Concise Physical Chemistry

c20 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

PROBLEMS AND EXAMPLES 343

(b) What is the energy of the hydrogen atom according to your STO-3G basis

set? What is the percent difference between your value and the exact (defined)

value ofEH= 0. 5000 Eh?

Problem 20.6

Carry out a 3-21G split valence calculation of the energy of atomic helium, He. What

is the approximation to the first ionization potential of helium in this model? The

experimental value is 0.903Eh.

Problem 20.7

(a) Carry out a 3-21G calculation of the energy of the molecular ion HeH+.

Do you predict that this molecular ion exists?

(b) Carry out a 3-21G calculation of the energy of the bond energy of the molecular

ion He 2 +. Do you predict that this molecular ion exists?

(c) Carry out a 3-21G calculation of the energy of the bond energy of the molecule

He 2. Do you predict that this molecule exists?

(d) Carry out a full 3-21G calculation of the total energy of the lithium hydride

molecule.

Problem 20.8

Suppose, on an exam, you are given the following Hartree–Fock 3-21G total en-

thalpiesH^298

Methane HF= 39. 9768776 Ethene HF=− 77. 6009881

Ethane HF= 78. 793948 1,3-Butadiene HF=− 154. 0594565

along with the enthalpies of formation of (^) fH^298 (methane)=− 74 .4kJmol−^1 ,
(^) fH^298 (ethene)= 52 .5kJmol−^1 , and (^) fH^298 (ethane)=− 83 .8kJmol−^1 .No
other information is given. Find the enthalpy of formation of 1,3-butadiene. See
also Chapter 21, Section 21.4.
Problem 20.9
Recalculate the previous result, this time using the 6-31G basis set to calculate the
HF values of the four hydrocarbons in the reaction
CH 2 CH CH CH 2 +2CH 4 →2CH 2 CH 2 +CH 3 CH 3
Is the agreement with the experimental result improved by using the higher-order
basis set? By how much?