Ch 13

The study of the interaction of energy with matter, which can be used to elucidate the structure of a molecule

A type of optical spectroscopy that measures the absorption of light in the visible and ultraviolet regions of the spectrum; UV-Vis spectra primarily provide structural information about the conjugation of multiple bonds in the compound being analyzed.

Various forms of electromagnetic waves that propagate at the speed of light; includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

The full range of electromagnetic frequencies from radio waves to gamma rays.

A plot of the wavelength of a region of the spectrum versus the absorbance at each wavelength.

 The wavelength at which the absorption is at a maximum. Also called absorbance peak.

 In a spectrometer, the amount of light transmitted through a sample divided by the amount of light transmitted through air; percent transmittance converts the fraction into a percent: %T = ( I / I0 ) ∙100, where I is the intensity of radiation transmitted through the sample at a particular wavelength, and I0 is the intensity of radiation incident on the sample at the same wavelength. The lower is %T, the greater the amount of incident radiation being absorbed by the sample.

A measure of the amount of light absorbed by a substance; in UV-Visible spectroscopy, the value of log(I0/I), where I0 is the intensity of the incident radiation, and I is the intensity of the beam after it has passed through the sample. Also called the optical density.

A physical law stating that the quantity of light absorbed by a substance dissolved in a nonabsorbing solvent is directly proportional to the concentration of the substance and the path length of the light through the solution, according to the equation: A = ε ∙ c ∙ l, where A is the absorbance at a particular wavelength, ε is the extinction coefficient, c is the concentration, and l is the path length in cm of the light beam through the sample cell.

The proportionality constant that relates the concentration of a compound to its absorbance of light, as defined by the Beer-Lambert Law: ε = A / ( c ∙ l ), where A is the absorbance at a particular wavelength, c is the concentration, and l is the path length in cm of the light beam through the sample cell. Also called molar absorptivity when units are M-1 cm-1 .

The distance between consecutive peaks (or troughs) of a wave; often expressed in μm; the product of the wavelength and the frequency is the speed of light: c = λ ∙ υ.

 The number of complete wave cycles that pass a fixed point in a second; or the number of reversals of the electromagnetic field per second; usually expressed in Hertz (Hz), which is cycles per second (cps or s-1 ).

A measure of the frequency of a wave in cycles per second (cps or s-1 ).

Excitation of a π electron in a HOMO bonding π orbital to a LUMO antibonding π* orbital by ultraviolet or visible light

Excitation of an electron in a lone-pair nonbonding n orbital to a LUMO antibonding π* orbital by ultraviolet or visible light.