formation or breaking of a hydrogen bond to some lone pair atom, with changes in
the relative energies of different conformations.
Chapter 3, Harder Questions, Answers
Q7
Why might the development of an accurate and versatile forcefield for inorganic
molecules be more of a challenge than for organic molecules?
For the purposes of this question we can consider “unproblematic organics” to
exclude molecules containing elements beyond calcium, element 20: our unevent-
ful organics can thus contain H, Li–F, Na–Cl, K and Ca. We’ll also give a pass to Br
and I. Problem elements are Sc, Ti, ..., As, Se, Rb, Sr ..., Sb, Te, Cs, Ba, ..., Bi, Po,
At, and the subsequent radioactive elements.
The problematic atoms are thus the heavier nonmetals, and the metals scan-
dium and beyond, most of which are transitionmetals(ortherelatedlanthanides):
pblock,dblockandfblockelements.Inthe context of electronic theories these
are, traditionally at least, considered toemploy d orbitals in their hypervalent
bonding [1]. Now, in molecular mechanics orbitals simply do not exist so the
difficulties must be for formulated without reference to them (parameterizing a
quantum mechanicalsemiempirical method like AM1 or PM3 to account for
dorbitaleffectsalsopresentsspecialproblems[2]).Insimplestterms,the
problems with these atoms lies in the unconventional (compared to the usual
organics) geometries encountered. Normalorganicshaveatetrahedralorsimpler
disposition of bonds around each atom, but problem elements (first paragraph
above) can have pentagonal bipyramidal, octahedral, and other geometries. There
are more bonds and more interbond anglestoaddress;someorganometallic
bonding is not even usually depicted in terms of bonds between individual
atoms, e.g. bonding to cyclopentadienyl,p-allyl and alkene ligands. A brief
discussion of MM applied to organometallic and inorganic compounds is given
by Rappe ́and Casewit [3].
References
- Lewars E (2008) Modeling marvels. Computational anticipation of novel molecules. Springer,
The Netherlands; chapters 4 and 5 - Thiel W, Voityuk AA (1996) J Phys Chem, 100, 616, and references therein
- Rappe ́AK, Casewit CJ (1997) Molecular mechanics across chemistry. University Science
Books, Sausalto, CA
606 Answers