Ring current effects

1

• Ring current effects
• Last time we finished with anisotropic effects
  from single,
• double, and triple bonds. One of the most
  pronounced
• effects arising from induced magnetic moments
  in a
• chemical group are due to aromatic rings.
• The induced magnetic dipole created by an
  aromatic ring is
• the easiest to understand. If we consider the
  ring current of
• the ring, it will generate a magnetic field
  perpendicular to the
• plane of the ring, that will be against the
  external magnetic
• field

Bring
e-
Bo
2

• Ring current effects (continued)
• As we had for simpler systems (single, double,
  and triple
• bonds), we can also estimate the degree of
  shielding as a
• function of the position of our nuclei around
  the ring.
• There are several formulas with different
  degrees of
• precision, but even the simplest ones give us a
  pretty decent
• estimate. The simplest one is the Polple
  point-dipole model

H
r
q
drc Cpople irc r-3 . ( 1 - 3 . cos2q )
3

• Ring current effects ()
• As was the case for single, double, and triple
  bonds, we can
• plot the value of the shielding as a function
  of the position in
• space of the 1H under study. It will also be
  cone-shaped,
• with a shielding regions (-, lower chemical
  shift), and
• deshielding regions (, higher chemical shift)
• Protons on the sides of the aromatic ring will
  feel a higher
• local magnetic field (higher ppms), while
  those on top or

7.27
7.79
7.41
4

• Ring current effects ()
• There are cases in which the protons of the ring
  end up
• inside the shielding cone of the aromatic ring,
  such as in
• 18annulene
• There is one last example of a ring with a
  considerable
• anisotropic effect. Cyclopropane is very
  strained, and has
• double bond character (carbons have sp2
  character). There
• is a magnetic dipole perpendicular to the plane
  of the ring

9.28
-2.99
-


-
5

• Electric field and Van der Waals effects
• Although there are many other factors affecting
  1H chemical
• shifts, well finish by describing the effect
  that polar groups
• and close contacts have on shifts.
• We can understand pretty intuitively how a
  charged group will
• affect the shielding of a proton. Depending on
  the charge, the
• electric field will pull or push on the
  electron density around
• the proton, deforming it, and therefore
  affecting the local field.
• Analogously, an uncharged group that sits close
  to the proton
• will disturb its electron density due to van
  der Waals
• contacts. Both effects are appropriately
  represented by the
• Buckingham equation

C
Ds - AEC-H - BE2
H
6

• Some examples
• To conclude this discussion of factors affecting
  chemical
• shift, lets take a look at some interesting
  examples in which
• chemical shift can be used to decide on the
  structure of
• different molecules.
• The first one deals with cyclopropane
  anisotropy. In the
• following compound, the chemical shift of the
  indicated
• protons appears were expected for aromatic
  protons
• However, if we just change the two methyls for a
  spiro

7.42
6.91
7

• Some examples (continued)
• In the following ketones, we can see the effects
  of the
• carbonyl group anisotropy
• Finally, the following example
• demonstrates that antiaromatic
• systems are paramagnetic (their
• induced field is in favor of the
• external magnetic field). In this
• dihydropyrene, the methyls show
• up were expected for an aromatic

7.27
6.57
5.47
d (CH3) -4 d (Ar-H) 8
d (CH3) 21 d (Ar-H) -4
8

• Some examples ()
• Another case in which several effects come into
  play is seen
• in a,b-unsaturated ketones. Here we resonance
  (electronic
• effects) dominating the shift at the b protons
• We also have CO group anisotropy
• In cis-malonates the deshielding is not as
  strong because

6.28
6.83
-1.0
-2.4
9

• Shoolery chemical shift rules for 1H
• As we have seen, most of the different effects
  on 1H
• chemical shifts have been tabulated in one way
  or another.
• Furthermore, we also saw that most of the
  effects are
• additive, meaning that if we can estimate the
  different effects
• on the chemical shift of a certain 1H from
  different groups
• and bonds, we can in principle estimate its
  chemical shift by
• adding all the effects together.
• There are several empirical rules, derived
  mostly by
• Shoolery in the late 50s/early 60s.
• In order to use them, we first have to identify
  the type of
• proton we have, such as aliphatic CH3, CH2, CH,
  olefinic
• CH2 or CH, aromatic, a or b to a ketone or
  alcohol,
• belonging to an a a,b-unsaturated system, etc.
  They will have
• a base value.

dH dHbase S contributions
10

• Shoolery rules (continued)
• Aliphatic compounds. There are two approaches to
  the
• calculation of additive effects on the 1H
  chemical shifts.
• The first one is very simple. We just use two
  skeletons with
• two base values, R1-CH2-R2 or R1-CH-(R2)-R3,
  and add the
• effects from the R1, R2, or R3 groups

Substituent
d
Alkyl
0.0
-CC-
0.8
R1-CH2-R2 d 1.25 R1 R2
-C?C-
0.9
-C6H5
1.3
-CO-R
1.3
-OH
1.7
-O-R
1.5
R1-CH2-(R2)-R3 d 1.50 R1 R2 R2
-O-CO-R
2.7
-NH2
1.0
-Br
1.9
-Cl
2.0
11

• Shoolery rules ()
• The second method is pretty more general. We
  start with
• methane (d of 0.23 ppm), and then we add
  substituent
• effects directly.
• Now, if instead of a susbtituent we have another
  carbon
• chain, we have to consider how many carbons it
  has, and
• each carbon will have an increment we need to
  add

CH3-
0.47
C6H5-
1.85
d 0.23 S S(d)
RO-
2.36
RC(O)O-
3.13
C3
0.248
0.244
0.147
0.006
C3
C2
C3
C2
C2
C3
C2
C3
C3
C1
C2
C3
HO-
2.47
0.048
0.235
Br-
1.995
0.363
0.023
CH3O-
-
-0.374
-
-O-CO-CR3
2.931
0.041
-0.086
12

• Shoolery rules ()
• Olefines. For alkenes we change the tables for
  the base
• values, but we also have to consider the
  stereochemistry of
• the substituent (cis, trans, or gem)

d 5.25 Rgem Rtrans Rcis
Substituent
dgem
dcis
dtrans
H-
0.0
0.0
0.0
Alkyl-
0.45
-0.22
-0.28
-OR
1.21
-0.60
-1.00
-COOH
0.80
0.98
0.32
-Ar
1.38
0.36
-0.07
-CC-
1.24
0.02
-0.05
-OH
1.22
-1.07
-1.21
-Cl
1.08
-0.40
-1.02
13

• Shoolery rules ()
• Aromatics. Finally, the Schoolery rules allow us
  to calculate
• the approximate chemical shifts in aromatic
  compounds.
• Again, we have a different base value of 7.26
  (benzene).

d 7.26 Rortho Rmeta Rpara
Substituent
dortho
dmeta
dpara
H-
0.0
0.0
0.0
CH3-
-0.18
-0.10
-0.20
-NO2
0.95
0.26
0.38
-COOH
0.85
0.18
0.25
-OCH3
1.38
0.36
-0.07
-Cl
1.24
0.02
-0.05
-F
1.22
-1.07
-1.21
-CONH2
1.38
0.36
-0.07
-CHCH2
1.24
0.02
-0.05
-SO3H
1.22
-1.07
-1.21
14

• Shoolery rules ()
• For p-Xylene
• dHa 7.26 - 0.18 - 0.10 6.98 (6.97)
• dHb dHa
• For 1-Chloro-4-nitrobenzene
• dHa 7.26 0.95 - 0.02 8.19 (8.17)
• dHb 7.26 0.03 0.26 7.55 (7.52)
• For mesitylene