dispersive IR spectrometers) as manifestations of optical pumping and energy transfer (2). I felt it was necessarv t o warn others of the questionable pedagogic value of an experiment designed to &fy a theory when, in fact. the actual experiment shows that the theory is not adequate.
with HC1 a preliminary. If true, however, i t would have the unfortunate effect that such easy-to-obtain IR spectra as HC1 and CO would not be rigorously representative of a Boltzmann eauilibrium. However.... eiven the sim~licitv . . of the spectroscopy, more advanced undergraduate experiments could actually investigate such possibilities. Mark Suikes
Llterature Cited 1. Henberg,O.SpemoofDiafomie Mo1rcules:VanNmtrand Reinhold:NewYotk, pp
124-127.
1950:
Tulane University New Orleans. LA 70118
2. Yang, Sze, Univemity of Rhcde laland, -rial communinition.
Gerald S. Ondrey Gannon Unlverslty Erie, PA 16541
Llterature Cited 1. See refs 5 and 13 of the original p a w . Fore more r e n t example nee Philips. L. A,: Levy, D. H.J. Phys. Chrm.1986,90.4921. 2. See ref 9 of original paper.
To the Editor:
The Boltzmann eauation eives relative auantum state populations for a system that-is in thermal equilibrium. T o the extent that thermal equilibrium has not been attained, deviations from the Boltzmann equation populations are resent. Such deviations must alwavs be a potential concern k i t h rotational simulations of samples coiled in supersonic expansions. since these are not true equilibrium situations. number of supersonic expansion-cooled laserinduced fluorescence spectra have been simulated with good reproduction of rotational detail using single temperature fits (I).In recent years rotationally resolved FTIR spectraof exnansion-cooled samoles also have been successfullv fit to single T distributions 72). Ondrev brines - UD. the ~ossibilitvthat room temperature samples bf gaseous HC1 and CO may give IR spectra that show deviations from room-T Boltzmann distributions, due to interaction with the IR probe beam. Nonetheless, as noted.our students haveobtained Tfitsof HCI that arepeneral. ly kithin 10 K of room by using the equation
ow ever, a
An,-
=
BT
as described in the paper. This equation works well, because i t depends only on the strongest few P and R rotational bands; it will not show sensitivity to more subtle changes for higher J transitions. As a secondary method, students generated "stick" transition simulations for HC1 as a function of T and qualitatively compared them with experimental results, as a prelude to the more complicated simulation case of 12. Though these comparisons are quite qualitative, the agreement with room T has been generally good since the major element of pattern recognition mainly involves a small number of intense features. If made more quantitative (minimum least squares differences between simulated and experimental sticks) this result may also carry over to yield best sinele !Ps near room. The qualitative "pattern fit" methods we have our students use are not as sensitive to non-Boltzmann peak ratios as Ondrey's method of considering ln(population/degeneracv) plots derived from the experimental spectra. However, the sensitivity of a log functibn on its argument makes i t important that this method be used with care. For example, a t a Boltzmann T = 298 K, the ratio of the P(1) to P(3) transition is 0.55. If the ratio is measured as 0.48, that peak ratio when solved for T would yield T = 411 K. Reviewing some of our past HCl spectra, we observe peak ratio discrepancies. esoeciallv for medium to small features. that can be on the eider of5-10%. This is no way impugns Ondrey's comments but instead noints out the care (verv eood quality spectra, careful tabulakon of peak heights a i d a r e a s ) that must be used to pin down such deviations. The possibilit; Ondrcy brings up is intriguing and possibly important. However, we do not feel it seriously damages the pedagogy of our experiment, the reason being that most of the pedagogy is concerned with spectrum simulation of 12, ~~
~
~
prbvocative Oplnlon: Descrlptlve Chernlstry versus Theoretical? To the Editor: Huddle [1987,64,765] makes statements about the scientific method and presents the following paradigm of the scientific method: organizingpostulates(laws) observations (facts) t I predikions theories (to explain)
-
Both this paradigm and the following statements are incorrect: They try to organize those facts by proposing postulates or laws that summarize many observations of how nature behaves. Thus, there is atendency toteach bothlaws and theories as if they were facta. A law, if i t summarizes facts (observations), is a general fad. This kind of law is another fact. Unfortunately, some theories are also called laws. If we would atop calling theories laws, some of the confusion about the scientific method would vanish. Postulates (assumptions) of hypotheses and theories are explanations of the facts and laws and are not directly related logically to the facts and laws. If assumptions were logicallv derivable from the facts and laws, science would be far easier. Postulates cannot and do not summarize observations. The stating of postulates (explanatory statements) is the step of the scientific method that puts a premium on human imagination. Some people would call this the "creative" step in the scientific method. What distinguishes a good hypothesis (a hypothesis that becomes a theory) from a bad hypothesis? The assumptions of the good hypothesis must not only explain the known facts and laws in its area of science, but also i t must predict new facts that have not been observed. If the new f& are observed, then the hypothesis is called a theory. If the new facts do not exist as predicted, then the hypothesis must be reiected or revised. A new hypothesis with different predict.. eci new facts is required. The postulates of a theory are outright assumptions that cannot be tested and verified directly. The deduced consequences of the theory will be specified facts. The facts are as specified; hence, the theory is true ( I ) . If a fact, old or new in the area of a theory, cannot be explained bythe assumptions of a theory, then the assumptions must be revised or a new hypothesis must be stated. Thus, a theory is a temporary truth. Newtan's theorv could not explain how light travelled through space; thus, Newton's theiry has heenreplaced by Einstein's theorv. Evolution isa hypothesis that hasasset to meet the test i f predicting new facts. One of evolution's principal predictions is that there should be transitional korms-between species. Such transitional forms have not been found. Volume 66
Number 6
June 1989
533
