Chapter 15: UltravioletVisible Spectroscopy

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Chapter 15 Ultraviolet-Visible Spectroscopy and
Mass Spectrometry
I. Ultraviolet-Visible (UV) Spectroscopy
A. Recall Orbitals

• Consider ethylene, ? bond is the chromophore
• Highest Occupied MO (HOMO)
• Lowest Unoccupied MO (LUMO)

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B. Electronic Transitions

• Energy
• Energy difference between HOMO and LUMO
• Thus, takes energy to excite the electron

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• Consider Formaldehyde

• n to ?
• ? to ?
• ? to ?
• SO?
• Energy corresponds to energy of UV-Vis
• Energy of ? to ? gt ? to ? gt n to ?

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• Sample Spectrum

• Recall Energy and ? are inversely related
• ?max 242 nm (A log(I0/I) ?cl)

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• Effect of Conjugation
• Energy gap between ? and ? decreases!
• So, ?? increases!

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• In the Visible Region!
• Multiple Conjugated Bonds B-Carotene
• Wavelength Absorbed vs. Transmitted

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• Color Wheel!

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• Relevance

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• Relevance Continued Ozone (O3)

UV Spectrum of O3

• UV-B (295 - 320 nm) sunburn skin cancer
  through DNA damage
• UV-A (320 - 400 nm) wrinkling and premature
  aging
• Ozone in the upper atmosphere absorbs much of
  the more damaging radiation (UV-B and UV-C)
• Problem? Concentration of O3 is going down

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• Relevance Continued Ozone (O3)

• Early sunscreens contained PABA (UV-B!)
• Newer formulations contain things like
  Oxybenzene which absorb in both the UV-B and UV-A
  region

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II. Mass Spectrometry
A. How it works

• Create charged species (high-energy electron
  beam)
• Separate based on Mass/Charge ration (m/z)
• Detect

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• Ionization

• Expulsion of an electron creates a radical
  cation (referred to as the Molecular Ion, )
• Cant necessarily identify exact location of
  radical cation
• Initial radical cation may fall apart (more on
  this later)

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• What does this tell you?

• High Resolution Mass Spec
• Can determine mass of Molecular Ion to several
  decimal places
• Based upon exact masses of common isotopes can
  calculate molecular formulas that fit the data!!!

• Say you saw mass of 44.0265
• Calculate that
• CO2 43.9898
• C2H4O 44.0262
• C3H8 44.0626
• Must be C2H4O
• Note doesnt tell you which isomer!

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• Isotopic Ratios
• Some elements are primarily one isotope
• Other elements are composed of multiple isotopes

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• Isotopic Ratios Continued
• So?
• Can see this by mass spec!
• 1.1 13C gives (M 1)

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• Isotopic Ratios Continued
• Chlorine (35Cl to 37Cl is 31, give M 2)
• Note Base Ion vs. Molecular Ion

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• Fragmentation
• Molecular Ion is not always stable, fragmentation
  can occur
• Fragmentation patterns can tell us something
  about the structure
• Consider octane

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• Fragmentation Continued
• Terminal C-C bond cleavage?

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• Fragmentation Continued
• Internal C-C bond cleavage?
• Note Relative stability of radicals and cations

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• Fragmentation Continued
• Now you predict!

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• Fragmentation Continued
• Other Patterns