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
