Renaissance of Concern with the Nascent Condition
S. H. Bauer Cornell Unlversity Ithaca. New Vork 14850
In 1927. when Mr. Davies. our hieh school teacher. introduced us to the exciting wohd of aioms and molecules, the term "nascent." occasionallv used to describe special . prop. erties of freshly prepared gases, was inserted in quotation marks. Mr. Davies was m i t e clear that there were observations which could not he readily explained, hut to ascribe stranee chemical behavior to the condition of being.newly born was not scientifically acceptable. It is worth mentioning, however, that as late as 1900, Russell' performed an extensive set of experiments in which he searched for the particular effectiveness of nascent gases for accelerating the rates of reaction between well dried gases, such as carbon monoxide and oxygen. Since thendecades passed, and so did Mr. Davies, and among the hundreds of thousands of words in the chemical literature that I have read, "n.sccentMdid nut appear. Indeed. \re remained true to the icientil~cimiwratio~ng.1 to sllhstitute names for explanations. It is now time to restore the term "nascent" to legitimacy. Much evidence has accumulated, both experimental and theoretical, which demonstrates that in many reactions the partition of nascent products among their characteristic rotational, vibrational, and electronic states differs substantially from th,at expected were these generated in statistical equilibrium. Nascent contrasts with state relaxed, i.e. a svstern for which a s i n-d e temoerature and correspunding thermodynamic functions can be defined. T o fullv . annreciate the current trend in chemical kinetics one must overcome a brainwashing to which most of us have been subjected during the past half century-that for all practical purposes all the systems we deal with are in statistical equilibrium even during chemical reaction, all the way from reactants, through the transition state, to the ~ r o d u c t s .When it is exoerimentally possible to demonstrate that the product-of a react& is so distributed among its excited states, such that no meaningful temperature can he assigned, and thus no thermodynamic fnn&ons evaluated, we are then forced to develop a more detailed structural-statistical model, one which provides an enhanced understanding of the hasic driving.forces for that chemical reaction. It is important to keep in mind that the dichotomy state relaxed versus nascent pertains to the manner in which product molecules are partitioned among the same set of molecular states. Hence it is a particularly significant distinction to the statistically sensitive chemist, and apparently of less concern to the structures oriented chemist. But all chemists subscribe to the axiom that the reacting molecules comprise that small fraction of the total which are in some manner excited. Thus, the strict accounting of their distribution amone" the hiahlv - " excited. mildlv energized, and close-to-ground states plays an important role in determining the observed kinetic behavior. This is inescapable if one wishes to understand the operation of chemical lasers. Atom transfer processes in highly exoergic 3-center reactions produce vihrationallv excited diatoms (v) ~
..
o+
ICSe CS
CO'~+ '
+
+
-
-
HF"' + F2 HF + 2F HF"' + H z HF + H&"' Hz("' + F2 Hz + 2F Only the most ardent proponent of absolute reaction-rate ASol for the nvertheory would attempt to specify AH,+, lapping steps in the hydrogenlfluorine reaction. Obviously, nascent H F has significantly different chemical properties from state-relaxed HF. The development of techniques for probing population distributions of nascent products presents a challenge to the experimentalists, while the results thus obtained place demands on the ingenuity of theorists. For both, the pursuit of the nascent condition is currently a highly respected enterprise. One extensive program, based on measurements of relative emission intensities under "arrested-relaxation" conditions [for HF'"' and HCI(")] was initiated by Polanyi3 and extended by Setser.' In another approach, developed by M. C. Lin,"he relative absorption intensities for selected state transitions (utilizing a highly stabilized CO laser for the source) permit one to deduce the nascent vihrational (and rotational) populations of CO(") generated in several interesting reactions. 0(3P) + C2H2 2CO("]+ 2H O('D) + C302 3COlV)('Zt)
-
In the meantime. Levine. Hofacker and Bernsteinqiscovered an information theory rationale for systematizing the non-Boltzmann distributions measured bv these and other investigators. The historical imperative directs us to recall Professor Randall's polemics, in the early 19302s,against the extension of the statistical-thermodynamic notation to chemical kinetics. In view of our present appreciation of the limitations of absolute-reaction-rate theory (which does not allow nascent products, in the sense defined above), his objections were not unjustified. Indeed, while the equation &(rate)= x(kTlh)exp(-AGatlkT) provides an excellent basis for establishing correlations hetween molecular properties and rate constants for reactions
' Russell, J. C h e m Soe., 77 361 (1900). Phys., 62,1714 (1975). 2 S ~ l l i ~ aJn Chem. . Volanyi, Disc. Faroday Soe., 44 (1967). Setaer, J Chem. Phys., 58, 2298 (1973). Lin, M. C., ACS Meeting, Philadelphia, 1975. filevine, Hofacker, and Bernstein, J Chem. Phys., (1974).
'
etc.
For these reactions thermochemical values, snch as enthal372 / Journal of Chemical Education
py and entropy increments, if interpreted in conventional terms, are meaningless unless one appends to these AH+', ASTO'S the statement, "after waiting until the system attained statistical equilibrium." Similarly,magnitudes for "overall" himolecular rate constants must be qualified; they relate to what one would observe in an efficiently cooled system, when care was taken to measure net cbemical composition changes. Even so snch statements are of questionable significance, since each of the above reactions is merely one step in an extended chain, the complete unscrambling of which is not unambiguous. This is particularly true for the HdF2 system. Evidence is accumulating2 Hz that the initially produced hydrogen fluoride IF HFi" HI leads to chain branching steps
that depart little from state-relaxed conditions, its universal acceptance would have prevented the serious search for
reactions which either generate overpopulations of excited products or require selected excitation of the reactants.
Volume 53, Number 6.June 1976 / 373
