Nuclear Magnetic Resonance
(NMR) - The electronics don’t lie
Purpose: We study molecules based on their electronic environments and nature
throughout the molecule
Common Use: To determine structural features and general molecule connectivity
Overview:
● How does it work/theory
● Chemical Shift and its factors
● Resonance shielding (s character + electron centralization)
● Instances where resonance deshields (partial positives)
● Types of 1H + 13C
● Spin-systems and how to recognize them
● Splitting Patterns (Neighbor Rule)
● DEPT-90/135 NMR
● Maybe mention 2D NMR?
Theory
Certain atoms whose nuclei have an odd atomic number are susceptible to spin.
These atoms can be spinning in one of two directions. We take advantage of this
property by exposing these atoms to a magnetic field; some atoms have a spin that
is aligned with the field while others are aligned against it. These differences result
in a measurable energy gap that we record as spectra. A stronger magnetic field
results in larger differences in energy levels between these states, improving
resolution.
, Chemical Shift and Its Factors
We read an NMR spectra based on its chemical shift values. These numbers are a
way of expressing how electrons behave in this magnetic field and are indicative of
various structural features.
Chemical Shift Equation:
The nice thing about this formula is that
it helps standardize our data, meaning
we will always get the same numbers no
matter what instrument we use to test
our sample! This mostly accounts for
differences in how good our magnet is.
sample refers to the measured frequency at a specific point in the molecule for a
given atom. The standard, usually TMS (tetramethylsilane), is calibrated such that it
will show up with a chemical shift of 0.0 ppm (numerator = 0)
Factors that Affect Chemical Shift
● Induction
● Hybridization (sp2 > sp > sp3)
● Electronegativity
● Resonance
These trends all affect the electron
density in a molecule. Some cause
better alignment with the
magnetic field and others push or
pull electrons around a molecule
(NMR) - The electronics don’t lie
Purpose: We study molecules based on their electronic environments and nature
throughout the molecule
Common Use: To determine structural features and general molecule connectivity
Overview:
● How does it work/theory
● Chemical Shift and its factors
● Resonance shielding (s character + electron centralization)
● Instances where resonance deshields (partial positives)
● Types of 1H + 13C
● Spin-systems and how to recognize them
● Splitting Patterns (Neighbor Rule)
● DEPT-90/135 NMR
● Maybe mention 2D NMR?
Theory
Certain atoms whose nuclei have an odd atomic number are susceptible to spin.
These atoms can be spinning in one of two directions. We take advantage of this
property by exposing these atoms to a magnetic field; some atoms have a spin that
is aligned with the field while others are aligned against it. These differences result
in a measurable energy gap that we record as spectra. A stronger magnetic field
results in larger differences in energy levels between these states, improving
resolution.
, Chemical Shift and Its Factors
We read an NMR spectra based on its chemical shift values. These numbers are a
way of expressing how electrons behave in this magnetic field and are indicative of
various structural features.
Chemical Shift Equation:
The nice thing about this formula is that
it helps standardize our data, meaning
we will always get the same numbers no
matter what instrument we use to test
our sample! This mostly accounts for
differences in how good our magnet is.
sample refers to the measured frequency at a specific point in the molecule for a
given atom. The standard, usually TMS (tetramethylsilane), is calibrated such that it
will show up with a chemical shift of 0.0 ppm (numerator = 0)
Factors that Affect Chemical Shift
● Induction
● Hybridization (sp2 > sp > sp3)
● Electronegativity
● Resonance
These trends all affect the electron
density in a molecule. Some cause
better alignment with the
magnetic field and others push or
pull electrons around a molecule
