21.10 More Advanced Treatments of Molecular Electronic Structure. Computational Chemistry 911
into the CAChe package of programs. As of 2004 the latest version is called MOPAC- Information is available at http://www.schrodinger.com.
Q-ChemThis is a program that requires high-level workstations such as DEC alpha
and IBM workstations. As of 2004, a version using a Macintosh G4 or G5 with OS X
is being developed. Information is available at http://www.q-chem.com.
SpartanThis is probably the most widely used program for undergraduate instruc-
tion and has appeared in versions for Macintosh and Windows computers. As of 2004,
the latest version is called Spartan’06 for Windows, and earlier versions were called
PCSpartan Pro and MacSpartan Pro. The new version contains a database with the
results of calculations on over 50,000 organic molecules. Semiempirical and ab initio
calculations are available, with several basis sets. Density functional methods can be
carried out. Molecular mechanics calculations are carried with the MMFF94 formu-
las. NMR chemical shifts can be calculated, as can vibrational and electronic spec-
tra. Vibrational normal modes are exhibited as movies. Information is available at
http://www.wavefun.com.
Any of these programs can be used without detailed knowledge of the calcu-
lations carried out by the program. A more complete list is available at
http://cmm.info.nih.gov/modeling/universal_software.html
PROBLEMS
Section 21.10: More Advanced Treatments of Molecular
Electronic Structure. Computational Chemistry
21.64a.Find the equilibrium conformation of ethylene
(ethene, C 2 H 4 ) using semiempirical calculations with
either the AM1 or PM3 parameters. Compare the bond
angles with the experimental bond angles and with the
experimental C–C–H bond angle of 122◦.
b.Can you find bonding C–C orbitals in your results that
resemble the sigma and pi bonding orbitals used in our
crude analysis in Example 21.4? Explain.
21.65Use a computer program such as CAChe or Spartan to
find the semiempirical approximations to the SCF orbitals
for benzene, using several methods, such as AM1, PM3,
and extended Hückel. Compare your orbitals with those
obtained using the Hückel method. Calculate the
wavelength of the lowest-energy transition (from the
highest occupied MO to the lowest unoccupied MO) for
each method. Compare the values with the experimental
value 180 nm and with each other.
21.66Use a computer program such as CAChe or Spartan that
carries out ab initio calculations for benzene, using
several different basis sets. Compare your orbitals with
those obtained using the Hückel method. Calculate the
wavelength of the lowest-energy transition (from the
highest occupied MO to the lowest unoccupied MO) for
each basis set. Compare the values with the experimental
value of 180 nm and with each other.
21.67Use a computer program such as CAChe or Spartan to
find the conformation of minimum energy for
trans-1,3-butadiene, using a semiempirical method such
as AM1 or PM3. Compare the bond angles with those
predicted by the use of the 2sp^2 hybrid orbitals. Inspect
the orbital regions and try to determine whether the
orbitals are eigenfunctions of any symmetry operators.
21.68Use a computer program such as Spartan that will carry
out ab initio calculations to find the conformation of
minimum energy fortrans-1,3-butadiene. Compare the
bond angles with those predicted by the use of the 2sp^2
hybrid orbitals. Inspect the orbital regions and try to
determine if the orbitals are eigenfunctions of any
symmetry operators. Compare your results with those of
Problem 21.67 if you did that problem.
21.69Use a computer program such as CAChe or Spartan to
find the conformation of minimum energy for
cis-1,3-butadiene, using a semiempirical method such as
AM1 or PM3. Compare the bond angles with those
predicted by the use of the 2sp^2 hybrid orbitals. Inspect
the orbital regions and try to determine whether the
orbitals are eigenfunctions of any symmetry operators.
Compare your results with those fortrans-1,3-butadiene.21.70Using an available computational chemistry program,
carry out a semiempirical calculation for the carbonate
ion, CO^23 −. Compare your LCAOMOs with those
obtained by the simple “guesswork” LCAOMO analysis.