alkenes and aromatics (sp^2 C) [4]. In fact ”...the difference is often ignored and an
average set of VSIPs is used for all carbons.” [5].
References
- Van Vleck JH (1934) J Chem Phys 2:20
- Mulliken RS (1934) J Chem Phys2:782
- Revealed for example, the increasing acidity of methane, ethene and ethyne, see e.g. Smith MB,
March J (2001) March’s advanced organic chemistry. Wiley, New York, pp 331, 348 - Hoffmann R (1963) J Chem Phys 39:1397
- Levine IN (2000) Quantum chemistry, 5th edn. Prentice Hall, Upper Saddle River, NJ, p 653
Chapter 4, Harder Questions, Answers
Q10
Which should require more empirical parameters: a molecular mechanics force
field (Chapter 3) or an EHM program? Explain.
The EHM will require far fewer parameters. This is easy to see, because each
atom requires just one parameter for each valence atomic orbital. For C, for
example, we need an ionization energy for the 2s, and one for the 2p orbitals, just
two parameters (strictly, valence state ionization energies, VSIEs – see Harder
Question 9).^3 Each H needs only one parameter, for its 1s orbital. So for an EHM
program that will handle hydrocarbons in general we need only three parameters
(as in Hoffmann’s pioneering paper on hydrocarbons [1]). In contrast, an early
but viable molecular mechanics forcefield limited to alkanes had 26 parameters
[2]. The Universal Force Field, which sacrifices accuracy for wide applicability, has
about 800 parameters, and the accurate and quite broadly applicable Merck Molec-
ular Force Field 1994 (MMFF94) has about 9,000 parameters [3].
References
- Hoffmann R (1963) J Chem Phys 39:1397
- Allinger NL (1971) J Am Chem Soc 93:1637
- Levine IN (2000) Quantum chemistry, 5th edn. Prentice Hall, Upper Saddle River, NJ, pp
674–675
(^3) Even the usually-ignored refinement (Harder Question 9) of using different VSIEs for sp (^3) , sp (^2) ,
and sp carbon would raise the number of C parameters only from two to six.
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