Spectroscopy Flashcards
Spectroscopy
measures the energy difference between possible states of a system by looking at absorbed electromagnetic radiation
Infrared Spectroscopy
measures molecular vibrations visualized as bond stretching, bending
infrared light is passed through a sample and the absorbance is measured
Infrared Light Range
700nm - 1 mm
useful absorptions are from 2500-25000
4 Types of Vibration
1) symmetric Bend
2) asymmetric bend
3) Symmetric Stretch
4) Asymmeric stretch
Fingerprint Region
1500 - 400 cm^-1 range
specific absorbance pattern is characteristic of each individual molecule
For an absorption to be recorded, the ____ must result in a change in the ________.
Therefore, molecules that are _____ or have atoms with _____ do not exhibit absorption/change in dipole moment
vibration
bond dipole moment
symmetrical
identical electronegativities
Hydroxyl Group (OH)
broad wide peak at:
3300 for alcohls
3000 for carboxylic acids as the carbonyl pulls some electron density from OH bond shifting absorption to lower wavenumber
Carbonyl Group (C=O)
1700 sharp peak
Bond between any atom and hydrogen is relatively ____ and as we add more bonds between carbon atoms, the absorption frequency ____
high
increases
Amines (N-H)
3100-3500
sharp peak instead of broad
Ultraviolet Spectroscopy
obtained by passing ultraviolet light through a sample dissolved in inert, nonabsorbing solvent and absorbance is recorded and plotted against wavelength
absorbance is caused by electronic transitions between orbitals
most important info we get is wavelength of maximum absorbance which tells us extent of conjugation within conjugated systems.
UV Q: The more _____ the compound, the ____ the energy of the transition and the _____ the wavelength of maximum absorbance.
conjugated
lower
higher
UV spectroscopy works because molecules with ________ or ______ can be excited by ultaviolet light to ________ antibonding orbitals.
pi electrons
nonbonding electrons
higher energy
HOMO
highest occupied molecular orbital
LUMO
lowest unoccupied molecular orbital
Molecules with a lower energy gap between ____ and _____ are more easily excited and can absorb longer wavelengths/_____ frequencies with lower energy.
HOMO
LUMO
lower
Conjugated Molecules
unhybridized p orbitals
can be excited by ultraviolet light and shifts absorption spectrum to higher wavelengths and lower frequencies
larger conjugated molecules can lead to color by absorbing light to visible range
Nuclear Magnetic Resonance (NMR)
based off the fact that certain atomic nuclei have magnetic moments oriented at random, in magnetic field the moments alight agaist or with the applied field
Alpha State
nuceli with magnetic moments aligned with the field (said to be in lower energy state)
Beta State
nuceli being irradiated with radiofrequency that match the energy gap between two states and excited lower energy nuclei into B state (higher energy)
Magnetic Resonance Imaging (MRI)
noninvasive diagnostic tool that uses proton NMR
multiple cross sectional scans of body are taken and various chemical shiftts of absorbing protons are translated into shades of grey
dark area tends to water, light area is fatty tissue
comparison with normal MRI then allows to detect abnormalities in scanned region
NMR spectrum
frequency vs absorption
chemical shift (delta) in ppm is the frequency
The chemical shift in NMR is plotted on the _______ and it increases toward the left which is refered to as ______
x axis
downstream
Tetramethylsilane (TMS)
a calibration standard to mark 0 ppm and does not count as a peak to see how downfield compounds are
Proton NMR
hydrogen nuceli come into resonance at 0-10 ppm downfield from TMS
each distinct nuclei gives rise to a seperate peak
If multiple protons are _____, having the same magnetic environment, they will lead to the same peak.
chemically equivalent
Deshielding vs Shielding
more deshielding the more downfield you go as there are more electronegative atoms pulling electron density away from the proton
more shielding the more upfield you go as there are electron donating groups that shield the proton
Integration
area under the peaks
ratio of protons that produced each peak (1:3 hence a could be 1 proton and b could be 3 protons)
Spin-Spin Coupling/Splitting
two nuceli in nonmagnetically identical enviornments are adjacent to each other and impact the splitting
DOublet
two peaks of identical intensity equally spaced around a chemical shift, both nuceli will have doublets where their shifts should be
n + 1 rule
used to determine number of peaks present (doublet, triplet, etc.)
if a proton has n protons that are three bonds away, it will be split into n+ 1 peaks (no not include protons attached to oxygen or nitrogen)
J
coupling constant which is the magnitude of splitting measured in hertz
Multiplet
peaks that have more than four shifts
H-NMR: Aldehyde
9-10 ppm
H-NMR: Carboxylic Acids
10.5-12 ppm
H-NMR: Aromatic Ring
6-8.5 ppm
H-NMR: sp3 Carbons/alkyl groups
0-3 ppm (higher if electron withdrawing groups are around)
H-NMR: sp Carbons/alkyne groups
2-3 ppm
H-NMR: sp2 Carbons/alkenes
4.6-6.0 ppm
