JACS AT 125
more complex shapes," says Paul R. Rablen, associate chemistry professor at Swarthmore College, in Pennsylvania. "However, authors who develop or use continuum solvation models still frequently cite Onsager, since he did lay out the original theoretical framework." ALE UNIVERSITY PROFESSOR The paper eventually opened new avecules. Onsager showed that with this corLars Onsagerfirstsent his paper rection the Debye theory properly pre- enues of research. For example, Onon electric moments of mole- dicts dielectric constants of pure polar sager's equation was adapted in the 1950s cules in liquids to Physikalische liquids from their dipole moments. to solvatochromism, the study of solvent Zeitschrifi, a journal edited by "Onsager single-handedly reversed the effects on electronic spectra of molecules Petrus J. W. (Peter) Debye, but Debye pessimism regarding the ability to under- in solution. turned it down. John G. Kirkwood, then a stand polar liquid behavior," says Patrik R. Onsager's model is also relevant for studyresearch fellow with Debye in Leipzig, con- Callis, a chemistry professor at Montana ing the time dependence ofthe polarization vinced Onsager to write an English version State University process. "Imagine a nonpolar molecule and submit it to theJournaloftheAmerican "The theory had far-reaching influence which becomes polar upon absorption of a Chemical Society, which published it \J.Am. on thefieldof solvent effects on chemical photon of light," Kauffman says. "In the Chem.Soc, 58,1486 (1936)}. ground state, there is no reactionfield(no The paper, which is the sixth most highoriented molecules surrounding the solute), ly citedJ ACS paper, corrected Debye's forbut upon excitation a polar molecule is cremula relating the dielectric constant of a ated and now we expect polarization. The polar substance to the molecular dipole time-dependent process of polarization is moment. Debye's formula includes the poknown as solvent relaxation, and it is a very larizability of the molecule, its permanent activefieldof research currently" electric moment, the dielectric constant, "Like Onsager's more important conand the energy of thermal agitation. Ontribution—the Onsager reciprocal relasager served as a research assistant for Detions, for which he was awarded the Nobel bye from 1926 to 1928. Prize—I believe this one was ahead of its "Debye's original theory is still used totime as regards applications by the chemiday to determine dipole moments of molcal community as a whole," Callis says. "Its ecules dissolved in nonpolar solvents by importance rose for a while and is now demeasuring the dielectric constant of the clining in terms of being cited or direct use solution," says John F. Kauffman, a chemof the equations. However, applications of istry professor at the University of Misthe concept of reactionfieldare probably souri, Columbia. growing, especially as the use of fluorescent The problem was that Debye's formudyes in biology increases exponentially" la didn't work for polar liquids. "At the "Onsager's paper had far-reaching contime," Kauffman says, "a common explasequences for essentially all of solutionnation was that 'association effects' due to NEW CONCEPT Onsager showed phase chemistry with respect to the fundipole-dipole interactions could not be how Debye's theory of dipoles could damental physics that underlies chemical properly accounted for in the Debye the- be extended to polar solvents. reactions," Kauffman says. "The reaction ory. Onsager sought to correct this misfield is a fundamental aspect of any soluconception by performing an analysis of reactions," Kauffman says. 'Any time a po- tion. Onsager was thefirstto recognize its the interaction of a dipolar solute mole- lar solute exists in a polar solvent, which of significance, and now everyone concerned cule with the electricfieldthat is produced course occurs often, the energy of the with solvent effects must include the reas the result of the polarization of the sursolute is affected by the reaction field." actionfieldin their picture of chemical rerounding solvent by the solute molecule." Theoretical chemist Jacopo Tomasi of action dynamics in solution. I would have Debye's theory predicted a "ferroelec- the University of Pisa, in Italy, wrote about to conclude that this will never diminish in tric Curie point," a temperature below Onsager's article in a special "New Centu- importance because it is so fundamental." which the molecule would be in a stable ry Issue" of Theoretical Chemistry Accounts Onsager received the 1968 Nobel Prize state of permanent electric polarization, [103,196 (2000)]. Tomasi indicated that in Chemistry for other work on the thermuch like the permanent magnetic polar- several characteristics have contributed to modynamics of irreversible processes, ization of iron. In his paper, Onsager point- the longevity of Onsager's paper, includ- which was published in two papers in 1931. ed out that such electric polarization states ing the mathematical simplicity and phys- He died in 1976 at the age of 72.—CELIA ical robustness of the model. "It may be HENRY are actually quite rare. Onsager introduced the concept of a modified with little effort and adapted to "reactionfield"to deal with the problem. many different problems," Tomasi wrote. CirEN is celebrating the 125th volume of the "The original theory by Onsager only Journal of the American Chemical Society by This reaction field is the electricfieldexperienced by a polar molecule when it's applies to a spherical molecule and cavity, featuring selectedpapersfromthe list ofits 125 surrounded by oriented polar solvent mol- and many modern implementations allow most cited. Onsagers paper ranks sixth on the list.
AN ELECTRIC MOMENT
Correction of Debye theory to predict dielectric constants of polar liquids had far-reaching effect
Y
70
C&EN / NOVEMBER 17, 2003
HTTP ://WWW.C EN-ONLINE. ORG
