Vocab 7

Question 1

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

Answer 1

Spin Quantum Number, I

Question 2

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

Answer 2

Spin l = 0 Nuclei

Question 3

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.

Answer 3

Spin I = 1⁄2 Nuclei

Question 4

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.

Answer 4

Spin I = 1 Nuclei

Question 5

Magnetic nuclei with non-zero spin angular momenta, and a non-spherical, ellipsoidal shape that have I > 1⁄2 .

Answer 5

Quadrupolar Nuclei

Question 6

Vector describing the magnitude and direction of the spin angular momentum of a nucleus

Answer 6

Nuclear Angular Momentum, P

Question 7

The number of possible orientations of the angular momentum in a magnetic field

Answer 7

Magnetic Quantum Number, m

Question 8

Magnetic dipole moment that arises from the circular electric current induced by a nucleus spinning about its axis

Answer 8

Nuclear Magnetic Moment,

Question 9

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.

Answer 9

Magnetogyric Ratio, g

Question 10

Behavior of the nuclear angular momentum P such that its components Pz must
point only along a reference direction, e.g. the z direction;

Answer 10

Directional Quantization

Question 11

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.

Answer 11

α Spin State

Question 12

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.

Answer 12

β Spin State

Question 13

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.

Answer 13

Nuclear Precession

Question 14

The frequency of the precession of a spinning nucleus in an applied B0 magnetic field

Answer 14

Larmor Precessional Frequency

Question 15

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.

Answer 15

Nuclear Magnetic Resonance

Question 16

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.

Answer 16

Net Magnetization Vector, M0

Question 17

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.

Answer 17

Saturated NMR Signal

Question 18

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.

Answer 18

Spin-Lattice or T1 Relaxation

Question 19

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

Answer 19

Spin-Spin or T2 Relaxation

Question 20

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.

Answer 20

Superconducting Magnet

Question 21

Method of maintaining the homogeneity of the magnetic field in superconducting magnets; a reference nucleus (typically 2H) is continuously irradiated;

Answer 21

Field-Frequency Lock

Question 22

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.

Answer 22

Shim Coils

Question 23

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

Answer 23

Sample Spinning

Question 24

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.

Answer 24

Continuous Wave NMR

Question 25

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

Answer 25

Pulsed Fourier Transform NMR

Question 26

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

Answer 26

Pulse Width

Question 27

The measure of the power with which the pulse is applied; determines the strength of
the B1 field.

Answer 27

Pulse Amplitude

Question 28

The time delay between pulses; can be in the range of 1 – 10 seconds.

Answer 28

Interpulse Delay

Question 29

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

Answer 29

Rotating Frame of Reference

Question 30

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

Answer 30

Free Induction Decay

Question 31

When placed in a B0 field, electrons surrounding the nucleus circulate in direction that produces a magnetic field that opposes the applied B0 field

Answer 31

Diamagnetic Circulation

Question 32

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

Answer 32

Screening or Shielding Constant

Question 33

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

Answer 33

Chemical Shift

Question 34

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.

Answer 34

Internal Standard

Question 35

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

Answer 35

Upfield

Question 36

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

Answer 36

Downfield

Question 37

The type of nuclear shielding that is dependent on the spherical charge density of the s electrons near the nucleus.

Answer 37

Diamagnetic Component of Shielding Constant

Question 38

The chemical shift values that directly correlate with the deshielding of the nucleus by the electronegativity effects of the neighboring groups.

Answer 38

Inductive Effects

Question 39

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.

Answer 39

Magnetic Anisotropy Effects

Question 40

Secondary fields arising in aromatic compounds that act in opposition to the B0 applied magnetic field.

Answer 40

Ring Currents

Question 41

A term that accounts for nuclear shielding that is dependent on non-spherical charge density effects.

Answer 41

Paramagnetic Component of Shielding Constant

Question 42

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

Answer 42

Spin-Spin coupling

Question 43

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

Answer 43

Coupling Constant, J

Question 44

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.

Answer 44

Fermi Contact Term

Question 45

NMR spectra in which the chemical shift between molecular resonances is large compared to the coupling constant

Answer 45

First-Order NMR Spectra

Question 46

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

Answer 46

Pascal’s Triangle