The first (second, ...) derivative @[email protected] of a molecule is defined as the first (second, ...) derivative, \(\frac{\text{d}A\left(\overset{˜}{\nu }\right)}{\text{d}\overset{˜}{\nu }}\)[\(\frac{\text{d}^{2}A\left(\overset{˜}{\nu }\right)}{\text{d}\overset{˜}{\nu }^{2}}\), ... ] of the @[email protected] \(A\) as a function of wavenumber, \(\overset{˜}{\nu }\). Wavelengths may be used in place of wavenumbers but the shape of the derivative spectra will be slightly different. When derivative spectra are obtained at low temperature, they are called first (second, ...) derivative @[email protected] (specifying the solvent, temperature and solute concentration).