Created with MindGenius Business 2005® Chemical Shift (1) Chemical Shift (1) ν is measured in Hz and varies with spectrometer frequency The position of a signal from a particular proton is therefore measured relative to TMS This is known as chemical shift (δ) in parts per million (ppm). δ = Distance from TMS signal (Hz) in ppm Spectrometer frequency (MHz) 1 H NMR spectra are normally of the range 0-10ppm (gives a 900 Hz range on a 90MHz machine).

Created with MindGenius Business 2005® Chemical Shift (2) Chemical Shift (2) Electronegative Neighbour Atoms: e.g. O, F 1 H δ for RCH 3 (1-2ppm)< RCH 2 Br (3-4ppm)< RCH 2 OH ( ppm) ALSO: Because hydrogen is more electropositive than carbon, increasing substitution (branching) also gives a downfield shift. i.e 1 H d for RCH 3 < RR’CH 2 < RR’R”CH Both effects decrease rapidly with distance. Neighbouring electronegative atoms Deshielded nucleus Increased ν needed for resonance Downfield shift in signal (increased ppm)

Created with MindGenius Business 2005® Chemical Shift (3) Chemical Shift (3) Unsaturated systems: i.e. Alkenes, alkynes Typical Benzene C-H is at 7-8.5ppm Aldehyde (RCHO) protons are observed the furthest downfield (>9.5ppm) as they have an electronegative atom and a double bond. Amines and Alcohols: RNH 2, ROH Protons attached directly to nitrogen or oxygen give broad, variable position signals, because they become involved in hydrogen-bonding which affects their electron density. π -bonds have high electron density Asymmetric magnetic fields Anisotropy Downfield shift for adjacent protons

Created with MindGenius Business 2005® Chemical Shift (4) Chemical Shift (4) Chemical Equivalence : Nuclei in identical environments have the same chemical shifts and therefore give only one signal. Atoms are said to be chemically equivalent if mentally substituting one of them would give identical results as substituting another. Equivalence can be identified using symmetry: Plane of symmetry: Atoms that are reflections of each other through a plane of symmetry are equivalent. Rotational symmetry: Atoms that can be interchanged by rotation about a chemical bond are equivalent (e.g. methyl protons) provided that bond is able to rotate freely.

Created with MindGenius Business 2005® Chemical Shift (5) Chemical Shift (5) Typical Chemical Shifts ( 1 H) δ/ppm Low field High field CH 3 Si CH 3 C CCH 2 C CH 3 C=C CH 3 C=O CH 3 N- CH 3 O, CH 2 O CH 2 Br CH 2 Cl CH0 2 HC=C Aromatic CONH, NH 2 COOH, Phenol OH CH=0