ib chemistry on nuclear magnetic resonance (nmr) spectroscopy

Electromagnetic Radiation and Spectroscopy

Radiowaves

Nuclear spin

Nuclear Magnetic Resonance Spectroscopy

• Organic structure determination • MRI and body scanning

Infra Red

Molecular vibration

Infra Red Spectroscopy

UV or visible

Transition of outer most valence electrons

• Organic structure determination • Functional gp determination • Measuring bond strength • Measuring degree unsaturation in fat • Measuring level of alcohol in breath

Electromagnetic Radiation

UV Spectroscopy Atomic Absorption Spectroscopy

• Quantification of metal ions • Detection of metal in various samples

Electromagnetic Radiation Interact with Matter (Atoms, Molecules) = Spectroscopy

Nuclear Magnetic Resonance Spectroscopy (NMR) • Involve nucleus (proton + neutron) NOT electrons • Proton + neutrons = Nucleons • Nucleons like electrons have spin and magnetic moment (acts like a tiny magnet)

Nuclei with even number of nucleon (12C and 16O) • Even number of proton and neutron – NO net spin • Nucleon spin cancel out each other –Nucleus have NO overall magnetic moment – NOT absorb radiowave radiation

Nuclei with odd number of nucleon (1H, 13C, 19F, 31P) -Nucleon have net spin – Nucleus have NET magnetic moment – Absorb radiowave radiation

• Nuclei with net spin – magnetic moment will interact with electromagnetic radiation/radio waves • Nuclei have a “spin” associated with them (i.e., they act as if they were spinning about an axis) due to the spin associated with their protons and neutrons. • Nuclei are positively charged, their spin induces a magnetic field • NMR spectroscopy does not work for nuclei with even number of protons and neutrons— nuclei have no net spin.

Nuclear Magnetic Resonance Spectroscopy (NMR)

Spin cancel each other

Main features of HNMR Spectra 1. Number of different absorption peaks – Number of different proton/chemical environment 2. Area under the peaks - Number of hydrogen in a particular proton/chemical environment (Integration trace) - Ratio of number of hydrogen in each environment 3. Chemical shift - Chemical environment where the proton is in - Spinning electrons create own magnetic field, creating a shielding effect - Proton which are shielded appear upfield. (Lower frequency for resonance to occur) - Proton which are deshielded appear downfield away. (Higher frequency for resonance to occur) - Measured in ppm (δ) 4. Splitting pattern - Due to spin-spin coupling - The number of peak split is equal to number of hydrogen on neighbouring carbon +1 (n+1) peak


Chemical Shift NMR spectrum of CH3CH2Br

Number of peaks

Area under peaks Chemical shift

Splitting pattern

http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Atomic_Theory/Electrons_in_Atoms/Electron_Spin

Absence of External magnetic Field (EMF) • The TWO nuclear spin have the same energy, (same energy level)

• External magnetic field applied to atomic nuclei, magnetic field of nuclei align themselves either with or against magnetic field

• Nuclei have a slight preference for the parallel alignment with the applied field as it has a slightly lower energy,

• Nuclei can absorb energy to move/flip to higher energy level by absorbing energy in radio frequency region

Presence of External Magnetic Field (EMF)

• The TWO nuclear spin split to TWO different energy level

Presence of External Magnetic Field (EMF)

Absence of EMF • Two spins in same energy level

Presence of EMF • Two spins in different energy level • Lower spin nuclei absorb radio frequency equivalent to ∆E • Move to higher energy level

Lower spin nuclei align with magnetic field

High spin nuclei align against magnetic field

∆E


Proton in nucleus – have spin – generate its magnetic field (MF) Electrons around nucleus – have spin- also generate its magnetic field Protons shielded by MF produced by electrons will appear UPFIELD Protons deshielded by electron withdrawing gps will appear DOWNFIELD

Chemical Shift (Shielding Effect)


∆E

∆E is smaller

Without any SHIELDING EFFECT • Energy of ∆E absorb by H to move to higher energy level

Upfield Downfield



Presence of EMF • Two spins at diff energy level


SHIELDING EFFECT • Electrons around H will produce MF and shield the H • H in CH3 will experience less EMF (SHIELDED) • Absorb at lower radiofrequency to move to higher level • ∆E absorb by H to move to higher energy level is less • Appear upfield.

∆E

∆E is higher

Without any DESHIELDING EFFECT • Energy of ∆E absorb by H to move to higher energy level

DESHIELDING EFFECT • Electrons are withdrawn away by C=O gp • Carbonyl gp has electron withdrawing effect • Less electron around the H in CH3 • H in CH3 will be deshielded, experience greater EMF • ∆E absorb by H, to move to high energy level is higher • Absorb at higher radiofreq, to move to high level • Appear downfield

Proton in nucleus – have spin – generate its magnetic field (MF) Electrons around nucleus – have spin- also generate its magnetic field Protons shielded by MF produced by electrons will appear UPFIELD Protons deshielded by electron withdrawing gps will appear DOWNFIELD

Upfield Downfield






Chemical Shift (Deshielding Effect)

∆E

∆E is smaller

Without any SHIELDING EFFECT • Energy of ∆E absorb by H to move to higher energy level

SHIELDING EFFECT • Electrons around H will produce MF and shield the H • H in CH3 will experience less EMF (SHIELDED) • Absorb at lower radiofrequency to move to higher level • ∆E absorb by H to move to higher energy level is less • Appear upfield.

Chemical Shift (Shielding and Deshielding Effect)

∆E is higher



DESHIELDING EFFECT • Electrons are withdrawn away by C=O gp • Carbonyl gp has electron withdrawing effect • Less electron around the H in CH3 • H in CH3 will be deshielded, experience greater EMF • ∆E absorb by H, to move to high energy level is higher • Absorb at higher radiofreq, to move to high level • Appear downfield

Downfield Upfield

Deshielding Effect

Shielding Effect

No shielding

Chemical Shift (Shielding and Deshielding Effect)

Shielding/Deshielding: • Electron circulates nucleus, creates magnetic field opposing the external magnetic field. • Hence, each nucleus experience a slightly different magnetic field • (Sum of external field and field from the electron cloud). • Energy a nucleus achieves resonance depends on its surroundings. • Frequency absorption depend on electron density around nucleus (chemical environment)

Chemical shift of various electron withdrawing groups

• Electron withdrawn from CH3 by C=O • Deshield the H in CH3 • Absorb at slightly higher radiofreq • Upfield ≈ 2.1

• Electron withdrawn from CH2 by COO • Stronger electron withdrawing effect • Higher ↑ Deshielding effect on H in CH2 • Absorb at Higher ↑ radiofreq • Slightly Downfield ≈ 4.1

• Electron withdrawn by benzene • Stronger electron withdrawing effect • Higher ↑ deshielding effect on H • Absorb at Very high ↑ radiofreq • Very Downfield ≈ 7.3 - 8

• Electron withdrawn from H by CHO • Very strong electron withdrawing effect • Higher ↑ Deshielding effect on H in CHO • Absorb at Very High↑ radiofreq • Very Very Downfield ≈ 9.7

• Electron withdrawn by COOH • Very strong electron withdrawing effect • Highest deshielding effect on H • Absorb at Very High↑ radiofreq • Very Very Very Downfield ≈ 12

Upfield Downfield

Tetramethyl Silane (TMS) as STD •Strong peak upfield (shielded) •Silicon has lower EN value < carbon • Electron shift to carbon • H in CH3 will be more shielded • Experience lower EMF, absorb ↓ radiofrequancy • UPFIELD ≈ 0


Click here for more complicated proton chemical shift

HO-CH2-CH3

• 3 different proton environment • Ratio of 3:2:1

CH3

• chemical shift ≈ 1

• integration = 3 H

• split into 3

CH2

• chemical shift ≈ 3.8


• split into 4

OH

• chemical shift ≈ 4.8


• No split (Singlet)

3 2 1

Upfield Downfield

12

Advantages using TMS • Volatile and can be removed from sample • All 12 hydrogens are in the same proton environment • Single strong peak, upfield, doesnt interfere with other peaks • All chemical shift, measured in ppm (δ) are relative to this STD, taken as zero

http://www.chem.wisc.edu/areas/reich/handouts/chem343-345/345-nmr-handout.pdf

NMR Spectrum

O ║

HO-C-CH2-CH3

3 diff proton enviroment, Ratio H - 3:2:3 • Peak A – split to 3 – 2H on neighbour C • Peak B - No split • Peak C – split to 4 – 3H on neighbour C

3 diff proton enviroment, ratio H - 3:2:1 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 4 – 3H on neighbour C • Peak C – No split

A B

C

B

A

C

O

║

CH3-C-O-CH2-CH3

12

3 2 3

3 2 1

HO-CH2-CH3

NMR Spectrum

O ║

CH3-C-CH2-CH2-CH3

3 diff proton enviroment, Ratio H - 3:2:1 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 4 – 3H on neighbour C • Peak C – No split

4 diff proton enviroment, Ratio H - 3:2:2:3 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 6 – 5H on neighbour C • Peak C – No split • Peak D – split to 3 – 2H on neighbour C

A

B C

3

B

A C D

2 1

3 2 2 3

O ║

H-C-CH3

NMR Spectrum

O ║

CH3-C-O-CH2-CH2-CH3

4 diff proton enviroment, Ratio H – 3:2:2:3 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 6 – 5H on neighbour C • Peak C – No split • Peak D – split to 3 – 2H on neighbour C

A

B C D

2 diff proton enviroment, Ratio H - 3:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – split to 4 – 3H on neighbour C

9.8

A

B

3 2 2 3

3 1

NMR Spectrum

Molecule with plane of symmetry

3 diff proton enviroment, Ratio H - 6:1:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – No split • Peak C – split to 7 – 6H on neighbour C

CH3

| H-C-OH

| CH3

O CH3

║ | CH3-C-O-CH

| CH3

A

B C

A B

C

3 diff proton enviroment, Ratio H - 6:3:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – No split • Peak C – split to 7 – 6H on neighbour C


6 1 1

6 3 1

NMR Spectrum


O

║

CH3-CH2-C-CH2-CH3

O CH3

║ | H-C-C-CH3

| CH3

2 diff proton enviroment, Ratio H – 6:4 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 4 – 3H on neighbour C

A

B

A

B

6 4

9 1

2 diff proton enviroment, Ratio H – 9:1 • Peak A – No split • Peak B – No split


NMR Spectrum


4 diff proton enviroment, Ratio H – 6:1:1:2 • Peak A – split to 2 – 1H on neighbour C • Peak B – split to 7 – 6H on neighbour C • Peak C – No split • Peak D – split to 2 – 1H on neighbour C

CH3 | HO-CH2-CH

| CH3

A

B D C

2 diff proton enviroment, Ratio H – 6:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – split to 7 – 6H on neighbour C

CH3-CH-CH3

| CI

A

B


6 1 1 2

6 1

Click here to animated Spectra Click here to search IR spectra

IR Spectra search for different Organic Molecules

http://undergrad-ed.chemistry.ohio-state.edu/anim_spectra/index.html

http://undergrad-ed.chemistry.ohio-state.edu/anim_spectra/index.html

http://webbook.nist.gov/chemistry/name-ser.html

http://webbook.nist.gov/chemistry/name-ser.html

ib chemistry on nuclear magnetic resonance (nmr) spectroscopy

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