IR & Raman Spectroscopy PDF

IR & Raman Spectroscopy Dr. Golfam Ghafourifar Winter 2025 CHEM341 1 Overview Infrared Spectroscopy Vibrational Transitions Quantum Treatment of Vibrations Group Frequency Region Infrared Instruments Fourier Transform In...

IR & Raman Spectroscopy Dr. Golfam Ghafourifar Winter 2025 CHEM341 1 Overview Infrared Spectroscopy Vibrational Transitions Quantum Treatment of Vibrations Group Frequency Region Infrared Instruments Fourier Transform Infrared Spectroscopy Raman Spectroscopy 2 Properties of Light 3 Infrared Spectroscopy The infrared region of the spectrum encompasses radiation with wavenumbers ranging from about 12,800 to 10 cm-1 or wavelengths from 0.78 to 1000 m. The infrared spectrum is divided into near-, mid-, and far-infrared radiation. 4  Infrared Spectroscopy Dipole Changes During Vibrations Infrared radiation is not energetic enough to bring about electronic transitions. Absorption of infrared radiation is thus confined largely to molecular species that have small energy differences between various vibrational and rotational states. In order to absorb infrared radiation, a molecule must undergo a net change in dipole moment as a consequence of its vibrational or rotational motion. 5  Vibrational Transitions Vibrational Transitions: Vibrational energy levels are quantized, and for most molecules the energy differences between quantum states correspond to the mid-infrared region. 6  Vibrational Transitions Number of Vibrational Modes: - for non-linear molecules, number of types of vibrations: 3N-6 - for linear molecules, number of types of vibrations: 3N-5 Examples: 1) HCl: 3(2)-5 = 1 mode 2) CO2: 3(3)-5 = 4 modes - + - moving in-out of plane See web site for 3D animations of vibrational modes for a variety of molecules 7 http://www.chem.purdue.edu/gchelp/vibs/co2.html  Vibrational Transitions - why so many peaks in IR spectra - observed vibration can be less then predicted because symmetry ( no change in dipole) energies of vibration are identical absorption intensity too low frequency beyond range of instrument 8  IR Active Vibrations - In order for molecule to absorb IR radiation: vibration at same frequency as in light but also, must have a change in its net dipole moment as a result of the vibration Examples: 1) CO2: 3(3)-5 = 4 modes  = 0; IR inactive d- 2d+ d-  > 0; IR active d- 2d+ d- - + -  > 0; IR active d- 2d+ d- degenerate –identical energy single IR peak 2d+  > 0; IR active d- d- 9 PtCl4 2- 10  Vibrational Frequency Vibrational Frequency: The natural frequency of the oscillation is 1 k m = 2 m m = natural frequency m = mass of the attached body k = force constant of the spring 11  Vibrational Frequency The equation may be modified to describe the behavior of a system consisting of two masses m1 and m2 connected by a spring. Here, it is only necessary to substitute the reduced mass  for the single mass m where m1m2 = m1 + m2 Thus, the vibrational frequency for such a system is given by ( µ is reduced mass (Kg)) 1 k 1 k ( m1 + m2 ) m = = 2  2 m1 + m2 12  Quantum Treatment of Vibrations h k  E = h m = 2  h k Eradiation = h =  E = h m = 2  The radiation in wavenumbers, 1 k − 12 k = = 5.3  10 2 c   13 Infrared Spectroscopy 14 15 Infrared Spectroscopy 16 Infrared Spectroscopy 17 Infrared Spectroscopy 18 Infrared Spectroscopy 19 Infrared Spectroscopy 20 Infrared Spectroscopy 21 Infrared Spectroscopy 22 Infrared Spectroscopy 23 Infrared Spectroscopy 24 Another Illustration of Molecular Vibrations 25 Group Frequency Region - approximate frequency of many functional groups (C=O,C=C,C-H,O-H) can be calculated from atomic masses & force constants - positions changes a little with neighboring atoms, but often in same general region - serves as a good initial guide to compound identity, but not positive proof. 26 Infrared Spectroscopy 27 Infrared Instruments An infrared spectrophotometer is an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. In infrared spectra the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample. Absorption of radiation lowers the percentage transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR spectra provide a considerable amount of structural data. 28  Instrumentation Basic Design - normal IR instrument similar to UV-vis - main differences are light source & detector 29  Instrumentation (Light Source) Light Source: - must produce IR radiation - can’t use glass since absorbs IR radiation - several possible types Zr, Ce, Th V Nernst Glower - rare earth metal oxides (Zr, Ce, Th) heated electrically - apply current to cylinder, has resistance to current flow generates heat (1200o – 2200o C). - causes light production similar to blackbody radiation - range of use ~ 670 – 10,000cm-1 - need good current control or overheats and damaged 30  Instrumentation (Light Source) Globar - similar to Nernst Glower but uses silicon carbide rod instead of rare earth oxides - similar usable range Incandescent Wire Source - tightly wound nichrome or rodium wire that is electrically heated - same principal as Nernst Glower - lower intensity then Nernst Glower or Globar, but longer lifetime 31  Instrumentation (Light Source) CO2 Laser - CO2 laser gas mixture consists of 70% He, 15% CO2, and 15% N2 - a voltage is placed across the gas, exciting N2 to lowest vibrational levels. - the excited N2 populate the asymmetric vibrational states in the CO2 through collisions. - infrared output of the laser is the result of transitions between rotational states of the CO2 molecule of the first asymmetric vibrational mode to rotational states of both the first symmetric stretch mode and the second bending mode - gives off band of ~ 100 cm-1’s in range of 900-1100 cm-1 - small range but can choose which band used & many compounds have IR absorbance in this region - much more intense than Blackbody sources Others - mercury arc (l > 50 m) (far IR) - tungsten lamp (4000 -12,800cm-1) (near IR) 32  Instrumentation (Detectors) Two main types in common IR instruments a) Thermal Detectors a.1.) Thermocouple - two pieces of dissimilar metals fused together at the ends - when heated, metals heat at different rates - potential difference is created between two metals that varies with their difference in temperature - usually made with blackened surface (to improve heat absorption) - placed in evacuated tube with window transparent to IR (not glass or quartz) IR “hits” and heats one of the two wires. h metal1 metal2 - + IR transparent material (NaCl) V 33  Instrumentation (Detectors) a.2.) Bolometer - strips of metal (Pt, Ni) or semiconductor that has a large change in resistance to current with temperature. - as light is absorbed by blackened surface, resistance increases and current decreases - very sensitive i h A 34  Instrumentation (Detectors) b) Photoconducting Detectors - thin film of semiconductor (ex. PbS) on a nonconducting glass surface and sealed in a vacuum. - absorption of light by semiconductor moves from non-conducting to conducting state - decrease in resistance → increase in current - range: 10,000 -333 cm-1 at room temperature h vacuum semiconductor glass Transparent to IR 35  Instrumentation (Detectors) c) Pyroelectric Detectors - pyroelectric (ceramic, lithium tantalate) material get polarized (separation of (+) and (-) charges) in presence of electric field. - temperature dependent polarization - measure degree of polarization related to temperature of crystal - fast response, good for FTIR 36  Instrumentation (Sample cell) Other Components Sample Cell - must be made of IR transparent material (KBr pellets or NaCl) Liquid Sample Holder NaCl plates 37  Instrumentation (monochromator) Other Components monochromator - reflective grating is common - can’t use glass prism, since absorbs IR 38  Overall Instrument Design -Monochromator after sample cell -Not done in UV-Vis since letting in all h to sample may cause photdegradation (too much energy) -IR lower energy -Advantage that allows monochromator to be used to screen out more background IR light Problems: Source weak , need long scans Detector response slow – rounded peaks 39 Fourier Transform Infrared Spectroscopy To observe all wavelengths at once and detect them simultaneously, an interferometer is used to produce an interference pattern that contains all of the IR data. Based on Michelson Interferometer 40 Principal: 1) light from source is split by central mirror into 2 beams of equal intensity 2) beams go to two other mirrors, reflected by central mirror, recombine and pass through sample to detector 3) two side mirrors; one fixed and other movable a) move second mirror, light in two-paths travel different distances before recombined b) constructive & destructive interference c) as mirror is moved, get a change in signal 41 https://www.youtube.com/watch?v=UA1qG7Fjc2A 42  Destructive Interference can be created when two waves from the same source travel different paths to get to a point. This may cause a difference in the phase between the two waves. If the paths differ by an integer multiple of a wavelength, the waves will also be in phase. If the waves differ by an odd multiple of half a wave then the waves will be 180 degrees out of phase and cancel out. 43 https://www.youtube.com/watch?v=j-u3IEgcTiQ 44 Advantages of FTIR compared to Normal IR: 1) much faster In a dispersive spectrometer, wavenumbers are observed sequentially, as the grating is scanned. In an FT-IR spectrometer, all the wavenumbers of light are observed at once. spectrum measured in 30 minutes on a dispersive spectrometer would be collected at equal S/N on an FT-IR spectrometer in 1 second, provided all other parameters are equal. 2) use signal averaging to increase signal-to-noise (S/N) 3) no slits, less optical equipment, higher light intensity FTIR instruments do not require slits (in the traditional sense) to achieve resolution. Therefore, much higher throughput with an FTIR can be achieved than with a dispersive instrument. This is called the Jacquinot Advantage. Disadvantages of FTIR compared to Normal IR: 1)requires collecting blank 2) can’t use thermal detectors – too slow 45  Attenuated total reflectance (ATR) Upon total reflection of the incident light at the interface where the crystal touches the sample a small fraction of the light extends into the sample as an evanescent wave. In areas where the sample is in contact with the evanescent wave specific parts of the IR beam are absorbed based on the sample’s composition. The totally reflected IR light lacks the absorbed parts and thus is attenuated, hence the name “attenuated total reflectance” (ATR). 46 Infrared Spectroscopy 47 Raman Scattering The Raman scattering effect differs from ordinary scattering in that part of the scattered radiation suffers quantized frequency changes. These changes are the result of vibrational energy level transitions that occur in the molecules as a consequence of the polarization process. 48  Raman Spectroscopy: Classical Treatment Number of peaks related to degrees of freedom DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms Energy related to harmonic oscillator Selection rules related to symmetry Rule of thumb: symmetric=Raman active, asymmetric=IR active CO2 H2O Raman: 1335 cm–1 Raman + IR: 3657 cm–1 IR: 2349 cm–1 Raman + IR: 3756 cm–1 IR: 667 cm–1 Raman + IR: 1594 cm–1 49 Inelastic Light Scattering Mechanisms Raman Shift Can be: To Longer WaveLengths (Stokes Scattering) To Shorter WaveLengths (AntiStokes Scattering) Raman Stokes Scattering Raman AntiStokes Scattering 50 51  Elastic Scattering (Raleigh) Anti-Stokes lines are appreciably less intense than the corresponding Stokes lines. For this reason, only the Stokes part of a spectrum is generally used. The magnitude of Raman shifts are independent of the wavelength of excitation. 52 The relative populations of the two upper energy states are such that Stokes emission is much favored over anti-Stokes. Rayleigh scattering has a considerably higher probability of occurring than Raman because the most probable event is the energy transfer to molecules in the ground state and reemission by the return of these molecules to the ground state. The ratio of anti-Stokes to Stokes intensities will increase with temperature because a larger fraction of the molecules will be in the first vibrationally excited state under these circumstances. 53  Selection rule for Raman spectrum Vibration is active if it has a change in polarizability, . For Raman-active vibrations, the incident radiation does not cause a change in the dipole moment of the molecule, but instead a change in polarizability. This is called the induced dipole moment, P. (Don’t confuse it with the molecule’s dipole moment, or change in dipole moment). 54 Factors affect Polarizability 1- Atomic number Z P  the amount of electrons, Electrons become less control by nuclear charge 2- Bond Length: P  Bond Length 3- Atomic or Molecular Size: P  Size 55  Factors affect Polarizability 4- Molecular orientation with respect to an electric field Parallel or perpendicular (Exp: Parallel has more effect) 5- Bond Strength (Bond order): P  1/strength of bond 6- Electronegativity difference: P  1/ difference in electronegativity 7- Covalent bonds more polarizable than ionic bonds. 56  Raman Depolarization Ratios Polarizability: describes a molecular property having to do with the deformability of a bond. Polarization: is a property of a beam of radiation and describes the plane in which the radiation vibrates. Raman spectra are excited by plane-polarized radiation. 57 58  z Asymmetric Sample y Incident laser Polarimeter x 59  Raman Spectroscopy: Summary 1. Raman is a vibrational spectroscopy akin to IR - Good for fingerprinting, probing molecular symmetry 2. Scattering-based, not transmission/reflection - Means no need for fancy sample preparation…gas, liquid, or solid - Virtually always use Stokes lines due to stronger signal 3. You need to pick excitation energy (laser line) 60