Concise Physical Chemistry

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

316 CLASSICAL MOLECULAR MODELING

C

Methane MM4 5

0 4 0 0 0 0 0 0 1

1 2 1 3 1 4 1 5

0.0 0.0 0.0 1

-1.0 1.0 0.0 5

-1.0 -1.0 0.0 5

1.0 0.0 1.0 5

1.0 0.0 -1.0 5

FILE 19.4 MM4 input geometry for methane. The 1 in column 65 requests calculation of


fH^298.

The experimental value is
fH^298 =− 74. 8 ± 0 .3kJmol−^1 =− 17. 9 ± 0 .1 kcal mol−^1.

Molecular properties beyond geometry and heat capacity, such as dipole moments,

spectra, partition functions, and entropy, are calculated in MM4.

Problem 19.1

(a) What is the enthalpy of formation
fH^298 of neopentane (dimethylpropane)

by the simple group additivity method in Section 19.1?

(b) The enthalpy of isomerization of isopentane (methylbutane) to neopentane is

–14 kJ mol−^1. What is the enthalpy contribution of the C H group in alkanes?

Problem 19.2

Determine the O H bond length from the Cartesian coordinates given in File 19.3 by

hand calculation using Pythagoras’s theorem. The experimental value is 97.0 pm=

0.970A. ̊

Problem 19.3

Find the H O H bond angle from the optimized geometry in File 19.3. The experi-

mental value is 104. 5 ◦.

Methane MM4 0 5 0 0 0 0 10.0

0 0 0.0000000 4 0 0 0 0 0 1 1 0

1 2 1 3 1 4 1 5

0.00000 0.00000 0.00000 C 1( 1)

-0.60766 0.92497 -0.02506 H 5( 2)

-0.66877 -0.88172 -0.02769 H 5( 3)

0.60089 -0.02167 0.92947 H 5( 4)

0.67554 -0.02158 -0.87672 H 5( 5)

FILE 19.5 MM4 output geometry for methane.