Chapter 13 Spectroscopy Nuclear Magnetic Resonance Spectroscopy Infrared Spectroscopy Ultraviolet-Visible Spectroscopy Mass Spectrometry

13.1 Principles of Molecular Spectroscopy: Electromagnetic Radiation

Propagated at the speed of light. Has properties of particles and waves. The energy of a photon is proportional to its frequency. Electromagnetic Radiation

The Electromagnetic Spectrum 400 nm 750 nm Visible Light Longer Wavelength ( ) Shorter Wavelength ( ) Higher Frequency ( ) Lower Frequency ( ) Higher Energy (E) Lower Energy (E)

The Electromagnetic Spectrum UltravioletInfrared Longer Wavelength ( ) Shorter Wavelength ( ) Higher Frequency ( ) Lower Frequency ( ) Higher Energy (E) Lower Energy (E)

Cosmic rays  Rays X-rays Ultraviolet (UV) light Visible light Infrared (IR) radiation Microwaves Radio waves Cosmic rays  Rays X-rays Ultraviolet (UV) light Visible light Infrared (IR) radiation Microwaves Radio waves The Electromagnetic Spectrum Energy

13.2 Principles of Molecular Spectroscopy: Quantized Energy States

Electromagnetic radiation is absorbed when the energy of the photon corresponds to the difference in energy between two states.  E = h  E = h

ElectronicVibrationalRotational Nuclear spin UV-VisibleInfraredMicrowaveRadiofrequency What Kind of States?

13.3 Introduction to 1 H NMR Spectroscopy

1 H and 13 C: Both have spin = ±1/2. 1 H is % at natural abundance. 13 C is 1.1% at natural abundance. The nuclei that are most useful to organic chemists are:

Nuclear Spin A spinning charge, such as the nucleus of 1 H or 13 C, 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. H0H0H0H0

There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. H0H0H0H0

No difference in absence of magnetic field. Proportional to strength of external magnetic field. Energy Differences Between Nuclear Spin States + + EEEE  E ' Increasing field strength

Some Important Relationships in NMR 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. UnitsHz 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: 1 H absorbs radiation having a frequency of 200 MHz (200 x 10 6 s -1 ) 13 C absorbs radiation having a frequency of 50.4 MHz (50.4 x 10 6 s -1 )

Some Important Relationships in NMR The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1 H) depends on its molecular environment. This is why NMR is such a useful tool for structure determination.

13.4 Nuclear Shielding and 1 H 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 H 0H 0H 0H 0

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 H 0H 0H 0H 0

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 H 0H 0H 0H 0 Chemical Shift

Chemical shifts (  ) are measured relative to the protons in tetramethylsilane (TMS) as a standard. Si CH 3 H3CH3CH3CH3C  = position of signal - position of TMS peak spectrometer frequency x 10 6 Chemical Shift

NMR Spectrometers

Chemical shift ( , ppm) measured relative to TMS Upfield Increased shielding Downfield Decreased shielding (CH 3 ) 4 Si (TMS)

Example: The signal for the proton in chloroform (HCCl 3 ) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz.  = position of signal - position of TMS peak spectrometer frequency x 10 6  = 1456 Hz - 0 Hz 200 x 10 6 Hz x 10 6  = 7.28 Chemical Shift

Chemical shift ( , ppm)  7.28 ppm H C Cl ClClChloroform

13.5 Effects of Molecular Structure on 1 H 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 Most shielded H CH 3 F CH 3 OCH 3 (CH 3 ) 3 N CH 3 CH 3 (CH 3 ) 4 Si  4.3  3.2  2.2  0.9  0.0

Electronegative Substituents Decrease Shielding H 3 C—CH 2 —CH 3 O 2 N—CH 2 —CH 2 —CH 3  0.9  1.3  1.0  4.3  2.0

Effect is Cumulative CH 3 Cl  3.1 CH 2 Cl 2  5.3 CHCl 3  7.3

Methyl, Methylene and Methine CH 3 more shielded than CH 2. CH 2 more shielded than CH. H3CH3CH3CH3C C CH3CH3CH3CH3 CH 3 H  0.9  1.6  0.8 H3CH3CH3CH3C C CH3CH3CH3CH3 CH 3 CH2CH2CH2CH2  0.9 CH 3  1.2

Protons Attached to sp 2 -hybridized Carbon are Less Shielded than Those Attached to sp 3 -hybridized Carbon HH HH HH C CHHHH CH 3 CH 3  7.3  5.3  0.9

But Protons Attached to sp-hybridized Carbon are More Shielded than Those Attached to sp 2 -hybridized Carbon C C HH HH  5.3  2.4 CH 2 OCH 3 C C H

Protons Attached to Benzylic and Allylic Carbons are Somewhat Less Shielded than Usual  1.5  0.8 H3CH3CH3CH3C CH 3  1.2 H3CH3CH3CH3C CH 2  2.6 H 3 C—CH 2 —CH 3  0.9  1.3

Proton Attached to C=O of Aldehyde is Most Deshielded C—H  2.4  9.7  1.1 CC O H H CH 3 H3CH3CH3CH3C

Type of proton Chemical shift (  ), ppm Type of proton Chemical shift (  ), ppm C HR C H CC C H CO C H NC C HAr C H CC 1 H Chemical Shifts of Some Common Groups

C HBr COH C HNR C HCl HAr C CH C H O Type of proton Chemical shift (  ), ppm Type of proton Chemical shift (  ), ppm

1-3HNR0.5-5HOR6-8HOAr10-13 CO HOHOHOHO 1 H Chemical Shifts of Some Common Groups Type of proton Chemical shift (  ), ppm

13.6 Interpreting Proton NMR Spectra

1. Number of signals. 2. Their intensity (as measured by area under peak). 3. Splitting pattern (multiplicity). Information contained in an NMR spectrum includes:

Number of Signals Protons that have different chemical shifts are chemically nonequivalent. Exist in different molecular environment.

Chemical shift ( , ppm) CCH 2 OCH 3 N OCH 3 NCCH 2 O Methoxyacetonitrile

Are in identical environments. Have same chemical shift. Replacement test: replacement by some arbitrary "test group" generates same compound. H 3 CCH 2 CH 3 chemically equivalent Chemically Equivalent Protons

H 3 CCH 2 CH 3 Chemically equivalent CH 3 CH 2 CH 2 Cl ClCH 2 CH 2 CH 3 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.

Replacement by some arbitrary test group generates diastereomers. Diastereotopic protons can have different chemical shifts. Diastereotopic Protons C CBr H3CH3CH3CH3C H H  5.3 ppm  5.5 ppm

Are in mirror-image environments. Replacement by some arbitrary test group generates enantiomers. Enantiotopic protons have the same chemical shift. Enantiotopic Protons

C CH 2 OH H3CH3CH3CH3C HH Enantiotopic Protons C CH 2 OH H3CH3CH3CH3C ClHC H3CH3CH3CH3C HCl R R S S

Not all peaks are singlets. Signals can be split by coupling of nuclear spins Spin-Spin Splitting in NMR Spectroscopy

Chemical shift ( , ppm) Cl 2 CHCH 3 1,1-Dicholoroethane 4 lines; quartet 2 lines; doublet CH3CH3CH3CH3 CHCHCHCH

Two-Bond and Three-Bond Coupling C C H H C C HH Protons separated by two bonds (geminal relationship). Protons separated by three bonds (vicinal relationship).

In order to observe splitting, protons cannot have same chemical shift. Coupling constant ( 2 J or 3 J) is independent of field strength and are measured in Hz. Two-Bond and Three-Bond Coupling C C H H C C HH

Chemical shift ( , ppm) Cl 2 CHCH 3 1,1-Dicholoroethane 4 lines; quartet 2 lines; doublet CH3CH3CH3CH3 CHCHCHCH Coupled protons are vicinal (three-bond coupling). CH splits CH 3 into a doublet. CH 3 splits CH into a quartet.

Why do the methyl protons of 1,1-dichloroethane appear as a doublet? C C HH Cl Cl HH Signal for methyl 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? C C HH Cl Cl HH Signal for methyl 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 C- 2 protons.

Why do the methyl protons of 1,1-dichloroethane appear as a doublet? C C HH Cl Cl HH Signal for methyl 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? C C HH Cl Cl HH “True” chemical shift of methyl protons (no coupling). This line corresponds to molecules in which the nuclear spin of the proton at C-1 reinforces the applied field. This line corresponds to molecules in which the nuclear spin of the proton at C-1 opposes the applied field.

Why does the methine proton of 1,1-dichloroethane appear as a quartet? C C HH Cl Cl HH Signal for methine proton is split into a quartet. 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.

C C HH Cl Cl HH There are eight combinations of nuclear spins for the three methyl protons. These 8 combinations split the signal into a 1:3:3:1 quartet. Why does the methine proton of 1,1-dichloroethane appear as a quartet?

For simple cases, the multiplicity of a signal for a particular proton is equal to the number of equivalent vicinal protons + 1. The Splitting Rule for 1 H NMR

13.8 Splitting Patterns: The Ethyl Group CH 3 CH 2 X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet).

Chemical shift ( , ppm) BrCH 2 CH 3 Ethyl bromide 4 lines; quartet 3 lines; triplet CH3CH3CH3CH3 CH2CH2CH2CH2

Splitting Patterns of Common Multiplets Number of equivalentAppearanceIntensities of lines protons to which H of multipletin multiplet is coupled 1Doublet1:1 2Triplet1:2:1 3Quartet1:3:3:1 4Pentet1:4:6:4:1 5Sextet1:5:10:10:5:1 6Septet1:6:15:20:15:6:1

13.9 Splitting Patterns: The Isopropyl Group (CH 3 ) 2 CHX is characterized by a doublet- septet pattern (septet at lower field than the doublet).

Chemical shift ( , ppm) ClCH(CH 3 ) 2 Isopropyl chloride 7 lines; septet 2 lines; doublet CH3CH3CH3CH3 CHCHCHCH

13.10 Splitting Patterns: Pairs of Doublets Splitting patterns are not always symmetrical, but lean in one direction or the other.

Pairs of Doublets Consider coupling between two vicinal protons. If the protons have different chemical shifts, each will split the signal of the other into a doublet. C C HH

Pairs of Doublets 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. C C HH

AX 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. C C HH JJ

AM 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. C C HH JJ

AB 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. C C HH JJ

A2A2A2A2 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. C C HH

Chemical shift ( , ppm) 2,3,4-Trichloroanisole (1,2,3-Trichloro-4-methoxybenzene) OCH 3 Skewed doublets HH Cl Cl Cl OCH 3

13.11 Complex Splitting Patterns Multiplets of multiplets

m-Nitrostyrene Consider the proton shown in red. It is unequally coupled to the protons shown in blue and white. J cis = 12 Hz; J trans = 16 Hz HH O2NO2NO2NO2N H

m-Nitrostyrene 16 Hz 12 Hz The signal for the proton shown in red appears as a doublet of doublets. HH O2NO2NO2NO2N H

m-Nitrostyrene HH O2NO2NO2NO2N H Doublet of doublets

H NMR Spectra of Alcohols What about H bonded to O?

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 D 2 O converts O—H to O—D. The O—H peak disappears. C O HH

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.

C NMR Spectroscopy

1 H and 13 C 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 1 H; it is standard practice for 13 C NMR.

1 H and 13 C NMR Compared 13 C NMR requires FT-NMR because the signal for a carbon atom is 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 13 C (most are 12 C).

1 H and 13 C NMR Compared 13 C signals are spread over a much wider range than 1 H signals making it easier to identify and count individual nuclei For 1-chloropentane, it is much easier to identify the compound by its 13 C spectrum than by its 1 H spectrum.

Chemical shift ( , ppm) ClCH 2 1-Chloropentane CH3CH3CH3CH3 ClCH 2 CH 2 CH 2 CH 2 CH 3 1H1H1H1H

Chemical shift ( , ppm) 1-Chloropentane ClCH 2 CH 2 CH 2 CH 2 CH C CDCl 3 a separate, distinct peak appears for each of the 5 carbons

C Chemical Shifts Measured in ppm (  ) from the carbons of TMS.

Factors Affecting 13 C Chemical Shifts Electronegativity of groups attached to carbon.Electronegativity of groups attached to carbon. Hybridization state of carbon.Hybridization state of carbon.

Electronegativity Effects Electronegativity has an even greater effect on 13 C chemical shifts than it does on 1 H chemical shifts.

Types of Carbons (CH 3 ) 3 CH CH4CH4CH4CH4 CH3CH3CH3CH3CH3CH3CH3CH3 CH 3 CH 2 CH 3 (CH 3 ) 4 C primarysecondary tertiary quaternary Classification Chemical shift,  1H1H1H1H 13 C Replacing H by C (more electronegative) deshields C to which it is attached.

Electronegativity Effects on CH 3 CH3FCH3FCH3FCH3F CH4CH4CH4CH4 CH 3 NH 2 CH 3 OH Chemical shift,  1H1H1H1H C

Electronegativity Effects and Chain Length Chemical shift,  Cl CH 2 CH Deshielding effect of Cl decreases as number of bonds between Cl and C increases.

Factors Affecting 13 C Chemical Shifts Electronegativity of groups attached to carbon.Electronegativity of groups attached to carbon. Hybridization state of carbon.Hybridization state of carbon.

Hybridization Effects sp 3 -Hybridized carbon is more shielded than sp sp-Hybridized carbon is more shielded than sp 2, but less shielded than sp 3. CH 3 HCC CH

Carbonyl Carbons Are Especially Deshielded O CH 2 C O CH

13 C Chemical Shifts for Some Common Groups Type of carbon Chemical shift (  ), ppm Type of carbon Chemical shift (  ), ppm RCH3RCH3RCH3RCH30-35 CR2CR2CR2CR2 R2CR2CR2CR2C65-90 CRCRCRCR RCRCRCRC R2CH2R2CH2R2CH2R2CH R3CHR3CHR3CHR3CH25-50 R4CR4CR4CR4C

RCH 2 Br RCH 2 Cl RCH 2 NH RCH 2 OH RCH 2 OR RCOR O RCRRCRRCRRCRO RCRCRCRCN C Chemical Shifts for Some Common Groups Type of carbon Chemical shift (  ), ppm Type of carbon Chemical shift (  ), ppm

C NMR and Peak Intensities Pulse FT-NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.

m-Cresol Chemical shift ( , ppm) carbons give 7 signals, but intensities are not equal CH 3 OH

13.20 Infrared Spectroscopy Gives information about the functional groups in a molecule.

Characteristic functional groups usually found between cm -1. From cm -1 called “fingerprint region.” Depends on transitions between vibrational energy states: Stretching.Bending. Infrared Spectroscopy

Stretching Vibrations of a CH 2 Group SymmetricAntisymmetric

Bending Vibrations of a CH 2 Group In plane “scissoring” “rocking”

Bending Vibrations of a CH 2 Group Out of plane “wagging” “twisting”

Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp C—H sp 2 C—H sp 3 C—H sp 2 C—O1200 sp 3 C—O Infrared Absorption Frequencies

Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. Infrared Spectrum of Hexane

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.

Structural unitFrequency, cm -1 Stretching vibrations (multiple bonds) C C —CN —CC— Infrared Absorption Frequencies

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.

Structural unitFrequency, cm -1 Stretching vibrations (carbonyl groups) Aldehydes and ketones Carboxylic acids Acid anhydrides and Esters Amides C O Infrared Absorption Frequencies

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.

Structural unitFrequency, cm -1 Bending vibrations of alkenes CH 2 RCH R2CR2CR2CR2C CHR' cis-RCH CHR' trans-RCH CHR' R2CR2CR2CR2C Infrared Absorption Frequencies

Infrared Spectrum of 1-Hexene Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

Structural unitFrequency, cm -1 Bending vibrations of derivatives of benzene Monosubstituted and ortho-Disubstituted meta-Disubstituted and para-Disubstituted Infrared Absorption Frequencies

Infrared Spectrum of Hexylbenzene Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

Structural unitFrequency, cm -1 Stretching vibrations (single bonds) O—H (alcohols) O—H (carboxylic acids) N—H Infrared Absorption Frequencies

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

Gaps between electron energy levels are greater than those between vibrational levels. Gap corresponds to wavelengths between 200 and 800 nm. Transitions between Electron Energy States  E = h  E = h

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. 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 (  ). Conventions in UV-VIS

Wavelength, nm max 230 nm max 230 nm  max 2630 Molar absorptivity (  ) UV Spectrum of cis,trans-1,3-Cyclooctadiene

Most stable  -electron configuration  -Electron configuration of excited state          * Transition in cis,trans-1,3-Cyclooctadiene HOMO LUMO  E = h  E = h

 * 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 C C H H H H C C H H CH 3 max 170 nm max 170 nm CH 3 max 188 nm max 188 nm

Effect of Conjugation Extending conjugation has a larger effect on max ; shift is again to longer wavelengths. C C H H H H C C H H max 170 nm max 170 nm max 217 nm max 217 nm H C C H H H

Effect of Conjugation max 217 nm for conjugated diene max 217 nm for conjugated diene H C CHH C C H H H C C H CH 3 H H C C H3CH3CH3CH3CH C C H H max 263 nm for conjugated triene plus two methyl groups max 263 nm for conjugated triene plus two methyl groups

Lycopene max 505 nm max 505 nm Orange-red pigment in tomatoes.

13.22 Mass Spectrometry

Atom or molecule is hit by high-energy electron. Principles of Electron-Impact Mass Spectrometry e–e–e–e–

Atom or molecule is hit by high-energy electron. Electron is deflected but transfers much of its energy to the molecule. e–e–e–e– Principles of Electron-Impact Mass Spectrometry

Atom or molecule is hit by high-energy electron. Electron is deflected but transfers much of its energy to the molecule. e–e–e–e– 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. Principles of Electron-Impact Mass Spectrometry Forming a positively charged, odd-electron species called the molecular ion. e–e–e–e– +

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.

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. Principles of Electron-Impact Mass Spectrometry + Usually several fragmentation pathways are available and a mixture of ions is produced.

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 Principles of Electron-Impact Mass Spectrometry

m/z m/z = Relative intensity Some Molecules Undergo Very Little Fragmentation Benzene is an example. The major peak corresponds to the molecular ion.

HHH HH H HHH HH H HHH HH H All H are 1 H and all C are 12 C. One C is 13 C. One H is 2 H. Isotopic Clusters %6.5%0.1%

m/z Relative intensity Isotopic Clusters in Chlorobenzene Visible in peaks for molecular ion. 35 Cl 37 Cl

m/z Relative intensity 77 Isotopic Clusters in Chlorobenzene No m/z 77, 79 pair; therefore, ion responsible for m/z 77 peak does not contain Cl. H H H H H

Alkanes Undergo Extensive Fragmentation m/z Decane CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 Relative intensity

Propylbenzene Fragments Mostly at the Benzylic Position m/z Relative intensity CH 2 —CH 2 CH

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. NH2NH2NH2NH NH2NH2NH2NH2 O2NO2NO2NO2N 183 NH2NH2NH2NH2 O2NO2NO2NO2N NO2NO2NO2NO2

Exact Molecular Weights CH 3 (CH 2 ) 5 CH 3 Heptane CH 3 CO O Cyclopropyl acetate Molecular formula Molecular weight C 7 H 16 C5H8O2C5H8O2C5H8O2C5H8O Exact mass Mass spectrometry can measure exact masses. Therefore, mass spectrometry can give molecular formulas.

Molecular Formulas Knowing that the molecular formula of a substance is C 7 H 16 tells us immediately that it is an alkane because it corresponds to C n H 2n+2. But C 7 H 14 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 Index of hydrogen deficiency C 7 H (molecular formula of alkane – molecular formula of compound) = 12 (C 7 H 16 – C 7 H 14 ) = 12 (2) = 1 = Therefore, one ring or one double bond.

Example 2 C 7 H (C 7 H 16 – C 7 H 12 ) = 1 2 (4) = 2 = Therefore, two rings, one triple bond, two double bonds or one double bond + one ring.

Oxygen Has no Effect CH 3 (CH 2 ) 5 CH 2 OH (1-heptanol, C 7 H 16 O) has same number of H atoms as heptane. Index of hydrogen deficiency = 1 2 (C 7 H 16 – C 7 H 16 O) = 0 = 0 No rings or double bonds.

Oxygen Has no Effect Index of hydrogen deficiency = 1 2 (C 5 H 12 – C 5 H 8 O 2 ) = 2 = 2 One ring plus one double bond. CH 3 CO O Cyclopropyl acetate

If Halogen is Present Treat a halogen as if it were hydrogen. C C CH 3 Cl H H C 3 H 5 Cl Same index of hydrogen deficiency as for C 3 H 6.

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.