Spectroscopy
Structure Elucidation
Organic molecules absorb Infrared light and generate spectra that are useful for structure determination. Organic functional groups absorb at different wavelengths to give reliable information on which functional groups are present in the molecule. Spectra are reported with an X-axis of wavenumber (related to frequency) and a Y-axis of either absorbance or transmittance of light. Combined with other techniques, IR spectroscopy is invaluable for structure analysis.
Click on the image to open an IR problem set (PDF).
When organic molecules are bombarded with high energy electrons in a magnetic field, the molecules break into smaller fragments, which then travel through the field and separate based on charge and size. When these fragments are detected a spectrum results in which the X-axis (m/z) is based on the mass of the fragments and the Y-axis relates to the number of each fragment. Since “z” usually equals one, the X-axis is therefore simply the mass of each fragment.
Click on the image to open a Mass Spec problem set (PDF).
Magnetically active nuclei (1H, 13C, 31P, etc.) respond to radiofrequency input when in the presence of an external magnetic field. When a sample contains these atoms in different magnetic environments it takes different frequencies of energy to promote different nuclei; we measure those energies and a spectrum results which is usually standardized using an x-axis of parts per million. Signal position, size, and shape all provide useful information on molecule structure.
Click on the image to open an H NMR tutorial (PDF).
Click here to open H NMR unknown problems (PDF).
Carbon 13 is the magnetically active form of carbon and although it is only present as less than 1% of the isotopic distribution it is measurable in NMR spectroscopy. The scale changes, with the X-axis representing signals in the 0-200 ppm region, however the same ideas of chemical shift and the number of signals still apply. Carbon NMR are often reported as the proton-decoupled variant to avoid complications from multiplet signal overlap.
Click on the image to open a C NMR tutorial (PDF).
Click here to open C NMR unknown problems (PDF).
These problems are more complicated but bring together IR, MS, and NMR spectra.