Nuclear Magnetic Resonance Spectroscopy Organic Chemistry . Nuclear Magnetic Resonance Spectroscopy

The Value of NMR Spectroscopy Provides information about molecular structure that is complementary to that provided by IR and Mass Spec. Mass spec.-Identifies Molecular Weight and Molecular Formula IR spec.-Identifies Functional Groups present NMR-Maps hydrocarbon framework of molecule =>

Nuclear Spins The nucleus of any atom contains only neutrons and protons and consequently has a positive charge Like electrons, the nuclei of atoms are considered to spin on their axis Physics tells us that a spinning charged nucleus generates a magnetic moment, B (acts like a tiny bar magnet). 13C and 1H nuclei behave this way. =>

Effect of an External Magnetic Field B0 The spinning of the 1H or 13C nucleus generates a magnetic moment (causes them to act like a tiny bar magnet) and this magnetic moment can align itself parallel to or against an external magnetic field, B0 Parallel orientation is lower in energy making this spin state more populated

External Magnetic Field When placed in an external field,B0, spinning protons act like bar magnets. =>

Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. The parallel alignment is lower in energy. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip from parallel to antiparallel =>

Effect Of The Strength of B0 The energy difference between the two spin states, parallel and anti-parallel, varies directly with the strength of the external magnetic field, B0

Nmr In principle, we could place a sample of hydrocarbon in the presence of a constant external magnetic field (B0) and vary the energy (frequency) of the applied radiation until the energy of the radiation matched the energy difference between the two possible orientations for the proton’s magnetic moment. At this point where the energy provided just matched the energy needed the proton would flip and we would record an absorption.

Nmr In practice, however, it has been found more convenient to keep the radiation energy (frequency) constant and to vary the strength of the external magnetic field (B0) until the energy difference between the two possible proton orientations exactly matches the constant energy available from the radiation. Please recall that as B0 varies so to does delta E. Consequently, one simply varies B0 until delta E is equal to the constant Energy available from the radiation.

Types of NMR Instruments In fact, Nmr instruments operate at one particular radiofrequency or energy. The instruments are identified by their operating radiofrequency There are 500 MHz (500 million cycles/sec) instruments, 360 MHz, 60MHz etc. Consequently, each Nmr instrument has one particular slug of energy available for absorption

Creating NMR Absorptions Radio energy of exactly correct frequency(Hertz) causes nuclei to flip from the parallel into anti-parallel state. Remember that each Nmr instrument has only one radioenergy slug available for absorption To cause a set of nuclei to absorb the radio energy available, one simply varies the strength of the external magnetic field, Bo, until the energy difference between the two spin states matches the energy slug that is available and an absorption is recorded.

The NMR Spectrometer =>

Magnetic Shielding Now, if the situation was as simple as we have described so far, all protons would absorb at the same applied external magnetic field (B0) and the spectrum would consist of a single signal that would tell us little about the molecule. The B0 value at which a proton absorbs depends upon the magnetic field that the proton “feels”. This effective magnetic field strength is not exactly the same as the applied external magnetic field strength B0 because, depending upon the amount of electron density around a proton, some protons are “shielded” from the effects of the applied external magnetic field and consequently it takes a higher applied external magnetic field strength to produce the same effective magnetic field strength. Circulating electrons create an induced magnetic field that opposes the external magnetic field and consequently electron density shields protons from the effects of the external magnetic field. =>

Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. =>

Electron Density Around Protons in a Molecule Causes Them To Be Shielded Depending on their chemical environment, protons in a molecule are shielded by different amounts-have different electron densities around them. =>

The Nmr Chart The external applied magnetic field, B0, increases from left to right on an Nmr chart. Consequently, deshielded protons absorb at a low value of B0 or downfield. Shielded protons absorb at a high value of B0 or upfield. In fact, tetramethylsilane (TMS) provides the most shielded 1H or 13C nuclei and is the internal standard used in NMR. It’s signal occurs further upfield than that of any other molecule.

We Will Now Begin To Study Four Aspects Of 1H Nmr The number of signals – This identifies the number of different sets of equivalent protons in a molecule. The position of the signals – This tells us about the electronic environment (shielded or deshielded) of a set of protons The area under each signal – This tells how many protons of each kind there are The splitting of each signal - shows the number of nonequivalent protons on adjacent carbon atoms. =>

The Number Of Signals To identify the number of signals that a molecule should give in its Nmr spectrum simply identify the number of different sets of equivalent protons that the molecule has. Theoretical equivalence can be predicted by seeing if replacing each H with “X” gives the same or different outcome (name) For example, how many Nmr signals would the following molecule present: CH3C(O)OCH3

Example # 2 How many Nmr signals would the following molecule give: HO-CH3?

Example # 3 Identify the number of signals and their relative positions on an Nmr chart for the following molecule: CH3OC(O)CH2C(O)CH3

Example #4 Identify the number of signals and their relative positions on an Nmr chart for the following molecule: CH3OC(CH3)3 =>

Tetramethylsilane Remember TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal The extent of this downfield absorbtion for a set of protons is referred to as their “chemical shift” downfield from TMS. =>

Chemical Shift Measured in delta units Each delta unit = 1ppm of the spectrometer operating frequency. For a 60MHz instrument, each delta unit = 60 Hz For a 100MHz instrument, each delta unit = 100 Hz For a 500MHz instrument, each delta unit = 500 Hz

Chemical Shifts More electronegative atoms deshield more and give larger shift values. Effect decreases with distance. Additional electronegative atoms cause increase in chemical shift. =>

Table of Chemical Shifts

O-H and N-H Signals Chemical shift depends on concentration. Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H. Proton exchanges between the molecules broaden the peak. =>

Carboxylic Acid Proton, 10+ =>

Intensity of Signals The area under each peak is proportional to the number of protons. Shown by integral trace. =>

How Many Hydrogens? When the molecular formula is known, each step height measured in terms of blocks can be assigned to a particular number of hydrogens. =>

To Identify The Number of Hydrogens Responsible for each Nmr signal Add up the total block heights of all steps Divide the total # of H’s in the molecular formula by the total #of blocks. This will give you a conversion factor of #H’s/block. Finally, multiply the block height of each absorbtion by the #H’s/block to get the # of H’s responsible for each absorbtion

Spin-Spin Splitting Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed. All possibilities exist, so signal is split. =>

1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. =>

Doublet: 1 Adjacent Proton =>

Triplet: 2 Adjacent Protons =>

The N + 1 Rule If a signal is split by N equivalent protons, it is split into N + 1 peaks. =>

Range of Magnetic Coupling Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent carbons normally will couple. Protons separated by four or more bonds will not couple. =>

Splitting for Ethyl Groups =>

Splitting for Isopropyl Groups =>

Coupling Constants Distance between the peaks of multiplet Measured in Hz Not dependent on strength of the external field Multiplets with the same coupling constants may come from adjacent groups of protons that split each other. =>

MRI Magnetic resonance imaging. Nmr on steroids. Humans are the sample. “Nuclear” is omitted because of public’s fear that it would be radioactive. Only protons in one plane can be in resonance at one time. Computer puts together “slices” to get 3D. Tumors readily detected. =>

End of Chapter 13