Chemistry 164, Spring, 2003
Molecular Structure and Modeling
Graded Exercise 4
Cotton and Wilkinson in their monograph on inorganic chemistry report that nitric
oxide, NO, forms a weakly bound dimer, (NO)2.1 Your task in this exercise is to employ
the ab initio quantum mechanics features of Spartan in the identification and
characterization of the stable form(s) of the dimer. Perform the search for stable species
at the Hartree-Fock level with 3-21G* basis functions. Note that you need to calculate
both the energy and the vibrational frequencies. Use the advanced drawing function of
Spartan to construct the structures of possible molecules. When you set up a calculation,
turn off the symmetry constraint in the Calculation window. Use the modeling tools that
we examine in class to characterize the stable species and discuss its/their stability.
If you determine that NO dimer has two or more stable isomers, determine the
standard enthalpy and entropy differences of the two species at 298.15 K and calculate
the equilibrium constant for the interconversion. Use the Hartee-Fock 3-21G* results for
the thermodynamic functions. However, explore a variety of methods to obtain an
optimum estimate of the difference in electronic energy.
Also, in the case of isomers, determine the barrier height for their interconversion.
Characterize the transition states.
1) F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th. ed., Wiley-
Interscience, New York, 1980.
c164_exer4_03.doc, WES, 31 Dec. 2002