c19 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come
19
CLASSICAL MOLECULAR MODELING
Although the chemical bond is a result of quantum mechanical interactions of elec-
trons with nuclei and with each other, the atom itself is large enough to be treated
classically. One way of understanding and predicting chemical properties is by treat-
ing the molecule as an aggregation of atoms bound by classical bonds and interacting
with each other in a classical way. Such a study is calledmolecular mechanics, MM.
19.1 ENTHALPY: ADDITIVE METHODS
It has long been known that extension of an alkane chain by one CH 2 group
CH 3 (CH 2 )nCH 3 →CH 3 (CH 2 )n+ 1 CH 3
brings about adecreasein thefH^298 of about 5 kcal mol−^1 =21 kJ mol−^1
(Fig. 19.1). For example, if we subtract 21 kJ mol−^1 fromfH^298 (ethane)=
−84 kJ mol−^1 , we get (correctly)fH^298 (propane)=−105 kJ mol−^1. Thus we can
call –21 kJ mol−^1 aCH 2 group enthalpyfor the extension of ethane to propane. The
group enthalpy istransferableto give the enthalpy change for extension of other
alkanes to an alkane larger by one CH 2 group.
If this works for CH 2 , why not try it with other alkyl groups? The enthalpy of
the CH 3 group must be one-half the enthalpy of formation of the ethane molecule
CH 3 CH 3 ,fH^298 (ethane)=–84 kJ mol−^1. Hence we take the CH 3 group enthalpy
Concise Physical Chemistry,by Donald W. Rogers
Copyright©C2011 John Wiley & Sons, Inc.
305