NMR Flashcards

Question

What do the different patterns mean

Answer 1

Spin-lattice relaxation Spin-spin relaxation

Answer 2

Proton-Decoupled Spectrum: Simplified, showing all carbons as singlets. Off-Resonance Decoupled Spectrum: Shows limited splitting, with carbon signals split based on the number of attached protons. DEPT Spectrum: Provides specific information about how many hydrogens are attached to each carbon and can distinguish between different types of carbon-hydrogen environments (CH₃, CH₂, CH).

Answer 3

It focuses on the interactions of nuclear spins, typically protons (¹H) and carbons (¹³C), to provide information about the molecular structure Chemical Shifts: The position of the peaks along the x-axis (chemical shift scale) indicates the electronic environment of the nuclei, providing insight into the type of chemical groups present. For instance, protons in different environments (like methyl, methylene, or aromatic protons) resonate at different frequencies. Multiplicity: The splitting patterns (multiplicity) of the peaks provide information about the number of neighboring hydrogen atoms (n + 1 rule), helping to deduce the connectivity of atoms in the molecule. Integration: The area under the peaks correlates with the number of protons contributing to each signal. This allows quantification of the relative number of hydrogen atoms in different environments

Answer 4

It provides more comprehensive data compared to traditional one-dimensional (1D) NMR by allowing the observation of interactions between different nuclei (like hydrogen and carbon) in a molecule In 2D NMR, the signals are spread out in two dimensions instead of just one. This allows for the simultaneous observation of multiple interactions, which can provide a more complete picture of the molecular structure. The two axes of the spectrum typically represent different nuclear spins (e.g., protons on one axis and carbons on another) or different interactions (such as chemical shifts or coupling constants

Answer 5

is a type of NMR (Nuclear Magnetic Resonance) experiment that allows researchers to identify correlations between hydrogen atoms (¹H) and carbon atoms (¹³C) that are directly bonded to each other, typically through one bond One peak for each proton-carbon pair

Answer 6

used to determine the connectivity between atoms in a molecule, specifically between hydrogen atoms (¹H) and carbon atoms (¹³C) that are separated by two to four bonds. It is particularly useful for figuring out which carbon atoms are connected to which hydrogen atoms when they are not directly bonded, helping to piece together the structure of complex organic molecules.

Answer 7

In pulsed NMR, instead of sweeping the frequency of the radiofrequency (RF) signal continuously (as in continuous wave NMR), a short, intense burst of RF energy is applied to the sample. This burst, known as an RF pulse, excites all nuclear spins in the sample at once across a range of frequencies. A common pulse in NMR is a 90-degree pulse, which flips the nuclear magnetization from its equilibrium position (along the z-axis in the laboratory frame) to the xy-plane. Other pulse angles, like 180-degree or 45-degree pulses, can also be used depending on the experiment. The pulse width and power determine the angle by which the nuclear spins are rotated After the RF pulse is applied, the nuclear spins start to relax back to their equilibrium positions. During this process, the nuclei emit a weak RF signal called the Free Induction Decay (FID). The FID contains information about all the frequencies at which the nuclei resonate, but it's recorded in the time domain as a decaying oscillation. The Fourier transform decomposes the FID into its constituent frequencies, yielding the familiar NMR spectrum where the x-axis corresponds to frequency (or chemical shift) and the y-axis shows signal intensity. Peaks in the spectrum correspond to specific resonances of different nuclei in the sample.

Answer 8

NOESY (Nuclear Overhauser Effect Spectroscopy) is a two-dimensional (2D) NMR technique that provides information about spatial proximity between nuclei in a molecule. It's particularly useful for determining 3D structures of molecules, especially in large biomolecules like proteins and nucleic acids. In a NOESY spectrum, cross-peaks indicate through-space interactions between nuclei that are close to each other, typically within 5 Å (angstroms)

Answer 9

A NOESY spectrum is a 2D plot with two frequency axes (usually in parts per million, ppm): The x-axis corresponds to the chemical shifts of one set of nuclei. The y-axis corresponds to the chemical shifts of the same or another set of nuclei.

Answer 10

The diagonal of the spectrum contains peaks where the chemical shifts on the x-axis and y-axis are the same. These are called diagonal peaks and correspond to the 1D NMR signals of individual nuclei. Diagonal Peaks: These are auto-correlation signals, representing nuclei interacting with themselves (i.e., no spatial interaction between different nuclei). Cross-Peaks: These appear off the diagonal and represent NOE interactions between nuclei that are spatially close but not chemically bonded (represents through space interactions between two protons that are spatially close)

Answer 11

Step 1: Identify Diagonal Peaks: Locate the diagonal line, where the x-axis equals the y-axis. Each peak on this line represents the chemical shift of a proton in the molecule, corresponding to the 1D NMR spectrum. Step 2: Look for Cross-Peaks: Cross-peaks off the diagonal show interactions between protons that are close in space but not directly bonded (through-space NOE). These are the key to determining spatial relationships. Step 3: Correlate Cross-Peaks with Protons: Identify which protons the cross-peaks correspond to by comparing the chemical shifts of the peaks on both axes. For example, if a cross-peak appears at the intersection of chemical shifts of proton A on the x-axis and proton B on the y-axis, it indicates that protons A and B are spatially close.

Answer 12

COSY (Correlation Spectroscopy) is a two-dimensional (2D) NMR technique used to identify through-bond couplings between nuclei, typically protons. In a COSY spectrum, cross-peaks indicate scalar (J) couplings between nuclei, providing information about the connectivity between atoms.

Answer 13

The x-axis and y-axis both correspond to the chemical shifts (usually in parts per million, ppm) of the same set of nuclei, typically protons. The spectrum is symmetric along the diagonal line, where the chemical shift of a nucleus on the x-axis equals its shift on the y-axis. This diagonal contains auto-correlation peaks (diagonal peaks), representing each proton's own signal.

Answer 14

Cross-peaks appear off the diagonal, indicating correlations (couplings) between different protons that are through-bond coupled. (Indicate scalar coupling) The diagonal peaks in a COSY spectrum are the same as what you would see in a 1D NMR spectrum. Each proton shows up as a peak along the diagonal. These diagonal peaks help you locate where each proton’s chemical shift is on both axes. They serve as reference points for finding cross-peaks.

Answer 15

Step 1: Identify the Diagonal Peaks: Find the diagonal peaks that correspond to individual protons (chemical shifts) in the molecule. These diagonal peaks represent the same information you would get from a 1D NMR spectrum. Step 2: Look for Cross-Peaks: Cross-peaks indicate which protons are coupled to each other through bonds. If a cross-peak exists between proton A (at position δA on the x-axis) and proton B (at position delta B on the y-axis), it means A and B are coupled via bonds, typically 2 or 3 bonds apart. Step 3: Determine Coupling Relationships: The cross-peaks tell you which protons are connected by chemical bonds. For example, if proton A shows a cross-peak with proton B, it suggests that proton A is directly bonded (or two bonds away) from proton B. This is useful for piecing together the structure of the molecule.