Block 2 - Spectroscopy Flashcards
Types of spectroscopy
Mass spectroscopy (MS)
Infrared spectroscopy (IR)
Ultraviolet-visible spectroscopy (UV-VIS)
Nuclear magnetic resonance spectroscopy (NMR)
Molecular spectroscopy
IR, UV-Vis and NMR are forms of spectroscopy collectively known as molecular spectroscopy
Depend on interaction of molecules with radiation of specific energy
Energy, frequency, and wavelength relationship
Energy directly proportional to frequency
Energy inversely proportional to wavelength
Mass spectrometry allows…
The determination of the mass of individual ions derived from compounds in the gas phase
Mass spectrometry - how to read the molar mass
The highest m/z (where z = 1, so m) value on the graph is the molar mass
Mass spectrometry - limitation(s)
Unable to distinguish between compounds with same molecular formula
Unable to distinguish between different molecular formulae that have same integer mass
IR spectroscopy - units
1/cm
UV radiation is of…
Appropriate energy to raise electrons in some molecules (generally those with pi e-) from lower energy bonding (or non-bonding (lone pairs)) molecular orbitals to higher energy anti-bonding molecular orbitals
UV-VIS radiation causes…
The transition of an e- from a lower energy (ground state) to a higher energy (excited state) level
Conjugation
Pi electron systems (double bonds) that are linked together
Double bonds are only conjugated when there is only ONE single bond between the double bonds, i.e. double-single-double
Conjugated vs non-conjugated systems
Exhibit different UV-VIS absorbance
ΔE for electron excitement is smaller in conjugated systems, so absorption is observed at longer wavelengths than for non-conjugated systems
Bonding molecular orbital
When 2 atoms share 2e- (one molecular orbit between 2 atoms)
UV-VIS - sigma bonds
Compounds containing only sigma bonds are generally transparent to UV-VIS light - won’t see an absorption
UV-VIS - number of double bonds?
Can’t tell how many C=C or C=O bonds there are; only one absorption seen
UV-VIS: non-conjugated C=C bond
Shows an absorption at ~170nm due to π-π* electron transition
UV-VIS: non-conjugated C=O bond
Shows an extra absorption (weak) at 280nm due to a n-π* electron transition
i.e. has absorption at 170nm and 280nm
Conjugated double bond - minimum absorption
200nm
The greater the number of double bonds in conjugation…
The smaller the ΔE and the larger the max wavelength
For a particular compound at a specified wavelength…
Absorbance is directly proportional to concentration and path length
Beer’s Law
A = ɛ b c Where A = absorbance (no unit) ɛ = molar absorptivity (L mol-1 cm-1) b = path length (cm) c = concentration (mol L-1)
Effectiveness of molecule absorbing light and absorbance
The more ‘effective’ a molecule is at absorbing light at a wavelength, the greater the absorbance
Bonding vs non-bonding orbitals - energy
Bonding orbitals lower in energy than anti-bonding orbitals
UV-VIS - least to most energy bonds
C=C: σ < π < π* < σ*
C=O: σ < π < n(on-bonding) < π* < σ*
What happens when nuclei with spin are placed in a magnetic field
They align themselves with (parallel to) or against (anti-parallel to) the external magnetic field
Nuclei aligned with the magnetic field have slightly lower energy than those aligned against the field
Nuclei magnetic field - irradiation
On irradiation with radio frequency of appropriate energy, energy is absorbed and the nuclear spin flips from the lower energy state to the higher energy state
NMR absorptions
Known as chemical shifts
Measured in 𝛿 (delta) units (ppm)
TMS = 0 –> ignore
0𝛿 - close and far
Closer to 0: shielded (C-H or C-C)
Further from 0: de-shielded (bonded to more electronegative atoms)
What Cs or Hs give rise to different signals
Each unique C or H will give rise to different signals
Chemical range for 13C NMR
Generally 𝛿0 - 210
𝛿0 - 90: sp3 carbon
𝛿100 - 210 (or sometimes higher): sp2 carbon
13C NMR: intensity of signal
Generally not an indication of the number of Cs giving rise to a signal
Interpreting 1H NMR - absorption signals
Number of absorption signals
Position of absorption signals
Relative areas under absorption signals
Splitting pattern for absorption signals
Chemical shift range for 1H NMR
Generally 𝛿0 - 15, with most signals observed in 𝛿0 - 10
NMR: signals for H bonded to sp2 carbon
Signals are at higher 𝛿 values
NMR: C and H link to deshielding
If C experiences less electron density around it, then the H bonded to the C will also experience less –> more de-shielded
Splitting pattern for absorption signals (H)
Splitting of an H absorption signal occurs when there are vicinal Hs for said H
Vicinal Hs
Neighbouring Hs
Signal for one H is split by vicinal Hs and vice versa
Number of H signals
In general, an absorption signal is split into N+1 lines, where N is the number of vicinal Hs
The line intensities within the split signal accord with Pascal’s triangle
