Infrared (IR) and Nuclear Magnetic Resonance (H NMR) Spectroscopy Flashcards
Week 3
The degree of unsaturation
the number of π bonds and/or rings in a molecule.
Each double bond or ring in a molecule results in a loss of 2 H from the formula CnH2n+2
Degree of unsaturation formula
DOU = {(2C+2) - (# of H) + Group V - Group VII} / 2
Group V
N, P
Group VII
F, Cl, Br
How many degrees does every successive DOU lose?
2
Mass Spectrometry Gives
Information about the mass of a compound and the fragments from which it is formed.
Mass Spectrometry Works…
- Molecules are ionized (made into charged species) and fragmented
- Ions are separated based on mass depending on their mass-to-charge ratio (m/z).
What is the most important peak on the MS
The most important peak is the molecular ion peak (usually the heaviest ion in large abundance, M+) as it represents the molecular mass of the compound.
Spectroscopy
The measurement of the interaction
between a molecule and electromagnetic radiation
Infrared spectroscopy consists of…
a high-quality infrared light source, a slit to create a parallel beam, a sample carrier, and a detector.
Infrared Spectroscopy works by
light being absorbed by compound only at its vibration frequencies, light absorbed by compounds is absent at detector, showing up as IR peaks
What kind of bonds do not give bands in the IR
Bonds that do not have dipole do not give bands in the IR
How does Nuclear Magnetic Resonance work?
Some atomic nuclei, like protons, behave like spinning spheres. Since the nuclei ( + charge) have electrons ( - charged) surrounding them, a small local magnetic field is created when they spin (magnetic moment).
When an external magnetic field is applied, the magnetic moments align, with some opposed to, and some parallel to the magnetic field.
1. The population of nuclei in the lower (more stable state) is greater
2. the electromagnetic radiation causes spin-flip, whose frequency is called resonance frequency.
3. As the nuclei relax, they emit a signal that provides information about their unique structure
Why is NMR such a powerful tool?
Every chemically distinct H nucleus in an organic molecule has a unique electronic environment: Each requires a slightly different radiofrequency to undergo resonance (so to create a distinct peak).
of signals shows the
of distinct Hydrogens
Integration
The peak area for each hydrogen type gives # of hydrogens associated with each hydrogen type
Chemical Shift
The position on the x-axis of each signal gives the chemical environment of each proton type. This includes hybridization of attached carbon, presence of adjacent functional groups, etc.
Shielding
the electrons around a nucleus create a magnetic field. This reduces the apparent field, thereby shielding it from applied magnetic field.
Up-field
Shielded, electron rich nuclei
Down-field
De-shielded, electron-withdrawing groups
Electronegative effects
Electronegative atoms ‘‘de-shield’ and shift protons towards the left; higher radio frequency needed for proton resonance.
Magnetic Anisotropy
Pi-electrons generate a local diamagnetic current that opposes the applied magnetic field (Bo). This causes a shielding cone where nuclei inside the cones are shielded and nuclei outside the cone are de-shielded.
What does the magnetic anisotropy cause?
Causes hydrogen atoms attached to pi-systems to have a dramatic de-shielding effect compared to alkane H’s
Hydrogen atoms on heteroatoms (OH or NH) show what signals?
are usually broad signals that have variable chemical shifts due to H-bonding
