Deuterium-induced isotope effects on the 13C chemical shifts of α- D -glucose pentaacetate

Article abstract of DOI:10.1002/mrc.3918

1,2,3,4,6-Penta-O-acetyl-α-d-glucopyranose and the corresponding [1-²H], [2-²H], [3-²H], [4-²H], [5-²H], and [6,6-²H₂]-labeled compounds were prepared for measuring deuterium/hydrogen-induced effects on ¹³C chemical shift ⁿΔ (DHIECS) values. A conformational analysis of the nondeuterated compound was achieved using density functional theory (DFT) molecular models that allowed calculation of several structural properties as well as Boltzmann-averaged ¹³C NMR chemical shifts by using the gauge-including atomic orbital method. It was found that the DFT-calculated C-H bond lengths correlate with ¹Δ DHIECS. Copyright

Full text of DOI:10.1002/mrc.3918

Research Article
Received: 26 September 2012
Revised: 6 December 2012
Accepted: 8 December 2012
Published online in Wiley Online Library: 14 January 2013
(wileyonlinelibrary.com) DOI 10.1002/mrc.3918
13
Deuterium-induced isotope effects on the C
chemical shifts of a-D-glucose pentaacetate
Nury Pérez-Hernández,a Celina Álvarez-Cisneros,b
Carlos M. Cerda-García-Rojas,b Martha S. Morales-Ríosb and
Pedro Joseph-Nathanb*
1,2,3,4,6-Penta-O-acetyl-a-D-glucopyranose and the corresponding [1-2H], [2-2H], [3-2H], [4-2H], [5-2H], and [6,6-2H2]-labeled
n
compounds were prepared for measuring deuterium/hydrogen-induced effects on 13C chemical shift Δ (DHIECS) values. A
conformational analysis of the nondeuterated compound was achieved using density functional theory (DFT) molecular mod-
els that allowed calculation of several structural properties as well as Boltzmann-averaged 13C NMR chemical shifts by using
the gauge-including atomic orbital method. It was found that the DFT-calculated C–H bond lengths correlate with 1Δ DHIECS.
Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: 13C NMR; deuterium/hydrogen-induced effects; a-D-glucose pentaacetate; molecular modeling; chemical shift calculations;
GIAO method
the C–H bond lengths calculated with density functional theory
Introduction
(DFT) molecular models. Additionally, 13C NMR chemical shifts
Glucose is maybe the second central substance in human life
maintenance after water. Besides its role as fuel in cells, its partic-
ipation in plural biochemical functions determines the human
metabolic profile in normal and pathologic states.[1] For a long
time, this important monosaccharide has been subjected to
many NMR analyses, from its signal assignment and a/b configu-
ration preference half a century ago,[2] to its recent uses as an
NMR tracer signal for delineating metabolic pathways in human
tissues and fluids.[3] In addition, some studies based on deute-
rium/hydrogen-induced effects have been used in the structure
analysis,[4] hydrogen bond,[5] conformation,[6] and tautomeric
equilibria[7] studies of glucose. Before the advent of 2D NMR
techniques, the structure of di- and oligosaccharides was
achieved by analysis of the characteristic splitting pattern of car-
bon signals in the glucopyranose unit with partially deuterated
hydroxyl groups under slow and fast exchange conditions.[4] In
another approach, deuterium-induced effects on 1H and 13C
shifts through intra- or intermolecular hydrogen bonds have also
been helpful to determine structural and conformational proper-
ties of important oligosaccharides, as cyclodextrins.[5] More re-
cently, isotope effects have been used to establish the transition
states in the a/b-glucose interconversion,[6] as well as for deter-
mining the percentages of acyclic forms of aldohexoses in their
interconversion with the cyclic forms on tautomeric equilibria.[7]
Particularly for D-glucose, isotope effects on 13C chemical shifts
induced by hydroxyl groups are very well described,[4] while iso-
tope effects induced by deuterium atoms directly attached to the
pyranose carbon atoms have been explored for the [2-2H], [3-2H],
[5-2H], and [6,6-2H2]-labeled sugar in D2O.[8]
have been compared with magnetic shielding constants calcu-
lated by the gauge-including atomic orbital (GIAO)–DFT method
using a correlation analysis.
Results and Discussion
1,2,3,4,6-Penta-O-acetyl-a-D-glucopyranose and their corresponding
[1-2H], [2-2H], [3-2H], [4-2H], [5-2H], and [6,6-2H2]-labeled compounds
were obtained by standard acetylation of the corresponding
D-glucopyranosides. Selected 13C NMR shifts and one-bond
CH coupling constants (1JCD) for the 2H-labeled compounds
are listed in Table 1. From these data, it can be observed that
1
the JCD values are approximately 24 Hz, in line with literature
values for deuterated xyloses and deoxygalactoses.[8]
n
The Δ DHIECS values were determined as the chemical shift
differences of the nondeuterated and the deuterated mole-
cules:[9]
nΔ13CðDÞ ¼ d13CðHÞ ꢀ d13CðDÞ
(1)
where n is the number of bonds between the site of deuteration
and the observed carbon atom. The nΔ DHIECS values were
*
Correspondence to: Pedro Joseph-Nathan, Departamento de Química, Centro
de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional,
Apartado 14-740, México, D. F., 07000 Mexico. E-mail: pjoseph@nathan.
cinvestav.mx
a Programa Institucional de Biomedicina Molecular, Escuela Nacional de Medicina
y Homeopatía, Instituto Politécnico Nacional, México, D. F. 07320, Mexico
In this work, we describe the deuterium/hydrogen-induced
effects on 13C chemical shift nΔ (DHIECS) values for [1-2H],
[2-2H], [3-2H], [4-2H], [5-2H], and [6,6-2H2]-D-glucose pentaacetates
in CDCl3 and evaluate them in terms of structural features such as
b Departamento de Química, Centro de Investigación y de Estudios Avanzados
del Instituto Politécnico Nacional, Apartado 14-740, México, D. F. 07000, Mexico
Magn. Reson. Chem. 2013, 51, 136–142
Copyright © 2013 John Wiley & Sons, Ltd.

Deuterium isotope effects on 13C chemical shifts of peracetylglucose
Table 1. Δ DHIECS values (in ppb),a selected 13C chemical shifts (in ppm), and 2H–13C coupling constants (in Hz) of deuterated
a-D-glucose pentaacetates
Carbon
D-[1-2H]-Glu
D-[2-2H]-Glu
D-[3-2H]-Gluc
D-[4-2H]-Glu
D-[5-2H]-Glu
D-[6,6-2H2]-Glu
C1
1Δ= ꢀ273
88.7, t, J = 27
2Δ= ꢀ73
2Δ= ꢀ39
C2
C3
C4
C5
C6
1Δ= ꢀ317
68.8, t, J = 23.2
2Δ= ꢀ54
2Δ= ꢀ67
3Δ= ꢀ20
1Δ= ꢀ298
69.5, t, J = 26.2
2Δ= ꢀ69
2Δ= ꢀ69
1Δ= ꢀ314
67.4, t, J = 24.1
2Δ= ꢀ69
2Δ= ꢀ49
3Δ= ꢀ10
3Δ= ꢀ20
1Δ= ꢀ365
69.3, t, J = 24.1
2Δ= ꢀ53
2Δ= ꢀ114
3Δ= ꢀ19
1Δ= ꢀ543
60.8, q, J = 22.8
C=O (2)
C=O (3)
C=O (4)
C=O (6)
Me (2)
3Δ= ꢀ3
3Δ= ꢀ10
3Δ= ꢀ9
5Δ= 5
3Δ= 13
5Δ= ꢀ2
4Δ= ꢀ2
5Δ= ꢀ3
Me (3)
5Δ= ꢀ3
6Δ= 3
Me (4)
Me (6)
4Δ= 3
aThe acetate carbons at C1 showed no measurable shifts.
measured under uniform conditions (see Experimental Section)
and are also summarized in Table 1. According to the general
trend,[9] the 1-3Δ DHIECS values decrease when the number of
bonds between the deuterium atom and the observed 13C atom
increases, while 4,5Δ DHIECS values are exceptions to this ten-
effect in the 13C signal. Also, 4Δ, 5Δ, and even 6Δ DHIECS values were
observed for [3-2H], [4-2H], and [6,6-2H2]-pentaacetates. For mono-
deuterated sugars, the values show in general a negative sign
contrary to the dilabeled compound. Thus, for [3-2H] pentaacetate,
a 4Δ DHIECS value of ꢀ2 ppb was observed at Me-3, and addition-
1
5
dency. The Δ DHIECS values exhibit a negative sign and are in
ally, two Δ DHIECS values at Me-2 and Me-4 with similar values,
the range of ꢀ273 to ꢀ365 parts per billion (ppb = 10–9) for
monodeuterated pentaacetates, while for the dideuterated pen-
taacetate, the magnitude increased almost twice, to ꢀ543 ppb,
keeping the direct proportionality between the shift magnitude
and the number of substituted deuterium atoms.[10] The smallest
1Δ DHIECS value was observed for [1-2H]-a-D-glucose pentaace-
tate, an expected consequence of the strong deshielding effect
of two electronegative oxygen atoms in the anomeric position.
The remaining 1Δ DHIECS in increasing order for the net
value are as follows: –298, –314, –317, and ꢀ365 ppb, for [3-2H],
[4-2H], [2-2H], and [5-2H] pentaacetates, respectively.
approximately ꢀ3 ppb, were also found. For [4-2H] pentaacetate, a
5Δ DHIECS value of ꢀ3 ppb was observed at Me-3, and a 6Δ DHIECS
with the same value but opposite sign (3 ppb) was found at Me-6.
For [6,6-2H2]-pentaacetate, a 4Δ DHIECS value of 3 ppb was observed
5
at C=O(3) and a Δ DHIECS value of 5 ppb was measured at Me-6.
The positive shift for 3Δ, 4Δ, and 5Δ DHIECS values in dilabeled
[6,6-2H2]-glucose pentaacetate could be explained by a combinatory
effect arising from several geometries adopted by the –CD2OCOCH3
group involving nonadditive and second-order terms.[11]
In order to find correlations between DHIECS and structural
properties of 1,2,3,4,6-penta-O-acetyl-a-D-glucopyranose (1), a
molecular model of this substance was obtained by means of
DFT calculations. The initial structure of 1 was built using its X-
ray atom coordinates. A search in the Cambridge Crystallographic
Data Centre (CCDC) database showed the four deposits
ZZZQPG01, ZZZRCA01, ZZZRCA, and ZZZRCA02 for 1. The first
two records provided no atom coordinates, while in the remain-
ing two files, data were refined to R1 values of 7.3% and 4.6%,
respectively. Therefore, to obtain a more precise set of atomic
coordinates, we performed a single crystal X-ray analysis of 1,
which was refined to R1 = 3.7% (Fig. 1). These coordinates were
loaded into the Spartan’04 program (Wavefunction, Irvine, CA,
USA), and the molecular model was subjected to an energy min-
imization routine employing the molecular mechanics force field
(MMFF). An exhaustive Monte Carlo search[12] afforded 31 confor-
mations within the first 3 kcal/mol. The whole set was subjected
to a DFT single-point energy calculation at the B3LYP/6-31G(d)
level of theory, reducing the number of conformers to eight
The 2Δ DHIECS values exhibit a negative sign and are in the range
of ꢀ49 to ꢀ114 ppb. As expected, the largest value was observed
1
for the dilabeled [6,6-2H2]-a-D-glucose that showed Δ=ꢀ114 ppb.
2
For monodeuterated compounds, the largest Δ was ꢀ73 ppb at
C2 of [1-2H]-pentaacetate, while the smallest value of ꢀ39 was ob-
3
served for C1 of [2-2H]-pentaacetate. Long-range Δ DHIECS were
observed for [1-2H], [2-2H], [3-2H], [4-2H], and [6,6-2H2]-glucose pen-
3
taacetates. For monolabeled glucoses, the Δ isotope effects are
negative and span the range ꢀ3 to ꢀ20 ppb. As seen from Table 1,
the 3Δ values for the glucopyranose ring are larger, –20 ppb in aver-
age, than those for the carbonyl carbons, –3 to ꢀ10 ppb range, in
agreement with the deshielding effect of the two oxygen atoms in
the carbonyl groups which affects more the deuterium shielding ef-
fect of the 13C chemical shift. On the contrary, in dilabeled [6,6-2H2]-
3
glucose pentaacetate, the Δ DHIECS magnitudes are inverted, as
was mentioned before, reflecting that the additive shielding effect
by two deuterium atoms is more effective than the electronegative
Magn. Reson. Chem. 2013, 51, 136–142
Copyright © 2013 John Wiley & Sons, Ltd.
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N. Perez-Hernandez et al.
Figure 1. X-ray diffraction structure of 1,2,3,4,6-penta-O-acetyl-a-D-glucopyranose (1).
minima. Geometry optimization, vibrational frequencies, and ther-
mochemical parameter calculations of these structures were car-
ried out at the B3PW91/DGDZVP and the B3LYP/6-311+G(2d,p)
levels of theory[13] in the presence of CHCl3 using the Gaussian 03
program (Gaussian Inc., Wallingford, CT, USA). This procedure gave
four main conformers (1a–1d) that, according to the ΔG = ꢀRT
ln K equation, accounted for 97.8% of the conformational pop-
ulation using the B3PW91/DGDZVP level of theory (Fig. 2) and
for 96.4% using the B3LYP/6-311+G(2d,p) level of theory
(Fig. 3). These structures essentially preserved the same spatial
arrangements of the O1–C1–C2–C3–C4–C5 framework, includ-
ing the ester residues at C1 to C4, but showed changes in
the rotation of the C5–C6 bond and in the orientation of
the acetyl group at C6. In both levels of theory, the global
minimum structure showed a gg conformation in the C5–C6
bond, followed by gt or gg conformational variants. The
absence of any tg conformer was remarkable and in line with
a previous experimental analysis of the rotameric distribution
for the C5–C6 bond of 1.[14] It is worth mentioning that
the B3PW91/DGDZVP models reproduced more faithfully the
observed gg:gt:tg conformational ratio.[14] In addition, the magnetic
shielding tensors were calculated for the selected conformers with
the GIAO method,[15] which were Boltzmann-weighted taking into
account the DFT population, referenced using TMS values
Figure 2. Minimum energy structures for 1,2,3,4,6-penta-O-acetyl-a-D-glucopyranose at the B3PW91/DGDZVP level of theory.
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Magn. Reson. Chem. 2013, 51, 136–142

Deuterium isotope effects on 13C chemical shifts of peracetylglucose
Figure 3. Minimum energy structures for 1,2,3,4,6-penta-O-acetyl-a-D-glucopyranose at the B3LYP/6-311+G(2d,p) level of theory.
calculated with the same basis sets, and scaled using a factor
obtained by iteration for each basis set. The scaling factors (ƒ) were
calculated to compensate for the moderately overvalued shielding
tensors arising from the limitations of the DFT–GIAO methods.[16]
The ƒ values were determined by an iterative process that mini-
mized the root-mean-square deviation between the calculated
and observed chemical shifts. For the B3PW91/DGDZVP, ƒ was
0.963, while for the B3LYP/6-311+G(2d,p) level of theory, ƒ was
0.953. In addition, a slight correction for the TMS chemical shift
of d 1.320 for the B3PW91/DGDZVP and of d 1.040 for the B3LYP/
6-311+G(2d,p) was applied. The calculated 13C NMR chemical shifts
of a-D-glucose pentaacetate for the two functionals/basis sets
are listed in Table 2, where they can be contrasted with the exper-
imental values.
Correlation between theoretical and experimental chemical shift
values is related to the accuracy of the used molecular models, as
shown in Table 2 and Fig. 4. The complete process involves geom-
etry optimization, vibrational frequencies, and GIAO chemical shift
Table 2. Experimental and calculated 13C NMR chemical shifts (in ppm) for a-D-glucose pentaacetate (1)
Carbon
Exp
B3PW91/DGDZVP
Correcteda
B3LYP/6-311+G(2d,p)
Correctedb
Calcd
Δexp-corr
Calcd
Δexp-corr
C1
89.0
69.1
95.5
73.0
90.7
69.0
ꢀ1.7
0.1
95.2
74.6
89.7
70.1
ꢀ0.7
ꢀ1.0
0.5
C2
C3
69.7
71.5
67.6
2.2
73.7
69.2
C4
67.8
69.3
65.4
2.3
72.5
68.1
ꢀ0.3
ꢀ2.3
ꢀ1.7
0.1
C5
69.7
74.8
70.7
ꢀ1.0
ꢀ0.3
ꢀ0.2
ꢀ0.4
0.5
76.6
72.0
C6
61.6
65.6
61.9
67.5
63.3
C=O (1)
C=O (2)
C=O (3)
C=O (4)
C=O (6)
OMe (1)
OMe (2)
OMe (3)
OMe (4)
OMe (6)
168.7
169.6
170.1
169.3
170.5
20.8
176.8
178.0
177.5
177.3
179.0
23.1
168.9
170.0
169.6
169.3
171.0
21.0
178.0
178.7
178.6
178.5
179.7
22.2
168.6
169.3
169.2
169.1
170.3
20.1
0.3
0.9
0.0
0.2
ꢀ0.4
ꢀ0.2
ꢀ0.3
ꢀ0.6
ꢀ0.4
ꢀ0.5
0.2
0.7
20.4
22.8
20.6
21.8
19.8
0.6
20.5
23.2
21.1
22.1
20.1
0.4
20.6
23.2
21.0
22.1
20.0
0.6
20.6
23.2
21.1
21.9
19.9
0.7
adcorrected = 0.963 (dcalculated – 1.320), factors obtained by iteration, RMS = 0.995 ppm.
bdcorrected = 0.953 (dcalculated – 1.040), factors obtained by iteration, RMS = 0.909 ppm.
Magn. Reson. Chem. 2013, 51, 136–142
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N. Perez-Hernandez et al.
200
150
100
50
the present study, it was found that the C–H bond lengths for
C1–H to C5–H, calculated at the DFT B3PW91/DGDZVP and
B3LYP/6-311+G(2d,p) levels of theory (Table 3) using CHCl3 as im-
(a)
1
plicit solvent, closely correlate with the magnitude of Δ DHIECS
(Fig. 5), suggesting that this relationship can be originated by the
same phenomenon that involves bond distance variations.[17]
1
Bond lengths and Δ DHIECS values for the C6–H2 moiety were
not included in the plot because they follow a more complex pat-
tern arising from differences in the substitution, the combinatory
effect of the previously mentioned changing geometries adopted
by the –CD2OCOCH3 group, and the influence of the two deute-
rium atoms.[17]
0
Assignment of the 13C NMR acetate signals was done using
0
50
100
150
200
1
HMBC spectra. Three-bond cross-peaks between the H signals
δ exp
of the pyranose ring and the 13C carbonyl signals were observed
as follows: H1 at d 6.34 with the signal at d 168.7, H2 at d 5.10
with the signal at d 169.6, H3 at d 5.48 with the signal at
d 170.1, H4 at d 5.15 with that of d 169.3, and the CH26 at
d 4.27 and 4.10 with the signal at d 170.5. Additionally, the methyl
200
150
100
50
(b)
1
group H signals were assigned using their correlations with the
already assigned carbonyl groups as follows: CO1 with the signal
at d 2.18, CO2 with the signal at d 2.02, CO3 with the signal at
d 2.03, CO4 with the signal at d 2.04, and CO6 with the signal at
d 2.10. The assignment of the corresponding 13C signals for the
methyl groups was made from their HSQC correlations.
0
Experimental
0
50
100
150
200
General experimental procedures
δ exp
[1-2H], [2-2H], and [6,6-2H2]-D-glucose were obtained from Sigma
Aldrich, while [3-2H], [4-2H], and [5-2H]-D-glucose were obtained
from Omicron Biochemicals, Inc. Sugars were used without fur-
ther purification. The melting point was determined on a
Fisher–Johns melting point apparatus and is uncorrected.
Figure 4. Correlation of the experimental and DFT calculated 13C NMR
chemical shifts for 1,2,3,4,6-penta-O-acetyl-a-D-glucopyranose (a) using
GIAO-DFT B3PW91/DGDZVP PCM shielding constants (dcal = 0.9997
d
exp + 0.0729, R2 = 0.9997); (b) using GIAO-DFT B3LYP/6-311+G(2d,p)
PCM shielding constants (dcal = 0.9992 dexp + 0.1138, R2 = 0.9998).
calculations, which for the B3PW91/DGDZVP functional/basis set
required approximately 24 h per conformer, while for the B3LYP/
6-311+G(2d,p) functional/basis set required approximately 96 h
per conformer. Given that calculations for eight conformers per
model were required, the data for both levels of theory are in-
cluded in Table 2, to provide diagnostic information to compare
accuracy and computer costs.
It has been reviewed that isotope effects arise from changes in
the average bond distance of the X–H bond upon deuteration,[10]
and it has been noticed that the magnitude of isotope effects
increases with increasing bond length,[17] as observed for axial
versus equatorial C–H bonds in cyclohexane.[18] Accordingly, in
Acetylation of D-glucoses
A solution of D-glucose or of 2H-labeled D-glucoses (30 mg,
0.08 mmol) in pyridine (1.5 ml) was cooled in an ice bath, and
acetic anhydride (1 ml) was added under stirring. The reaction
mixture was kept for 72 h at room temperature. Then, in an ice
bath, H2O (1.5 ml) was added, and the reaction mixture was
extracted with AcOEt (3 ꢁ 20 ml). The combined organic extract
was washed with 10% HCl (3 ꢁ 20 ml), with H2O (20 ml), with a
5% solution of NaHCO3 (20 ml), and with H2O again, dried over
anhydrous Na2SO4, filtered, and evaporated under vacuum. The
corresponding acetylated glucoses (62 mg, 96% in average) were
Table 3. 1Δ DHIECS values (in ppb) for deuterated a-D-glucose pentaacetates and DFT-calculated C–H bond lengths (in Å) of 1,2,3,4,6-penta-O-ace-
tyl-a-D-glucopyranose (1) at the B3PW91/DGDZVP and the B3LYP/6-311+G(2d,p) levels of theory
2H position
1Δ DHIECS
Bond
B3PW91/DGDZVP C–H bond lengtha
B3LYP/6-311+G(2d,p) C–H bond lengtha
C1
C2
C3
C4
C5
ꢀ272.5
ꢀ316.8
ꢀ297.6
ꢀ314.4
ꢀ364.6
C1–H
C2–H
C3–H
C4–H
C5–H
1.091
1.093
1.092
1.092
1.097
1.088
1.091
1.089
1.091
1.094
aBoltzmann-averaged values.
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Magn. Reson. Chem. 2013, 51, 136–142

Deuterium isotope effects on 13C chemical shifts of peracetylglucose
1.098
(a)
were collected at 293(2) K in the o-2θ scan mode. Unit cell refine-
ments using 25 machine-centered reflections were done using the
CAD4 Express v2.0 software. Crystal data were C16H22O11,
M= 390.34, orthorhombic, space group P212121, a= 5.584(2) Å,
b= 14.713(1) Å, c= 23.958(2) Å, V= 1968.4(6) Å3, Z=4, r =1.32mg/
mm3, m(CuKa) = 0.975/mm, total reflections = 1634, unique reflec-
tions 1535 (Rint 0.01%), observed reflections 1352, final R indices
[I> 2s(I)] R1 =3.7%, wR2 = 10.0%. The structure was solved by direct
methods using the SHELXS-97 program included in the WinGX
v1.70.01 crystallographic software package. For the structural refine-
ment, the nonhydrogen atoms were treated anisotropically, and the
hydrogen atoms, included in the structure factor calculation, were
refined isotropically. Crystallographic data (excluding structure fac-
tors) have been deposited at the CCDC. The CCDC deposition num-
ber is 911297. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 IEZ, UK
(fax: +44-(0)1223-336033; e-mail: deposit@ccdc.cam.ac.uk).
1.097
1.096
1.095
DFT C–H
bond length
(Å)
1.094
1.093
1.092
1.091
1.090
240
290
340
390
1.095
1.094
1.093
1.092
1.091
1.090
1.089
1.088
1.087
(b)
DFT C–H
bond length
(Å)
Computational analysis
The molecular model of 1,2,3,4,6-penta-O-acetyl-a-D-glucopyra-
nose was constructed using the X-ray atom coordinates and
subjected to a full minimization routine employing molecular
mechanics (MMFF), in the Spartan’04W package (Wavefunction,
Irvine, CA, USA). The conformational search was made with the
Monte Carlo protocol,[11] obtaining a total of 31 conformers in
the first 3 kcal/mol. These structures were then submitted to
geometry optimization by DFT calculations at the B3LYP/6-31G
(d) level of theory to provide eight minimum energy structures,
which were reoptimized at the B3PW91/DGDZVP and B3LYP/6-
311+G(2d,p) levels of theory in the presence of CHCl3 perturbation
using the polarizable continuum model as implemented in the
Gaussian 03W program (Gaussian Inc., Wallingford, CT, USA), pro-
viding four representative minimum energy conformers (1a–1d)
for each level. The geometries of these four DFT conformers were
then used for estimate the NMR shielding tensors through
the GIAO method.[14] The calculated NMR shielding tensors were
converted into chemical shifts (d), considering the isotropic values
of the shielding tensors of the 13C atom in TMS, calculated at
the same levels of theory as d 184.63 for B3PW91/DGDZVP and
d 183.61 for B3LYP/6-311+G(2d,p).
240
290
340
390
1Δ DHIECS (ppb)
Figure 5. Correlation between the DFT-calculated C–H bond lengths of
1,2,3,4,6-penta-O-acetyl-a-D-glucopyranose and the magnitude of 1Δ
DHIECS at (a) the B3PW91/DGDZVP level of theory (DFT C–H bond
length = 7 ꢁ 10–5 1Δ DHIECS + 1.072, R2 = 0.926) and (b) the B3LYP/6-311
+G(2d,p) level of theory (DFT C–H bond length = 7 ꢁ 10–5 1Δ DHIECS +
1.069, R2 = 0.978).
obtained as a 4:1 mixture of a/b isomers. The a-isomers were pu-
rified by successive recrystallizations using THF-hexanes (5:1) to
provide colorless plates mp 100–101 ꢂC.
NMR experiments
NMR measurements were carried out on a Varian Mercury spec-
trometer at 75.4 MHz for 13C using a 5-mm probe from CDCl3
solutions. Coupling constant values are given in Hz, and chemical
shifts are reported in ppm from internal TMS registered with a
magnet homogeneity better than W1/2 = 0.20 Hz measured at
the TMS signal. For the DHIECS measurements, 0.83 M CDCl3 solu-
tions of the unlabeled/labeled glucoses in a 7:3 ratio were
degassed by bubbling argon, and measurements were per-
formed at 300 (ꢃ1) K. The 1H interactions were eliminated using
the low-energy Waltz-16 decoupling pulse technique.[19] For data
acquisition, the pulse width was set to 20.5ꢂ (5 ms), the recycling
time was set to 5 s, and at least 26 544 data points were acquired.
The FID data were zero-filled to 128 k data points prior to Fourier
transformation providing digital resolution of at least 0.081 Hz
per point (1.1 ppb). The NMR data were processed on a SUN
ULTRA 60 workstation using the SOLARIS 9 platform and VNMR
6.1c software.
Acknowledgements
Partial financial support from Conacyt-México, grant 61122,
is acknowledged.
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