Infrared Spectroscopy (IR) Vibrational spectroscopy is an energy sensitive method and is used to characterize compounds in terms of the strengths and number of the bonds present. One can get/detect: the presence of known compounds (finger print) the components of an unknown compound (functional groups) and thus a likely structure of a compound changes in the concentration of a species during reaction the properties of bonds (bond strength, force constants) Since a bond in a molecule behaves like a spring, the harmonic/ anharmonic oscillator model is used to describe the 3N-6 or 3N-5 different ways a nonlinear or linear molecule, respectively, consisting of N atoms can vibrate. These vibrational modes (normal modes) give rise to absorption bands of characteristic energies/frequencies/wave numbers, intensities, and widths, which are detected and analyzed. Vibrational energy levels in harmonic approximation 6. Please note that vibrations normally are more or less anharmonic Normal modes of vibration 3N – 6 modes (3N – 5 if linear) ~ ~ The three normal modes of H2O and their wavenumbers f /m The four normal modes of CO2 Typical wavenumbers of stretching/bending vibrations “molecule“ stretching C-H 2800 - 3000 N-N 3300 - 3500 H2O 3600 - 3000 C=O 1700 C=C 1600 SO32- 970 (s) 930 (as) bending 1600 620 () 470 () IR - Spectrometer grating, double beam Sample Fourier Transform (FT) Sample Examples BaSO3 Cs2CrCl5·4H2O (PH) 758 Sr(HPO2OH)2 686 1226 RT 2439 2433 1168 779 694 Transmission KMn(SeO2OH)3 3214 3080 2881 (OH) 1235 TT 3191 3032 2899 Sr(SeO2OH)2 RT 2440 2434 1170 (OH) 2300 2400 1225 1155 3176 3019 2303 2432 TT 3500 2963 B(OH) 3000 2500 2783 3175 685 1241 1164 1300 1150 1000 850 700 /cm -1 400 Abb. 7: FT-Infrarotspektren von Sr(HPO OH) und Sr(SeO OH) bei