Question

Explain the following terms: (5) (i) Auxochrome (ii) Fingerprint region (iii) Chemical shift (iv) Chemical exchange (v) Molecular ion

Answer 1

An auxochrome (Greek αὐξάνειν auxánein: "to increase" and χρῶμα chrōma: "colour") is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light.
2)The region  (from about 1500 to 500 cm-1) usually contains a very complicated series of absorptions. These are mainly due to all manner of bending vibrations within the molecule. This is called the fingerprint region.
3)The chemical shift is the resonant frequency of a nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule.

4)So-called chemical exchange occurs when microsecond to millisecond motions cause a modulation in the isotropic chemical shift. Chemical exchange is relevant both in the situation where a protein undergoes internal motions (e.g. a conformational change) or where it interacts with another molecule (e.g. complex formation).
5)The formation of molecular ions. When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion.