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

Cl Cl NO 2 Cl H E H G H C Br Ring the hydrogen bells!

H c H e H g H c H e H g frequency H c H e H g time

frequency time

Cl NO 2 Cl Cl H E H G Br H C How do 1 H nuclei ring (or “resonate”)? � the force required to cause ringing is radio wave fequency electromagnetic radiation the 1 H nuclei only ring when placed in a very strong magnetic field � � the pitch (frequency) of the 1 H 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 sample O sample* sample O detector, amplifier, & recorder source 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:

2477.32 MHz Chloroform + 0.03% TMS Cl C Cl Cl H 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, B o , that the nucleus feels: � B o ) / 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 10 6 ) - (299.95 x 10 6 ) � � � = _________________________ = 7.26 ppm � 299.95 x 10 6

downfield upfield deshielded shielded higher frequency lower frequency

( 2477.32 MHz ) Cl C Cl Cl H CHCl B eff CHCl3 = B 0 + B H - B e 3 CH 3 Si CH 3 H 3 C CH 3 TMS B eff TMS = B 0 + B H - B e TMS > B e CHCl As TMS H’s are more electron rich than the CHCl 3 H, B e 3 . So, B eff TMS < B eff CHCl3 and therefore ν TMS < ν CHCl3 . ( 299.95 MHz ) downfield upfield deshielded shielded higher frequency lower frequency

The chemical shift reflects the electronic environment CHCl 3 δ 7.3 ppm CH 2 Cl 2 δ 5.2 ppm CH 3 Cl δ 3.1 ppm

The chemical shift reflects the electronic environment CH 3 I δ 2.2 ppm CH 3 Br δ 2.7 ppm CH 3 Cl δ 3.1 ppm

Methanol 1 H NMR spectrum H H C H O H

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

Methanol 1 H NMR spectrum H H C H O H

para -Xylene 1 H NMR spectrum CH 3 H H H H CH 3

BASIC 1 H NMR CHEMICAL SHIFTS (R = H or Alkyl) Approximate δ , ppm H Type methyl 0.9 methylene 1.3 methine 1.5 R 2 C=C H R vinylic 4.6-5.9 RC � C– H acetylenic 2-3 R 2 C=CRC H R 2 ayllic 1.7 Ar– H aryl 6.5-8.0 Ar–C H R 2 benzylic 2.2-3 Cl–C H R 2 3-4 Br–C H R 2 2.5-4 I–C H R 2 2-4 RO–C H R 2 3.3-4 RCO 2 –C H R 2 3.7-4.1 O �� R 3 C–C–C H R 2 2-2.6 O �� 9-10 R–C– H O �� 10.5-13 R–C–O H R–O H anywhere R 2 N– H anywhere