SpectroscopyNotes

SpectroscopyNotes - Chem 2262 Notes Spectroscopic...

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Chem 226 2 Notes Spectroscopic Identification of Organic Compounds (Bruice, Chapter 1 2, 13 ) 1 Spectroscopy Review electromagnetic spectrum (Bruice, p. 528 ) UV 200-400 nm 71-143 kcal/mole Bond DIssociation Energy RANGE ENERGY IR 625-4000 cm –1 1.8-11 kcal/mole Bond Deformation (stretch/bend) CORRESPONDS TO: NMR 100-1000 MHz 10 –5 - 10 –4 kcal/mole Nuclear Spin Flip With the limited time we have available, we will mostly discuss IR (Chapter 1 2 ) and NMR (Chapter 1 3 ), which are two of the most useful techniques for identifying organic compounds. Units and Conversion Factors Wavelength (UV spectroscopy) λ : 1 nm = 10 Å = 10 –9 m = 10 –7 cm Wavenumber (IR spectroscopy) ν :cm –1 Frequency (NMR spectroscopy) ν : Hz (s –1 ) Note that λ ν has units of velocity λ ν = 2.998 x 10 10 cm•s –1 = 2.998 x 10 17 nm•s –1 So Energy E = h ν = hc λ h = 9.537 x 10 –14 kcal•mol –1 •s E (kcal/mol) = 9.537 x 10 –14 ν (Hz) = 28,591 λ (cm) Infrared (IR) Spectroscopy By measuring characteristic frequencies associated with bond stretching and bending, IR provides information about the types of functional groups present in an organic molecule. CH H C H stretch (change in bond length) bend (change in bond angle) higher ν lower ν ν = (cm –1 ) 1 λ (cm) = 10 7 λ (nm) λ = (nm) 2.998 x 10 17 ν (Hz)
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Chem 226 2 Notes Spectroscopic Identification of Organic Compounds (Bruice, Chapter 1 2, 13 ) 2 3500 4000 2500 2000 1550 650 cm –1 2000 1800 1650 1550 cm –1 CO CN CC very few bonds OH NH CH CO 2 strongly coupled & complex FINGERPRINT REGION 3000 Requirement for IR absorption: DIPOLE MOMENT MUST CHANGE DURING VIBRATION OF MOLECULE R C R O δ + δ C O stretch STRONG BAND H C H C H H no C=C dipole NO BAND SEEN Hooke's Law k μ ν = 1 2 π c force constant reduced mass μ = m 1 m 2 m 1 + m 2 Force constants are related to bond strength; stronger bonds correspond to large force constants and therefore higher stretching frequency. However, vibrational frequency is also a ffected by the mass of the vibrating atoms. Bonds connecting lighter atoms vibrate faster (higher frequency) than those connecting heavier atoms (for lighter atoms there is less inertia to overcome. This is why C-H, O-H, and N-H bond stretches occur at higher frequencies (because of the lightness of H) than even strong bonds like C C and C N.
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Chem 2262 Notes Spectroscopic Identification of Organic Compounds Bruice, Chapter 12, 13
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Chem 2262 Notes Spectroscopic Identification of Organic Compounds Bruice, Chapter 12, 13
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Chem 2262 Notes Spectroscopic Identification of Organic Compounds
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This note was uploaded on 04/21/2008 for the course CHEM 2262 taught by Professor Smith during the Spring '08 term at LSU.

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SpectroscopyNotes - Chem 2262 Notes Spectroscopic...

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