The Electromagnetic Spectrum The Electromagnetic Spectrum Visible X-Rays Microwave UY Rays Radio meters 1013 10 109 10 105 103 10 10 m Wavelength 1011 10 10 105 10-3 10 10 nm 104 10 102 10 10 10 Frequency Hz 1021 1019 10 1015 1013 10 10 Energy kcal⊥ 10 10 102 10 10 U=C/\ U=frequency, n=wavelength, c=velocity of light(c=3.1010 cm/sec) AE=hu E=energy U=frequency h=planck s constant(h=6.6.10-27 erg sec)
The Electromagnetic Spectrum
Electronic transitions The absorption of uv or visible radiation corresponds to the excitation of outer electrons There are three types of electronic transition which can be considered Transitions involving p, S, and n electrons Transitions involving charge-transfer electrons Transitions involving d and f electrons(not covered in this unit When an atom or molecule absorbs energy electrons are promoted from their ground state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each other These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level Rotational Vibrational electronic levels electronic levels
Electronic transitions The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition which can be considered; Transitions involving p, s, and n electrons. Transitions involving charge-transfer electrons Transitions involving d and f electrons (not covered in this Unit) When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each other. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level
UV-Visible Absorption Spectra The energies noted above are sufficient to promote or excite a molecular electron to a higher energy orbital. Consequently, absorption spectroscopy carried out in this region is sometimes called"electronic spectroscopy g(anti-bonding) π(anti- bonding) n+兀 n(non-bonding) π( bonding) o(bonding)
UV-Visible Absorption Spectra The energies noted above are sufficient to promote or excite a molecular electron to a higher energy orbital. Consequently, absorption spectroscopy carried out in this region is sometimes called "electronic spectroscopy
Molar absorptivity Molar absorptivity, e=A/cI A= absorbance, c= sample concentration in moles/liter 1=length of light path through the sample in cm 25,000 9 osk mux=222 nm 3 max 222 nm 20,000 15,000 Isoprene 0.4 10,000 C=4·105 moles/liter I= 1 cm 0 5,000 Isoprene in hexane soln 200220240260280300320340 200220240260280300320340 入(nm 入r
Molar absorptivity Molar Absorptivity, e = A/ c l A= absorbance, c = sample concentration in moles/liter l = length of light path through the sample in cm
Molar absoptivities may be very large for strongly absorbing chromophores >10,000)and very small if absorption is weak(10 to 100). The magnitude of e reflects both the size of the chromophore and the probability that light of a given wavelength will be absorbed when it strikes the chromophore a general equation stating this relationship may be written as follows e=0.87*1020R*a (R is the transition probability(o to 1)& a is the chromophore area in cm 兀- orbita C o2 n-orbitals 元*- orbita electron
Molar absoptivitiesmay be very large for strongly absorbing chromophores (>10,000) and very small if absorption is weak (10 to 100). The magnitude of e reflects both the size of the chromophore and the probability that light of a given wavelength will be absorbed when it strikes the chromophore. A general equation stating this relationship may be written as follows: e = 0.87*1020 R * a (R is the transition probability (0 to 1) & a is the chromophore area in cm2 )