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 Gamma Rays UV IR Radio Microwave 10- 11 10- 9 10- 7 10- 5 10- 3 10- 1 10 3 10 Wavelength (cm) School of Chemistry

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 DE Before Absorption Energy Energy School of Chemistry

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 After Energy Before School of Chemistry

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

Interpretation depends on the energy range investigated. 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. In the excitation the energy of the whole molecule increases. After DE Before Energy Absorption Energy School of Chemistry

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 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. Bonding Anti-bonding Energy Orbitals of Butadiene Again note that lowest energy transition may lie in visible. But we can also excite to higher orbitals with sufficiently energetic (UV) photons. School of Chemistry

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 trans-Crocetin Xanthopterin 16,17-DimethoxyViolanthrone School of Chemistry

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 ORANGE PURPLE BLUE School of Chemistry

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 HO O- HO O- -2H+ pink colourless School of Chemistry

Methyl orange red orange-yellow H+ School 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). Atoms may be considered as weights connected by springs. Each vibrational mode has its own resonant frequency. symmetric stretch asymmetric stretch bending School of Chemistry

Appropriate energy for this is infra-red 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’ Appropriate energy for this is infra-red symmetric stretch no dipole no dipole asymmetric stretch change in dipole - IR active bending change in dipole - IR active School of Chemistry

The IR spectrometer School of Chemistry

The symmetric stretch is not IR active CO2 IR spectra 2800 2400 2000 1600 1200 800 400 100 Transmittance /% The bigger the change in dipole, the more intense the absorption Stretching higher energy than bending Wavenumber /cm-1 The symmetric stretch is not IR active (no change in dipole) School of Chemistry

IR spectra of organic compounds More complex: 2000 4000 3000 1500 1000 Wavenumber/cm-1 500 Ethyl ethanoate (CH3COOCH2CH3) C=O bond C-O stretch School of Chemistry

But functional groups have characteristic frequencies 1000 Wavenumber / cm- 1 650 1500 2000 3000 4000 N H C O Cl (all types) School of Chemistry

Four regions in the spectrum: Wavenumber / cm-1 School of Chemistry

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. Identify two main functional groups on the spectrum. 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. 4000 3600 3200 2800 2400 2000 1900 1800 1700 1600 Wavenumber / cm-1 10 20 30 40 50 60 70 80 90 100 Transmittance / % C=C C=O? C=N? C-H CC CN? School of Chemistry

Combine this information with the following data to deduce its structure Mr 53 Likely structure: Cyanoethene C So, formula = C3H3N 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. 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 NMR Spectrum of Ethyl Acetate School of Chemistry

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 Connectivity can be deduced to be School of Chemistry

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 Data System Ion Source Mass Analyzer Ion Detector Vacuum Pumps School of Chemistry

Ion Generation School of Chemistry ~70 Volts Electron Collector (Trap) Neutral Molecules Positive Ions + Inlet Repeller _ _ + + + To Analyzer + + + + Electrons e- e- e- _ Filament Extraction Plate School of Chemistry

The magnetic field exerts a force on these fast-moving 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 Correct m/z ratio ion detected Ion Source Detector S N Electromagnet ion not detected m/z too small ion not detected m/z too large School of Chemistry

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-to-charge ratio) Mass Spectrum of Water 18 [H2O]+ Relative Abundance 17 [OH]+ 1 H+ O+ 16 Mass (mass-to-charge ratio) School of Chemistry

Examples Alcohols Pentan-3-ol School of Chemistry

An alcohol's molecular ion is small or non-existent 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. 59 m/z(parent ion) = 88 School of Chemistry

Alkanes Hexane 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). m/z(parent ion) = 86 15 43 71 57 29 School of Chemistry

Aromatics Naphthalene School of Chemistry

Molecular ion peaks are strong due to the stable structure.   100 80 60 40 20 mass / charge (m/z) relative abundance 128 120 140 m/z(parent ion) = 128 School of Chemistry

Esters Ethylethanoate School of Chemistry

Fragments appear due to bond cleavage next to C=O (alkoxy group loss, -OR) and hydrogen rearrangements. 100 80 60 40 20 mass / charge (m/z) relative abundance -OCH2CH3 -C2H3 43 45 88 61 H m/z(parent ion) = 88 School of Chemistry

Halo-organics Chloroethene 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 27 m/z(parent ion) = 62/64 64 62 35 37 100 80 60 40 20 30 50 H2C = CH-Cl 26 School of Chemistry

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