4. Molecular spectroscopy. Basel, 2008

Transcription

1 4. Molecular spectroscopy Basel, 2008

2 4. Molecular spectroscopy Contents: 1. Introduction 2. Schema of a spectrometer 3. Quantification of molecules mouvements 4. UV-VIS spectroscopy 5. IR spectroscopy 6. MW spectroscopy 7. Magnetic resonance 8. Nullpunct energy Literature: P. Atkins, J. de Paula - Atkins Physical Chemistry, 8th Ed., Oxford University Press 2007, chapters P. Atkins, J. de Paula - Physical Chemistry for the Life Sciences, Ed., Oxford University Press 2006, chapter 13. C.E. Housercroft, E.W. Constable - Chemistry, Ed. Pearson Education Ltd, 2002, chapters Supplementary material: Vibrations of CO 2 and H 2 O (molecules gallery). MacSpartan (PC room at groundfloor)

3 4.1 Introduction Origin of spectral lines in molecular spectroscopy: absorption or emission of a photon when the energy of the molecule changes: - electronic transitions - rotational states change - vibrational states change Absorption spectroscopy: Emission spectroscopy E f E i ΔE = hν (4.1) E i E f Emission or absorption spectroscopy: - give the same information about the energy levels - practical considerations determine which technique is better to use.

4 Spectrum I Spectrum λ Energy levels Particularities: - lines occur with a variety of intensities - some lines which are expected to occur, do not appear at all. Selection rules

5 4.1.1 Beer-Lamber law Beer- Lambert law: transmitted intensity varies with the length of the sample, l, and the molar concentration [c], of the absorbing species (Absorption spectroscopy). I 0 sample l I I = I ε[c]l ε - molar absorption coefficient (so called extinction coefficient, too) I 0 incident intensity I transmitted intensity (4.2) ε - depends on ν of I 0, and is greater where the absorption is more intense. [ε] = dm 3 mol -1 cm -1

6 Beer-Lamber law Absorbance, A of a sample at a given wavenumber is: A I 0 = log A = ε [ c]l I (4.3) (4.4) Calibration curve Transmitance is the ratio between the transmitted light and the incident one: I 0 T = I (4.5) l : - short for liquids - very long for gases (multiple passage of the beam between parallel mirrors at each end of the cavity where the sample is introduced)

7 4.1.2 Linewidths Spectral lines: individual transitions between two energy levels. Spectral bands: simultaneous transitions between very close energy levels, thus the summ of many spectral lines. δe i Lifetime broadening: finite lifetime of the states involved in the transition are inducing a broadening of the spectral lines: δe = h τ (4.6) δ E = hcδν~ (4.7) δν~ = 5.3cm τ / ps Example: the width of a transition from a state with a lifetime of 5.0 ps is 1.1 cm (4.8) E f Why lifetimes? Uncertainty principle (Heisenberg) - collisional deactivation (intermolecules or with the walls) - spontaneous emission

8 Absorption coefficient of a transition Maximum value of the molar absorption coefficient, ε max represents a measure of the intensity of a transition. Absorption bands Integrated absorption coeficient, A : summ of the absorption coeficients over the entire spectral band. A = band ε ~ d ~ ( ν ) ν (4.9) Integrated absorption coefficient: area under the plot of the molar absorption coefficient against ~ ν of the incident radiation. A is proportional to the heights of the lines if they have similar widths

9 4.1.3 Electromagnetic energy domain Frequency: ν Wavelength: λ = c/ν Energy: ΔE = h ν

10 4.2 Schema of a spectrometer Spectrometer: an instrument that detects the characteristics of light which is emmited, absorbed or scattered by atoms or molecules Absorption spectrometer I Ox axis λ Oy axis Sourse Monochromator λ I 0 Sample c I Detector d

11 Absorption Spectrometer Source: - Far IR (35 cm -1 < <200 cm -1 ): mercury arc inside a quartz envelope - Vis: tungsten iodine lamp (white light) - Near UV: deuterium lamp - MW: klistron ν ~ If the source is emmiting only one frequency, the monocromator in not needed, for example in the case of a klistron. Monocromator can be replaced by an interferometer, if necessary. Detectors: UV-Vis - photodiode (photoelectric effect) (Uv- VIS) - CCD camera (charged couple device) - 2D array of photodiodes - photovoltaic device of Hg-Cd-Te (IR) - Detector sensitive to the phase (EPR)

12 4.2.1 Aims of spectroscopy 1. Qualitative analysis of a new substance/mixture (to identify the molecular species present) 2. Quantitative determination of a known substance (Beer-Lamber law) 3. Structural characterisation of a substance (from the energy levels and various transitions > establish the physical and chemical properties of the substance).

13 Molecular spectroscopy ν ~ ν hν λ

14 Types of spectroscopy λ Domain Spectroscopy 10-8 m bis m X-ray Electron spectroscopy 10-6 m bis 10-8 m UV-VIS Electron spectroscopy 10-4 m bis 10-6 m IR Vibration spectroscopy 10-2 m bis 10-4 m MW Rotation spectroscopy 10-2 m bis 1m Magnetic resonance - EPR >1m Radio ν Magnetic resonance - NMR

15 4.2.2 Physical background for transitions X-ray Electron spectroscopy Internal electrons are excited, and after the rearrangement of the electronic configuration, photons are emitted. UV-VIS Electron spectroscopy Electrons of valence are excited (possible ionization), and atoms will emit photons with the difference in energy between the two states where the transition took place. IR Vibration spectroscopy Oscillations of atoms in a molecule modify its energy levels between which transitions occur > vibration modes MW Rotation spectroscopy Molecules as rigid rotors have a rotation movement associated with various rotational energy levels, between which transitions occur (for gas phase molecules). MW EPR The spins of unpaired electrons of a paramagnetic substance placed in a magnetic field change their orientation when they are irradiated with microwave energy equal to the difference between two energy levels of the system. Radio ν NMR The spins of the nuclei placed in a magnetic field change their orientation when they are irradiated with a energy which corresponds to the difference between two energy levels.

16 4.3 Quantification of the mouvement a particle in a box, formula 3.32, and (K3-12 Huber course) Energy levels structure: E = h2 8mL 2 n2 (4.10) m - particle mass (electron, atom, molecule) n - principal quantum number = 1,2,... h Planck constant L dimension of the box where de Broglie wave of the particle is confined. Example For: n =1 > n =2 ΔE = 3h 2 8mL 2 (4.11) λ = 8mL2 c 3h ν = 1 λ = 3h 8mL 2 c (4.12) (4.13) ν = 3h 8mL 2

17 Mouvement > spectroscopy Using 4.12 and 4.13 for specific values of particle mass and box lenght: Mouvement Domain λ [m] ν ~ [cm -1 ] Observations Electron excitation UV-VIS m el = 0.001/1800 kg /N A ; L (molecule) = 200 pm Vibration IR Red. Weight: 0.01Kg/N A ; L (10%bondlenght) =15pm Rotation MW M W =0.1Kg/N A ; L (molecule) = 200pm Rotation Translation continuum x10-18 M W =0.1Kg/N A ; L (box lenght) =0.1m Vibration Electronic Groundstate Rotation energy levels Vibration energy levels Electronic groundstate energy To calculate the energies for these experimental values!!! Schema is not at scale concerning various energy levels for different types of movement.

18 4.3.1 Selection rules For a transition involving the absorption of a photon (IR, optical etc.) there must be a change in dipole moment on going from the ground (i) to the excited state (f): Transition probability: μ d τ Ψ i Ψ f (4.14) μ - dipol moment operator Ψ i/f wavefunctions Allowed transition: Forbidden transition: Ψ Ψ μ Ψ d τ 0 i f i μ Ψ f d τ = 0 (4.15) (4.16) The integral is nonzero only if the integrand is unchanged by any operation of symmetry of the molecule.

19 Selection rules The integral is non-zero only when the overall product ψ 1 µψ 2 has g symmetry. Conditions: - µ transforms like the functions x, y and z, (u symmetry for a centrosymmetric species). - Ψ1 and ψ2 differ or not in symmetry, and can be: centrosymmetric (g) or anticentrosymmetric (u). Laporte selection rules (for UV-Vis spectra): For centrosymmetric species (free ions and octahedral complexes): u g and g u transitions are allowed g g and u u transitions are forbidden

20 4.4 UV-Vis spectroscopy Ground state of the molecule: a given configuration of nuclei of the atoms forming the molecular structure. hν Excitated state of the molecule: an electon has migrated in an other molecule region > rearrangement of the nuclei configuration by vibration. Vibrational structure of an electronic transition

21 Electronic transitions Vibrational structure of an electronic transition: - in solutions > broad bands - in gas phase > resolved vibrational structure. A λ/nm Absorption spectrum of I 2 molecule in gas phase. Absorption spectra of chlorophylls a and b, the main pigments in the visible region of the electromangnetic spectrum.

22 4.4.1 Franck-Condon principle Franck-Condon principle explains the vibrational structure of an electronic band by considering that the electronic transition takes place faster than the change of the nuclear configuration in the excited state of the molecule. Predict the most likely vibrational state (in the excited electronic state). All other vibrational states in the excited state of the molecule are less probable (less intense): Absorption band

23 4.4.2 Electronic transitions Chromophore: molecular groups with characteristic optical absorptions > Presence of a chromophore induces the color of many substances. Electronic transitions: n-to-π* transitions transition form a non-bonding orbital to an antibonding π* orbital (energy = 4eV > λ = 290nm) π-to-π* transition electron from a π orbital moves to a π* orbital (energy = 7eV > λ = 180nm for unconjugate double bonds, and higher λ for conjugate double bonds ) d-d transitions in metal complexes where d orbitals of the metal ar involved. CT transitions transfer of an electron from ligend to the metal d orbitals or inverse (LMCT or MLCT)

24 Examples of chromophores π to π* transition for carboncarbon double bond (excitation of an electron from a π orbital to an antibonding π* orbital). n to π* transition for carbonyl groups (excitation of a nonbonding O lone-pair electron to an antibonding CO π* orbital).

25 4.4.3 Transition intensity: experimental CT bands are often very strong: ε ~ L mol -1 cm -1 d-d transitions are intermediate in intensity:» centrosymmetric molecules: ε ~ L mol -1 cm -1» non-centrosymmetric molecules: ε ~ 250 L mol -1 cm -1 n-to-π* transitions are usually weak, for example in carbonyls they are symmetry forbidden. Selection rules: ΔS = 0 the intensity of a transition depends on the symmetry of the states involved (Laporte selection rules).

26 Transition intensity- example d-d transitions in octahedral complexes are formally forbidden as they are g g. But, d-d transitions in octahedral complexes appear, even if they are weak. Explanation: Complexes undergo vibrational motion that can distort the complex so as to remove its center of symmetry. Coupling between these vibration modes and the electronic transition leads to the transitions being weakly allowed (essentially we excite the complex to a final state that is both electronically excited and vibrationally excited in such as way that the vibration removes the center of symmetry). This interaction is called vibronic coupling that leads to the rather broad nature of most d-d transitions.

27 Transition intensity- examples Transition ε Complexes, CX Spin forbidden Laporte forbidden CX, as [Mn(OH 2 ) 6 ] 2+ Spin allowed Laporte forbidden Cx, as [Ni(OH2)6]2+ square planar Cx, as [PdCl4]2-6-coordinate Cx of low symm. Spin allowed Laporte allowed MLCT bands in Cx with unsaturare ligands non-centrosymm. CX with P atoms CT in organic compounds

28 4.4.4 Radiative and non-radiative decay Excitation energy of a molecule is degraded by: - termal motion of the surrounding - radiative decay (electronic transition to a lower-energy orbital and emission of a photon) -Dissociation (fragmentation of the molecule) Radiative decay: 1. Fluorescence: spontaneously emmited radiation which ceases very soon after the exciting radiation is extinguish 2. Phosphorescence: spontaneously emmited radiation which may persist long periodes after the exciting radiation is extinguish (energy storage in a reservoir from which is slowly released).

29 The excited molecule: Fluorescence radiation - is subject to collisions with the surrounding molecules and gives up energy by decreasing the vibrational levels (vibrational relaxation). - the rest of energy is released by emission of a photon > fluorescence occur at a lower frequency that of the incident radiation! Fluorescence microscopy Fluorescence correlation spectroscopy CLSM micrograph of convolaria