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Chapter 13

Organic Chemistry

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Nuclear magnetic resonance spectroscopy (NMR) tells about the carbon skeleton and the environments of the hydrogens attached Infrared spectroscopy (IR) reveals the presence or signals the absence of key functional groups Ultraviolet-visible spectroscopy (UV-VIS) probes the electron distribution especially in molecules that have conjugated pi electron systems Mass spectroscopy (MS) gives the molecular weight and formula both of the molecule itself and various structural units within it all based on how molecules absorb energy all techniques use electromagnetic radiation except MS which uses charges particles like electrons 13.1 Principles of Molecular Spectroscopy: Electromagnetic Radiation EMR had properties of particles and waves particles are photons which have energy referred to as quantum E = hv SI units for v (frequency) are hertz (s -1 ) h is planck's constant (6.63x10 -34 J*s) EMR travels at speed of light (3.0x10 8 m/s) c = vλ λ is wavelength visible light is 400nm-800nm (violet-red) frequency is inversely proportional to wavelength energy is directly proportional to frequency gamma>x-ray>ultra violet>visible>infrared>microwave>radio in terms of energy (shorter wavelength, higher frequency) molecules absorb only certain frequencies of EMR (measured by spectrometers) 13.2 Principles of Molecular Spectroscopy: Quantized Energy States energy of photon must equal energy difference between two states of molecule to be absorbed (two nuclear spin states-NMR, two vibrational states-IR, two electronic states- UV-VIS) called resonance transfer of E between two objects when v matches energy states of molecule are quantized so they can only have certain values spectrometer detects which frequencies a molecule absorbs and produces a spectrum relation of v and absorption graph is interpreted to determine structure 13.3 Introduction to 1 H NMR Spectroscopy NMR depends on absorption of E when nucleus of an atom is excited from lowest energy spin state to the next higher doesn't work for all atoms 1H and 13C both work and both are very common in orgo proton has two spin states with quantum numbers +/- ½
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no E difference in presence of magnetic field (Bo) they have different E and EMR can be absorbed spin state aligned with Bo is lower energy than that opposed E difference directly proportional to strength of field EMR absorption means lower energy states must be in excess so strong Bo needed response of atom to Bo depends on element and isotope 1H (200MHz at 4.7 T) and 13C (50.4MHz) must be taken in two different runs NMR has powerful magnet, radiofrequency (rf) transmitter and a detector which monitors rf absorption and displays information continuous wave technique (CW) is when you either keep Bo constant and vary v or keep v constant and vary Bo CW not really used anymore replaced by pulsed Fourier-transform NMR (FT-NMR)
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