Block 2: Spectroscopy Flashcards
What does mass spectroscopy determine?
The mass of individual ions derived from compounds in the gas phase
What is the concept of mass spectrometry?
The molecule is bombarded with a high energy electron beam where the ionised molecule results from the expulsion of one electron (therefore s radical cation)
What is a mass spectrum?
The plot of Kok abundance versus m/z (mass of ion/charge of ion)
Which value on a mass spectrum is the mass of the molecule?
The highest value
What size has a larger amount of deflection?
Small is more deflected (and only bends with magnetic field if a positive charge)
What happens when the mass of the M+ is measure to the nearest unit value?
Sometimes not possible to distinguish between certain molecular formulae
Limitation of MS (mass spectrometry)?
Not possible to distinguish between molecules with same molecular formula
What is spectroscopy?
The study of the interaction of electromagnetic radiation with matter
What is characteristic of specific structural units in molecules?
Absorption of light of particular frequencies
What spectrum has the lowest energy?
NMR (radio waves)
Whet does a longer wavelength correlate with?
Lower frequency and lower energy
What happens when the energy of the electromagnetic radiation exactly corresponds to the energy difference between two molecular energy levels?
The molecule can absorb the energy
Equation for velocity of light?
Wavelengthxfrequency
Equation for energy of radiation?
E = planck’s constant (h) x frequency
E= planck’s constant x velocity of light (c) / wavelength
Does energy have to match the energy gap exactly?
Yes, for transition state to occur
What is ultraviolet-visible (UV-VIS) radiation?
It’s of an appropriate energy to raise electrons in some molecules (especially pi bonds) from low energy bonding (or non bonding) to high energy anti-bonding molecular orbitals
What compounds are transparent to UV-VIS?
Those that contain only sigma bonds
What systems have absorption observed at a longer wavelength under UV-VIS?
Those in conjugated systems (ie alternating single and double bonds)
What is the absorption of any molecule containing conjugated double bonds greater than?
200nm
What is absorbance proportional to?
Concentration and path length
The more effective a molecule is at absorbing light at that wavelength…
The greater the absorbance
What is Beer’s Law?
Absorbance (A) = molecular absorbitivity in L mol-1 cm-1 (E) x path length in cm (b) x concentration in mol L-1 (c)
How do you rearrange Beer’s Law to find concentration?
C = A/(E x b)
Remember units (mol L-1)
What is IR (infra red) spectroscopy?
When IR radiation absorption corresponds to vibrational excitation m; that increasing the amplitude of the vibration
Two types of bond vibrations in IR?
Stretching
Bending
Do symmetrical bonds that are symmetrically substituted show absorption?
No (as a vibrational mode has to be accompanied by a change in dipole moment)
What determines the position of IR absorption?
Relative masses of atoms and bond strengths
What is the IR range for c=O?
1850-1600
What’s the IR absorption for N-H?
3500-3100
What’s the IR absorption for C-H?
3100-2800
What is the utility of IR?
- Rapid identification of functional groups
- As a fingerprint as no two compounds have identical spectra (although you cant distinguish between functional groups)
Do MS, UV-VIS or IR give information in regards to the hydrocarbon skeleton?
No
What spectroscopic technique had the greatest impact on molecule structure determination?
Nuclear Magnetic Resonance (NMR)
What does NMR provide information about?
Carbon and hydrogen framework
What do nuclei do when placed in a magnetic field?
They align them selves with (parallel) or against (anti parallel) the external magnetic field
Which nuclei have lower energy?
Those aligned with the magnetic field
In NMR, is the absorption frequency the same for 1H or 13C?
No, it’s different for all unique 1H or 13C because the nuclei are in different chemical environment
In NMR, what are positions of absorption called and what is it measured in?
Chemical shifts measured in delta units (ppm) from a peak of reference compound tetramethylsilane (TMS)
What does a bigger PPM mean?
An increasing frequency
What is 13C NMR?
A proton decoupled spectra: proton decoupling means one does not observe splitting of signals which would otherwise be seen because of 13C-1H
What’s the chemical shift range for 13C?
Delta 0-210
What carbons have the higher delta values?
Those bonded to electronegative substitutents (O, N, halogen) as opposed to hydrogen or carbon
What does the electronegative atom do in regards to the attached carbon?
It pulls itself away from it, as its nucleus is deshielded
In 13C NMR, what’s the signal of sp3 carbons?
Between 0-90
In 13C NMR, what’s the signal for sp2 carbons?
Between 100-210
In 13C NMR, is the signal strength indicative of the number of carbons giving rise to a signal?
No
Which carbons display a signal in 13C NMR?
Every one in a unique environment
Which hydrogens give rise to an absorption signal in H NMR?
Only hydrogens in unique environments (same environment=same signal)
What increases a chemical shift range for hydrogen in H NMR?
An increased electronegativity (eg, C-N-O)
Which hydrogens have a higher value?
Those bonded to sp2 carbons (as substituents containing double bonds deshield)
What are signal areas of H proportional to?
The number of hydrogens giving rise to the signals
When do absorption signals of H split?
When, on the atom directly bonded to one bearing the hydrogen(s) giving rise to the absorption signal, hydrogens are attached (ie vicinal/neighbouring Hs)
What is the rule for absorption splitting due to vicinal hydrogens?
N+1
