Molecular Orbital Calculations of Hydrogen Chloride
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AM1 geometry optimization

AM1 geometry optimization gave a bond length value of 1.28 angstroms.
 
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PM3 geometry optimization

PM3 geometry optimization gave a bond length value of 1.27 angstroms. This proved to be the best level of theory for geometry optimization because the value came closest to the literature value of 1.27 angstroms.7
 
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DZV geometry optimization

DZV geometry optimization gave a bond length value of 1.26 angstroms. DZV is the highest level of theory used for geometry optimization.
 
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6-21G geometry optimization

6-21G geometry optimization gave a bond length value of 1.27 angstroms. This also proved to be the best level of theory for geometry optimization because the value came closest to the literature value of 1.27 angstroms.7 6-21G is the lowest level of ab initio theory used for geometry optimization.
 
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6-31G geometry optimization

6-31G geometry optimization gave a bond length value of 1.26 angstroms. 6-31G is the second highest level of ab initio theory for geometry optimization.
 
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Highest occupied molecular orbital of HCl

This is the highest occupied molecular orbital (HOMO) at orbital 9. Orbital 9 was chosen for the HOMO because HCl has a total number of 18 electrons, and the number of total electrons was divided by two.
 
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Lowest unoccupied molecular orbital of HCl

This is the lowest unoccupied molecular orbital (LUMO) at orbital 10. Orbital 10 was chosen as the LUMO because the HOMO is in orbital 9.
 
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The electrostatic potential of HCl

This is the electrostatic potential of HCl. The red area represents the lowest electrostatic potential and the blue area represents the highest electrostatic potential. Intermediate colors represent intermediate potentials.
 
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Partial atomic charges on each atom of HCl

The partial atomic charge on each atom is shown in the diagram on the right. The values of the partial charges on each atom were created by the asymmetric distribution of electrons in the chemical bond.
 
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Sigma bonding present in the H-Cl bond

The diagram on the left shows the sigma bonding orbital present in the H-Cl bond as shown in figure 2. This is the lowest occupied energy level.
 
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Sigma-pi bonding present in the H-Cl bond

The diagram on the right shows the non-bonding orbital present in the H-Cl bond as shown in figure 2 below. This is the highest occupied energy orbital and is also degenerate.
 
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Pi non-bonding orbital present in the H-Cl bond

The diagram on the left shows the pi non-bonding orbital present in the H-Cl bond as shown in figure 2 below. This is the highest occupied energy orbital and is also degenerate.
 
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Vibrational stretch of HCl

The vibrational stretch of the HCl molecule can be visualized.  The calculated vibrational frequency associated with this motion is 3170.14 cm¯¹. The literature vibrational frequency 2990.10 cm¯¹.7
      Figure 1 shows the potential energy of the HCl bond as it is stretched and compressed.  Potential energy minima for all three basis sets occur at the ideal bond length, where there is no stretching or compression.  Figure 1 is a direct comparison of the three ab initio basis sets that were used.  The lowest potential energy well is the most stable configuration of the molecule, and therefore the most accurate calculation. It is surprising that the largest basis set, DZV, is found in the middle of the other two, as opposed to at the bottom. The lowest potential energy actually comes from the mid-sized basis set, the 6-31G.  This suggests that the size of the basis set is not the only important parameter to consider when evaluating the validity of any quantum calculations.
Potential Energy plot for 3 ab initio basis sets
Figure 1: Graph of potential energy vs bond length at different levels of theory. The graph was created in IGOR Pro.


                                                                               
Figure 2: molecular orbital diagram for hydrogen chloride                                                                                                                                Table 1: Calculated dipole moments at different levels of theory and the percent error (compared                                                                                                                                                                                                                                 literature value of 1.08.7

The dipole moment was calculated at different levels of theory with the inclusion of many different combinations of diffuse functions.  The values for "d", "f", and "light" functions were varied between 0 and 3 for each of the three ab initio basis sets. Nine different combinations were run in the 6-21G basis set with the best (closes to literature value) dipole moment of 1.174649 D resulting from d=3, f=0, light=3.  Fourteen different combinations were run in the 6-31G basis set with the best dipole moment of 1.170069 D resulting from d=3, f=1, light=3. Fourteen different combinations were also run in the DZV basis set with the best dipole moment of 1.153389 D resulting from d=3, f=1, light=3.  The DZV basis set plus diffuse functions produced the closest to literature value for the dipole moment of 1.08 D7, with a 6.80% margin of error.  The best dipole calculations for each ab initio basis set are summarized above in Table 1.
Based on template by A. Herráez as modified by J. Gutow
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