Ochem 345, Part 1 Flashcards
Basic principle of Absorption Spectroscopy
- Make light (radiation)
- Pass light through a sample (which absorbs some amount of radiation)
- Measure transmitted (unabsorbed) light with detector
UV/Vis Absorbtion Spectroscopy
Good for measuring conjugated pi systems.
Pi to pi* transitions are common absorptions; electron excites by a quantum leap from the HOMO to the LUMO (making a SOMO).
All about Photons
Particles of light
Units of light energy
Must be deposited in discrete amounts aka quantum leaps.
E=hc/wavelength
and
E=hf
(Energy of a photon = Planck’s Constant * frequency of light wave)
The energy of a photon is directly proportional to the light’s ________________ and inversely proportional to the _____________.
The energy of a photon is directly proportional to the light’s frequency and inversely proportional to the light’s wavelength.
When measuring a conjugated pi system via UV/Vis Absorbtion Spectroscopy, the more extensive the pi system, the ___________ the wavelength.
The more extensive the conjugated the pi system, the smaller the energy gap between the HOMO and LUMO; the smaller that gap, the lower energy the photon needs to be to make the leap.
Therefore, the more extensive the conjugated pi system, the longer the wavelength.
Light spectrum of wavelengths
Molecules need to absorb light in the visible range in order to be colored. <400 nm = not visible UV 400 nm 475 nm (blue) 510 nm 570 nm (yellow) 590 nm 650 nm (red) >650 nm = not visible infrared
Colored organic molecules (ex. Carrots, crystal violet, saffron)
Typically an indicator of extensive pi systems or N=N pi bonds
IR Spectroscopy
Measure of vibrations of molecular bonds.
When graphed, vertical axis is % of light transmitted (or sometimes absorption).
Horizontal axis is wavenumber (cm^-1) signals; higher frequency vibration, higher wavenumber.
So, the reverse peaks represent molecular IR radiation absorptions at particular signals.
3 modes of simple molecular bond vibration
1) bond length changes/stretching in symmetric fashion
2) bond length changes/stretching in asymmetric fashion
3) scissor-like motion
If a molecule absorbs a photon of a frequency of light that matches the frequency of a vibration, an absorption occurs and that vibrational state is excited.
This happens in the IR region of the spectrum.
IR intensity is related to a change in dipole upon vibration.
Bonds as harmonic oscillators
Can treat bonds as springs, as harmonic oscillators governed by Hooke’s Law.
The frequency (v) is dependent on the BOND STRENGTH (force constant k) and the MASS of the atoms in the molecule.
How C-C bond strength affects vibrations and IR Absorption frequency
Bond strength (force constant k) increases when you add bonds, so Frequency increases.
Approximate vibrational frequency
C-C, 1000 cm-1
C=C, 1600 cm-1
C(triple)C, 2200 cm-1
How C hybridization in C-H bonds affect IR Absorption Frequency
More “s” character = stronger C-H bonds, so force constant k increases and frequency increases.
Bond Approximate vibrational frequency C(sp)-H 3300 cm-1 C(sp?)-H 3100 cm-1 C(sp3) H 2900 cm-1
Mass affecting IR Absorption Frequency (example)
Bond Approximate vibrational frequency
C(sp3)-D, 2200 cm-1
C(sp3)-H, 2900 cm-1
IR Absorption Spectrum Regions
Functional groups identified on spectra, by wavenumber:
(Highest energy signals)
-3400 - 2800: great for detecting O-H or N-H stretching.
-2250 - 2100: great for detecting asymmetric triple bonds R-C(triple)N, R-C(triple)C-R’.
-1880 - 1600: great for detecting C=O, C=N, C=C, and aromatic rings.
-Much of the spectra <1500 cm^-1 can be ignored at our level
(Lowest energy signals)
Uses for IR Spectroscopy
- Studying and comparing a molecule’s conformations (ex. syn-syn conformation absorbing much more that syn-anti, indicating much more vibrational activity)
- Watching reactions, comparing computationally predicted molecular vibration spectrum to experimental to determine product (photochemistry)
- IR works at very cold temperatures
Electron Impact Mass Spectrometry (EI-MS)
Used to determine
- molecular mass
- connectivity of atoms in a molecule
Bombards a molecule with high energy electrons at 70eV, blows the molecule apart, and detects the mass of the resulting cationic particle fragments.
The resulting fragments that we observe at the detector tell a lot about the original molecule.
Spectrometry vs spectroscopy
Spectrometry measures particles, Spectroscopy measures photons
EI-Mass Spectrometry plots
Products of EI-mass spec are some weird, high energy fragments.
Vertical access is relative abundance, with the heaviest, most abundant partial set at 100.
Horizontal axis is mass-to-charge ration (m/z).
EI-Mass Spectrometry resulting fragments
EI-Mass Spectrometer produces high energy, unstable, strange particles! Only cationic species are responsive to the magnetic field and detected (not anions or neutral species).
Molecule —> 70 eV e-
—> [molecule]+•
—> fragments
Work backwards from fragments to study original molecule.
Lost electron can be depicted with a dotted bond and series of applicable resonance structures, or a single dot e in place of bond.
Homolytic bond cleveage
Bond breaks and each atom gets 1 electron
Heterolytic bond cleveage
Bond breaks and both electrons go to a single atom.
Abundance of ions detected in EI-Mass Spectrometry depends on…
Abundance of ions detected depends on
- rate of formation
- rate of fragmentation
***More stable ions that are easy to form tend to be responsible for more intense signals!!!
NMR Spectroscopy
Nuclear Magnetic Resonance Spectroscopy takes advantage of the magnetic spin properties of some nuclei to obtain detailed structural information.
Molecules must have an odd number of protons and/or neutrons to be active on NMR.
NMR active nuclei: 1H, 13C, 15N, 19F, 31P
NMR inactive nuclei: 12C (by far most abundant!), 16O, 32S
How NMR works
Start with sample. Randomly-oriented nuclear spins have magnetic moments.
Add an external magnetic field which orients the spins aligned either with the magnetic field (+1/2, parallel configuration) or against the magnetic field (-1/2, anti parallel). Small energy gap between these two states so both heavily populated, but slightly more in lower E +1/2 state. Pulse the system with radiation to equalize the two states; as system returns to EQM, it emits light which is detected, measuring the energy gap.
MRI works the same way.
Interpreting NMR outputs
Pay attention to:
- number of signals (how many distinct environments).
- chemical shift in ppm (type of environment).
- integration value (intensity of signals provide the relative number of atoms making that signal).
- coupling pattern (splitting of the same peak; the number of H atoms in close proximity in nonequivalent environments).
- coupling constant value (substitution pattern or stereochemistry based on size odd coupling constants)
- compare the integration of two signals in separate molecules to determine the purity or relative abundance.
