Chemistry Reference
In-Depth Information
chemists with models of considerable predictive power.
8
Although there are
exceptions to the Woodward-Hoffmann rules, orbital symmetry models have con-
siderable explanatory power relative to more detailed and accurate lower-level
veridical models of computational quantum chemistry. In fact, the orbital symmetry
control of organic reactions has received demonstrable theoretical support from
lower-level ab initio computations (Houk et al.
1995
).
Idealization plays a crucial in the construction of explanatory models because
it is an ineliminable part of the procedure used to classify the symmetries of
molecular orbitals. The use of the LCAO method demonstrates the interconnected-
ness of idealization and difference-making. It is by adopting this method that one
can extract the crucial explanatorily relevant details in spite of the serious
limitations of that method. This might be an illegitimate move if one were
attempting a
causal
explanation along the lines of Strevens
approach. But unlike
'
Strevens
account, the procedure of abstraction does not begin with physical causal
influence. At the very least, one can think of it as a methodological hedge: in order
to distil those explanatorily relevant factors from the kinds of systems of interest to
chemist, one is going to have to get a bit dirty. The ontological cost of the
idealizations and approximations can be offset since it is a step
towards
a lower
level explanation in terms of the difference-making
state
symmetries of the target
system even if one is thereby undermining the causal story itself. The process
simply goes beyond the idea of a kairetic procedure applied to causal systems
because one is attempting to distil from an idealized putative causal representation
of a chemical reaction another model containing those properties of states that make
a non-causal difference to the explanatory target. Building up molecular orbitals
and then populating them with electrons, which if taken at face value is highly
approximate and idealized since the use of the LCAO method is underwritten by the
assumption of electron configurations, we ultimately leave the causal story behind.
In this case, idealization is a crucial feature of the procedure for determining
explanatory relevance. Idealization therefore has a more intrinsically positive
function in the sense that it is not just limited to a communicative sense of
explanation constrained to non-difference-makers. The idea of non-casual differ-
ence-making can be more flexible and inclusive. One can admit of an intrinsically
constructive explanatory role for idealizations in the sense that they are an
ineliminable feature of the procedure for determining those states of a system that
make a difference to their explanatory targets.
'
8
For example, a thermally allowed cycloaddition is the 4 + 2 cycloaddition of ethene and
butadiene, i.e. the Diels-Alder reaction. The corresponding selection rule is: m + n
4q + 2,
ΒΌ
where m and n are numbers of pi-electrons, q is an integer 0, 1, 2
(Hoffmann and Woodward
...
1968
, p. 827).
