Spectroscopy Techniques for Molecular Analysis PDF

🌈 9/8/24, 7:28 PM Spectroscopy Techniques for Molecular Analysis Infrared (IR) Spectroscopy Infrared (IR) spectroscopy is a tool used to determine the structure of molecul...

🌈 9/8/24, 7:28 PM Spectroscopy Techniques for Molecular Analysis Infrared (IR) Spectroscopy Infrared (IR) spectroscopy is a tool used to determine the structure of molecules. It works by shooting IR light at the molecule, causing the bonds to wiggle and absorb energy. The energy that is not absorbed is turned into an IR spectrum, which has peaks that are unique to each type of bond. How IR Spectroscopy Works IR spectroscopy is based on the principle that molecules absorb IR radiation, causing their bonds to vibrate. The energy absorbed by the molecule is proportional to the frequency of the radiation. Types of Bonds and Their IR Peaks Bond IR C-H C=O (carbonyl) C=C or C=N N-H CoN (nitrile) NO2 1500 Peaks to Memorize Small to medium peak at 1600-1700 cm-1: C=C or C=N Big, pointy peak at 1700-50 cm-1: C=O (carbonyl) Large, broad trough far to the left: alcohols and OH Big, pointy peaks coming straight down around 3000 cm-1: C-H Sharp peak to the left of 3000 cm-1: N-H Medium-sized peak at 2200 cm-1: CoN (nitrile) Vampire teeth at 1500-1600 and 1300-1400 cm-1: NO2 The C=O (Carbonyl) Peak The C=O peak is a big, pointy peak that appears at 1700-50 cm-1. It is a characteristic peak for carbonyl groups. The OH Stretch (Alcohols) The OH stretch appears as a large, broad trough far to the left of the IR spectrum. It is a characteristic peak for alcohols. The OH Stretch (Carboxylic Acids) The OH stretch for carboxylic acids appears on top of the C-H peaks at 3000 cm-1. It is a characteristic peak for carboxylic acids. UV-Vis Spectroscopy 🔍 UV-Vis spectroscopy is a technique used to analyze compounds with conjugated double bonds. It works by bombarding the compound with UV and visible light, and collecting the transmitted light to produce a spectrum. How UV-Vis Spectroscopy Works UV-Vis spectroscopy is based on the principle that compounds with conjugated double bonds absorb UV and visible light, causing them to transmit light at specific wavelengths. https://www.turbolearn.ai/content/a7e5b910-8787-4608-8876-8944c46e208d 1/4  ⚗️ 9/8/24, 7:28 PM Spectroscopy Techniques for Molecular Analysis Mass Spectrometry Mass spectrometry is a technique used to determine the mass of a compound. It works by ionizing the compound and separating the ions based on their mass-to-charge ratio. How Mass Spectrometry Works Mass spectrometry is based on the principle that ions with different mass-to-charge ratios are separated by their mass. Example of Mass Spectrometry Compound Molecular Formula C3H6O C3H6O C6H5CH3 C7H8 C2H6 C2H6 Degrees of Unsaturation 📊 Degrees of unsaturation refer to the number of double bonds or rings in a compound. Each degree of unsaturation decreases the number of hydrogens by 2. Formula for Degrees of Unsaturation of degrees of unsaturation = # of double bonds + # of rings Examples of Degrees of Unsaturation Compound Molecular Formula Alkane CnH2n+2 Alkene CnH2n Alkyne CnH2n-2 Cycloalkane CnH2n Degrees of unsaturation is a measure of the number of double bonds or rings in a molecule. A molecule with no double bonds or rings has zero degrees of unsaturation. The formula to calculate degrees of unsaturation is: (A - B) / 2 Where: A = the number of hydrogen atoms the compound would have if it had no double bonds or rings (CnH2n+2) B = the number of hydrogen atoms the compound actually has Determining Degrees of Unsaturation To determine the number of degrees of unsaturation, follow these steps: 1. Count the number of carbons in the given formula. 2. Draw a molecule with the same number of carbons, but no double bonds or rings. 3. Add non-hydrogen atoms to the structure, if present in the formula. 4. Add hydrogen atoms to the structure, as needed. 5. Determine the formula of the drawn structure. https://www.turbolearn.ai/content/a7e5b910-8787-4608-8876-8944c46e208d 2/4 9/8/24, 7:28 PM Spectroscopy Techniques for Molecular Analysis 6. If the formula matches the given formula, the compound has zero degrees of unsaturation. 7. If the formula does not match, add a double bond and repeat steps 3-6. 🔍 8. Continue adding double bonds until the formula matches. 13C-NMR Spectroscopy A typical 13C-NMR spectrum shows peaks corresponding to different kinds of carbon atoms. Different kinds of carbon atoms are those that are not in the same environment, i.e., they do not have the same atoms surrounding them in all directions. The number of peaks (signals) in a 13C-NMR spectrum equals the number of non-equivalent carbon atoms in the molecule. Where Different Peaks Show Up Kind of Carbon Alkane carbons Alkene carbons Aromatic carbons (like Cs in benzene rings) Carbonyl (C=O) carbons (Esters, amides, and carboxylic acids) Carbonyl (C=O) carbons (Aldehydes and ketones) TMS (tetramethylsilane) 1H-NMR Spectroscopy 🔍 A typical 1H-NMR spectrum shows peaks corresponding to different kinds of hydrogen atoms. Different kinds of hydrogen atoms are those that are not in the same environment, i.e., they do not have the same atoms surrounding them in all directions. The number of peaks (signals) in a 1H-NMR spectrum equals the number of non-equivalent hydrogen atoms in the molecule. Where Different Peaks Show Up Kind of Hydrogen Hs stuck to sp3-hybridized carbon atoms Hs stuck to a C=C carbon Hs stuck to aromatic rings like benzene Aldehyde Hs Phenol Hs (sometimes) Carboxylic acid Hs Amide Hs (sometimes) Alcoholic OHs Amine NHs TMS (tetramethylsilane) Integrals Integrals are numbers written above the peaks in a 1H-NMR spectrum, indicating the number of hydrogens in each signal. https://www.turbolearn.ai/content/a7e5b910-8787-4608-8876-8944c46e208d 3/4 9/8/24, 7:28 PM Spectroscopy Techniques for Molecular Analysis Splitting Hydrogen atoms can split the signal of neighboring hydrogen atoms. The n+1 rule is used to determine the splitting pattern: Hydrogens A look next-door and see n hydrogens B. The signal for hydrogens A gets split into n+1 peaks. For example: Hydrogens A see 2 hydrogens B, so the signal for hydrogens A is split into 3 peaks (2+1). Hydrogens B see 3 hydrogens A, so the signal for hydrogens B is split into 4 peaks (3+1). Textbook Versus Reality The actual 1H-NMR spectrum may differ from the textbook spectrum due to various factors. https://www.turbolearn.ai/content/a7e5b910-8787-4608-8876-8944c46e208d 4/4