Visible and Ultraviolet Spectroscopy An obvious difference between certain compounds is their color In this respect the human eye is functioning as a spectrometer analyzing the light reflected from the surface of a solid or passing through a liquid Wavelength is defined on the left below as the distance between adjacent peaks(or troughs), and may be designated in meters centimeters or nanometers(10-9 meters ). Frequency is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz(Hz) Visible wavelengths cover a range from approximately 400 to 800 nm
Visible and Ultraviolet Spectroscopy An obvious difference between certain compounds is their color. In this respect the human eye is functioning as a spectrometer analyzing the light reflected from the surface of a solid or passing through a liquid. Wavelength is defined on the left below, as the distance between adjacent peaks (or troughs), and may be designated in meters, centimeters or nanometers (10-9 meters). Frequency is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz (Hz). Visible wavelengths cover a range from approximately 400 to 800 nm
DIspersion white Angle Light Red orange Yell Green Prism B n ed Violet
“ ROY G BIV wAvelength Amplitude Violet: 400-420 nm 420-440 Blue: 440-490 nm Green: 490-570 nm Higher Visible Spectrum L。wer Yellow 570-585nm Frequency Frequency ge:585-620 IR Red:620-780nm 400 500 60o 700800 Wavelength in nanometers
Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm “ROY G BIV
Color wheel 620nm 800 nm 580nm 400nm 560 nm 430nm 490nm Complementary colors are diametrically opposite each other Thus absorption of 420-430 nm light renders a substance yellow. and absorption of 500-520 nm light makes it red green is unique in that it can be created by absoption close to 400 nm as well as absorption near 800 nm
Complementary colors are diametrically opposite each other. Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red. Green is unique in that it can be created by absoption close to 400 nm as well as absorption near 800 nm. Color wheel
Some Natural Organic Pigments OH o CH3 O H CO2H HO OH OH O Kermesic Acid (Carminic Acid Z=H from the insect coccus cacti Indigo from Isatis tinctoria (woad) Z=Br Punicin or Tyrian Purple from mollusks of the genus Murex Crocetin CH3 CH3 from saffron B-carotene from carrots extensively conjugated pi-electrons
“extensively conjugated pi-electrons
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 )
