William H. Brown & Christopher S. Foote

Transcription

1 Requests for permission to make copies of any part of the work should be mailed to:permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida William H. Brown & Christopher S. Foote 13-1

2 Chapter

3 Electromagnetic Radiation u Electromagnetic radiation: light and other forms of radiant energy u Wavelength ( ): the distance between consecutive identical points on a wave u Frequency ( ): the number of full cycles of a wave that pass a point in a second u Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read per second ) 13-3

4 Electromagnetic Radiation u Wavelength Relation Unit to Meter meter (m) ---- millimeter (mm) 1 mm = 10-3 m micrometer ( m) nanometer (nm) Angstrom (Å) 1 mm = 10-6 m 1 nm = 10-9 m 1 Å = m 13-4

5 Molecular Spectroscopy u Molecular spectroscopy: the study of which frequencies of electromagnetic radiation are absorbed or emitted by substances and the correlation between these frequencies and specific types of molecular structure 13-5

6 Molecular Spectroscopy u We study three types of molecular spectroscopy Region of the Electromagnetic Spectrum radio frequency infrared ultraviolet-visible Absorption of Electromagnetic Radiation Results in Transition Between nuclear spin energy levels vibrational energy levels electronic energy levels 13-6

7 Molecular Spectroscopy u Nuclear magnetic resonance (NMR) spectroscopy: a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of hydrogens using 1 H-NMR spectroscopy carbons using 13 C-NMR spectroscopy phosphorus using 31 P-NMR spectroscopy 13-7

8 Nuclear Spin States u An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 this spinning charge creates an associated magnetic field in effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic moment u Pauli exclusion principle: two electrons can occupy the same atomic or molecular orbital only if they have paired (opposite) spins 13-8

9 Nuclear Spin States u Any atomic nucleus that has an odd mass, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment. u The allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus. u A nucleus with spin quantum number I has 2I + 1spin states. If I = 1/2, there are two allowed spin states 13-9

10 Nuclear Spin States u Spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds Element 1 H 2 H 12 C 13 C 14 N 16 O 31 P nuclear spin quantum 1/ / /2 0 number ( I ) 32 S number of spin states

11 Nuclear Spins in B 0 u Within a collection of 1 H and 13 C atoms, nuclear spins are completely random in orientation u When placed in a strong external magnetic field of strength B 0, however, interaction between nuclear spins and the applied magnetic field are quantized, with the result that only certain orientations of nuclear magnetic moments are allowed 13-11

12 Nuclear Spins in B Nuclear Spins in B 0 u For 1 H and 13 C, only two orientations are allowed

13 Nuclear Spins in B 0 u In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1 H is approximately cal/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz) 13 C is approximately cal/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz) 13-13

14 Nuclear Magnetic Resonance u When nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy state u The nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out cone-shaped surface as it precesses in the earth s gravitational field u We express the rate of precession as a frequency in hertz 13-14

15 Nuclear Magnetic Resonance u If the precessing nucleus is irradiated with electromagnetic radiation of the same frequency as the rate of precession, the two frequencies couple, energy is absorbed, and the nuclear spin is flipped from spin state +1/2 (with the applied field) to -1/2 (against the applied field) 13-15

16 Nuclear Magnetic Resonance u Coupling of precession frequency and the frequency of electromagnetic radiation 13-16

17 Nuclear Magnetic Resonance u Resonance: the absorption of electromagnetic radiation by a precessing nucleus and the flip of its nuclear spin from a lower energy state to a higher energy state u The instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal 13-17

18 Nuclear Magnetic Resonance u If we were dealing with 1 H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1 H would produce a signal for all 1 H. The same is true of 13 C nuclei u But hydrogens in organic molecules are not isolated from all other atoms; they are surrounded by electrons, which are caused to circulate by the presence of the applied field 13-18

19 Nuclear Magnetic Resonance u The circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding u The difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small 13-19

20 Nuclear Magnetic Resonance u The difference in resonance frequencies for hydrogens in CH 3 Cl compared to CH 3 F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency 360 Hz 300 x 10 6 Hz = = 1.2 ppm 13-20

21 Nuclear Magnetic Resonance u It is customary to measure the resonance frequency (signal) of individual nuclei relative to the resonance frequency (signal) of a reference compound u The reference compound now universally accepted is tetramethylsilane (TMS) CH 3 H 3 C Si CH 3 CH 3 Tetramethylsilane (TMS) 13-21

22 Nuclear Magnetic Resonance u For a 1 H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS u For a 13 C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS u Chemical shift ( ): the shift in ppm of an NMR signal from the signal of TMS = Shift in frequency from TMS (Hz) Frequency of spectrometer (Hz) 13-22

23 NMR Spectrometer u Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector u The sample is dissolved in a solvent, most commonly CDCl 3 or D 2 O, and placed in a sample tube which is then suspended in the magnetic field and set spinning u Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds 13-23

24 NMR Spectrum u Downfield: the shift of an NMR signal to the left on the chart paper u Upfield: the shift of an NMR signal to the right on the chart paper 13-24

25 Equivalent Hydrogens u Equivalent hydrogens: have the same chemical environment (Section 2.3C) u Molecules with 1 set of equivalent hydrogens give 1 NMR signal 2 or more sets of equivalent hydrogens give a different NMR signal for each set Cl CH 3 CHCl 1,1-Dichloroethane (2 signals) O Cyclopentanone (2 signals) Cl C C CH 3 H H (Z)-1-Chloropropene (3 signals) Cyclohexene (3 signals) 13-25

26 Signal Areas u Relative areas of signals are proportional to the number of hydrogens giving rise to each signal u All modern NMR spectrometers electronically integrate and record the area of each signal 13-26

27 Chemical Shift - 1 H-NMR Type of H (C H 3 ) 4 Si RCH Type of H ROH RCH 2 OR RCH 2 R R 2 NH R 3 CH O R 2 C=CRC HR 2 RC CH ArC H RCCH 3 O RCCH 2 R ArC H 2 R

28 Chemical Shift - 1 H-NMR Type of H Type of H O RCOCH 3 O RCOCH 2 R RCH 2 I RCH 2 Br RCH 2 Cl R 2 C=C H 2 R 2 C=C HR ArH O RCH O RCH 2 F RCOH

29 Chemical Shift u u Depends on (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) magnetic induction within an adjacent pi bond Electronegativity CH 3 -X Electronegativity of X of H CH 3 F CH 3 OH CH 3 Cl CH 3 Br CH 3 I (CH 3 ) 4 C (C H 3 ) 4 Si (by definition 13-29

30 Chemical Shift u Hybridization of adjacent atoms Type of Hydrogen (R = alkyl) RCH 3, R 2 CH 2, R 3 CH R 2 C=C(R)C HR 2 RC CH R 2 C=C HR, R 2 C=C H 2 RCHO Name of Hydrogen alkyl allylic acetylenic vinylic aldehydic Chemical Shift ( )

31 Chemical Shift u Magnetic induction in pi bonds of a a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield (to the right) to a smaller value carbon-carbon double bond deshields vinylic a hydrogens and shifts their signal downfield (to the left) to a larger value Type of H Name RCH 3 RC CH R 2 C=CH 2 alkyl acetylenic vinylic

32 Signal Splitting (n + 1) u Peak: the units into which an NMR signal is split; doublet, triplet, quartet, etc. u Signal splitting: splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens u (n + 1) rule: the 1 H-NMR signal of a hydrogen or set of equivalent hydrogens is split into (n + 1) peaks by a nonequivalent set of n equivalent neighboring hydrogens 13-32

33 Signal Splitting (n + 1) n = 1. Their signal is split into (1 + 1) or 2 peaks ; a doublet Cl CH 3 -CH-Cl n = 3. Its signal is split into (3 + 1) or 4 peaks; a quartet u Problem: predict the number of 1 H-NMR signals and the splitting pattern of each O O (a) CH 3 CCH 2 CH 3 (b) CH 3 CH 2 CCH 2 CH 3 O (c) CH 3 CCH(CH 3 )

34 Origins of Signal Splitting u When the chemical shift of one nucleus is influenced by the spin of another, the two are said to be coupled u Consider nonequivalent hydrogens H a and H b on adjacent carbons the chemical shift of H a is influenced by whether the spin of H b is aligned with or against the applied field H a H b C C 13-34

35 Origins of Signal Splitting 13-35

36 Origins of Signal Splitting u Table 13.8 Observed signal splitting patterns for an H with 0, 1, 2, and 3 equivalent neighboring hydrogens Structure H a Spin States of H b Signal of H a C C H a C H b C

37 Origins of Signal Splitting u Table 13.8 (contd.) Ha Hb C C Hb Ha Hb C C Hb Hb

38 Coupling Constants u Coupling constant (J): the distance between peaks in an NMR multiplet, expressed in hertz J is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled H a H b -C-C- 6-8 Hz H a H b 8-14 Hz 0-5 Hz H a H b H a H a H b H a C C C C C C H b H b Hz 5-10 Hz 0-5 Hz 13-38

39 13 C-NMR Spectroscopy u Each nonequivalent 13 C gives a different signal u A 13 C is split by the 1 H bonded to it according to the (n + 1) rule u Coupling constants of Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine u The most common mode of operation of a 13 C-NMR spectrometer is a hydrogendecoupled mode 13-39

40 13 C-NMR Spectroscopy u In a hydrogen-decoupled mode, a sample is irradiated with two different radio frequencies one to excite all 13 C nuclei a second is a broad spectrum of frequencies that causes all hydrogens in the molecule to undergo rapid transitions between their nuclear spin states u On the time scale of a 13 C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that 1 H- 13 C spin-spin interactions are not observed; they are decoupled 13-40

41 Chemical Shift - 13 C-NMR Type of Carbon RCH 3 RCH 2 R R 3 CH Chemical Shift ( ) Type of Carbon O C R Chemical Shift ( ) RCH 2 I RCH 2 Br RCH 2 Cl R 3 COH R 3 COR RC CR R 2 C=CR RCOR O RCNR 2 O RCOH O O RCH, R CR

42 The DEPT method u In the hydrogen-decoupled mode, information on spin-spin coupling between 13 C and attached hydrogens is lost u The DEPT method is an instrumental mode that provides a way to acquire this information u Distortionless Enhancement by Polarization Transfer (DEPT) is an NMR technique for distinguishing among 13 C signals for CH 3, CH 2, CH, and quaternary carbons 13-42

43 The DEPT method u The DEPT methods uses a complex series of pulses in both the 1 H and 13 C ranges, with the result that CH 3, CH 2, and CH signals exhibit different phases; signals for CH 3 and CH carbons are recorded as positive signals signals for CH 2 carbons are recorded as negative signals quaternary carbons give no signal in the DEPT method 13-43

44 Interpreting NMR spectra u Alkanes: all 1 H-NMR signals fall in the narrow range of C signals fall in the considerably wider range of 0-60 u Alkenes: vinylic hydrogens typically fall in the range coupling constants are generally larger for trans vinylic hydrogens (J= Hz) compared with cis vinylic hydrogens (J= 5-10 Hz) the sp 2 hybridized carbons of alkenes give 13 C- NMR signals in the range , which is downfield from the signals of sp 3 hybridized carbons 13-44

45 Interpreting NMR spectra u Alcohols: the chemical shift of the hydroxyl hydrogen is variable. It normally falls in the range , but may be as low as 0.5. hydrogens on an sp 3 hybridized carbon adjacent to the -OH group are deshielded by the electronwithdawing inductive effect of the oxygen and their signals appear in the range u Ethers: a distinctive feature in the 1 H-MNR spectra of ethers is the chemical shift, , of hydrogens on carbon attached to the ether oxygen

46 Index of H Deficiency u Index of hydrogen deficiency (IHD): the sum of the number of rings and pi bonds in a molecule u To determine IHD, compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds the molecular formula of the reference hydrocarbon is C n H 2n

47 Index of H Deficiency IDH = (H reference - H molecule ) u For each atom of a Group VII element (F, Cl, Br, I) added to the reference hydrocarbon, subtract one H u No correction is necessary for the addition of atoms of Group VI elements (O,S) to the reference hydrocarbon u For each atom of a Group V element (N, P) added to the reference hydrocarbon, add one hydrogen

48 End Chapter