Vocab 7 Flashcards
A quantum number describing the overall spin of a nucleus. Due to the very complex vector summation properties of p protons and n neutrons for an individual nucleus, it cannot be determined from theory, but instead is an empirical quantity defined for various nuclear configurations
Spin Quantum Number, I
Refers to nuclei with zero spin angular momenta, and are therefore magnetically invisible to the NMR experiment. Occurs for nuclei with even atomic mass, and even atomic numbers. Examples 12C and 16O
Spin l = 0 Nuclei
Refers to nuclei with non-zero spin angular momenta, and that possess a spherical shape with uniform charge distribution. possess 2 energy states. Occurs for nuclei with odd atomic mass, and odd or even atomic numbers. Examples include nuclei such as 1H, 13C, 15N, and 31P.
Spin I = 1⁄2 Nuclei
Refers to nuclei with non-zero spin angular momenta, and that posse a non-spherical, ellipsoid shape. possess 3 energy states. Occurs for nuclei with even atomic mass, and odd atomic numbers. Examples include nuclei such as 2H, 14N, and 17O.
Spin I = 1 Nuclei
Magnetic nuclei with non-zero spin angular momenta, and a non-spherical, ellipsoidal shape that have I > 1⁄2 .
Quadrupolar Nuclei
Vector describing the magnitude and direction of the spin angular momentum of a nucleus
Nuclear Angular Momentum, P
The number of possible orientations of the angular momentum in a magnetic field
Magnetic Quantum Number, m
Magnetic dipole moment that arises from the circular electric current induced by a nucleus spinning about its axis
Nuclear Magnetic Moment,
Proportionality constant between nuclear magnetic moment and P such that nuclear magnetic moment = γ P ℏ ; a fundamental property of any given nucleus; units = rad T-1 s-1 ; also known as the gyromagnetic ratio.
Magnetogyric Ratio, g
Behavior of the nuclear angular momentum P such that its components Pz must
point only along a reference direction, e.g. the z direction;
Directional Quantization
Refers to a nuclear dipole aligned with the external applied B0 field; this is the state of lower energy; corresponds to m = + 1⁄2 for an I = 1⁄2 nucleus.
α Spin State
Refers to a nuclear dipole aligned opposed to the external applied B0 field; this is the state of higher energy; corresponds to m = - 1⁄2 for an I = 1⁄2 nucleus.
β Spin State
The phenomenon where a spinning nucleus in an external magnetic field experiences a torque on its magnetic moment imposed by the applied β0 field; this torque causes the axis of the spinning nucleus to move in a circular motion, and at a constant angle, about the direction of the applied β0 field.
Nuclear Precession
The frequency of the precession of a spinning nucleus in an applied B0 magnetic field
Larmor Precessional Frequency
The coherent exchange of energy between an applied external electromagnetic field B1 having an associated frequency ν1 , and a nucleus having a Larmor frequency of νL ; the resonance condition occurs when ν1 =νL ; resonance supplies the energy that allows transitions between ground and excited nuclear energy states to occur.
Nuclear Magnetic Resonance
The vector summation of the z components of all nuclear dipole moments in a sample to give a bulk, or net, magnetization vector pointing along the +z direction.
Net Magnetization Vector, M0
The situation that occurs when the NMR ground and excited states become equally populated; this leads to no net difference in absorption vs. emission, and thus no observable NMR signal.
Saturated NMR Signal
Dissipation of excess spin energy in excited state nuclei by equilibration with the surrounding sample environment, i.e. the lattice; absorbed energy is dissipated as heat throughout the lattice; refers to the return to thermal equilibrium that re-establishes the equilibrium Na and Nb values; first-order process characterized by the return of net magnetization along the z axis with relaxation time T1 ; strongly affected by mobility of the lattice.
Spin-Lattice or T1 Relaxation
Dissipation of excess spin energy in excited state nuclei by exchange of spin with other surrounding ground-state nuclei; refers to the loss of phase coherence in the transverse (x’ - y’) plane; first-order process characterized by relaxation time T2
Spin-Spin or T2 Relaxation
Electromagnets that maintain a magnetic field, i.e. an electrical current in the coils of the electromagnet, by keeping the coils of the magnet at or below 4 K, where the material comprising the coils is superconducting; no external current is needed – superconductivity is maintained at 4 K using liquid He; field strengths of 1.4 T – 21.1 T (60 – 900 MHz) are commercially available.
Superconducting Magnet
Method of maintaining the homogeneity of the magnetic field in superconducting magnets; a reference nucleus (typically 2H) is continuously irradiated;
Field-Frequency Lock
Method of maintaining the homogeneity of the magnetic field in superconducting magnets; wires through which current is passed, producing small magnetic fields; these fields are used to compensate for inhomogeneity in the main B0 field.
Shim Coils
Method of maintaining the homogeneity of the magnetic field in superconducting magnets; B0 inhomogeneity is damped by spinning the sample using an air turbine at 20 – 50 rpm along the longitudinal axis of the sample NMR tube
Sample Spinning
Technique that uses a radiofrequency sweep to excite the Larmor frequencies of the nuclei in the sample; older NMR technique that is not currently widely used.
Continuous Wave NMR
Technique that excites the Larmor frequencies of the nuclei in a sample; nuclei in static B0 field are subjected to periodic, short, high power, radiofrequency pulses from a second B1 magnetic field; the RF pulse provides a bandwidth of frequencies centered around a particular frequency, ν1 , thereby exciting the Larmor frequencies of all nuclei in the sample simultaneously
Pulsed Fourier Transform NMR
The length of time the B1 field interacts with the sample; tp is usually very short, i.e. 1
– 10 μs; also known as the pulse length or pulse duration
Pulse Width
The measure of the power with which the pulse is applied; determines the strength of
the B1 field.
Pulse Amplitude
The time delay between pulses; can be in the range of 1 – 10 seconds.
Interpulse Delay
The concept of a transformed coordinate system that rotates about the L is constant, laboratory z axis at the Larmor frequency; a net magnetization vector M0 rotating at the Larmor frequency in a laboratory coordinate system appears stationary in a frame of reference rotating about the z axis at νL
; the rotating coordinate system is distinguished from the x, y, and z laboratory coordinate system by primes on the axes
Rotating Frame of Reference
In the NMR experiment, an RF pulse is applied to a nuclei with pulse width and v1; this rotates M0 away from the z’ axis into the transverse plane; during the interpulse delay, an oscillating RF signal is emitted by the nuclei as they return to the ground state
Free Induction Decay
When placed in a B0 field, electrons surrounding the nucleus circulate in direction that produces a magnetic field that opposes the applied B0 field
Diamagnetic Circulation
Quantity that describes the magnetic shielding of a nucleus by its surrounding electronic and magnetic environment, leading the nucleus to experience a smaller, “effective” magnetic field; accounts for local charge density and opposes B0, leading to a new resonance value
Screening or Shielding Constant
The difference in the NMR absorption frequency of protons bound to chemically distinct groups in molecules; d is a dimensionless quantity defined in relation to an internal standard
reference frequency
Chemical Shift
Use of a reference molecule to act as a frequency standard for calculation of chemical shifts; in 1H and 13C NMR spectroscopy, tetramethylsilane (CH3)4Si is commonly used since it contains 12 highly shielded protons resulting in a single, sharp NMR peak occurring at low resonance frequency; by convention, NMR spectra are displayed with the magnetic field strength increasing from the left to right; therefore, the TMS resonance, defined as ppm = 0, lies at the far right of the spectrum.
Internal Standard
NMR naming convention that corresponds to the region of the NMR chemical shift spectrum with higher diamagnetic shielding and lower resonance frequencies; indicates a shift towards smaller values of the chemical shift parameter
Upfield
NMR naming convention that corresponds to the region of the NMR chemical shift spectrum with lower diamagnetic shielding and higher resonance frequencies; indicates a shift towards larger values of the chemical shift parameter
Downfield
The type of nuclear shielding that is dependent on the spherical charge density of the s electrons near the nucleus.
Diamagnetic Component of Shielding Constant
The chemical shift values that directly correlate with the deshielding of the nucleus by the electronegativity effects of the neighboring groups.
Inductive Effects
Anomalous chemical shift values that are not explained by electronegativity arguments, and are due to the anisotropic magnetic properties of multiply bonded and/or aromatic compounds.
Magnetic Anisotropy Effects
Secondary fields arising in aromatic compounds that act in opposition to the B0 applied magnetic field.
Ring Currents
A term that accounts for nuclear shielding that is dependent on non-spherical charge density effects.
Paramagnetic Component of Shielding Constant
A term that describes the effect that two nuclei have when they are sufficiently close in space that they can exert a magnetic effect on one another; this magnetic effect is manifested through the observed splitting of their NMR resonances – the splitting is caused by the presence of the 2nd nucleus; the origin of spin-spin interactions takes place through the interactions of nuclei and bonding electrons – not through space; also known as through-bond coupling, scalar coupling, indirect coupling, or J-coupling
Spin-Spin coupling
The spacings between the multiplet splitting patterns seen in the NMR resonances that are observed in spin-spin coupling; units = Hz; when measured in Hz, J is independent of the magnitude of B0
Coupling Constant, J
Describes the mechanism of spin-spin coupling; direct interaction between the magnetic moments of nuclei and those of the bonding electrons in s states; seen in the striking dependence on the s character, i.e. the hybridization of the bonds between the nuclei.
Fermi Contact Term
NMR spectra in which the chemical shift between molecular resonances is large compared to the coupling constant
First-Order NMR Spectra
Triangular array of binomial coefficients; also corresponds to the relative areas of a spin-spin splitting multiplet in NMR spectroscopy; areas of the split resonances are symmetric around the midpoint of the peak and proportional to the coefficients of the term
Pascal’s Triangle
