Frequency of Nuclei in Matte
a> = y B
ALL TENSORS
1) Shielding, chemical shift
electron distribution around nuclei, induced magnetic field
2) Dipolar interactions
magnetic nuclei distribution, through space interactions, in solids
3) Electric field gradient
distribution of nuclei (positive charge) and electrons (negative charge) quadrupolar nuclei
4) Scalar coupling
dipolar interactions through electrons in molecules, bonds
1
Chemical Shift Information
The presence of an element in the sample
Number of signals = number of chemically different atoms Symmetry of the molecule
Relative intensity = ratio of atoms — integration
Position = chemical shielding — electronic environment
Type of bonds, oxidation state, coordination number
Multiplicity = connectivity of functional groups
2
Chemical Shift Information
The presence of an element in the sample
Chemical Shift
Chemical Shift
Chemical shift for a given molecule:
• Number of signals = nonequivalent nuclei molecular symmetry
• Intensity = number of nuclei
• Position in the spectrum = shielding electronic structure
• Multiplicity = connectivity of atoms and groups
CH3CH2OH
Methyl triplet
—i-1-1-1-1-1-1-1-1—
876543210
6
Molecular Symmetry
Number of signals = nonequivalent nuclei, molecular symmet
Flow chart for point group determination
Symmetry Elements and Operations
Symbol	Element	Operation	
	Mentis	Identity	No change, (= 1)
• i	Center of symmetry (inversion center) POINT	Invert	Inversion through the central point every point x,y,z translated to -x,-y,-z
	Rotation axis LINE	True (proper) rotation	Rotation by an angle 360/n
a	Plane of symmetry, mirror PLANE	Reflection	Reflection through a mirror plane
Sn	Improper axis Roto-reflection axis LINE	Improper rotation	Rotation by an angle 360/n followed by reflection through a mirror plane 7
NO inversion center
(b)
10
12
Rotatio
Xe
. / \
FF
Pt
Ci Ci
2©
Cu
/ \
H3N NH3
2 ®
Plane of Symmetry a
Planar molecules - symmetry plane of the molecule ah = horizontal plane, perpendicular to principal axis
av = vertical plane, parallel to principal axis, bisects the most atoms
ad = dihedral plane, colinear with principal axis, bisecting two C2'
Plane of Symmetry a
6
Improper Axis Sn
1
Rotation-reflection = a compound operation, rotation (Cn) followed by a reflection through a plane perpendicular to the Cn axis
S1 = C x a = a
S2 = C2 x a = i
Symmetry Elements in a Molecule
Equivalent atoms = exchanged by symmetry operations
19
Figure 2. The Founding Fathers. R. M. Barrer (19101996) (right) and R. M. Milton (1920-2000) photographed
Condition of chirality: no Sn pr
Chemical Shift
Number of signals = nonequivalent nuclei
Chemically different atoms
13C NMR
3 x t-Bu groups
No C3 axis
Geometrical difference = chemical difference
22
Molecular Symmetry
1 x Me group signal C2 axis
23
Chemical Shift
R
R
25
Chemical Shift
26
1H NMR (ÓGG MHz, CDCl3, TMS): 3.2S (s, 2H, H-2' and H-Ó'), 1.?G (br s, 2H, H-1 and H-3), 1.S? (br s, 2H, H-S and H-?), 1.SS (br s, 4H, H-4, S, 9, 1G), 1.4S (m, ÓH, H-4, S, 9, 1G and H-Ó), 1.34 (m, 4H, H-3' and H-S'), G.9Ó ppm (m, 2H, H-4')
13C NMR (?S MHz, CDCl3, TMS): 1G2.2 (C-8'=C-2), ^^^^^H SG.S (2C, C-2' and C-Ó'), 4G.G (4C, C-4, S, 9, 1G),
39.1 (C-Ó),
29.2 (2C, C-S and C-?), 2S.? (2C, C-1 and C-3),
22.3 (2C, C-3' and C-S'), 2G.S (C-4'),
2G.2 ppm (2C, C-1' and C-?').
29
Chemical Shift
11B NMR
Mono- and Disubstituted B12H122- Molecules with
Identical Substituents
1 c5v	Civ		Dm	
| 1:5:5:1	2:2:4:2:2	2:2:4:2:2	2 : 10	
30				
32
34
Chemical Shift
8 (13C) = 143 ppm
de
Geminal Groups
Geminal groups — paired ligands
38
Chemical Shift Nonequivalence of Geminal Ligands in Prochiral Groups
Chiral Group X
H
X
|
H
a
Prochiral Group X
H
C2 Group X
X
Gem nal groups
Me, Me Enantiotopic
Equivalent (isochronous) in achiral media
Nonequivalent (anisochronous) in chiral
media
Me, Me Homotopic Equivalent (isochronous)
X H
No a present
Me, Me Diastereotopic Nonequivalent (anisochronous) If X is chiral, the paired ligands in a prochiral group are always diastereotoppic
Substitution Test of Geminal Groups
	replace Ha	replace Hb
		
		
	enantionners	
	\     H-, pnd Hb ptp pnpntinl-npir	
diastereormers
41
Chemical Shift Nonequivalence in
Prochiral Groups
42
Chemical Shift Nonequivalence in
Prochiral Groups
S
43
Chemical Shift Nonequivalence in
Prochiral Groups
R = Me
R = 2-butyl
Pyramidal N - Fast inversion on N
44
Chemical Shift Nonequivalence in
Prochiral Groups
R = iPr
45
6.3
62
i
63 ^^jiii
62
H 1
—r 23
-&4
-i—i—iip-—i—i—i i i i S3 ei
4 Win
rid UBE I NMR spectra of CH(P) moiaty of compound 2: a), b) lH NMRspec-■ra inCDCt and CDCla, DaO respectively; c)31 ?\1H} NMR spectrum in CDCl^; [) CH COSY diagram in CDCla.
47
48
1H NMR spectrum
6 CH aromatic signals How many CH2 signals ?
6 CH3 mesityl signals
49
1H NMR spectrum The methylene hydrogens are diastereotopic
6 CH aromatic signals        two signals at 3.69 and 4.81 ppm 6 CH3 mesityl signals
Chemical Shift
Chemical shift for a given molecule:
• Number of signals = nonequivalent nuclei molecular symmetry
•Intensity = number of nuclei
• Position in the spectrum = shielding electronic structure
Multiplicity = connectivity of atoms and groups
Integration
—
The area under each signal is proportional to the number of protons that give rise to that signal
The height of each integration step is proportional to the area under a specific signal
The integration tells us the relative number of protons that give rise to each signal, not absolute number
Relative Signal Intensity
9 H / 2 H
CH,
I 3 CHXCH9Br
I 2
7.0
integration line
1.6
4 3
5 (ppm)
- frequency
0
53
Polyphosphate Chain Length
Relative Signal Intensity