13-1 Nuclear Magnetic Resonance Spectroscopy Part-1 Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry

13-2 Molecular Spectroscopy  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  hydrogen atoms using 1 H-NMR spectroscopy.  carbon atoms using 13 C-NMR spectroscopy.  phosphorus atoms using 31 P-NMR spectroscopy.

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13-4 NMR: Basic Experimental Principles Imagine placing a molecule, for example, CH 4, in a magnetic field. We can probe the energy difference of the  - and  - state of the protons by irradiating them with EM radiation of just the right energy. In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radio waves). So, if we bombard the molecule with 300 MHz radio waves, the protons will absorb that energy and we can measure that absorbance. In a magnet of Tesla, it takes EM radiation of about 500 MHz (stronger magnet means greater energy difference between the  - and  - state of the protons)

13-5 The Chemical Shift of Different Protons NMR would not be very valuable if all protons absorbed at the same frequency. You’d see a signal that indicates the presence of hydrogens in your sample, but any fool knows there’s hydrogen in organic molecules. What makes it useful is that different protons usually appear at different chemical shifts ( . So, we can distinguish one kind of proton from another. Why do different protons appear at different  ? There are several reasons, one of which is shielding. The electrons in a bond shield the nuclei from the magnetic field. So, if there is more electron density around a proton, it sees a slightly lower magnetic field, less electron density means it sees a higher magnetic field: How do the electrons shield the magnetic field? By moving. A moving charge creates a magnetic field, and the field created by the moving electrons opposes the magnetic field of our NMR machine. It’s not a huge effect, but it’s enough to enable us to distinguish between different protons in our sample.

13-6 The Hard Part - Interpreting Spectra 1) Chemical shift data - tells us what kinds of protons we have. 2) Integrals - tells us the ratio of each kind of proton in our sample. 3) 1 H - 1 H coupling - tells us about protons that are near other protons. Learning how an NMR machine works is straightforward. What is less straightforward is learning how to use the data we get from an NMR machine (the spectrum of ethyl acetate is shown below). That’s because each NMR spectrum is a puzzle, and there’s no single fact that you simply have to memorize to solve these spectra. You have to consider lots of pieces of data and come up with a structure that fits all the data. What kinds of data do we get from NMR spectra? For 1 H NMR, there are three kinds each of which we will consider each of these separately:

13-7 Acetone in the Absence of a magnetic Field Axis of nuclear spin is random محور الغزل عشوائي

13-8 Acetone in the presence of a magnetic Field The magnetic moment of The proton will line up with Or against the external magnetic field These are known as nuclear spins and each represents an energy level

13-9 Nuclear spin in the presence of an external Magnetic field الاشعة التي طاقتها تقابل فرق الطاقة بين مستويات الغزل النووية هي اشعة الراديو اذا عرضنا البروتون الموجود في المجال المغناطيسي بشعاع راديو يتطابق مع فرق الطاقة ، البروتون الموجود في المستوى الارضي سيمتص هذه الاشعة وينتقل الى المستوى المثار

13-10 NMR Active Nuclei الأنوية النشيطة  The number of protons or neutrons or both should be odd يشترط في هذه الانوية ان تحتوي على عدد ذري مفرد او عدد نترونات  مفرد او كلاهما معا Isotope# of Protons # of neutrons # of Spin States 1H1H H2H C C N O F P Cl

13-11 Nuclear Spin  Any atomic nucleus which possesses either odd mass, odd atomic number, or both has spin angular momentum and a magnetic moment IAtomic MassAtomic No.Example (I) Half integerOddOdd or even IntegerEvenOdd ZeroEven

Bruker AV600Bruker AV400Bruker DRX500JEOL 270Bruker DRX300 NMR Safety Precautio ns : Very Strong Magnetic Field! 5G10G

13-13 Nuclear Spin States  An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2.  This spinning charge has 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.  The same effect holds for certain atomic nuclei.  Any atomic nucleus that has an odd mass number, an odd atomic number, or both, also has a spin and a resulting nuclear magnetic moment.  The allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus.

13-14 Nuclear Spin States I2I + 1  A nucleus with spin quantum number I has 2I + 1 spin states. If I = 1/2, there are two allowed spin states.  Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds.

13-15 Nuclear Spins in B o  Within a collection of 1 H and 13 C atoms, nuclear spins are completely random in orientation.  When placed in a strong external magnetic field of strength B 0, however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed.

13-16 Nuclear Spins in B 0  Figure 13.1 for 1 H and 13 C, only two orientations are allowed.

13-17 Nuclear Spins in B 0  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 J ( cal)/mol, which corresponds to a frequency of 300 MHz (300,000,000 Hz).  13 C is approximately J ( cal)/mol, which corresponds to a frequency of 75MHz (75,000,000 Hz).

13-18 Nuclear Spin in B 0 Figure 13.2 The energy difference between allowed spin states increases linearly with applied field strength.  Values shown here are for 1 H nuclei.

13-19 Nuclear Magnetic Resonance  When nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins align with the applied field in the lower energy state.  The nucleus begins to precess and traces out a cone- shaped surface, in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it precesses in the earth’s gravitational field.  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).

13-20 Nuclear Magnetic Resonance  Figure 13.3 The origin of nuclear magnetic “resonance”. (a) Precession and (b) after absorption of electromagnetic radiation.

13-21 Nuclear Magnetic Resonance  Resonance:  Resonance: In NMR spectroscopy, resonance is the absorption of energy by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state.  The precessing spins induce an oscillating magnetic field that is recorded as a signal by the instrument.  Signal:  Signal: A recording in an NMR spectrum of a nuclear magnetic resonance.

13-22 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 is true of 13 C nuclei.  Hydrogens in organic molecules, however, are not isolated from all other atoms. They are surrounded by electrons, which are caused to circulate by the presence of the applied field. diamagneticcurrent diamagnetic shielding  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.

13-23 NMR Spectrometer  Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio- frequency detector.  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.  Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds.

13-24 NMR Spectrometer  Figure 13.4 Schematic diagram of a nuclear magnetic resonance spectrometer.

13-25 Nuclear Magnetic Resonance  The difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small.  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.

13-26 Nuclear Magnetic Resonance  NMR signals are measured relative to the signal of the reference compound tetramethylsilane (TMS).  The signal from TMS becomes the reference point and is assigned a shift of zero ppm.  Chemical shift (  ):  Chemical shift (  ): The shift in ppm of an NMR signal from the signal of TMS.  For a 1 H-NMR spectrum, signals are reported by their shift from the signal of the 12 equivalent H atoms in TMS.  For a 13 C-NMR spectrum, signals are reported by their shift from signal of the 4 equivalent carbons in TMS.

13-27  Figure H-NMR spectrum of methyl acetate.  High frequency:  High frequency: The shift of an NMR signal to the left.  Low frequency:  Low frequency: The shift of an NMR signal to the right. Terms used in an NMR spectrum. Downfield Upfield Less shielding More shielding Low induced field High induced field High frequency -  Low frequency-  300 Hz ppm =  = = 5  or 5 ppm 60 MHz

13-28 يحتوي على 6 ذرات هيدروجين. لو كانت انوية الذرات الست متكافئة كيميائيا ومغناطيسيا بمعنى انها تتاثر بالمجال المغناطيسي الخارجي بنفس الدرجة فانها ستمتص اشعة الراديوا عند نفس التردد وعندها تكون تقنية الرنين النووي المغناطيسي بدون فائدة. لحسن الحظ نجد بروتونات الايثانول غير متكافئة لاختلاف البيئة التركيبية التي توجد فيها Equivalent Hydrogens Equivalent hydrogens: Equivalent hydrogens: Hydrogens that have the same chemical environment.

13-29 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات لمعرفة ما اذا كانت ذرتا هيدروجين في جزيئ ما متكافئتين ام لا نستبدل كل منهما بمجموعة ولتكن برومين ونقارن نواتج الاستبدال. اذا نتج نفس المركب كانت الذرتان متكافئتين والعكس صحيح. مثال: اولا: هل انوية ذرات مجموعة المثل متكافئة ام لا؟؟

13-30 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات مثال: اولا: هل انوية ذرات مجموعة المثل متكافئة ام لا؟؟

13-31 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات مثال: اولا: هل انوية ذرات مجموعة المثل متكافئة ام لا؟؟ الخلاصة: انوية ذرات مجموعة المثل متكافئة كيميائيا ومغناطيسيا A molecule with one set of equivalent hydrogens gives one NMR signal because each hydrogen is in the same environment.

13-32 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات ثانيا: هل انوية ذرات مجموعة المثلين متكافئة ام لا؟؟

13-33 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات ثانيا: هل انوية ذرات مجموعة المثلين متكافئة ام لا؟؟ الخلاصة: انوية ذرات مجموعة المثلين متكافئة كيميائيا ومغناطيسيا

13-34 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات ثالثا: هل انوية ذرات مجموعة المثل متكافئة مع المثلين ام لا؟؟

13-35 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات ثالثا: هل انوية ذرات مجموعة المثل متكافئة مع المثلين ام لا؟؟ الخلاصة: انوية ذرات مجموعة المثلين غير متكافئة كيميائيا ومغناطيسيا مع مجموعة المثل

13-36 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات OH رابعا: ذرة الخلاصة: انوية ذرات مجموعة المثلين والمثل والكحول غير متكافئة كيميائيا ومغناطيسيا

13-37 How to determine Chemical and magnetic equivalence التكافؤ الكيميائي والمغناطيسي وتصنيف البروتونات الخلاصة: انوية ذرات مجموعة المثلين والمثل والكحول غير متكافئة كيميائيا ومغناطيسيا مع بعضها – هنالك 3 اصناف من البروتونات – 3 اشارات

13-38 number of signals?

13-39 Equivalent Hydrogens  Equivalent hydrogens:  Equivalent hydrogens: Hydrogens that have the same chemical environment.  A molecule with one set of equivalent hydrogens gives one NMR signal because each hydrogen is in the same environment.

13-40 Equivalent Hydrogens  A molecule with two or more sets of equivalent hydrogens gives a different NMR signal for each set. In the following structures, the sets of equivalent hydrogens are distinguished by the labels a, b, and c.

H NMR Problems  How many unique proton environments are there in:

H NMR Problems

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13-45 Signal Areas  Relative areas of signals are proportional to the number of H giving rise to each signal. Modern NMR spectrometers electronically integrate and record the relative area of each signal.  Figure H-NMR spectrum of tert-butyl acetate.

13-46 Good Luck