Chemistry Reference
In-Depth Information
2.4 The Unity of Chemistry and Physics: Belief Revision
in the Theory of Absolute Reaction Rates
In this section, I will focus on an example from the chemistry: the theory of absolute
chemical reaction rates.
9
This theory is a typical
theory in the sense that
it draws on many underlying theories to synthesise a new theory. Another motiva-
tion is that it can be argued that many of the current problems that plague current
philosophers of chemistry - such as the problem of molecular structure
10
and the
problem of inconsistent use of quantum theoretical notions - were introduced to
accommodate this theory.
11
A brief overview and philosophical evaluation of the
theory was given in Hettema (
2012b
), which I refer to for some of the details.
chemical
'
'
9
As the topic by Nye (
2011
) illustrates, this theory was sometimes jokingly referred to as the
'
absolute
'
theory of reaction rates. Many of his contemporaries found Eyring
'
s ideas too radical, as
the proceedings of the 1937 workshop at the University of Manchester illustrate. In my earlier
paper (Hettema (
2012b
)) I had this wrong, and used the designation of
'
absolute theory
'
through-
out. At the time I was unaware of the earlier ironic use, and thought that
absolute theory of
'
reaction rates
was a neater choice to designate the theory than the somewhat more clumsy
'
sounding
. Of course, I now see the error of my ways.
10
The notion of a (reactive) potential energy surface for the nuclear motion is key to the
development of the theory. While the idea was introduced by Born and Heisenberg (
1924
) and
Born and Oppenheimer (
1927
) it may be argued that the idea of a potential energy surface only
reached its full fruition with the development of a theory of chemical reaction rates. The idea of a
potential energy surface is part of Wigner
theory of absolute reaction rates
'
'
(see below).
11
Especially illustrative for this is the motivation Eyring (
1938
) gave for his introduction of
various
s
three threes
'
'
'
methods in quantum chemistry, which lead to various inconsistencies
between these semi-empirical theories and quantum theory.
For the purposes of calculating the potential energy surface for a chemical reaction, Eyring first
classifies theories as
semi-empirical
'
'
when they have the following characteristics:
(a) that each electron can be assigned a separate eigenfunction which involves the coordi-
nates of only this one electron. (b) Multiple exchange integrals are negligible,
(c) Normalising integrals for overlapping orbitals are negligible in comparison with
unity. (d) The exchange and coulombic integrals for a complicated molecular system
may be estimated from a potential curve for the isolated pair of atoms. (e) For distances
involved in activation energy calculations this percentage is around 20 per cent. coulombic
and 80 per cent. exchange binding, and this varies but little from atom pair to atom pair.
(Eyring
1938
,p.8)
Eyring then remarks that more detailed calculations, as well as principled considerations, give
no support for the construction of these theories:
None of these assumptions have been rigorously derived from theory, and, as has been
emphasised by Coolidge and James, if one assumes for H
3
, the approximate eigenfunctions
used by Heitler and London and Sugiura for H
2
, the assumptions can all be shown to fail
badly. (Eyring
1938
,p.8)
Thus stated, these sort of theories seem to be counterexamples to a theory of reduction: the sort
of reduction that derives chemical
semi-empirical
'
'
directly from basic quantum theory can only be achieved
on the basis of theoretical assumptions that are unjustified from the viewpoint of basic theory and
which can moreover be shown up as factually wrong in a large number of practical cases.
laws
'
'
