CCL: G3B3 Atomization Energy Methane

From: John Bushnell <bushnell : chem.ucsb.edu>
Subject: CCL: G3B3 Atomization Energy Methane
Date: Thu, 23 Feb 2006 01:38:37 -0800 (PST)

 Sent to CCL by: John Bushnell [bushnell],[chem.ucsb.edu]
 So the ZPE of methane is approximately equal to the singlet/triplet
 splitting in atomic carbon.  What a coincidence!
 Also, I guess I was a little confused.  The enthalpy of formation
 involves the difference in energy going from graphite and H2 gas
 to methane gas.  But you also refer to the atomization energy,
 which is what you are calculating below.  I wonder what all goes
 into the reported experimental value for the enthalpy of atomization.
 They're certainly not mixing triplet carbon and doublet hydrogen. :-)
 I can see how to measure (experimentally) enthalpies starting from
 graphite, but I wonder how this is related to atomic triplet carbon?
           - John
 On Wed, 22 Feb 2006, Pablo F. Salazar Pablo.Salazar#%#chemail.tamu.edu wrote:

 Sent to CCL by: "Pablo F. Salazar" [Pablo.Salazar||chemail.tamu.edu]
 It is not fortuitous, it is actually much better:
 Apparently the singlet was used instead of the triplet for C.
 Also notice that the data from the log file at 0K and the G3(0K), already
 contains the ZPE correction . Thus, with the following energies at 0K:
 H -0.501087
 C -37.82845
 CH4 -40.458277,
 the atomization energy for methane at 0K is:
 (-37.82845+4*-0.501087)- -40.458277 = 1642.20 kJ/mol
 in perfect agreement with the experiment at 0K(1642.27 kJ/mol).
 For the atomization energy for methane at 298K, you must use the corrections
 listed in the log file. So the energy of a molecule (or a atom) at 298K
 would be:
 H -0.498727
 C -37.82609
 CH4 -40.454469
 and the atomization energy for methane at 298K is:
 (-37.82609+4*-0.498727)- -40.454469 = 1663.2 kJ/mol
 also in perfect agreement with the experiment at 0K(1663.3 kJ/mol).
 Pablo F Salazar
 Department Chemical Engineering
 3122 TAMU
 College Station, TX 77843-3122
 Phone 979-862-1329
 -----Original Message-----

 From: "John Bushnell bushnell,+,chem.ucsb.edu"
 <owner-chemistry^ccl.net>

 To: "Salazar, Pablo Felix " <pablo.salazar^chemail.tamu.edu>
 Date: Mon, 20 Feb 2006 19:43:34 -0500
 Subject: CCL: G3B3 Atomization Energy Methane
 Sent to CCL by: John Bushnell [bushnell**chem.ucsb.edu]
 I'm not at all familiar with the "G3B3" method, but right off hand
 I would think that this is very good agreement with experiment.
 You show an error of 13 kJ/mol at zero K.  But the calculation involves
 the difference of some very large numbers.  The difference of only
 3 kJ/mol at 298 K seems fortuitously small in fact.  If enthalpies
 of formation could be routinely calculated to this accuracy, we
 wouldn't have to do so many experiments. :-)
   Just my offhand impression...
           - John
 On Mon, 20 Feb 2006, Roger Kevin Robinson r.robinson .. imperial.ac.uk wrote:

 Sent to CCL by: Roger Kevin Robinson [r.robinson,,imperial.ac.uk]
 Hi,
    I've asked about this before but i still seem to be having some
 trouble. Im just using Methane as an example.
 Using G3B3 methods.
 At 298K I get
 Name    G3-Energy(G3B3)    ZPE
 C          -37.778738
 H          -0.499671
 CH4     -40.455401             0.043410
 Using this values to calculate Atomization Energy
 = (-37.778738 + 4* -0.499671) - -40.455401  -   0.043410= 0.634569 =
 1666.06091 kJ/mol
 this fits in well with an experimental value of 1663.3
 to calculate Enthalpy of formation you need the atomization Energy at 0K
 as far as im aware.
 Right at 0K
 Name    G3(0K) - (G3B3)      ZPE
 C         -37.780154
 H         -0.501087
 CH4     -40.458277             0.043410
 = ( -37.780154 +4 * -0.499671) - -40.455401  -   0.043410 = 0.630365 =
 1655.02331.
 But the experimental value is 1642.27
 Does any one know where im making the mistake ?

Thanks