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Introduction • Spectroscopy is an analytical technique which helps determine structure. • It destroys little or no sample. • The amount of light absorbed by the sample is measured as wavelength is varied. =>
Dorothy Crowfoot Hodgkin 1910-1994
Cholesterol Story • Isolated in 1832 • Structure First reported in 1927 • Structure Determined in 1942 by Dorothy Crowfoot Hodgson • Synthesized in 1971
Chapter 13Spectroscopy Infrared spectroscopy Ultraviolet-Visible spectroscopy Nuclear magnetic resonance spectroscopy Mass Spectrometry
Electromagnetic Radiation is propagated at the speed of light has properties of particles and waves the energy of a photon is proportional to its frequency
Shorter Wavelength () Longer Wavelength () Higher Frequency () Lower Frequency () Higher Energy (E) Lower Energy (E) Figure 13.1: The Electromagnetic Spectrum 400 nm 750 nm Visible Light
Shorter Wavelength () Longer Wavelength () Ultraviolet Infrared Higher Frequency () Lower Frequency () Higher Energy (E) Lower Energy (E) Figure 13.1: The Electromagnetic Spectrum
Figure 13.1: The Electromagnetic Spectrum Cosmic rays Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Energy
E = h Electromagnetic radiation is absorbed when theenergy of photon corresponds to difference in energy between two states.
What Kind of States? UV-Vis infrared microwave radiofrequency electronic vibrational rotational nuclear spin
The nuclei that are most useful toorganic chemists are: 1H and 13C both have spin = ±1/2 1H is 99% at natural abundance 13C is 1.1% at natural abundance
+ + Nuclear Spin A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.
+ + + + The distribution of nuclear spins is random in the absence of an external magnetic field. +
NMR Spectroscopy Brief Intro Certain nuclei behave as bar magnets - consider proton DE = g h/2pHo 499,990 Using magnet Ho = 25,000 Gauss DE = 0.019 cals/mol!!! Radio region 500,010 only 20 nuclei excess!!! Ho
+ + + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + H0
+ + + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. + H0
+ E ' E + Energy Differences Between Nuclear Spin States increasing field strength
Requirement for nuclei to behave as a barmagnet 2) odd atomic number 1) odd mass number 14 N 1H 7 NOTE: nuclei with even mass no and atomic number do not behave as a bar magnet! 13C 19F 12C and 16O WONDERFUL : can obtain proton resonances NMR without interference by these ubiquitous isotopes. 35P Questions: Since many atoms are NMR active, will we not get overlapping signals in 1H-NMR spectra? NO!! We are saved by the fact that each NMR active nuclei has a different gyromagneto ratio - different region of radio radiation
Units Hz kJ/mol(kcal/mol) tesla (T) Some important relationships in NMR The frequency of absorbedelectromagnetic radiationis proportional to the energy difference betweentwo nuclear spin stateswhich is proportional to the applied magnetic field
150th Birthday • http://www.teslasociety.com/happybirthday.htm • Most of us, think of Guglielmo Marconi as the father of radio, and Tesla is unknown for his work in radio. Marconi claimed all the first patents for radio, something originally developed by Tesla. Nikola Tesla tried to prove that he was the creator of radio but it wasn't until 1943, where Marconi's patents were deemed invalid; however, people still have no idea about Tesla's work with radio.
Some important relationships in NMR The frequency of absorbed electromagneticradiation is different for different elements, and for different isotopes of the same element. For a field strength of 4.7 T:1H absorbs radiation having a frequency of 200 MHz (200 x 106 s-1)13C absorbs radiation having a frequency of 50.4 MHz (50.4 x 106 s-1)
Some important relationships in NMR The frequency of absorbed electromagneticradiation for a particular nucleus (such as 1H)depends on its molecular environment. This is why NMR is such a useful toolfor structure determination.
13.4Nuclear Shieldingand1H Chemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?"
C H Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. H0
C H Shielding The induced field shields the nuclei (in this case, C and H) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. H0
C H Chemical Shift Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. H0
CH3 Si CH3 H3C CH3 position of signal - position of TMS peak x 106 d = spectrometer frequency Chemical Shift Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard.
10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 DownfieldDecreased shielding UpfieldIncreased shielding (CH3)4Si (TMS) Chemical shift (, ppm)measured relative to TMS
1456 Hz - 0 Hz position of signal - position of TMS peak x 106 d = x 106 d = spectrometer frequency 200 x 106 Hx Chemical Shift Example: The signal for the proton in chloroform (HCCl3) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz. d = 7.28
Cl Cl H C Cl 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 7.28 ppm Chemical shift (, ppm)
13.5Effects of Molecular Structureon1H Chemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts
CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si d 4.3 d 3.2 d 2.2 d 0.9 d 0.0 least shielded H most shielded H Electronegative substituents decreasethe shielding of methyl groups
d 0.9 d 1.3 d 0.9 Electronegative substituents decrease shielding H3C—CH2—CH3 d 4.3 d 2.0 d 1.0 O2N—CH2—CH2—CH3
Effect is cumulative CHCl3 7.3 CH2Cl2 5.3 CH3Cl 3.1
d 0.9 d 0.9 CH3 CH3 d 1.2 d 0.8 H3C H d 1.6 C C H3C CH2 CH3 CH3 CH3 Methyl, Methylene, and Methine CH3 more shielded than CH2 ; CH2 more shielded than CH
H H H H H C C H H H H H Protons attached to sp2 hybridized carbonare less shielded than those attachedto sp3 hybridized carbon CH3CH3 7.3 5.3 0.9
5.3 H CH2OCH3 2.4 C C H H C C H H But protons attached to sp hybridized carbonare more shielded than those attachedto sp2 hybridized carbon
d 0.9 d 1.3 d 0.9 H3C CH3 0.8 1.5 1.2 2.6 H3C CH2 Protons attached to benzylic and allyliccarbons are somewhat less shielded than usual H3C—CH2—CH3
O d 2.4 H d 9.7 H3C C C H d 1.1 CH3 Proton attached to C=O of aldehydeis most deshielded C—H