In order to determine the nature of an unknown compound correctly, other techniques such as NMR (C, HETCOR, DEPT), Mass spectrometry, the chemical reactivity (solubility tests, sometimes derivatives) and physical properties (melting point, boiling point, refractive index) are required as well.
Based on all these data, a structural suggestion for an unknown compound can be made.
A medium or strong peak in this area corresponds to aromatic ring.
A nitro group shows two very intense peaks in the range between 1300-1400 cm (out of plane modes, C-X: X=Cl, Br, I, heavier atoms) This range belongs to the fingerprint area, where assignments are a little bit uncertain.
With the help of quantum mechanical calculations (Schroedinger equation) you can find the frequencies of basic stretching and bending modes: Based on the equation, one would always expect a sharp lines at a well defined wavenumber.
Unfortunately, the change in vibration modes is always accompanied with change in rotational mode (Stokes and Anti-Stokes).The ones who get accepted are the cream of the industry.And one shall for sure benefit by ordering his papers from the best scribes.For non-linear molecules 3N-6 (2N-5 bending, N-1 stretching) vibrations are observed (e.g. If there is no symmetry in the molecule most of them will be observed the IR spectrum; the remaining modes will be observed in a Raman spectrum.The more complicated the molecule is (the more atoms it possesses and the lower the symmetry), the more peaks can be observed in the IR spectrum (see example 3 and 4).Although IR spectroscopy can reveal a lot of information about a molecule, but it is not enough to identify an unknown compound for sure.It is also important to keep in mind that an attempt to get too many details out of the fingerprint area can be very misleading.However, if there is a carbonyl group present, it might be difficult to locate a weak or medium sized peak right next to it. If there is a coupling between a C=C-group and other double bonded systems e.g.C=O or aromatic systems, the intensity will increase due to the increase in dipole momentum in the double bond.The required energy for this process is much smaller (1-5 cm) and causes together with some other effects the broading of the 'lines'.The number of basic stretching and bending modes expected for a molecule increases with the number of atoms in the molecule.