Can NMR calculations be used as fingerprints for the determination of structural features in Monolayer Protected Clusters (MPCs)? The effect of the chiral 2-methyl-1-butanethiol (SMeBut) ligand on the inherently achiral Au25 nanocluster system Au25(SMeBut)180, is being studied. To explain the unique features found in the 1H NMR spectra as a function of decreasing enantiomeric excess (e.e.) percentage, we are using computational spectroscopic characterization. One of the unique features to this cluster is that when the e.e. of the ligand is systematically changed the 1H NMR peak for the α-methylene protons evolves by splitting and broadening. We are currently investigating the origin of such splitting. Computational methods involving quantum chemistry and molecular dynamics simulations may be able to answer these questions. Currently, our results agree with experiment in terms of the maximum peak for the -methylene protons for 100% S enantiomer. Our hypothesis is that as the e.e. is changed, steric constraints lock specific populations of ligand conformations throughout the systems monolayer.

1. References:
1) Bertini, Ivano, Claudio Luchinat, and Giacomo Parigi. Solution NMR of paramagnetic
molecules: applications to metallobiomolecules , Elsevier, 2001.
2) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scusera, G. E.; Robb, M. A.; Cheeseman, J. R.;
Scalmani, G.; Basrone, V.; Mennucci, B.; Gaussian, Inc Wallingford CT, 2009.
3) Guberman-Pfeffer, M.,Ulcickas,J., Gascón JA. (2015) J Phys Chem C 119: 27804
4) Knoppe, S.; Bürgi, T. Acc. Chem. Res. 2014, 47 (4), 1318–1326
Acknowledgements:
S. G. would like to thank Dr. Angelo Rossi for support in simulations and for helpful discussions.
She also acknowledges support from NSF Northeast LSAMP Bridge to the Doctorate Award #
1400382.
Computational NMR Characterization of Chiral Au25(SMeBut)18
0
Svetlana Gelpí-Domínguez and José A. Gascón
Department of Chemistry, University of Connecticut, 55 North Eagleville Rd, Storrs, CT 06269
Biomolecular Simulations and Electronic Structure Laboratory
http://gascon.chem.uconn.edu/
αOuter - 1H-NMR in C6D6, 3mM, 25°C
Sample
FWHM
[Hz]
───── 100 % (S) 46
───── 95 % (S) 47
───── 91 % (S) 51
───── 88 % (S) 48
───── 85 % (S) 50
───── 75 % (S) 67
───── 65 % (S) 69
───── 55 % (S) 107
───── 44 % (S) 132
───── 29 % (S) -
───── 15 % (S) -
───── 0 % (S) -
Can NMR calculations be used as fingerprints for the determination of
structural features in Monolayer Protected Clusters (MPCs)? The effect of the
chiral 2-methyl-1-butanethiol (SMeBut) ligand on the inherently achiral Au25
nanocluster system Au25(SMeBut)18
0, is being studied. To explain the unique
features found in the 1H NMR spectra as a function of decreasing enantiomeric
excess (e.e.) percentage, we are using computational spectroscopic
characterization. One of the unique features to this cluster is that when the e.e. of
the ligand is systematically changed the 1H NMR peak for the α-methylene protons
evolves by splitting and broadening. We are currently investigating the origin of such
splitting. Computational methods involving quantum chemistry and molecular
dynamics simulations may be able to answer these questions. Currently, our results
agree with experiment in terms of the maximum peak for the a-methylene protons
for 100% S enantiomer. Our hypothesis is that as the e.e. is changed, steric
constraints lock specific populations of ligand conformations throughout the
systems monolayer.
Catalysis
G. Li, H. Qiana, R. Jin
Nanoscale, 2012,4, 6714-6717
JACS 2009, 131:888-889
J. Xie, Y. Zheng, J. Ying
Protein Bioconjugation
X-D. Zhang, Z. Luo, J. Chen, X. Shen, S. Song, Y.
Sun, S. Fan, F. Fan, D. Leong, J. Xie
Advanced Materials, 2014, 26, 4565-4568
Cancer Radiotherapy (radiosensitisers)
Abstract
Applications
Methods and models
System Au25(SMeBut)18
0
(6) dimeric staples Icosahedral core
Hydrogen atoms
Sulfur atoms
+ =
Gold atoms
Carbon atoms
+
(18) 2-methyl-1-butanethiol
System: Au25(SMeBut)18
0
Outer
Inner
References and Acknowledgements
Experimental Characterization: 1H NMR
Figure 3. Variation of the width and splitting of the outer α-CH2 for 3 mM A25(SMeBut)18
0 for
samples with different e.e.. Experiments carried out by Maran’s group, University of Padova.
Figure 4. Calculated computational chemical shifts for the α-CH2 protons on the outer SMeBut
ligands for the 100% e.e. (S) cluster using full quantum mechanical methods. The average
chemical shift for the α-methylene protons was calculated at 5.42 ppm which is in good agreement
with the experimental value of 5.05 ppm.
Figure 2. Au25(SR)18 consists of six dimeric staples (Fig2.2) and one icosahedral core (Fig 2.3.)
. Fig 2.4. shows an X-ray structure of the Au25(SMeBut)18. Orange atoms are gold, yellow atoms
are sulfur, gray atoms are carbon, and white atoms represent hydrogen.
1.
2.
4.
3.
Results and Discussion
Figure 1. Advancements and applications of gold monolayer protected clusters.
100% e.e. (S)-Au25(SMeBut)18
Experimental
shift
Obtain X-Ray structure
Perform Molecular
Dynamics (MD)
Simulations in vacuum
using AuSBio TiNKER.
Perform single point
NMR calculations on
optimized structures
Analyze dihedral angle
combinations and
distance distributions
Create 100%R e.e.
cluster and optimize
Optimize 100% S e.e. Create 0% S e.e.
cluster and optimize
Results and Discussion
Dihedral Angle Combinations and 1H NMR
D1 D2
The theoretical 1H NMR of the S and R enantiomer of the
SMeBut ligand was taken at 9 different dihedral angle
combinations. Splitting between the α and α’ methylene
protons range from 0.05 ppm to 0.94 ppm depending on the
dihedral angle combination and the chirality.
Dihedral Angle Combination
(D2,D1)
Expected ppm
(-60,-60)
R = 0.94
S = 0.1
(60,60)
R =0.1
S =0.94
-165 -132 -99 -66 -33 0 33 66 99 132 165 198 231
-165
-132
-99
-66
-33
0
33
66
99
132
165
198
231
100% S
D2
D1
-165 -132 -99 -66 -33 0 33 66 99 132 165 198 231
-165
-132
-99
-66
-33
0
33
66
99
132
165
198
231
100% R
D2
-165 -132 -99 -66 -33 0 33 66 99 132 165 198 231
-165
-132
-99
-66
-33
0
33
66
99
132
165
198
231
0% Alternating
D2
0.000
3.812E-05
7.625E-05
1.144E-04
1.525E-04
1.906E-04
2.288E-04
2.669E-04
3.050E-04
Density
Figure 5. Molecular dynamics dihedral angle combination frequency analysis for three models 100% S, 100% R,
and 0%. As shown in 0% model, a uniformity arises in the possibility of dihedral angle combination that can
exist in the cluster's monolayer giving rise to the possibility for either (-60,-60) and (60,60) dihedral
combinations that cause larger splitting.
Perform single point
NMR calculations
selected optimized MD
structures.
Create and analyze
average NMR shifts to
the models
Conclusions
• The dynamics, chirality, and the enantiomeric excess of the cluster are
responsible for locking in specific ligand conformations in the monolayer.
+ +
D1 D2
• The dihedral angle combinations frequency in the monolayer
depends on the enantiomeric excess of the cluster being studied.
• When the cluster is at 0% e.e. a uniform distribution for the
probability of all angle combinations is found. Combinations of
(60,60) for S and (-60,-60) for R which have a high expected
splitting (0.94 ppm) have a higher probability of being found.