Introduction to NMR Spectroscopy and Imaging

Lecture 02 Chemical Shift and J-Coupling

(Spring Term, 2011)

Department of ChemistryNational Sun Yat-sen University

核磁共振光譜與影像導論

Chemical Shift and J-Coupling

In the beginning….

Norman Ramsey (Phys.Rev. 1950, 78,699):"Furthermore, with heavier nuclei the ratios of the resonance frequencies for the same nucleus in different molecules have been measured with high precision and discrepancies have been found by various observers that are sometimes called chemical shifts".

(Proctor says: "until it is clearly understood, the accuracy of magnetic moments determined under certain chemical conditions remains somewhat in doubt").

W.G.Proctor, F.C.Yu(虞福春 ), Phys Rev 1950,77,717. W.C.Dickinson, Phys Rev 1950 77, 736.

All spins were of no difference…same, identical, equal, I/You/He/We/You/They wereall the same…or believed to be so….then...the apple of discern came in…

?Dickinson?Who can find his photo?

Ramsey

Yu

Proctor Valuable reading: http://www.ebyte.it/library/hist/ProctorWG_Reminiscences.html

1922-2006

1913-2003

Chemical Shift (Shielding)

0

0

loc induced

induced

B B B

B Bchemical shift tensor

0B

Induced shielding field

Chemical Shift:a molecule becomes a dipole

0

0

loc induced

induced

B

B Bchemical shift tensor

0B

The induced dipole moment shifts the resonance frequency of the nuclear spin.

m

Shielding Depends on Chemical Environment

Different environments cause different shieldings

0 0

0 0

0 0( ) (1 ( ))s

B BB

B r B B B r

(Representing different local chemical environments, Proctor and Yu, 1951)

Hence chemical shift (of resonance frequency relativeto Zeeman frequency ω0.)

0

0

Anti-shielding Is Possible

))(1()( 00 rBBBrB s

Anybody cares to find some interesting literature?

Some proton chemical shifts

0 0

0 0

( ) [( ) ( )]

6

( )

=( ) 10 ppm

ref ref

relative

ref absolute

ref absolute

Reference shift

Less shielded.

Increasing δ

(downfield)

The OH bonding in vaporized water clearly differs from that in liquid water! (hydrogen bonding has significant effect on chemical shift)

The more localized AO/MO, the more shieldingStronger/more bonds mean smaller CS.

You can tell a lot from this diagram

Upfield (low freq)Downfield (high freq)

These words were from CW NMR. ‘Downfield’ means for agiven resonance frequency, the magnetic field used is lower. ‘High frequency’ means at a fixed magnetic field, the spins inthis region have higher resonance frequencies.

(these trends for σ,δ,B, |ν| are same for γ>0 or γ<0 nuclei. However, for γ<0, ν is negative.)

Small shift/large shieldingLarger shift/small shielding

Why CHn have smaller CS than H atom?

B0

Some people said: An H in CHn seems to be less shielded because the C has larger electronegativity so it ‘draws’ electrons to its side. But why an H in CHn has smaller CS than H atom? Answer: The electron density at the C-H bonding area is larger than that of an H atom albeit the electron density at other places is smaller. Overall, the H in CHn is more shielded than in H atom.

H atomHC

This can also explain why H > H2, H>OH>H2 More bonds, more shielded.

Why this CS order: CH>CH2>CH3?

More bonds, more shielded.

The bonding regions correspondto large shielding (small CS).The CS is smallest when the magneticfield is along the bonding direction.

Why this CS order: HF>HCl>HBr>HI?

The fewer number of electrons of the bonding partner, the less the shielding the bonded H. (The more clothes you dress, the more you’re shielded.)

The shielding of s orbitals is smaller than that of p orbitals which is even smaller than that of d orbitals etc. (The more localized the orbitals, the more shielding)

Isotopic Effect• Because CS is generated by electrons, nuclei of isotopic elements

(e.g. H1/D2, N14/N15, Cl35/Cl37) have very similar chemical shift but isotope shift does exist: e.g., 1H CS of HOD is 0.035 ppm upfield (more shielded) of that of HOH (the electrons in HOD is a little ‘heavier’ than in HOH lower vib freq/amp more shielding (you are more shielded by your clothes if you shake yourself less violently.)

• There is a general rule which says that when one substitutes a nuclide in a chemical group with a heavier isotope then all other nuclides in the group become a bit more shielded (this has to do with an overall reduction in vibrational amplitudes).

• Consequently, the chemical shift of protons in standard bulk water should be 4.795 + 0.035 = 4.830 ppm, give or take 0.02 ppm. Of course, heavy water and normal water do not even have the same bulk properties (such as density and magnetic susceptibility) and this introduces a further uncertainty when trying to deduce the chemical shift of normal water from the data on HDO traces in D2O.

You may be able to memorize this table or you may explain it based on your good scientific intuition

Proton and Carbon Standards for Organic Solents

Chemical Name Chemical Formula Chemical Structure Boiling Point (oC)

Chemical Shift

H1 C13

Tetramethylsilane (TMS) C4H12Si                 27 0.000 0.000

Dioxane C4H8O 2          -- 3.75 --

Proton Standards for Aqueous Solents

3-(Trimethylsilyl)- Propionic acid-D4, sodium salt (TSP)

C6H9D4NaO2SI                                302 0.000 0.000

2,2-Dimethyl-2-silapentane- 5-sulfonate sodium salt (DSS)

C6H15NaO3SSi                                120 0.000 (labelled as DSS)

--

P31 Standards

Chemical Name Chemical Formula Chemical Structure B. P. (oC) Chemical Shift

85% Phosphoric Acid (external) H3PO4              -- 0.00

10% trimethylphosphate (internal) (CH3O)3PO -- 0.00

N15 Standards

Chemical Name Chemical Formula Chemical Structure B. P. (oC) Chemical Shift

liquid NH3> (external) NH3 -- 0.00

OTHER NMR RESOURCES

More info http://www.bmrb.wisc.edu/home/iupac.pdf.

Most internal NMR referencing standards are pH and temperature sensitive. N

MR

Intern

al Referen

cing

Stan

dard

S

amp

les

Solvent 1H Chemical Shift (multiplicity) JHD (Hz) HOD in solvent (approx.)

13C Chemical Shift (multiplicity)JCD

(Hz) B.P. (oC) M.P. (oC)

Acetic Acid-d4 11.652.04

15

--2.2

11.5 178.9920.0

17

--20

118 17

Acetone-d6 2.05 5 2.2 2.8 206.6829.92

137

0.919.4

57 -94

Acetonitrile-d3 1.94 5 2.5 2.1 118.691.39

17

--21

82 -45

Benzene-d6 7.16 1 -- 0.4 128.39 3 24.3 80 5

Chloroform-d 7.27 1 -- 1.5 77.23 3 32.0 62 -64

Cyclohexane-d12

1.38 1 -- -- 26.43 5 19 81 6

Deuterium Oxide 4.80 (DSS) 1 -- 4.8 -- -- -- 101.4 3.8

N,N-Dimethyl-formamide

8.032.922.75

155

--1.91.9

3.5 163.1534.8929.76

377

29.421.021.1

153 -61

Dimethyl Sulfoxide-d6

2.50 5 1.9 3.3 39.51 7 21.0 189 18

p-Dioxane-d6 3.53 m -- 2.4 66.66 5 21.9 101 12

Ethanol-d6 5.293.561.11

11

m -- 5.3

--56.9617.31

--57

--2219

79 <-130

Methanol-d4 4.873.31

15

--1.7

4.9--

49.15 --5

--21.4

65 -98

Methylene Chloride-d2

5.32 3 1.1 1.5 54.00 5 24.2 40 -95

Pyridine-d58.747.587.22

111

-- 5.0150.35135.91123.87

335

27.524.525

116 -42

Tetrahydrofuran-d8

3.581.73

11

-- 2.4 - 2.567.5725.37

55

22.220.2

66 -109

Toluene-d8

--7.097.006.982.09

--m1m5

--------

2.3

0.4

137.86129.24128.33125.4920.4

13337

--23242419

111 -95

Trifluoroacetic Acid-d

11.50 1 -- 11.5164.2116.6

44

  72 -15

Trifluoroethanol-d3

5.023.88

14x3

--2 (9)

5 126.361.5

44x5

--22

75 -44

H1 an

d C

13 Ch

emical S

hifts o

f NM

R

So

lvents

NOTES: o1H chemical shifts are in PPM, relative to 0.05% TMS (v/v), at 295 K. o13C chemical shifts are in PPM, relative to 1.0% TMS (v/v), at 295 K. o'm' denotes broad peak with some fine structures (at 200 MHz). oHOD peak positions may vary depending upon concentration in solvent, pH and temperature. oM.P. and B.P. values are for the corresponding non-deuterated solvent (except for D2O). o(DSS) denotes chemical shifts relative to 2,2-Dimethyl-2-silapentane- 5-sulfonate sodium salt.o See NMR Referencing for more information

CH

AR

AC

TE

RIS

TIC

PR

OT

ON

CH

EM

ICA

L S

HIF

TS

Type of Proton Structure Chemical Shift, ppm

Cyclopropane C3H6 0.2

Primary R-CH3 0.9

Secondary R2-CH2 1.3

Tertiary R3-C-H 1.5

Vinylic C=C-H 4.6-5.9

Acetylenic triple bond,CC-H 2-3

Aromatic Ar-H 6-8.5

Benzylic Ar-C-H 2.2-3

Allylic C=C-CH3 1.7

Fluorides H-C-F 4-4.5

Chlorides H-C-Cl 3-4

Bromides H-C-Br 2.5-4

Iodides H-C-I 2-4

Alcohols H-C-OH 3.4-4

Ethers H-C-OR 3.3-4

Esters RCOO-C-H 3.7-4.1

Esters H-C-COOR 2-2.2

Acids H-C-COOH 2-2.6

Carbonyl Compounds H-C-C=O 2-2.7

Aldehydic R-(H-)C=O 9-10

Hydroxylic R-C-OH 1-5.5

Phenolic Ar-OH 4-12

Enolic C=C-OH 15-17

Carboxylic RCOOH 10.5-12

Amino RNH2 1-5

Carbon-13 Chemical Shifts

Carbon-13* Environment

Chemical ShiftRange (ppm)

(CH3)2C*O -12

CS2 0

CH3C*OOH 16

C6H6 65

CHCl=CHCl (cis) 71

CH3C*N 73

CCl4 97

dioxane 126

C*H3CN 196

CHI3 332

You may be able to memorize this table or you may explain it based on your good scientific intuition

• For most organic compounds, the 1H chemical shift is in the range of 12 ppm, but the chemical shift range for hydrides (organometallic compounds) is approximately +25 to -60 ppm, the largest range could possibly reach 200 ppm!. The downfield shifts are most common in d0, d10 and early transition metal cases whereas those with other dn counts and late transition metals tend to be upfield of zero.

• Similar phenomenon occurs for other nuclei such as 13C, 31P etc.

                                                    

Temperature dependence of the 1H NMR spectrum of Ni Ni dissolved in toluene-d8.

The temperature runs from 183 (lowest trace) to 385 K. S = solvent.

Be aware of “abnormal” chemical shifts …...

Harald Hilbig and Frank H. Koehler, New J Chem, 2001.

1H

13C

1H

Phosphorous-31 Chemical Shifts

Phosphorous-31 Environment

Chemical ShiftRange (ppm)

PBr3 -228

(C2H5O)3 P -137

PF3 -97

85% phosphoric acid 0

PCl5 80

PH3 238

P4 450

Compound

Chemical Shift (ppm)Relative to 85% H3PO4

PMe3 -62

PEt3 -20

PPr(n)3 -33

PPr(i)3 +19.4

PBu(n)3 -32.5

PBu(i)3 -45.3

PBu(s)3 +7.9

PBu(t)3 +63

PMeF2 245

PMeH2 -163.5

PMeCl2 +192

PMeBr2 +184

PMe2F +186

PMe2H -99

PMe2Cl -96.5

PMe2Br -90.5

Phosphorous (III) Chemical Shift Table (from Bruker Almanac 1991)

Phosphorous (V) Chemical Shift Table

(from Bruker Almanac

1991)

CompoundChemical Shift (ppm)Relative to 85% H3PO4

Me3PO +36.2

Et3PO +48.3

[Me4P]+1 +24.4

[PO4]-3 +6.0

PF5 -80.3

PCl5 -80

MePF4 -29.9

Me3PF2 -158

Me3PS +59.1

Et3PS +54.5

[Et4p]+1 +40.1

[PS4]-3 +87

[PF6]-1 -145

[PCl4]+1 +86

[PCl6]-1 -295

Me2PF3 +8.0

Fluorine-19 Chemical Shifts

Fluorine-19 Environment

Chemical ShiftRange (ppm)

UF6 -540

FNO -269

F2 -210

bare nucleus 0

C(CF3)4 284

CF3(COOH) 297

fluorobenzene 333

F- 338

BF3 345

HF 415

Nitrogen-14 Chemical Shifts

Nitrogen-14* Environment

Chemical ShiftRange (ppm)

NO2Na -355

NO3- (aqueous) -115

N2 (liquid) -101

pyridine -93

bare nucleus 0

CH3CN 25

CH3CONH2 (aqueous) 152

NH4+ (aqueous) 245

NH3 (liquid) 266

B-11 Chemical Shift

Almost all quadrupolar nuclei have rather small CS range.

Factors Affecting Chemical Shift• Temperature• Solvents (pH,

concentration)• Pressure• Sample shape

(susceptibility)• ……

NMR can be used as a thermometer, a pH meteror a barometer. (Only very smart guys would like to buy an NMR spectrometerfor those purposes though)

Solvent Shift *(H2O)

H2O 4.83

D2O 4.79

DMSO 3.3

acetone 2.5

CD Cl3 1.4

C6D6 0.3* Relative to TMS.

Amide proton chemical shifts of NHA in CDCl2CDCl2 as a function of temperature and concentration.

Derr et al. J. Chem. Soc., Perkin Trans. 1, 2000.

Chemical Shift

The surrounding electrons cause a shielding magnetic field at the nucleus

)1(00 BBBB s

Shielding Anisotropy (CSA)

B0

B0

)1(0 BB

Chemical shift anisotropy (CSA) tensor

In liquids, CSA is averaged out by rapid molecular tumbling; in solids, CSA is kept.

Electron clouds are seldomspherically symmetrical. Theyare anisotropic in almost all molecules.

Oriented Molecules

B0

Oriented Single Crystals

B0

Powder (Polycrystalline Solid)

B0

Chemical Shift Tensor

)(0 rBE

http://bouman.chem.georgetown.edu/nmr/interaction/chmshf6.gif

http://bouman.chem.georgetown.edu/nmr/interaction/chmshf9.gif

Applications of Chemical Shift

Ap

plicatio

ns o

f Ch

emical S

hift

http://www.bmrb.wisc.edu/data_access/outlier_selection_grid.html

Applications of Chemical Shift

Applications of Chemical Shift

Relaxation, dynamics

Solid state NMR

CS Imaging

……

Story Goes On

Indirect Dipolar Interaction (J-Coupling)

N

SN

S

Interaction between spins mediated by electrons around them.J-coupling is usually much smaller than direct dipolar coupling.

J-CouplingNMR/I

Homonuclear system

A Heteronuclear System AX System

X

X

A X

AXJ AXJ

11 32: :1 1: :nn nnn CC C C

1 2 1: : : : :1 1mmmmC C C

Spin A:

Spin B:

00000…000

10000…000

11111…101

11111…111

11111…110

01000…000

00100…000

…

11000…000

01100…000

00110…000

…

1=“up”0=“down”

General Cases of Two-Site Homonuclear Systems000…00

100…00

111…01

111…11

111…10

010…00

001…00

…

110…00

011…00

0011..00

…

Spin BSpin A

Exercise: Who are They?

ABC System

200 MHz 1H-NMR spectrum of dibromo benzonorbornene derivative in CDCl3 and expansions of the signals.

ABCD system

Equivalent Spins

Coupled with Quadrupolar Spins

Strong Coupling and Quantum Mechanical Treatment

Example

Ｅ　ｉｓ　ｂｒｏａｄｂｅｃａｕｅ　ｏｆ　ｅｘｃｈａｎｇｅ．

Ha

Hb

Ｈｃ

Ha（Ｈｏｙｅ）

Ａｎａｌｙｓｉｓ

Ａｎａｌｙｓｉｓ

Ｈｃ

Ｈｄ

Ｈｄ

Ｒｅｓｕｌｔ

Ｒｅｓｕｌｔ

Karplus Equation

Karplus Equation showing the relationship between the observed couplingconstant and the φ(=θ-135o) angle. Note that unique solutions are obtained only for J > 8 Hz and J <5 Hz .

Φ

Karplus Equations

Karplus Equations3JH-C-C-H = 10 cos2 for 0 £ 900, and3JH-C-C-H = 12 cos2 for 90 £ £ 1800

Typical J-coupling constants• 3JCOCH Mulloy et al. Carbohydr. Res. 184 (1988) 39-46 • Tvaroska et al. Carbohydr. Res. 189 (1989) 359-362 • Anderson et al. J. Chem. Soc., Perkin 2 (1994) 1965-1967 • 3JCOCC B. Bose et al. J. Am. Chem. Soc. 120 (1998) 11158-11173 • Q. Xu and A. Bush Carbohydr. Res. 306 (1998) 335-339 • M.J. Milton et al. Glycobiology 8 (1998) 147-153 • 3JCCCH R. Aydin & H. Günther Mag. Reson. Chem. 28 (1990) 448-

457 • A. de Marco et al. Biochemistry 18 (1979) 3847- • 3JPOCH Lankhorst et al. J. Biomol. Struct. Dyn. 1 (1984) 1387-1405 • 3JCCOP Lankhorst et al. J. Biomol. Struct. Dyn. 1 (1984) 1387-1405 • 3JHNCH S. Ludvigsen et al. J. Mol. Biol. 217 (1991) 731- A. Pardi et

al. J. Mol. Biol. 180 (1985) 741- • V.F. Bystrov ， Prog. NMR Spectrosc. 10 (1976) 41- • 3JCNCH L.-F. Kao et al. J. Am. Chem. Soc. 107 (1985) 2323-

3JCNCC L.-F. Kao et al. J. Am. Chem. Soc. 107 (1985) 2323- • 3JHCOH R.R. Fraser et al. Can. J. Chem. 47 (1969) 403-409

Applying the Karplus Equation

Applying the Karplus Equation

Long Range Coupling

The doublet splitting arises from the coupling with the geminal proton Ha. The fact that the Hb, proton does not couple with the bridgehead protons Hc is attributed to the dihedral angle, which is nearly 90°. At the same time, proton Ha couples with the geminal proton Hb and bridgehead protons Hc. Furthermore, proton Ha has long-range coupling to the Hj protons. This can be clearly seen by the further triplet splitting of the signals. This long-range coupling arises from the zigzag orientation of protons Ha and Hd. The zigzag orientation of protons Hb and Hd is impossible because of the rigid geometry. Consequently, there is no long-range coupling between these protons. The fact that proton Ha has long-range coupling to Hd protons clearly indicates the exo configuration of the bromine atoms. In the case of the endo configuration we should not observe any long-range coupling.

Amino Acids

Amino Acid, Name, Abbr. R =

Alanine, ala,A CH3-

Arginine, arg,R H2N-C(=NH2+)-, NH-(CH2)3-

Asparagines,asn,N H2NC(O)CH2-

Aspartic acid, asp,D HOOC-CH2-

Cysteine, cys,C HS-CH2-

Glutamic acid, glu,E HOOC-(CH2)2-

Glutamine, gln,Q H2NC(O)CH2-, CH2-

Glycine, gly,G H-

Histidine, his,H

Isoleucine, ile,ICH3CH2-

CH(CH3)-

Leucine, leu,L (CH3)2CHCH2-

Lysine, lys,K +H3N(CH2)4-

Methionine, met,M CH3SCH2CH2-

Phenylalanine,phe,F Ph-CH2-

Praline, pro,P

Serine, ser,S HOCH2-

Threonine,thr,T CH3CH(OH)-

Tryptophan,trp,W

Tyrosine,tyr,Y HO-Ph-CH2-

Valine,val,V (CH3)2CH-

Summary of one-bond heteronuclear couplings along the polypeptide chain utilized in 3D and 4D NMR experiments

Structure of an A-U ( Ａ ) and a C-G ( Ｂ ) Watson-Crick base pair. Notice that in each case, there is a single N-H ... N hydrogen bond. Scalar coupling across this bond was determined to be approximately 6.3 Hz for the GC bp and 6.7 Hz for the AU bp. Non-Watson Crick bp schemes (such as Hoogsteen) contain different hydrogen bonds that can be distinguished from traditional Watson-Crick.

(CH3)2CH

(CH3)2CH

Coupled

Decoupled

Varian parameters: dn, dm, dmm, dpwr

C-H Coupling and 13C Broadband Decoupling

13C-1H Coupling and 13C Broadband Decoupling

Selective Decoupling of 1H-1H

Selective Decoupling of 1H-1H