Unit 3.5 - Molecular Absorption Theory PDF

MOLECULAR ABSORPTION THEORY: UV-VISIBLE SPECTROSCOPY By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 26 Chemical Analysis, Chapter 9 Lab Manual, Experiment 3  Atomic Absorption/Emission Spectroscopy 2. Type of radia...

MOLECULAR ABSORPTION THEORY: UV-VISIBLE SPECTROSCOPY By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 26 Chemical Analysis, Chapter 9 Lab Manual, Experiment 3  Atomic Absorption/Emission Spectroscopy 2. Type of radiation:  UV: 180-380 nm  Visible: 380-780 nm Interaction of UV- visible radiation with metals (atoms) results in change of electron distribution of valence electrons.  Internal Energy of Molecules 3 Etotal = Eelec + Evib + Erot + Enucl Eelec: electronic transitions → UV-vis, X-ray spectroscopy Evib: vibrational transitions → infrared spectroscopy Erot: rotational transitions → microwave spectroscopy Enucl: nucleus spin → nuclear magnetic resonance spectroscopy (or MRI; magnetic resonance imaging) Organic molecules are polyatomic: more E levels than atoms  Vibrational and Rotational Energy States Within Electronic Levels 4 v3 v2 v1 r2 r1  Energy Transitions for Molecules 5 1. Rotational:  Far-infrared → microwave + radiowaves (λ = 100 µm – 10 cm)  Applies to gas molecules  Gas molecules rotate about their 1 axes  Absorption of long λ’s (low energy) causes energy of molecules to increase, resulting 0 0 in transitions to higher rotational states within the ground state  NO promotion of electrons  Energy Transitions for Molecules 6 2. Vibrational:  Infrared → λ = 780nm – 3.75 µm (low energy)  Absorption of IR radiation by molecules causes changes in the amplitude of the vibration (stretching or bending) and results in transitions to higher vibrational states within the ground state  NO promotion of electrons  Energy Transitions for Molecules 7 3. Electronic Transitions:  UV – Vis spectroscopy  Conventional UV → λ = 180 – 380 nm  Visible → λ = 380 nm – 780 nm  Absorption of high energy radiation by molecules causes promotion of bonded/lone pairs of electrons to higher energy levels  π and n electrons are promoted to antibonding orbitals  Summary of Energy Transitions For molecules: Eoverall = Eelectronic + Evibrational + Erotational + Enuclear Energy changes occur during absorption of IR, VIS and UV radiation by organic molecules Ground state and excited states contain both vibrational and rotational energy levels: 8  Electronic (Molecular) Spectroscopy 9  Phenomenon of interaction of molecules with ultraviolet and visible light  Conventional UV-Vis spectroscopy: 180-780 nm  Vacuum ultraviolet (VUV) region: < 180 nm  Absorption of photon results in electronic transition of a molecule → electrons are promoted from ground state to higher electronic states 1. Absorption Process (UV-Visible absorption): ABS Mo + hf → M* Mo: ground state of organic molecules hf: photons of UV or visible light M*: molecule in excited state 2. Decay Process (Fluorescence): IE Mo + hc/λUV/Vis → M* → Mo + hc/λVis high E low E shorter λ longer λ  Molecular Spectrum 10 Result of electronic transitions: Continuous broad band spectrum (in contrast to atomic absorption line spectrum) λ (nm) λ (nm) Atomic Spectrum Molecular Spectrum Continuous Broad Band  Limited structural info  Unique for given compound (fingerprint)  λmax can be read  Molecular Absorption 11  Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain unsaturated functional groups (chromophores)  Chromophore: an atom or functional group in an organic molecule, that contains 𝝅𝝅 or n electrons of low excitation energy, capable of absorbing UV –visible radiation  UV-VIS spectroscopy can be used to detect the presence of chromophores such as dienes, aromatics, polyenes, and conjugated ketones, etc.  Molar Absorptivity for Chromophores 12  ε large for strongly absorbing chromophores (> 10,000 L mol-1 cm-1)  ε small for weakly absorbing chromophores (10 – 100 L mol-1 cm-1)  Magnitude of ε reflects both:  Size of the chromophore  Probability that light of a given wavelength will be absorbed when it strikes the chromophore  ε useful for detection of λmax  Molecular Absorption 13  Absorption of UV-Vis photons by molecules results in electronic excitation of the molecule containing chromophoric groups  The electronic transition involves promotion of electrons from the electronic ground state to higher energy states, usually from the HOMO to the LUMO  HOMO: highest occupied molecular orbital  LUMO: lowest unoccupied molecular orbital  Molecular Orbital Theory 14 Overlap of 2 atomic orbitals = 2 molecular orbitals: bonding (lower energy) + antibonding (higher energy) Types of Molecular Orbitals: 1. Sigma (σ) orbitals: Cylindrical symmetrical e- density around internuclear axis Overlap of the following orbitals results in σ bonds: a) Two ‘s’ atomic orbitals b) Two ‘px’ atomic orbitals → head to head overlap c) ‘s’ + ‘p’ atomic orbitals d) ‘s’ + ‘spx’ hybridized orbital e) Two ‘spx’ hybridized orbitals  Molecular Orbital Theory 15 Sigma (σ) bonds:  Single bonds between two atoms  One of the bonds that comprise a double or triple bond in an unsaturated molecule (e.g. C=C, C=N, C=O, C≡C)  When a sigma bond forms, this gives:  σ bonding molecular orbital → lowest energy  σ* antibonding molecular orbital → highest energy  Molecular Orbitals of H2 16 Electrons in sigma bonds are normally not excited to higher energy levels → only π and n electrons are excited  Molecular Orbital Theory 17 2. π orbitals: clouds of electron density above and below the internuclear axis  π bonds found in unsaturated molecules:  Double bond: 1 σ bond, 1 π bond  Triple bond: 1 σ bond, 2 π bonds  When unhybridized ‘py’ or ‘pz’ orbitals overlap, a π bond is formed, to give:  π bonding molecular orbital → lowest energy  π* antibonding molecular orbital → highest energy 3. Nonbonding (n) orbitals  Consist of unshared e- pairs on electronegative atoms (in atomic-like orbitals) of a molecule  n orbitals do not influence the energy of a molecule  Lone pair of electrons present in n orbitals  Overlap of p Orbitals 18  Molecular Orbitals in Ethylene 2 sp2 hybridized carbon atoms Molecular Orbitals in Acetylene Molecular Orbitals in Formaldehyde 19  Electronic Energy Transitions 20  When molecules are electronically excited, electrons are promoted from bonding to antibonding orbitals.  Shown below are the regions of the electromagnetic spectrum and the types of transitions that occur in each:  Vacuum UV or Far UV ( λ < 180 nm ) UV/VIS LUMO HOMO 21  σ → σ∗ Transitions 22  An electron in a bonding σ orbital is excited to the corresponding antibonding orbital.  The energy required for this transition is large.  For example, methane (which has only C-H bonds, and can only undergo σ → σ* transitions) shows an absorbance maximum at 125 nm.  Absorption maxima due to σ → σ* transitions are not seen in conventional UV-Vis spectra (180 – 780 nm)  σ → σ∗ Transitions 23  σ → σ* transitions only occur in vacuum UV system  large ΔE, λ < 150 nm  ε = 10 – 10,000 L mol-1 cm-1  n → σ∗ Transitions 24  Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n → σ* transitions  These transitions need less energy than σ → σ* transitions  Transitions are initiated by light whose wavelength is in the range 150 – 250 nm  The number of organic functional groups with n → σ* peaks in the UV region is small  n → σ∗ Transitions 25  Smaller ΔE, λ = 150 – 250 nm  Small number of organic functional groups with n  σ* peaks in UV region  ε = 200 – 2000 L mol-1 cm-1  n → π∗ and π → π∗ Transitions 26  Most absorption spectroscopy of organic compounds is based on transitions of n or π electrons to the π* excited state  These transitions fall in an experimentally convenient region of the spectrum (180 – 780 nm)  These transitions need an unsaturated group in the molecule to provide the π electrons  Molar absorptivity (ε) relatively low for n to π* transitions  10 – 100 L mol-1 cm-1  Molar absorptivity (ε) relatively high for π to π* transitions  1000-10,000 L mol-1 cm-1  n → π∗ and π → π∗ Transitions 27 Ideal for UV-Vis spectroscopy of  π to π* and n to π* transitions organic chromophores  Small ΔE, λ = 200-780 nm (unsaturated functional group or  ε = 10 – 10,000 L mol-1 cm-1 electronegative functional group with non-bonding electrons) Common Electronic Transitions of Chromophores Chromophore Transition λmax (nm) Solvent C=C π→π* 171 hexane n→π* 290 hexane C=O π→π* 180 hexane n→π* 275 ethanol N=O π→π* 200 ethanol C-X n→σ* 205 hexane (X=Br, I) n→σ* 255 hexane Note that if no π bonds are present in the molecule, this means that no π* orbitals are available, and therefore transitions to the π* orbital are not possible 28  UV Spectrum of Isoprene 29 π → π∗  UV Spectrum of Benzoic Acid Problem 1: Absorbance Label the parts of the structure of benzoic acid that contribute to the transitions indicated in the UV spectrum. Label the spectrum with the type of transition responsible for each peak. 30 UV Spectrum of Vanillin Problem 2: Label the parts of the structure of vanillin that contribute to the transitions indicated in the UV spectrum. Label the spectrum with the type of transition responsible for each peak. 31  Up Next: UV-Visible Spectrophotometer 32 Double beam grating spectrometer  References 33  Tyrer, N.; Kogut, E. CHEM25415 Lecture on Molecular Absorption/Emission Theory. Presented at Sheridan College, Brampton, ON, Fall 2012.  Tyrer, N. CHEM25415 Instrumental Analysis 1 Laboratory Manual; Sheridan College: Brampton, ON; Experiment 3.  Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 9th ed.; Brooks/Cole: California, 2014; Chapter 26.  Rouessac, F.; Rouessac, A. Chemical Analysis, 2nd ed.; John Wiley & Sons: New Jersey, 2007; Chapter 9.