Microhydration of 2-Naphthol at Ground, First Excited Triplet, and First

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Microhydration of 2-Naphthol at Ground, First Excited Triplet and First Excited Singlet States: A Case Study on Photo Acids Parvathi Krishnakumar, Rahul Kar, and Dilip Kumar Maity J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09579 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

Microhydration of 2-Naphthol at Ground, First Excited Triplet and First Excited Singlet States: A Case Study on Photo Acids

Parvathi Krishnakumara,b, Rahul Karc and Dilip Kumar Maity*a,b

a

Homi Bhabha National Institute, Training School Complex, Anushaktinagar,

Mumbai-400094, India b

Bhabha Atomic Research Centre,

Mumbai-400085, India c

Department of Chemistry, Dibrugarh University,

Dibrugarh-786004, Assam, India

*Email for correspondence: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Molecular interactions of 2-naphthol (nap) with water molecules are studied at the ground, first excited triplet and first excited singlet states, applying DFT and TD-DFT methods. The minimum energy structure of hydrated clusters of 2-naphthol up to four water molecules are selected from several possible input geometries. It is observed that the minimum energy conformer of the tetra-hydrate of 2-naphthol has proton transfer occurring from nap to solvent water molecules, in its first excited singlet state. This is however not observed in case of its ground or first excited triplet state. It is consistent with the fact that the pKa of nap in the first excited singlet state is very much lower compared to the ground and first excited triplet state. This is also reflected in the O-H potential energy profile of tetra-hydrate of nap, obtained by performing a rigid potential energy scan of the dissociating O-H bond of nap at ground, first excited triplet and first excited singlet states. Frequency of O-H stretching vibration of 2-napthol and its hydrated clusters in the ground (S0) as well as in the first excited singlet (S1) state are calculated and compared with the available experimental data. The performance of macroscopic solvation model is also examined in the ground and these excited states.


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1.

Introduction Studies on microhydration of a Brønsted acid can provide better insight into

hydration process and proton transfer phenomena, which are fundamental in many chemical, biological and environmental processes. The study may be performed considering a solute acid molecule surrounded by a small number of solvent water molecules. Electronic structure calculations of such a hydrated cluster of an acid molecule can help to have molecular level understanding about the hydration process. Microhydration of acids can also help to link between hydration and acidity of the systems. There have been a few reports on the microhydration of carboxylic acids leading to dissociation in the ground state.1–4 A correlation has been reported between the pKa of the acids and the number of water molecules needed to dissociate the acid, in the ground state.2 Photo acids are molecules which have marked differences in their pKa values, at ground and certain excited states. Studies on microhydration of a photo acid may provide basic understanding to have large differences in acid constants in ground and its excited states. Ground and excited state electronic structure and the nature of electronic transitions involved in excited states of these photo acids can explain the differences in pKa values and in reactivity of these molecules in ground and excited states.5–8 Photo acids undergo significant enhancement in their acidity upon electronic excitation. This leads to excited state proton transfer to the solvent. The change in electron density distribution of a photo acid can be determined from its excited state acidity constant. The significant change in acidity of these molecules makes them an ideal system to probe solute-solvent interactions and solvent polarities.9 Photo acids can be used as molecular probes for determining the structural transitions of proteins under various conditions.10 Water accessibility in biological surfaces can also be probed using photo acids.11,12 The excited state pKa of a photo acid is generally determined from the 3 ACS Paragon Plus Environment


Fӧrster cycle, based on absorption and emission data.13 It can also be determined using photo-potentiometry.14 In aqueous solution, aromatic compounds like phenol and 2-naphthol have ground state pKa values of 10.0 and 9.2 respectively, while their excited state pKa values are just 3.6 and 2.8 respectively.15,16 Theoretical calculations to explain excited state proton transfer in phenols has been reported.17 Two different explanations have been proposed for the increase in acidity in the first excited singlet state. One proposition attributes the increase in acidity to a charge transfer in the photo acid.18,19 According to the other proposition, the enhanced excited state acidity is largely determined by the deprotonated photo acid.20 It is reported that the acidity constants of ground and first excited triplet states of 2-naphthol are comparable while that of the first excited singlet state differ by a factor of ~106.21 The studies were based on flash photolysis, fluorescence and phosphorescence measurements. They provide a qualitative explanation for this observation based on electron density. The difference in the pKa value of ground state and excited states of 2naphthol could be reflected in the number (n) of water molecules needed for their ionization. Present study explores the microhydration of 2-naphthol (nap) at ground (napS0), first excited triplet (napT1) and first excited singlet (napS1) states, to understand the correlation between n and pKa of 2-naphthol, at different electronic states. There have been a few studies on the complex excited state dynamic of 2-naphthol and its clusters with water.22–26 O-H stretching vibrations of 2-naphthol-H2O clusters in the S0 state have been studied previously applying IR-UV double resonance spectroscopy.27 Investigations have also been reported on hydrogen bonded clusters of 2-naphthol with water, methanol and ammonia using IR and IR-UV double resonance spectroscopy.28,29 Frequency of OH stretching vibration of nap and 1:1 H2O/CH3OH/NH3 cluster of nap in the ground (S0)


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and first excited singlet (S1) states have been reported. To the best of our knowledge a systematic theoretical study on the microhydration of 2-naphthol at different electronically excited states have not been reported so far. Molecular interactions of 2naphthol (nap) with finite number of solvent water molecules are studied at the ground, first excited triplet and first excited singlet states, based on first principle electronic structure methods. Present study also compares the theoretical O-H stretching frequency of 2-naphthol and its 1:1 water cluster in the ground (S0) and first excited singlet (S1) states with the reported experimental values by Mikami and co-workers. An implicit solvation model is also applied to examine the effect of macroscopic hydration on nap. 2.

Theoretical Methods The ground state geometries are optimized at ωB97X-D/aug-cc-pVDZ level of

theory. The first excited singlet and first excited triplet states are optimized at CAMB3LYP/aug-cc-pVDZ level of theory, with their input geometries based on ground state geometries. CAM-B3LYP is a long-range corrected hybrid exchange-correlation functional designed to better describe electronic excitations and charge-transfer processes.20,30–32 The excited state calculations are done using Time-Dependent DFT (TD-DFT). The initial input geometries for ground state are generated applying Atom Centered Density Matrix Propagation (ADMP) molecular dynamics model, at B3LYP/631G(d) level of theory. The simulations are allowed to run for 2000 steps. It must be noted that for the initial guess structures for MD simulation, the water molecules are placed in such a way so as to increase the number of hydrogen bonds formed between the solvent water molecules and nap, facilitating the easy dissociation of the O-H bond of nap. The minimum energy geometry and all geometries with relative energy 5.0 kcal/mol or less, with respect to minimum energy geometry, are selected and optimized at ωB97X-D/aug-cc-pVDZ level of theory. The final minimum energy equilibrium 5 ACS Paragon Plus Environment


structure of each size cluster at ground, first excited triplet and first excited singlet state is selected to determine the minimum number of water molecules needed to dissociate nap. A rigid potential energy scan of the most stable conformer of each size cluster, at S0, T1 and S1 states, is carried out by keeping all but the dissociating O-H bond of nap constant. The potential energy profile of the hydrated nap system is obtained by plotting the relative energy, with respect to most stable structure, at each O-H bond distance against the O-H bond distance. All calculations are carried out using the general ab initio quantum package, Gaussian 16.33 3.

Results and Discussion

3.1

Microhydration at ground state At ground state, two conformers are possible for napS0. Structures of both the

conformers, along with their relative energy, are given in Figure 1. The two structures differ in the relative position of the O-H bond with respect to the naphthalene ring. In the most stable conformer (see Figure 1 (i)), the O-H bond faces the ring while in the higher energy conformer, the O-H bond faces away from the ring. This is consistent with the results obtained from MP2/aug-cc-pVDZ level of theory reported recently.34 The O-H bond length is 0.961 Å and the C-O bond length is 1.364 Å in the most stable conformer at the present level of theory.

Figure 1 Equilibrium structures of 2-naphthol, at ground state, calculated at ωB97X-D/aug-ccpVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.


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Calculated harmonic O-H stretching vibration frequency at ωB97X-D/aug-cc-pVDZ level for cis- and trans-2-napthol (see Fig. 1) is 3899 and 3907 cm-1 respectively. Based on fluorescence detected IR spectra of cis- and trans-2-napthol, the values are 3654 and 3661 cm-1 respectively. Thus, the scaling factor of O-H stretching vibration frequency of 2-napthol is considered as 0.937 for its water clusters to account anharmonicity of the stretching mode.

TABLE 1: Comparison of calculated and available experimental frequencies (cm-1) of OH stretching vibrartions of 2-napthol.nH2O (n=0-4) clusters in the ground (S0) and the first excited state (S1) states. Ground and excited state values are calculated at ωB97XD/aug-cc-pVDZ and CAM-B3LYP-D/aug-cc-pVDZ level of theory.

S0 System

a

Calculated Shift (∆ν)

S1 b

Expt

c

Calculated Shift (∆ν)

d

Expt

n=0

3654

0

3654

3609

0

3609

n=1

3453

201

3512

3330

279

3408

n=2

3395

259

3376

3143

466

-

n=3

3196

458

3226

2942

667

-

n=4

3129

525

-

297

3322

-

a

Each frequency is scaled by a factor of 0.937 based on calculated frequency of cisconformer in S0 state. b Reference 27. c Each frequency is scaled by a factor of 0.934 based on calculated frequency of cisconformer in S1 state. d Reference 28.



With the addition of a single water molecule, the most stable conformer has the H atom of the nap forming hydrogen bond with the O atom of the water molecule (see Figure 2 (i)). The O-H bond length of the naphthol molecule increases to 0.972 Å and the C-O bond length decreases to 1.356 Å. the hydrogen bond distance between the acidic hydrogen atom of nap and the O atom of the water molecule is calculated to be 1.853 Å. Another structure is also observed for the mono-hydrate of nap (napS0.1H2O), which is higher in energy than the most stable conformer by just 0.3 kcal/mol, as shown in Figure 2 (ii). The difference between this conformer and the most stable conformer is in the relative position of the O-H bond of the nap with respect to the naphthalene ring.

Figure 2 Equilibrium structures of mono-hydrate of 2-naphthol, at ground state, calculated at ωB97X-D/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Calculated scaled O-H stretching vibration frequency of 2-napthol in its 1:1 water cluster is 3453 cm-1 in the napthol site and in the H2O site the values are 3734 (ν3) and 3632 (ν1). Thus, a red shift of 201 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol in its 1:1 water cluster compared to the experimental value of 142 cm-1. Present calculated frequency values are compared with the available experimental data along with higher clusters in S0 state and the first excited singlet state results in Table 1.

With two water molecules, the most stable conformer has the O-H bond of nap hydrogen bonded to the oxygen atom of one of the water molecules. The other water molecule is


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hydrogen bonded to the first water molecule, without direct bonding with the nap. A higher energy equilibrium geometry obtained for the di-hydrate of nap (napS0.2H2O) along with its zero point corrected relative energy is given in Figure 3. Unlike as observed in carboxylic acids, a closed ring motif is absent in case of the di-hydrates, possibly due to the steric strain involved in such a ring formation.2 The O-H and C-O bond lengths of nap in the most stable conformer are 0.994 and 1.326 Å respectively.

Figure 3 Equilibrium structures of di-hydrate of 2-naphthol, at ground state, calculated at ωB97X-D/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Scaled O-H stretching vibration frequency of 2-napthol in its 1:2 water cluster is 3395 cm-1 in the napthol site and in the H2O site the values are 3691 (ν3) and 3537 (ν1). Thus, a red shift of 259 cm-1 is calculated for the O-H stretching vibration frequency in 2napthol in its 1:2 water cluster compared to the experimental value of 276 cm-1. Six equilibrium structures are obtained for the tri-hydrate of nap (napS0.3H2O). The most stable conformer has the three water molecules and the O-H bond of the nap forming hydrogen bonded close-ring motif. Open chain structures are found to be higher in energy than the closed ring conformers. The O-H and C-O bond length of nap are 0.985 and 1.362 Å respectively, for the most stable conformer of napS0.3H2O, while the hydrogen bond distance between the acidic H of nap and the O atom of the nearest water



molecule is 1.627 Å. Two of the most stable conformers of napS0.3H2O are given in Figure 4.

Figure 4 Equilibrium structures of tri-hydrate of 2-naphthol, at ground state, calculated at ωB97X-D/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Calculated scaled O-H stretching vibration frequency in S0 state of 2-napthol in its 1:3 water cluster is 3196 cm-1 in the napthol site and in the H2O site the values are in the range of 3702 to 3368 cm-1. A red shift of 458 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol in its 1:3 water cluster compared to the experimental value of 428 cm-1. A closed ring conformer is the most stable geometry obtained for the tetra-hydrate of nap (napS0.4H2O), as given in Figure 5. The O-H and C-O bond lengths of nap in the most stable conformer are 0.987 and 1.360 Å respectively. It is seen that the conformer does not show proton transfer from nap to the solvent water molecules at the ground state.

Figure 5 Equilibrium structures of tetra-hydrate of 2-naphthol, at ground state, calculated at ωB97X-D/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also


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given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Calculated scaled O-H stretching vibration frequency in S0 state of 2-napthol in its 1:4 water cluster is 3129 cm-1 in the napthol site. Thus, a red shift of 525 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol in its 1:4 water cluster. 3.2

Microhydration at first excited triplet state Of the two equilibrium conformers obtained for nap at the first excited triplet

state (T1), the most stable conformer has the O-H bond facing away from the naphthalene ring. The other conformer, with O-H bond facing towards the ring, is higher in energy than the most stable conformer by 16.8 kcal/mol. The O-H and C-O bond lengths of the most stable conformer are 0.961 and 1.364 Å respectively. Both the conformers are shown in Figure 6.

Figure 6 Equilibrium structures of 2-naphthol, at first excited triplet state, calculated at CAMB3LYP/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

With the addition of a single water molecule, the most stable conformation at first excited triplet state has the O-H bond facing away from the ring and the O atom of the water molecule forming hydrogen bond with H atom of nap (see Figure 7(i)). The O-H and C-O bond length of nap are respectively, 0.975 and 1.358 Å. The hydrogen bond distance between the hydroxyl H atom of nap and O atom of water molecule is 1.840 Å. Another conformer of napT1.2H2O is also obtained which is higher in energy than the the minimum energy conformer by 16.3 kcal/mol. It differs from the minimum energy 11 ACS Paragon Plus Environment


conformer by the orientation of the O-H group relative to the naphthalene ring (see Figure 7 (ii)).

Figure 7 Equilibrium structures of 2-naphthol.1H2O, at first excited triplet state, calculated at CAM-B3LYP/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

The most stable conformation of the first excited triplet state of di-hydrate of nap (napT1.2H2O) has an open chain conformation with the O-H bond facing way from the naphthalene ring as shown in Figure 8 (i). The O-H and C-O bond lengths are 0.979 and 1.353 Å respectively. Higher energy conformation of napT1.2H2O is also given as Figure 8 (ii).


For the tri-hydrate of nap (napT1.3H2O), the most stable conformation in the first excited triplet state also has an open chain motif, with one water molecule forming direct 12 ACS Paragon Plus Environment

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hydrogen bond with nap and the other two water molecules forming hydrogen bonds with the first water molecule. This conformation is more stable than the next stable conformation by 9.6 kcal/mol, which has closed ring geometry (see Figure 9). The O-H and C-O bond lengths of nap in the tri-hydrate are 0.994 and 1.354 Å respectively.


It is observed that even with four water molecules, proton transfer does not take place in case of first excited triplet state of tetra-hydrate of nap (napT1.4H2O). The most stable conformation has a closed ring conformation which is more stable than the other conformations obtained, by more than 19.2 kcal/mol (see Figure 10). The O-H and C-O bond length of nap are 0.993 and 1.370 Å respectively.




3.3

Microhydration at first excited singlet state The most stable conformer of nap, at the first excited singlet state (S1), has the O-

H bond facing away from the naphthalene ring. The O-H and C-O bond lengths of nap are 0.966 and 1.346 Å respectively. This structure is more stable than the structure in which the O-H bond is facing towards the naphthalene ring, by 0.7 kcal/mol. Both the structures are given in Figure 11.

Figure 11 Equilibrium structures of 2-naphthol, at first excited singlet state, calculated at CAMB3LYP/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Calculated harmonic O-H stretching vibration frequency at CAM-B3LYP/aug-cc-pVDZ level for cis--napthol (see Fig. 11(i)) in its first excited singlet state (S1) is 3827 cm-1. Based on photofragment detected IR spectroscopy, the value is reported as 3609 cm-1. Thus, the scaling factor of O-H stretching vibration frequency of 2-napthol is considered as 0.943 for its water clusters to account anharmonic effect.

Three equilibrium conformers are obtained for the first excited singlet state of monohydrate of 2-naphthol (napS1.1H2O). The most stable conformation of napS1.1H2O, at first excited singlet state is shown in Figure 12 (i). The O atom of the water molecule forms hydrogen bonding with the hydrogen atom of the nap. The O-H and C-O bond lengths of 14 ACS Paragon Plus Environment

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nap are 0.984 and 1.323 Å respectively. Another conformer is also obtained for napS1.1H2O which is higher in energy than the minimum energy conformer by 31.6 kcal/mol (see Figure 12 (ii)).

Figure 12 Equilibrium structures of 2-naphthol.1H2O, at first excited singlet state, calculated at CAM-B3LYP/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Calculated scaled O-H stretching vibration frequency of 2-napthol in its 1:1 water cluster in S1 state is 3330 cm-1 in the napthol site and in the H2O site the values are 3711 (ν3) and 3611 (ν1). Thus, a red shift of 279 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol in its 1:1 water cluster compared to the experimental value of 201 cm-1. It is clearly visible from Table 1 that the effect of microhydration on 2-napthol is larger in the S1 state than in S0 state as depicted by ∆ν values in the two state. With two water molecules, three conformers are observed for the first excited singlet state of di-hydrate of nap (napS1.2H2O). The most stable conformer has the O-H bond of nap facing away from the ring and forming open chain hydrogen bonds with the water molecules (see Figure 13(i)). The O-H and C-O bond lengths of nap are 0.994 and 1.326 Å respectively. Another conformer, similar to the first conformer except in the relative orientation of the O-H bond with respect to the naphthalene ring, is also shown in Figure 13.




Calculated scaled O-H stretching vibration frequency of 2-napthol in its 1:2 water cluster in S1 state is 3143 cm-1 in the napthol site and in the H2O site the values are in the range of 3678 to 3387 cm-1. A red shift of 466 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol of this water cluster compared to free 2-napthol . The most stable conformation at the first excited singlet state of tri-hydrate of nap (napS1.3H2O) has hydrogen bonded closed ring conformation (see Figure 14 (i)). The OH and C-O bond length of nap are 1.001 and 1.341 Å respectively. A slightly higher energy structure is also obtained, differing only in the relative position of the O-H bond of nap (see Figure 14 (ii)). It must be noted that closed ring conformation are more stable than the open chain ones.

Figure 14 Equilibrium structures of 2-naphthol.3H2O, at first excited singlet state, calculated at CAM-B3LYP/aug-cc-pVDZ level of theory. Zero point energy corrected relative energy (in kcal/mol) of the higher energy conformer with respect to minimum energy conformer is also


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given. Red balls represent oxygen, grey balls represent carbon and blue balls represent hydrogen atoms.

Calculated scaled O-H stretching vibration frequency of 2-napthol in its 1:3 water cluster in S1 state is 2942 cm-1 in the napthol site and in the H2O site the values are in the range of 3681 to 3175 cm-1. A red shift of 667 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol in its 1:3 water cluster compared to free 2-napthol . Interestingly, the most stable conformer of tetra-hydrate of nap (napS1.4H2O), at the first excited singlet state, shows proton transfer from nap to the neighboring water molecule. The Mulliken charge distribution of this conformer gives a charge of -1.071 a.u. on oxygen atom of naphthol and +0.891 a.u. on the hydronium ion as a whole, indicating that the proton transfer has taken place from the naphthol to solvent water molecule. This conformer is more stable than the next stable conformer by 1.0 kcal/mol (see Figure 15). The most stable conformer has an open chain conformation with two water molecules stabilizing the first water molecule through hydrogen bonding and the H atom of the fourth water molecule forming hydrogen bond with the oxygen atom of nap. The O..H hydrogen bond distance between O atom of nap and nearest H atom of the hydrated proton is 1.598 Å.




Calculated scaled O-H stretching vibration frequency values in the H2O site of tetrahydrated 2-napthol are in the range of 3691 to 3127 cm-1. A large red shift of 3322 cm-1 is calculated for the O-H stretching vibration frequency in 2-napthol in this 1:4 water cluster and a O-H symmetric stretching vibration frequency of 1699 cm-1 is calculated for H3O+ site of the cluster. This clearly indicates the dissociation of O-H bond in 2napthol in the S1 state of tetra-hydrated cluster. Calculated ∆ν values in S0 state indicate a smaller effect of microhydration on 2-napthol and as a result even four solvent water molecules do not lead to dissociation of O-H bond. 3.4

Effect of macroscopic solvation on nap At this point it would be interesting to examine the effect of macroscopic

solvation on the nap molecule at S0, T1 and S1 states. Popular solvation model, SMD35, is applied to obtain the optimized geometry of nap at S0, T1 and S1 states, at the same level of theory as is used in the gas phase. In this implicit solvation model, the solute electron density is allowed to interact with the solvent, which is represented as a dielectric continuum. It is observed that the application of SMD model does not change the geometric parameters significantly. The dissociation of nap does not occur in S0, T1 or S1 states. The O-H bond distance of nap in S0, T1 and S1 states, applying SMD scheme, are 0.965, 0.968 and 0.970 Å respectively. However, even with the addition of a single water molecule, the monohydrated nap has O-H bond length of 0.972, 0.975 and 0.965 Å respectively at S0, T1 and S1 states. This implies that the macroscopic model cannot be used to accurately describe solute-solvent interactions, especially when non-covalent interactions such as hydrogen boding are involved. Explicit solvation model must be considered at least up to the first solvation shell to obtain a better description of such systems.


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3.5

O-H Potential Energy Profile of nap The dissociating O-H bond potential energy profile of the hydrated clusters of

nap gives the barrier for proton transfer from nap to the neighboring solvent water molecule. The potential energy profiles for the dissociating O-H bond of tetra-hydrate of nap, at the ground, first excited triplet and first excited singlet state are given in Figure 16. The equilibrium bond length of O-H in nap is ~1 Å for mono, di, tri and tetrahydrates of nap in the ground and first excited triplet state. In these cases, it is seen that when the O-H bond distance is increased, the energy increases and the system becomes less stable. The potential energy profile of the dissociating O-H bond of nap at the first excited singlet state of mono, di and tri-hydrates also show a minimum at O-H bond distance ~1 Å. Further increase in O-H bond length leads to destabilization of the system.

120

T1

2-naphthol.4H2O

S0

90

-∆E (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

30

0

S1 -30 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

ROH (Å) Figure 16 Rigid potential energy scan of O-H bond of 2-naphthol.4H2O, at ground, first excited triplet and singlet state. ∆E gives the relative energy of the system at each point of the scan of the dissociating O-H bond, with respect to the energy of the equilibrium geometry. Curves marked as S0, T1 and S1 represent potential energy surfaces of ground, first excited triplet and singlet states respectively of 2-naphthol.4H2O, upon increasing the bond distance of dissociating O-H bond of the acid.



However, a rigid potential energy scan of the acidic O-H bond of nap.4H2O at the first excited singlet state reveals that in the presence of four water molecules, the acidic proton transfer from nap to the neighboring water molecule becomes a barrier-less process, as shown in Figure 16. In fact, in the first excited singlet state, the tetra-hydrate becomes unstable at O-H bond distances less than 1.4 Å. The potential energy profiles show that a barrier exists for proton transfer from nap to solvent water molecules in all cases except the tetra-hydrate of nap at the first excited singlet state. In case of napS1.4H2O, spontaneous dissociation of the O-H bond is observed. This is in line with the experimentally observed fact that first excited singlet state is more acidic than the ground and first excited triplet state. 4.

Conclusion Microhydration studies of 2-naphthol at ground, first excited triplet and singlet

states are carried out using TD-DFT. It is seen that even in the presence of four water molecules, proton transfer from naphthol to water molecules is not observed for either ground or first excited triplet state. However, proton transfer becomes a barrier-less process for first excited singlet state of tetra-hydrate of naphthol. O-H stretching vibration frequency of 2-napthol in free and its hydrated clusters are calculated in the ground and first excited singlet state. Effect of successive hydration on the O-H stretching frequency of 2-napthol is clearly depicted as large red shift in both S0 and S1 states. The pKa of the first excited singlet state is very small compared to the ground and first excited triplet state, which is reflected in its ‘n’. Thus, a correlation can be seen between the pKa of the ground, first excited triplet and first excited singlet states of the 2naphthol, and the number of water molecules (n) needed for dissociating the acid molecule. However, the correlation between pKa and ‘n’ of a system that exists for the ground state,2 may not be applicable in the excited state, as suggested by this case study. 20 ACS Paragon Plus Environment

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Another point observed is that explicit solvation model, SMD, does not accurately depict hydration of 2-naphthol. For a better description of solute-solvent interaction involving hydrogen bonding, implicit solvation must be considered at least up to the first solvation shell. Single determinant methods are not enough to accurately describe the excited states of a system. To completely define the excited state chemistry, multi-reference calculations are required. Hence excited state calculations are complicated and timeconsuming. Although TD-DFT is a powerful and widely used procedure, higher multireference methods have to be employed to get a better picture of the photo-chemistry of hydrated-naphthol systems. The present article deals with the structures and the potential energy profiles of hydrated clusters of 2-naphthol, at different energy states, to understand the correlation between pKa and the number of water molecules needed for the dissociation of the acid. To get a comprehensive understanding of the microhydration of photo acids, the following actions are planned to be undertaken in the future. A multireference method, like CASSCF, can be used to verify the present results. Further studies to understand the reason behind the difference in pKa of S0, T1 and S1 states of 2naphthol could be carried out. We plan to study a wider range of systems so that a general expression can be derived to predict the pKa of photo acids from the number of water molecules (n) required to dissociate the acid.

Supporting Information: Cartesian coordinates are provided for the most stable conformers of 2-naphthol and its hydrated clusters at S0, T1 and S1 states. Higher energy equilibrium conformers of mono, di, tri and tetra-hydrates of nap at S0, T1 and S1 states are also given.



Acknowledgement We would like to thank BARC computer centre for providing generous computing time. Parvathi wishes to thank Homi Bhabha National Institute for research fellowship. RK thanks the Science Academies’ (IASc-INSA-NASI) for Summer Research Fellowship.


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