Subject: A question about enthalpy

From: [email-masked] (Anthony P Scott)
Date: Sat, 14 May 94 23:33:30 EST

Given the following:

             A \/\/\/\/\/\/\/\/\/ A

A molecule A2 in which the internuclear distance is large and hence the 
vibrational frequency that can be ascribed to A2 is small.

Now if we work out the enthalpy contributions for A2 we get:

          translational: 3/2RT
          rotational   : 2/2RT
          vibrational  : RT
          Total        : 7/2RT

The vibrational contribution tends towards RT as the vibrational frequency
tends to zero.

Now consider two isolated A atoms.

            A                     A

separated by a large distance. The total enthalpy for the two A atoms
is:

          Total       : 6/2RT

Why is there a difference of 1/2RT for these two systems that appear to
be similar?

Any pointers to this would be appreciated.

Thanks in advance.

Anthony P. Scott
Research Officer
Computational Chemistry Group
Research School of Chemistry
Australian National University
Canberra, ACT, Australia