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gsu spectrum

gsu spectrum

Description

School: Georgia Southern University
Department: Biochemistry
Course: Organic Chemistry II
Professor: Christine whitlock
Term: Spring 2017
Tags: infraredspectroscopy, massspectrometry, NMR, and HNMR
Cost: 25
Name: Organic Chemistry II Week 4
Description: These notes cover IR Spectroscopy, Mass Spectroscopy, and NMR Spectroscopy
Uploaded: 02/10/2017
110 Pages 121 Views 0 Unlocks
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• WHAT type of fragmenting is responsible for the “groupings” of peaks observed?




• What is a radical cation?




• Which bond is strongest, and WHY?



Chapter 15 Infrared Spectroscopy and Mass SpectrometryCopyright 2012 John Wiley & Sons, Inc. 7 -1 Klein, Organic Chemistry 1e 15.1 Introduction to Spectroscopy Spectroscopy-using a molecule’s response to  energy to determine its structure Infrared (IR) Spectroscopy ???? IR radiation Ultraviolet/Visible(UV/Vis) Spectroscopy ???? UV/Visible  radiation Nuclear Magnetic Resonance (NMR) Spectroscopy ???? radio waves Mass Spectrometry ???? electron beam Copyright 2012 John Wiley & Sons, Inc. 15-2 Klein, Organic Chemistry 1e15.1 Introduction to Spectroscopy Energy (Electromagnetic radiation) γ rays > X rays > UV > visible > IR > microwave > radio λ(m)    10-10            10-8         10-7 10-6 10-4 10-2 102 E (kJ•s) 106 104 102 10 1          10-4 10-6 Effect:            ionization            electron          vibrations        molecular       nuclear  spin transitions                                   rotation          transitions Copyright 2012 John Wiley & Sons, Inc. 7 -3 Klein, Organic Chemistry 1e15.1 Introduction to Spectroscopy • There are many wavelengths of light that cannot be  observed with your eyes. Copyright 2012 John Wiley & Sons, Inc. 15-4 Klein, Organic Chemistry 1e15.1 Introduction to Spectroscopy Remember… C = λν E = hν = 1/λ wave number (1/cm) Copyright 2012 John Wiley & Sons, Inc. 15-5 Klein, Organic Chemistry 1e15.1 Introduction to Spectroscopy • In the IR region, bond lengths vibrate. – Stretch – Bend – Scissor – Rock/wag – Twist • λ = 2.5 x 10-4cm – 2.5 x 10-3cm. • =  4000-400cm-1 Copyright 2012 John Wiley & Sons, Inc. 15-6 Klein, Organic Chemistry 1e15.2 IR Spectroscopy • Molecular bonds can vibrate by stretching or by bending  in a number of ways. • This chapter will focus mostly  on stretching frequencies. • Some night vision goggles  can detect IR light that is  emitted. Copyright 2012 John Wiley & Sons, Inc. 15-7 Klein, Organic Chemistry 1e15.2 IR Spectroscopy • Fingerprint region: – Most complex – 1400-600cm-1 – Where bending usually occurs • Functional group region: – 4000-1400cm-1 1400Copyright 2012 John Wiley & Sons, Inc. 15-8 Klein, Organic Chemistry 1e Instrument sample beam DETECTOR SPECTRUM source of IR radiationreference beam Copyright 2012 John Wiley & Sons, Inc. 15-9 Klein, Organic Chemistry 1e IR Spectrum O-H (broad) N-H (spiked) %T C-H  alkene alkyne aromatic C-H  alkane4000 3000 2200 1700 1600 (cm-1) Copyright 2012 John Wiley & Sons, Inc. 7 -10 Klein, Organic Chemistry 1e 15.2 IR Spectroscopy Copyright 2012 John Wiley & Sons, Inc. 15-11 Klein, Organic Chemistry 1e15.3 IR Signal Wavenumber • Analyze the diagnostic and fingerprint regions below. Copyright 2012 John Wiley & Sons, Inc. 15-12 Klein, Organic Chemistry 1e15.3 IR Signal Wavenumber • Compare the IR  spectra. Copyright 2012 John Wiley & Sons, Inc. 15-13 Klein, Organic Chemistry 1e15.3 IR Signal Wavenumber • Compare the IR stretching wavenumbers below. • Are the differences due to mass or bond strength? • Which bond is strongest, and WHY? Copyright 2012 John Wiley & Sons, Inc. 15-14 Klein, Organic Chemistry 1e15.3 IR Signal Wavenumber • Note how the region ≈3000 cm-1 in the IR spectrum can  give information about the functional groups present. Copyright 2012 John Wiley & Sons, Inc. 15-15 Klein, Organic Chemistry 1e15.4 IR Signal Strength • Note the general strength  of the C=O stretching signal  vs. the C=C stretching  signal. • Imagine a symmetrical  molecule with a completely  nonpolar C=C bond: 2,3- dimethyl-2-butene. • 2,3-dimethyl-2-butene does  not give an IR signal in the  1500–2000 cm-1 region. Copyright 2012 John Wiley & Sons, Inc. 15-16 Klein, Organic Chemistry 1e15.7 Using IR to Distinguish Between  Molecules • For the reactions below, identify the key functional  groups, and describe how IR data could be used to  verify the formation of product. • Is IR analysis qualitative or quantitative? Copyright 2012 John Wiley & Sons, Inc. 15-17 Klein, Organic Chemistry 1eIR Practice • Match the following spectra with the structures below: Copyright 2012 John Wiley & Sons, Inc. 7 -18 Klein, Organic Chemistry 1eSpectrum A Copyright 2012 John Wiley & Sons, Inc. 7 -19 Klein, Organic Chemistry 1eSpectrum B Copyright 2012 John Wiley & Sons, Inc. 7 -20 Klein, Organic Chemistry 1eSpectrum C Copyright 2012 John Wiley & Sons, Inc. 7 -21 Klein, Organic Chemistry 1eSpectrum D Copyright 2012 John Wiley & Sons, Inc. 7 -22 Klein, Organic Chemistry 1eSpectrum E Copyright 2012 John Wiley & Sons, Inc. 7 -23 Klein, Organic Chemistry 1e15.8 Introduction to Mass  Spectrometry • Mass spectrometry (MS) is primarily used to determine  the molar mass and formula for a compound: 1. A compound is vaporized and then ionized. 2. The masses of the ions are detected and graphed. Copyright 2012 John Wiley & Sons, Inc. 15-24 Klein, Organic Chemistry 1e15.8 Introduction to Mass  Spectrometry • The most common method of ionizing molecules is by  electron impact (EI). • The sample is bombarded with a beam of high energy  electrons (1600 kcal or 70 eV). • EI usually causes an electron to be ejected from the  molecule. HOW? WHY?  • What is a radical cation? Copyright 2012 John Wiley & Sons, Inc. 15-25 Klein, Organic Chemistry 1e15.8 Introduction to Mass  Spectrometry • The parent ion can then fragment into smaller ions. • Anything with a “+” charge is detected and reported as  M/z. • Often, the molecular ion undergoes some type of  fragmentation. WHY? Copyright 2012 John Wiley & Sons, Inc. 15-26 Klein, Organic Chemistry 1e15.8 Introduction to Mass  Spectrometry • The resulting fragments may undergo even further  fragmentation. • The ions are deflected by a magnetic field. • Smaller mass and higher charge fragments as affected  more by the magnetic field. WHY? • Neutral fragments are not detected. WHY? Copyright 2012 John Wiley & Sons, Inc. 15-27 Klein, Organic Chemistry 1eMass Spectrometer Copyright 2012 John Wiley & Sons, Inc. 7 -28 Klein, Organic Chemistry 1eMass Spectrum Base peak- strongest peak (100%), most abundant fragment Molecular ion peak- M+ (unfragmented)Copyright 2012 John Wiley & Sons, Inc. 15-29 Klein, Organic Chemistry 1e 15.9 Analyzing the M+• Peak • In the mass spectrum for benzene, the M+• peak is the base  peak. • The M+• peak does not easily fragment. Copyright 2012 John Wiley & Sons, Inc. 15-30 Klein, Organic Chemistry 1e15.9 Analyzing the M+• Peak • Like most compounds, the M+• peak for pentane is NOT  the base peak. • The M+• peak fragments easily. Copyright 2012 John Wiley & Sons, Inc. 15-31 Klein, Organic Chemistry 1e15.12 Analyzing the Fragments • WHAT type of fragmenting is responsible for the  “groupings” of peaks observed? Copyright 2012 John Wiley & Sons, Inc. 15-32 Klein, Organic Chemistry 1eIsotope Effect M M+1 M+2 12C (99%)               13C (1%) 35Cl (76%)                                                  37Cl (24%) 79Br (51%)                                                 81Br (49%) 1H (100%) 127I (100%) Copyright 2012 John Wiley & Sons, Inc. 15-33 Klein, Organic Chemistry 1e15.11 Analyzing the (M+2)+• Peak • Chlorine has two abundant isotopes: – 35Cl=76% and 37Cl=24% • Molecules with chlorine often have strong (M+2)+• peaks. • WHY is it sometimes difficult to be absolutely sure which  peak is the (M)+• peak? Copyright 2012 John Wiley & Sons, Inc. 15-34 Klein, Organic Chemistry 1e15.11 Analyzing the (M+2)+• Peak • 79Br=51% and 81Br=49%, so molecules with bromine  often have equally strong (M)+• and (M+2)+• peaks. • Practice with CONCEPTUAL CHECKPOINTs  15.23 and 15.24. Copyright 2012 John Wiley & Sons, Inc. 15-35 Klein, Organic Chemistry 1eMass Spec Practice Copyright 2012 John Wiley & Sons, Inc. 7 -36 Klein, Organic Chemistry 1eMass Spec Practice Copyright 2012 John Wiley & Sons, Inc. 7 -37 Klein, Organic Chemistry 1e15.13 High Resolution Mass  Spectrometry • High resolution (high-res) MS allows m/z to be  measured with up to 4 decimal places. • Masses are generally not whole number integers: – 1 proton = 1.0073 amu and 1 neutron = 1.0086 amu • One 12C atom = exactly 12.0000 amu, because the amu  scale is based on the mass of 12C. • All atoms other than 12C will have a mass in amu that  can be measured to four decimal places by a  high-res MS instrument. Copyright 2012 John Wiley & Sons, Inc. 15-38 Klein, Organic Chemistry 1e15.13 High Resolution Mass  Spectrometry • Why are the values in Table 15.5 different from those on  the periodic table? • Imagine you want to use  high-res MS to distinguish  between the molecules  below. • Why can’t you use low  resolution (low-res) MS? Copyright 2012 John Wiley & Sons, Inc. 15-39 Klein, Organic Chemistry 1e15.13 High Resolution Mass  Spectrometry • Using the exact masses and natural abundances for  each element, we can see the difference high-res  makes. • The molecular ion results from the molecule with the  highest natural abundance. • What if the molecular ion is not observed? • Practice with CONCEPTUAL CHECKPOINTs 15.19 and  15.30. Copyright 2012 John Wiley & Sons, Inc. 15-40 Klein, Organic Chemistry 1e15.16 Degrees of Unsaturation • Unsaturation: Δ = C + 1 - H2Copyright 2012 John Wiley & Sons, Inc. 15-41 Klein, Organic Chemistry 1e 15.16 Degrees of Unsaturation • Heteroatoms: Δ = C + 1 – H/2 – X/2 + N/2 Copyright 2012 John Wiley & Sons, Inc. 15-42 Klein, Organic Chemistry 1e 15.16 Degrees of Unsaturation • Consider the isomers of C4H6. • How many degrees of unsaturation are there? • 1 degree of unsaturation = 1 unit on the hydrogen  deficiency index (HDI) Copyright 2012 John Wiley & Sons, Inc. 15-43 Klein, Organic Chemistry 1e15.16 Degrees of Unsaturation • Propose a structure for a molecule with the formula  C7H12O. The molecule has the following IR peaks:  – A strong peak at 1687 cm-1 – NO IR peaks above 3000 cm-1 Copyright 2012 John Wiley & Sons, Inc. 15-44 Klein, Organic Chemistry 1eChapter 15 Nuclear Magnetic Resonance Spectroscopy Copyright 2012 John Wiley & Sons, Inc. 7 -1 Klein, Organic Chemistry 1e16.1 Introduction to NMR  Spectroscopy • What is spectroscopy? • NUCLEAR MAGNETIC RESONANCE (NMR) spectroscopy  may be the most powerful method of gaining structural  information about organic compounds. • NMR uses environments of hydrogen atoms (and other  atoms) to determine structure. Copyright 2012 John Wiley & Sons, Inc. 16-2 Klein, Organic Chemistry 1e16.1 Introduction to NMR  Spectroscopy • We study different nuclei. • If the total number of neutrons and protons is an ODD  number, the atoms will have net nuclear spin. • Examples:  Odd atomic # or atomic mass!• The spinning charge in the nucleus creates a MAGNETIC  MOMENT. Copyright 2012 John Wiley & Sons, Inc. 16-3 Klein, Organic Chemistry 1e 16.1 Introduction to NMR  Spectroscopy • Like a bar magnet, a MAGNETIC MOMENT exists  perpendicular to the axis of nuclear spin. Copyright 2012 John Wiley & Sons, Inc. 16-4 Klein, Organic Chemistry 1e16.1 Introduction to NMR  Spectroscopy • If the normally disordered magnetic moments of atoms  are exposed to an external magnetic field, their  magnetic moments will align. Copyright 2012 John Wiley & Sons, Inc. 16-5 Klein, Organic Chemistry 1e16.1 Introduction to NMR  Spectroscopy • The aligned magnetic moments can be either WITH or  AGAINST the external magnetic field. • The α and β spin states are not  equal in energy. WHY? Copyright 2012 John Wiley & Sons, Inc. 16-6 Klein, Organic Chemistry 1e16.1 Introduction to NMR  Spectroscopy • When an atom with an α spin state is exposed to radio  waves of just the right energy, it can be promoted to a β spin state. • The stronger the magnetic field,  the greater the energy gap. Copyright 2012 John Wiley & Sons, Inc. 16-7 Klein, Organic Chemistry 1e16.1 Introduction to NMR  Spectroscopy • The magnetic moment of  the electrons generally  reduces the effect of the  external field. • The more SHIELDED a  nucleus is with electron  density, the smaller the α ???? β energy gap. WHY? Copyright 2012 John Wiley & Sons, Inc. 16-8 Klein, Organic Chemistry 1e16.2 Acquiring a 1H NMR Spectrum • Solvents are used such as chloroform-d. WHY? • The magnet is super-cooled, but the sample is generally  at room temperature. Copyright 2012 John Wiley & Sons, Inc. 16-9 Klein, Organic Chemistry 1e16.3 Characteristics of a 1H NMR  Spectrum • NMR spectra contain a lot of structural information: – Number of signals – Signal location—shift – Signal area—integration – Signal shape—splitting pattern Copyright 2012 John Wiley & Sons, Inc. 16-10 Klein, Organic Chemistry 1e16.4 Number of Signals • Identify all the groups of equivalent protons in the  molecules below and describe their relationships. OH • Integration Br OHCH3CHCH3 Copyright 2012 John Wiley & Sons, Inc. 16-11 Klein, Organic Chemistry 1e 16.5 Chemical Shifts • Tetramethylsilane (TMS) is used as the  standard for NMR chemical shift. • In many NMR solvents, 1% TMS is added as an internal  standard. • The shift for a proton signal is calculated as a  comparison to TMS: • For benzene on a 300 MHz instrument: Copyright 2012 John Wiley & Sons, Inc. 16-12 Klein, Organic Chemistry 1e16.5 Chemical Shifts • The shift for a proton signal is calculated as a  comparison to TMS: • The shift relative to TMS (δ) is a dimensionless number  because the Hz units cancel out. • Units for δ are often given as ppm (parts per million),  which simply indicates that signals are reported as a  fraction of the operating frequency of the spectrometer. • Most 1H signals appear between 0 and  10 ppm. Copyright 2012 John Wiley & Sons, Inc. 16-13 Klein, Organic Chemistry 1e1H Chemical Shifts Intensity O O C H O C H C H C H O C H OHH C C C X C H C H 12 10 8 6 4 2 0 Chemical shift (δ) downfield upfield Copyright 2012 John Wiley & Sons, Inc. 16-14 Klein, Organic Chemistry 1e 16.5 Chemical Shifts • Predict chemical shifts for all of the protons in the  molecule below. OH Br OHCH3CHCH3 Copyright 2012 John Wiley & Sons, Inc. 16-15 Klein, Organic Chemistry 1e 16.5 Chemical Shifts • Explain all of  the shifts in  Table 16.2. Copyright 2012 John Wiley & Sons, Inc. 16-16 Klein, Organic Chemistry 1e16.6 Integration • The INTEGRATION or area under the peak quantifies the  relative number of protons giving rise to a signal. • A computer will calculate the area of each peak  representing that area with a STEP-CURVE. • The curve height represents the integration.  Copyright 2012 John Wiley & Sons, Inc. 16-17 Klein, Organic Chemistry 1e16.6 Integration • The computer operator sets one of the peaks to a whole  number to let it represent a number of protons. • The computer uses the integration ratios to set the  values for the other peaks. 1.48 1.561.00 1.05 Copyright 2012 John Wiley & Sons, Inc. 16-18 Klein, Organic Chemistry 1e 16.6 Integration • Integrations represent numbers of protons, so you must  adjust the values to whole numbers. • If the integration of the first peak is doubled, the  computer will adjust the others according to the ratio. 2.96 3.122.002.10 Copyright 2012 John Wiley & Sons, Inc. 16-19 Klein, Organic Chemistry 1e 16.6 Integration • The INTEGRATION are relative quantities rather than an  absolute count of the number of protons. • Predict the 1H shifts and integrations for tert-butyl methyl ether. O• Symmetry can also affect integrations. • Predict the 1H shifts and integrations for 3-pentanone. Copyright 2012 John Wiley & Sons, Inc. 16-20 Klein, Organic Chemistry 1e 16.7 Multiplicity • When a signal is observed in the 1H NMR, often it is split  into multiple peaks. • Multiplicity or a splitting patterns results. Copyright 2012 John Wiley & Sons, Inc. 16-21 Klein, Organic Chemistry 1e16.7 Multiplicity • Multiplicity results from magnetic effects that protons  have on each other. • Consider protons Ha and Hb. • We already saw that protons align with or against the  external magnetic field.  • Hb will be aligned with the magnetic field in some  molecules. Other molecules in the sample will have Hb aligned against the magnetic field.  • Some Hb atoms have a slight shielding affect on  Ha and others have a slight deshielding effect. Copyright 2012 John Wiley & Sons, Inc. 16-22 Klein, Organic Chemistry 1e16.7 Multiplicity • The resulting multiplicity or splitting pattern for Ha is a  doublet. • A doublet generally results when a proton is split by  only one other proton on an adjacent carbon. Copyright 2012 John Wiley & Sons, Inc. 16-23 Klein, Organic Chemistry 1e16.7 Multiplicity • Consider an example where there are two  protons on the adjacent carbon. • There are three possible affects the Hb  protons have on Ha. Copyright 2012 John Wiley & Sons, Inc. 16-24 Klein, Organic Chemistry 1e16.7 Multiplicity • Half of the Ha atoms will not be affected by the Hb atoms. WHY? • ¼ of the Ha atoms will be shielded and ¼  deshielded. Copyright 2012 John Wiley & Sons, Inc. 16-25 Klein, Organic Chemistry 1e16.7 Multiplicity • Ha appears as a triplet. • WHY? • The three peaks in the triplet have an  integration ratio of 1:2:1. • WHY? Copyright 2012 John Wiley & Sons, Inc. 16-26 Klein, Organic Chemistry 1e16.7 Multiplicity • Consider a scenario where Ha has three  equivalent Hb atoms splitting it. • Explain how the magnetic fields cause shielding  or deshielding. Copyright 2012 John Wiley & Sons, Inc. 16-27 Klein, Organic Chemistry 1e16.7 Multiplicity • Ha appears as a quartet. • What should the integration ratios be  for the four peaks of the quartet? Copyright 2012 John Wiley & Sons, Inc. 16-28 Klein, Organic Chemistry 1e16.7 Multiplicity • The trend in Table 16.3 also allows us to predict splitting  patterns. • Explain how the n+1 rule is used. Copyright 2012 John Wiley & Sons, Inc. 16-29 Klein, Organic Chemistry 1e16.7 Multiplicity • Predict splitting patterns for all of the protons in the  molecule below. CH3CH2CH2OCH3Copyright 2012 John Wiley & Sons, Inc. 16-30 Klein, Organic Chemistry 1e 16.7 Multiplicity • The degree to which a neighboring proton will shield or  deshield its neighbor is called a COUPLING CONSTANT. – The coupling constant  or J value is the  distance between peaks  of a splitting pattern  measured in units of Hz. – When protons split  each other, their  coupling constants will  be equal. – Jab = Jba Copyright 2012 John Wiley & Sons, Inc. 16-31 Klein, Organic Chemistry 1e16.7 Multiplicity • Sometimes recognizable  splitting patterns will stand  out in a spectrum. • An isolated ethyl group gives  a triplet and a quartet. • Note the integrations. • The triplet and quartet must  have the same coupling  constant if they are splitting  each other. Copyright 2012 John Wiley & Sons, Inc. 16-32 Klein, Organic Chemistry 1e16.7 Multiplicity • Complex splitting results when a  proton is split by NONEQUIVALENT  neighboring protons. • In the molecule shown, Hb is split  into a quartet by Ha and into a  triplet by Hc. • If Jab is much greater than Jbc, the  signal will appear as a quartet of  triplets. Copyright 2012 John Wiley & Sons, Inc. 16-33 Klein, Organic Chemistry 1e16.7 Multiplicity • Splitting is not observed for some protons. Consider  ethanol: • The protons bonded to carbon split each other, but the  hydroxyl proton is not split. Copyright 2012 John Wiley & Sons, Inc. 16-34 Klein, Organic Chemistry 1e16.9 Using 1H Spectra to Distinguish  Between Compounds • The three molecules below might be difficult to  distinguish by IR of MS. WHY? • Explain how 1H NMR could distinguish between them. Copyright 2012 John Wiley & Sons, Inc. 16-35 Klein, Organic Chemistry 1e16.10 Analyzing a 1H NMR Spectrum • With a given formula and 1H NMR spectrum, you can  determine a molecule’s structure by a four-step  process: 1. Calculate the degree or unsaturation or hydrogen deficiency  index (Δ). What does the HDI tell you? 2. Consider the number of NMR signals and integration to look  for symmetry in the molecule. 3. Analyze each signal, and draw molecular fragments that  match the shift, integration, and multiplicity. 4. Assemble the fragments into a complete structure like puzzle  pieces. Copyright 2012 John Wiley & Sons, Inc. 16-36 Klein, Organic Chemistry 1e

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