Title: Ned H. Martin
1Computation of Through-Space NMR Chemical Shift
Effects
- Ned H. Martin
- Department of Chemistry
- University of North Carolina at Wilmington
2Introduction to NMR
- In a strong magnetic field Bo, hydrogen nuclei
have two possible spin states aligned with or
against the magnetic field. - These states differ only slightly in energy.
- The energy difference between the two spin states
corresponds (by Einsteins equation E hn) to
energy in the radiofrequency region of the
electromagnetic spectrum. - When hydrogen nuclei are irradiated with the
appropriate radiofrequency in a strong magnetic
field, they absorb energy and spin-flip. This is
NMR.
3Introduction to NMR
- Hydrogen nuclei are shielded from the full
applied magnetic field Bo by the electrons
surrounding them. - Nearby electronegative atoms
such as
oxygen or chlorine
attract electrons, thus
reducing electron density
and causing deshielding
of nearby hydrogens. - Each type of hydrogen
has a unique position
of absorption (called
the
chemical shift) in
the NMR spectrum.
4Estimation of Proton Chemical Shifts
- Proton chemical shifts are usually estimated
based on additive through-bond substituent
effects. - However, in some instances, through-space
(shielding or deshielding) effects may be more
important.
Observed deviations from estimated chemical
shifts caused by through-space effects (- is
shielding, is deshielding)
2.2 ppm
- 0.3 ppm
- 3.1 ppm
0.2 ppm
5Through-Space (de)Shielding
- Structures that exhibit NMR shifts that deviate
from those predicted by substituent effects
generally have one or more protons near a p bond. - Theoretical predictions of through-space magnetic
effects resulted in the familiar shielding
cones, such as the one for the CC
shown below.
shielding
deshielding
deshielding (q less than 54.7o)
shielding
6Shielding Cones
- Shielding cones are based on the McConnell
equation - which predicts the magnetic shielding
increment at a point in space due solely to the
magnetic anisotropy of the
functional group (CC in this case). - Based on the McConnell equation,
protons close to and over a CC
(within a 54.7º
cone) should be
shielded (shifted upfield) in
fact they
are deshielded and
are shifted downfield.
Ds 1/3 Dc (1 - 3 cos2 q)/4pR3
McConnell Ds 0.12 Observed Ds -2.12
7Our Groups Reseach
- Our groups research over the past eight years
has focused on the use of ab initio quantum
chemical calculations to - study through-space shielding effects of various
functional groups - Try to understand their origin and
- develop corrections to estimated shifts based on
through-bond substituent effects.
8Our Approach
- Our approach has been to use ab initio
computational methods to calculate isotropic
shielding values of protons in simple model
systems incorporating functional groups that
exert through-space effects. - For instance, to examine the effect of a CC
bond, our model system uses methane as a probe in
various positions over ethene, the simplest
molecule containing a CC bond. - Subtraction of the isotropic shielding value of
protons in an isolated methane gives
the shielding effect of the CC functional group.
9Methodology
- The subroutine GIAO (gauge-including atomic
orbital) in Gaussian was employed to calculate
isotropic shielding values. - HF/6-31G(d,p) calculations
were performed
on a simple
model system composed
of
previously-optimized
methane and ethene
structures juxtaposed
variously. - Symmetry reduced the
number of calculations
required.
10Methodology
- Single-point calculations were done on a series
of supramolecules each having methane at a
different position over the plane of ethene. - This process was repeated at several distances
(2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 Å) between
the proximate proton of methane and the plane of
ethene. - The isotropic shielding values of the proximate
proton of methane were extracted from the
Gaussian output. - The isotropic shielding value of methane (by
itself) calculated in the same way was subtracted
from each of the above values. We define this
difference as the shielding increment (Ds).
11Methodology
The shielding increment (Dd) for each H position
was plotted against Cartesian coordinates to
obtain a shielding surface at each distance above
ethene.
3.0 Å
2.0 Å
Positive values are shielding negative
values are deshielding.
12Methodology
- A function was matched to each surface using
TableCurve3D. - The same form of mathematical function
- . was found to give a good fit to the
shielding surface at each of
several distances of methane over ethene - 2.0 Å Ds 2.73 1.88X 2.20Y 0.29X2
0.22Y2 0.77XY - 2.5 Å Ds 0.79 0.82X 0.70Y 0.22X2
0.14Y2 0.21XY - 3.0 Å Ds 0.12 0.35X 0.23Y 0.14X2
0.065Y2 0.038XY - (etc., up to 5.0 Å)
Ds a bX cY dX2 eY2 fXY
13Methodology
- The values of the constant a and coefficients b,
c, d, e f in the equations of
the form
.
varied smoothly as a
function of distance. - Each of these variables could be related to the
distance above ethene using quadratic equations.
Ds a bX cY dX2 eY2 fXY
14Methodology
- Substitution of these quadratic equations into
the general shielding surface equation resulted
in one equation (18 terms too big to show!) for
predicting the through-space shielding increment
as a function of Cartesian coordinates relative
to the center of the CC. - This increment is useful as a correction for
estimated chemical shifts. - Fcn. Calc. Ds - 0.28
- 0.30 - 1.99 - Obs. Dev. Ds - 0.24
- 0.27 - 2.12
15Discrepancy rel. to Mc Connell Eqn.
- Our results show deshielding over the center of a
CC the McConnell equation predicts shielding. - McConnells equation considers only the magnetic
anisotropy of the CC (or other functional
group) it disregards all other factors that
affect the chemical shift!
Martin et al., J. Am. Chem. Soc. 1998, 120(44),
11510-11511.
16McConnell Eqn. vs. Our Function
Calcd NMR shielding increments along the CC
bond axis of ethene (Blue shielding Red
deshielding)
McConnell Equation
Our Shielding Function
Martin et al., Int. J. Mol. Sci., 2000, 1,
84-91.
17Results
- We have reported results of such computational
studies of the NMR shielding (or deshielding)
surfaces over aromatic rings1,2 and alkenes3.
benzene rings1,2 and alkenes3.
Positive values are shielding negative
values are deshielding.
1. Martin et al., J. Mol. Struct. (THEOCHEM)
1998, 454, 161-166. 2. Martin et al., J. Mol.
Graphics Mod. 2000, 18(3), 242-246. 3. Martin
et al., Struct. Chem. 1998, 9(6), 403-410,
Struct. Chem. 1999, 10(5), 375-380,
J. Molec. Graph. Mod. 2000, 18(1), 1-6.
18Results
- The function-predicted through-space shielding
increment compares favorably to the observed
deviation from the estimated chemical shifts
(based on additive substituent effects).
Observed deviations from estimated chemical
shifts (Here, - is shielding, is
deshielding) and Function-computed chemical
shift increments
2.1 ppm 2.0 ppm
- 0.3 ppm - 0.3 ppm
- 3.1 ppm - 3.0 ppm
0.2 ppm 0.3 ppm
19Results
- We have also reported on the NMR shielding
surfaces of the ethynyl, cyano, and nitro
groups using CH4 as a probe, with good prediction
of chemical shift effects.
Chemical shifts Predicted 8.7d 8.7d
8.7d Adjusted 9.8d 10.9d
8.2d Observed 9.9d 10.3d 8.1d
Martin et al., J. Mol. Graphics Mod. 2002, 21,
51-56.
20Results
- Our most recently published NMR shielding surface
study was of the carbonyl group, CO. - The traditional (McConnell) shielding cone
picture of the carbonyl group in textbooks shows
a cone of deshielding along the CO bond axis,
with shielding above the CO group. Our results
differ
of deshielding along the CO bond axis, with
shielding above the CO group. Our results differ
substantially
shielding
shielding
Martin et al., J. Mol. Graphics Mod. 2003, 22,
127-131.
21Origin of NMR (de)Shielding Effects
- In collaboration with P.v.R. Schleyer (U. Ga.),
IGLO-HF was used to analyze the localized orbital
origins of the through-space shielding effects
due to the ethenyl, ethynyl, cyano, nitro and
carbonyl groups. - The results indicated that in each of these
systems, the proximate C-H bond of the methane
probe accounts for over 40 of the shielding
increment. - Thus, McConnells approach, based solely on
magnetic anisotropy of the functional group can
not be expected to predict chemical shift effects
accurately.
Martin et al., Org. Lett. 2001, 3(24),
3823-3826.
22Origin of (de)Shielding
- The strong deshielding of a proton in the face of
a CC may be the result of mutual perturbation of
the interacting orbitals of the probe
and the test molecules. - An indication of this is seen in the
representation (right) of the HOMO of ethene
(wiremesh) superimposed with the HOMO of a
methane-ethene pair (solid), separated by 2.0 Å
Martin et al., in Modeling NMR Chemical Shifts
Gaining Insights into Structure and
Environment," ed. Facelli, J.C and deDios,
A.C., ACS, Washington, D.C., 1999, 207-219.
23Polarization of C-H Bond
- Such a perturbation should be accompanied by a
change in the calculated atomic charge. - NPA charges were calculated for the proximal H of
the probe methane over each functional group and
also for the Hs on isolated methane. - The difference between these was plotted vs.
distance of the
proximal H above ethene. - Similar results were observed over ethyne a
similar pattern but with less charge difference
was seen over HCN and over benzene.
24Effect of Choice of Probe?
- Several referees and researchers in this field
have suggested using other probes, such as a
ghost atom (Bq in Gaussian ), a H atom, or a He
atom. - It was also suggested that constrained
geometry-optimized probe-test supramolecules (as
opposed to the single point calculations we had
performed) would give more accurate results. - Our most recent work has involved investigating
alternative computational probes of through-space
NMR shielding effects to assess their validity
and computational efficiency.
25Methodology
- The following probes were used to calculate the
through-space shielding effect of several test
molecules - Bq (ghost atom)
- H atom (single point)
- H atom (geometry optimized)
- He atom (single point)
- He atom (geometry optimized)
- H2 molecule (single point)
- H2 molecule (geometry optimized)
- CH4 (single point) (the probe used in our
previous work) - CH4 (geometry optimized)
26Methodology
- The test molecules, simple structures containing
common organic functional groups, included - We are also examining the effect of the choice of
probe over some small-ring hydrocarbons. Of
these, only cyclopropane will be discussed today.
27Single Point Calculations
- Each HF/6-31G(d,p) geometry-optimized probe (Z)
was placed over the HF/6-31G(d,p)
geometry-optimized test structures in separate
Cartesian coordinate input files. - The probes position was moved 0.5 Å
incrementally in the Z direction. - Single point calculations were performed using
GIAO in Gaussian 98.
28Geometry-optimized calculations
- Each HF/6-31G(d,p) geometry-optimized probe (Z)
was placed over the HF/ 6-31G(d,p)
geometry-optimized test structures in separate
Z-matrix input files. - A dummy atom X was placed at the reference point
(here, the center of CC bond). The distance
between the probe and the dummy atom was fixed,
but all other structural parameters were allowed
to optimize.
29Z-Matrix Description of He over Ethene
- 0 1
- X
- C1 X halfcc
- He X hX C1 a
- C2 X halfcc He a C1 b
- H1 C1 1.076 X 121.7 He1 90.0
- H2 C1 1.076 X 121.7 He1 -90.0
- H3 C2 1.076 X 121.7 He1 90.0
- H4 C2 1.076 X 121.7 He1 -90.0
- Variables
- halfcc0.65746
- Constants
- hX2.0
- a90.0
- b180.0
30Shielding over the Center of the Carbon-Carbon
Single Bond in Ethane
31Shielding over the Center of the Carbon- Carbon
Double Bond in Ethene
32Shielding over the Center of the Carbon-Carbon
Triple Bond in Ethyne
33Shielding over the Center of the Carbon-Nitrogen
Triple Bond in HCN
34Shielding over the Center of the Benzene Ring
35CH4 Shielding Surface over Cyclopropane
- NMR shielding increments over cyclopropane were
computed using CH4 as a probe and using H2 as a
probe in much the same way that the
shielding increments over ethene and
other models for functional groups were obtained. - The resulting shielding increments were plotted
vs. Cartesian coordinates to provide shielding
surfaces.
36 CH4 Shielding Surface over Cyclopropane, 2.5 Å
Top view
Ds
Positive (blue) is shielding Negative (red) is
deshielding.
Note vdW deshielding!
37Comparison of Probes over Cyclopropane, 2.5 Å
CH4 probe
H2 probe
38CH4 vs. H2 Probe
- The shielding surfaces obtained using the two
different molecular probes are very similar. - They differ slightly in the magnitude of
shielding. - The ratio of the isotropic shielding values for
the two probes (H2 / CH4) was 0.84, regardless of
the test molecule and independent of the position
over the test molecule in the systems studied
(ethane, ethene, ethyne, benzene, HCN). - Both probes provide good agreement with
experimental chemical shift effects in example
structures. - The H2 probe is considerably easier to employ
and is more economical computationally.
39Summary of Shielding Probes
- Bq (ghost atom) gives the poorest agreement with
observed chemical shift effects. It completely
ignores the mutual perturbation of orbitals. - Monatomic probes (H and He) are not much better.
- There is no appreciable difference between
single-point calculations and geometry-optimized
calculations. - H2 and CH4 give generally similar results, with
H2 providing isotropic shielding values 0.84 of
those obtained with CH4. - CH4 has been found previously to give accurate
predictions of chemical shift effects of
through-space shielding H2 (with or without a
minor correction) can do so also. - H2 is simpler and cheaper computationally.
40Ongoing Research
- We have begun to study the NMR shielding surfaces
of molecules and complexes of biochemical
interest, modeling through-space NMR shift
effects that are operative in peptides
41Acknowledgments
- Student collaborators
- Noah W. Allen, III Luong Vo Jill C. Moore
Everett K. Minga Sal T. Ingrassia Justin D.
Brown H. Lee Woodcock David M. Kmiec, Jr.
Kimberly H. Nance Dustin C. Wade David M.
Loveless Kristin L. Main. - The donors of the American Chemical Society
Petroleum Research Fund for support of this
research (1996-2003) - The (former) North Carolina Supercomputing Center
- The UNCW Information Technology Services Division
- The UNCW College of Arts and Sciences
- The UNCW Department of Chemistry