Characterization of Complex Chemical Systems PDF

Characterization of complex chemical systems Department of Part A Chemical Sciences ElectroMagnetic Radiation / Matter interactions Spectroscopy studies the interaction of...

Characterization of complex chemical systems Department of Part A Chemical Sciences ElectroMagnetic Radiation / Matter interactions Spectroscopy studies the interaction of matter with electromagnetic radiation. The interaction involves an exchange of energy between electromagnetic radiation and matter. In other words, electromagnetic radiation does work on matter, which after the interaction can have higher or lower energy. The principle of energy conservation imposes that the energy given up by the electromagnetic radiation must be equal to the energy absorbed by matter or vice versa. In order to describe it, we must first define the properties of both. 1  ElectroMagnetic Radiation The electromagnetic (em) radiation is energy that propagates in time and space. The primary source of electromagnetic radiation is the sun. Electromagnetic radiation exchanges energy with charges (electrons and nuclei) that are present in matter. From a classical point of view (wave description) it is described as the combination of oscillating electric and a magnetic fields that propagate in time and space. Light is the portion of em radiation that is detected by the human eye. 2  Electromagnetic radiation Propagation in space and time: The wave moves periodically in space : the amplitude of the electric/magnetic field of the radiation changes periodically and returns to its initial value after a path of length λ: wavelength, having dimensions of length (1 nm=1x10-9 m). The wave moves also in time: the amplitude of the electric/magnetic field of the radiation changes periodically and returns to its initial value after a time t equal to T: period, having   æ 2p x 2pt ö E ( x, t ) = E0 cosç - ÷ dimension of s. è l T ø   æ 2p x 2pt ö B( x, t ) = B0 cosç - ÷ è l T ø 3 In vacuum, the ratio of the wavelength and the period is a constant called “speed of light” in vacuum c λ /T = speed =c Other quantities used to characterize the periodicity of the wave are: frequency [Hz = s-1] n =1 T Wavenumber [cm-1] n =n c = 1 l Example: in the visible a monochromatic radiation of λ=500 nm T = λ c = 500 ⋅10 −9 3⋅108 = 1, 67 ⋅10 −15 s ν = 1 T =6, 0 ⋅1014 Hz ν = 1 λ = 1 500 ⋅10 −9 = 2 ⋅10 6 m −1 = 2 ⋅10 4 cm −1 The energy density (intensity) carried by the em radiation is proportional to the maximum amplitudes E0 and B0:     E0  2 I µ E0 ´ B0 , B0 = Þ I µ E0 c 4  Particle description The em radiation is formed by particles called PHOTONS ü Each photon has and energy Ephoton = hν, ü h = Planck constant = 6.626 × 10-34 J s, and ν is the oscillation frequency of the radiation. ü Photons have mass equal to zero ü The number of photons carried by the em radiation depends by its amplitude. Example: The photons emitted in 1 s by a sodium lamp (λ = 550 nm) of 0.1 W (1W =1 Js-1) are: nphotons=Power×time/Ephoton= 0.1 × 1 / 3.61 ×10-19 = 2.77 ×1017 NB: The energy of a single photon is so small that the energies emitted by lamps in everyday life can be varied continuously. 5 Energy, Frequency and Wavelength 6  Molecules: Molecules are formed by atoms, which in turn are formed by a positively charged nucleus and electrons. Electrons have a finite probability of being in a certain region of space around the nucleus, and valence electrons are often engaged in chemical bonds, that is, they are in molecular orbitals shared by multiple nuclei. The energies possessed by molecules are quantized, that is, they can take on only discrete and not continuous values. Atomic orbitals 7  The energy levels of molecules are distinguished in: Electronic states: energy levels associated to the kinetic and potential energies of the electrons; Nuclear vibrational states: energy levels associated with the displacements of the nuclei around their equilibrium positions in the molecule. Rotational states: energy levels associated with the rotational motion of the whole molecules considered as a single body. Spin states: energy levels associated with the spin moments of electrons and some nuclei of the molecule. The energy of the molecule can be approximated to a sum of these single energies ETOT≈ Eel+Evib+Erot+Espin with Eel > Evib> Erot > Espin 8  Molecular energy levels v’=1 Energia v’=0 e J=2 J=1 J=0 v=1 J=2 v=1 J=1 v=0 J=0 g v=0 Electronic transitions: ~ 4×10−19 J , Vibrational transitons: ~ 1×10−20 J 1eV= 1.602x10−19 J Rotational transitions: ~ 1×10−23 J = 8065 cm-1 Spin transitions: ~ 4 x10−25 J 9 General description of spectroscopy The interaction between an em radiation and matter can generate various phenomena: absorption of radiation, emission of radiation, scattering of radiation (scattering), photoionization, photochemical reactions, … We will consider only three types of phenomena: UV-VIS Absorption/Emission IR Absorption Raman scattering 10  Jabłoński diagram the “all-in-one” diagram for spectroscopists (ISC is also a path, but not considered here) Aleksander Jabłoński “The emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity.” [M. Kasha] M. Kasha, Discuss. Faraday Soc., 1950, 9, 14–19 Michael Kasha 11 Valle, Catalan, Phys. Chem. Chem. Phys., 2019,21, 10061-10069  Selection Rules To have energy exchange between the em wave and the molecules it is necessary to have: Resonance: the energy of the Selection: the distribution of one photon of the em wave must the charges in the molecule, in be equal to the energy difference the transition between the two between two energy levels of the energy levels, must allow an molecule energy exchange with the em wave (e.g. form an electric E2 dipole). It follows that not all the molecular transitions will give rise to em wave hνem = E2-E1 absorption E1 As a result, absorption occurs only at discrete frequencies (wavelengths) of the em wave which satisfy the previous criteria. 12  Interaction with photons ofSpettroscopia di frequency which satisfy the an em wave having 22. Spettroscopia UV-VIS (hnrem = DE) allow resonance condition:Assorbimento ( elettronica Elettronico exciting the molecule )form the lowest electronic state (only the lowest molecular orbitals are occupied by electrons) to a higher energy states (one electron moves to a higher energy molecular Spettri UV-VIS prodotti da transizioni tra orbitali molecolari degli orbital) elettroni. Le transizioni che richiedono minore energia coinvolgono gli The lowest orbitali energy chiamati HOMOtransition e LUMO:involves the molecular orbitals called HOMO and LUMO: HOMO = highest occupied molecular orbital HOMO = highest occupied molecular orbital LUMO = lowest unoccupied molecular orbital LUMO = lowest unoccupied molecular orbital Energia The absorption of the photon energy moves Assorbimento di unthe LUMO hν ν electron from the HOMO fotone UV-VIS da to the LUMO. energy parte di una molecola: promozione di un HOMO elettrone dall’ HOMO hν rem = EL − EH al LUMO. 1  Stati Gli di Singoletto elettroni possonoeavere di Tripletto spin +1/2 (α) o spin -1/2 (β). In un orbitale (atomico o molecolare) ci possono essere al massimo due elettroni con Gli elettroni Singlet and spin Triplet possono states opposto. and Per avere selection due spinsu elettroni +1/2 due(α) rules o spindiversi orbitali -1/2 (β). In un si può orbitale avere: (atomico o molecolare) ci possono essere al massimo due elettroni con Electrons canspin haveopposto. Per due spin +1/2 elettroni (α) or su due spin -1/2 (β).orbitali diversi si può In a molecular avere: orbital there can be a maximum of two electrons with opposite spin (Pauli exclusion principle). If the two electronsStati reside in two different di Singoletto S: orbitals (excited state) there can be LUMO i due elettroni più esterni hanno”spin Stati di Singoletto 1) Singlet electronic state antiparallelo” (S): S: quindi the twototale lo spin electrons have è nullo. HOMO LUMO i due elettroni più esterni hanno”spin opposite spin and the total spin momentum is 0. antiparallelo” quindi lo spin totale è nullo. HOMO 2) Triplet electronic state (T):T:the Stati di Tripletto two elettroni i due electronspiù have LUMO the same spinesterni and the total ”spin hanno spin momentum is 1. lo parallelo” quindi HOMO Statitotale spin di Tripletto T:1.i due elettroni più è pari ad LUMO Multiplicity àesterni 2 S+1hanno ”spinspin (S = total parallelo” angularquindi lo momentum quantum HOMO number) spin totale è pari ad 1. Absorption of photons (em radiation) is impossible for a transition from a singlet 2 electronic state (S) to a triplet electronic state (T), because it can not change the 2 spin momentum. Selection rule for electronic states: S0 → Sn The ground electronic state for many organic molecules is a singlet state S0. Absorption of em radiation will then occur only from the ground singlet state S0 to excited singlet states Sn. Calculate the multiplicity of these molecules O2 N2 Shape of the electronic absorption bands The broad range of the absorption band of an electronic transition is due to the presence of the vibrational energy level associated with each electronic state. v2 B v1 E1 2 v0 Assorbanza 1 0 20000 25000 30000 35000 v2 E0 -1 n (cm ) v1 v0 Anthracene in ethanol 16 Example: Absorption spectra of the pigments responsible for light-harvesting in photosynthesis. 17 UV-Visible Absorption Spectra UV-Visible spectroscopy is an absorption spectroscopy that uses UV- Visible light in the wavelength range 200-800 nm, where the electronic transition bands for most molecules fall. Nanometers (nm = 1x10-9 m) are used as the unit of measurement to characterize the em wave: With current spectrophotometers, this range can be extended to 1800 nm. In a UV-Visible absorption spectrum one measures the absorbance of the sample vs. the wavelength of the incident radiation.  Absorption Spectra Absorption spectra are experiments in which the transmittance (T) or the absorbance (A) are measured as a function of the wavelength (l) / frequency (n) of the em radiation. They produce a graph in which T or A are plotted against l or n. 19 Absorption Absorption involves transfers of energy from the em radiation to matter. From a macroscopic point of view, absorption is measured by recording the intensity (energy per unit area per unit time) of the em radiation impinging on the sample and the intensity of the em radiation which has passed through (transmitted by) the sample. The Transmittance T is defined as the ratio of the impinging light intensity (I0) and the intensity of the transmitted light (I(L)). L L= length of the path done by the radiation in the sample 20 Absorption in a solution (microscopic point of view): Under the assumption of weak interaction between em radiation and the solute molecule, the change in light intensity, crossing the sample in the x direction, is linearly proportional to the intensity itself. The proportionality constant is given by the product of a coefficient a, characteristic of the absorbing molecule, with the concentration C of the molecule itself : L There is an exponential decrease of the light intensity along space, and L is the length of the light path in the absorbing solution. 21 Absorbance For practical reasons, the intensity decrease of the light inside the sample is described by the Absorbance: the decimal logarithm of the ratio of the incident light with the transmitted light. : Lambert-Beer Law: The absorbance is proportional with the concentration C of the solute molecule that absorbs and the path length L. The proportionality constant ε is called extinction coefficient. A = εCL A is dimensionless, L is usually measured in cm, and the concentration C in molarity and is a function only of the chemical characteristic of the absorbing species. The extinction coefficient e is a specific property of the chemical substance which absorbs and varies with the λ of the e.m. wave. Its unit of measurements is cm-1mol-1 22 Determination of CuSO4 concentration in solution 125 mg, 250 mg, 500 mg e 750 mg of CuSO4 are dissolved in 25 ml of water solution at pH=4.0. The absorption spectra between 400nm and 800nm are: 750 mg 500 mg 250 mg 125mg Assorbanza nm From the absorption spectra, choosing a specific wavelength, at which the substance absorbs, it is possible to make a graph in which the absorbance at that wavelength is plotted against the substance concentration The data will show a linear dependence from concentration (Lambert Beer law) and from the slope it is possible to compute the extinction coefficient ε(λ), since L is known. A(l ) = e (l )CL Assorbanza 750 nm Concentrazione normalizzata Cell viability test The Trypan Blue (TB) assay is used to assess cell viability,. In this test, a solution of Trypan Blue (TB) dye is injected into the cell culture. If the cells are dead/moribund, the TB penetrates the cell membrane and stains the cells dark blue. In contrast, if the cell membrane is intact the TB is not internalized. HUVEC cells treated with TB HUVEC cells exposed to Control HUVEC cells (not exposed toxicants to toxicants) Changes do to metal complexation From UV-Visible measurements, the equilibrium constant of the complexation reaction can be obtained. If the reaction occurs slowly (seconds to minutes), the reaction kinetics can also be followed. Same behaviour for pH indicators. Dalton Trans.,(2009) Vol. 24, p. 4735 Information? From the UV-Visibile absorption spectrum it is possible to gain information on: Lambert-Beer law: Lambert-Beer law: determination of the concentration of specific molecules in the sample Molecular recognition (not much used) Changes of the chemical structure of the molecule or of the surrounding of the molecule