Microhydration of Neutral and Charged Acetic Acid - The Journal of

Article pubs.acs.org/JPCA

Microhydration of Neutral and Charged Acetic Acid Parvathi Krishnakumar†,‡ and Dilip Kumar Maity*,†,‡ †

Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India

‡

S Supporting Information *

ABSTRACT: A systematic theoretical study has been carried out on the effect of sequential addition of water molecules to neutral and mono positively charged acetic acid molecules by applying first principle based electronic structure theory. Geometry, dipole moment, and polarizability of hydrated clusters of neutral and mono positively charged acetic acid of the type CH3COOH·nH2O (n = 1−8) and [CH3COOH·nH2O]+ (n = 1, 2) are calculated at the ωB97X-D/aug-cc-pVDZ level of theory. Free energies of formation of the hydrated acid clusters, at different temperatures and pressures are determined. Solvent stabilization energy and interaction energy are also calculated at the CCSD(T)/6311++G(d,p) level of theory. It is observed that in the case of neutral acetic acid, proton transfer from the acid molecule to solvent water molecules does not occur even with eight water molecules and the acid molecule remains in the undissociated form. High-energy equilibrium structures showing dissociation of acetic acid are obtained in case of hexahydrated and larger hydrated clusters only. However, dissociation of mono positively charged acetic acid occurs with just two water molecules. Interestingly, it is noted that in the case of dissociation, calculated bond dipole moments of the dissociating bonds of acetic acid in microhydated clusters shows a characteristic feature. IR spectra of CH3COOH·nH2O (n = 1−8) and [CH3COOH·nH2O]+ (n = 1−3) clusters are simulated and compared with the available experimental data.

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INTRODUCTION The study on microhydration of neutral and charged chemical species has been a subject of intense research to understand the structural, energetic, spectroscopic, and dynamic aspects of hydration at molecular level. When a solute is added to a solvent water pool, the water molecules in the immediate neighborhood of the solute get rearranged to form a hydrogenbonded cluster of solute and solvent water molecules. The electron distribution pattern of the added solute plays the key role to form a stable hydrogen-bonded water network around the solute. Off late, cluster spectroscopy and first principle based quantum chemical studies have been successfully applied to determine the detailed structure of a hydration shell around a solute molecule. The properties of these hydrated clusters have been studied to understand the fundamental interactions between the solute and solvent molecules that are responsible for the process of solvation.1−10 A few experimental and theoretical studies have also been reported on microhydration of acid molecules leading to proton transfer from the acid molecule to surrounding water molecules.11−15 Carboxylic acids are one of the dominant classes of organic species found in the troposphere up to the lower stratosphere.16 In particular, lower molecular weight carboxylic acids, namely, formic acid, acetic acid, and oxalic acid, play a significant role in cloud formation due to their polarity and characteristic hygroscopicity.17 They are the prevalent carboxylic acids found in rain, fog, and snow samples from both urban and rural sites.18 These pollutants significantly contribute to important environmental concerns, such as rain acidification and aerosol formation. Thus, understanding the interactions of © XXXX American Chemical Society

these acid molecules with water molecules is very important. Acetic acid is highly soluble in water and this is due to strong interaction of acetic acid molecules with solvent water molecules through hydrogen bonding. IR and Raman spectroscopic studies in the condensed phase have suggested that in dilute aqueous solution of acetic acid, major components are hydrates of acetic acids.19,20 Under atmospheric conditions, formation of hydrates of acetic acid is expected due to strong H-bonding interaction between acetic acid and water. Microwave spectroscopic studies have been carried out on acetic acid and its small size hydrates, CH3COOH·nH2O (n = 1, 2) to extract rotational constants and geometrical information.21 Recently, a combined experimental and theoretical study has been reported on two isomers of acetic acid and water complex (1:1) trapped in an argon matrix at 11 K.22 The most stable isomer has a six-membered ring structure involving two hydrogen bonds: one between the hydroxyl hydrogen atom of the acid molecule and the oxygen atom of the water molecule and the other between the hydrogen atom of the water molecule and carbonyl oxygen atom of the acid molecule. Noncovalent interaction of only 1:1 acetic acid and water cluster (CH3COOH·1H2O) has been studied experimentally by IR spectroscopy in a nitrogen matrix and theoretically at MP2 and CCSD(T) level.23 Only a few theoretical studies have been reported in the literature on small size (n = 1, 2) hydrated clusters of acetic Received: September 13, 2016 Revised: December 20, 2016 Published: December 21, 2016 A

DOI: 10.1021/acs.jpca.6b09257 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Most stable equilibrium structures of (a) acoh, (b) acoh·1H2O, (c) acoh·2H2O, (d) acoh·3H2O, (e) acoh·4H2O, (f) acoh·5H2O, (g) acoh· 6H2O, (h) acoh·7H2O, and (i) acoh·8H2O calculated at the ωB97X-D/aug-cc-pVDZ level of theory. The gray, red, and blue balls represent carbon, oxygen, and hydrogen atoms, respectively.

acid, elucidating mainly hydrogen bonding interactions.24,25 Gao and Leung reported DFT results on equilibrium structures and binding energies of mono- and dihydrated clusters of acetic acid applying B3LYP functional.24 Structure, energy parameters, and infrared vibational frequencies of CH3COOH·nH2O (n = 1, 2) clusters have also been calculated at the MP2 level of theory. In what follows, we report a systematic study on hydrated clusters of acetic acid, CH3COOH·nH2O (n = 1−8), applying first principle based electronic structure theory. Special emphasis is given for finding equilibrium structures having the ionized form of acetic acid on transferring the proton to solvent water molecule. Structure, dipole moment, polarizability, and IR spectra of the most stable structure of different size hydrated clusters are presented. To examine the effect of charge on microhydration, we have also studied hydrated clusters of mono positively charged acetic acid, [CH3COOH·nH2O]+.

functional is able to account for such interactions quite well. All calculations to locate equilibrium structures of acetic acid and its hydrated clusters are carried out by applying this functional along with Dunning’s correlation consistent double-ζ basis sets augmented with diffusion functions. It is a daunting task to obtain an accurate potential energy surface for such complexes having intermolecular interactions, as the number of local minima are numerous for large size clusters. The initial geometries, in all cases, were made so as to facilitate the dissociation of hydroxyl bond of the acetic acid molecule and to have a maximum number of solute−solvent and solvent− solvent hydrogen bonds possible. Geometry optimization is performed to obtain minimum energy equilibrium structures based on the Newton−Raphson algorithm. Hessian calculations are done for the minimum energy structures to confirm that the structures obtained are true local minima and also to simulate the IR spectra. Whenever an equilibrium structure is predicted having imaginary frequency, the optimization is repeated, making an appropriate change in input geometry. Temperature and pressure dependence on the free energy formation of the microhydrated clusters are studied at the ωB97X-D/aug-ccpVDZ level of theory. Solvent stabilization energies and the interaction energies of acetic acid−water clusters are calculated at the CCSD(T)/6-311++G(d,p) level of theory. These

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COMPUTATIONAL METHODS From the previous studies, it is observed that the long-range corrected hybrid density functional, ωB97X-D that includes empirical atom−atom dispersion corrections can described the carboxylic acid−water systems well.14,15,26 This is due to the fact that hydrogen bonding interaction is crucial in shaping the structure of these hydrated clusters and this particular DFT B


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Figure 2. Minimum energy structures of the acoh·6H2O cluster calculated at the ωB97X-D/aug-cc-pVDZ level of theory showing formation of a charge-separated ion pair. The gray, red, and blue balls represent carbon, oxygen, and hydrogen atoms, respectively. ΔE (zero-point energy corrected) values represent relative stability with respect to the most stable structure.

Structure. Acetic Acid (acoh) and Its Hydrated Clusters (acoh·nH2O). The most stable structure of acetic acid (acoh) in isolated gas phase conditions calculated by applying the ωB97X-D functional considering the aug-cc-pVDZ atomic basis set is shown in Figure 1a. The calculated O1H bond length is 0.967 Å (O1 is hydroxyl oxygen atom) compared to the reported experimental value of 0.97 Å.30 Calculated CO1 and CO2 (O2 is carbonyl oxygen atom) bond lengths are 1.352 and 1.207 Å whereas the reported experimental values are 1.365 and 1.196 Å, respectively.31 Previous theoretical bond distance values calculated at the B3LYP/6-311++G(3df,3pd) level of theory for CO1, CO2, and O1H bonds are 1.354, 1.202, and 0.967 Å, respectively.24 On addition of a single water molecule to acetic acid by various possible ways followed by geometry optimization, three equilibrium structures of monohydrated cluster (acoh·1H2O) are obtained similar to previous reports based on B3LYP and MP2 calculations. The most stable structure of the monohydrated cluster is shown in Figure 1b with selected bond distance parameters. On addition of a single solvent water molecule, the O1H bond length of acoh is increased to 0.985 Å, which is longer than what is observed by applying macroscopic hydration models. The higher stability of this complex of monohydrated cluster is due to two strong hydrogen bonds compared to only one hydrogen bond in the weaker complexes. Structures and relative energies (zero-point energy corrected) of all the conformers obtained in monohydrated clusters of acoh as well as higher hydrated clusters, acoh·nH2O (n = 2−8), are provided in the Supporting Information.

calculations are carried out using the general ab initio quantum chemistry package GAMESS.27

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RESULTS AND DISCUSSION Macroscopic Hydration of Acetic Acid. To determine the effect of macroscopic hydration on acetic acid (acoh), popular implicit solvation models are applied to find optimized geometries. It is observed that bulk hydration of acetic acid adopting macroscopic solvent models like the polarized continuum model (PCM) or solvent density model (SMD) at the ωB97X-D/aug-cc-pVDZ level of electronic structure theory does not change the geometrical parameters significantly. It is worthwhile to mention that in the case of the PCM model, the cavity is formed by the union of spheres centered on each atom. The PCM model takes into account the electrostatic interaction between the solute’s wave function and a dielectric model of the bulk solvent, obtained from a set of surface charges on the finite elements of the cavity.28 In SMD, full solute electron density is allowed to interact with solvent represented as a dielectric medium.29 The O−H bond length of acetic acid is calculated as 0.968 Å by applying the PCM model and as 0.970 Å by applying the SMD technique for macroscopic solvation compared to the gas phase value of 0.967 Å. However, the O−H bond of acetic acid (acoh) is expected to be much longer in water as it is known that acetic acid ionizes in water. Hence for accurate description of acetic acid hydration shell, application of an explicit solvation model following sequential addition of solvent water molecules to acetic acid molecule is necessary. C


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Out of four distinctly different equilibrium structures obtained for the dihydrated acetic acid cluster (acoh·2H2O), the most stable geometry is the one that forms a closed-ring structure with both the water molecules, as shown in Figure 1c. Note that input structures having hydrogen bonding interactions of H2O molecules with −CH3 groups of acetic acid are

not considered at present. On addition of a second solvent water molecule, the O1−H bond of the acid is further elongated to 0.996 Å. The CO1 bond length decreases to 1.321 Å and the CO2 bond increases to 1.222 Å. A hydrogen-bonded closed-ring structure (Figure 1d) is calculated to be energetically most favorable out of seven D


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shown in Figure 1e. These ring structures are formed due to Hbonding interactions between CH3COOH and solvent H2O molecules and interwater H-bonding. O1H, CO1, and CO2 bond lengths in the most stable tetrahydrated cluster are calculated as 1.011, 1.311, and 1.227 Å, respectively. It is to

equilibrium structures obtained in the case of trihydrated cluster. The calculated O1H bond length is seen to be increased marginally. In the case of the tetrahydrated cluster (acoh·4H2O), ten equilibrium geometries are obtained, and the most stable tetrahydrated cluster having a ring structure is E


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Table 1. Comparison of Selected Bond Lengths (Å) and Certain Properties of acoh·nH2O (n = 0−8) and [acoh·nH2O]+ (n = 0− 3) Clusters Calculated at the ωB97X-D/aug-cc-pVDZ Level of Theory bond dipole moment (Debye)b

bond distance (Å)a system

O1H

CO2

acoh acoh·H2O acoh·2H2O acoh·3H2O acoh·4H2O acoh·5H2O acoh·6H2O acoh·7H2O acoh·8H2O

0.967 0.985 0.996 0.997 1.011 1.007 0.999 1.048 1.019

1.207 1.220 1.222 1.221 1.227 1.225 1.225 1.225 1.227

[acoh]+ [acoh·1H2O]+ [acoh·2H2O]+ [acoh·3H2O]+

0.976 1.051 1.403 1.554

1.255 1.266 1.288 1.290

CO1

polarizability (Bohr3)

CH3COOH and Its Hydrated Clusters 1.352 33.8 1.333 43.6 1.321 53.9 1.319 63.9 1.311 73.2 1.314 83.3 1.318 92.3 1.320 100.7 1.317 109.6 [CH3COOH]+ and Its Hydrated Clusters 1.284 30.2 1.265 39.9 1.241 50.0 1.239 60.3

Dipole moment (Debye)

O1H

HO3

1.8 1.4 0.5 1.0 3.3 2.3 0.5 5.0 3.7

1.1 1.2 1.4 1.4 1.5 1.4 1.5 1.4 1.3

0.2 0.4 0.5 0.9 0.7 1.2 0.8 0.7

0.5 1.2 4.2 4.0

0.4 0.5 0.3 0.2

0.2 0.3 0.4

a O1 and O2 refer to the hydroxyl and carbonyl oxygen atoms of the acid molecule. bO3 refers to the oxygen atom of the nearest water molecule; C is the carboxylic carbon atom, and H is the transferable hydrogen atom of the acid molecule.

minimum structure shown in Figure 1h are 1.320 and 1.225 Å, respectively, whereas those in the structure displayed in Figure 3i are 1.293 and 1.239 Å, respectively. An un-ionized cagelike structure consisting of four and five hydrogen-bonded rings is the energetically most favored one out of 24 minimum energy structures obtained for octahydrated cluster, acoh·8H2O. Thirteen equilibrium structures of higher energy are also obtained in this size cluster where the proton of the acid is transferred to the solvent water molecule. In this case, the most stable structure (Figure 4i) out of these ionized structures is 3.3 kcal/mol higher in energy than the most stable un-ionized form (Figure 1i). It is worthwhile to note that in some of these ionized structures, proton is transferred to the second hydration shell of the acetic acid as in the case of hexa- and heptahydrated acetic acid. Overall, in all these cases it is observed that hydrogenbonded closed-ring structures are favored in smaller clusters and cagelike structures are preferred in larger size clusters. Hydrogen bonds are formed between solute acetic acid and solvent water molecules as well as between two water molecules. Carbonyl O atom, hydroxyl H, and O atoms of CH3COOH participate in H-bonding with the solvent H2O molecule. There is a delicate balance in the formation of solute−solvent and solvent−solvent hydrogen bonding in shaping conformational structures of these hydrated clusters of acetic acid. Selected bond lengths of the acid−water clusters along with certain calculated properties are provided in Table 1. With nine water molecules, the number of possible input geometries becomes very large, and determining the global minimum structure becomes very difficult with the present procedure. The possibility of missing out important structures also becomes high with the increase in the number of water molecules, and thus the present study confines us to the interaction of acetic acid up to eight water molecules. Though there is a polarization of the hydroxyl bond of the neutral acid molecule due to hydrogen bonding with water, the formation of the most stable structure having the ion-pair form does not occur even with eight water molecules.

be noted that the growth motif of these hydrated clusters is similar to what is observed in hydrated clusters of HCOOH and CF3COOH. For the pentahydrated cluster of acoh (acoh· 5H2O), the most stable geometry consists of a hydrogenbonded ring structure formed between the acid molecule and three solvent water molecules, as in the case of a trihydrated cluster. Two water molecules having the interwater H-bonded ring remains in the second hydration shell, as displayed in Figure 1f. It is to be noted that the O1H bond length of acetic acid is shortened to 1.007 Å compared to that in the tetrahydrated cluster. For this system, 16 minimum energy structures are predicted having no negative frequency. The most stable conformer of the hexahydrated acetic acid cluster (acoh·6H2O) has a cyclic H-bonded structure and is shown as Figure 1g. It is interesting to point out that five equilibrium structures are obtained in the case of the hexahydrated cluster in which a dissociating proton is transferred to a neighboring solvent water molecule. These structures are displayed in Figure 2 along with selected bond distance parameters and relative stability with respect to the most stable structure shown in Figure 1g. As one can see from the values of relative stabilization energy, all these structures showing dissociation of acetic acid are higher in energy than the most stable one. Calculated bond lengths clearly indicate breaking of the O1H bond and transfer of a proton to the neighboring water molecule. The most stable structure having a charge-separated ion pair is 3.3 kcal/mol higher in energy than the predicted global minimum. Twenty minimum energy structures are also predicted in the case of the heptahydrated cluster, acoh·7H2O; out of which eight structures (Figure 3) show formation of a charge-separated ion pair and hydronium ion, H3O+. The most stable conformer has a cagelike structure (Figure 1h), and it is more stable by 1.3 kcal/mol than the most stable ion-pair structure shown in Figure 3i. Note that in the case of the hexahydrated cluster, this difference of energy is 3.3 kcal/mol. The calculated O1H distance in the most stable structure of a heptahydrated cluster is 1.048 Å and that in structure 3i is 1.348 Å, indicating dissociation of acid. Calculated CO1 and CO3 bond distances in the global F


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Figure 5. Minimum energy equilibrium structures of (a) [acoh]+, (b) [acoh·H2O]+, (c) [acoh·2H2O]+, and (d) [acoh·3H2O]+ and its conformers (i−v) calculated at the ωB97X-D/aug-cc-pVDZ level of theory. The gray, red, and blue balls represent carbon, oxygen, and hydrogen atoms, respectively. ΔE (zero-point energy corrected) values represent relative stability with respect to the most stable structure of [acoh·3H2O]+.

Hydrated Clusters of Mono Positively Charged Acetic Acid. The calculated O−H bond length of [acoh]+ is 0.976 Å, which is slightly longer than the neutral acoh molecule. The predicted equilibrium structure of the monohydrated cluster of unipositively charged acetic acid is little different than that in neutral acoh·1H2O. In the case of neutral acetic acid, the two hydrogen atoms of the water molecule lie in the plane of the COOH group of acoh, whereas for [acoh·1H2O]+, they are perpendicular (Figure 5b). The O−H bond length of the acid molecule in the monohydrate cluster is calculated as 1.051 Å. With two water molecules, the hydroxyl proton of the mono positive charged acetic acid molecule, [acoh·2H2O]+, is transferred to the nearest solvent water molecule, forming a hydrated proton, H3O+ which is stabilized by the second water molecule. The distance between the transferred proton and the

nearest oxygen of acetic acid is 1.403 Å, as shown in Figure 5c. Only one equilibrium structure is obtained in the dihydrated cluster of acoh+ in contrast to four equilibrium structures predicted for the neutral one. Adding a third water molecule to [acoh·2H2O]+, five minimum energy structures having no negative frequency are obtained. The most stable structure (Figure 5d−i) shows ionpair formation with two solvent water molecules attached to the hydronium ion, H3O+ and stabilizing it through the formation of H-bond. The distance between the hydrated proton and the nearest oxygen atom of acetic acid in [acoh·3H2O]+ is 1.554 Å. Thus, unlike the neutral acetic acid molecule, the mono positively charged acetic acid molecule undergoes dissociation in the presence of just two water molecules and transfers its proton to solvent water molecules. The structure of di- and G


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addition of water molecules in the case of the most stable structures of neutral or charged acetic acid. Calculated values of O1−H and H−O3 bond dipole moments are listed in Table 1. Calculated bond dipole moments of the most stable structures of previously studied formic acid and trifluoroacetic acid systems show a dip in O1−H bond dipole moment for the size of the cluster in which the formation of an ion pair occurs. A similar characteristic feature is absent in the case of the most stable structures of hydrated acetic acid clusters as the formation of an ion pair does not take place. The bond dipole moment becomes insignificant when the distance between the atoms connected by the bond becomes large, as in the case of the ionized acid cluster. Figure 7 shows the variation of O1−H

trihydrated clusters of charged acetic acid molecules are different from their neutral analogs; they prefer open chain structures and form contact ion pairs. Selected bond distance parameters of these charged systems is listed in Table 1. Polarizability and Dipole Moment. Intermolecular interactions in a solvated system can be understood by the system’s response to an external electric field, which is given by its electronic polarizability. The isotropic polarizability of a hydrated cluster is calculated as the average of the diagonal elements of the polarizability tensor. Isotropic polarizability (α) of neutral acetic acid molecule is calculated as 33.8 Bohr3 at the present level of theory. A linear increase in the isotropic polarizability with an increase in the size of the acetic acid hydrated clusters is observed as depicted in Figure 6.

Figure 7. Variation of O1−H bond dipole moment (μO1−H) in Debye of microhydrated clusters of acetic acid, trifluoroacetic acid, and formic acid with the number of water molecules (n) calculated at the ωB97XD/aug-cc-pVDZ level of theory for the most stable structures.

Figure 6. Variation of isotropic polarizability of acoh·nH2O, with an increase in the number of water molecules (n) present in the system calculated at the ωB97X-D/aug-cc-pVDZ level of theory for the most stable conformers.

bond dipole moment of acetic acid, formic acid, and trifluoroacetic acid with an increase in the number of water molecules in the corresponding most stable equilibrium structure. In the case of formic acid and trifluoroacetic acid, proton transfer from the acid molecule to solvent water molecules occurs in the presence of seven and six water molecules, respectively.14,15 The plot shows a decrease in the O1−H bond dipole moment corresponding to n = 6 for trifluoroacetic acid and n = 7 for formic acid whereas there is no such significant variation for acetic acid. Energy Parameters. Free Energy. The feasibility of the formation and thermodynamic stability of the microhydrated acoh clusters can be determined from its free energy of formation, ΔG. It is seen that the neutral molecular clusters have positive free energy of formation at room temperature and atmospheric pressure. The free energy of formation at 100 K and very low pressure are also calculated (μTorrΔG100K). As is seen from Table 2, acoh·nH2O clusters (where n is the number of water molecules present in the hydrated cluster) are stable only at very low temperature and pressure (100 K and 1 μTorr). Thus, there is a possibility that such molecular clusters exist in the upper atmosphere, thereby making the studies on acoh·nH2O clusters environmentally relevant. At 100 K and 1 μTorr pressure, it is seen that there is a near linear increase in the −ΔG value of the acoh·nH2O clusters, with an increase in the number of water molecules. The free energy change for acoh·1H2O is −2.7 kcal/mol at 100 K and 1

For neutral monohydrate acetic acid cluster, the isotropic polarizability is 43.6 Bohr3 and that for neutral dihydrate, trihydrate, and tetrahydrate are 53.9, 63.9, and 73.2 Bohr3, respectively, for the most stable conformers. With five, six, seven, and eight water molecules, the isotropic polarizability of the neutral hydrated acetic acid clusters increases to 83.3, 92.3, 100.7, and 109.6 Bohr3, respectively. This linearity relation is best fitted with a regression value of 0.999, as α = 34.6 + 9.5n, where n is the number water molecules present in the cluster. In the case of charged systems also the isotropic polarizability increases linearly with an increase in the number of water molecules, showing no characteristic features indicating ion pair formation. Table 1 gives isotropic polarizability of microhydrated clusters of neutral and charged acetic acid molecule. The dipole moment, μ, of the neutral acetic acid molecule is calculated to be 1.8 D and that for charged acetic acid is 0.5 D. The calculated total dipole moments of the most stable conformers of microhydrated clusters of acetic acid, acoh·nH2O (n = 1−8), are provided in Table 1, but the direction of these vectors are not along the O−H bond. For comparing the polarization of the O−H bonds in each hydrated acid cluster, the O1−H and H−O3 bond dipole moments are determined using Multiwfn, A Multifunctional Wave function Analyzer.32 It calculates bond dipole moment as the sum of products of density matrix and dipole moment integrals between the basis functions involved in the selected bond. No characteristic change in the O1−H bond dipole moment is observed with the H


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The Journal of Physical Chemistry A Table 2. Comparison of Different Energy Parameters of acoh·nH2O and [acoh·nH2O]+ Systemsa energy (kcal/mol) system

μTorrΔG

100K

1atmΔG

298K

EStab

EInt

Neutral Hydrated Clusters of CH3COOH acoh·1H2O −2.7 +1.8 −10.2 −11.1 acoh·2H2O −6.9 +0.3 −22.1 −19.7 acoh·3H2O −8.6 +0.9 −32.0 −21.5 acoh·4H2O −10.9 +2.9 −42.2 −27.4 acoh·5H2O −12.7 +3.0 −52.3 −25.4 acoh·6H2O −14.8 +5.7 −63.4 −23.1 acoh·7H2O −18.9 +6.3 −76.3 −35.5 acoh·8H2O −22.8 +7.2 −89.3 −32.8 Mono Positively Charged Hydrated Clusters of CH3COOH [acoh·1H2O]+ −20.0 −17.2 −27.4 −30.6 [acoh·2H2O]+ −32.0 −27.0 −46.7 −95.0 [acoh·3H2O]+ −43.2 −35.8 −66.7 −134.8

Figure 8. Variation of stabilization and interaction energy of acoh· nH2O, with an increase in the number of water molecules, calculated at the CCSD(T)/ 6-311++G(d,p) and ωB97XD/aug-cc-pVDZ levels of theory. The green and maroon lines represent the calculated stabilization energy applying ωB97XD/aug-cc-pVDZ and CCSD(T)/ 6-311++G(d,p) levels of theory, respectively. The orange and black lines represent the calculated interaction energy applying ωB97XD/ aug-cc-pVDZ and CCSD(T)/ 6-311++G(d,p) levels of theory, respectively.

a

EStab and EInt are the solvent stabilization and interaction energies, respectively, calculated at the CCSD(T)/ 6-311++G(d,p) level of theory. μTorrΔG100K and 1atmΔG298K give the free energy of formation of the system at 1 μTorr pressure, 100 K temperature and 1 atm pressure, 298 K temperature, respectively, calculated at the ωB97X-D/aug-ccpVDZ level of theory.

μTorr pressure. With the addition of another water molecule, the free energy change for dihydrate is −6.9 kcal/mol. At 100 K and 1 μTorr pressure, ΔG for acoh·3H2O, acoh·4H2O, acoh· 5H2O, acoh·6H2O, acoh·7H2O, and acoh·8H2O are, respectively, −8.6, −10.9, −12.7, −14.8, −18.9, and −22.8 kcal/mol. The presence of such clusters determines many atmospheric phenomena such as cloud nucleation, aerosol formation, etc. Hence understanding of intermolecular interactions and formation these hydrated clusters is important. However, microhydrated clusters of the charged acetic acid molecule have negative ΔG values even at room temperature and atmospheric pressure, implying that they are stable under normal conditions. Decreasing the temperature to 100 K and 1 μTorr pressure shows an increase in stability of the microhydrated charged acid clusters, as clearly seen in Table 2. The interactions of the solvent water molecules with solute acid molecule can be qualitatively analyzed by solvent stabilization energy and interaction energy. Solvent Stabilization and Interaction Energy. The solvent stabilization energy of the system is defined as Estab = Eacoh·nH2O − Eacoh − nEw, where Eacoh·nH2O is the energy of the hydrated acid cluster, Eacoh is the energy of the acetic acid molecule, n is the number of water molecules present in the hydrated cluster, and Ew is the energy of a single water molecule. Estab gives the stabilization of the hydrated cluster on account of addition of solvent molecules, which includes both solute−solvent and solvent−solvent interactions. To isolate the solute−solvent interactions from the solvent stabilization energy, excluding any solvent−solvent stabilization, interaction energy, Eint is defined. Eint = Eacoh·nH2O − Eacoh* − Enw, where Eacoh* and Enw are energies of the acid molecule and water cluster at the geometries as they are present in the hydrated acid cluster. For microhydrated clusters of neutral acid molecule, Estab and Eint calculated at the level of CCSD(T)/6-311++G(d,p) follows the same trend as calculated at ωB97X-D/aug-cc-pVDZ level of theory. These calculated energy parameters are plotted in Figure 8 against the size (n) of the hydrated clusters. As is seen

in the figure, Estab varies linearly with n whereas Eint varies irregularly with the addition of water molecules, depending on the geometry of the neutral hydrated acid cluster. Table 2 gives the Estab and Eint of neutral and charged hydrated clusters of acetic acid, calculated at the CCSD(T)/6-311++G(d,p) level of theory. In the case of charged acid cluster, the Estab calculated at the CCSD(T)/6-311++G(d,p) level of theory, for [acoh· 1H2O]+, [acoh·2H2O]+, and [acoh·3H2O]+ are −27.4, −46.7, and −66.7 kcal/mol, respectively, whereas the Eint calculated at the same level of theory are −30.6, −95.0, and −134.8 kcal/ mol, respectively. It should be noted that the solvent stabilization energies of neutral acetic acid clusters are higher than solvent interaction energies. This is justified as the solvent stabilization energy includes solvent−solvent dispersion interactions whereas interaction energy accounts only for the net dispersion interaction between solute and solvent clusters. In the case of hydrated clusters of charged acetic acid, the interaction between solute and solvent is electrostatic in nature and very strong compared to weak van der Waals interactions between solvent molecules. This is reflected in calculated values of solvent stabilization energy and interaction energy in [acoh· nH2O]+ systems. Spectroscopic Features. Microwave Spectra. The microwave and infrared spectral property of neural and monopositively charged microhydrated acetic acid systems are calculated. Calculated rotational constants A, B, and C and IR frequency of free mono- and dihydrated clusters at the ωB97XD/aug-cc-pVDZ level of theory along with the available experimental data are supplied in Table 3. It is noted that for acoh, the present theoretical values of rotational constants A, B, and C differ from the experimental values by +45, +17, and +14 MHz, respectively.33 For the monohydrated cluster of acetic acid, acoh·1H2O, the calculated theoretical rotational constants A, B, and C are 11064, 2639, and 2169 MHz and these are higher than the reported experimental values by 4, 55, 42, and 84 MHz, respectively. In the case of the dihydrated cluster of acetic acid, calculated theoretical values of rotational constants I


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Table 3. Comparison of Rotational Constants and Selected IR Stretching Frequencies for the Most Stable Structures of acoh, acoh·nH2O, [acoh]+, and [acoh·nH2O]+ Systems Calculated at the ωB97X-D/aug-cc-pVDZ Level of Theory and the Available Experimental Dataa IR freq (cm−1)

rotational constants (MHz) A

system

a

acoh acoh·1H2O acoh·2H2O acoh·3H2O acoh·4H2O acoh·5H2O acoh·6H2O acoh·7H2O acoh·8H2O

11380 (11335)22 11064 (11060)11 4529 (4445)11 2707 1830 1442 808 729 522

[acoh]+ [acoh·1H2O]+ [acoh·2H2O]+ [acoh·3H2O]+

11774 11532 6761 2284

B

C

νOH

Neutral Hydrated Clusters of CH3COOH 9462 (9479)22 5339 (5325)22 3549 (3566)34 2639 (2584)11 2169 (2127)11 3222 (3208)22 1646 (1618)11 1223 (1200)11 3014 (2992)22 996 736 3002 738 722 2745 503 427 2815 515 439 2962 546 463 2168 490 459 2619 Mono Positively Charged Hydrated Clusters of CH3COOH 9370 5401 3450 2390 2025 2174 974 920 241 806 619 211

ΔνOH

νCO

327 535 547 804 734 587 1381 930

1718 (1779)34 1679 (1732)22 1661 (1714)22 1665 1638 1638 1650 1631 1641

1276 3209 3239

1470 1486 1448 1476

Experimental values are provided in parentheses.

shifted compared to that of the free acid by 734 cm−1, is slightly blue-shifted relative to that for acoh·4H2O (ΔωOH = 804 cm−1). Similarly, acoh·6H2O also has a slight blue shift compared to acoh·5H2O but is red-shifted by 587 cm−1 compared to free acoh. The cagelike neutral heptahydrate has a largely red-shifted ωOH of 2168 cm−1. However, the calculated ωOH of acoh·8H2O is 2619 cm−1, showing that the OH bond is not as weakened as in the case of the heptahydrate. The IR stretching frequencies of the O−H bond in the −COOH group of acetic acid in the most stable structures of all the studied hydrated clusters are given in Table 3. There is no characteristic change observed in the scaled CO stretching frequencies (ωCO) of hydrated acetic acid clusters. Simulated scaled IR spectra of acetic acid (acoh) and its hydrated clusters, acoh· nH2O (n = 1−8), are provided in the Supporting Information. In the case of mono positively charged acetic acid water clusters, as experimental values are not available, the same scaling factor is considered. The ωOH values of hydrated clusters of charged acetic acid are given in Table 3, along with the shift in O−H frequency of the hydrated clusters with respect to the O−H stretching frequency of charged acetic acid molecule (ΔωOH). For free mono positive acetic acid molecule, the ωOH is 3450 cm−1. With the addition of one water molecule, ΔωOH is −1276 cm−1, indicating the weakening of the OH bond. In the presence of two water molecules, the proton transfer occurs and there is a large red shift of the ωOH by 3209 cm−1. For the trihydrate of charged acetic acid also the red shift is substantial (3239 cm−1). Such a trend is absent in the case of the carbonyl stretching frequency.

differ from the experimental values by +84, +28, and +23 MHz, respectively.21 Overall, the calculated rotational constants at the present level of theory are in good agreement with the available experimental values with the highest deviation of 1.9%. IR Spectra. The addition of water molecules to acoh weakens the hydroxyl bond due to hydrogen bonding between the hydroxyl hydrogen atom of acoh and the oxygen atom of water molecule. This manifests as a red shift in its IR spectrum. Thus, simulating IR spectra can serve to keep track of the polarization occurring to the O−H bond. It can also help in determining the stage of proton transfer from the solute molecule to solvent water molecules, as the presence of hydrated proton produces new peaks in the IR spectrum. Comparing the simulated IR spectra with that of gas phase IR spectra of acoh·nH2O clusters can help in verifying the structures predicted. IR spectrum simulated is based on harmonic approximation. To account for the anharmonic nature of the chemical bonds, a scaling factor needs be incorporated to the calculated spectrum. In the case of free neutral acoh, the experimentally reported O−H stretching frequency (νOH) is 3566 cm−1 whereas the calculated value is 3829 cm−1.34 Computation of the anharmonic frequency of a system is computationally demanding at the ab initio level of theory with atomic basis functions. The vibrational mode of interest in the present systems is mainly the O−H stretching; thus it may be scaled appropriately. The scaling factor is obtained by taking the ratio of available experimental to the calculated values of the O−H stretching frequency. The average scaling factor for the O−H stretching frequency, considering acoh, acoh·1H2O, and acoh·2H2O is 0.93. The scaled O−H stretching (ωOH) in free neutral acid is thus 3549 cm−1. With the addition of a water molecule to neutral acetic acid, the OH stretching frequency of the −COOH group, ωOH in acoh·1H2O, is observed to be red-shifted to 3222 cm−1 from 3549 cm−1 in free acetic acid. Similarly, for the dihydrated cluster, the OH stretching frequency of acetic acid is calculated to be red-shifted to 3014 cm−1 compared to the available experimental value of 2992 cm−1. In the case of acoh· 3H2O and acoh·4H2O clusters, the calculated ωOH is 3002 and 2745 cm−1, respectively. The O−H stretching frequency of acetic acid in the neutral pentahydrated cluster, though red-

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CONCLUSIONS Available macroscopic solvation models cannot fully describe acid−water systems that are important in many aspects ranging from biological to atmospheric applications. With explicit addition of a single water molecule, the O−H bond length of the acid molecule is calculated as 0.985 Å, whereas the solvent density model produces the O−H bond length as 0.970 Å. It is observed that even with eight water molecules there is no transfer of proton from the neutral acetic acid molecule to the solvent water molecules. However, in the case of mono J


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(4) Weber, J. M.; Kelley, J. A.; Nielson, S. B.; Ayotte, P.; Johnson, M. A. Isolating the Spectroscopic Signature of a Hydration Shell With the Use of Clusters: Superoxide Tetrahydrate. Science 2000, 287, 2461− 2463. (5) Myshakin, E. M.; Jordan, K. D.; Robertson, W. H.; Weddlle, G. H.; Johnson, M. A. Dominant Structural Motifs of NO−·(H2O)n Complexes: Infrared Spectroscopic and Ab Initio Studies. J. Chem. Phys. 2003, 118, 4945−4953. (6) Pathak, A. K.; Mukherjee, T.; Maity, D. K. Microhydration of NO3−: A Theoretical Study on Structure, Stability and IR Spectra. J. Phys. Chem. A 2008, 112, 3399−3408. (7) Pathak, A. K.; Mukherjee, T.; Maity, D. K. Distinctive IR Signature of CO3− and CO3−2: A Theoretical Study. J. Phys. Chem. A 2009, 113, 13443−13447. (8) Re, S.; Osamura, Y.; Morokuma, K. Coexistence of Neutral and Ion-Pair Clusters of Hydrated Sulphuric Acid H2SO4(H2O)n (n = 1− 5): A Molecular Orbital Study. J. Phys. Chem. A 1999, 103, 3535− 3547. (9) Aloisio, S.; Hintze, P. E.; Vaida, V. The Hydration of Formic Acid. J. Phys. Chem. A 2002, 106, 363−370. (10) Zhou, Z.; Shi, Y.; Zhou, X. Theoretical Studies on the Hydrogen Bonding Interaction of Complexes of Formic Acid with Water. J. Phys. Chem. A 2004, 108, 813−822. (11) Scott, J. R.; Wright, J.B. Computational Investigation of the Solvation of Nitric Acid: Formation of the NO3− and H3O+ Ion Pair. J. Phys. Chem. A 2004, 108, 10578−10585. (12) Weber, K. H.; Tao, F. M. Ionic Dissociation of Perchloric Acid in Microsolvated Clusters. J. Phys. Chem. A 2001, 105, 1208−1213. (13) Leopold, K. R. Hydrated Acid Clusters. Annu. Rev. Phys. Chem. 2011, 62, 327−349. (14) Maity, D. K. How Much Water Is Needed to Ionize Formic Acid? J. Phys. Chem. A 2013, 117, 8660−8670. (15) Krishnakumar, P.; Maity, D. K. Effect of Microhydration on Dissociation of Trifluoroacetic Acid. J. Phys. Chem. A 2014, 118, 5443−5453. (16) Gonzalez Abad, G.; Bernath, P. F.; Boone, C. D.; McLeod, S. D.; Manney, G. L.; Toon, G. C. Global distribution of upper tropospheric formic acid from the ACE-FTS. Atmos. Chem. Phys. 2009, 9, 8039− 8047. (17) Yu, S. Role of Organic Acids (Formic, Acetic, Pyruvic and Oxalic) in the Formation of Cloud Condensation Nuclei (CCN): A Review. Atmos. Res. 2000, 53, 185−217. (18) Tsai, Y. I.; Kuo, S.-C. Contributions Of Low Molecular Weight Carboxylic Acids To Aerosols And Wet Deposition In A Natural Subtropical Broad-Leaved Forest Environment. Atmos. Environ. 2013, 81, 270−279. (19) D'Amico, F.; Bencivenga, F.; Gessini, A.; Principi, E.; Cucini, R.; Masciovecchio, C. Investigation of Acetic Acid Hydration Shell Formation through Raman Spectra Line-Shape Analysis. J. Phys. Chem. B 2012, 116, 13219−13227. (20) Pu, L.; Sun, Y.; Zhang, Z. Hydrogen Bonding in Hydrates with One Acetic Acid Molecule. J. Phys. Chem. A 2010, 114, 10842−10849. (21) Ouyang, B.; Howard, B. J. The Monohydrate and Dihydrate of Acetic Acid: A High-Resolution Microwave Spectroscopic Study. Phys. Chem. Chem. Phys. 2009, 11, 366−373. (22) Haupa, K.; Bil, A.; Barnes, A.; Mielke, Z. Isomers of the Acetic Acid-Water Complex Trapped in an Argon Matrix. J. Phys. Chem. A 2015, 119, 2522−2531. (23) Lopes, S.; Fausto, R.; Khriachtchev, L. Acetic Acid−water Complex: The First Observation of Structures Containing the HigherEnergy Acetic Acid Conformer. J. Chem. Phys. 2016, 144, 084308. (24) Gao, Q.; Leung, K. T. Hydrogen-Bonding Interactions in Acetic Acid Monohydrates and Dihydrates by Density-Functional Theory Calculations. J. Chem. Phys. 2005, 123, 074325. (25) Fedotova, M. V.; Kruchinin, S. E. Hydration of Acetic Acid and Acetate Ion in Water Studied by 1D-RISM Theory. J. Mol. Liq. 2011, 164, 201−206.

positively charged acetic acid, just two water molecules are able to ionize acetic acid and form a contact ion pair. In the case of hexa-, hepta-, and octahydrated clusters of neutral acetic acid, many high-energy structures having a contact/separated ion pair are predicted. Free energy calculations show that the microhydrated clusters of neutral acetic acid molecules are not favored at room temperature and atmospheric pressure. They are stable only at very low temperature and pressure (100 K and 1 μTorr). However, the free energy formation of hydrated clusters of the charged acetic acid molecule is negative even at room temperature and atmospheric pressure. The solvent stabilization energy of acetic acid increases linearly with the increase in the number of water molecules, showing no indication of ion-pair formation or acid dissociation. No characteristic feature is observed in the plot of interaction energy vs the number of water molecules present in the cluster. Polarizability, net dipole moment, and bond dipole moment of neutral and charged acetic acid−water clusters are calculated. The bond dipole moment of the dissociating O−H bond does show a characteristic feature of acid dissociation in case of formic and trifluoroacetic acids. To compare with experimental data, microwave and IR spectral properties of the acid hydrates are simulated. A strong correlation is found between the reported values and the present calculated data. On the basis of this study, we conclude that the nature of interaction of neutral and charged acetic acid molecules with water molecules differs greatly. Although even eight water molecules are not sufficient to bring about proton transfer in neutral acetic acid, for mono positive acetic acid just two would suffice.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b09257. Equilibrium structures, relative energies (zero-point energy corrected), and simulated scaled IR spectra of acetic acid and its hydrated cluster, CH3COOH·nH2O (n = 1−8) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*D. K. Maity. E-mail: [email protected]. ORCID

Dilip Kumar Maity: 0000-0003-4284-3578 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank BARC computer centre. P.K. thanks Homi Bhabha National Institute for a research fellowship. REFERENCES

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Microhydration of Neutral and Charged Acetic Acid - The Journal of

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