Spectroscopy is the study of interaction of matter with light. It is used as a tool to explore the composition (IR and NMR), physical structure(XRD) and electronic structure(UV) of matter at atomic scale. Molecules are known to undergo internal processes like nuclear, vibrational, rotational and electronic transitions between energy levels corresponding to different regions in the electromagnetic(EM) spectrum. The underlying principle to interact with incoming EM radiation is that the molecules must possess some electric and magnetic properties which can be influenced by the electric and magnetic part of the radiation. Some examples with respect to common spectroscopic techniques are given below.

1. In NMR spectroscopy, the nuclear spins have dipoles (magnetic) which are either aligned along or against a magnetic field. Interaction with a radiowave of appropriate energy results in the transition among these dipoles.
2. Vibrations among the molecules can also produce changes in the dipole (electric) that can interact with the electrical component of the EMR.
3. Molecules/Atoms also possess internal electronic energy levels (AOs or MOs) among which transitions of electrons can take place on absorption of appropriate energy from incoming EMR. These interactions also arise from changes in the electric dipole due to size, geometry and spatial organization of different orbitals.
4. Molecules possessing a permanent net dipole moment when undergoing rotation produce a change in dipole which can interact with the electrical part of the incoming radiation and hence undergo transition among its rotational energy levels.

From the above examples, it can be inferred that change in either of the dipoles (electric or magnetic) is a necessary condition for the absorption or emission of EMR.

Infrared spectroscopy

The vibrational energy levels are more closely spaced in each electronic energy level and hence require less energy for transition among them and these transitions fall in the IR region. Hence IR spectroscopy is used to study the vibrational transitions and thereby the vibrational modes of any molecule.

For a non-linear molecule there are 3N-6 vibrational degrees of freedom. For a linear molecule there are 3N-5 vibrational degrees of freedom. Any molecule has some IR active modes and some inactive modes. All modes which possess a change in dipole moment are IR active. A particular IR active vibration has a unique frequency and absorbs radiation of the same frequency. These frequencies depend upon the electronic environment of the bond involved and hence tell a lot about the molecular identity. Because of this feature, IR spectroscopy is used to identify functional groups in molecules, for example Carbonyl (C=O), which has a double bond with a high spring constant, k, with high polarity. Its vibration produces large changes in dipole moment across the bond resulting in a very intense absorption band.

Mass spectroscopy

Mass spectroscopy (MS) is also a powerful tool in analyzing organic molecules, for example biomolecules. An instrument separates the molecules and its fragments based on their charge and mass. The underlying principle is that a moving charged particle is deflected by an applied electric and magnetic field. The range of this deflection depends on its mass and charge. A mass spectrum is a two dimensional plot with ion abundance and m/z ratio in its axis.

Nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance spectroscopy also known as NMR spectroscopy or magnetic resonance spectroscopy (MRS). the technique observes the local magnetic field around atomic nuclei. When the sample is placed in a magnetic field, an NMR signal is produced by the excitation of nuclei spin by interacting with incoming radiowaves (known as nuclear magnetic resonance), detected by sensitive radio detectors. The resonance frequency depends upon the intramolecular magnetic field around an atom which can give details of the electronic structure of a molecule and its individual functional groups.

The principle of NMR involves three steps :

1. The alignment of the magnetic nuclear spin on application of an applied constant magnetic field B^,0.
2. The change in the alignment of the nuclear spin on interaction with an oscillating magnetic field from the incoming radiation, referred to as radio - frequency (RF) pulse.
3. Detecting the electromagnetic wave emitted by the sample due to the produced perturbation.

NMR spectroscopy also investigates the structure, reaction dynamics and state of the molecule. Most common types of NMR instruments are proton (H¹) and carbon-13 (C¹³). NMR spectroscopy possesses the speciality of being well-resolved, analytically tractable and is highly predictable for small molecules. Different functional groups with different chemical environments give well resolved signals. NMR has successfully replaced the traditional wet chemistry methods like color reagents or chromatography.