Transcript Slide 1

Comparison of multi-standard and
TMS-standard calculated NMR shifts
for coniferyl alcohol
Heath D. Watts, Mohamed N.A. Mohamed,
James D. Kubicki
7 April 2011
Goal – Build a reasonably accurate model
of lignin testable against spectroscopic data
www.lbl.gov/Publications/YOS/Feb/
Experimental 13-C NMR data for coniferyl alcohol in acetone
Monomer provides less convoluted spectrum, but has ambiguous
shifts
g
b
a
1
2
6
5
3
4
Me
Carbon
1
2
3
4
5
6
a
b
g
Me
d13C (ppm)
130.2
109.9
148.4
147.1
115.7
120.6
130.4
128.0
63.4
56.1
http://ars.usda.gov/Services/docs.htm?docid=10491
Can computational chemistry methods
reproduce the observed NMR chemical
shifts for coniferyl alcohol?
Energy minimization method: Structure
B3LYP/6-311++G(d,p)
Cheeseman et al. Journal of
Chemical Physics. 1996,
104(14), 5497.
NMR Theory: Chemical shielding
B3LYP/6-311+G(2d,p);
NMR standard: TMS
Inorganic
character
Si
13
d C
= sTMS - ssample
160
d13Ccalc (ppm)
140
1:1 line
120
MG5
(Watts, 2011)
100
B3LYP/6-311+G(2d,p)
Slope: 1.01
y-intercept (ppm): 7.41
r2=0.975
Mean-unsigned error (MUE) (ppm): 8.1
Root mean-squared error (RMSE) (ppm): 9.4 ppm
Max Error (ME) (ppm): 21.4
80
60
40
40
60
80
100
120
d13Cexp (ppm)
140
160
http://ars.usda.gov/Services/
docs.htm?docid=10491
Is there a conformational
isomer effect?
g
b
a
6
5
1
MG1
4
2
MG2
MG3
MG5
MG6
3
Me
MG4
NMR Theory: mPW1PW91/6-31G(d);
NMR standard: benzene  sp2 C;
CH3OH sp3 C Organic
standards
Multi-standard
d13C = sM-S – ssample + dexp,ref
Sarotti & Pellegrinet; Journal of Organic Chemistry. 2009, 74, 7254.
NMR Theory:
B3LYP/6-311+G(2d,p);
NMR standard: TMS
NMR Theory:
HF/6-311+G(2d,p);
NMR standard: TMS
TMS, single standard
d13C = sTMS - ssample
Cheeseman et al. Journal of Chemical Physics. 1996, 104(14), 5497.
160
d13Ccalc (ppm)
140
120
MG3
100
mPW1PW91/6-31G(d)
Slope: 1.00
y-intercept (ppm): -0.42
r2=0.994
MUE (ppm): 2.2
RMSE (ppm): 2.4 ppm
Max Error (ppm): 3.7
80
60
Watts et al. Journal
of Physical
Chemistry B. 2011,
115(9), 1958.
40
40
60
80
100
120
d13Cexp (ppm)
140
160
NMR Theory: mPW1PW91/6-31G(d);
NMR standard: benzene  sp2 C;
CH3OH sp3 C
NMR Theory:
B3LYP/6-311+G(2d,p);
NMR standard: TMS
NMR Theory:
HF/6-311+G(2d,p);
NMR standard: TMS
Reviewer comments:
…the authors conclude that the MG3 should be
the “experimentally observable conformer”. In
the case of flexible compounds, the generally
accepted protocol is to calculate the Boltzmannaveraged shielding constants, which gives a
more “realistic” result, because it takes into
account the effect of all significantly populated
conformations.
In addition, the authors did not mention the
relative energies of the different conformers.
The Gibbs free energy of solution (G°soln) was calculated by:
(Foresman, 1996; www.gaussian.com/g_whitepap/thermo.htm)
G°soln = G°IEFPCM + G°TCDG
G°IEFPCM  total free energy in solution with all nonelectrostatic terms from the polarized continuum calculation
(solvents were acetone, DMSO, & CHCl3)
G°TCDG  thermal correction to Gibbs free energy from the
gas-phase frequency calculation
The relative G°soln for each model was
determined by setting the model with the
lowest G°soln to 0 kJ/mol
Coniferyl
Relative DG°soln
alcohol model (kJ/mol; acetone)
MG1
MG2
MG3
MG4
MG5
MG6
10.2
0.7
4.4
10.9
0.0
4.0
Boltzmann-weighted NMR chemical shifts to account for
contribution of each conformer based the energy distribution
(Barone, 2002)
N
δ13 CX 

i 1




 e xp( ΔG ºi /RT) 
δ13 CXi  N
 


º
 e xp( ΔG i /RT) 
 i 1


N

i 1
d13CX  Boltzmann averaged chemical shift of atom X
d13CXi  Chemical shift of atom X in conformer i
δ13CXi
 Ni 
N
 
Probability
Relative DG°soln
Acetone (kJ/mol)
[Ni/N] acetone
MG1
MG2
Sum
10.2
0.7
4.4
1.00
0.01
0.35
0.08
Weighted
shifts MG1
MG3 MG4
MG5
MG6
10.9
0.0
4.0
0.01
0.46
0.09
MG2 MG3* MG4
MG5
MG6
128.0
128.4
Carbon
Experimental
1
130.2
128.0
128.7
2
109.9
111.1
120.1 106.5 107.4 126.8 114.1 115.2
3
148.4
145.2
145.9 145.4 146.4 145.1 144.7 145.5
4
147.1
145.3
150.2 145.5 145.7 149.7 144.9 144.8
5
115.7
115.0
116.7 114.4 115.3 117.6 115.2 116.2
6
120.6
121.0
127.8 125.1 124.2 121.1 118.1 117.0
a
130.4
133.4
132.9 133.4 133.4 132.9 133.4 133.5
b
g
OMe
128.0
63.4
56.1
125.9
65.2
53.4
126.4 125.5 125.3 126.5 126.3 126.1
65.2 65.1 65.1 65.3 65.3 65.3
57.2 53.4 53.1 57.0 53.4 53.1
127.8
128.2 128.8
Boltzmann-weighted d13Ccalc (ppm)
Watts et al. Journal
of Physical
Chemistry B. 2011,
115(9), 1958.
160
140
120
100
Boltzmann-weighted
mPW1PW91/6-31G(d)
Slope: 1.00
y-intercept (ppm): 0.06
r2: 0.996
MUE (ppm): 2.1
RMSE (ppm): 2.1
Max Error (ppm): 3.2
80
60
40
40
60
80
100
120
d13Cexp (ppm)
MG3 only
mPW1PW91/6-31G(d)
Slope: 1.00
y-intercept (ppm): -0.42
r2=0.994
MUE (ppm): 2.2
RMSE (ppm): 2.4 ppm
Max Error (ppm): 3.7
140
160
Conclusion: coniferyl alcohol
• For d13C NMR calculations on coniferyl alcohol
– Performance of multi-standard method >> TMSstandard method
• Linear correlation
• Statistical errors
• Multiple, Boltzmann-weighted conformers better
predict chemical shifts than did comparison of a
particular conformer with data
Acknowledgments
USDA National Needs Graduate Fellowship Competitive Grant 2007-38420-17782 from
the National Institute of Food and Agriculture to H.D. Watts through Nicole Brown.
Instrumentation funded by the National Science Foundation through grant OCI0821527.
JDK, MNAM, and HDW acknowledge support of the U.S. Department of Energy grant
for the Energy Frontier Research Center in Lignocellulose Structure and Formation
(CLSF) from the Office of Science, Office of Basic Energy Sciences under Award
Number DE-SC0001090.
HDW acknowledges support from Shell Geosciences Energy Research Facilities Award
MNAM was supported by the USDA grant “Improved Sustainable Cellulosic Materials
Assembled Using Engineered Molecular Linkers” through Jeff Catchmark.
Computational support was provided by the Research Computing & Cyberinfrastucture
group at the Pennsylvania State University.
Discussions with Ming Tien, Brett Diehl, Nicole Brown and other members of the Center
for Nanocellulosics and CLSF are also acknowledged.
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