Download presentation
Presentation is loading. Please wait.
Published byBartholomew Allen Modified over 9 years ago
1
Nuclear Magnetic Resonance Spectroscopy Dr. Sheppard Chemistry 2412L
2
Introduction NMR is the most powerful technique for organic structure determination Number and type of atoms in a molecule Connectivity of atoms Used to study a wide variety of nuclei: 1 H 13 C 15 N, 19 F, 31 P Radio-frequency radiation used to transition between energy states 30 – 900 MHz Transition = nuclear spin
3
Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin The spinning charged nucleus generates a magnetic field
4
External Magnetic Field When placed in an external field, spinning nuclei act like bar magnets
5
Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field A photon with the right amount of energy can be absorbed and cause the spinning nucleus to flip Spin flip = resonance Detected and recorded by the spectrometer as a signal
6
The NMR Spectrometer
7
Magnetic Shielding If all nuclei absorbed the same amount of energy in a given magnetic field, not much information could be obtained But nuclei are surrounded by electrons that shield them from the external field Circulating electrons create an induced magnetic field that opposes the external magnetic field Effective magnetic field
8
Shielded Nuclei Magnetic field strength must be increased for a shielded nucleus to flip at the same frequency Differences detected by machine, cause differences in signals (chemical shift, )
9
Nuclei in a Molecule Depending on their chemical environment, atoms in a molecule are shielded by different amounts Chemically equivalent nuclei Interchanged through bond rotation or element of symmetry Have same absorption Chemically different nuclei have different absorption
10
1 H-NMR Spectrum for Methanol
11
Tetramethylsilane TMS is added to the sample Since silicon is less electronegative than carbon, TMS protons are highly shielded Signal defined as zero Organic protons absorb downfield (to the left) of the TMS signal Deuterated solvent signal
12
Chemical Shift Measured in parts per million Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz) Same value for 60, 100, or 300 MHz machine Called the delta ( ) scale
13
Delta Scale downfieldupfield
14
Location of Signals More electronegative atoms deshield more and give larger shift values (downfield) Effect decreases with distance Additional electronegative atoms cause increase in chemical shift
15
Hydrogen and Carbon Chemical Shifts
16
NMR Spectra
17
13 C-NMR 12 C has no magnetic spin 13 C has a magnetic spin, but is only 1% of the carbon in a sample Signals are weak, get lost in noise Hundreds of spectra are taken, averaged Signal = one sharp line for each different type of carbon
18
3-Pentanone How many signals? Chemical shifts: sp 3 C upfield sp, sp 2 C downfield C adjacent to en atom downfield
20
2-Butanone How many signals? Chemical shifts?
22
How is 13 C-NMR useful for reactions we have studied? Zaitsev vs. non-Zaitsev 7 signals5 signals
23
Interpreting 13 C-NMR The number of different signals indicates the number of different kinds of carbon The chemical shift indicates the functional group Use to support 1 H-NMR analysis
24
1 H-NMR More info than 13 C-NMR The number of signals shows how many different kinds of protons are present The location of the signals shows how shielded or deshielded the proton is The intensity of the signal shows the number of protons of that type Signal splitting shows the number of protons on adjacent atoms
25
1 H-NMR Given a structure, how many signals are expected? How many sets of H in each molecule? Isomers Same molecular formula Same IR stretches Different NMR
26
Another example:
27
Chemical shifts in 1 H-NMR Info about type of H giving rise to signal Strongly shielded = upfield (to the right) Less shielded = downfield (to the left) Most common shifts: McMurry, Table 13-3
28
Typical Values
29
O-H and N-H Signals Chemical shift depends on concentration Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H
30
Using chemical shifts Given a structure, predict Use to distinguish between two structures Example: Constitutional isomers Each with 2 sets of H’s
31
Which isomer best fits this spectrum? or
32
Which isomer best fits this spectrum? or
33
Intensity of Signals The area under each peak is proportional to the number of protons Shown by integration line Height area under peak # H’s in set Measure height with ruler or look at graph paper Ratio of height = ratio of hydrogens
36
So far… Determine the number of sets of equivalent hydrogen atoms Number of signals on spectrum Determine the number of hydrogen atoms in each set Integration line Determine general information about adjacent groups Chemical shift ( )
37
Next… Determine specific information about adjacent groups In particular, how many H atoms on the adjacent atoms Signal splitting
Similar presentations
© 2024 SlidePlayer.com. Inc.
All rights reserved.