Lecture 2 nmr Spectroscopy

Transcription

1 Lecture 2 nmr Spectroscopy Pages and Chapter 13

2 Molecular Spectroscopy Molecular spectroscopy: the study of the frequencies of electromagnetic radiation that are absorbed or emitted by substances and the correlation between these frequencies and aspects of molecular structure

3 E = hν Energy per photon

4 W A V E L E N G T H E N E R G Y

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

6 Radio Frequency and Microwave

7 Electromagnetic Radiation Wavelength λ Relation Unit to Meter meter (m) ---- millimeter (mm) micrometer ( µm) nanometer (nm) Angstrom (Å) 1 mm = 10-3 m microwaves 1 µ m = 10-6 m infrared 1 nm = 10-9 m soft x-rays 1 Å = m Hard x or γ rays

8 Electromagnetic Radiation Important relationships: νλ = c Where C = 3.00 x 10 8 m/s hc E = hν = Where h = x 10 λ -14 kcal sec/mol See examples of calculations on pages 455 and 456 (Carefully follow example 13.1)

9 Absorbance promotes atom or molecule to higher energy state

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

11 Absorption of electromagnetic radiation Described by quantum mechanical theories Only discrete (unique) energy states are allowed (accessible) Therefore only discrete (unique) amounts of radiation can be absorbed (or emitted) How many potential energy states are available to the ball??

12 Summary Molecular Spectroscopy Concept... Discrete transitions in energy levels Transitions with varying energy (areas of spectrum) Nmr : nuclear spin, radio frequency region IR : vibration, infrared region UV-Vis : electronic transitions, UV to visible Please know relationships between frequency, wave length and energy. Know length scale conversions micron, millimeter, nanometer, angstrom...

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14 NMR Spectrosopy Nuclear magnetic resonance (NMR) spectroscopy: a powerful spectroscopic technique that provides information about the number and types of atoms in a molecule, hydrogen : 1 H-NMR spectroscopy carbons: 13 C-NMR spectroscopy Phosphorus: 31 P-NMR spectroscopy Not all isotopes can be detected 12 C is not detectable, for example Must have a certain nuclear spin property to work

15 Spin States Electrons have a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 this spinning charge creates an associated magnetic field electrons have what is called a magnetic moment and they behave as if they were a were a tiny bar magnet Remember the Pauli exclusion principle? Spin up and spin down

16 Nuclear Spin States Nuclei with an odd mass, an odd atomic number, or both also have a net spin and a resulting nuclear magnetic moment. The allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus. For each I there are 2I + 1 spin states If I = 1/2, there are two allowed spin states

17 Nuclear Magnetic Resonance If the nucleus is irradiated with radiation having energy (E=hυ) that is exactly the same as the difference between the nuclear spin states, 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)

18 Nuclear Spins in B 0 E=hν or ν = E/h

19 What is spin Anyhow?? A fundamental property of nature like electrical charge or mass. Spin comes in multiples (quanta) of 1/2 Protons, electrons, and neutrons all possess spin. Individual unpaired electrons, protons, and neutrons each possesses a spin of 1/2. Deuterium ( 2 H ) 1unpaired electron, 1 unpaired proton, and 1unpaired neutron total electronic spin = 1/2 and the total nuclear spin = 1. Two or more particles with spins having opposite signs pair up to eliminate the observable manifestations of spin. An example is helium. In nuclear magnetic resonance, it is unpaired nuclear spins that are of importance.

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

21 Nuclei Unpaired Unpaired Net G(MHz/T) Protons Neutrons Spin 1 H 1 0 1/ H P 0 1 1/ Na 2 1 3/ N C 0 1 1/ F 0 1 1/ When placed in a magnetic field of strength B, a particle with a net spin can absorb a photon, of frequency ν. The frequency, ν depends on the gyromagnetic ratio γ, of the particle. E = ν = γb For hydrogen, γ = MHz / Tesla

22 E = hν = f ( B 0 )

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

24 Nuclear Spins in B 0 In an applied field strength of 7.05T (BIG!) E 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) This E is quite small low frequency radiation induces flip (resonance)

25 Resonance The transition from the lower state to the higher occurs at a unique combinations of magnetic field and frequency of electromagnetic radiation. When placed in a magnetic field of strength B, a particle with a net spin can absorb a photon, of frequency ν. The frequency, ν depends on the gyromagnetic ratio γ, of the particle. ν = γb For hydrogen, γ = MHz / Tesla Allows you to calculate spectrometer frequency for 1 H!!

26 In principle, we could hold field constant and scan frequency looking oking for resonance it is equally effective to scan field strength and hold frequency constant

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28 500MHz 1 H-nmr Spectrometer Magnet

29 The 100MHz nmr Chart CH H 3 C Si CH CH Frequency field The TMS resonance is defined as having 0 frequency

30 100MHz Spectrum 20 C H 3 Cl Hz Hz

31 100MHz nmr Spectrum C H 3 Br Hz Hz

32 Nuclear Magnetic Resonance 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 for 13 C nuclei 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

33 Nuclear Magnetic Resonance The circulation of electrons around a nucleus in an applied field is called diamagnetic current. This current generates a field that opposes the applied field...diamagnetic nuclear shielding results. Lenz s Law?? The difference in resonance frequencies between the various hydrogen nuclei within a molecule due to shielding/deshielding is very small but very important

34 Conditions for Resonance It is the frequency of the radiation and the NET field at the nucleus that matters. The NET field is the sum of all incident magnetic fields including those from: The Giant Magnet (applied field) Diamagnetic Shielding field (electrons) Coupling (spin fields of adjacent nuclei) credit card strips, earth s field, etc..

35 Chemical Shift The difference in resonance frequencies for hydrogens in CH 3 Cl compared to CH 3 Br under an applied field of 2.34T is only 35.5Hz, which is 0.35 parts per million (ppm) compared with the irradiating frequency 35 Hz 100 x 10 6 = 0.35 = Hz ppm Here, 35Hz is the resonance frequency (difference in energy) and 100MHz is the 1 H resonance frequency for B=2.34T and γ = MHz / Tesla

36 The Chemical Shift The difference betweeen TMS and CH 3 Cl is 304HZ for the 2.35T (100MHz) magnet the chemical shift is 304Hz It is also: 304 Hz x 10 6 = Hz 10 6 = 3.04 ppm δ (ppm)= Shift in frequency from TMS (Hz) Frequency of spectrometer (Hz) Chemical Shift in ppm is independent of field strength

37 The Chemical Shift At 7.05T, the resonance is at 912 Hz What is the Chemical shift in ppm? ν = γb, = x 7.05 = 300MHz (spectrometer frequency) 912 Hz x 10 6 = Hz 10 6 = 3.04 ppm!! Chemical shift in ppm is not a function of field strength

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

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

40 Chemical Shift 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 CH 3 F CH 3 OH CH 3 Cl CH 3 Br CH 3 I (CH 3 ) 4 C (C H 3 ) 4 Si Electronegativity of X δ of H (by definition