6.3.2 Spectroscopy

nuclear magnetic resonance

• the nuclear spins of the C13 and H1 isotopes • have an odd number of nucleons (protons and neutrons) • H1 is just a proton, so proton spectroscopy

• nucleus absorbs energy from strong magnetic field • and radio frequency radiation • radio frequency radiation has less energy than IR radiation, so needs a strong magnetic field to be able to detect it • this makes the nucleus rapidly flip between 2 spin states (the resonance) • same technique used in MRI machines

the frequency shift of the electrons surrounding the atomic radius - measured on a chemical shift scale δ - units ppm (parts per million)

• tetramethylsilane • (CH3)4Si • used as the standard reference chemical • all chemical shifts are measured against this • and has a δ value of 0 ppm • (all referenced against TMS at δ=0ppm)

• determined by the chemical environment • means that absorption peaks at different chemical shifts for certain compounds

can find the C/H arrangement of a compound without carrying out chemical tests, or needing to destroy the compound

must be dissolved in a solvent first - (and whole sample is placed inside the NMR spectrometer with TMS too)

• most common solvents contain C/H too • so will produce C13 and H1 signals

• a deuterated solvent: • the H1 atoms are replaced with H2/D deuterium atoms • so no signals are produced in the frequency range of H1/C13 • most commonly use CDCl3 (deuterated trichloromethane) • still produced a peak in the C13 NMR spectrum, but is filtered out before displaying

• the number of different carbon environments present (via the number of peaks) • the type of carbon environments present (via the chemical shifts)

• the type of chemical environments • start with C-C, to C-EN atom, to C=C/ring, to C=O

• different solvent used • different concentrations used • different substituents used

• the position of the atom within the molecule • so C-atoms bonded to different atoms/groups will have different environments and absorb at different chemical shifts • and if 2 carbon atoms are positioned symmetrically within a molecule, they will be equivalent and have the SAME chemical environment ( so absorb radiation at the same shift and contribute the same peak)

3 - one next to CH2COH group - one between CH3 and CHO group - one next to CH2H3 group

2 - one as a part of C=O - 2 symmetrical carbons, in a CH3 group and next to a C=O-CH3

• 3 peaks for 3 environments • one at C=O • and 2 in the C-C range (one shifted more to the left as closer to the EN O atom)

• 2 peaks for the 2 environments • one at C=O • and one at C-C (relatively proportional, but doesn’t really matter)

• symmetry is present based on the group, and what it is bonded too • even if it doesn’t look symmetrical, remember how it would look in the 3D structure, and that bonds can rotate • remember to be careful as to which C you are looking at, IN BOLD • even if you are comparing isomers, may still be able to distinguish between due to the number of peaks and chemical shifts of the NMR

1) the number of different proton environments (via the number of peaks) 2) the types of proton environments (via chemical shift) 3) the relative numbers of each type of proton (via the integration traces/ratio numbers of the relative peak areas) 4) the number of non-equivalent protons adjacent to the given proton (from spin-spin splitting patterns)

• concentration • solvent • substituents

• protons with the same chemical environment absorb at the same chemical shift value [are equivalent] • and increase the size of the peak • protons of different types have different chemical environments and absorb at different chemical shifts [non-equivalent]

• look for planes of symmetry

• 4 • as even the 2 CH2 groups are bonded to 2 different groups, and there are no lines of symmetry