Mostafa Abediab,
Hossein Farrokhpour*a and
Solaleh Farniab
aDepartment of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran. E-mail: farrokhphossein@gmail.com
bDepartment of Chemistry, University of Zanjan, Zanjan 38791-45371, Iran
First published on 10th December 2013
The acidity constants (pKa) of two important series of selenium oxoacids including H2SeOn (n = 1–5) and H2Se2On (n = 1, 3, 4, 6 and 7) were predicted by using two different computational methods for the first time. The calculations were performed using MP2 level of theory employing a very large basis set, 6-311++G(3df,3pd), and a new density functional method, M06-2X, with the same basis set, separately. A new continuum solvation model, SMD, based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent, was used to account the solvent effects. The calculated values of pKa were corrected using the different correlation equations, reported in the literature, to improve the accuracy of results. Also, the calculated results in this work were compared with the corresponding results related to sulfur oxoacids obtained in our previous work. The same trend was observed for the variation of the acidity constants of selenium oxoacids and corresponding sulfur oxoacids (with the number of oxygen atoms). Comparison of the pKa1 of selenium and sulfur oxoacids showed that the acidity strength of sulfur is higher than selenium while for pKa2, selenium oxoacids are stronger acids than sulfur oxoacids. The predicted values of pKa of selenium oxoacids are important and useful in the different chemical aspects of these compounds in chemistry and biochemistry.
It is well-known that the density functional (DFT)2 methods, specially the hybrid B3LYP method,3–5 is capable of reaching average errors of 1–2 kcal mol−1 in calculation of deprotonation energies for small molecule test cases. Recently, two new hybrid meta exchange correlation functional, called M06 and M06-2X were presented by Zhao and Truhlar.6 The M06 functional is parameterized including both transition metals and nonmetals, whereas the M06-2X functional is a high nonlocality functional with double the amount of non-local exchange (2X), and it is parameterized only for nonmetals. The M06-2X functional applies for main-group thermochemistry, kinetics, non-covalent interactions, and electronic excitation energies to valence and Rydberg states.6
The self-consistent reaction field (SCRF)7–9 methods have been employed in combination with DFT method to calculate solvation energies in aqueous solution. A new continuum solvation model based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent known as SMD, (where the “D” stands for “density” to denote that the full solute electron density is used without defining partial atomic charges) is a new recommended option for calculating free energy of solvation which accomplished by performing gas phase and SCRF = SMD calculations for the considered system.10
Inorganic selenium compounds are widely available in dietary supplements and have been extensively studied for their antioxidant and anticancer properties. Although many in vivo and clinical trials have been conducted using these compounds, their biochemical and chemical mechanisms of efficacy are the focus of much current research. Selenium compounds also have similar abilities to act as both antioxidants and pro-oxidants.11
In our previous work, we predicted the acidity constants (pKa) of some important sulfur oxoacids using expensive theoretical methods including MP2 and CCSD methods.12 In this work, the selenium oxoacids were selected for prediction their acidities because of their importance in inorganic chemistry and biochemistry. We were reported the aqueous phase acidities constant (pKa1 and pKa2) for some important selenium oxoacids including, H2SeOn (n = 1–5) and H2Se2On (n = 1, 3, 4, 6 and 7) for the first time. The calculations were performed at the second-order Moller–Plesset perturbation (MP2)13 and M06-2X methods, separately. Single point calculations on the optimized structures of neutral and related anions were performed according to PCM model and the SMD model was used for calculating the solvation free energies of the species. To the best of author knowledge, there is no experimental report on the values of the pKas of the considered compounds in the literature except, for H2SeO3 and H2SeO4. Therefore, it is worthwhile to obtain information about the acidity constants of these compounds, computationally and compare them with the acidity constant of sulfur oxoacids. Among the different considered selenium oxoacids, H2Se2O3 and H2Se2O4 are more well-known than the others. The results obtained in this work are interesting and challenging from the inorganic chemistry and biochemistry aspects and help to better understand of these oxoacids.
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The deprotonation of a compound in aqueous solution can be assumed as a part of the following thermodynamic cycle (Scheme 1):14
One part of this cycle, ΔGg, is the gas-phase deprotonation free energy of the molecule. Three other parts, , are the free energy of solvation of the protonated and deprotonated forms of the molecule and the proton, respectively. The fifth part of the cycle, ΔGa, is the desired free energy changes of deprotonation in solution. As the sum of free energies around the cycle must add to zero, the fifth term, ΔGa, can be calculated from the other three terms as:
![]() | (2) |
ΔG°gas = G°gas (A−) + G°gas (H+) – G°gas (AH) | (3) |
Aqueous phase calculations were performed based on the polarizable continuum model (PCM) of Tomasi and co-workers17–23 employing SMD solvation model for geometry optimization, frequency calculations and also single energy calculations to obtain solvation free energy.
According to the thermodynamic cycle shown in Scheme 1, the total change in Gibbs free energy in solution is equal to the sum of the gas-phase Gibbs free energy (ΔG°gas) and the change in solvation energies, ΔΔG°sol:
ΔΔG°sol = ΔG°sol (A−) + ΔG°sol (H+) – ΔG°sol (AH) | (4) |
For G°solv(H+) the experimental value of −264.61 kcal mol−1 is used.16
The gas phase optimized structures were used for the frequency calculations, at the MP2/6-311++G(3df,3pd) and M06-2X/6-311++G(3df,3pd) levels of theory which are expensive and reasonable methods, to obtain the Gibbs free energies of the considered species. Tables S2 and S3† tabulated the electronic and Gibbs free energies of the selected species, calculated using MP2 and M06-2X methods, respectively. The gas phase acidity (GA) values, both experimental and theoretical, are widely used to characterize the acidic strength of acids. Table S1† reports the GA of all acids obtained based on eqn (3) for both applied methods.
Tables S4 and S5† show the solvation energies of neutral species and their anions were calculated at the MP2/6-311++G(3df,3pd) and M06-2X/6-311++G(3df,3pd) levels of theory, respectively. To calculate the solvation energy, it is necessary to fully optimize the geometries of oxoacids in the presence of the solvent.
The calculated GA and the solvation free energy of the species can be used to obtain free energy of deprotonation in solution (ΔGa) through the thermodynamic cycle presented in Scheme 1. It should be mentioned that the calculated gas-phase energies are for a standard state of 1 atm but solvation energies use a standard state of 1 mol L−1, therefore, the value of 1.89 kcal mol−1 which corresponds to RT ln(24.46) must be considered and added to gas-phase energies.24–26
The values of ΔGa can be used along with eqn (1) to calculate the pKa values for acids in aqueous solution. Table S6† reports the values of ΔG°gas, ΔΔG°sol (eqn (4)), ΔG°a and pKa1 for the first dissociation reactions of the considered acids obtained from the MP2 calculations. As seen, by increasing the number of oxygen atoms in the series of oxoacids containing one selenium atom, the acidity increases (the pKa1 decreases). It is important to notice this behavior and trend, also, was seen for the acidity of the sulfur oxoacids in our previous work and also the acidity of selenium oxoacids is lower than the corresponding sulfur oxoacids except for H2SeO (see Fig. 2a). The variation of the acidity of oxoacids containing two selenium atoms is similar to the trend was observed for the corresponding sulfur oxoacids (See Fig. 2b) and with lower acidity strength than the corresponding sulfur oxoacids. For example, H2Se2O4 and H2Se2O7 show the opposite acidity order. The reason for this deviation may be attributed to the intramolecular hydrogen bonding which decreases the tendency for dissociation of proton. As seen in Fig. 2b, it is interesting that this effect is stronger for H2Se2O4 than H2Se2O7 in comparison with that in the sulfur oxoacids. This observation may be attributed to the effect of selenium atom compared to sulfur atom on the intramolecular hydrogen bonding. It is expected that the variation of acidity for sulfur and selenium oxoacids should be the same because they are in the same group in the periodic table but, the calculations show different results in this study. Similar results obtained from M06-2X method, have been reported in Table S7.† Comparison of Tables S6 with S7† shows that, the MP2 level of theory presents the effect of the hydrogen bonding on the acidity much better than the M06-2X method.
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Fig. 2 Variation of pKa1 versus number of oxygen atoms for selenium and sulfur oxoacids. (a) One selenium and sulfur oxoacids (b) two selenium and sulfur oxoacids. |
As mentioned before, there are the experimental values only for the pKa1 and pKa2 of H2SeO4 and H2SeO3 in the literature.27,28 For example, the experimental value of pKa1 is 2.6 for H2SeO3 and has been considered the acidity similar to H2SO4 for H2SeO4 and classified it as strong acid. It is seen that the corresponding calculated values obtained at two employed theoretical levels have errors from the experimental values. These differences are common in the theoretical calculation of acidity. One of the methods for decreasing this error is finding a linear correlation between the theoretical pKas and the corresponding experimental data reported in the literature.29–31 Unfortunately, there are no experimental values for the pKa of the most of the selected oxoacids in this work to find this correlation. Therefore, the best way for solving this problem is using the different correlation equations reported for the other acids in the literature.
Zimmermann and Tossell29 calculated the acidity of arsenic(III) and arsenic(V) thio- and oxoacids (H3AsS3, H2AsS3−, HAsS32−, AsS33−, H3AsS4, H2AsS4−, HAsS42−, AsS43−, H3AsO3, H2AsO3−, HAsO32−, AsO33−, H3AsO4, H2AsO4−, HAsO42−, and AsO43−) using different theoretical methods including CBS-QB3/CPCM, CCSD(T)/6-311G(2d,p), CCSD(T)/aug-cc-pVDZ and CCSD(T)/CEP-121G. They have reported a linear correlation equation for each applied theoretical methods with different r2 values. The first reason for using the correlation equations of Zimmermann and Tossell29 is that, the selected acids in their work are similar to selenium oxoacids considered in this work because both series of acids are oxoacids and inorganic acids. The second reason is that these linear correlations were applied to improve the acidity constants of the sulfur oxoacids in previous work and improved our results very well. Therefore, these correlations are used in the present study, too.
Similar to previous work12 the different linear correlations used to improve the calculated values of pKas in this work. The predicted pKa1 values of selenium oxoacids, considered in this work and obtained using different linear correlation equations of Zimmermann and Tossell29 have been reported in Tables 1 and 2 for MP2 and M06-2X methods, respectively. As reported in Tables 1 and 2, the best agreement between the experimental and theory, for the pKa1 of H2SeO3 and H2SeO4, is reached using the linear correlation equation obtained based on the CCSD(T)/aug-cc-pVDZ + optimized with CPCM. Comparison of Tables 1 with 2 show that the pKa1 calculated at the MP2 and M06-2X methods are nearly equivalent and propose that the M06-2X can be applied for the calculation of acidity instead of MP2 method with the low cost of computation.
Species | pKa1 (cal.) | (m = 0.444; b = 0.767)a | (m = 0.429; b = −0.233)b | (m = 0.273; b = −1.593)c | (m = 0.428; b = −0.597)d |
---|---|---|---|---|---|
r2 = 0.952 | r2 = 0.974 | r2 = 0.979 | r2 = 0.944 | ||
a CBS-QB3 method; CBS-QB3 CPCM pKa (all structures optimized & energy calculated in PCM) (ref. 29).b CBS-QB3 with explicit salvation; CBS-QB3 CPCM pKa w/H2O (ref. 29).c CCSD(T)/6-311G(2d,p) + CPCM salvation energy (ref. 29).d CCSD(T)/aug-cc-pVDZ + optimized with CPCM (ref. 29). | |||||
H2SeO | 9.66 | 5.05 | 3.91 | 1.04 | 3.54 |
H2SeO2 | 16.12 | 7.92 | 6.68 | 2.81 | 6.30 |
H2SeO3 | 6.06 | 3.46 | 2.36 | 0.06 | 2.00 |
H2SeO4 | −6.09 | −1.93 | −2.84 | −3.25 | −3.20 |
H2SeO5 | −10.07 | −3.70 | −4.55 | −4.34 | −4.90 |
H2Se2O | 5.27 | 3.10 | 2.02 | −0.15 | 1.65 |
H2Se2O3 | −9.68 | −3.53 | −4.38 | −4.23 | −4.74 |
H2Se2O4 | 14.38 | 7.15 | 5.93 | 2.33 | 5.55 |
H2Se2O6 | −12.42 | −4.75 | −5.56 | −4.98 | −5.91 |
H2Se2O7 | −8.72 | −3.10 | −3.97 | −3.97 | −4.33 |
Species | pKa1 (cal.) | (m = 0.444; b = 0.767)a | (m = 0.429; b = −0.233)b | (m = 0.273; b = −1.593)c | (m = 0.428; b = −0.597)d |
---|---|---|---|---|---|
r2 = 0.952 | r2 = 0.974 | r2 = 0.979 | r2 = 0.944 | ||
a CBS-QB3 method; CBS-QB3 CPCM pKa (all structures optimized & energy calculated in PCM) (ref. 29).b CBS-QB3 with explicit salvation; CBS-QB3 CPCM pKa w/H2O (ref. 29).c CCSD(T)/6-311G(2d,p) + CPCM salvation energy (ref. 29).d CCSD(T)/aug-cc-pVDZ + optimized with CPCM (ref. 29). | |||||
H2SeO | 7.34 | 4.02 | 2.91 | 0.41 | 2.54 |
H2SeO2 | 15.96 | 7.85 | 6.61 | 2.76 | 6.23 |
H2SeO3 | 7.11 | 3.92 | 2.82 | 0.35 | 2.44 |
H2SeO4 | −5.04 | −1.47 | −2.39 | −2.97 | −2.75 |
H2SeO5 | −8.95 | −3.20 | −4.07 | −4.03 | −4.43 |
H2Se2O | 2.63 | 1.93 | 0.89 | −0.87 | 0.53 |
H2Se2O3 | −8.85 | −3.16 | −4.03 | −4.00 | −4.38 |
H2Se2O4 | 11.21 | 5.74 | 4.57 | 1.46 | 4.20 |
H2Se2O6 | −10.88 | −4.06 | −4.90 | −4.56 | −5.25 |
H2Se2O7 | −6.10 | −1.94 | −2.85 | −3.25 | −3.21 |
Tables S8 and S9† report the values of pKa2 calculated using MP2 and M06-2X levels of theory, respectively. Again, the effect of the intramolecular hydrogen bonding can be seen on the pKa2 of the acids and the MP2 method shows this effect much better than the M06-2X method. The values of the pKa2 were improved using the Zimmermann and Tossell's correlation equations and tabulated in Tables 3 and 4. Based on the experimental values of pKa2 for H2SeO3 and H2SeO4, the predicted values using the correlated equation of CCSD(T)/aug-cc-pVDZ + optimized with CPCM calculations is more reliable for both applied methods in this work.
Species | pKa2 (cal.) | (m = 0.444; b = 0.767)a | (m = 0.429; b = −0.233)b | (m = 0.273; b = −1.593)c | (m = 0.428; b = −0.597)d |
---|---|---|---|---|---|
r2 = 0.952 | r2 = 0.974 | r2 = 0.979 | r2 = 0.944 | ||
a CBS-QB3 method; CBS-QB3 CPCM pKa (all structures optimized & energy calculated in PCM) (ref. 29).b CBS-QB3 with explicit salvation; CBS-QB3 CPCM pKa w/H2O (ref. 29).c CCSD(T)/6-311G(2d,p) + CPCM salvation energy (ref. 29).d CCSD(T)/aug-cc-pVDZ + optimized with CPCM (ref. 29). | |||||
H2SeO | 31.28 | 14.65 | 13.18 | 6.94 | 12.79 |
H2SeO2 | 22.19 | 10.62 | 9.28 | 4.46 | 8.90 |
H2SeO3 | 14.28 | 7.11 | 5.89 | 2.30 | 5.51 |
H2SeO4 | 6.60 | 3.70 | 2.60 | 0.21 | 2.23 |
H2SeO5 | 19.82 | 9.57 | 8.27 | 3.82 | 7.88 |
H2Se2O | 41.01 | 18.97 | 17.36 | 9.60 | 16.95 |
H2Se2O3 | 4.15 | 2.61 | 1.55 | −0.46 | 1.18 |
H2Se2O4 | 8.79 | 4.67 | 3.54 | 0.80 | 3.16 |
H2Se2O6 | 0.49 | 0.98 | −0.02 | −1.46 | −0.38 |
H2Se2O7 | −3.18 | −0.64 | −1.60 | −2.46 | −1.95 |
Species | pKa2 (cal.) | (m = 0.444; b = 0.767)a | (m = 0.429; b = −0.233)b | (m = 0.273; b = −1.593)c | (m = 0.428; b = −0.597)d |
---|---|---|---|---|---|
r2 = 0.952 | r2 = 0.974 | r2 = 0.979 | r2 = 0.944 | ||
a CBS-QB3 method; CBS-QB3 CPCM pKa (all structures optimized & energy calculated in PCM) (ref. 29).b CBS-QB3 with explicit salvation; CBS-QB3 CPCM pKa w/H2O (ref. 29).c CCSD(T)/6-311G(2d,p) + CPCM salvation energy (ref. 29).d CCSD(T)/aug-cc-pVDZ + optimized with CPCM (ref. 29). | |||||
H2SeO | 29.62 | 13.92 | 12.47 | 6.50 | 12.08 |
H2SeO2 | 20.51 | 9.87 | 8.56 | 4.00 | 8.18 |
H2SeO3 | 12.24 | 6.20 | 5.02 | 1.74 | 4.64 |
H2SeO4 | 6.09 | 3.47 | 2.38 | 0.07 | 2.00 |
H2SeO5 | 19.20 | 9.30 | 8.00 | 3.64 | 7.62 |
H2Se2O | 38.14 | 17.70 | 16.13 | 8.82 | 15.72 |
H2Se2O3 | 2.30 | 1.79 | 0.75 | −0.96 | 0.38 |
H2Se2O4 | 11.14 | 5.71 | 4.55 | 1.44 | 4.17 |
H2Se2O6 | 1.64 | 1.50 | 0.47 | −1.14 | 0.10 |
H2Se2O7 | −3.07 | −0.60 | −1.55 | −2.43 | −1.90 |
Fig. 3a and b show the variation of the pKa2 of selenium and sulfur oxoacids with the number of oxygen atoms. It is seen in Fig. 3 that the trend of variation of pKa2 for the selenium and sulfur oxoacids is almost the same. Interestingly, Fig. 3a shows that the pKa2 of selenium oxoacids are lower than the corresponding sulfur oxoacids which is opposite with what observed for corresponding pKa1 (Fig. 2a).
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Fig. 3 Variation of pKa2 versus number of oxygen atoms for selenium and sulfur oxoacids. (a) One selenium and sulfur oxoacids (b) two selenium and sulfur oxoacids. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47167e |
This journal is © The Royal Society of Chemistry 2014 |