NMR Concepts Flashcards
What is upfield in an NMR spectrum. Downfield?
A signal of an NMR spectrum that is shifted toward the right (smaller chemical shift). Downfield is the converse.
What is observed on an NMR spectrum due to increased shielding?
The signal will be shifted upfield (to the right).
What is observed on an NMR spectrum due to decreased shielding?
The signal will be shifted downfield (to the left). Deshielded = Downfield
What is the relationship between shielding and the electromatic radiation frequency in NMR?
The more the nucleus is shielded by electron density, the lower the magnetic field experienced by the nucleus in an NMR spectrometer. The energy of electromagnetic radiation (radio waves) required to bring that nucleus into resonance (i.e. “flip its spin”) also decreases. This correlates with a lower chemical shift on an NMR spectrum. The converse is true for increased shielding as well.
What is chemical shift?
The shift in parts per million (ppm) of an NMR signal relative to the signal of TMS (tetramethylsilane). The difference in resonance frequencies (in Hz) between a given hydrogen and a hydrogen on TMS divided by the irradiating frequency used (in Hz). The irradiating frequency used will be on the order of 10^6, corresponding to ppm (1/10^6). The range of chemical shift isabout 0-13 ppm. This scale is a more convenient scale for interpreting the differences than if the actual frequency differences were used.
What is resonance in NMR spectroscopy?
The absorption of electromagnetic radiation by a precessing nucleus and the resulting “flip” of its nuclear spin form the lower energy state to the higher energy state.
What is diamagnetic current in NMR?
The circulation of electron density in a molecule in an applied magnetic field.
What is shielding in NMR?
Also called diamagnetic shielding; refers to the reduction in magnetic field strength experienced by a nucleus underneath electron density induced to circulate when the molecule is placed in a strong magnetic field.
What is a coupling constant?
Coupling constant (J) is the separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet and a quantitative measure of the influence of the spin-spin coupling with adjacent nuclei.
What is the range of coupling constants in proton NMR?
0 - 18 Hz.
What effect does the applied field strength or operating frequency have on the coupling constant (J) in proton NMR?
The value of J depends only on interactions with other nuclei within a molecule, and so it is independent of the applied field strength and NMR spectrometer operating frequency.
What is spin-spin coupling?
An interaction in which nuclear spins of vicinal hydrogens (H atoms on two C atoms that are bonded to each other) influence each other and lead to the splitting of NMR signals. The spin state of nuclei of non-equivalent hydrogens no more than three bonds away influence th enet magnetic field experienced by a given nucleus.
What effect does the operarting frequency of the NMR spectrometer have on the chemical shift?
None, chemical shift is by definition independent of the operating frequency. It is always just relative to TMS (the reference compound that is assigned a value of 0 ppm)
What is the (n +1) rule?
If a hydrogen has n hydrogens nonequivalent to it, but equivalent among themselves on the same or adjacent atom(s), its proton NMR signal will be split into (n+1) peaks.
How do you determine the number of peaks due to combined couplings of adjacent hydrogen nuclei?
In general, if H nucleus is coupled to a set of n hydrogen nuclei on one side and a set of m hydrogen nuclei on the other, the signal will be split into a maximum of (n + 1)(m + 1) peaks. In flexible molecules this isn’t always observed however due to similar coupling constants, the splitting will simplify to (m+n+1)
