spectroscopy and proof of structure element 8: NMR part 1

Outline of NMR

• nuclei behave as small magnets - have a small magnetic dipole associated with them

• this means that nuclei can interact with an applied magnetic field to give rise to sets of energy levels

• magnetic field of radio waves induces transitions between these energy levels

• frequency of transition is very sensitive to local electronic structure around the nucleus so gives chemical information on the molecule

• magnetic moments on different nuclei within a molecule can interact - coupling which tells us about bonding pathways

nuclear spin

• spin quantum number (I) - quantum mechanical property of nuclei

• spin quantum number of 0 - NMR silent, do not come up on the spectrum

• magnetogyric ratio - tells us how big the magnetic dipole is associated with a particular nucleus

• magnetic dipole will line up if in a magnetic field - can align in 2I + 1 ways

nuclear spin energy levels

• in a magnetic field (B), the magnetic moment can only take certain orientations with respect to B

• orientations described with second quantum number - m_I

• for a proton, I = ½ so m_I = + or - ½ 

• proton in absence of magnetic field doesn’t know the difference between the m_I values since they are degenerate

• in presence of magnetic field, the m_I values split so one is aligned with the magnetic field and one goes against the magnetic field

• the energy of a nucleus depends on if m_I = + or - ½ in the presence of a magnetic field

• the energy difference between the 2 spin states is very small - NMR very sensitive

how NMR is carried out

• energy difference between spin states depends on strength of magnetic field applied - energy absorbed in radiofrequency region

• absorption means that nucleus excited from lower to higher energy spin state, relaxation then occurs from upper to lower spin state - different nuclei have different relaxations

• assess sensitivity in terms of signal to noise ratio of recorded spectrum - plot emission intensity against time and use Fourier transform to convert it to absorption against frequency spectrum

• free induction decay - when a mixture of frequencies are used to excite all nuclei in different environments at once and record emission as a function of time

• different nuclei resonate at different frequencies - NMR spectrometer tuned at particular resonance frequency to detect a selective NMR active nucleus

NMR active isotopes

• if I=0, nucleus is NMR silent

• NMR strongly isotope specific as different isotopes have different spin states since they each have their own magnetogyric ratio

chemical shift

• nucleus experiences shielding from molecular orbitals so magnetic field you see is not the same as magnetic field experienced by nucleus

• different nuclei with different electron densities have different levels of shielding, more shielding = weaker magnetic field experienced by nucleus

• if electrons put in magnetic field they start to orbit around axis of rotation - generates opposing magnetic field

• B_eff = B_app(1-sigma) where sigma is shielding constant, shielding constant very small but very significant

• energy gap is proportional to the magnetic field which is proportional to the frequency

chemical shift scale

• reference sample used and frequency at which reference sample absorbs used as standard

• compare frequency which sample absorbs to reference

• shift to positive chemical shift = shifted to higher frequency

• shift to negative chemical shift = shifted to lower frequency

• chemical shift remains constant no matter what magnetic field is whilst frequency does vary with magnetic field

• if electron density around nucleus is lower, less shielding so frequency higher and chemical shift larger

• TMS usually used as reference - chemically inert, cheap, lots of protons all in same environment so strong signal

solvents for NMR

• use deuterated solvents

• these have a different resonance frequency to molecules being observed in the spectrum so are silent and do not affect the spectrum

factors affecting ¹H chemical shifts: local electron density

• more electrons around a nucleus, the more shielded the nucleus is from the applied magnetic field so lower frequency and lower chemical shift

• electronegative substituents pull electron density away from the proton so there is less shielding and shift the resonance to a higher frequency so higher chemical shift

factors affecting ¹H chemical shifts: aromatic systems

• have circular pi system which can support a ring-current - electrons move due to magnetic field so electrons in pi system begin to circulate

• ring-current - movement of electrons and induced current around the ring

• ring-current opposes applied magnetic field at centre of motion and reenforces it around the edge - higher chemical shift as protons in nucleus more exposed to the magnetic field

peak intensities

• height of signals proportional to the number of nuclei of that type in the sample

• computer can be used to integrate the peak and tell us the area - can use integrals to work out ratios of molecules in a mixture

¹³C NMR

• ¹³C - stable isotope, low abundance (1.1%) but has spin of ½ so can be used in NMR

• less sensitive than protons as less abundant, larger chemical shift range

• cannot use integrals of ¹³C spectra - different relaxation effects from different nuclei so only get accurate representation of ones which relax more rapidly

• chemical shift related to hybridization