Che 440/540 Proton Nuclear Magnetic Resonance (NMR) Spectroscopy

Fundamental NMR EquationsNumber of energy levels = 2I + 1I = the spin quantum number (1/2 for 1H and 13C)Therefore, there are two possible spin states for these nuclei.

E = h; h = Planck’s constant, = resonant frequency, Hz

= Bo/2Bo = applied magnetic field = gyromagnetic ratio (unique for each NMR active nucleus)

E = hBo/2

IsotopeNatural %Abundance

Spin (I)MagneticMoment (μ)*

MagnetogyricRatio (γ)†

1H 99.9844 1/2 2.7927 26,753

2H 0.0156 1 0.8574 4,107

11B 81.17 3/2 2.6880 --

13C 1.108 1/2 0.7022 6,728

17O 0.037 5/2 -1.8930 -3,628

19F 100.0 1/2 2.6273 25,179

29Si 4.700 1/2 -0.5555 -5,319

31P 100.0 1/2 1.1305 10,840

* μ in units of nuclear magnetons = 5.05078•10-27 JT-1 † γ in units of 107 rad T-1 sec-1

Characteristics of Some NMR Active Nuclei

no magnetic field present Bo magnetic field present

Nuclear Spins in the Absence and Presence of a Magnetic Field

Slide by Joanna LeFevre

spin state

spin state(slight excess)

N/N = e-E/kT

For a 300 MHz instrument, N/N = 1,000,000/1,000,048.Therefore, for every two million nuclei, there are only 48 excess nuclei in the spin state!! Therefore NMR is an

inherently insensitive technique.http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm

magnetic moment, μ

http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr2.htm#nmr12b

A spinning gyroscope

in a gravitational field A spinning charge

in a magnetic field

I = +1/2

I = -1/2

Net Macroscopic Magnetization of a Sample in an External Magnetic Field Bo

Excitation by RF Energy and Subsequent Relaxation

T1 = spin-lattice relaxation time; establishes the z axis equilibrium. T1’sare usually short (<1 sec) in 1H NMR. They can be quite long (>1 min) in 13C NMR.

T2 = spin-spin relaxation time; causes a decrease in magnetization in the x-y plane.For a good magnet, T2 = 1-2 sec.

http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr2.htm#pulse

Four different frequencies

Complex summation wave (FID)

Fourier transformation

Generation and Fourier Transformation (FT) of a Free Induction Decay (FID) Pattern

http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr2.htm#pulse

seconds0 1 2 3 4 5

Free Induction Decay (FID) Signal: A Decaying Cosine Curve

5 Hz signalAssume T2 = 2 secIt = Ioe-t/T2

~35% of signal remains after 2 sec.

0.020.02 0.040.04 0.060.06 0.080.08 0.100.10

Time (sec)

0.02 0.04 0.06 0.08 0.1

Portion of the FID of BetulinH

0.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.04.54.55.05.0

1H Spectrum of Betulin after Fourier Transformation

Presentation of NMR Data

= chemical shift (Hz) – shift of tetramethyl silane (TMS; 0 Hz) = ppm spectrometer frequency (MHz)

For example, in CH2Cl2 a sharp singlet occurs at 1,590 Hz using a 300 MHz spectrometer frequency.

The chemical shift is:(1,590 – 0) Hz = 5.3 ppm 300 MHz

Shielding vs Deshielding

Increasing deshielding

Increasing shielding, Bo

ClCH2 C CH2Cl

Downfield Upfield

Compound, CH3X CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si

X F O Cl Br I H Si

Electronegativity of X 4.0 3.5 3.1 2.8 2.5 2.1 1.8

Chemical shift, / ppm 4.26 3.4 3.05 2.68 2.16 0.23 0

Electronegative groups attached to the C-H system decrease the electron density around the protons, and there is less shielding (i.e. deshielding) so the chemical shift increases.

These effects are cumulative, so the presence of more electronegative groups produce more deshielding and therefore, larger chemical shifts.

Compound CH4 CH3Cl CH2Cl2 CHCl3

/ ppm 0.23 3.05 5.30 7.27

http://www.chem.ucalgary.ca/courses/351/Carey/Useful/nmr1.gif&imgrefurl

Anisotropic Shielding and Deshielding

1H NMR Spectrum of 4-Methylbezaldehyde

deshieldedprotons

CH3CH3

CH3OCH3

Ph CH3

Upfield regionof the spectrum

Downfield regionof the spectrum

TMS = Me Si

012345678910

CH3HO(R)

http://orgchem.colorado.edu/hndbksupport/nmrtheory/NMRtutorial.html

Chemical Shifts of Various Protons

Integrations: Relative Numbers of Protons

CH3CH3

1H NMR Spectrum of Isopentyl Acetate

Spin-Spin Splitting: The n+1 rule in Vicinal Coupling (HA-C-C-HB)

Equivalent nuclei do not couple each other.

The number of lines in a multiplet is determined by the number of equivalent protons on neighboring atoms plus one, i.e. the n + 1 ruleThe distance between the peaks is called the coupling constant (3J).

The coupling constant is not dependent on the applied field strength.

JAB = 7 Hz

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm

The Origin of Spin-Spin Splitting

CH3 CH2 OH

Some Common Splitting Patterns

Condition for Applying the n+1 Rule

HB HA'

If JHA-HB = JHA’-HB then the n+1 rule applies and HB appears as a 1:2:1 triplet.

However, if the relevant J values are not the same, the splitting is more complex.

O16 Hz

Example: Cis and Trans Coupling in a Carbon-Carbon Double Bond

OCH2CH2Cl

The Karplus Relationship

The vicinal coupling constant (3J) is dependent upon the dihedral angle, .

3.103.103.203.203.303.303.403.403.503.503.603.603.703.70

H3C CH3

H2(ax)

H6(eq)

H6(ax)

Menthol

Some Alkene Splitting Patterns

Typical 1H-1H Coupling Constants

http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm

First-Order CouplingThe splitting pattern shown below displays the ideal or"First-Order" arrangement of lines. This is usually observedif the spin-coupled nuclei have very different chemical shifts.

The condition that must be met is /J > 6. Consider ethyl acetate.

HA = 1.26 ppm x 90 Hz/ppm = 113.4 HzHC = 4.11 ppm x 90 Hz/ppm = 369.9 Hz

/J = (369.9 – 113.4)Hz/7.2 Hz = 35.6

However, if the ratio of Δν to J decreases to less than 10 a significant distortion of the expected pattern will take place.

Second-Order Coupling

First-order

Example of a Second-Order Coupling Pattern

Magnetic Non-equivalence

H(A) and H(B) are magnetically non-equivalent.H(A) and H(A)* couple differently to H(B) [and to H(B*)].

Para and Meta-Disubstituted Benzene RingsNO2

Increasing the field strength leads to greater dispersion of signals.

Mono-Substituted Benzene Ring

Chemical Shift Equivalence

H’s are homotopic:Related by a 180o rotational axis, Cn.They have the same chemical shift.

Chemical Shift Equivalence

H’s are enantiotopic:Related by a mirror plane, .They have the same chemical shift.

Chemical Shift EquivalenceHA and HB are diastereotopic: They are not related by a rotational axis or a mirror plane. They have different chemical shifts, and they split each other.

C HAHB

Homotopic Methyl Groups

These methyls are homotopic: they arerelated by a 180o rotational axis, Cn.They have the same chemical shift.

Enantiotopic Methyl Groups

These methyls are enantiotopic:They are related by a mirror plane, .They have the same chemical shift.

Diastereotopic Methyl Groups

These methyls are diastereotopic: They are not related by a rotational axis or a mirror plane. They have different chemical shifts.Notice that a chiral center* is present.

H3CCOCH2CH2 H

Compare with isopentyl acetate(enantiotopic methyls)

For the following molecules label any groups that are homotopic, enantiotopic, or diastereotopic.

OCH3OCCH3

http://www.cis.rit.edu/class/schp740/docu/avance/noediff.pdf

NOE Difference Spectra of Pamoic Acid

NOE Difference Spectrum of Betulin

4.404.504.604.704.804.90

NOE difference spectrum

normal spectrum

Hcis Htrans

Pople Notation: Describes sets of spins

If /J > 8, the pattern is called AX (i.e. ethyl acetate).

CH3CO2CH2CH3:A3X2

If /J < 8, the pattern is called AB (i.e. 2-chloroacrylonitrile)

Three weakly coupled sets are designated AMX; (i.e. styrene)

AA’XX’ pattern

AA’BB’ pattern

A compound whose 1H NMR spectrum appears below has a molecularformula of C7H14O2. The IR spectrum shows a strong absorbance at1739 cm-1. Suggest a structure for this compound.

Che 440/540 Proton Nuclear Magnetic Resonance (NMR) Spectroscopy

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