Exam 4 Review PDF

​ FT-IR ○​ Fourier transform infrared spectroscopy ​ Old spectrum ○​ Collected with a grating, not as clean and sharp ​ New spectrum ○​ Sample collected with FT-IR, sharper fine features ​ Interferogram ○​ IR radiation source ○​ Half-silvered mirror- beam splitter...

​ FT-IR ○​ Fourier transform infrared spectroscopy ​ Old spectrum ○​ Collected with a grating, not as clean and sharp ​ New spectrum ○​ Sample collected with FT-IR, sharper fine features ​ Interferogram ○​ IR radiation source ○​ Half-silvered mirror- beam splitter ○​ Fixed mirror ○​ Translating mirror ○​ Sample ○​ Time domain data- interferogram ○​ Spectrum (x-axis is cm-1) ​ Transmission mode ○​ IR radiation is transmitted through the sample ​ FT-IR is a single beam instrument ○​ Take a background spectrum ​ Noisy backgrounds ○​ Take a sample measurement ○​ The software subtracts out the background ​ With FT-IR we usually take multiple scans and use ensemble averaging to increase S/N ​ Sample holders ○​ For gases ​ Windows are made of KBr, NaCl, CaF2 (salt plates) ○​ For liquids ​ Place a neat liquid on a NaCl or KBr plate (salt plate) ​ press the second plate on top ​ Sandwich it ○​ For solids ​ ​ Mix KBr powder with the sample in a mortar and pestle ​ Press with pellet press (high pressure- don’t break) ​ ATR ○​ Attenuated total reflection ○​ Big difference between diamond tip and atr crystal- results in TIR (total internal reflectance) ○​ ATR crystal ​ IR radiation travels to sample and back out many times ​ Part of the IR light is absorbed ​ IR “light” sources ○​ Black body radiation ​ A heated material that emits light ○​ IR sources ​ Nernst glower  ​ Coiled wire sources ​ Globar ​ Solid state sources ​ ​ Nernst glower ○​ A ceramic heated to incandescent ○​ Old technology ​ Silicon carbide ○​ Acts as a black body ○​ Current technology ○​ Radiation emits from this aperture ​ Pyroelectric detectors ○​ Generate electrical signal when heated or cooled ​ Noisy detector ○​ LiTaO3 ○​ DLaTGS ​ Deuterated lanthanum alpha- alanine doped triglycine sulfate ○​ Black coating absorbs IR radiation ​ Comparing IR detectors ○​ A pyroelectric detector is a noisy detector ○​ LiTaO3 is noisier than DLaTGS but is more rugged and durable ○​ DLaTGS can provide 4x higher S/N than LiTaO3 ○​ If we can't change detectors, how can we increase S/N ​ Ensemble averaging ​ Beer's law: UV/Vis vs FT-IR ○​ uv/vis high sensitivity ○​ FT-IR poor sensitivity ​ FT-IR can be used for qualitative measurements, but not as sensitive as UV/Vis ​ FT-IR is best for qualitative measurements ​ Raman ​ A complementary technique to FT-IR spectroscopy, for qualitative organic structure identification ​ Water is not an interference ○​ Aqueous samples can be analyzed ​ Glass or quartz sample holders can be utilized ​ Rapid analysis ​ non-destructive ○​ Can measure contents of a bottle without opening it ​ History ○​ Using the sun as a source, raman filtered the green light to pass through a sample of CHCl3 (chloroform) ○​ Most of the light that passed through was green but a small portion was yellow ○​ Yellow- inelastic raman scattering ​ Some loss of energy ​ Generating a raman signal  ○​ Raman signal is light that is inelastically scattered (different from rayleigh, mie scattering) ​ Both elastic ○​ Raman signal is tiny, requires a laser as the light source to generate a detectable signal ○​ Laser wavelength will also be scattered (rayleigh scatter) elastically must be blocked from the detector with a filter ​ 999,999 photons for every 1 raman scattered photon ​ Raman effect ○​ Some (very small amount) of scattered light will have a different wavelength than the incident light ​ The change is due to the chemical structure of the molecule ○​ The signal that is detected is the inelastically scattered light ​ Raman ○​ The shift in lambda is caused by absorption of the molecule of a small amount of the photons energy ○​ That energy corresponds to a vibrational level ○​ Comparing FT-IR to raman ​ FT-IR ​ Molecule must experience a change in the dipole moment to be IR active ​ Raman ​ No change in dipole moment is necessary ○​ I2, O2, raman active ○​ Raman spectra look similar to IR spectra ​ Both have peaks due to exciting vibrational levels ○​ For some functional groups that give weak (or zero) IR signals, a strong raman signal can be detected ○​ Excitation line= rayleigh scattering ​ FT (fourier transform) ○​ What is a fourier transform ​ Math- an algorithm that converts a signal in the time domain to the frequency domain ○​ Why would we use it ​ Greatly improves S/N ​ In some instruments ​ Can isolate a weak signal from environmental noise ​ When the instrument is detector noise limited ​ IR, microwave, NMR ○​ Frequencies are dispersed ​ Mathematically rather than physically ○​ Non-dispersive instrument ​ No grating ​ No need for a monochromator  ​ Instead of grating, it uses an interferometer ○​ Requires a powerful computer ○​ Fourier transform spectroscopy is done by including an interferometer in the instrument ​ Incoming light is split (with a beamsplitter) and then recombines to cause interference which results in an interferogram ​ Fourier transform converts the interferogram (time domain) into a spectrum (frequency domain) ○​ Michelson interferometer ○​ f= 2 v(mirror)/ lambda ○​ What kind of resolution can we get with the fourier transform dispersing our radiation ​ It depends on the distance the translating mirror travels ○​ Advantages ​ FT provides an advantage over physical dispersion with monochromator ​ When instrument is detector noise limited- same amount of noise in signal whether you are measuring a small signal intensity or a large one ​ Jacquinot advantage ○​ Because you’re dispersing light mathematically instead of physically/ optically you do not use as many optics that can cause light to be lost (reflection losses and narrow slits) ​ Higher signal, same noise= higher S/N and better resolution ​ Fellgett advantage ○​ Because you are measuring all the wavelengths simultaneously, you can save time ​ Or can use time wisely by taking numerous scans and using ensemble averaging to improve S/N ​ Interferometers are very precise- great wavelength reproducibility and resolution ○​ Makes it easier to see fine spectral features ○​ Michelson-morley experiment 1887 ​ The bottom line on detectors ○​ Diffraction grating monochromatic with CCDs are best for UV and visible light ​ Can provide good S/N in real times ​ They are cheap, small, and can be portable ○​ FT is best for IR because it improves S/N ​ IR detectors are noisy ​ FT is best for techniques that are detector noise limited including NMR and microwave spectra ​ NMR ​ 1952 nobel prize ​ 1953 first instrument was sold ​ Most powerful tool available for structural elucidation ​ Provides structural info about how the molecule is put together ​ Either the atomic number, the mass number, or both must be odd ​ PURE SAMPLE ​ Structural qualitative analysis ○​ NMR ○​ FT-IR ○​ GC-MS ​ NMR: all about the nucleus ​ A spinning nucleus creates a magnetic field ○​ In the absence of an external magnetic field (Bo) all energies are identical ​ In an NMR, an external magnetic field is applied ○​ The application of magnetic (external) field causes the energy level to split ​ Zeeman splitting ○​ The magnetic moment of the nucleus becomes oriented in one of two directions ​ One with a higher energy and one with a lower energy ○​ The nuclear magnetic energy levels of an atom located in a magnetic field will be split by the field ​ The difference will correspond to radio frequency ​ Between 3KHz and 300GHz ​ Magnetic moment ○​ Created by the spinning nucleus ​ Magnetogyric ratio ○​ Constant for a particular isotope ​ In the RF range ○​ This is the absorption frequency, also called the resonance frequency, also called the Larmor frequency ○​ It is dependent on both the magnet in the NMR instrument and what kind of isotope you are investigating ○​ Comparison of data for different RF ​ Higher magnetic field= higher resolution ​ Nj is the number of higher energy protons (M-½) ​ No is the number of lower energy protons (M+½) ○​ We need a difference between the two to get a net signal ○​ An excess of lower energy nuclei is needed to create an NMR signal (Nj>No) ​ Very little difference between the two ○​ We do what we can to increase S/N ​ Decrease temp ​ Increase magnetic field ​ A nucleus absorbs RF energy ​ The frequency of the precession (staggering spin) is called the Larmor frequency ○​ This is the frequency difference between split energy levels ○​ It is the frequency absorbed then emitted by the nuclei ○​ It is in the RF range ​ After the energy is absorbed ○​ The nucleus wants to get rid of it ​ We call this relaxation ○​ In NMR this is the emission of the RF energy ○​ The emitted RF energy is measured as the NMR signal ○​ It is slightly different for different functional groups ​ Fourier transform NMR ○​ The RF energy is pulsed, allowing the nuclei to relax before being excited again ○​ The signal can be observed as a decay from the excited state, which lasts as long as the relaxation processes are going on ○​ This is called a free induction decay ​ FID ​ It is an interferogram ​ Instrumentation ○​ Superconducting magnet ​ Niobium/tin or titanium bathed in liquid He ​ High field strength, stable, relatively small size ○​ Sample cell: glass tube, spinning in a magnetic field ​ For H NMR use solvent with no H ​ CDCl3 ​ 2-15% solution ​ RF transmitter/receiver ○​ RF receiver ​ Coiled around the sample tube ○​ Signal received is very small, it must be amplified ○​ NMR signal is detector noise limited ​ Ensemble averaging can help increase S/N ​ Fluorescence ​ Types (light is emitted) ○​ Fluorescence ○​ Phosphospherence ○​ Chemiluminescence ○​ Bioluminescence ​ The intensity of the light is proportional to the concentration of the analyte ​ Fluorescence ○​ The molecule emits photons, some amount of time after absorbing photons ​ The emitted light is the luminescence ​ It is different than UV Vis spectroscopy because the wavelength of the emitted light is longer than the wavelength of absorbed light ○​ Some energy has been lost ​ Not all molecules fluoresce ​ Fluorescence is a more sensitive technique than UV/vis absorption- one of the most sensitive techniques we have ​ The intensity of the fluorescence can be measured independently of the source intensity (beers law) ​ Very intense light sources can be used to increase the fluorescence signal: lasers, pulsed Xe lamp ​ Quantum yield ○​ Photons emitted/absorbed ○​ Strongly fluorescent compounds have Q- 1 ○​ Weakly fluorescent compounds have Q near 0 ​ Electronic transitions ○​ One feature fluorescent molecules have is delocalized electrons ​ For this reason transitions are typically ​ Pi to pi star ​ N to pi star ○​ Quinine ​ Exc 350nm ​ Em 450nm ​ Q= 0.5-0.6 ○​ Another fluorescent molecule ​ Green fluorescent protein ​ Intramolecular bonding leads to rigidity ​ External influences on a molecules fluorescence ○​ Decrease temp will increase fluorescence ○​ If solvent contains heavy atoms this will decrease fluorescence ○​ Increase dissolved O2 will quench fluorescence ​ Quench means decrease ○​ pH ​ Both em and fluorescence intensity can change depending on whether the molecule is protonated or not ​ Excitation and emission spectra ○​ The absorbed photons have shorter wavelengths than fluorescence (emitted) photons ​ Instrumentation ○​ Fluorimeter ○​ Exc UV ○​ Em vis ○​ Light source, slit, mirrors, diffraction grating, sample cuvette (quartz), mirrors, diffraction grating, detector (PMT) ○​ Simpler filter-based instrumentation ​ Biochemists ​ Pulsed Xe lamp ○​ Fluorescent tags ​ Proteins can be tagged ​ Fluorescein ​ Exc 494nm  ​ Em 521nm ​ Reacts with protein amine groups ○​ Bioluminescence ​ A biochemical reaction provides energy to excite fluorescence