHD CD3
−4
0
−1
2
0.810
CD CD3
0.800
3
ples by using Eq. (2) are about the same as in the
tification of the rest is somewhat hindered by the
overlap of many of the features in the spectrum,
but the data is nevertheless of good enough qual-
ity to render individual peaks for almost all un-
equivalent deuteriums, allowing for the complete
identification of all compounds in the equimolar
1
3
case of the C NMR, but that the relative errors
are much larger because the absolute shifts in-
duced by deuterium substitutions are more than
2
an order of magnitude smaller in the H NMR
case: the worst cases in Table 1 display differences
between the experimental and calculated chemical
shifts of less than 1%, while those in Table 4 can
show discrepancies of the order of 10%. In addi-
2
tion, there are many overlaps in the H NMR
spectra, a problem compounded by the low reso-
lution of the individual peaks. For instance, the
signals for the asymmetric ethane-d overlap with
4
those for CH DCHD and ethane-d , and one of
2
2
5
the peaks for CH DCHD is close to that for the
2
2
asymmetric ethane-d2.
In order to further test the analytical power of
H NMR for the quantitative analysis of isoto-
2
pomeric samples, a spectrum was taken for an
equimolar mixture of all the deuterated ethanes
used in this work (all possible isotopomers except
for the symmetric ethane-d ). The left panel of
2
Fig. 5 shows that spectrum, which displays about
ten distinguishable resonances. The end reso-
nances at 0.869 and 0.805 ppm clearly correspond
Fig. 5. 1H-decoupled 2H NMR spectra for an equimolar
mixture of all possible ethane isotopomers except for the
symmetrically-substituted ethane-d (CH DCH D). Left: raw
2 2 2
data and peak assignment. Right: spectra obtained by using a
double exponential window function before Fourier transfor-
mation of the time-based data, as described by Brown et al.
to ethane-d1 and ethane-d , respectively. Also,
6
using the data from the reference spectra (Table
4
), the peak at 0.814 ppm can be easily assigned
[
23]. The peak assignment and relative peak areas are also
to the –CD deuteriums of ethane-d . The iden-
provided in Table 5.
3
5

2
490
A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
Table 5
2
H NMR chemical shifts and relative intensities for an equimolar mixture of all possible deuterated ethanes except for ethane-1,2-d2.
Structure
l2H (ppm)
Relative intensities
Experimental
Experimental
Calculated
Expected
CH CH D
0.869
0.850
0.851
0.869 (reference)
0.850
0.851
0.841
0.831
0.842
0.822
0.832
0.823
1.0
2.3
0.6
1.3
3.8
1.5
2.3
2.7
1.4
4.5
4.4
1
2
1
2
3
1
3
4
2
3
6
3
2
CH CHD2
3
CH DCHD2
2
0.843
CH CD3
0.833
0.840
3
CH DCD3
2
0.821
CHD CHD2
0.830
0.824
2
CHD CD3
2
0.814
0.813
0.804
CD CD3
0.805
3
mixture; Table 5 contrasts the chemical shifts
observed experimentally with those calculated us-
ing the formula provided above. This is however
no longer true in the case of the mixtures that
results from the catalytic studies, because in those
systems the concentration of some of the isoto-
pomers is too low to be well resolved in the H
NMR spectra (Fig. 6).
It can also be seen from the example above that
even though the effect of deuterium substitution
on the chemical shift appears to be additive, there
is insufficient resolution to adequately quantify
most mixtures (see the comparison between the
experimental and calculated H NMR peak areas
2
in Table 5). It is in principle possible to improve
the resolution of the spectra by using zero filling
and resolution-enhancement approaches in which
the accumulated decay signal is multiplied by a
weighting function before the Fourier transforma-
tion of the data [21,22]. A direct comparison with
the approach described by Brown et al. [23] was
done here by using a double exponential weight-
ing function on the data from the equimolar
ethane mixture, the result of which is displayed in
the right panel of Fig. 5. The results of that
analysis show that in practice this procedure dis-
torts the peak shapes and renders the quantitative
analysis unreliable. Alternatively, the raw spectra
could be fitted to a set of Lorentzian peaks. This
was also attempted for the equimolar mixture
used in Fig. 5, but even though the overall fit was
quite good, the individual areas obtained this way
displayed rather large errors (see Table 5). It is
worth mentioning that the original data appears
to reflect the composition of the mixture quantita-
tively since, after normalizing the total area of the
spectrum to the 28 deuterium atoms expected
from the mixture, the area of the peak for
CH CH D (the only one clearly separated from
2
Fig. 6. 1H-decoupled 2H NMR spectrum for the reaction
mixture obtained after catalytic exchange of normal C H with
3
2
2
6
the rest) amounted to the expected value of one
the actual experimental value was 1.004). The
failure of the fitting analysis in yielding an accu-
D over a platinum foil. This is the same sample as in the left
2
(
panel of Fig. 3. The assignment of the peaks to the different
isotopomers is also provided.

A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
2491
rate estimate of the composition of the sample is
most likely due to the fact that the shapes for the
2
H NMR peaks of the three species which have
two types of unequivalent deuterium atoms may
be dominated by unresolved deuterium–deu-
2
terium couplings. Overall, H NMR proved to be
adequate for the identification of the components
of complex ethane mixtures, but not good enough
to obtain quantitative information about their
composition.
1
3
.3. H Nuclear magnetic resonance
In a further attempt to identify and quantify
the composition of complex mixtures of deuter-
ated ethanes, proton NMR was also employed in
Fig. 7. 2H-decoupled 1H NMR spectrum of the equimolar
mixture of ethane isotopomers used in the experiment reported
in Fig. 5. A partial peak identification was attempted, but the
complete assignment was hindered by the splitting of the
signals for hydrogens in asymmetric ethane molecules due to
1
3
2
this study. As in the C and H NMR character-
1
ization, H NMR spectra of 1:1 mixtures of
ethane with the various deuterated ethanes were
run first. The normal non-decoupled spectra dis-
played only broad and nondescript multiplets.
The spectra were simplified significantly by em-
1
1
H– H spin couplings.
4
. Discussion
2
ploying H decoupling, but still presented com-
1
1
The main goal of this research was to develop a
plex multiplets due to H– H coupling for all but
CH CD , CHD CD and CHD CHD , which col-
reliable method for the analysis of mixtures of
isotopomers of hydrocarbons, in particular of
ethane. In our evaluation of NMR as a technique
to be used for this task, a few general observa-
tions were made. First, examining the results ob-
3
3
2
3
2
2
lapsed to single resonances. It would be normally
2
1
expected for the H-decoupled H NMR spectra
of the deuterated ethanes to collapse to multiplets
containing no more than four peaks, but that is
not the case; higher-order splittings appear here
because the chemical shift differences between the
coupled protons is of the same order as the cou-
pling constant for these same protons. Therefore,
while it is possible to partially identify the chemi-
cal shifts and coupling constants for the 1:1
ethane reference mixtures used in this work, it is
much more difficult to analyze mixtures of deuter-
ated ethanes, especially those resulting from H–D
exchange reactions. To illustrates the degree of
difficulty of this task, Fig. 7 shows the spectrum
obtained for the equimolar mixture used before,
together with a partial assignment of the peaks.
13
2
1
tained by C, H and H NMR, it was found that
deuterium isotope substitutions always induce
upfield chemical shifts in the signals, as reported
before [8]. Also, this effect was proven to be
13
greater on C than the other nuclei, most likely
because of the correspondingly greater chemical
13
shift range in the C NMR. Third, it was shown
1
2
13
that H- and H-decoupling in the C NMR
spectra gives rise to simple spectra for each
deuterated compound. The effect of deuterium
substitution on the ethane chemical shift was di-
rectly correlated with the number of deuterium
substituents and the positions occupied by each
deuterium in the ethane molecule. Thus, two iso-
1
The H NMR spectra could be simplified some-
what by running them on a higher field instru-
ment, but that would not completely eliminate the
many overlapping resonances expected for the
mixture. As the objective here is to analyze com-
plex mixtures of deuterated ethanes quantita-
1
13
tope effects were observed: (1) D C, the effect
observed when the deuterium substituted is
bonded to the nucleus under observation; and (2)
2
13
D C, the effect observed when the substituted
deuterium is one carbon away from the nucleus
1
tively, H NMR can be deemed unsatisfactory.

2
492
A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
13
1
under observation. The value for D C was
found to be −290 ppb per deuterium, quite
close to those reported by Wesener et al. for
monodeuterated ethane (−284 ppb), propane
accurate to within a few percent provided that
the spectra are acquired employing a sufficient
relaxation time. The overall composition ob-
tained with NMR matched reasonably well that
from mass the spectrometry analysis [4,7].
(
[
−299 ppb), and ethyl benzene (−296 ppb)
10], suggesting that these values are transferable
to other compounds. In addition, the value of
2
It has also been shown here that H NMR can
in principle be used in a similar manner to ana-
lyze mixtures of deuterated ethanes. However,
even though a resolution enhancement routine
and an increased field as compared with that in
the report by Brown et al. [23] were used in this
work, the resolution was still insufficient to accu-
rately quantify the composition of the mixture.
In order to obtain a complete separation of the
signals in the deuterium spectra peak fitting
2
13
D C was found to be −100 ppb per deuterium,
and both effects were found to be additive.
13
The large separation in the C NMR spectra
among the signals from the different isotopomers
of ethane and the additive nature of the effect of
isotopic substitution on chemical shifts make this
technique ideal for the analysis of isotopomeric
mixtures. In particular, it was shown here that it
is clearly possible to distinguish between symmet-
rically and asymmetrically substituted ethanes.
This turned out to be key in the determination
of the mechanism involved in the H/D exchange
of ethane over platinum catalysts [4]. In particu-
lar, it was found that the yield for CH DCH D
production was more than twice that for
CH CHD , a result that suggests that adsorbed
ethylene is one of the main intermediates in the
mechanism for the complete exchange of normal
ethane to the perdeutero isotopomer [1,3,4].
It is also worth noticing that a particular chal-
lenge in the analysis of mixtures of isotopomers
from catalytic processes is the fact that the sev-
eral products are produced in significantly differ-
ent quantities. First of all, it is desirable to keep
the overall conversion low in order to avoid sig-
nificant contributions from secondary reactions,
which means that a large fraction of the mixture
is still composed of the reactants (more than 50%
of the mixtures analyzed here was C H ). In ad-
needed to be employed, and that procedure
proved unreliable as well. Consequently,
2
H
NMR is obviously a less desirable technique
13
than C NMR for these analyses, even though
2
H NMR does have the advantage that it is
2
2
13
more sensitive than C NMR and therefore re-
13
quires less acquisition time, especially if C un-
enriched samples are used. Finally, proton NMR
suffers from the complications of second order
splittings, which causes severe overlap of the ob-
served resonances. Fitting theses types of spectra
is in principle possible, but requires knowledge of
3
2
1
1
the exact chemical shifts and H– H coupling
constants.
It should be straightforward to extend the ana-
lytical approach reported here to the study of
more complex systems. In particular, it would be
useful to extend the work on platinum to other
metals, since different catalysts display different
exchange patterns: while some only favor single
exchange steps, others catalyze complete ex-
change exclusively, and a third group (which in-
cludes platinum) is capable of promoting both
pathways [5]. It would also be interesting to
study the H-D exchange in larger alkanes, start-
ing with either propene or neopentane, for which
a new mechanistic channel involving h, k-diad-
sorbed intermediates is open. It would be ex-
pected for the values of the isotope-induced
chemical shifts measured in this work to be ap-
plicable to those problems.
2
6
dition, the particular system studied in this work
favors either mono- or per-deutero substitutions;
the concentrations of CH CH D and CD CD in
3
2
3
3
the sample reported in Table 2 were about 13
and 19%, respectively. The challenge was to de-
termine the ratio of symmetric to asymmetric
ethane-d2 produced from a total of about 5%
13
C H D in a complex matrix, and C NMR did
2
4
2
a nice job of it. As shown above, quantification
of NMR signals is easily accomplished by inte-
1
3
gration of the observed C resonances, and is

A. Loaiza et al. / Spectrochimica Acta Part A 53 (1997) 2481–2493
2493
5
. Conclusions
fred T. Tysoe for providing most of the deu-
terium-labelled pure samples.
The use of NMR was evaluated as an analytical
method to study a complex mixture of deuterated
ethanes. The results indicate that the isotope sub-
stitution effects on ethane chemical shifts are ad-
ditive and can therefore be predicted by using a
general simple equation, thus allowing for the
identification of each individual deuterated ethane
in complex mixtures. The power of this NMR
approach is that not only the degree but also the
regiospecificity of deuterium substitution can be
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[
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1
3
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[
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[
[
7] A. Loaiza, F. Zaera, 1997, to be published.
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(
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[
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2
to provide only slight improvements in the H
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1
[
[
[
13] R.A. Bernheim, R.J. Lavery, J. Chem. Phys. 42 (1965)
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complications induced by overlapping multiplets
1
1
due to H– H coupling, which makes it unreliable
for the study of deuterated ethanes. Of the three
nuclei available for NMR in the study reported
7
5
1
3
here, C was shown to be by far the best choice
because of the superior resolution and degree of
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(1977) 2057.
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50 (1969) 1897.
1
simplicity in the spectra obtained under H and
2
H decoupling conditions.
[
[
[
[
[
[
18] A.K. Jameson, C.J. Jameson, J. Magn. Reson. 32 (1978)
55.
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Acknowledgements
This work was supported by the National Sci-
ence Foundation (NSF) under grant No. CHE-
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9
530191, and by the Consejo de Desarrollo
Cient ´ı fico, Tecnol o´ gico y Human ´ı stico (CDCHT)
of the Universidad de Los Andes under contract
C-554–92. We are also grateful to Professor Wil-
1
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