IAP5301 S petros copy Chemists use spectroscopy (Ir, UV-Vis, NMR) to determine molecular structure. spectroscopy: technique that measures the amount of radiation a substance absorbs at various wavelengths(based on the quantization of molecular energy levels Molecules exist in quantized energy states (rotational, vibrational, and electronic quantum states) Molecules absorb discrete amounts of energy(ae)and are excited to higher energy states We can use electromagnetic radiation to excite molecules h=6.62608x1034j △E=hv c=2998x10cm/s 入 length(cI Molecules have unique absorption spectra that depend on their structure Specific absorptions provide useful structural information. How do you know what type of electromagnetic radiation to use? EM Radiation Wavelength(cm Excitation Approximate AE cosmic ravs 10-12to10-10 amma ravs 10-10to103 10-8to10-6 ultraviolet 106to3:x105 electronic 70-300 kcal/mol visible 3.8x10-5 to 7.8x10-5 electronic 40-70 kcal/mol infrared 7.8x10-5 to 3x 10-2 vibrational 1-10 kcal /mol microwave 3x10-2to3x102 rotational 1 cal/mol 102to3x105 Radio frequency EM radiation is used in NMr spectroscopyIAP 5.301 Spectroscopy Chemists use spectroscopy (IR, UV-Vis, NMR) to determine molecular structure. Spectroscopy: technique that measures the amount of radiation a substance absorbs at various wavelengths (based on the quantization of molecular energy levels) • Molecules exist in quantized energy states (rotational, vibrational, and electronic quantum states). • Molecules absorb discrete amounts of energy (∆E) and are excited to higher energy states. • We can use electromagnetic radiation to excite molecules. E1 E2 ∆E = hν (ν = c/λ) 10 cm/s ν ( ) λ h = 6.62608 x 10–34 J·s c = 2.998 x 10 = frequency Hz = wavelength (cm) • Molecules have unique absorption spectra that depend on their structure. • Specific absorptions provide useful structural information. How do you know what type of electromagnetic radiation to use? EM Radiation Wavelength (cm) 10 –10 10 –8 10–8 –6 10–6 –5 –5 –5 –5 –2 –2 2 2 5 Excitation –– –– –– –– Approximate ∆E –– –– –– / / –– cosmic rays gamma rays X-rays ultraviolet visible infrared microwave radio* –12 to 10 –10 to 10 to 10 to 3.8 x 10 3.8 x 10 to 7.8 x 10 7.8 x 10 to 3 x 10 3 x 10 to 3 x 10 3 x 10 to 3 x 10 electronic electronic vibrational rotational 70–300 kcal mol 40–70 kcal mol 1–10 kcal/mol ~ 1 cal/mol * Radio frequency EM radiation is used in NMR spectroscopy. 1