Ring the hydrogen bells! H c H e H g H c H e H g frequency H c H e H - - PowerPoint PPT Presentation

HE

HG

Cl Br

HC

Cl Cl NO2

Ring the hydrogen bells!

Hc He Hg Hc He Hg frequency Hc He Hg time

frequency time

HE HG Cl Br HC Cl Cl NO2

How do 1H nuclei ring (or “resonate”)?

• the force required to cause ringing is radio wave fequency electromagnetic radiation
• the 1H nuclei only ring when placed in a very strong magnetic field
• the pitch (frequency) of the 1H nuclei when ringing is
• proportional to the magnetic field strength
• proportional to the electronic environment of the molecule

Nuclear Magnetic Resonance (NMR) Spectroscopy: A kind of emission spectrometry

EM radiation source sampleO sample* sampleO detector, amplifier, & recorder excitation relaxation

in NMR,

• 1. EM source is a radio transmitter, typically in the range of ~60 - 750 MHz

(compare with the FM radio band 88 - 108 MHz)

• 2. Sample holder is a magnet with a test tube in it
• 3. Excitation occurs by absorption of Rf radiation, causing a net change in nuclear spin (“bulk

magnetization”)

• 4. Relaxation occurs by Rf emission and a return of the bulk magnetization to equillibrium
• 5. Detector is a radio receiver

Block diagram of an NMR instrument:

NMR magnet size: old and new

2001: Scripps superconducting 21.1 Tesla electromagnet, 17' 4" tall. Contains ~ 27 MJ of stored energy 1968: Creighton T60 with 1 Tesla permanent iron core magnet, ~ 18" tall

A typical 300 MHz NMR instrument:

Cl C Cl H Cl

2477.32 MHz

Chloroform + 0.03% TMS 299.95 MHz

The frequency of resonance,

• , of a hydrogen in a molecule is a function of the intrinsic

magnetic properties (the magnetogyric ratio,

• ) of the hydrogen nucleus and the magnetic field,

Bo, that the nucleus feels:

• = (
• Bo) / 2
• To make the frequency of resonance magnetic-field independent, we use a relative scale

called chemical shift which is measured in ppm or

• :
• sample -
• TMS
• = _________
• TMS

In our case, (2477.32 x 106) - (299.95 x 106)

• = _________________________ = 7.26 ppm

299.95 x 106

downfield upfield deshielded shielded higher frequency lower frequency

(2477.32 MHz) Beff CHCl3 = B0 + BH - Be

CHCl 3

Beff TMS = B0 + BH - Be

TMS

As TMS H’s are more electron rich than the CHCl3 H, Be

TMS > Be CHCl 3 .

So, Beff TMS < Beff CHCl3 and therefore νTMS < νCHCl3 . (299.95 MHz) downfield upfield deshielded shielded higher frequency lower frequency Cl C Cl H Cl H3C Si CH3 CH3 CH3

The chemical shift reflects the electronic environment CHCl3 δ 7.3 ppm CH2Cl2 δ 5.2 ppm CH3Cl δ 3.1 ppm

The chemical shift reflects the electronic environment CH3I δ 2.2 ppm CH3Br δ 2.7 ppm CH3Cl δ 3.1 ppm

H C O H H H

Methanol 1H NMR spectrum

H C O H H H

Methanol 1H NMR spectrum Relative ratio of areas: 70 / 23 = 3.0 23 / 23 = 1 3H 1H

H C O H H H

Methanol 1H NMR spectrum

CH3 CH3 H H H H

para-Xylene 1H NMR spectrum

BASIC 1H NMR CHEMICAL SHIFTS (R = H or Alkyl) H Type Approximate δ, ppm methyl 0.9 methylene 1.3 methine 1.5 vinylic R2C=CHR 4.6-5.9 acetylenic RCC–H 2-3 ayllic R2C=CRCHR2 1.7 aryl Ar–H 6.5-8.0 benzylic Ar–CHR2 2.2-3 Cl–CHR2 3-4 Br–CHR2 2.5-4 I–CHR2 2-4 RO–CHR2 3.3-4 RCO2–CHR2 3.7-4.1 O

• R3C–C–CHR2

2-2.6 O

• 9-10

R–C–H O

• 10.5-13

R–C–OH R–OH anywhere R2N–H anywhere