Chapter Summary & Important Vocab

• Molecular Orbital Theory

• Electromagnetic Spectrum

• UV/Vis Spectroscopy

• IR Spectroscopy

• Microwave Spectroscopy

Hybrid Orbital Theory

• Atomic orbitals on the same atom combine in order to form hybrids.

• Atomic orbitals on different atoms overlap in order to form covalent bonds.

• Each atom in the compound retains its associated orbitals and electrons.

• This theory correlates with observed bond angles in molecules.

Molecular Orbital (MO) Theory

• Views a molecule as a whole instead of a collection of individual atoms.

• MO’s are similar to atomic orbitals.

  • They both have specific energy levels.

  • They both have specific sizes and shapes.

  • They can both hold a maximum of two electrons that spin in opposite directions.

• Atomic orbitals combine to for MO’s.

  • When two atomic orbitals combine, two MO’s are formed.

  • Orbitals are always ‘conserved’.

Molecular Orbitals

• Bonding Orbital

  • A MO that is lower in energy than any atomic orbital from which it was derived.

  • Electrons that occupy these orbitals cause stability.

• Anti-Bonding Orbital

  • A MO that is higher in energy than any atomic orbitals from which it was derived.

  • Electrons that occupy these orbitals cause instability.

• Non-Bonding Orbital:

  • A MO that is the same energy level as the one atomic orbital that it was derived from.

  • Electrons that occupy these orbitals do not cause stability or instability.

  • Orbitals that contain lone pairs.

The Electromagnetic Spectrum

Spectroscopy

• Spectroscopy:

  • A Method of analysis which is based upon the absorbance of electromagnetic radiation by matter.

  • Used to acquire data pertaining to the structure of a molecule or the concentration of a species.

  • Io = It + Ia

    • Io - Intensity of electromagnetic radiation striking sample

    • It - Intensity of electromagnetic radiation exiting sample

    • Ia - Intensity of electromagnetic radiation absorbed by the sample.

• Two methods used for Spectroscopic analysis:

  • 1. Ultraviolet/Visual (UV/Vis) Spectroscopy

    • Examines transitions in the electronic energy levels

      • Is used to probe the electronic structure of certain compounds.

    • Is used to determine concentrations of solutions that contain certain compounds.

  • 2. Infrared (IR) Spectroscopy

    • Examines transitions in molecular vibrations

      • Is used to detect the presence of different types of bonds and to identify molecules.

Ultraviolet/Visual (UV/Vis) Spectroscopy

• An absorption spectrometer is used to measure the absorbance of a sample at wavelengths between about 200 nm and 800 nm.

• The peaks represent wavelengths that correspond to the energy associated with possible electronic transitions within the molecule.

• Colorless species can only absorb UV light between about 200 nm and 400 nm in these experiments.

• Colored species will always absorb light from the visual spectrum, but could also absorb UV light in these experiments.

Beer-Lambert Law

A = Ebc

• A = Absorbance

• E = Molar Absorptivity

• b = Path Length

• C = Concentration (M)

  • E describes how intensely a sample of ions or molecules absorbs light at a specific wavelength.

  • In most experiments, the path length and wavelength remain constant, so E also remains constant and absorbance A is only proportional to concentration, c.

Infrared (IR) Spectroscopy

• All covalent bonds in molecules are vibrating.

• Bond length is the average distance between nuclei.

• Covalent bonds have a vibrational frequency that is the IR region of the electromagnetic spectrum.

• IR radiation of exactly the same frequency will be absorbed by the molecule.

• Vibrational frequencies depend on the mass of the atoms and the strength of the bonds.

• Frequency is related to wavelength -c = h x v

• IR Spectra can be used to identify bond types, functional groups, and compounds.

• Molecules that share the same functional group, such as alcohols (-OH) or carboxylic acids (-COOH) can be identified through IR spectroscopy, as they will have absorption peaks within the same range.

• Every compound has a characteristic IR spectrum that it can be identified through.

Microwave Spectroscopy

• Microwaves cause polar molecules to rotate.

• Each type of polar molecules has specific rotational frequencies that it can exhibit.

• The peaks in the microwave spectra blow correlate with the different rotational frequencies for a specific polar molecule.

• Data from microwave spectra can be used to calculate the bond length or diatomic polar molecules and to determine the shapes of polar molecules.