Microscopic Origin of Different Hydration Patterns of para-Nitrophenol

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Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

Microscopic Origin of Different Hydration Patterns of paraNitrophenol and Its Anion: A Study Combining Multiconfigurational Calculations and the Free-Energy Gradient Method Carlos Bistafa,†,‡,§ Yukichi Kitamura,‡,§ Masataka Nagaoka,*,‡,§ and Sylvio Canuto*,† †

Institute of Physics, University of São Paulo, Rua do Matão, 1371, Cidade Universitária, São Paulo-SP 05508-090, Brazil Graduate School of Informatics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan § Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Honmachi, Kawaguchi 332-0012, Japan

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‡

ABSTRACT: A theoretical study of the solvatochromic shifts of para-nitrophenol (pNP) and para-nitrophenolate anion (pNP−) in aqueous solution is presented using a QM/MM methodology with molecular dynamics simulation. The optimized structures in aqueous solution are obtained using both the polarizable continuum and the free-energy gradient methods. For pNP, the calculated redshifts at the CASPT2 (12,10) level are, respectively, 0.71 and 0.94 eV, in good agreement with the experimental ones (0.80−0.83 eV), whereas for pNP−, they are small. The difference between the solvatochromic shifts of pNP and pNP− is calculated as 0.71 eV in good agreement with the experimental one (0.79− 0.81 eV). Finally, these shifts are understood in terms of the solvent effect on the solute structure, accurately calculated by the present theoretical treatment.

1. INTRODUCTION para-Nitrophenol (pNP) (Figure 1a) and its anion, 4-paranitrophenolate anion (pNP−) (Figure 1b), have been studied experimentally and theoretically. Although the ultraviolet− visible (UV−vis) absorption spectra of both species in gas1−3 and in solution2,4,5 have been measured, a number of theoretical calculations,1−3 limited to the gas phase, were performed to understand the experimental observations. The

solvent effects on the excited states of these push−pull molecules influence also the hyperpolarizability that is essential to understand the nonlinear optical behavior of these molecules and other similar compounds.6,7 Very early, Biggs studied experimentally the pNP and pNP− spectra in different pH conditions of aqueous solutions.4 He observed that the maxima of the bands were located, respectively, at 3.91 and 3.05 eV. He also observed, as expected, that as the pH increases, the intensity of the pNP band decreases, whereas the intensity of pNP− band increases. In addition, he determined the isosbestic point of the system as 3.54 eV. Later, Abe5 studied experimentally the pNP molecule and some of its derivatives. He reported the absorption spectra of these molecules in aqueous solution and pointed out the absorption band maxima at 3.88 and 3.09 eV for pNP and pNP−, respectively, in agreement with the earlier study done by Biggs.4 More recent experiments obtained similar values. For instance, Ando et al.1 combined theoretical and experimental efforts to analyze the push−pull changes of pNP after excitation and upon deprotonation. They also obtained the

Figure 1. Structures of (a) pNP and (b) pNP− species. The atomic labels are used in the text and tables.

Received: July 5, 2018 Revised: August 30, 2018 Published: August 30, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.jpcb.8b06439 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 2. Active space used in the CASPT2 calculations of (a) pNP and (b) pNP−. The nomenclature used for the molecular orbitals comes from the ordering presented in the SCF calculation. (Contour value used: ±0.04 e/bohr3; green color is used to represent the negative density, whereas red color is used to represent the positive one).

absorption spectra of pNP and pNP− in aqueous solution and found the experimental values of 3.90 and 3.09 eV, in very good agreement with the previous experimental values by Biggs4 and Abe.5 In addition, Ando et al. also measured the pNP spectrum in gas phase and found a band maximum at 4.71 eV. They corroborated this value by performing a CASPT2 calculation in gas phase, obtaining the value of 4.87 eV, larger than the experimental value by only 0.16 eV. Kirketerp et al.2 studied experimentally several 4-nitrophenolate ions in gas phase and in several solvents, but they did not consider the case of aqueous solution. For pNP− in gas phase, they obtained a value of 3.16 eV, in agreement with their own calculated values of 3.19 and 3.27 eV, obtained by using CC2 and time-dependent density functional theory (TDDFT), respectively. Wanko et al.3 more recently studied the different nitrophenolate isomers in gas phase, by using an electrostatic ion-storage ring, and found the values of 3.15 eV (para), 3.11 eV (ortho), and 3.42 eV (meta). Other recent studies explored the charge-transfer characteristics of these molecules, via substitutions, longer carbon chains, and so forth.8,9 Thereby, from the experimental point of view, consistent results of the absorption bands of pNP and pNP−, in gas phase as well as in solution, are available in the literature.1−5 These experimental results present accurate information about the intensity of the bands, the change of these intensities depending on pH conditions, the wavelength of band peak, and the isosbestic point. However, from the theoretical point of view,1−3 calculations have been limited, mainly considering isolated molecules, providing limited explanations about the

atomistic mechanism behind the differences observed on the absorption bands of pNP and pNP−, in gas phase as well as in solution. For instance, the gas−water experimental solvatochromic shift of pNP has the sizable value 0.80−0.83 eV,1,4,5 whereas the solvatochromic shift of pNP− can be inferred as being so small as 0.06−0.11 eV,1−5 but the reason for this difference is not completely clarified. Thus, in the present work, we address the solvent effects on the absorption bands of pNP and pNP−, providing an atomistic explanation for the different solvatochromic shift observed experimentally, through QM/MM simulations. The electronic transition energies, related to the observed absorption bands of pNP and pNP−, were calculated accurately by employing the CASPT2 method. We also paid special attention to obtain the optimized structure of the solute molecules in aqueous solution since the electronic transition energies are sensitive to structural changes.

2. COMPUTATIONAL METHODS The optimized structure of pNP and pNP− in gas phase was obtained at the B3LYP/aug-cc-pVDZ level,10,11 using the GAUSSIAN 09 package.12 For the pNP molecule, no symmetry constraint was imposed for optimization, and the planar structure with Cs symmetry was obtained, whereas for pNP−, C2v symmetry was imposed. Electronic transition calculations were performed using the MOLCAS 7.6 package.13 We used the multistate CASPT2 method,14 that is, a perturbative method based on stateaveraged CASSCF wave functions. Because the previous theoretical and experimental studies pointed out that the B


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The Journal of Physical Chemistry B lowest electronic transition is a π−π* transition,1,2 we included in the active space all of the π orbitals (full-π space): 6 occupied and 4 virtual orbitals with a total of 12 electrons in 10 orbitals (Figure 2a for pNP and Figure 2b for pNP−). This is usually denoted as CASPT2(12,10). In this calculation, five roots were included (ground state +4 lowest excited states). We adopted the atomic natural orbital (ANO)-type basis set with the large (ANO-L) primitive sets contracted as C, O, N(14s9p4d)/[4s3p1d] and H(8s4p)/[2s1p], which generally presents converged results for low-lying electronic transitions of organic molecules.15 An imaginary level shift16 of 0.1 a.u. was used to reduce the presence of intruder states, as suggested before.1 To include the solvent effect in the electronic transitions, the sequential QM/MM methodology was used.17,18 For this purpose, we use classical molecular simulations [in the present study, molecular dynamics (MD) simulations] for sampling solute−solvent configurations and then select some of these configurations for performing electronic structure calculations. To include the polarization effect of the solute molecule caused by the interaction with the solvent, the optimized structures and charges of the solute molecules used in the simulation were obtained using the polarizable continuum model (PCM)19,20 at the B3LYP/6-31+G(d) level of calculation. In this case, no symmetry constraint was imposed to any of the species. We also used the free-energy gradient (FEG) method to free-energetically optimize the solute structures in aqueous solution21−26 and adopted the QM/ MM framework using the AMBER-GAUSSIAN interface27 that connects GAUSSIAN 0912 and AMBER 0928 programs. Although the computational cost of FEG optimization is very demanding, the QM/MM treatment, therefore, is a justifiable approximation to reduce the size of the basis set to a reasonable one in treating a large number of solvent molecules at the same time. As it will be clear in our results, therefore, the optimized structures obtained with the FEG method are able to provide a reasonably improved description of the electronic transition energies in solution. MD simulations were carried out with the AMBER 09 package,28 with the general AMBER force field.29 One simulation was performed for each optimized structure obtained as described above. The simulation box, with one side of 27.5 Å, was composed by including 1 solute (pNP or pNP−) and 854 TIP3P30 water molecules. For each model system of pNP and pNP− in aqueous solution, an equilibration NPT MD simulation was carried out for 100 ps at room conditions (300 K and 1 atm) with a time step of 1 fs. Then, a production NVT MD simulation was performed for 300 ps in the same room conditions. In the simulation, the optimized structure of the solute molecule was kept fixed. This allows us to sample the configurations to construct an average solvent electrostatic configuration (ASEC),31 which is used in the CASPT2 calculation of the electronic transition energies. In ASEC, the water molecules are represented by a number of effective partial point charges localized at the centers of the atomic sites (Figure 3a). Several configurations obtained from the classical MD simulation are selected and used to represent the solvent. In the present work, we used 100 configurations, collected every 3000 steps of the MD simulation, that is, 3 ps. All of the water molecules in the box were included to construct the ASEC, totalizing 256 200 point charges.

Figure 3. (a) ASEC model used in the CASPT2 electronic transition energy calculations. The pNP solute molecule is treated by a QM method, whereas the surrounding solvent molecules are represented by the point charges localized in the atomic sites. Several configurations are overlapped to construct an average configuration, and the charge values are normalized by the number of snapshots used. A total of 256 200 point charges were included. Red points represent the Owater atoms, whereas white ones represent the Hwater atoms. (b) Detailed view of the region around the pNP species shows the cavity formed by the solute−solvent interaction, as also the formation of hydrogen bonds, represented by the high concentration of red points close to the hydroxyl group. (c) Detailed view of the region around the pNP− species also shows the formation of hydrogen bonds, represented by the high concentration of red points close to the oxygen atoms. Figure constructed with VMD software.41

3. RESULTS AND DISCUSSION 3.1. Necessity of Structure Optimization for Including the Solvent Effect: Energetic Perspectives. First, we show the results for the lowest π−π* electronic transition energies of pNP and pNP− in gas phase. These results are summarized in Table 1 together with the available experimental data1−3 and previous theoretical results.1−3 We can see that the calculated values in gas phase are in agreement with the experimental ones within 0.2 eV. As C


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The Journal of Physical Chemistry B Table 1. Lowest π−π* Transition Energies of pNP and pNP− in Gas Phase, Calculated at CASPT2(12,10)/ANO-L Levela this work previous theoretical works EXP a

pNP

pNP−

4.93 4.87b 4.71e

3.18 3.19c, 3.21d 3.16f, 3.15g

Table 2. Lowest π−π* Transition Energies of pNP and pNP− in Aqueous Solution, Calculated at the CASPT2(12,10) Level, using Optimized Structures Obtained with Different Treatments (GAS, PCM, and FEG)a model used to obtain the solute optimized structure

b

Values in eV. CASPT2(12,10) calculation, but using a different basis set: ANO-L-VDZP (see ref 1). cCC2/aug-cc-pVDZ (see ref 2). d CC2/aug-cc-pVDZ (see ref 3). eReference 1. fReference 2. g Reference 3. 32

discussed by Merchán and Serrano-Andrés, this accuracy is possible if all of the molecular orbitals relevant to describe the transition are included in the active space. Our results are also consistent with previous theoretical calculations.1−3 These results also indicate the adequacy of the basis set and active space used to describe the electronic transitions in the gas phase. It should be noted that the mild difference of 0.06 eV between the value obtained in the CASPT2(12,10) calculations performed by Ando et al.1 (4.87 eV) and that by us (4.93 eV) should be because of our adoption of a larger ANOL basis set that provides converged results, as discussed before.15 On the basis of the above results, we used the same calculation level for obtaining the lowest π−π* electronic transition energies of pNP and pNP− in aqueous solution. In addition, to analyze the importance of the solvent effect on the structure of the solute molecules, we obtained the optimized structure of the solute molecules in three different treatments: (i) considering the solute molecule in gas phase; (ii) using a PCM representation of the solvent; and (iii) using the QM/ MM FEG method, that is, including the solvent effect atomistically. The optimized structures obtained were used in subsequent MD simulations. In these MD simulations, the optimized structures were kept fixed to construct the ASEC31 model, which we use to represent the solvent in the CASPT2 calculation to obtain the electronic transition energies in solution. These results are summarized in Table 2. Available experimental data are also presented.1,4,5 In this subsection, we will devote our attention to the case of pNP species and make appropriate comparisons with the pNP− in the Subsection 3.3. As one would expect, as we obtain the optimized structure of the pNP molecule in a more realistic description of the solvent, the electronic transition values become closer to the experimental value. For instance, by using the optimized structure obtained in gas phase, and including the ASEC model to represent the solvent only on the electronic transition calculation, we obtained the lowest π−π* electronic transition energy as 4.51 eV, overestimating the experimental value1,4,5 (3.88−3.91, see Table 2) by 0.60−0.63 eV. This means that with respect to the value we calculated in the gas phase, the electronic transition calculated in aqueous solution presents a solvatochromic redshift of 0.42 eV, only half of the experimental redshift value1,4,5 (0.80−0.83 eV). This suggests that the solvent effect on the optimized structure of the solute is responsible for about half of the observed solvatochromic shift, and is, therefore, inevitable to reproduce the transition energy in solution. For obtaining an estimate of the solvent effect on the optimized structure of the solute, we have employed the PCM.19,20 We obtained the optimized structure of the solute

pNP−

pNP

GAS PCM FEG

4.51 (−0.42) 4.22 (−0.71) 3.99 (−0.94)

EXP

3.88d, 3.90e, 3.91f, (−0.83, −0.81, −0.80)g

spectral shift (neutral-anion)

b

3.37 3.38, 3.28 ± 0.02c 3.05f, 3.09d,e

0.85 0.61, 0.71c 0.79−0.81

a

To account for the solvent effect in the electronic transition energy, all of the water molecules of 100 MD snapshots were included as point charges (ASEC Model). In parentheses, the solvatochromic shifts of pNP (with respect to the electronic transition values in gas phase shown on Table 1) are presented. Values in eV. bIn gas phase, the geometry used for pNP− has C2v symmetry, whereas in aqueous solution, no constraint was used. cIncluding explicit water molecules in the electronic transition calculation. dReference 5. eReference 1. f Reference 4. gThe experimental electronic transition value in gas phase is 4.71 eV (ref 1).

molecule considering the PCM. The resulting optimized structure was used to perform another MD simulation for constructing the ASEC. The value of the electronic transition energy obtained at CASPT2 level using this ASEC decreased to 4.22 eV, that is, 0.29 eV smaller than the value (4.51 eV) obtained with the structure optimized in gas phase. This result implies a redshift of 0.71 eV, which is in better agreement with the experimental values of 0.80−0.83 eV.1,4,5 This is a clear evidence of the importance of obtaining the optimized structure in aqueous solution to describe correctly the electronic transition energy values and the solvatochromic shift. Once we verified the importance of obtaining the optimized structure in solution, we proceeded further and obtained the optimized structure of pNP by employing the QM/MM FEG method. We repeated once more the same procedure aforementioned, using this optimized structure in an MD simulation and generating the ASEC model. The electronic transition energy obtained with CASPT2(12,10) by using this ASEC model is 3.99 eV (Table 2), in very good agreement with the experimental values of 3.88−3.91 eV.1,4,5 This result is a clear evidence of the relevance of atomistic description for obtaining the optimized structures of solute molecules that are able to reproduce the experimentally observed values of electronic transitions. 3.2. Necessity of Structure Optimization for Including the Solvent Effect: Molecular Orbital Perspectives. From the molecular orbitals (Figure 2a) involved in the pNP lowest π−π* electronic transition, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO), we can note the reason for the improvement of the electronictransition energy value using the optimized structure obtained with FEG. In the HOMO, the electron density is delocalized over the ring. Upon excitation, the density migrates to the nitro group, as presented in the LUMO of pNP (see Figure 2a). This change in the electron density increases the dipole moment of pNP, in agreement with the experimental estimation33 that suggests that the dipole moment of pNP increases from 4.83 D D


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Table 3. Bond Lengths (Å) and Angle (degrees) of the Nitro Group and C−O Group for the Optimized Structures of the pNP and pNP− Species Obtained in Gas Phase, in PCM and by Using the FEG Methoda pNP

pNP−

r(C6−N1) r(N1−O2) r(N1−O3) r(C3−O1) θ(O2−N1−O3) r(C6−N1) r(N1−O2) r(N1−O3) r(C3−O1) θ(O2−N1−O3)

gas

PCM

FEG

EXPb

1.466 1.231 1.231 1.362 124.2 1.413 1.254 1.254 1.258 121.3

1.450 1.240 1.239 1.356 122.8 1.405 1.260 1.260 1.269 120.7

1.412 1.257 1.254 1.327 120.3 1.406 1.256 1.259 1.289 120.5

1.450 1.243 1.241 1.361 122.8 1.430 1.253 1.238 1.295 121.3

a For the atomic labels of pNP and pNP−, refer to Figure 1a,b, respectively. Available experimental data are also shown. bExperimental values obtained for the crystal structure.34−36

that in aqueous solution is very small, no significant improvement in the electronic transition energy value is expected by using more sophisticated approaches to obtain the optimized structure (see discussion in the next subsection). These smaller changes in the nitro group of pNP− are due to substantial charge density of the group when compared to the neutral case. Because of the deprotonation, the hydroxyl group becomes negatively charged, whereas in the nitro group, the charge separation between oxygen and nitrogen increases. Then, the nitro group is already considerably saturated, not being able to accept more charge density and, consequently, interacting less with the water molecules. Figure 4 presents the

in the ground state to 9.51 D upon excitation. At CASSCF level, we obtained 4.28 and 9.81 D for the dipole moment of pNP in gas phase for ground and π−π* excited states, respectively. This change in the electron density can be held responsible for the redshift upon solvation. In addition, because electronic transition energy values are very sensitive to small structural deformations, the changes in the bond distances and bond angles of the nitro group will affect significantly the electronic transition energies. This observation indicates that a more accurate treatment for obtaining the optimized structure in aqueous solution, for instance, the atomistic treatment of solvent, would provide a theoretical value closer to the experimental one. Thus, when we employed the FEG for obtaining an optimized structure through QM/MM calculations, treating the pNP quantum-mechanically and the water molecules classically, we achieved a very good agreement between our calculated values for the electronic transition energy and the values observed experimentally. The importance of the theoretical treatment used for obtaining the optimized structure can be readily understood comparing the deformations in the nitro group obtained for the three optimized structures discussed in the previous subsection. In Table 3, we compare our calculated values of the bond distances and the bond angle of the nitro group in gas phase and in aqueous solution obtained in the three different treatments, GAS, PCM, and FEG in addition to the available experimental data.34−36 From gas phase to aqueous solution, the bond length r(C6−N1) and the bond angle θ(O2−N1− O3) decrease, whereas both bond lengths r(N1−O2) and r(N1−O3) increase. The increase in the N−O lengths influences the electronic transition energy. The differences between the optimized structure obtained in the gas phase and that in the aqueous solution are more pronounced when using the FEG method. This observation does not hold true for the case of anionic species, pNP−. The changes in the bond distances and angle in the nitro group of pNP− species are much smaller than those for the neutral species (Table 3). For example, if we compare the r(C6−N1) bond distances, we can understand that, in the case of pNP−, the value slightly decreases from 1.413 Å (GAS) to 1.406 Å (FEG), whereas in the case of pNP, it significantly decreases from 1.465 Å (GAS) to 1.412 Å (FEG). Similar comments can be made for the other bond distances of the nitro group. Thus, for the pNP− species, because the difference observed between the optimized structure in gas phase and

Figure 4. Total density difference, at CASSCF level, obtained by subtracting the pNP density distribution from the pNP− density distribution (contour values used ±0.04 e/bohr3; green color is used to represent the negative density, whereas red color is used to represent the positive one). It is clear that pNP− species has a substantial density on nitro and hydroxyl groups when compared to pNP.

density difference between two species, obtained by subtracting the pNP density distribution from the pNP− density distribution. It is clear that pNP− species has a substantial density on nitro and hydroxyl groups when compared to pNP. 3.3. Differences between pNP and pNP− in Aqueous Solution: Electronic Transitions and Spectral Shifts. In line with the discussion in the previous subsection, given the absence of changes in the different optimized structures of pNP− species, the value obtained for the electronic transition E


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The Journal of Physical Chemistry B energy of pNP− is the same by using optimized structures obtained in both approaches, PCM and FEG, within the CASPT2 accuracy, with the values of 3.37 and 3.38 eV, respectively (Table 2). In this case, because the optimized structure of the nitro group seems to have no effect on the electronic transition energy values, the most important interaction with the solvent occurs in another group. Different from the pNP species, for the pNP−, O1 is not bonded to any hydrogen and is susceptible to interact with the surrounding water molecules (Figure 3c). Because the O1 atom is negatively charged, a polarization effect on the water molecules hydrating the O1 atom is expected, but such a polarization effect is neglected in ASEC because the atomic charge values of the water molecules are constant. One possibility to overcome this limitation is, instead of employing ASEC itself, to employ the configurations used to composed ASEC and treat quantum-mechanically the water molecules interacting with the O1 atom (Figure 3b,c). However, CASPT2 calculations including explicit solvent molecules are very demanding computationally, especially if several water molecules are used. Nevertheless, to proceed further, we considered the calculations with some explicit solvent water molecules. To decrease the computational cost, from the 100 configurations used to construct the ASEC, we selected 10 configurations to perform CASPT2(12,10) calculations including explicit water molecules. On the basis of the analysis of the solvation structures employing the radial distribution function, the water molecules nearest to the CO group within a distance of 3.0 Å (dCO···Hw ≤ 3.0 Å) were selected to be included explicitly. By using this criterion, the number of explicit water molecules ranged from 3 to 5 in each of the 10 configurations. The remaining water molecules in the simulation box were included as point charges (Figure 5).

0.2 eV in the value of the electronic transition energy. We have previously observed a similar behavior in other organic molecules, that is, a decrease of the π−π* electronic transition energy obtained with ASEC when including explicit water molecules (0.1 eV in the case of 5-fluorouracil calculated with TDDFT37 and 0.3 eV in the case of acrolein calculated with CIS(D)38). The reduction of ca. 0.1−0.2 eV in the present case would bring the π−π* electronic transition value of pNP− molecule close to approximately 3.20 eV, implying, indeed, a very small value for the solvatochromic shift. Thus, for the case of the pNP molecule, the polarization of the closest water molecules can be neglected, whereas this is not the case for the pNP− species. These differences in the hydration mechanism explain the different electronic transition energy, consequently, the different solvatochromic shift, observed for pNP and pNP− in aqueous solution. The solvatochromic shift for pNP is found to be approximately 0.71 and 0.94 eV when using PCM and FEG values (Table 2), respectively, both in good agreement with the experimental value ranging 0.80−0.83 eV.1,4,5 Although the solvatochromic shift is relatively well-described by using the geometries obtained in PCM, by using FEG method, further improved agreement of the pNP electronic transition energy was obtained. Moreover, the comparison between the solvatochromic shifts calculated with the optimized structures obtained in gas phase (−0.42) and with FEG (−0.94) clearly suggests that the solvent effect on the optimized structure of pNP in aqueous solution is responsible for nearly half of the solvatochromic shift. On the other hand, for pNP−, the electronic transition energy in aqueous solution is correctly described only by using explicit water molecules. The solvatochromic shift of pNP− can be estimated experimentally1−5 as a mild redshift of ∼0.06− 0.11 eV. Using our electronic transition energy value of pNP− obtained with FEG and explicit water molecule (3.28 eV, Table 2) and our value obtained in aqueous solution (3.18 eV, Table 1), we successfully obtained a small solvatochromic shift of 0.1 eV, but it is a blueshift. This discrepancy deserves some comments. First, it should be noted that the experimental values are obtained from different studies in the gas phase2,3 and in the aqueous solution,1,4,5 with none of them performing the measurements in both conditions, and this can pose a limitation to the accuracy of such a small value. In addition, we adopted, as it is usual, the experimental values as the maxima of the experimentally observed bands, but in fact, these bands have considerable linewidths. On the other hand, from the theoretical point of view, this small redshift is a quantity whose order of magnitude is the same as that of the accuracy of our electronic transition calculations used to obtain the shift. Moreover, such an FEG optimization including QM water molecules should also improve the description of the solute optimized structures, as it has been discussed before.39,40 Keeping these theoretical and experimental limitations in mind, we can say that the obtained results are satisfactory within the present approximation. Finally, using the calculated values in aqueous solution for both species, we obtain a neutral-anion shift of 0.71 eV in good agreement with the experimental value ranging 0.79−0.81 eV.

Figure 5. One of the 10 configurations used in the CASPT2 calculation of pNP− electronic transition including explicit water molecules. The other water molecules of the simulation box were included as point charges (omitted in the figure for easy visualization).

From these 10 calculations, we obtained an average value of 3.28 ± 0.02 eV for the π−π* electronic transition energy value of pNP− in aqueous solution. This clearly shows a trend for decreasing the value of the electronic transition energy by including explicit water molecules, corroborating our considerations about the need for describing the polarization effect in the closest water molecules. On the basis of these results, we can infer that the inclusion of explicit water solvent molecules in the QM calculations contributes to a reduction of ca. 0.1−

4. CONCLUSIONS In this work, we combined the ab initio multiconfigurational CASPT2 method with the FEG method to study spectroscopic properties of pNP and pNP− in aqueous solution. Our F



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calculations show that for pNP, the lowest electronic transition is mainly a HOMO−LUMO transition, involving a migration in the electron density from the ring to the nitro group. Thus, an accurate description of this group interacting with the solvent is very important to study the electronic transitions. We achieved the necessary accurate description by employing the FEG method to obtain the optimized structure of pNP in solution. On the other hand, for the anionic species, pNP−, the change in geometry caused by the interaction with the water is very minor, much smaller than that in the neutral species. In this case, the electronegativity of the O1 atom seems to play a significant role, polarizing the nearby water molecules that need to be treated quantum-mechanically for the correct description of the electronic transition energy. This difference in the hydration mechanisms of both species, pNP and pNP−, explains the different solvatochromic shift observed experimentally. Indeed, taking it into account these different hydration mechanisms in our calculations, we succeeded to describe very well the shifts of the electronic transitions of both species. One important point to achieve this result was to obtain the optimized structures in aqueous solution. Using the optimized structure obtained with FEG, the calculated electronic transition energy yields a solvatochromic redshift of 0.94 eV in good agreement with the experimental values ranging 0.80−0.83 eV. The neutral-anion spectral displacement in aqueous solution is calculated as 0.71 eV, in good comparison with the experiment (0.79−0.81 eV). For pNP−, the present CASPT2 (12,10) result including explicit solvent water molecules is 3.28 eV, overestimating the experiment value by only 0.19 eV, within the CASPT2 accuracy. The pNP− solvatochromic shift is founded mild, as in the experiments.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.N.). *E-mail: [email protected] (S.C.). ORCID

Masataka Nagaoka: 0000-0002-1735-7319 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.B. and S.C. thank FAPESP, CNPq, and CAPES (Brazil) for the continuous support. In particular, C.B. thanks FAPESP for a Ph.D. fellowship and an Abroad Research Internship Fellowship, which allowed him to visit Japan to start the development of the present collaborative work. Y.K. and M.N. thank the support from the Core Research for Evolutional Science and Technology (CREST) “Establishment of Molecular Technology towards the Creation of New Functions”, granted by the Japan Science and Technology Agency (JST). They also thank the support from the Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan, that is, the MEXT programs “The Strategic Program for Innovation Research (SPIRE)” and “Elements Strategy Initiative for Catalysts and Batteries (ESICB)”. G


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