Molar Absorption Coefficients
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.
"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.
(However, the IUPAC Gold Book definition states: "The term molar absorptivity for molar absorption coefficient should be avoided.")
"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.
Where to Look for Them - Online
- Reaxys (Beilstein and Gmelin)
- The UV.EAC field in Beilstein contains numerically searchable extinction/absorption coefficient data for organic compounds. In Reaxys this will be found under the Spectra/Ultraviolet heading. The pre-1976 Gmelin file contains some UV data and references, but not ε.
- 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 then, for older papers, "extinction coefficient". The original document usually has to be consulted, and much of the relevant literature is not recent. This search method is not precise and depends on the absorption coefficient being mentioned in the abstract, title, or indexing terms. If the measurement is not a key point of the paper, it usually is not mentioned. A fallback search would be to search for documents with ultraviolet data on the compound and consult them directly.
- Combined Chemical Dictionary
- Database containing chemical information on over 500,000 substances and their derivatives. The UV data fields for some compounds display ε values in context with λ max, but ε is not numerically searchable.
- Properties of Organic Compounds
- POC entries often contain ε values in the UV data section. Covers about 25,000 important organics.
- 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. The WebBook includes only several hundred UV spectra from a Russian source.
Where to Look for Them - Print Sources
Numeric values of ε can be found scattered through spectral reference sources, although they are not always clearly labeled. They generally appear in context with wavelength values.
All titles listed are shelved in the library's Spectra section unless otherwise indicated. Because so many people outside UT-Austin use this page, links to WorldCat.org are provided so that other owning libraries can be quickly found. Just click the button.
- 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.
- Correlation tables for the structural determination of organic compounds by ultraviolet light absorptiometry.
- QD 272 S6 P447 1974 Spectra (Verlag Chemie, 1974)
- log ε is given in parentheses after the wave number. Compounds are ordered by R (ring or unsaturation) number, then by number and kind of heteroatoms in alphabetical order. No indexes.
- Data for biochemical research.
- QP 520 D37 1986 Reference (3rd ed., 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.
- Handbook of biochemistry and molecular biology.
- QP 514.2 H34 1975 Reference (3rd ed., CRC, 1975-77)
- 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." This table was reprinted in the Practical handbook of biochemistry and molecular biology (CRC, 1989), pp.196-358.
- Handbook of biochemistry and molecular biology.
- QH 345 H347 2010 Reference (4th ed., CRC, 2010)
- Table on pp.269-358 contains UV and pKa data and references for 246 purines, pyrimidines, nucleosides and nucleotides.
- 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 as "Properties of Organic Compounds" (see above).
- Handbook of natural products data.
- QD 415.7 R34 1990 Spectra (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. v.1 - Diterpenoid and steroidal alkaloids; v.2 - Pentacyclic triterpenoids; v.3- Isoquinoline alkaloids.
- 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 annual series, so you have to search year by year, making it a last resort.
- 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 usually provided in the spectrum's data table, denoted as am. Use the main index set to locate the compound, as described on the Sadtler page. A one-volume abridgement with 1600 important compounds, The Sadtler handbook of ultraviolet spectra, was published in 1979.
- Specifications and criteria for biochemical compounds.
- QP 520 N38 1972 Storage (3rd ed., NAS, 1972)
- ε is given at selected wavelengths for a number of carotenoids, coenzymes, nucleotides, and porphyrins. A 1977 supplement gives ε for 30 biogenic amines.
- 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."
- 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.