Environmental Influence (7) Flashcards
What are chemical shifts?
The difference in the NMR absorption frequency of protons bound to chemically distinct groups in molecules
How is the frequency of RF absorption influenced by the local chemical environment?
differences occur in the absorption frequency of protons bound to chemically different groups in molecules.
What is diamagnetic circulation?
term that accounts for spherical charge density and opposes B0
What is the shielding constant?
dimensionless, independent of B0, determined by electron density and distribution
How does shielding influence the local magnetic field felt by the nucleus?
shielding decreases with increasing electronegativity of adjacent groups
How does the shielding constant affect the Larmor precessional frequency?
precess with a slightly smaller Larmor frequency
What is the shielding constant for an isolated hydrogen nucleus?
zero
How are shielding and the observed chemical shifts related?
Identical 1H nuclei give rise to different absorption positions when in different chemical surroundings, reflected in the chemical shift
How are chemical shifts measured?
internal standards report the position of the NMR peaks
Why is frequency not directly measured?
its much easier to measure the magnitude of change in field strength
Why is TMS used as the internal standard for proton NMR?
all protons are equivalent with a large screening constant
What is δ?
NMR chemical shift parameter
How does δ depend on magnetic field strength?
it does not; independent
What are the units of δ?
ppm
How can δ be defined relative to the spectrometer frequency?
(∆v/spectrometer frequency) * e6
