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Chapter 13 Spectroscopy Nuclear Magnetic Resonance Spectroscopy Infrared Spectroscopy Ultraviolet-Visible Spectroscopy Mass Spectrometry 13.1 Principles of Molecular Spectroscopy: Electromagnetic Radiation Electromagnetic Radiation Propagated at the speed of light. Has properties of particles and waves. The energy of a photon is proportional to its frequency. The Electromagnetic Spectrum Shorter Wavelength () 400 nm Longer Wavelength () 750 nm Visible Light Higher Frequency () Higher Energy (E) Lower Frequency () Lower Energy (E) The Electromagnetic Spectrum Shorter Wavelength () Ultraviolet Higher Frequency () Higher Energy (E) Longer Wavelength () Infrared Lower Frequency () Lower Energy (E) The Electromagnetic Spectrum Cosmic rays Rays X-rays Energy Ultraviolet (UV) light Visible light Infrared (IR) radiation Microwaves Radio waves 13.2 Principles of Molecular Spectroscopy: Quantized Energy States E = h Electromagnetic radiation is absorbed when the energy of the photon corresponds to the difference in energy between two states. What Kind of States? Electronic UV-Visible Vibrational Infrared Rotational Microwave Nuclear spin Radiofrequency 13.3 Introduction to 1H NMR Spectroscopy The nuclei that are most useful to organic chemists are: 1H and 13C: Both have spin = ±1/2. 1H is 99.985% at natural abundance. 13C is 1.1% at natural abundance. Nuclear Spin + + A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2. The distribution of nuclear spins is random in the absence of an external magnetic field. + + + + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + + + H0 + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. + + + H0 + + Energy Differences Between Nuclear Spin States + E E ' + Increasing field strength No difference in absence of magnetic field. Proportional to strength of external magnetic field. Some Important Relationships in NMR Units The frequency of absorbed electromagnetic radiation is proportional to: the energy difference between two nuclear spin states, which is proportional to: the applied magnetic field. Hz kJ/mol (kcal/mol) tesla (T) Some Important Relationships in NMR The frequency of absorbed electromagnetic radiation is different for different elements and for different isotopes of the same element. For a field strength of 4.7 T: 1H absorbs radiation having a frequency of 200 MHz (200 x 106 s-1) 13C absorbs radiation having a frequency of 50.4 MHz (50.4 x 106 s-1) Some Important Relationships in NMR The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment. This is why NMR is such a useful tool for structure determination. 13.4 Nuclear Shielding and 1H Chemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?" Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. C H H0 Shielding The induced field shields the nuclei (in this case, C and H) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. C H H0 Chemical Shift Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. C H H0 Chemical Shift Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard. d = CH3 H3C Si CH3 position of signal - position of TMS peak spectrometer frequency CH3 x 106 NMR Spectrometers Downfield Decreased shielding Upfield Increased shielding (CH3)4Si (TMS) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 Chemical shift (d, ppm) measured relative to TMS 1.0 0 Chemical Shift Example: The signal for the proton in chloroform (HCCl3) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz. d = d = position of signal - position of TMS peak spectrometer frequency 1456 Hz - 0 Hz 200 x 106 Hz d = 7.28 x 106 x 106 Chloroform Cl H d 7.28 ppm 10.0 9.0 8.0 C Cl Cl 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 13.5 Effects of Molecular Structure on 1H Chemical Shifts Protons in different environments experience different degrees of shielding and have different chemical shifts. Electronegative Substituents Decrease the Shielding of Methyl Groups Least shielded H CH3F CH3OCH3 d 4.3 d 3.2 Most shielded H (CH3)3N d 2.2 CH3CH3 (CH3)4Si d 0.9 d 0.0 Electronegative Substituents Decrease Shielding d 0.9 d 1.3 d 0.9 H3C—CH2—CH3 d 4.3 d 2.0 d 1.0 O2N—CH2—CH2—CH3 Effect is Cumulative CH3Cl CH2Cl2 CHCl3 d 3.1 d 5.3 d 7.3 Methyl, Methylene and Methine CH3 more shielded than CH2. CH2 more shielded than CH. d 0.9 CH3 H3C C CH3 H d 1.6 d 0.9 CH3 H3C C CH3 d 1.2 CH2 d 0.8 CH3 Protons Attached to sp2-hybridized Carbon are Less Shielded than Those Attached to sp3-hybridized Carbon H H H H H C H H CH3CH3 C H H H d 7.3 d 5.3 d 0.9 But Protons Attached to sp-hybridized Carbon are More Shielded than Those Attached to sp2-hybridized Carbon d 5.3 H H C H C H d 2.4 H C C CH2OCH3 Protons Attached to Benzylic and Allylic Carbons are Somewhat Less Shielded than Usual H3C CH3 d 0.8 d 1.5 d 0.9 d 1.3 d 0.9 H3C—CH2—CH3 d 1.2 H3C d 2.6 CH2 Proton Attached to C=O of Aldehyde is Most Deshielded C—H d 2.4 H H3C C O C CH3 d 1.1 H d 9.7 1H Chemical Shifts of Some Common Groups Type of proton Chemical shift (d), ppm Type of proton Chemical shift (d), ppm H C R 0.9-1.8 H C C N 2.1-2.3 H C C C 1.5-2.6 H C C C 2.5 2.0-2.5 H C Ar O H C C 2.3-2.8 1H Chemical Shifts of Some Common Groups Type of proton Chemical shift (d), ppm H C NR 2.2-2.9 Type of proton H C H H C C Cl Br Chemical shift (d), ppm C 4.5-6.5 3.1-4.1 2.7-4.1 H Ar 6.5-8.5 O H C O 3.3-3.7 H C 9-10 1H Chemical Shifts of Some Common Groups Type of proton Chemical shift (d), ppm H NR 1-3 H OR 0.5-5 H OAr 6-8 O HO C 10-13 13.6 Interpreting Proton NMR Spectra Information contained in an NMR spectrum includes: 1. Number of signals. 2. Their intensity (as measured by area under peak). 3. Splitting pattern (multiplicity). Number of Signals Protons that have different chemical shifts are chemically nonequivalent. Exist in different molecular environment. N Methoxyacetonitrile CCH2OCH3 OCH3 NCCH2O 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 Chemically Equivalent Protons Are in identical environments. Have same chemical shift. Replacement test: replacement by some arbitrary "test group" generates same compound. H3CCH2CH3 chemically equivalent Chemically Equivalent Protons Replacing protons at C-1 and C-3 gives same compound (1-chloropropane). C-1 and C-3 protons are chemically equivalent and have the same chemical shift. ClCH2CH2CH3 CH3CH2CH2Cl H3CCH2CH3 Chemically equivalent Diastereotopic Protons Replacement by some arbitrary test group generates diastereomers. Diastereotopic protons can have different chemical shifts. Br C H3C H d 5.3 ppm H d 5.5 ppm C Enantiotopic Protons Are in mirror-image environments. Replacement by some arbitrary test group generates enantiomers. Enantiotopic protons have the same chemical shift. Enantiotopic Protons H H C CH2OH H3C H Cl Cl H C H3C CH2OH R C H3C CH2OH S 13.7 Spin-Spin Splitting in NMR Spectroscopy Not all peaks are singlets. Signals can be split by coupling of nuclear spins. Cl2CHCH3 1,1-Dicholoroethane 4 lines; quartet 2 lines; doublet CH3 CH 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 Two-Bond and Three-Bond Coupling H C H C C C H H Protons separated by two bonds (geminal relationship). Protons separated by three bonds (vicinal relationship). Two-Bond and Three-Bond Coupling H C H C C C H H In order to observe splitting, protons cannot have same chemical shift. Coupling constant (2J or 3J) is independent of field strength and are measured in Hz. 1,1-Dicholoroethane 4 lines; quartet Cl2CHCH3 2 lines; doublet CH CH3 Coupled protons are vicinal (three-bond coupling). 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 CH splits CH3 into a doublet. CH3 splits CH into aChemical quartet. shift (d, ppm) 0 Why do the methyl protons of 1,1-dichloroethane appear as a doublet? Cl H H C C Cl H Signal for methyl H protons is split into a doublet. To explain the splitting of the protons at C-2, we first focus on the two possible spin orientations of the proton at C-1. Why do the methyl protons of 1,1-dichloroethane appear as a doublet? Cl H H C C Cl H Signal for methyl H protons is split into a doublet. There are two orientations of the nuclear spin for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C2 protons. Why do the methyl protons of 1,1-dichloroethane appear as a doublet? Cl H H C C Cl H Signal for methyl H protons is split into a doublet. The protons at C-2 “feel” the effect of both the applied magnetic field and the local field resulting from the spin of the C-1 proton. Why do the methyl protons of 1,1-dichloroethane appear as a doublet? H Cl H C C Cl H H “True” chemical shift of methyl protons (no coupling). This line corresponds to molecules in which This line corresponds to molecules in which the nuclear spin of the proton at C-1 the nuclear spin of the proton at C-1 reinforces the applied field. opposes the applied field. Why does the methine proton of 1,1-dichloroethane appear as a quartet? Signal for methine proton is split into a quartet. H Cl H C C Cl H H The proton at C-1 “feels” the effect of the applied magnetic field and the local fields resulting from the spin states of the three methyl protons. The possible combinations are shown on the next slide. Why does the methine proton of 1,1-dichloroethane appear as a quartet? H Cl H C C Cl H There are eight combinations of nuclear spins for the three methyl protons. H These 8 combinations split the signal into a 1:3:3:1 quartet. The Splitting Rule for 1H NMR For simple cases, the multiplicity of a signal for a particular proton is equal to the number of equivalent vicinal protons + 1. 13.8 Splitting Patterns: The Ethyl Group CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet). BrCH2CH3 Ethyl bromide 4 lines; quartet 3 lines; triplet CH3 CH2 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 Splitting Patterns of Common Multiplets Splitting Patterns of Common Multiplets Number of equivalent protons to which H is coupled Appearance Intensities of lines of multiplet in multiplet 1 Doublet 1:1 2 3 4 5 6 Triplet Quartet Pentet Sextet Septet 1:2:1 1:3:3:1 1:4:6:4:1 1:5:10:10:5:1 1:6:15:20:15:6:1 13.9 Splitting Patterns: The Isopropyl Group (CH3)2CHX is characterized by a doubletseptet pattern (septet at lower field than the doublet). ClCH(CH3)2 Isopropyl chloride 2 lines; doublet 7 lines; septet CH3 CH 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 13.10 Splitting Patterns: Pairs of Doublets Splitting patterns are not always symmetrical, but lean in one direction or the other. Pairs of Doublets H C C H Consider coupling between two vicinal protons. If the protons have different chemical shifts, each will split the signal of the other into a doublet. Pairs of Doublets H C C H Let be the difference in chemical shift in Hz between the two protons. Let J be the coupling constant between peaks for each proton in Hz. AX H C C J H J When is much larger than J the signal for each proton is a doublet, the doublet is symmetrical, and the spin system is called AX. AM H C C J H J As /J decreases the signal for each proton remains a doublet, but becomes skewed. The outer lines decrease while the inner lines increase, causing the doublets to "lean" toward each other. AB H C H C J J When and J are similar, the spin system is called AB. Skewing is quite pronounced. It is easy to mistake an AB system of two doublets for a quartet. A2 H C C H When = 0, the two protons have the same chemical shift and don't split each other. A single line is observed. The two doublets have collapsed to a singlet. 2,3,4-Trichloroanisole (1,2,3-Trichloro-4-methoxybenzene) H Skewed doublets H OCH3 Cl Cl Cl OCH3 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 13.11 Complex Splitting Patterns Multiplets of multiplets m-Nitrostyrene H H O2N H Consider the proton shown in red. It is unequally coupled to the protons shown in blue and white. Jcis = 12 Hz; Jtrans = 16 Hz H m-Nitrostyrene H O2N The signal for the proton shown in red appears as a doublet of doublets. 16 Hz 12 Hz H 12 Hz m-Nitrostyrene H H O2N H Doublet of doublets 13.12 1H NMR Spectra of Alcohols What about H bonded to O? O—H H C O H The chemical shift for O—H is variable and depends on temperature and concentration. Splitting of the O—H proton is sometimes observed but usually is not. It usually appears as a broad singlet peak. Adding D2O converts O—H to O—D. The O—H peak disappears. 13.13 NMR and Conformations NMR is “Slow” Most conformational changes occur faster than NMR can detect them. An NMR spectrum is the weighted average of the conformations. For example, cyclohexane gives a single peak for its H atoms in NMR. Half of the time a single proton is axial and half of the time it is equatorial. The observed chemical shift is half way between the axial chemical shift and the equatorial chemical shift. 13.14 13C NMR Spectroscopy 1H and 13C NMR Compared Both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons). Both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.). It is convenient to use FT-NMR techniques for 1H; it is standard practice for 13C NMR. 1H and 13C NMR Compared 13C NMR requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom, because of differences in the magnetic properties of the two nuclei and, at the “natural abundance” level, only 1.1% of all the C atoms in a sample are 13C (most are 12C). 1H and 13C NMR Compared 13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei For 1-chloropentane, it is much easier to identify the compound by its 13C spectrum than by its 1H spectrum. 1H 1-Chloropentane ClCH2CH2CH2CH2CH3 10.0 9.0 8.0 7.0 6.0 CH3 ClCH2 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 13C 1-Chloropentane ClCH2CH2CH2CH2CH3 a separate, distinct peak appears for each of the 5 carbons 200 180 160 140 120 CDCl3 100 80 60 Chemical shift (d, ppm) 40 20 0 13.15 13C Chemical Shifts Measured in ppm (d) from the carbons of TMS. Factors Affecting 13C Chemical Shifts • Electronegativity of groups attached to carbon. • Hybridization state of carbon. Electronegativity Effects Electronegativity has an even greater effect on 13C chemical shifts than it does on 1H chemical shifts. Types of Carbons Classification CH4 Chemical shift, d 1H 13C 0.2 -2 CH3CH3 primary 0.9 8 CH3CH2CH3 secondary 1.3 16 (CH3)3CH tertiary 1.7 25 (CH3)4C quaternary 28 Replacing H by C (more electronegative) deshields C to which it is attached. Electronegativity Effects on CH3 Chemical shift, d 1H 13C CH4 0.2 -2 CH3NH2 2.5 27 CH3OH 3.4 50 CH3F 4.3 75 Electronegativity Effects and Chain Length Cl Chemical shift, d CH2 CH2 CH2 CH2 CH3 45 33 29 22 14 Deshielding effect of Cl decreases as number of bonds between Cl and C increases. Factors Affecting 13C Chemical Shifts • Electronegativity of groups attached to carbon. • Hybridization state of carbon. Hybridization Effects sp3-Hybridized carbon is more shielded than sp2. 36 114 138 36 126-142 sp-Hybridized carbon is more shielded than sp2, but less shielded than sp3. H C C CH2 68 84 22 CH2 20 CH3 13 Carbonyl Carbons Are Especially Deshielded O 127-134 CH2 C 41 171 O CH2 CH3 61 14 13C Chemical Shifts for Some Common Groups Type of carbon Chemical shift (d), ppm Type of carbon Chemical shift (d), ppm RCH3 0-35 RC CR 65-90 R2CH2 15-40 R2C CR2 100-150 R3CH 25-50 110-175 R4C 30-40 13C Chemical Shifts for Some Common Groups Type of carbon Chemical shift (d), ppm RCH2Br RCH2Cl 20-40 25-50 Type of carbon RC Chemical shift (d), ppm N 110-125 RCOR 160-185 O RCH2NH2 35-50 RCH2OH 50-65 O RCH2OR 50-65 RCR 190-220 13.16 13C NMR and Peak Intensities Pulse FT-NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive. m-Cresol CH3 7 carbons give 7 signals, but intensities are not equal OH 200 180 160 140 120 100 80 60 Chemical shift (d, ppm) 40 20 0 13.20 Infrared Spectroscopy Gives information about the functional groups in a molecule. Infrared Spectroscopy Characteristic functional groups usually found between 4000-1600 cm-1. From 1300-625 cm-1 called “fingerprint region.” Depends on transitions between vibrational energy states: Stretching. Bending. Stretching Vibrations of a CH2 Group Symmetric Antisymmetric Bending Vibrations of a CH2 Group In plane “scissoring” In plane “rocking” Bending Vibrations of a CH2 Group Out of plane “wagging” Out of plane “twisting” Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp C—H 3310-3320 sp2 C—H 3000-3100 sp3 C—H 2850-2950 sp2 C—O 1200 sp3 C—O 1025-1200 Infrared Spectrum of Hexane Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Benzene Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Dihexyl Ether Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (multiple bonds) C 1620-1680 —C C— 2100-2200 —C N 2240-2280 C Infrared Spectrum of 1-Hexene Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Hexanenitrile Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (carbonyl groups) C O Aldehydes and ketones 1710-1750 Carboxylic acids 1700-1725 Acid anhydrides 1800-1850 and 1740-1790 Esters 1730-1750 Amides 1680-1700 Infrared Spectrum of 2-Hexanone Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Hexanoic Acid Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Methyl Hexanoate Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Absorption Frequencies Frequency, cm-1 Structural unit Bending vibrations of alkenes RCH CH2 910-990 R2C CH2 890 cis-RCH CHR' trans-RCH R2C CHR' CHR' 665-730 960-980 790-840 Infrared Spectrum of 1-Hexene Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Bending vibrations of derivatives of benzene Monosubstituted 730-770 and 690-710 ortho-Disubstituted 735-770 meta-Disubstituted 750-810 and 680-730 para-Disubstituted 790-840 Infrared Spectrum of Hexylbenzene Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) O—H (alcohols) 3200-3600 O—H (carboxylic acids) 3000-3100 N—H 3350-3500 Infrared Spectrum of 1-Hexanol Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Hexylamine Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Hexanamide Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. 13.21 Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated electron systems Transitions between Electron Energy States E = h Gaps between electron energy levels are greater than those between vibrational levels. Gap corresponds to wavelengths between 200 and 800 nm. Conventions in UV-VIS X-axis is wavelength in nm (high energy at left, low energy at right). max is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially electron system. Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity (). UV Spectrum of cis,trans-1,3-Cyclooctadiene 2000 max 230 nm Molar absorptivity () max 2630 1000 200 220 240 260 Wavelength, nm 280 * Transition in cis,trans-1,3-Cyclooctadiene LUMO HOMO Most stable -electron configuration E = h -Electron configuration of excited state * Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond. As HOMO-LUMO energy difference decreases (smaller E), max shifts to longer wavelengths. Effect of Substitution Methyl groups on double bond cause max to shift to longer wavelengths H H C H H CH3 C C H max 170 nm H C CH3 max 188 nm Effect of Conjugation Extending conjugation has a larger effect on max; shift is again to longer wavelengths. H H C H H H C C H C H H C C H max 170 nm max 217 nm H H Effect of Conjugation H C C H H C C H H H3C max 217 nm for conjugated diene H C C H H C H C H C H C CH3 max 263 nm for conjugated triene plus two methyl groups Lycopene Orange-red pigment in tomatoes. max 505 nm 13.22 Mass Spectrometry Principles of Electron-Impact Mass Spectrometry Atom or molecule is hit by high-energy electron. e– Principles of Electron-Impact Mass Spectrometry Atom or molecule is hit by high-energy electron. e– Electron is deflected but transfers much of its energy to the molecule. Principles of Electron-Impact Mass Spectrometry Atom or molecule is hit by high-energy electron. e– Electron is deflected but transfers much of its energy to the molecule. Principles of Electron-Impact Mass Spectrometry This energy-rich species ejects an electron. Principles of Electron-Impact Mass Spectrometry This energy-rich species ejects an electron. + • e– Forming a positively charged, odd-electron species called the molecular ion. Principles of Electron-Impact Mass Spectrometry Molecular ion passes between poles of a magnet and is deflected by magnetic field. Amount of deflection depends on mass-to-charge ratio (m/z). Highest m/z deflected least. Lowest m/z deflected most. + • Principles of Electron-Impact Mass Spectrometry If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose. However, the molecular ion often fragments to a mixture of species of lower m/z. Principles of Electron-Impact Mass Spectrometry The molecular ion dissociates to a cation and a radical. + • Principles of Electron-Impact Mass Spectrometry The molecular ion dissociates to a cation and a radical. + • Usually several fragmentation pathways are available and a mixture of ions is produced. Principles of Electron-Impact Mass Spectrometry Mixture of ions of different mass gives separate peak for each m/z. Intensity of peak proportional to percentage of each ion of different mass in mixture. Separation of peaks depends on relative mass. + + + + + + Principles of Electron-Impact Mass Spectrometry Mixture of ions of different mass gives separate peak for each m/z. Intensity of peak proportional to percentage of each atom of different mass in mixture. Separation of peaks depends on relative mass. + + + + + + Some Molecules Undergo Very Little Fragmentation Benzene is an example. The major peak corresponds to the molecular ion. Relative intensity 100 m/z = 78 80 60 40 20 0 20 40 60 80 100 120 m/z Isotopic Clusters H H 79 H H 93.4% All H are 1H and all C are 12C. H H H H 78 H H H H 79 H H H H 6.5% One C is 13C. H H 0.1% One H is 2H. Isotopic Clusters in Chlorobenzene 35Cl 37Cl Visible in peaks Relative for molecular ion. intensity 100 112 80 60 40 114 20 0 20 40 60 80 100 120 m/z Isotopic Clusters in Chlorobenzene Relative intensity 100 80 H No m/z 77, 79 pair; therefore, ion responsible for m/z 77 peak does not contain Cl. 60 + H H H 77 40 m/z 20 0 H 20 40 60 80 100 120 Alkanes Undergo Extensive Fragmentation CH3—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH3 Relative intensity 43 57 100 80 Decane 60 71 40 85 20 0 142 99 20 40 60 80 m/z 100 120 Propylbenzene Fragments Mostly at the Benzylic Position Relative intensity 100 91 80 CH2—CH2CH3 60 40 120 20 0 20 40 60 80 m/z 100 120 13.23 Molecular Formula as a Clue to Structure Molecular Weights One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula. However, we can gain some information even from the molecular weight. Mass spectrometry makes it relatively easy to determine molecular weights. The Nitrogen Rule A molecule with an odd number of nitrogens has an odd molecular weight. A molecule that contains only C, H, and O or which has an even number of nitrogens has an even molecular weight. O2N NH2 93 NH2 138 NO2 O2N NH2 183 Exact Molecular Weights O CH3(CH2)5CH3 Heptane CH3CO Cyclopropyl acetate Molecular formula C7H16 C5H8O2 Molecular weight 100 100 Exact mass 100.1253 100.0524 Mass spectrometry can measure exact masses. Therefore, mass spectrometry can give molecular formulas. Molecular Formulas Knowing that the molecular formula of a substance is C7H16 tells us immediately that it is an alkane because it corresponds to CnH2n+2. But C7H14 lacks two hydrogens of an alkane; therefore, it contains either a ring or a double bond. Index of Hydrogen Deficiency Relates molecular formulas to multiple bonds and rings. Index of hydrogen deficiency = 1 2 (molecular formula of alkane – molecular formula of compound) Example 1 C7H14 Index of hydrogen deficiency = 1 (molecular formula of alkane – 2 molecular formula of compound) = 1 (C7H16 – C7H14) 2 = 1 (2) = 1 2 Therefore, one ring or one double bond. Example 2 C7H12 = 1 (C7H16 – C7H12) 2 = 1 (4) = 2 2 Therefore, two rings, one triple bond, two double bonds or one double bond + one ring. Oxygen Has no Effect CH3(CH2)5CH2OH (1-heptanol, C7H16O) has same number of H atoms as heptane. Index of hydrogen deficiency = 1 2 (C7H16 – C7H16O) = 0 No rings or double bonds. Oxygen Has no Effect O CH3CO Cyclopropyl acetate Index of hydrogen deficiency = 1 (C H – C H O ) = 2 5 12 5 8 2 2 One ring plus one double bond. If Halogen is Present Treat a halogen as if it were hydrogen. H Cl C H C3H5Cl C CH3 Same index of hydrogen deficiency as for C3H6. Rings versus Multiple Bonds Index of hydrogen deficiency tells us the sum of rings plus multiple bonds. Catalytic hydrogenation tells us how many multiple bonds there are.