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basic-princ-1 - NMR Spectroscopy: 1 Sophomore organic NMR:...

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NMR Spectroscopy: 1
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This course: -what gives rise to the signals; microscopic and macroscopic -what determines signal frequencies, intensities and multiplicities (chemical shifts/couplings/relaxation) -how are spectra acquired and processed (pulsed Fourier transform methods) -how can we manipulate nuclear magnetism to get other types of information (complex pulse sequences) -2D methods / chemical shift correlation (COSY, TOCSY) -the nuclear Overhauser effect (NOE, NOESY) -etc. Sophomore organic NMR: -1D, 1 H spectra -small organic molecules -one signal for each type of 1 H -signals split into doublets, triplet, quartets, etc., due to presence of 1 H nuclei on neighboring carbon atoms (“coupling”) -frequencies (“chemical shifts”) of signals determined by electron density
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Most elements are comprised of one or more isotopes that are NMR active http://bouman.chem.georgetown.edu/NMRpt/NMRpertbl.html
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Classical Angular Momentum -any rotating macroscopic object possesses angular momentum Quantum Angular Momentum -any rotating system (molecule) possesses angular momentum -the set of stable rotational states leads to quantized angular momentum = Planck’s constant (~6.6 x 10 -34 Js)/2 π - J is integral (0,1,2…) -angular momentum depends on the molecular rotational state -angular momentum depends on rate of rotation Spin Angular Momentum -spin is a quantum mechanical concept/phenomenon…there is no good classical analogy -spin is also a form of angular momentum -it is not , however, produced by rotation of the particle, but is an intrinsic property of the particle -nuclear magnetism is a manifestation of nuclear spin angular momentum L = ± × J(J + 1) ±
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NMR is a manifestation of nuclear spin angular momentum ( P ) -spin angular momentum is quantized -spin (spin angular momentum) is an intrinsic property of a particle I = angular momentum quantum number = nuclear spin = Planck’s constant/2 π = h /2 π h 6.6 x 10 -34 Js I can be 0, integral or half-integral - I is zero for elements of even mass number (A) and even atomic number (Z) - I is integral (nonzero) when A is even and Z is odd - I is half-integral when A is odd NMR behavior - I = 0, NMR inactive ; examples and , unfortunately -these nuclei have no magnetic moment (see below) and are not NMR active - I > 1/2, quadrupolar nuclei : examples -these nuclei possess an electric quadrupole moment due to non-spherical nuclear charge distribution (short magnetic state life times, broad line widths, complex spectra) - I = 1/2, “spin 1/2” nuclei : examples -these are the mainstay nuclei for organic chemistry and biochemistry P = ± × I(I + 1) 6 12 C 8 16 O 7 14 N 1 1 H, 6 13 C, 7 15 N, 15 31 P, 9 19 F ±
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some nuclear properties important for NMR spectroscopy
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The angular momentum P has associated with it a magnetic (dipole) moment ( μ ) = γ P The for the electron is well understood (predicted value from Dirac’s integration of relativity and quantum mechanics, and in agreement to 11 signi f cant f gures with measured value). For nuclei no so well understood.
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basic-princ-1 - NMR Spectroscopy: 1 Sophomore organic NMR:...

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