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
How is the area of the observed NMR peak related to the number of magnetically equivalent nuclei?
area of the absorption peak is proportional to the number of equivalent nuclei giving
rise to the absorption
How are NMR resonance frequencies related to shielding?
high shielding = low frequency = upfield
low shielding = high frequency = downfield
what does the term upfield refer to in an NMR spectrum?
increased δ, high shielding, low frequency
what does the term downfield refer to in an NMR spectrum?
decreased δ, low shielding, high frequency
What is the range of δ for 1H atoms?
17.8 x 10^-6
What is the range of δ for 13C atoms?
260.7 x 10^-6
What is the range of δ for 31P atoms?
961.1 x 10^-6
What is the range of δ for 19F atoms?
What is the diagmagnetic component of the shielding constant?
local induced magnetic field opposes B0, is strongly dependent on the spherical charge density of s electrons near the nucleus
How is the diagmagnetic component of shielding dependent on the spherical charge density of the s electrons near the nucleus?
strongly
How is shielding related to inductive effects?
local induced magnetic field opposes the B0 field
How is shielding related to electronegativity?
shielding decreases with increasing electronegativity of adjacent groups
How is shielding related to magnetic anisotropy?
shielding of neighboring groups dependent on their location with respect to the bond
What are ring currents?
secondary fields that act in opposition to the applied field
How do ring currents influence the value of σ?
includes effects from both shielding and deshielding contributions
What is the paramagnetic shielding term?
accounts for non-spherical charge density
How is the paramagnetic shielding term related to electronic energy levels?
inversely proportional
