Absorption Spectroscopy GPS 7

Absorption Spectroscopy Spectroscopic technique that measures the absorbance of radiation by a sample at varying wavelengths.

Absorption Spectroscopy: Example in Visible Range

Radiation Smaller wavelength(λ), the more energy (E) the radiation has (E = hc/λ)

Radiation Effects: Electronic Transitions Electronic transition within atoms of the molecules may occur if ultraviolet (UV) radiation is applied.

Electronic Transitions Electronic transitions require a relatively large amount of energy from UV wavelengths. Electronic transitions describe the movement of electrons to other orbitals (s, p, d, f, etc.).

Radiation Effects: Molecular Rotation Molecular rotation requires weaker photons in the infrared (IR) range.

Molecular Rotation A molecule in the gas phase may rotate around its center of mass. Rotational energy describes the energy required to transition between different rotational states, which is often used to investigate and confirm molecular structure.

Molecular Rotation: Application Infrared spectroscopy may be used to determine the major functional groups (like and –OH group or a C=O bond) within the molecules of a sample.

Molecular Rotation: Application Infrared spectroscopy may also be useful in identifying a substance within a sample by comparing the IR spectrum of a known substance to the IR spectrum of an unknown substance. The example to the right shows the comparison of the known IR spectrum of SO2 to the sample in Io’s atmosphere (Io is one of Jupiter’s moons).

Radiation Effects: Molecular Vibrations Even less energy is required for molecular vibration, from the microwave region.

Molecular Vibrations Atoms in a molecule bond at ideal distances (equilibrium bond length) to minimize energy required to bond. When excited by a relatively low amount of energy (microwaves), atoms may vibrate within a molecule while still maintaining the bond length equilibrium.

Molecular Vibrations: Animated examples Symmetrical stretching Asymmetrical stretching Scissoring (bending) Rocking Wagging Twisting

Absorbance Spectroscopy: Practice Do electronic, vibrational, or rotational transitions within molecules require radiation with the shortest wavelengths? Justify your answer.

References http://web.gps.caltech.edu/classes/ese148a/lecture6.pdf https://en.wikipedia.org/wiki/Molecular_vibration