HBr

Hydrogen Bromide

Here are the results of the computations on HBr. In general the larger the basis set the more accurate the calculation, although some basis sets are more well suited to certain types of calculations than others. This is demonstrated by comparing the calculated bond lengths and dipole moments with the literature.


	Figure 1 - HBr with bond length displayed.

Basis set	Bond Length, Angstroms
3-21G	1.432407
6-31G	1.429702
Double Zeta Valence, DZV	1.416607
Literature	1.4145¹

Table 1 - We see that with increasing basis size going down the table the bond length becomes closer to the measured value.

Figure 2 - HBr with HOMO displayed.

Here is displayed HBr's HOMO. It's difficult to evaluate the accuracy of this result without a basis for comparison.

Number of diffuse orbital functions included in basis set	3-21G	6-31G	DZV
0	1.322598	1.429702	1.500531
3	failure	1.146224	1.0979278
5	failure	0.978147	0.951230
7	failure	0.951641	0.970366

Table 2 - Dipole moments, Debye.

Literature Value: 0.827 Debye
The failures of the 3-21G basis set can likely be attributed to the insufficient size of the set. There is a general trend of more diffuse functions leading to more accurate results, with the exception of the DZV with 7 difuse functions. This is another example of how care must be taken when performing these calculations.

UV VIS data

3-21G	6-31G	DZV	Literature
62460.53	62625.91	61888.18	2500
113392.79	110478.65	110387.62
194016.17	117185.39	169565.38
194281.11	129661.68	171294.34
197612.77	133333.86	171821.6
198249.03	133687.31	178705.02
202884.99

Table 3 - ultraviolet visual range electronic transition frequencies, cm^-1.

Below is a graph of potential energy vs. bond stretch. In general it is accepted that the larger the basis set, the lower the calculated energy will be. However, these results are an exception to that trend, as the DZV set is larger than the 6-31G set. Heavier atoms are harder to deal with in computations, and this result may be explained by an inability to accurately compute the properties of bromine.

Figure 3 - Graph of potential energy vs. bond stretch. The larger basis set DZV calculated a higher energy than the 6-31G basis set, which is contrary to the expectation that larger basis sets produce lower energies.

Below is a table of vibrational frequencies for HBr.

3-21G	6-31G	DZV	Literature
2491	2672	2654.5	2649
		14.01	2505
		13.55
		4.17

Table 4 - Vibrational frequencies, cm^-1. Each basis set came close to the literature value of 2649,
with the largest basis set DZV coming the closest in this instance.

Based on template by A. Herráez as modified by J. Gutow

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