Block 2 - Spectroscopy

Mass spectroscopy (MS) Infrared spectroscopy (IR) Ultraviolet-visible spectroscopy (UV-VIS) Nuclear magnetic resonance spectroscopy (NMR)

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 directly proportional to frequency Energy inversely proportional to wavelength

The determination of the mass of individual ions derived from compounds in the gas phase

The highest m/z (where z = 1, so m) value on the graph is the molar mass

Unable to distinguish between compounds with same molecular formula Unable to distinguish between different molecular formulae that have same integer mass

1/cm

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

The transition of an e- from a lower energy (ground state) to a higher energy (excited state) level

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

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

When 2 atoms share 2e- (one molecular orbit between 2 atoms)

Compounds containing only sigma bonds are generally transparent to UV-VIS light - won’t see an absorption

Can’t tell how many C=C or C=O bonds there are; only one absorption seen

Shows an absorption at ~170nm due to π-π* electron transition

Shows an extra absorption (weak) at 280nm due to a n-π* electron transition i.e. has absorption at 170nm and 280nm

200nm

The smaller the ΔE and the larger the max wavelength

Absorbance is directly proportional to concentration and path length

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)

The more ‘effective’ a molecule is at absorbing light at a wavelength, the greater the absorbance

Bonding orbitals lower in energy than anti-bonding orbitals

C=C: σ < π < π* < σ* C=O: σ < π < n(on-bonding) < π* < σ*

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