文档介绍：IAP
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 ic radiation to excite molecules.
E2
h = x 10–34 J·s
∆ν
E = h c = x 1010 cm/s
ν
(ν= c/λ) = frequency( Hz )
λ= wavelength (cm)
E1
• Molecules have unique absorption spectra that depend on their structure.
• Specific absorptions provide useful structural information.
How do you know what type of ic radiation to use?
EM Radiation Wavelength (cm) Excitation Approximate ∆E
cosmic rays 10–12 to 10–10 ––––
gamma rays 10–10 to 10–8 ––––
X-rays 10–8 to 10–6 ––––
ultraviolet 10–6 to x 10–5 electronic 70–300 kcal/ mol
visible x 10–5 to x 10–5 electronic 40–70 kcal/ mol
infrared x 10–5 to 3 x 10 –2 vibrational 1–10 kcal/mol
microwave 3 x 10–2 to 3 x 102 rotational ~ 1 cal/mol
radio* 3 x 102 to 3 x 105 ––––
* Radio frequency EM radiation is used in NMR spectroscopy.
1
Infrared Spectroscopy (vibrational energy)
• Chemic