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CHEM 2331H
T.Andrew Taton

Chapter 12 Book Notes Infrared Spectroscopy and Mass Spectroscopy 12.1) Introduction • Ochem = about determining organic structures o when interesting compound isolated from natural source, structure must be completely determined before can do any synthesis o for rxns, must determine if product has desired structure and know structure of unwanted product  can alter rxn conditions to favor wanted product • Can identify compound chemically: analyze elemental composition/determine molecular weight molecular formula o compare physical properties (melting pt, etc) w/ published values o chemical tests suggest functional groups, narrow down possible structures before using physical properties to identify molecule • But above techniques = insufficient for complex compounds that have never been synthesized & characterized o & impractical w/ hard to obtain compounds b/c need large sample for elemental analysis/functional group tests o need techniques that use small sample and don’t destroy it (chemical techiniques destroy) spectroscopy (can measure many different spectra types w/ little/no loss) • Absorption spectroscopy: measurement of amt light absorbed by compound as f(wavelength of light) o spectrometer irradiates sample w/ light, measures amt light transmitted as f(wavelength), plots results on graph • 4 Spectroscopic techniques covered o 1) Infrared (IR) Spectroscopy: observes vibrations of bonds, provides evidence of functional groups present o 2) Mass Spectrometry (MS): not spectroscopic b/c doesn’t measure light absorption/emission; spectrometer bombards molecules w/ electrons, breaks them into fragments analyze fragment masses molecular weight, possibly molecular formula, clues about structure/functional groups; less than 1 mg sample destroyed in process o 3) Nuclear Magnetic Resonance (NMR) spectroscopy: observes chemical environments of H’s and C’s, gives evidence of alkyl group structure, clues of functional groups o 4) Ultraviolet (UV) spectroscopy: observes electronic transitions, gives info on electronic bonding in sample • Use spectroscopic techniques together and use multiple spectra to determine compound structure 12.2) The Electromagnetic Spectrum • Electromagnetic radiation (like visible, infrared, ultraviolet, microwaves, radio waves) travels @ speed of light (3*10 cm/s) but different wavelengths and frequencies o frequency: # complete wave cycles that pass fixed pt. in 1 sec; represented by ν, units = Hz (cycles/sec) o wavelength: distance btwn any 2 peaks/troughs of wave; represented by λ • Wavelength and frequency = inversely proportional: νλ = c (speed of light) • Electromagnetic waves travel as photons (massless energy packets); energy of photon: E = hν = hc/λ o h = Planck’s constant: 6.62*10 -37kJsec or 1.58*10 -3kcalsec o Molecule can be struck by photon and absorb its energy molecule’s energy increased by amt energy of photon represent irradiation by (hν) • Electromagnetic spectrum: range of all possible frequencies from 0 to ∞ (but in reality, goes from low frequency (low energy) radio waves to high frequency (high energy) gamma rays) • Electromagnetic spin really = continuous, exact dividing line positions = arbitrary o top of spectrum: high frequency, short wavelength, high energy o bottom: low frequency, longer wavelengths, low energy o X rays = so energetic, excite electrons past all energy levels ionization o UV excites to high energy levels w/in molecules o IR excites molecular vibrations o Microwaves excite rotations o Radio waves excite nuclear spin transitions observed in NMR spectroscopy 12.3) The Infrared Region • Infrared region = just below visible frequencies (8*10 cm- 1*10 cm wavelengths) (just below visible to just above highest microwave and radar frequen-4es -4 o Common Infrared spectrometers operate btwn 2.5*10 to 25*10 cm (= 4.6-46 kJ/mol or 1.1-11 kcal/mol) o Infrared photons don’t have enough energy to cause electronic transitions, but cause atom groups to vibrate w/ respect to bonds that connect them; correspond to distinct energies like electronic transitions; molecules only absorb IR @ certain wavelengths/frequencies • Can specify infrared band position by wavelength (λ) measured in microns (µm) -6 (micrometers = 10 meter) o wavenumber: more common unit; represented by νbar; = # cycles in 1 cm; reciprocal of wavelength (cm) o since 1cm = 10000 µm, calculate wavenumber by 10000/wavelength in microns -1 o wavenumber units = cm ; • Wavenumber = proportional to frequency, proportional to energy 12.4) Molecular Vibrations • Covalent bond btwn 2 atoms acts like spring o If stretched, restoring force pulls 2 atoms together to equilibrium length o If compressed, restoring force pushes 2 atoms apart o If stretched/compressed then released, atoms vibrate • Frequency of stretching vibration depends on atoms masses and bond stiffness o Frequency decreases w/ atomic weight o ex: C-D frequency < C-H frequency • Frequency ↑ w/ bond energy o Stronger bonds = stiffer need more force to stretch/compress vibrate faster o ex: O-H frequency > C-H frequency o Triple bonds stronger than double stronger than single bonds o applies if atoms = similar masses • Infrared spectrum = graph of energy molecule absorbs as f(frequency/wavelength of light) o get absorptions from exciting vibrational modes of molecule bonds o get many different absorptions even for simple molecule, not just 1 for each bond; like stretch, bending (bond lengths = constant, but angles vibrate about eqbm values): scissoring and twisting • For water, get symmetric, antisymmetric, and scissoring stretching • Nonlinear molecule w/ n atoms generally has 3n – 6 fundamental vibrational modes o also get combinations and multiples (overtones) of simple fundamental vibrational modes • Very unlikely IR spectra of 2 different molecules (except enantiomers) have same frequencies for all complex vibrations o IR spectrum gives fingerprint of molecule o fingerprint region: region (600-1400 cm ) (bending vibrations) w/ most of the complex vibrations -1 • Simple stretching vibrations = in 1600-3500 cm region, most characteristic and predictable; but do get lot of info from fingerprint region too 12.5) IR-Active and IR-Inactive Vibrations • Not all molecular vibrations absorb IR o molecular bond interacts with electromagnetic field b/c of bond polarity (dipole moment) o think of bond as + and – charge separated by spring, put it in electric field stretched/compressed depending on field direction • Rapidly reversing electric field = one of electromagnetic wave components; field alternately stretches/compresses polar bond o when electric field in same direction as dipole moment bond compressed, dipole moment ↓ o When field opposite dipole moment bond stretched, dipole moment ↑ o If stretching/compressing happens @ molecule’s natural vibration state energy may be absorbed o IR-active: when bond vibrations w/ dipole moments generally result in IR absorptions • If symmetrical bond w/ 0 dipole moment electric field doesn’t interact w/ bond (or does so very weakly) even when bond = stretched/compressed o no dipole moment change no energy absorption IR-inactive (no absorption in IR spectrum) o IR-active vibration must change dipole movement of molecule • Bonds w/ 0 dipole moments sometimes produce weak absorptions b/c molecular collisions, rotations, and vibrations make them unsymmetrical part of time • Overtone peaks: when strongly polar bonds (like C=O) absorb so strongly, produce relatively small peaks @ a multiple (usually double) of fundamental vibration frequency 12.6) Measurement of the IR Spectrum • Can measure IR spectra w/ solid, liquid, or gaseous samples placed in IR light beam o liq: put drop btwn 2 salt plates (NaCl or KBr = transparent to IR light @ most important frequencies) o solid: ground w/ KBr, then press into disk, then place into light beam; Or grind it into pasty mull w/ paraffin oil place btwn 2 salt plates; Or dissolve in common solvents like CH 2l 2 CCl ,4or CS 2hat don’t have absorptions in areas of interest o Gases: place in longer cell w/ polished salt windows; gas cells often have mirrors that reflect beam through cell many times stronger absorption • Infrared spectrometer: measures frequencies of IR light absorbed by compound o simple IR spectrometer has 2 beams: sample beam of light passes through sample cell, reference beam passes through reference cell that has only solvent o rotating mirror alternately allows light from each of 2 beams to enter monochromator • Monochromator uses prisms/diffraction gratings to allow only 1 light frequency to enter detector @ a time o scans IR frequencies range as pen moves along corresponding frequencies on x axis of chart paper o higher frequencies @ left of chart paper o detector signal = proportional to difference in sample light intensity and reference beams (which compensate for absorption by air/solvent) o detector signal controls pen movement along y axis (100% transmittance, no absorption @ top of paper; 0% transmittance, all light absorbed @ bottom) • Dispersive instrument: disperses light into all different frequencies, then measures them individually o need expensive prisms and diffraction gratings, must be manually aligned and calibrated regularly o only 1 frequency observed at a time, so need strong IR sources and need 2-10 min to scan th
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