Nuclear Magnetic Resonance (NMR) Spectroscpoy
a stectroscopic technique that gives information about the number and types of atoms in a molecule
nuclear spin states
- an electron has a spin quantum number of 1/2 (can be +1/2 or -1/2)
- a moving charge has an associated magnetic field
- nuclei need multiple spin states to be NMR active, which include those with an odd mass and /or atomic number
orientation of nuclear spin in an applied magnetic field
- placing nuclei in a strong magnetic field aligns their molecular spin with (+1/2 = lower energy) and against (-1/2 =higher energy) the field
- the difference in energy between these states is on the order of radio waves and is of the magnet
- stronger magnets have greater sensitivity and require higher radio frequency
nuclear magnetic resonance
- irradiating the nuclei with a radio frequency equal to the difference in energy between the spin states excites the spin from +1/2 to -1/2
- upon return to the ground state, the nuclei emit radio waves equal in energy to that absorbed, which is measured and recorded as signal
- a nucleus is in resonance when absorption occurs
- an applied magnetic field causes electrons to circulate, and the orientation of this field dictates the direction of circulation
deshielding
electron density circulation adds to the applied magnetic field, thus requiring higher energy to cause resonance
chemical shift
the difference in resonance frequencies caused by shielding/deshielding of nuclei
NMR Spectrometer
-NMR samples are prepared by dissolving the compound of interest in a deuterated solvent, to avoid H resonances from the solvent that would saturate the signal
equivalent hydrogens
hydrogens that have the same chemical environment, and thus show up as a single signal in H-NMR
rules to determine equivalency
- bonded to the same sp3 hybridized carbon that can rotate freely
- a plane or point of symmetry in a molecule exists and hydrogens equidistant from the plane or point are the same
signal area
- the area under the signal is proportional to the number of equivalent hydrogens giving rise to that signal
- integrating the peaks in a H-NMR spectrum tells you the relative number of hydrogens represented by each peak
chemical shift
- incredibly valuable in determining structure since the “electronic environment” around the H influences its chemical shift
- influenced by electronegativity, hybridization of nearby atoms, and magnetic induction within an adjacent pi bond
electronegativity of nearby atoms
-EN atoms pull electron density away from atoms bonded to it, thus deshielding them, and causing a downfield shift in resonance
hybridization of adjacent atoms
-more s character for sp2 and sp hybridized carbon than sp3, making it more EN
diamagnetic effects from pi-Bonds
-shielding vs. E.N=> “deshielding”
ring current
circulation of pi-electrons on an aromatic ring under an applied magnetic field
-this current induces a strong magnetic field that opposes the applied field in the middle of the ring, but reinforces it outside the ring
signal splitting
spin-spin coupling with adjacent nuclei split NMR signals depending on the extent of coupling and the number of adjacent nuclei
(n+1) rule
if a hydrogen has n hydrogens nonequivalent to it, but equivalent among themselves on the same or adjacent atoms, its H-NMR signal is split into (n=1) peaks
-for molecules that can freely rotate, sp3 hybridized, the rule simplifies to m+n+1 peaks
vicinal coupling
spin-spin coupling involving the H atoms on two C atoms that are bonded to each other
germinal coupling
spin-spin coupling that occurs between nonequivalent H atoms bonded to the same C atom (often due to restricted bond rotation, such as on a terminal double bond
13C-NMR
- 13C-NMR active, and 13C has a natural abundance of 1%
- (n+1) rule applies, but splitting is not observed due to the nearly negligible probability of two 13C isotopes being adjacent to each other in a molecule
- 1H will split 13C signals resulting in complex splitting patterns, which is why H signals are decoupled and no splitting is observed, resulting in relative clear spectra
