Chapter 17: Spectroscopy

Mass spectra can be used to identify the molecular mass of an organic compound and to gain further information about its structure.

x axis = m/z ratio y axis = relative intensity (or relative abundance)

It loses an electron and forms a positive ion. Molecular ion formed: M —> [M]+ + e-

• m/z value for [M]+ gives the Mr • high resolution measures m/z to enough accuracy to find the molecular formula • to find the molecular mass, the molecular ion peak (M+ peak) has to be located • the molecular ion peak is the clear peak at the HIGHEST m/z value on the right hand side of the mass spectrum

• M+1 peak • exists because 1.1% of carbon is present as the carbon-13 isotope

number of carbon atoms = height of M+1 peak / height of M peak * 100

mass-to-charge ratio m/z = mass of cation / charge of cation the charge of the cation is almost always +1, so m/z is usually the mass of the cation anyway

in the mass spectrometer some molecular ions break down into smaller pieces known as fragments

the other peaks in a mas spectrum are caused by fragment ions, formed from the breakdown of the molecular ion this is because the molecular ion is energetically unstable.

• the simplest fragmentation breaks a molecular ion into 2 species - a positively charged fragment ion (detected), and a radical (not detected) e.g. CH3CH2CH2OH+ —> CH2OH+ + CH3CH2. General: [M]+ —> X+ + Y.

• the more stable the ion, the greater the peak intensity • you can work out the structure of a compound from the other peaks

A radical is not a charged particle. A mass spectrometer only accelerates, deflects and detects ions. A radical will simply get lost in the machine and eventually removed via vacuum pump.

Ions with different isotopes produce different signals. Most important for Br and Cl: Br is 50% 79 and 81 Cl is 75% 35 and 25% 37

IR causes covalent bonds to vibrate more and absorb energy

Yes - covalent bonds in molecules posses energy and vibrate naturally around a central point. - the amount of vibration increases and temperature increases - atoms in molecules are therefore in constant motion

• stretching • bending

• mass of atoms in the bond: the lighter the atom the more it vibrates • strength of the bond: stronger bonds vibrate faster than weaker bonds • temperature: as temperature increases the amount of vibration increases

• it’s a measure of wavelength/frequency of absorption • it’s proportional to wavelength: wavenumber = 1 / wavelength (cm) • chemists use this scale to characterise covalent bonds since covalent bonds have characteristic bond frequencies

IR (infrared radiation) is a form of energy. Bonds can absorb IR, which makes them bend or stretch more.

IR spectroscopy is used as a means of identifying functional groups present in organic molecules. Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond. That’s why covalent bonds have such characteristic bond frequencies.

An instrumentation method of analysis that identifies bonds from absorption of the IR radiation of their different wavelengths.

wavenumber 0-4000 is read from right to left on the x-axis.

peaks above 1500cm-1 is used to identify functional groups.

Used for fingerprinting