Chapter 13 Spectroscopy

Introduction Spectroscopy is a technique used to determine the structure of a compound. Most techniques are nondestructive (destroys little or no sample). Absorption spectroscopy measures the amount of light absorbed by the sample as a function of wavelength.

Types of Spectroscopy Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the mass. MS can give the molecular weight of the compound and functional groups. Nuclear magnetic resonance (NMR) spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present. Ultraviolet (UV) spectroscopy uses electronic transitions to determine bonding patterns.

Electromagnetic Radiation Is propagated at the speed of light Has properties of particles and waves The energy of a photon is proportional to its frequency 2

E = h Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states. 6

The Electromagnetic Spectrum

Introduction to 1H NMR Spectroscopy 8

The Nuclei that are Most Useful to Organic Chemists are: 1H and 13C both have spin = ±1/2 1H is 99% at natural abundance 13C is 1.1% at natural abundance 9

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. 10

The distribution of nuclear spins is random in the absence of an external magnetic field. + + + + + 11

An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + + + B0 + + 11

There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. + + + B0 + + 11

Energy Differences Between Nuclear Spin States + + increasing field strength No difference in absence of magnetic field Proportional to strength of external magnetic field 12

Some Important Relationships in NMR Units Hz kJ/mol (kcal/mol) tesla (T) 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. 6

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) 6

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. 6

Nuclear Shielding and 1H Chemical Shifts What do we mean by "shielding"? What do we mean by "chemical shift"? 14

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 B 0 15

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 B 0 15

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 B 0 15

Chemical Shift Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard. Si CH3 H3C d = position of signal - position of TMS peak spectrometer frequency x 106 15

Downfield Decreased shielding Upfield Increased shielding (CH3)4Si (TMS) 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) measured relative to TMS 1

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 = position of signal - position of TMS peak spectrometer frequency x 106 d = 1456 Hz - 0 Hz 200 x 106 Hx x 106 d = 7.28 15

H C Cl  7.28 ppm Chemical shift (, ppm) 1 1.0 2.0 3.0 4.0 5.0 6.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

Effects of Molecular Structure on 1H Chemical Shifts Protons in different environments experience different degrees of shielding and have different chemical shifts. 17

Electronegative Substituents Decrease the Shielding of Methyl Groups least shielded H most shielded H CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si d 4.3 d 3.2 d 2.2 d 0.9 d 0.0 21

Electronegative Substituents Decrease Shielding H3C—CH2—CH3 d 4.3 d 2.0 d 1.0 O2N—CH2—CH2—CH3 21

Effect is Cumulative CHCl3  7.3 CH2Cl2  5.3 CH3Cl  3.1 21

Methyl, Methylene, and Methine CH3 more shielded than CH2 ; CH2 more shielded than CH H3C C CH3 H d 0.9 d 1.6 d 0.8 CH2 d 1.2 21

Protons Attached to sp2 Hybridized Carbon are Less Shielded than those Attached to sp3 Hybridized Carbon H C H CH3CH3  7.3  5.3  0.9 21

But Protons Attached to sp Hybridized Carbon are More Shielded than those Attached to sp2 Hybridized Carbon  5.3 C H  2.4 CH2OCH3 C H 21

Protons Attached to Benzylic and Allylic Carbons are Somewhat Less Shielded than Usual  1.5  0.8 H3C CH3 d 0.9 d 1.3 H3C—CH2—CH3  1.2 H3C CH2  2.6 21

Proton Attached to C=O of Aldehyde is Most Deshielded C—H H3C 21

Table 13.1 C H R 0.9-1.8 C H N 2.1-2.3 C H 1.5-2.6 C H Ar 2.3-2.8 C H Type of proton Chemical shift (), ppm Type of proton Chemical shift (), ppm C H R 0.9-1.8 C H N 2.1-2.3 C H 1.5-2.6 C H Ar 2.3-2.8 C H O 2.0-2.5 25

Table 13.1 C H NR 2.2-2.9 C H 4.5-6.5 C H Cl 3.1-4.1 C H Br H Ar Type of proton Chemical shift (), ppm Type of proton Chemical shift (), ppm C H NR 2.2-2.9 C H 4.5-6.5 C H Cl 3.1-4.1 C H Br H Ar 6.5-8.5 2.7-4.1 C O H C H O 9-10 3.3-3.7 25

Table 13.1 H NR 1-3 H OR 0.5-5 H OAr 6-8 C O HO 10-13 Type of proton Chemical shift (), ppm H NR 1-3 H OR 0.5-5 H OAr 6-8 C O HO 10-13 25

Interpreting 1H NMR Spectra Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

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

They exist in different molecular environments. Number of Signals Protons that have different chemical shifts are chemically nonequivalent. They exist in different molecular environments. 3

N Figure 13.12 CCH2OCH3 OCH3 NCCH2O Chemical shift (, ppm) 1 1.0 2.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

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 7

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 7

Diastereotopic Protons Replacement by some arbitrary test group generates diastereomers. Diastereotopic protons can have different chemical shifts. C Br H3C H  5.3 ppm  5.5 ppm 9

Are in mirror-image environments. Enantiotopic Protons Are in mirror-image environments. Replacement by some arbitrary test group generates enantiomers. Enantiotopic protons have the same chemical shift. 10

Enantiotopic Protons C CH2OH H3C H C CH2OH H3C Cl H C CH2OH H3C H Cl R 11

13.7 Spin-Spin Splitting in 1H NMR Spectroscopy Not all peaks are singlets. Signals can be split by coupling of nuclear spins. 12

Figure 13.13 Cl2CHCH3 4 lines; quartet 2 lines; doublet CH3 CH 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

Two-bond and Three-bond Coupling protons separated by two bonds (geminal relationship) protons separated by three bonds (vicinal relationship) 14

Two-bond and Three-bond Coupling In order to observe splitting, protons cannot have same chemical shift. Coupling constant (2J or 3J) is independent of field strength. 14

Figure 13.13 Cl2CHCH3 4 lines; quartet 2 lines; doublet CH3 CH coupled protons are vicinal (three-bond coupling) CH splits CH3 into a doublet CH3 splits CH into a quartet 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

Why Do the Methyl Protons of 1,1-Dichloroethane Appear as a Doublet? Cl 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. 19

Why Do the Methyl Protons of 1,1-Dichloroethane Appear as a Doublet? Cl 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. 19

Why Do the Methyl Protons of 1,1-Dichloroethane Appear as a Doublet? Cl 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. 19

Why Do the Methyl Protons of 1,1-Dichloroethane Appear as a Doublet? Cl "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. 19

Why Does the Methine Proton of 1,1-Dichloroethane Appear as a Quartet? Cl 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. 19

Why Does the Methine Proton of 1,1-Dichloroethane Appear as a Quartet? Cl 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. 20

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. 22

Splitting Patterns: The Ethyl Group CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet). 24

Figure 13.16 BrCH2CH3 4 lines; quartet 3 lines; triplet CH3 CH2 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

Splitting Patterns of Common Multiplets Table 13.2 Splitting Patterns of Common Multiplets Number of equivalent Appearance Intensities of lines protons to which H of multiplet in multiplet is coupled 1 Doublet 1:1 2 Triplet 1:2:1 3 Quartet 1:3:3:1 4 Pentet 1:4:6:4:1 5 Sextet 1:5:10:10:5:1 6 Septet 1:6:15:20:15:6:1 23

Splitting Patterns: The Isopropyl Group (CH3)2CHX is characterized by a doublet-septet pattern (septet at lower field than the doublet). 24

Figure 13.18 ClCH(CH3)2 2 lines; doublet 7 lines; septet CH3 CH 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

Splitting Patterns: Pairs of Doublets Splitting patterns are not always symmetrical, but lean in one direction or the other. 24

Consider coupling between two vicinal protons. Pairs of Doublets C H 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. 6

Let J be the coupling constant between them in Hz. Pairs of Doublets C H H Let  be the difference in chemical shift in Hz between the two hydrogens. Let J be the coupling constant between them in Hz. 6

C H AX J 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. 6

C H AM J 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. 6

C H AB 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. 6

C H H A2 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. 6

H Figure 13.20 skewed doublets Cl OCH3 OCH3 Chemical shift (, ppm) 1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

Complex Splitting Patterns Multiplets of multiplets 24

Consider the proton shown in red. m-Nitrostyrene H O2N Consider the proton shown in red. It is unequally coupled to the protons shown in blue and green. Jcis = 12 Hz; Jtrans = 16 Hz 6

H O2N m-Nitrostyrene 16 Hz The signal for the proton shown in red appears as a doublet of doublets. 12 Hz 12 Hz 6

Figure 13.21 H O2N doublet doublet doublet of doublets 1

1H NMR Spectra of Alcohols What about H bonded to O? 24

Adding D2O converts O—H to O—D. The O—H peak disappears. The chemical shift for O—H is variable ( 0.5-5 ppm) and depends on temperature and concentration. Splitting of the O—H proton is sometimes observed, but often is not. It usually appears as a broad peak. Adding D2O converts O—H to O—D. The O—H peak disappears. 6

NMR and Conformations NMR is “Slow” Most conformational changes occur faster than NMR can detect them. An NMR spectrum shows 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 halfway between the axial chemical shift and the equatorial chemical shift. 6

13C NMR Spectroscopy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

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. 6

1H and 13C NMR Compared: 13C requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom. A signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C). 6

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. Figure 13.26 (a) shows the 1H NMR spectrum of 1-chloropentane; Figure 13.26 (b) shows the 13C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13C spectrum than by its 1H spectrum. 6

1H Figure 13.26(a) ClCH2 CH3 ClCH2CH2CH2CH2CH3 Chemical shift (, ppm) 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (, ppm) 1

13C Figure 13.26(b) ClCH2CH2CH2CH2CH3 A separate, distinct peak appears for each of the 5 carbons. CDCl3 20 40 60 80 100 120 140 160 180 200 Chemical shift (, ppm) 1

are measured in ppm () from the carbons of TMS 13C Chemical Shifts are measured in ppm () from the carbons of TMS 3

13C Chemical Shifts are Most Affected By: Electronegativity of groups attached to carbon Hybridization state of carbon 6

Electronegativity Effects Electronegativity has an even greater effect on 13C chemical shifts than it does on 1H chemical shifts. 6

1H 13C Types of Carbons Classification Chemical shift, d (CH3)3CH CH4 0.2 -2 8 16 25 28 primary secondary tertiary quaternary 0.9 1.3 1.7 Replacing H with C (more electronegative) deshields C to which it is attached. 6

Electronegativity Effects on CH3 Chemical shift, d 1H 0.2 2.5 3.4 4.3 13C -2 27 50 75 CH4 CH3NH2 CH3OH CH3F 6

Electronegativity Effects and Chain Length Cl CH2 CH3 Chemical shift, d 45 33 29 22 14 Deshielding effect of Cl decreases as number of bonds between Cl and C increases. 6

13C Chemical Shifts are Most Affected By: Electronegativity of groups attached to carbon Hybridization state of carbon 6

Hybridization Effects 114 138 36 126-142 sp3 hybridized carbon is more shielded than sp2. sp hybridized carbon is more shielded than sp2, but less shielded than sp3. CH3 H C CH2 68 84 22 20 13 6

Carbonyl Carbons are Especially Deshielded CH2 C O CH2 CH3 41 171 61 14 127-134 6

Table 13.3 RCH3 0-35 CR RC 65-90 R2CH2 15-40 CR2 R2C 100-150 R3CH Type of carbon Chemical shift (), ppm Type of carbon Chemical shift (), ppm RCH3 0-35 CR RC 65-90 R2CH2 15-40 CR2 R2C 100-150 R3CH 25-50 110-175 R4C 30-40 25

Table 13.3 RCH2Br 20-40 RC N 110-125 O RCH2Cl 25-50 RCOR 160-185 Type of carbon Chemical shift (), ppm Type of carbon Chemical shift (), ppm RCH2Br 20-40 RC N 110-125 O RCH2Cl 25-50 RCOR 160-185 RCH2NH2 35-50 RCH2OH 50-65 O RCH2OR 50-65 RCR 190-220 25

13C NMR and Peak Intensities Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive. 3

7 carbons give 7 signals, but intensities are not equal. Figure 13.27 CH3 OH 7 carbons give 7 signals, but intensities are not equal. 20 40 60 80 100 120 140 160 180 200 Chemical shift (, ppm) 1

13C—H Coupling 3

Peaks in a 13C NMR Spectrum are Typically Singlets 13C—13C splitting is not seen because the probability of two 13C nuclei being in the same molecule is very small. 13C—1H splitting is not seen because spectrum is measured under conditions that suppress this splitting (broadband decoupling). 9