Concise Physical Chemistry

c19 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

PROBLEMS AND EXAMPLES 317

Problem 19.4

What is the dipole moment of water according to an MM4 calculation? Compare this

value with the value given in a current textbook.

Problem 19.5

Expand the input file for water, File 19.2, which has the geometry>to an input file

for ethene, which has the geometry>=<. The atom designator for ansp^2 carbon is 2

and the hydrogens are designated 112. Run the resulting program. The steric energy

for a successful run should be approximately 2 kcal mol−^1.

Problem 19.6

Modify either the input or the output files resulting from Problem 19.5 after the

manner of Example 19.3 to obtain an input file for ethane. Run the file and obtain the

full output file TAPE9.MM4 and TAPE4.MM4, part of which is given as File 19.1.

Problem 19.7

Be sure that there is a 1 in column 65 of both the ethene and ethane programs obtained

from Problems 19.5 amd 19.6 to produce values for the enthalpy of formation of

each. Determine the enthalpy of hydrogenation of ethene to ethane, each in the

standard state. The result from a successful geometry optimization should be about

–32 kcal mol−^1. The experimental value is− 32. 60 ± 0 .05 kcal mol−^1 =− 136. 4 ±

0 .2kJmol−^1.

Problem 19.8

In the previous problem, why don’t you need to compute the standard state enthalpy

of formation of H 2?

C 2 H 4 (g)+H 2 (g)→C 6 H 6 (g)

Problem 19.9

If you have access to a GUI, find the enthalpies of formation of mono- di- tri- and

tetra(t-butyl)methane. Explain the curious pattern of these results. Print the structure

of tetra(t-butyl)methane.

Problem 19.10

There are three vibrational frequencies given for water in the output file TAPE4.MM4.

Two are for the symmetric and antisymmetric O H stretch. Diagram these two modes

of motion. What molecular motion corresponds to the third frequency? Why it this

frequency so different from the other two?