DFT study of Lithium, Sodium and Potassium salts of Amino Acids; Global Reactivity Descriptors, Optimized parameters and Hydration Free Energy

Introduction

Major greenhouse gas carbon dioxide (CO₂) is accountable for the global warming that has been observed over the past few decades, as well as for the worries about relatedclimate change and its potential impact on humanity. Consequently, the removal of CO₂ from a process gas is a crucial step in many industrial processes. Prior to the flue gas being released into the atmosphere, CO₂ is separated from the flue gas using CO₂ capture methods. Many researchers have examined the responses of the amino acid salt solution with CO₂^1-3, which is developing into a CO₂ capture absorbent. Their physical characteristics, optimum geometrical parameters, and molecular characteristics (Global Reactivity Descriptors) may be useful for assessing and characterizing them as a CO₂ collecting absorbent and for other industrial uses.

For lithium cysteinate, sodium cysteinate and potassium cysteinate, lithium prolinate, sodium prolinate and potassium prolinate such properties have not yet been reported in the open literature. Thus, in the present work, we presented new data on the optimized geometrical parameters like bond length L_O-M and bond angle O-C=O of –COOM group(M=Na/k), molecular properties including chemical hardness(η), softness (S), chemical potential (µ) and electronegativity (χ) and physical parameters like hydration free energies(HFE), total dipole moment, HOMO/LUMO band gap energy, C=O vibration of COOH group, bond length and bond angles inlithium cysteinate, sodium cysteinate and potassium cysteinate, lithium prolinate, sodium prolinate and potassium prolinate.

Figure 1: Structures of Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro and K-pro. Click here to View Figure

Experimental Section

Computational Work

Computational work was performed using Gaussian 09 software. Lithium cysteinate(Li-cyst), sodium cysteinate(Na-cyst), and potassium cysteinate(K-cyst), as well as lithium prolinate(Li-pro), sodium prolinate(Na-pro) and potassium prolinate(K-pro), hydration free energies and other optimal parameters were estimated in the current work in both the gas phase and the aqueous phase. The geometry of amino acids salts (AAS) was fully optimised to calculate the various parameters and hydration free energy and the frequencies were calculated  by using the density functional theory (DFT) B3LYP method at the 6-31++ G (d, p) basic set⁴ using the PCM (Polarizable Continuum Model) solvation Model.

Results and Discussion

Optimized geometrical parameters (bond length (L) in Å and angle (A) in Deg.)

For Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro, and K-pro, the optimized geometrical parameters, suchas bondlength L_O-M, and O-C=O of the -COOM group (M=Na/k), were computed⁵and are presented in Table 1. The O-M bond length increases from Li-AAS to Na-AAS and from Na-AAS to K-AAS. This is most likely attributed to an increase in the metal’s size in gas and aqueous phases. The O-M bond length increases for M= Li, Na and K when transition from the gas phase to the aqueous phase. From Li-AAS to Na-AAS and from Na-AAS to K-AAS, the O-C=O bond angles increases. This O-C=O bond angel increase is also observed from gas to aqueous phase. This pattern is seen across all examined AAS. In Li-cyst, Na-cyst, K-cyst, the O-M bond length and O-C=O bond angles are less than that of corresponding Li-pro, Na-pro and K-pro salts.

Li-AAS (O-M) L< Na-AAS (O-M) L< K-AAS (O-M)L

pro-AAS(O-M) L < cyst-AAS (O-M) L

Li-AAS (O-C=O) A< Na-AAS(O-C=O) A< K-AAS(O-C=O) A

pro-AAS (O-C=O) A< cyst-AAS (O-C=O) A

Molecular properties (Global Reactivity Descriptors)

The eigenvakue of the highest-occupied molecular orbital(HOMO), lowest occupied molecular orbital(LUMO),electronegativity, HOMO-LUMO gap and chemical hardness are the most well-known of these parameters (Global Reactivity Descriptors) that are based on the DFT^6,7. The 1930s-era Koopmans theorem⁸ provides a different method for predicting the ionization energy and electron affinities of chemical species and serves as a link between conceptual density functional theory and molecular orbital theory. This theory states thatthe ionization energy and electron affinity of a molecule are roughly similar to the negative values of those orbitals’ HOMO and LUMO energies. The energy difference between the HOMO and LUMO states is crucial aspect in defining the molecular electrical transport capacities⁹. Global chemical reactivity descriptors¹⁰, such as chemical hardness(𝜼), chemical potential(μ), softness(S) and electronegativity(χ) of molecules have been defined using the HOMO and LUMO energy values for a molecule^11–13. Using Koopmans’ theorem¹⁴ and the HOMO and LUMO energy values, chemical hardness, chemical potential, electronegativity, and softnessof closed-shell molecules are defined as follows,

where, I denotes the ionisation potential and Ais the electron affinity. HOMO and LUMO orbital energies can be used to express I and A as I = -E HOMO and A = -E LUMO¹⁵.

Resistance to changes in electronic configurationof a chemical speciesis evaluated by their chemical hardness^16–18. It also has some important applications in topic like complex stability, chemical reactivity¹⁹, solubility of molecules and in estimation of formed product of the reaction²⁰. Calculated values of ionization product, electron affinity, hardness, potential and softness for Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro and K-pro, are presented in Table 2.Hardness and softness can also have an impact on a molecule’s stability²¹. In thegaseous and aqueous phases, the chemical hardness and electronegativity values of K-AAS are marginally lower than those of corresponding Na-AAS and for Na-AAS, they are marginally lower than those of corresponding Li-AAS. The only exception is for the value for Na-AAS in the gaseous phase. Cyst-salt values are higher than pro-salt readings. All of the examined AAS experience an increase in and a decrease in electronegativity as they transition from a gaseous to an aqueous solution.

It is clear from the information in Tables 2 that the η value for Li-cyst is maximum in both the aqueous phase and the gas phase.

The combined molecular properties data for the studied structures Na-gly, Na-ala, Na-val, Na-leu, K-gly, K-ala, K-val, and K-leu indicate that:

Chemical hardness and chemical potential values are higher in aqueous phase as compare to gas phase; whereas, softness and electronegativity show lower values for all studied structures.

Li-AAS, Na-AAS, and K-AAS have been shown to have the following observed trends for η and χ.

 cyst-salt < η pro-salt          

 Li-salt >η Na-salt > η  K-salt          

χ Li-salt >χ Na-salt >χ K-salt  -aqueous phase

Figure 2: Optimized structures for Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro and K-pro. Click here to View Figure

Physical parameters viz.total dipole moment(TDM) , HOMO-LUMOband gap energy(ΔE) and the C=Ovibration in carboxylic group.

In addition, the computed ΔE, TDM values, and C=O vibration of the carboxylic group of the investigated AAS in both gas and water phase. The two physical parameters TDM and E measure a given compound’s reactivity^22,23 and stability²⁴ with the molecules around it. Reactive chemicals are indicated by a high TDM and a low ΔE²⁵. From Li-AAS to Na-AAS and from Na-AAS to K-AAS, the total dipole moment increases while the band gap energy marginally lowers for both amino acids.

(ΔE) Li-salt >(ΔE) Na-salt > (ΔE) K-salt,

(TDM) Li-salt <(TDM) Na-salt < (TDM) K-salt,

Aqueous phase (TDM and ΔE) > Gas phase (TDM and ΔE)

Trend of total dipole moment(TDM) in both phase for AAS as shown below

(TDM) pro-salts>(TDM) cyst-salts

Trend of HOMO/LUMO band gap energy(ΔE)in both phase for AAS as

(ΔE)pro-salts <(ΔE)cyst-salts

The aforementioned trend clearly indicates that for the examined AAS, the TDM and ΔE values are larger in aqueous solutions compared to gas phase. Figures 3 and 4 present a graphic comparison of the ΔE and TDM of Li-AAS, Na-AAS, and K-AAS, respectively. In both phases, it was discovered that K-pro had the lowest ΔE values, indicating that it was highly reactive, whereas Li-cyst had the highest ΔE values, indicating that it was the least reactive.

Due to an increase in the ionic radius of the metal ion, the C=O vibrational characteristic band of the carboxyl group was significantly shifted toward higher wavenumber from Li-AAS to Na-AAS and from Na-AAS to K-AAS. Observations show that K-pro has the highest and Li-pro has the lowest values.

Calculated Hydration Free Energies for all studied AAS molecules

Understanding the free energy of solvation, which is the energy required for a molecule to go from gas phase to solution, is essential for understanding how chemistry works in solutions. When water is the solvent, the term “solvation free energy” is often used to describe the hydration process. The hydration free energy²⁶ determines how stable an ion is in its hydrated state compared to how stable it is in its anhydrate condition. Hydration free energy was calculated using the DFT B3LYP method at the basis set of 6-31++G (d, p) (HFE),which is the work required to transfer a molecule from the gas phase into the solution phase. To calculate HFE, use the following equation.

Δ G⁰_{Hydr   =}    ΔG⁰_S  – ΔG⁰_g                                

where ΔG⁰_g  and ΔG⁰_Srepresents the standard free energy of solute in the gas phase and in the solvent respectively, and G⁰_Hydr refers as HFE. Table 4 and Figure 5 compare the HFE of the interested AAS. HFE for all AAS under investigation is negative, indicating a strong solute-solvent interaction. Na-salts have greater HFE values than the corresponding Li- and K-salts for amino acids. Among the examined AAS, Na-cyst was shown to have the highest HFE values.

The observed trend for the HFE values for all studied AAS is

(HFE) K-AAS<(HFE) Li-AAS<(HFE) Na-AAS

Table 1: Optimized geometrical parameters: (bond length (L) in Å and bond angle (A) in Deg.) for all studied AAS molecules obtained at B3LYP/6-311+G(d,p) level.

Optimized geometrical parameters	Li-cyst	Na-cyst	K-cyst
	Gas Phase
L(O-M)	1.8610	2.1968	2.5537
A(O-C=O)	121.29	123.73	124.8
	Aqueous Phase
L(O-M)	2.0518	2.3561	2.7667
A(O-C=O)	122.71	124.33	125.27
	Li-pro	Na-pro	K-pro
	Gas Phase
L(O-M)	1.8525	2.1956	2.5497
A(O-C=O)	120.92	123.43	124.47
	Aqueous Phase
L(O-M)	2.0251	2.3559	2.7597
A(O-C=O)	122.39	124.12	125.04

 

Table 2: Molecular propertiesviz.chemical hardnes (η), softness(S), chemical potential (m) and electronegativity(c).

Molecular Properties	Li-cyst	Na-cyst	K-cyst
	Gas Phase
I	6.2706	5.9792	5.7648
A	1.1023	1.6183	1.4294
chemical hardness(η)	2.5842	2.1805	2.1677
softness (S)	0.3870	0.4586	0.4613
chemical potential(m)	-3.6865	-3.7988	-3.5971
electronegativity(c)	3.6865	3.7988	3.5971
	Aqueous Phase
I	6.6622	6.6266	6.5950
A	0.4060	0.4169	0.4229
chemical hardness(η)	3.1281	3.1049	3.0861
softness (S)	0.3197	0.3221	0.3240
chemical potential(m)	-3.5341	-3.5218	-3.5090
electronegativity(c)	3.5341	3.5218	3.5090
	Li-pro	Na-pro	K-pro
		Gas Phase
I	5.6461	5.3313	5.0690
A	1.1083	1.6117	1.4278
chemical hardness(η)	2.2689	1.8598	1.8206

Table 3: Physical parameters for all studied AAS viz. ΔE, TDM and the C=O vibration in carboxyl group.

Molecular Properties	Li-cyst	Na-cyst	K-cyst
		Gas Phase
ΔE (eV)	5.1683	4.3609	4.3354
TDM (D)	3.6075	5.8056	7.8980
C=O vibration	1580.3700	1592.5600	1601.5300
	Aqueous Phase
ΔE (eV)	6.2562	6.2097	6.1721
TDM (D)	5.1733	7.2182	9.5197
C=O vibration	1573.3900	1577.5800	1582.1400
	Li-pro	Na-pro	K-pro
	Gas Phase
ΔE (eV)	4.5378	3.7196	3.6412
TDM (D)	4.3832	6.7758	7.3595
C=O vibration	1567.7000	1587.5300	1600.4900
	Aqueous Phase
ΔE (eV)	5.8722	5.7210	5.6277
TDM (D)	6.4383	8.5160	8.9823
C=O vibration	1571.4000	1579.2400	1584.3100

 

Table 4: Calculated Hydration Free Energies for all studied AAS molecules.

Molecular Properties	Li-cyst	Na-cyst	K-cyst
Hydration Free Energy (Kcal/mol)	-33.08	-33.16	-29.26
	Li-pro	Na-pro	K-pro
Hydration Free Energy (Kcal/mol)	-30.66	-31.12	-27.52

Figure 3: Comparison of DE of Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro and K-pro. Click here to View Figure

Figure 4: Comparison of TDM of Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro and K-pro. Click here to View Figure

Figure 5: Comparison of Hydration Free Energies of Li-cyst, Na-cyst, K-cyst, Li-pro, Na-pro and K-pro. Click here to View Figure

Conclusion

As the O-M bond length increases from Li-AAS to Na-AAS and from Na-AAS to K-AAS with a change in the (C-O=C) bond angle, the geometry of the studied amino acid salt changes. All of the examined AAS experience an increase in η andχas they transition from a gaseous to an aqueous solution. It is known that both the aqueous phase and the gas phase of Li-cyst have the greatest η values. As a result, we can say that it is the least reactive.

Both the TDM and ΔE varies as a result of increased size of metal and the alkyl part of studied amino acid. Lower ΔE and higherTDM values are the indicative of greater reactivity of K-AAS compare to Na-AAS. ΔE values of cyst-salt are higher than that of pro-salt. K-cyst has found to be highest dipole moment among all studied salts in both phases. The change in the geometrical parameters shiftsC=O characteristic band toward lower wavenumbers.Negative value of the HFE of AAS suggests strong interaction between AAS and water. The hydration free energy is highest for Na-AAS compare to Li-AAS and K-AAS.

Acknowledgement

MBD thanks the Director of the Government Vidarbha Institute of Science and Humanities in Amravati and the Principal of the HPT Arts and RYK Science College in Nashik for providing research facilities and their kind cooperation.

Conflicts of Interest

Authors declares that there is no conflict of interest.

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