BS4103 Fundamentals of Analytical Chemistry Week 3 Infrared Spectroscopy PDF

BS4103 Fundamentals of Analytical Chemistry Week 3 Infrared Spectroscopy Dr Kejing Shi [email protected]  Learning Outcomes ❑ To have an overview of the IR spectroscopy ❑ To underst...

BS4103 Fundamentals of Analytical Chemistry Week 3 Infrared Spectroscopy Dr Kejing Shi [email protected]  Learning Outcomes ❑ To have an overview of the IR spectroscopy ❑ To understand the Hooke’s Law ❑ To be able to describe various vibrational modes ❑ To be able to interpret IR spectrum ❑ To be familiar with the IR spectrometer  Infrared (IR) Spectroscopy Infrared (IR) spectroscopy is a chemical sample (a solid, liquid, or gas) is exposed to radiation in the IR region of the electromagnetic spectrum. Applications: ❑identify functional groups in a molecule ❑confirm the structure of a compound ❑measure concentrations ❑measure vibrational frequencies and force constant of chemical bonds within a compound  Molecular Vibration Atoms in a molecule do not maintain fixed positions with respect to each other but vibrate back and forth about an average corresponding to the interatomic distance. A spring is a good model for the vibration of a chemical bond. Molecular rotation involves the motion of the whole molecule, Molecular vibration involves motion of the atoms in individual chemical bonds  Hooke’s Law The only vibration possible is stretching/compression, called oscillation of the bond. As the bond vibrates, the spring stretches and contracts around its equilibrium position, which corresponds to the equilibrium bond length of the molecule, r0. 𝐹 = −𝑘𝑥 F is the restoring force, x is the deformation of the spring (e.g. extension), k is the spring constant, a characteristic of the particular bond The force is proportional to the extension  Hooke’s Law What factors influence the frequency of vibration of a chemical bond? 1 𝑘 1/2 ν= ( ) 2𝜋 𝑚 Bond stretching frequencies depend on: ν is the frequency of vibration, Bond Strength – the frequency absorbed is directly m is the mass of atoms, proportional to the bond stiffness (k) k is the spring constant Atomic weight – the frequency absorbed is inversely proportional to the atomic weight (m)  IR Active For a molecule to be IR active, there must be a change in dipole moment as a result of the vibration that occurs when IR radiation is absorbed. Dipole moment is a vector quantity and depends on the orientation of the molecule and the photon electric vector. the symmetric stretch of carbon dioxide is not infrared active whilst asymmetric stretch and bending are infrared active  Selection Rule The gross selection rule for infrared spectroscopy is that: there must be a change in the dipole moment of the molecule during the vibration if it is to give rise to a peak in the spectrum. For diatomic molecules this means that the molecule must have a permanent dipole moment. Homonuclear diatomics, such as H2, do not have dipole moments and so do not give IR spectra.  Vibrational Mode The vibrations of bonds between atoms in a molecule are detected and used to identify the functional groups that are present in a molecule. Each of the possible vibrations is known as a vibrational mode. In a diatomic molecule, the only possible vibration is the bond stretch/compression. Bending Stretching Mainly two types ( scissoring, (symmetrical and of vibrations asymmetrical) rocking, wagging and twisting)  Numbers of Vibrational Mode As more atoms are introduced, several other types of vibration become possible. The bond angles can change due to bending, and symmetric and asymmetric stretches are possible. For a molecule containing N atoms the number of vibrational modes is: linear molecule: (3N − 5) vibrational modes, non-linear molecule: (3N − 6) vibrational modes.  Two types of stretching vibrations : With each vibration, atoms first approach and then move away from one another. The potential energy of such a system at any instant depends on the extent to which the spring is stretched or compressed. Four types of bending vibrations : The energies associated with these vibrational states usually differ from one another and from the energies associated with stretching vibrations. the plus sign indicates motion out of the page; the minus sign indicates motion into the page.  Vibrations of a methylene group (–CH2–) 1. Symmetrical stretching 2. Asymmetrical stretching 3. Scissoring (Bending) 4. Rocking 5. Wagging 6. Twisting Within the CH2 group, commonly found in organic compounds, the two low mass hydrogens can vibrate in six different ways Characteristic Group Wavenumbers Double bonds are stiffer than single bonds and require more force to cause them to stretch or bend. As a result, they vibrate faster and hence at higher wavenumber (higher energy). IR Spectrum IR spectrum is a plot of transmission of radiation versus the wavenumber (ν ҧ ) of the radiation transmitted (typically between 4000 cm-1 -1 and 600 cm )  Wavenumber Wavenumber or wave number is the spatial frequency of a wave, measured in cycles per unit distance (ordinary wavenumber) or radians per unit distance (angular wavenumber) c = velocity v = frequency Pay attention to the unit: Wavelength in micrometer (μm), Wavenumber in inverse centimeter (cm-1)  Wavenumber of Absorption Band For a given bond, the stretching vibration gives rise to an absorption band at a higher wavenumber than a bending vibration, because it requires more energy to stretch a bond than to bend it. For example, in the IR spectrum of ethanol, the stretching vibration of a C–H bond is at a higher wavenumber than the bending vibration. Normally, short strong bonds vibrate at a higher wavenumber than longer weaker bonds because more energy is required to vibrate a short strong bond. For example, the absorption band due to the stretching vibration of a C=C bond is at a higher wavenumber than that for a C–C bond.  Wavenumber of Absorption Band Bonds involving atoms with low mass vibrate at a higher wavenumber than those involving atoms with higher mass (as predicted by Hooke’s law). For example, the absorption band due to the stretching vibration of a C–H bond (~ 3000 −1 cm ) is at a higher wavenumber than that for a C–O bond (~ 1150 −1 cm ). The wavenumber of an absorption band depends on: the type of bond vibration, the bond enthalpy, the masses of the atoms in the bond  Wavenumber of Absorption Band From the wavenumber of the absorption band, you can identify the type of bond in the molecule that undergoes the stretching or bending vibration. In the IR spectrum of ethanol, absorption bands due to the stretching vibrations of O–H and C–H bonds have much higher wavenumbers than for a C–O bond. Not only do the O–H and C–H bonds contain a low mass hydrogen atom, but they are also stronger than a C–O bond.  Characteristic Group Wavenumbers more difficult to identify vibrations of individual bonds in fingerprint region than it is for the functional group region because of complicated series of absorption bonds Distinctive bands are called characteristic group wavenumbers. They are due to stretching vibrations and provide useful structural information— they are normally found in functional -1 group region (between 4000 cm and 1400 cm-1) in the IR spectrum. -1 The area between 1400 cm and 600 cm-1 in the IR spectrum is called the fingerprint region. There are certain groups of atoms within a molecule that give rise to absorption bands at the same or similar wavenumber, irrespective of the rest of the structure.  Assigning an IR Spectrum The most useful information obtained from an IR spectrum is what functional groups are present within the molecule. When analysing a spectrum avoid the temptation to try and assign every absorption band. Look for the most characteristic absorption bands using the following sequence of steps. 1. Use summary table identify functional groups present in molecules 2. Often, the molecular structure can be identified from an IR spectrum. Focus on the functional group region (above 1500 cm −1 ) a. 1600-1850 cm −1 : check for double bonds b. 2100-2300 cm −1 : check for triple bonds c. 2700-400 cm −1 : check for X-H bonds d. Analyse wavenumber, intensity, and shape for each signal 3. When listing IR data, presented as a list of peaks in decreasing order of magnitude e.g. νmax 3600-3150, 2980, 2920, 1563 … cm −1  Assigning an IR Spectrum 1. Check whether there is an intense absorption band at around 3000 cm−1. This band indicates the presence of a C–H bond so it is usually found in the spectra of organic compounds. 2. Look for an intense, broad absorption band between 3600 −1 cm and 2500 −1 cm , which indicates the presence of a hydrogen-bonded O–H or N–H bond. The O–H bond indicates an alcohol or a carboxylic acid (RCO2H). The N–H bond indicates a primary or secondary amine (RNH2 or R2NH) or a primary or secondary amide (RCONH2 or RCONHR). 3. Look for an intense absorption band between 1820 cm −1 and 1660 −1 cm , which indicates the presence of a C=O bond. If a C=O bond is present, the wavenumber of the absorption band can be used to assign the specific functional group.  Assigning An IR Spectrum 4. Look for medium intensity absorption bands around 2150 cm −1 or 1650 −1 cm , which indicate the presence of a C≡C or a C=C bond, respectively. 5. Look for 2 or 3 medium to weak intensity absorption bands between 1600 cm −1 and 1500 cm −1, which indicate the presence of a benzene ring. 6. The absence of an absorption band can be as useful as the presence of an absorption band, in assigning a structure. If no absorption bands are identified from steps 2–5, then the compound may be an alkane (RH), an ether (ROR), or a halogenoalkane (RX). IR Window  Example 1: Characteristic bands of the organic carbon-hydrogen methylene CH2 bond Also included in the spectrum is the band for the carbonyl functional group C=O stretch at 1750 cm−1 for ketones, an important spectral range area associated with the carbonyl group −1 five bands listed in the figure ranging from 1165 to 2850 cm (C–H bend).  Example 2: Assigned IR spectrum of aspirin in a Nujol mull A Nujol mull is a finely divided suspension of a solid sample (such as aspirin) in a hydrocarbon oil, called Nujol. Two intense C=O absorption bands: a band at 1750 cm −1 due to the C=O bond in the ester group a band at 1690 cm −1 due to the C=O bond in the carboxylic acid group When analysing an IR spectrum of a Nujol mull, Nujol gives absorption bands around 2950 - 2800 cm−1 (C–H stretch) and 1470 - 1350 cm −1 (C–H bend). Example 3: IR spectrum of the organic compound vanillin a band at ~3500 cm −1 for the CH3 CH hydroxyl OH group aromatic C–H bands: 3000-3200 cm −1 CH3 bands: 2700-2850 cm−1 OH aromatic aldehyde: 1700 cm −1 -CHO aromatic ether: 1220-1260 cm −1 -O-  IR Spectrometer Samples are usually prepared in solution, as a solid, or nujol mull. Thank you for listening Any questions?