information content available through nmr

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Information Content Available Through NMR

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Page 1: Information Content Available Through NMR

Information Content Available

Through NMR

Page 2: Information Content Available Through NMR

We need to measure the NMR signals…

Page 3: Information Content Available Through NMR

Tetramethylsilane (TMS) is our reference for 1H, 13C and 29Si.The chemical shift (denoted δ)

varies in different chemical environments.

δ = [ (νobs – νTMS ) / νTMS ] x 106

Page 4: Information Content Available Through NMR

Chemical shifts depend on the electronegativities of nearby

atoms

Page 5: Information Content Available Through NMR

Having 2n+2 π electrons in a ring allows for charge circulation

Image courtesy of the U.S. Geological Survey

Page 6: Information Content Available Through NMR
Page 7: Information Content Available Through NMR

We also want to know how much of each signal we have

Page 8: Information Content Available Through NMR

NMR provides information on the relative amounts of

chemically distinct molecular sites (integrals and intensities)

Page 9: Information Content Available Through NMR

Quantitation requires…

• Waiting five times the longest spin-lattice relaxation time T1

Mz(t) = M0 [1 - exp (-t / T1)]

• The absence of any enhancement, such as that arising from decoupling

η = γX / (2 γA)

Page 10: Information Content Available Through NMR

1H 1-D integration

Page 11: Information Content Available Through NMR

13C spectra provide useful intensity information

Page 12: Information Content Available Through NMR

How do spins talk to each other?

Image courtesy of U.S. Air Force

Page 13: Information Content Available Through NMR

Through-bond connectivity is revealed

through J-couplings

Page 14: Information Content Available Through NMR

The magnitude of J1J’s (ca. 125-160 Hz for 1H-13C, sometimes

more, used in HMQC, HSQC, and INADEQUATE)

• 2J’s (geminal couplings, 2-15 Hz) • 3J’s (vicinal couplings, 0-15 Hz) , • sometimes 4J’s (0-4 Hz)• and even 5J’s (0-2 Hz)

Page 15: Information Content Available Through NMR

Geometry controlling magnitude of 2J’s and 3J’s is well

understood and expressed through the Karplus relationship

Page 16: Information Content Available Through NMR

2J Karplus relationship

Page 17: Information Content Available Through NMR

3J Karplus relationship

Page 18: Information Content Available Through NMR

Measurable 4J’s are found in rigid molecules

Page 19: Information Content Available Through NMR

1H-1H gCOSY

(gradient COrrelation

SpectroscopY)

Page 20: Information Content Available Through NMR

1H-13C HMQC and HSQC(Heteronuclear Multiple Quantum Correlation &

Heteronuclear Single Quantum Correlation)

Page 21: Information Content Available Through NMR

1H-13C gHMBC

Page 22: Information Content Available Through NMR

Other important through-bond experiments:

1H-1H DQFCOSY(Double Quantum Filtered COrrelation SpectroscopY)

and13C-13C INADEQUATE

(Incredible Natural Abundance DEtection blah blah blah)

Page 23: Information Content Available Through NMR

We can also detect interactions…

THROUGH SPACE

Page 24: Information Content Available Through NMR

Through-space effects: the dipolar interaction gives us the

nuclear Overhauser effect (NOE)

Page 25: Information Content Available Through NMR

Consider a 1H-13C spin pair at equilibrium

Page 26: Information Content Available Through NMR

Now we irradiate (decouple) the 1H transitions

Page 27: Information Content Available Through NMR

The double quantum transition relaxes the fastest (sometimes)

Page 28: Information Content Available Through NMR

1H-1H NOESY(Nuclear Overhauser Effect SpectroscopY)

Page 29: Information Content Available Through NMR

Those are the basic tools from which we choose

(Photo courtesy of JM, http://www.logodesignweb.com/stockphoto)

Page 30: Information Content Available Through NMR

We start with assigning a molecule of known structure: ethyl nipecotate

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Entry points

• Ethyl group• Carbonyl carbon• Methylenes α to N• Lone methine @ 3

Page 32: Information Content Available Through NMR

In simple problems:Following the identification of entry points, we use 3JHH’s to assign 1H’s adjacent to those we already know using the

gCOSY

Page 33: Information Content Available Through NMR

In complex problems:Following the identification of

entry points, we use 3-5JHH’s and 2-5JHC’s to assign spins

Page 34: Information Content Available Through NMR

1H 1-D

Page 35: Information Content Available Through NMR

13C 1-D

Page 36: Information Content Available Through NMR

1H-13C HMQC

Page 37: Information Content Available Through NMR

1H-1H gCOSY

Page 38: Information Content Available Through NMR

For six-membered rings, we normally assume a chair

conformation, and so the dihedrals are either trans or gauche