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Spectroscopy Workshop School of Chemistry The Queen’s University of Belfast School of Chemistry Workshop Content Spectroscopy overview Ultra-violet/visible (UV-vis) Infra-Red (IR) Nuclear Magnetic Resonance (NMR) Mass Spectrometry School of Chemistry Spectroscopy In spectroscopy, transitions between different energy levels within atoms and molecules are recorded and then used to give information on chemical structure. School of Chemistry The range of energies that can be used for spectroscopy is very large and spans a large proportion of the electromagnetic spectrum. Visible X-Rays UV Gamma Rays 10- 11 10- 9 Microwave IR Radio 10- 7 10- 5 10- 3 Wavelength (cm) School of Chemistry 10- 1 10 10 3 Energy Energy In a typical experiment, the molecules or atoms start at lower energy and go to a higher energy level upon absorption of radiation of appropriate wavelength. After School of Chemistry Before DE Absorption can only occur when the energy of the radiation (calculated from the frequency or wavelength) matches the energy gap. If there are several different upper levels (and there usually are) then several transitions will be observed. After After Energy After School of Chemistry Before For current purposes we look only at: UV/visible ( highest energy) Infra red (intermediate) Radio frequency (lowest energy). But in all cases : School of Chemistry To record a spectrum, sweep through the appropriate range of energies and look for absorption at particular values. School of Chemistry Absorption gives peaks, when these have been measured this gives the energy gaps within the sample. These can then be related to structure. Interpretation depends on the energy range investigated. School of Chemistry UV/visible Spectroscopy Chemical compounds are coloured because they absorb visible light. In general, even organic compounds that are colourless will absorb UV light. School of Chemistry Absorption of visible light Where has the energy that was within the photons gone to ? School of Chemistry In UV/visible spectroscopy the energy of the absorbed photon is used is used to drive the molecule into an excited electronic state. Energy Energy In the excitation the energy of the whole molecule increases. After School of Chemistry Before DE This overall change is typically due to promotion of a single electron from a lower to higher energy orbital. The energy of the transition depends on the gap between the two orbitals. In organic compounds which have only single bonds between the atoms the excitation energy is very high- lies in deep UV. School of Chemistry Even if have a simple p bond, the excitation from highest occupied to lowest unoccupied orbitals still lies in the UV. This excitation gives a dramatic decrease in bond order due to excitation from a bonding to an anti-bonding orbital. e.g. ethene H H H H School of Chemistry If we have a highly conjugated molecule the energy separation between the orbitals is smaller. Excitation of the electron thus has a proportionately smaller effect and requires less energy- energy gap may lie in the visible region. School of Chemistry Again note that lowest energy transition may lie in visible. Anti-bonding Bonding Energy Orbitals of Butadiene H H H H School of Chemistry H H But we can also excite to higher orbitals with sufficiently energetic (UV) photons. With increasing conjugation, the decreasing energy gap is reflected by absorption at longer wavelengths. School of Chemistry The structures of many coloured compounds show they are very extensively conjugated. beta-Carotene O COOH HOOC H H N N trans-Crocetin H2N N O O School of Chemistry N Xanthopterin OMe OMe 16,17-DimethoxyViolanthrone O Substituents added to the compound may alter the energy of the orbitals which e- is excited from or to. Auxochromes: substituents that alter the wavelength or intensity of the absorption due to the chromophore O NH2 O NH2 PURPLE ORANGE O O O NHCH3 BLUE School of Chemistry O NHCH3 OH Changes in chemical composition can give rise to pronounced colour changes since this can dramatically alter the energies of the orbitals involved in the transitions e.g. pH indicators. Phenolphthalein O O O -2H+ O O OH HO School of Chemistry O- colourless pink Methyl orange H3C H3C N red N N SO3- H+ H3C H3C N H + orange-yellow N N SO3School of Chemistry Summary Absorption of UV-vis radiation occurs via excitation of electrons from filled to unfilled orbitals i.e. they are electronic transitions. Molecules have characteristic absorption spectra. The absorption can lead to coloured materials. pH Indicators use the change in colour between the acid and alkali forms of the molecules. School of Chemistry IR Spectroscopy Origin of the absorption The spectrometer The spectra Organic compounds Example problem School of Chemistry Origin of IR absorptions CO2 Atoms within a molecule are never still. They vibrate in a variety of ways (modes). symmetric stretch Atoms may be considered as weights connected by springs. asymmetric stretch Each vibrational mode has its own resonant frequency. School of Chemistry bending If the vibrational mode involves a change in molecular dipole moment, the vibration can be induced by absorption of a photon - it is ‘IR-active’ no dipole no dipole symmetric stretch asymmetric stretch change in dipole - IR active Appropriate energy for this is infra-red bending change in dipole - IR active School of Chemistry The IR spectrometer School of Chemistry CO2 IR spectra The bigger the change in dipole, the more intense the absorption Transmittance /% 100 0 2800 2400 2000 1600 1200 800 400 Wavenumber /cm-1 Stretching higher energy than bending School of Chemistry The symmetric stretch is not IR active (no change in dipole) IR spectra of organic compounds More complex: Ethyl ethanoate (CH3COOCH2CH3) C=O bond C-O stretch School of Chemistry Wavenumber/cm-1 4000 3000 2000 1500 1000 500 But functional groups have characteristic frequencies Wavenumber / cm- 1 4000 3000 C 2000 H C C O School of Chemistry H C N 1000 650 C C C H (all types) N 1500 C C O O Cl Four regions in the spectrum: 4000 3500 3000 2500 2000 O-H N-H C-H stretching C C C O C N N O stretching N-H bending C C C N X Y Z stretching School of Chemistry Wavenumber / cm-1 1500 1000 other stretching, bending and combination bands: fingerprint region Example problem Identify two main functional groups present in the compound which gave this spectrum Explain why infrared radiation is absorbed by molecule HCl but not by molecules H2 and Cl2. Explain what occurs in the HCl molecule when infrared radiation is absorbed. The simplified infrared spectrum below is that of an organic compound. Transmittance / % (a) (b) (c) 100 90 80 70 60 50 40 30 20 10 4000 3600 3200 2800 2400 2000 1900 1800 1700 1600 Wavenumber / cm-1 (i) Identify two main functional groups on the spectrum. (ii) This compound has composition by mass C, 67.9%; H, 5.7%; N, 26.4%, and Mr of 53. Suggest a structural formula for the compound. C-H School of Chemistry CC CN? C=C C=O? C=N? Combine this information with the following data to deduce its structure C H N Mr 67.9% 5.7% 26.4% 53 So, formula = C3H3N Likely structure: H H C N H Cyanoethene School of Chemistry Summary Absorption of IR can occur if a vibrational mode is associated with a change in dipole. Functional groups have characteristic absorption frequencies. In combination with other analytical data, the structure of an organic compound can often be deduced. School of Chemistry NMR Spectroscopy The Basis of NMR Spectroscopy The Spectrometer Chemical Shifts Signal Intensity and Integration Coupling Constants Example Spectra School of Chemistry The Basis of NMR Spectroscopy Atomic nuclei behave like small bar magnets as a result of their charge and spin. In the presence of an applied magnetic field the spin states have different energy and the magnetic moment can align with or against the applied field. School of Chemistry The difference in energy between the two spin states is dependent on the external magnetic field strength. Irradiation of a sample with radio frequency energy corresponding to the spin state separation (DE) will excite nuclei in the +½ state to the higher energy –½ state. School of Chemistry The 1H NMR Experiment For example, consider a water sample in a 2.3487 T external magnetic field irradiated by 100 MHz radiation. If the magnetic field is increase to 2.3488 T the water protons will at some point absorb rf energy (DE) and a resonance signal will appear, School of Chemistry The Chemical Shift Not all protons give resonance signals at the same field frequency. Electrons move in response to the applied field and generate a secondary magnetic field which opposes the applied field. The secondary field shields the nucleus from the applied field and nuclei in different environments resonate at different frequencies. The difference in resonance frequency is measured as a chemical shift, units d School of Chemistry Proton Chemical Shift Ranges School of Chemistry Signal Intensity The relative area of the absorption signals can provide valuable structural information. The area under a peak is proportional to the number of a given type of nuclei in the molecule. O H 3C CH3 H H MEK School of Chemistry The keto-enol equilibrium ratio of 2,4-pentandione can determined by 1H NMR spectroscopy School of Chemistry Spin-Spin Coupling The applied magnetic field experienced by a proton Ha will be modified by the local field produced by its neighbouring Hb Ha modifies the field at Hb by aligning with or against the applied field and and gives 2 resonant frequencies for Hb (doublet) Similarly Hb modifies the field at Ha in 3 different ways (triplet) School of Chemistry Splitting pattern can provide valuable structural information Chemically equivalent protons act as a group and a peak due to n adjacent protons is split into n+1 lines, with a coupling constant J School of Chemistry 1H School of Chemistry NMR Spectrum of Ethyl Acetate 1H NMR Spectrum of 1,3-Dichloropropane School of Chemistry Example Problem Given the formula, deduce what you can about the structure Integration corresponds to 2H : 2H : 3H A triplet must correspond to 2 near neighbour protons A sextet corresponds to 5 near neighbour protons Therefore CH2, CH2 and CH3 groups are present School of Chemistry Solution H Connectivity can be deduced to be H H H H School of Chemistry H H NO2 Summary NMR spectroscopy involves irradiating a sample with radio frequency radiation Protons in different chemical environments have different chemical shifts d Protons in different environments can couple to each other with a coupling constant J The combination of chemical shifts and coupling constants provides valuable structural information School of Chemistry Mass Spectrometry The basic principles Applications School of Chemistry What is a mass spectrometer ? A mass spectrometer is an instrument which produces charged particles (ions) from chemical substances under analysis. It then uses magnetic and/or electric fields to separate those ions and to measure their mass. School of Chemistry Mass Spectrometer Schematic Sample Introduction Data Output Inlet Ion Source School of Chemistry Data System Mass Analyzer Vacuum Pumps Ion Detector Ion Generation ~70 Volts Electron Collector (Trap) Neutral Molecules + Repeller + Electrons Inlet _ _ + ++ + + + e- e- e_ Filament School of Chemistry Positive Ions Extraction Plate To Analyzer The magnetic field exerts a force on these fastmoving ions and causes them to move in a circular path, the radius of which is dependent upon their mass to charge ratio (m/z) and speed. School of Chemistry Magnetic Mass Separation ion not detected m/z too small Correct m/z ratio ion detected S Ion Source Detector N Electromagnet School of Chemistry ion not detected m/z too large Applications Mass spectrometers are used for all kinds of chemical analyses: - Chemical analysis (Chemical Research) - Environmental analysis - Analysis of petroleum products - Trace metals - Biological materials School of Chemistry How is mass spectral information used? Let us use water (H2O) as an example. If a beam of electrons is directed through water vapour in the source of a mass spectrometer, some of the electrons will hit water molecules and knock off an electron, producing charged ions from the water: H2O + 1 (fast) electron [H2O]+ + 2 electrons School of Chemistry Electron impact on a water molecule Some of the collisions between water molecules and electrons will be so hard that the water molecules will be broken into fragments. For water, those fragments will be [OH]+, O+, and H+ with the following masses: 1 = H+ 16 = O+ 17 = [OH]+ 18 = [H2O]+ School of Chemistry Mass Spectrum of Water [H2O]+ 18 Relative Abundance 17 1 H+ O+ [OH]+ 16 Mass (mass-to-charge ratio) School of Chemistry Examples Alcohols Pentan-3-ol OH CH3CH2 CH2CH3 H School of Chemistry An alcohol's molecular ion is small or non-existent. Cleavage of the C-C bond next to the oxygen usually occurs. A loss of H2O may occur as in the spectra below. OH CH3CH2 59 CH2CH3 H m/z(parent ion) = 88 School of Chemistry Alkanes Hexane H3C C C C C CH3 H2 H2 H2 H2 School of Chemistry Molecular ion peaks are present, possibly with low intensity. The fragmentation pattern contains clusters of peaks 14 mass units apart (which represent loss of (CH2)n CH3). 43 15 School of Chemistry 71 m/z(parent ion) = 86 H3C C C C C CH3 H2 H2 H2 H2 29 57 Aromatics Naphthalene School of Chemistry relative abundance Molecular ion peaks are strong due to the stable structure. 128 100 80 60 40 20 0 0 20 40 60 80 100 120 140 mass / charge (m/z) m/z(parent ion) = 128 School of Chemistry Esters Ethylethanoate O H3C School of Chemistry O CH2CH3 relative abundance Fragments appear due to bond cleavage next to C=O (alkoxy group loss, -OR) and hydrogen rearrangements. 100 80 60 40 20 0 -OCH2CH3 43 -C2H3 45 0 61 88 20 40 60 80 mass / charge (m/z) 100 61 H O H3C School of Chemistry H O CH2CH3 43 45 m/z(parent ion) = 88 Halo-organics Chloroethene Cl H H H School of Chemistry Isotopes are shown by mass spectrometry The natural abundance of each isotope gives characteristic fragmentation e.g. 35Cl:37Cl is in a 3:1 ratio therefore the peaks containing Cl are in a 3:1 ratio and separated by 2 mass units 100 H2C = CH-Cl 80 H 62 60 H Cl 40 26 64 20 H 27 School of Chemistry 27 0 35 30 37 40 50 m/z(parent ion) = 62/64 60 Summary Mass spectrometry involves the ionisation of molecules and atoms. The mass spectrometer measures the mass to charge ratio. On ionisation the molecule can break up giving fragments of different m/z ratios . Each molecule has a characteristic fragmentation pattern which can be used to identify the molecule. School of Chemistry