Nuclear Magnetic Resonance Spectroscopy - Physical Chemistry - Lecture Notes, Study notes for Physical Chemistry. Agra University
zoya24 December 2012

Nuclear Magnetic Resonance Spectroscopy - Physical Chemistry - Lecture Notes, Study notes for Physical Chemistry. Agra University

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Nuclear Magnetic Resonance Spectroscopy, Intrinsic Spin, Angular Momentum, Solutions for Nuclei, Spin Quantum Number, Property of Nucleus, Spin Eigenfunction, Protons and Neutrons. Before every lecture in Physical Chemis...
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NMR Spectroscopy

Nuclear Magnetic Resonance Spectroscopy

Nuclei have an intrinsic spin angular momentum just like electrons. The

solutions for nuclei are structured in a similar manner:



Iz m






)1(ˆ 22

nuclear angular momentum vector

spin quantum number (property of nucleus)

, ( 1) , ( 2), ... ,

spin eigenfunction ( , , ,...)



m I I I I

  

      


Exercise: Write out specific equations for the cases of I=1/2 and I=1. Use the

vector model to interpret them. Discuss the relationship between I and the

numbers of protons and neutrons present in an atom.

In a magnetic field, the states are no longer degenerate in energy. There are

different energy levels possible (two for nuclei that have spin ½) and transitions

between them can be detected just like in other forms of spectroscopy. The

transition energy depends on several things:

 Strength of the applied field (Bo)  Magnetogyric ratio of nucleus (contained in the “g” factor)  Electronic environment of the nucleus

The transition energy can be expressed (N is the nuclear magneton):

N (1 )

275.051 x 10 / N

[ ] 7.623 (1 )

I o N

I o N

h g B

J Tesla

MHz g B

  

 

 


 

Examine the data for selected nuclei and compute more specific formulae

OLD NMR: Field (B) was varied and frequency of radiation was held constant

(60 MHz).

NEW FT-NMR: Field (B) is held constant and frequency is varied in the

neighborhood of 300 MHz (hence the name of the spectrometer).

To ensure that spectra are the same on all instruments, we use a relative scale

called the chemical shift:

6( ) 10H TMS


   

  

Exercise: What is the field strength of our instrument? What is the frequency

range corresponding to a 10 ppm window on our instrument?

Exercise: How do you compute chemical shifts in Gaussian? What are the

units? How can you compare to experiment? What is Absolute Shielding?

Conventions and Terminology for C-13 Chemical Shifts

Rationalize the chart above in terms of electronic structure and the induced

magnetic field which can either oppose or align with the applied magnetic field.

Where do Solvents Appear? Additivity Rules for functional Groups

J-resolved spectra: example ethylbenzene

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