ε: Molar Absorption Coefficients (UV-VIS)

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These spectral properties can be found in various reference sources. First, some definitions:

Molar absorption coefficient (ε)
Synonyms: Molar extinction coefficient, Molar absorptivity
"The recommended term for the absorbance for a molar concentration of a substance with a path length of l cm determined at a specific wavelength. Its value is obtained from the equation ε = A / cl
Strictly speaking, in compliance with SI units the path length should be specified in meters but it is current general practice for centimeters to be used for this purpose.
Under defined conditions of solvent, pH and temperature the molar absorption coefficient for a particular compound is a constant at the specified wavelength."
-- Denney, R.C. Dictionary of Spectroscopy, 2nd ed.; Wiley: New York, 1982; pp 119-20.

Molar absorptivity
"Synonym: Molar (decadic) absorption coefficient.
Decadic absorbance divided by the path-length l and mole concentration c, of the absorbing material. ε = A₁₀/cl. The molar absorptivity is a Beer-Lambert absorption coefficient. SI unit: m² mol^-1."
-- Handbook of Vibrational Spectroscopy; Chalmers, J.M., Griffiths, P.R. Eds.; Wiley: New York, 2002; Vol.5, p 3772.

Extinction coefficient
"A term that has been widely used for the molar absorptivity, unfortunately often with values given in ill-defined units. Use of this term has been discouraged since the 1960s, when international agreement with non-chemical societies reserved the word "extinction" for diffusion of radiation, i.e. the sum of the effects of absorption, scattering, and luminescence."
-- Ibid.; Vol.5, p 3760.

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Where to Look for Them - Print

Values of ε and log ε can be found scattered through spectral and general reference sources, although they are not always clearly indicated. They generally appear in context with wavelength numbers (λ max). Organic compounds dominate in handbooks and the literature. Values for inorganics and other types of substances are much more difficult to find. Here are some suggested places to look.

Atlas of spectral data and physical constants for organic compounds.

QD 272 S6 G73 1975 Spectra (6 vols., 2nd ed., CRC, 1975)

Molar absorption coefficients appear in parentheses next to the wavelengths in the Ultraviolet column.

CRC handbook of biochemistry and molecular biology.

QP 514.2 H34 1975 Reference (CRC, 1975-)

Vol. A2, pp.383-545, table titled "Molar absorptivity and A(1%1cm) values for proteins at selected wavelengths of the ultraviolet and visible region."

Data for biochemical research.

QP 520 D37 1986 Reference (Oxford, 1986)

ε or log ε is occasionally given in parentheses next to the λ max in the General Remarks column in the chapters on amino acids, amines, amides, peptides; carboxylic acids, alcohols, aldehydes, ketones; phosphate esters; vitamins and coenzymes, etc.

Gmelin handbook of inorganic and organometallic chemistry.

Reference Stacks (8th ed., Springer, 1924-98)

Consult the formula indexes for a compound's location in the series. ε if given will be found in the Optical Absorption (Lichtabsorption) section of a compound's entry. Gmelin information

Handbook of data on organic compounds. (HODOC)

Handbook Table (7 vols., 3rd. ed., CRC, 1994)

Molar absorption coefficients when available are given in parentheses next to major bands in the UV section of the entry. Also available online (see below).

Handbook of fluorescence spectra of aromatic molecules.

QC 463 H9 B4 1971 Spectra (I.B. Berlman, ed.; 2nd ed., Academic Press, 1971)

Graphed molar absorption coefficients of 200 compounds. Name and formula indexes.

Handbook of natural products data.

QD 415.7 R34 1990 (Elsevier, 1990-94)

3-volume set with UV (λ max and log ε), IR, 1H-NMR and 13C-NMR spectral data for plant-derived natural products.

Handbook of ultraviolet and visible absorption spectra of organic compounds.

QC 462.85 H5 1967 Spectra (Plenum, 1967)

log ε appears in the column next to the λ max.

Merck index.

Handbook Table

ε is given in parentheses next to the UV max in the properties paragraph of some entries.

Organic electronic spectral data.

QC 459 O64 Spectra (Wiley, 1946-89)

log ε is given in parentheses next to the λ max. There is no cumulative index to this series.

Practical handbook of spectroscopy.

QD 95 P73 1991 Spectra (CRC, 1991)

The UV table (pp.565-646) data are sorted into groups based on the intensity of the strongest band, then in decreasing order by wavelength of the strongest band. If you already have ε you can identify an unknown this way.

Sadtler ultraviolet spectra.

Sadtler Alcove

Molar absorptivities are provided in the spectrum's data table for many organic compounds, denoted as a_m. Use the main index set to locate the compound, as described on the Sadtler page.

Tables of spectrophotometric absorption data of compounds used for the colorimetric determination of elements.

QD 113 I55 Spectra (Butterworths, 1963)

Entries for anions and cations include extinction coefficient values "with respect to one gram-atom of the element to be determined per liter of solution, not to one mole of the colored complex."

Ultraviolet spectra of aromatic compounds.

QC 459 F7 Spectra (Wiley, 1951)

600 spectra plotted by log ε as a function of wavelength.

UV-VIS atlas of organic compounds.

QC 462.85 P47 1992 Spectra (2nd ed., VCH, 1992)

Exact values of ε max are indicated on the spectral traces.

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Where to Look for Them - Online

Beilstein Crossfire

The UV.EAC field contains numerically searchable extinction/absorption coefficient data for organic compounds.

SciFinder (Chemical Abstracts)

The best approach is to search for the compound by Registry Number, name, or structure; get All References, and then refine the results by topic, using the phrases "molar absorption coefficient" and/or "extinction coefficient" (for older articles). The original document usually has to be consulted, and much of the relevant literature is pre-1960. This method is not foolproof and depends on the absorption coefficient being mentioned in the abstract, title, or CA indexing.

Combined Chemical Dictionary

Database containing chemical information on over 500,000 substances and their derivatives. The UV data fields display ε values in context with λ max, but ε is not numerically searchable. UV data are not available for all compounds in the database.

Properties of Organic Compounds

Online equivalent of the HODOC (see above).

NIST Chemistry WebBook

UV-VIS spectra in this database graph log ε on the Y axis, and by placing the mouse pointer over the desired wavelength you can see log ε. However the WebBook includes only several hundred UV spectra from a Russian source.