The enthalpy of an isolated primary amine was computed using Gaussian software for various temperature, say T= 100 K, 200K ...500K. When the enthalpy of an isolated secondary amine (derived from that primary amine) was calculated, the enthalpy was larger than those of primary amine. How to interpret the results?
The calculation was done as shown in the link https://www.youtube.com/watch?v=An35qX0fX7Q
The values obtained are very rough. These are rough estimates of quantities
The total electronic energy E0, ZPEs, as well as the thermal corrections to enthalpy (Hcorr) and free energy (Gcorr) are functions of the composition of the molecule (and hence the number of chemical bonds). Therefore, they cannot be directly compared. In fact, enthalpies and Gibbs' free energy are calculated as differences between this calculated for the products and the reagents of a balanced reaction. All DH/DG values should be calculated at the same temperature and with the same computational setup to be physically meaningful.
Hence, if you are considering a reaction leading from a primary amine RNH2 to a secondary amine R,R'NH you have to:
1) draw a balanced chemical reaction leading from your reagents to your products
2) optimize the geometries of all the species participating to that reaction
3) carry out a thermochemical calculation on all these species at their optimized geometries
4) sum up the enthalpies/free energy of the products and the reagents (keeping into account the stoichiometric ratios) so to calculate a net enthalpy/free energy variation of the reaction (DHr/DGr)
5) you can repeat the previously calculations at a different temperature T if needed.
As for the meaning of the mentioned thermodynamic state functions and how to evaluate them in Gaussian, my advice is to read carefully the paper at https://gaussian.com/wp-content/uploads/dl/thermo.pdf by J.W. Ochterski.
I hope this may help.
Massimiliano Arca Thank you for your detailed response.
The thing is, I do not consider any reaction from primary amine leading to secondary amine.
In Gaussian 09, the isolated neutral molecule is drawn and then submitted for calculation. After the calculation, we obtain, heat capacity, entropy and enthalpy in the output file for a specific temperature.
It is noted that, when the temperature increases, the enthalpy also increases linearly. I want to know the reason for this behaviour.
S. Xavier, when dealing with an isolated molecules, its formation enthalpy depends on the temperature as stated by Kichhoff's rule, which provides the definition of heat capacity.
Yes, the answer of Dr. Massimilano A. is correct, should be calculate K, then gibbs free energy equation, at different temperatures.
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