nmr (theory and applications) 1

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Nuclear Magnetic Resonance Spectroscopy

(Theory and application)

Lecture 1

By Dr. Ahmed M. Metwaly

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• Nuclear magnetic resonance spectroscopy is a powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules.

• Two common types of NMR spectroscopy are used to characterize organic structure: 1H NMR is used to determine the type and number of H atoms in a molecule; 13C NMR is used to determine the type of carbon atoms in the molecule.

• The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency.

• When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1H and 13C.

Introduction to NMR Spectroscopy

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Basic theory: The NMR technique depends on the magnetic properties of certain nuclei possessing a numerical value of spin quantum (I). 1. Any atomic nucleus with odd mass No., odd atomic No. or both, has a quantized spin No. (≠ 0) 2. Any atomic nucleus with even mass No. and even atomic No. has spin No. (=0) 3. Spin quantum number (I): describes the number of orientations of a spinning nucleus under applied magnetic field. 4. Number of possible spin states (n) = 2I + 1

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1 H 1

13C

6 31P

15 19F

9 35Cl

17 15N

7 17O 8

I 1/2 1/2 1/2 1/2 3/2 1/2 5/2 n 2 2 2 2 4 2 7

Nucleuses with odd mass No

Nucleuses with even mass No and even atomic No

Nucleuses with even mass No and odd atomic No

16O 8

12C

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I 0 0

2 H 1

14N

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I 1 1

n 3 3

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Why 15N cant be used as source for NMR? Because it has a very low natural abundance = 0.38% Why 17O cant be used as source for NMR? Because it has a very low natural abundance = 0.04% And ??????? In case of 2H I= 1 so n=3 (Explain CDCl3 peakes). Why 14N doesn't give a splitting with protons? Due to very rapid relaxation time in 14N.

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• When a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet.

• Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of an external magnetic field B0, they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy.

• The energy difference between these two states is very small (<0.1 cal).


Ensemble of Nuclear Spins

Bo

Bo > 0 Bo = 0 Highly oriented Randomly oriented

Each nucleus behaves Like a bar magnet.

N

S

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• In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B0, and a higher energy state in which the nucleus aligned against B0.

• When an external energy source (hn) that matches the energy difference (DE) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another.

• The energy difference between these two nuclear spin states corresponds to the low frequency RF region of the electromagnetic spectrum.


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• Thus, two variables characterize NMR: an applied magnetic field B0, the strength of which is measured in tesla (T), and the frequency n of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz)


Basic equation of NMR ν = γ Bo /2π

ν - resonance frequency γ - gyromagnetic ratio B - external magnetic field (the magnet) Gyromagnetic ratio (γ): nuclie-specfic constant relating how fast spins will precess (radians/sec) per unit of applied magnetic field (Tesla)

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h=Planck’s constant

Example: (Protons) For a Bo=11.74 T v= 267.513 x 106 rad/T/s x 11.74 T/2π rad/cycle = 5 x 108 cycles/sec = 500 MHz

Nucleus γ (106 rad.s-1.T-1) 1H 267.513 2H 41.065 3He -203.789 7Li 103.962 13C 67.262 14N 19.331 15N -27.116 17O -36.264 19F 251.662 23Na 70.761 31P 108.291

Gyromagnetic ratios (γ) For some nucleuses

Hertz, was formally cycles/sec (cps)

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Explain?? Experimental: NMR spectra were recorded on a Bruker Avance DRX- 500 instrument at 500 (1H) and 125 MHz (13C)

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Larmor equation predicts all protons would have same resonance frequency !!!! What good is NMR for ????? Chemical Shift Phenomena is the power of NMR. Chemical Shift Phenomena can be defined as the alternation of the resonance frequency of chemically distinct NMR- active nuclei due to the resistance of the electron cloud to the applied magnetic field

Chemical Shift Phenomena

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The distance in ‘δ’ values from TMS to each signal (absorption or peak ) in the spectrum is called the chemical shift for the proton or proton giving that signal.

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Measurement of Chemical shift

chemical shift is measured in frequency unit ‘Hertz’. Most of routine instruments operate at 60 to 900 MHz. The chemical shift recorded in Hz may vary with the spectrometer. To avoid this complication the chemical shift values are expressed in terms of delta or tau scale. Which are independent of field strength. Chemical shift in delta scale are expressed in parts per million (ppm).

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Delta Scale

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1. It is chemically inert. 2. It gives unique line position. 3. It is symmetrical molecule and gives single and strong , sharp absorption

peak as all 12 protons are equivalent. 4. Methyl protons of TMS are strongly shielded and hence absorption occurs

at high field . It is taken as zero ppm. 5. It is soluble in most of organic solvents. 6. It is volatile(b.p.270c)and hence the recovery of sample is possible.

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• An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm).

1H NMR—The Spectrum

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• NMR absorptions generally appear as sharp peaks. • Increasing chemical shift is plotted from left to right. • Most protons absorb between 0-10 ppm. • The terms “upfield” and “downfield” describe the relative location of peaks.

Upfield means to the right. Downfield means to the left. • NMR absorptions are measured relative to the position of a reference peak at 0

ppm on the d scale due to tetramethylsilane (TMS). TMS is a volatile inert compound that gives a single peak upfield from typical NMR absorptions.

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• Four different features of a 1H NMR spectrum provide information

about a compound’s structure: a. Number of signals b. Position of signals c. Intensity of signals. d. Spin-spin splitting of signals.

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• The number of NMR signals equals the number of different types of protons in a compound.

• Protons in different environments give different NMR signals. • Equivalent protons give the same NMR signal.

1H NMR—Number of Signals

• To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen.

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• In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis (or trans) to the same groups.


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• Proton equivalency in cycloalkanes can be determined similarly.


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• In the vicinity of the nucleus, the magnetic field generated by the circulating electron decreases the external magnetic field that the proton “feels”.

• Since the electron experiences a lower magnetic field strength, it needs a lower frequency to achieve resonance. Lower frequency is to the right in an NMR spectrum, toward a lower chemical shift, so shielding shifts the absorption upfield.

1H NMR—Position of Signals

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• The less shielded the nucleus becomes, the more of the applied magnetic field (B0) it feels.

• This deshielded nucleus experiences a higher magnetic field strength, to it needs a higher frequency to achieve resonance.

• Higher frequency is to the left in an NMR spectrum, toward higher chemical shift—so deshielding shifts an absorption downfield.

• Protons near electronegative atoms are deshielded, so they absorb downfield.


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• Protons in a given environment absorb in a predictable region in an NMR spectrum.

1H NMR—Chemical Shift Values

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• The chemical shift of a C—H bond increases with increasing alkyl substitution (increase S character).


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• In a magnetic field, the six π electrons in benzene circulate around the ring creating a ring current.

• The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons.

• The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield.


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• In a magnetic field, the loosely held π electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons.

• The protons now feel a stronger magnetic field, and require a higher frequency for resonance. Thus the protons are deshielded and the absorption is downfield.


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• In a magnetic field, the π electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B0).

• Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield.


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1H NMR—Chemical Shift Values)

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• The area under an NMR signal is proportional to the number of absorbing protons.

• An NMR spectrometer automatically integrates the area under the peaks, and prints out a stepped curve (integral) on the spectrum.

• The height of each step is proportional to the area under the peak, which in turn is proportional to the number of absorbing protons.

• Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units.

• The ratio of integrals to one another gives the ratio of absorbing protons in a spectrum. Note that this gives a ratio, and not the absolute number, of absorbing protons.

1H NMR—Intensity of Signals

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1H NMR—Intensity of Signals

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• Consider the spectrum below:

1H NMR—Spin-Spin Splitting

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