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Entropy 2014, 16(7), 4101-4120; doi:10.3390/e16074101
Abstract: The spectroscopic features of the multilayer honeycomb model of structured water are analyzed on theoretical grounds, by using high-level ab initio quantum-chemical methodologies, through model systems built by two fused hexagons of water molecules: the monomeric system [H19O10], in different oxidation states (anionic and neutral species). The findings do not support anionic species as the origin of the spectroscopic fingerprints observed experimentally for structured water. In this context, hexameric anions can just be seen as a source of hydrated hydroxyl anions and cationic species. The results for the neutral dimer are, however, fully consistent with the experimental evidence related to both, absorption and fluorescence spectra. The neutral π-stacked dimer [H38O20] can be assigned as the main responsible for the recorded absorption and fluorescence spectra with computed band maxima at 271 nm (4.58 eV) and 441 nm (2.81 eV), respectively. The important role of triplet excited states is finally discussed. The most intense vertical triplet→triplet transition is predicted to be at 318 nm (3.90 eV).
The detailed understanding of interfacial water, as well as the specific role of solutes on it, has been during the last decade an area of increasing interest and active research [1–12]. A number of properties different from bulk water, such as viscosity, density, freezing temperature, relative permittivity, and distinct absorption and fluorescence spectra, have been characterized next to solid surfaces by different groups [2,8,10–12]. Thus, the wide body of accumulated experimental experience up to date seems to point out to the actual existence of water structuring. Structured (or ordered) water is observed adjacent to hydrophilic surfaces, a solute-free region, known for that reason as “exclusion zone” (EZ) [2,12,13]. Since structured water can be therefore seen as interfacial water, and most of the water in living organisms can be considered as interfacial, the name “liquid crystalline” has also been employed . Consequently, “structured”, “ordered”, “EZ water”, or “liquid crystalline” are indistinctly used in practice. Furthermore, experimental support has been recently given describing interfacial water as a “photonic crystal” . Since the recent publication of the latest book produced by Pollack , one of the most active researchers in the field over the last decade, the term “fourth phase of water” to denote structured water is also becoming progressively popular. It is well-recognized by now that water is capable to shape and control biomolecules, a fundamental aspect in cell biology . For instance, the right protein function lies ultimately in a proper balance between ordered and bulk water [17,18]. Whereas water inside of relaxed myofibrils is extensively hydrogen bonded, muscle contraction has been recently characterized by a loss of order in the muscle-protein complex, accompanied by a destructuring of hydration water .
The absorption spectrum with a band peaking at ∼270 nm (∼4.59 eV) has been revealed to be a unique characteristic of structured water in solutions of many different substances (salts, amino acids, sugars), similarly to the feature recorded in the EZ by using Nafion 117 solution/film . All the fluorescence emissions for those same systems lay in the range 300–500 nm (4.13–2.48 eV), most of them in interval 400–490 nm (3.10–2.53 eV) . Interestingly, a transition 270-nm phase, eventually leading to bulk water, has also been identified as ice melts . On the contrary, the lowest-energy band of the absorption spectrum in the gas phase has a maximum at 7.447 eV , whereas the spectra of the liquid is dominated by a pronounced peak at about 8.2 eV, showing the absorption spectra of amorphous, hexagonal, and cubic ice a similar structure . The different electronic spectra of water in the gas phase (formed by water molecules) with respect to liquid and solid water (composed by different water aggregates) reflects the distinct structural water motifs responsible for the recorded absorption in the respective phases. It is therefore expected that the basic geometry of structured water responsible for the observed peak at ∼4.59 eV would also be different. The multilayer honeycomb ice-like model depicted in Figure 1, which has been recently proposed by Pollack and co-workers [12,19], seems to be consistent with the experimental evidence. It is worth recalling that water hexamers have been observed next to diverse surfaces, including metals [22,23], protein subunits , and graphene [25,26], as well as in supercooled water .
In the present contribution, the hexameric model for structured water is analyzed through the H19O10 monomer composed by two fused water hexagons (see Figure 1c). In order to better mimic the actual situation, both the anionic and the neutral systems of the monomer, involving 100 and 99 electrons, respectively, shall be considered.
Based on our previous experience on the issue, where the relevance of π stacking has been highlighted [28,29], the corresponding π-stacked dimers [H38O20], comprising 200 and 198 electrons, would be subsequently considered. We pursue to get a satisfactory response on theoretical grounds to the following basic question: is the hexagonal ice-like model capable of rendering the spectroscopic fingerprints of structured water? For this purpose, high-level ab initio (from the first principles) quantum-chemical computations have been carried out in order to describe the relevant excited states and electronic transitions in the monomers (anion and neutral) and dimers (dianion and neutral) under study. In this manner, knowledge gathered on theoretical grounds may hopefully provide further insight into the water arrangements responsible for the spectroscopic features of structured water, which might be potentially relevant in the realms of biology, nanotechnology, and materials science.
2. Quantum Chemistry and Computational Strategies
The study has been performed within the framework of nonrelativistic Quantum Chemistry within the Born-Oppenheimer approximation . Therefore, the electronic wave functions have been computed in the field of the bare nuclei. The electronic energy (Eelec), thus obtained, is added to the classical repulsion exerted by the nuclei (Erep) leading to the total energy (Etot). The latter describes the complex potential energy hypersurface of a given molecular system. It is precisely the delicate balance between Eelec (attractive) and Erep (repulsive), which makes the bonding possible. As an illustrative example, the full configuration interaction (FCI) solution (exact solution within a given basis set) for the H2 molecule employing a minimal STO-3G basis set, using the values for the molecular integrals from appendix D of , is shown in Figure 2. By a close inspection of the solutions around the equilibrium distance, as compared to the two isolated hydrogen atoms, one is able to conclude that the matrix elements related to the operator for the kinetic energy of the electrons and the Coulomb attraction between electrons and nuclei are able to compensate the repulsion between the nuclei, 1/R in atomic units (a.u.), where R stands for the interatomic distance, as well as the repulsion between the electrons. Thus, the sum of the terms ascribed to an electron in the field of the nuclei (core Hamiltonian) can be seen as the main responsible of the binding interaction of the system, bringing a completely new quantum-mechanical view on the stability of the molecule. The possibility of sharing the two electrons in the same space around the two fixed nuclei makes possible the birth of a new species: the H2 molecule. It is worth recalling at this point that the treatment of the spin of the electrons is within the nonrelativistic paradigm described phenomenologically. In short, the interaction between the nuclei is taken into account from a classic perspective, whereas the electronic energy is accounted for on the basis provided by Quantum Mechanics. Quantum Chemistry deals with the application of the basic quantum-mechanical principles to the study of atoms and molecules. The zero energy reference corresponds to an electron separated infinitely from a proton, that is, the energy of the proton is zero. The nature of the proton itself is therefore outside of the scope of standard Quantum Chemistry. At this resolution level, the proton is simply considered as a positive charge.
The major effort from a quantum-chemical standpoint is related, of course, to the correct description of the attractive potential Eelec, that is, the proper treatment of electron correlation. The movement of the electrons being correlated implies that the probability of finding two electrons at the same point in space becomes zero (the so-called Coulomb hole). Unfortunately, it is not the case for many approximate wave functions. Except for small molecular systems, exact solutions are not technically feasible. Therefore, one has to deal with truncated quantum-chemical algorithms, making special emphasis on the magnitude of the property of interest to be calculated, e.g., an electronic transition energy. The predicted value should become stable upon further improvements, reflecting so the balanced treatment performed in the electron correlation effects, both in the ground and excited states. In this sense, the CASPT2 (Complete-Active-Space Second-Order Perturbation Theory) method proposed by Roos and co-workers in the early nineties represented a fundamental breakthrough in ab initio computational work . Electronic transitions could then be characterized on theoretical grounds, within a few tenths of eV in practice, with respect to comparable experimental data in a variety of compounds of different molecular size, ranging from ethene to free-base porphyrin [32–37]. It represented quite an outstanding outcome two decades ago because error bars of about one eV used to be more a rule that an exception in previous ab initio research. We are quite confident by now in the predictive character of the CASPT2 methodology, which makes possible a suited analysis of experimental and theoretical work on an equal footing. Normally, the experimental-theoretical combined joint efforts produce a fruitful synergy favoring the deepest understanding of a given subject. The underlying reason clearly relies on the fact that we do not simulate but predict the intrinsic molecular properties of the treated compounds. Of course, the theoretical results are independent of any past, present, or future experimental work.
The CASPT2 method can be framed within single-reference second-order perturbation theory where a multiconfigurational CASSCF (Complete Active Space Self-Consistent Field) wave function is used as zeroth-order wave function. The zeroth-order Hamiltonian as originally implemented has been used  employing the MOLCAS-7 software package [38,39]. The CASSCF wave function is defined by the user, given by a number of active electrons and active orbitals based on the needs of the system according to the target to be achieved. The active electrons are distributed among the active orbitals in all possible ways consistent with the spatial and spin symmetry of the wave function required. Namely, the CAS space is of FCI type within the subspace spanned by the active orbitals and electrons. Both orbitals (one-electron functions) used to build the many-electron wave function (spin-adapted linear combination of determinants) and coefficients of the CASSCF expansion are optimized by means of the variation principle (search of stationary point). Details on the active spaces employed will be next given as appropriate. The oxygen 1s electrons have been kept frozen in the perturbation step. In order to minimize weakly interacting intruder states, the imaginary level-shift technique, with a value of 0.2 a.u., has been employed . Ground-state geometry optimizations have been performed at the MP2 (Second-Order Møller-Plesset Perturbation Theory) level through the program GAUSSIAN 09 . The basis set 6-31G** has been used throughout, as a compromise between accuracy and computational resources [32–37,42,43].
3. Results and Discussion
The results are presented in three subsections. Firstly, the vertical electronic absorption spectrum for the anionic system formed by two fused hexagons built by water molecules, that is, the [H19O10]− monomer, is considered. The structural evolution of the anionic dimer [H38O20]2− is then analyzed. Secondly, the absorption spectrum for the corresponding neutral species, the radical system [H19O10] and its π-stacked dimer [H38O20], is reviewed. For the π-stacked neutral dimer, the fate of the absorbed energy is subsequently explored and the results are related to the observed fluorescence features. Finally, the relevance of the triplet manifold is put forward and the most intense electronic transition of the triplet-triplet spectrum is predicted.
3.1. Anionic Systems
As originally proposed [12,44], the ice-like multilayer model of structured water is formed by a network of fused hexagon units (see Figure 1) negatively charged. Assuming that the charge contribution made by each oxygen atom is minus two while that for each hydrogen atom is plus one, the net charge of the ten-hexagon structural diagram drawn in Figure 1b is −9. In general, for N-fused hexagons, the charge acquired by the global system is 1-N . Therefore, from a molecular standpoint, the smallest model unit corresponds to the anionic monomer [H19O10]− bearing a negative charge −1. For each couple of fused hexagons built, a proton is assumed to be released. Thus, the model is particularly appealing because it accounts for the local charge separation between the EZ and the regions beyond observed by using pH measurements .
The main geometrical parameters for the ground-state equilibrium structure obtained for the [H19O10]− anionic system at the MP2/6-31G** level, together with the charge reorganization derived from Mulliken population analysis, are shown in Figure 3. Instead of two fused hexagons with bond-equalized O-H bond lengths, as it occurs for the ground state of the neutral species , the results for the anionic system can be seen as a reminiscent of a Zundel cation H5O2+ in the middle of the system plus two anionic H3O2− species situated on the left- and right-side, those being hydrated by four nearly neutral water molecules (top and bottom). The trends described are maintained for layers formed by a larger number of fused hexagons. For instance, in the ten-hexagon system depicted in Figure 1b, specifically the [H55O32]9− anionic system, the structure of the two fused hexagons displayed in Figure 3 prevails in the center, which is surrounded by a number of hydrated hydroxyl anions. Thus, anions do not favor the formation of honeycomb-like layers but causes a pronounced charge polarization ending up in positively and negatively charged species. Among the latter, a clear contribution of hydroxyl anions is noted.
The vertical electronic transitions for the [H19O10]− anionic system have been computed at the ground-state geometry. The selection of the active space has been based on the nature and distribution of the canonical orbitals in the closed-shell system. There is a clear gap in orbital energies between the HOMO–5 and HOMO–6 and the LUMO+5 and LUMO+6, where HOMO and LUMO stand for the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively. Consequently, the MOs topologically equivalent to the highest six occupied canonical MOs and the lowest six unoccupied MOs have been employed. It corresponds to 12 electrons distributed in 12 active MOs, hereafter denoted as CASSCF(12,12). Specifically, the number of active orbitals employed belonging to the irreducible representations ag, b3u, b2u, b1g, b1u, b2g, b3g, au, of the D2h symmetry point group are 2, 1, 2, 1, 2, 2, 1, 1, respectively. In each symmetry, the lowest root at the CASSCF(12,12) level has been computed. For the sake of completeness, the inactive orbitals employed are also given: 11, 2, 10, 2, 9, 1, 8, 1. The core orbitals (3, 0, 3, 0, 2, 0, 2, 0) have been kept frozen at the second-order level. The corresponding results corrected up to second order shall be labeled as CASPT2(12,12).
Table 1 compiles the results for the vertical electronic transitions obtained at the CASPT2(12,12) level for the [H19O10]− anionic system, together with the associated oscillator strengths, and information on the nature of the electronic excited states in terms of the corresponding CASSCF(12,12) natural orbitals (NOs), which are shown in Figure 4. As expected, the ground state is well described by the single closed-shell Hartree-Fock (HF) type configuration with a weight in the CASSCF wave function of 98%. The computed excited states show however multiconfigurational character.
The six electronic transitions obtained for the [H19O10]− anionic system can be grouped in three pairs of degenerate features centered at 7.4, 7.5, and 8.3 eV. For the monomer, the [H19O10]− anionic system, the lowest-lying excited states are energetically far away from that characteristic of structured water. The two lowest excited states 11B1g and 11Au, appearing at 7.42 eV (167 nm), are mainly described by a linear combination of one-electron promotions involving charge reorganization from the π system of the molecule towards the σ region. The states resemble to the lowest excited state of the water molecule itself, both in nature and energetic placement [46,47]. The corresponding vertical electronic transitions are forbidden. In increasing ordering of energy, the electronic states 11B3g and 11B2u of σ-σ* character are next found. The vertical electronic transition 11Ag → 11B2u placed at 7.51 eV (165 nm) has the largest oscillator strength with a computed value of about 0.24. The 11B2u state is described by the linear combination of the configurations HOMO–1 → LUMO and HOMO → LUMO+1 with equal weights (49%). Finally, the 11B3u and 11B2g states of π-σ* nature are predicted at 8.34 eV (149 nm). The vertical transition from the ground state to the former has an oscillator strength of around 0.04.
When two [H19O10]− anionic systems are situated in a parallel orientation at 2.4 Å within the constraints of D2h symmetry (see Figure 5a), geometry optimization of the resulting global system [H38O20]2− leads, after a long optimization process where the original spatial symmetry is lost and breaking of the wave function occurs, to the bulk-type water aggregate displayed in Figure 5b.
According to the present results, a negatively charged monolayer formed by two fused water hexagons tends to produce hydrated hydroxyl anions and Zundel-like cations. Thus, in a layer, a negative charge is not equally spread over the entire system. Moreover, the findings do not support a stabilizing interaction activity between the different negatively charged layers (cf. Figure 5) as originally proposed for the hexagonal ice-like model [12,44]. The interaction of such anionic layers collapse towards the formation of hydrated hydroxyl anions in bulk-type structures, which are not expected to contribute to the absorption recorded at ∼270 nm (4.59 eV) . Therefore, they have not been considered further. For these reasons, in our quest to elucidate the basic molecular units responsible of the unique spectroscopic absorption of structured water, anionic species are discarded.
3.2. Neutral Systems
3.2.1. Absorption Spectroscopic Features
The vertical electronic spectra at the respective MP2/6-31G** geometries of the ground-state neutral monomer and dimer, the radical [H19O10] and the π-stacked system [H38O20], have been previously reported . As stated above, in contrast to the optimized parameters for the ground-state anionic monomer, for the ground-state neutral monomer all the O-H bond distances are similar (around 1.2 Å, see Figure 6a). On the other hand, the interaction of two radical species leads to a stabilization of the dimer by more than 2 eV, characterized for a relatively short inter-monomer distance between the fused central oxygen atoms belonging to different monomers (2.115 Å, see Figure 6).
The radical monomer [H19O10] certainly constitutes a versatile unit, having two spectroscopic features in the near infrared region (50,000–770 nm; 0.02–1.61 eV), two in the visible range (770–390 nm; 1.61–3.32 eV), and two in the near ultraviolet zone (390–200 nm; 3.22–6.62 eV). The six vertical electronic transitions found in the energy range up to 5 eV are shown in Figure 7. The doublet states are in all cases well described by the corresponding singly occupied natural orbital (SONO) of the CASSCF(13,12) wave function employed . The vertical electronic transitions 12B2u → 12Au and 12B2u → 12B2g placed at 4.42 and 4.45 eV, respectively, are consistent with the spectroscopic signature of structured water with a maximum recorded at ∼4.59 eV . However, the dipole-forbidden nature of those features pushed us to keep searching for a more notorious molecular system that would become without any hesitation an excellent candidate of the 270-nm absorption determined experimentally . As shall be discussed next, the π-stacked dimer [H38O20] is able to fulfill such expectation.
A total of twelve singlet→singlet electronic transitions have been computed for the [H38O20] system, seven in the visible range and five in the near ultraviolet region. In the latter, the electronic transition 11Ag → 21B1u of π character has been predicted at 4.58 eV (271 nm), with a computed oscillator strength of 0.29 , which perfectly matches with the experimental band maximum recorded at ∼270 nm . In terms of the natural orbitals (NOs) derived from the complex CASSCF(12,15) wave function employed , which corresponds to 12 electrons distributed among 15 active molecular orbitals, the electronic transition is depicted in Figure 8.
Based on the analysis of the occupation numbers of the active molecular orbitals in the respective CASSCF(12,15) wave functions, the ground state 11Ag (0.80 and 1.23) and the excited state 21B1u (1.20 and 1.22) can be regarded as biradicals, the ground state having singly occupation numbers in the LUMO- and HOMO-like NOs and the brightest excited state in the LUMO- and (HOMO–4)-like NOs.
3.2.2. Fluorescence Spectroscopic Features
In order to determine the fate of the absorbed energy once the 21B1u state has been populated, potential energy curves (PECs) with respect to the distance of the central fused oxygen atoms belonging to different monomers, R(O-O) in Figure 6, have been built for the twelve excited states, keeping the intra-monomer geometry fixed at that obtained for the ground state. In this manner, the effect of the variation of the inter-monomer distance, which can in principle be considered as the primary structural relaxation between the corresponding layers, can be analyzed. The most relevant results obtained from such comprehensive study are compiled in Figure 9. The results can be related to the fluorescence feature observed experimentally for many different kinds of compounds in the range 300–500 nm (4.13–2.48 eV), most of them in the interval 400–490 nm (3.10–2.53 eV) .
According to the relatively large oscillator strength calculated for the brightest 11Ag → 21B1u electronic transition , upon light absorption at the Franck-Condon (FC) region (around the ground-state equilibrium structure), the 21B1u state is expected to be populated. The 21B1u and 41Ag states computed vertically at 4.58 and 4.68 eV, respectively, are energetically close and can further interact. The PEC of the 21B1u state has a flat character in an ample R(O-O) distance interval (2–2.6 Å). Conversely, the 41Ag state becomes relaxed adiabatically towards the minimum displayed in the 31Ag PEC placed at R(O-O) = 2.390 Å. Therefore, the occurrence of a crossing between the 21B1u and 41Ag states in the vicinity of the FC region makes possible the transfer of population between the two hypersurfaces. From the minimum characterized for the 31Ag state, the system may then dissipate energy by emitting radiation in the form of fluorescence. The predicted fluorescence peak is computed to be at 2.81 eV (441 nm), with an origin of band (Te) of 3.20 eV (energy difference between the excited-state and ground-state minima). The results can be considered as upper bounds of the actual spectroscopic theoretical features (involving the full geometry optimization of the excited state). For the sake of completeness, let us recall that the 31Ag state, has a vertical excitation energy of 3.47 eV (357 nm) . The PEC for the 21Ag state, as it also occurs for other low-lying excited states, is repulsive and it is not expected to be involved in the fluorescent process. The repulsive states lead to crossing regions with the ground state at large R(O-O) distances. The corresponding conical intersections may mediate in this way an effective nonradiative decay channel towards the repopulation of the ground state, as it occurs in DNA/RNA nucleobases [48–54]. The well found on the 31Ag hypersurface at 2.390 Å constitutes the only minimum accessible in the excited system. The fact that the nature of the excited state responsible for the absorption (21B1u) and the fluorescence (31Ag) is different, nicely rationalizes the large Stokes shift observed experimentally . The large difference between positions of the band maxima of the absorption and fluorescence spectra is then related to the different inter-monomer R(O-O) distance obtained for the ground state (2.115 Å) and the excited state (2.390 Å). The fluorescent state 31Ag at R(O-O) = 2.390 Å has a complex CASSCF(12,15) wave function with a marked multiconfigurational character dominated by the doubly excited configurations (HOMO-like)2 → (LUMO-like)2 (44%) and (HOMO–4) → (LUMO-like)2 (26%), contributing also the closed-shell HF (13%), where the corresponding weights are reported within parentheses. The HOMO- and (HOMO–4)-like NOs have occupation numbers of 1.06, and 1.36, respectively. Thus, as can be seen schematically in Figure 10, the fluorescent state can also be considered approximately as a biradical.
From an experimental standpoint, the wavelength of the emission peak (λmax) depends on the wavelength of the excitation applied (λexc) in a variety of aqueous solutions employing different solutes (salts, Nafion 117 solution/film, L-lysine, D-alanine, D-glucose, and sucrose) . For instance, in the case of Nafion 117 solution, λmax varies from 431 nm (2.88 eV) to 463 nm (2.68 eV), ranging λexc from 400–250 nm (3.10–4.96 eV). Moreover, the intensity increases when the λexc used is 270 nm (4.59 eV) or 250 nm (4.96 eV), peaking λmax at 458–459 nm (2.71–2.70 eV) . According to the present results depicted in Figure 9, at least 4.58–4.68 eV are required to reach the crossing region between the bright and the dark excited states, a process which ultimately causes the relaxation of the population towards the equilibrium region of the 31Ag hypersurface, from where the system may then emit light. The fluorescence feature computed here at 2.81 eV, which corresponds to the theoretical prediction for λmax, is therefore consistent with the experimental findings .
3.2.3. On the Relevance of the Triplet Manifold
The importance of the triplet manifold becomes apparent taking into consideration the degeneracy computed between the lowest triplet state 13B2u and the lowest singlet state 11Ag at the CASPT2(12,15) level. In other words, the ground state of the π-stacked dimer [H38O20] can be equally described by the singlet state 11Ag and the triplet state 13B2u. The latter, has as the leading configuration in the CASSCF(12,15) wave function the singly excited configuration HOMO-like→LUMO-like NOs (97% weight percentage). Therefore, a number of questions arise at this stage:
What is the “glue” that makes the singlet/triplet ground-state so stable?
What determines the population of the singlet or triplet ground state?
What about the most intense transition in the triplet manifold?
The understanding of the stability of the ground-state π-stacked dimer [H38O20] relies on the same foundations as the description of the hydrogen molecule considered in Section 2. The high-level calculations performed at the CASPT2 level are in a large extent able to account for the attractive potential Eelec, characterizing correctly the system, which makes possible the balanced description of the intriguing π-stacked dimer through Etot. Thus, the “glue” that enables the existence of the ground-state H2 and the ground state of the [H38O20] system in a seagull-like structure is the same: a purely quantum-mechanical effect able to describe properly the attractive potential of the electrons in the field of the nuclei, compensating the repulsive terms between the electrons and the nuclei.
The answer to question B will probably come from an interdisciplinary arena in the future. Just to remark on the importance of triplet states, it is worth recalling that they are especially sensitive to external forces such as electric and magnetic fields , which might be especially relevant in living organisms. Nanotechnological measurement of the electromagnetic activity of cells has been recently proposed in order to disclose the main features of pathological disturbances of coherent states in living organisms . Moreover, information transfer without any transfer of matter (epitaxy), a well-documented and popularized technique in materials science can also be conceived as similar vectors .
The answer to question C is much easier to us and it also gives some clue to the issue put forward in B. The 13B2u→23B3g triplet→triplet vertical electronic transition is predicted at 3.90 eV (318 nm) at the CASPT2(12,15) level with an oscillator strength of 0.33. The 23B3g state is described mainly by the doubly excited configuration (HOMO–3)-like, HOMO-like → (LUMO-like)2 NOs (95% weight percentage). Thus, the feature is about 0.7 eV red shifted with respect to the main absorption found in the singlet manifold at 4.58 eV (271 nm) . Good tasting and clear uncontaminated water from a deep mountain has been reported to develop an absorption maximum in the range 290–310 nm (4.28–4.00 eV) . On the other hand, it loses its capacity to absorb ultraviolet radiation in that range within a few days or weeks after being removed from groundwater, yielding so a criterion to the term “fresh water” . As a reasonable working hypothesis, we tentatively assign the absorption feature of good mountain well water to the involvement of the most intense triplet→triplet electronic transition (see Figure 11).
Ongoing work in our group is currently devoted to fully characterize the triplet manifold of the π-stacked dimer [H38O20], especially whether a possible phosphorescent state could contribute to the bluish color in the range of 450–490 nm (2.76–2.53 eV) ubiquitously encountered in pristine natural environments.
Within the framework of our own field of expertise in Theoretical and Computational Chemistry, on the basis of high level ab initio results, several conclusions can be drawn regarding the multilayer honeycomb model of structured water:
For negatively charged multilayers, i.e., anionic systems, the charge is not expected to be delocalized over the entire honeycomb layer. The system becomes polarized into hydrated hydroxyl anions and cationic moieties. Accordingly, the negative potential observed inside the EZ can be related to the presence of hydroxyl anions. The lowest-lying excited states found for the charged monolayer considered are about ∼7.4 eV (∼168 nm), well above the structured-water spectroscopic fingerprints determined experimentally ∼4.59 (∼270 nm). Therefore, we confidently conclude that heavily negatively charged hexagonally arrangements are not responsible for the observed spectroscopy.
The spectroscopic features recorded for structured water are fully consistent with those computed for the neutral molecular models used to describe the essentials of the multilayer honeycomb arrangements. The neutral π-stacked dimer [H38O20] has been revealed to be particularly suited for this purpose, both for the absorption and fluorescence events. The ground state of this unique system is equally well described by the 11Ag and 13B2u states. Namely, the lowest singlet and the lowest triplet states show degeneracy. The relative large attractive interaction (more than 2 eV) between the two monomers [H19O10] exerted by the (singlet/triplet) ground state of the dimer [H38O20] can be related to the exclusion of many substances from the zone adjacent to various hydrophilic surfaces determined experimentally. The neutral π-stacked dimer [H38O20] can be regarded as the main responsible for the recorded absorption with a computed band maximum at 271 nm (4.58 eV). On the other hand, the most intense vertical triplet→triplet transition is predicted to be at 318 nm (3.90 eV), which has been tentatively related to the absorption maximum observed in the range 290–310 nm (4.28–4.00 eV) for pristine water. The computed fluorescence features with band maximum at 441 nm (2.81 eV) is consistent with that registered experimentally in the range 400–490 nm (3.10–2.53 eV). In the singlet manifold, the absorbing bright excited state (21B1u) and the fluorescent emitting state (31Ag) have distinct nature, which nicely rationalizes the large Stokes shift concomitant to the spectroscopy of structured water. The population transfer from the bright to the dark state occurs in the vicinity of the ground-state equilibrium structure (R(O-O) = 2.115 Å). The release of radiation as fluorescence can be accomplished by a slight separation of the two monomers in the π-stacked dimer [H38O20] (R(O-O) = 2.390 Å).
The underlying reason behind the successful description of the molecular systems considered is grounded on the ability of the CASPT2 method to treat in a balance manner the electron correlation effects of the electronic states in the different hypersurfaces explored. Despite the latter statement is supported up to date by more than two thousand calculations of excited states dealing with many different types of compounds, the fact is particularly outstanding in this case involving crystalline-like systems formed by hexagonally arranged water moieties, where a basic concept such as atomic valence becomes uncertain and quantum-mechanical effects able to describe the attractive potential properly have to be accounted for in a large extent.
The research work presented emphasizes the quantization of matter at the molecular level, yielding the resonance conditions for the interaction with the electromagnetic radiation. Both aspects are complementary to describe the event actually taking place. Therefore, the results provided fully support the scenario envisioned in the realm of biology by Albert Szent-Györgyi. In the conclusion section of the book entitled “Bioenergetics” , published in 1957, he stated: “Lucretian biochemistry involves the assumption that no interaction can take place between molecules without their touching one another. Support is given in this book to the idea that manifold interactions can take place without such bodily contact, either through energy bands or through the electromagnetic field, which thus appears with water and its structures as the matrix of biological reactions.” Quantum Chemistry shows indeed that water may form structures that absorb and transmit energy precisely in the same energy range as that arising from aromatic amino acids and DNA/RNA nucleobases.
The research reported has been supported by the Spanish MINECO project CTQ2010-14892 and the Juan de la Cierva programme (Ref. JCI-2012-13431).
Calculations were mainly carried out by Javier Segarra-Martí. Daniel Roca-Sanjuán was primarily involved in the analysis and the discussions of the obtained findings. Manuela Merchán designed the current research project and took care of the writing of the paper. All authors have read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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|State||ΔE [eV]||ΔE [nm]||Oscillator strength||Assignment (Weight) a|
|11Ag (ground state)||closed-shell HF||(98%)|
|11B1g||7.42||167||forbidden||HOMO–2 → LUMO||(46%)|
|HOMO–3 → LUMO+1||(45%)|
|11Au||7.42||167||forbidden||HOMO–3 → LUMO||(46%)|
|HOMO–2 → LUMO+1||(45%)|
|11B3g||7.49||166||forbidden||HOMO → LUMO||(51%)|
|HOMO–1 → LUMO+1||(48%)|
|11B2u||7.51||165||0.2431||HOMO–1 → LUMO||(49%)|
|HOMO → LUMO+1||(49%)|
|11B3u||8.34||149||0.0382||HOMO–4 → LUMO||(44%)|
|HOMO–5 → LUMO+1||(42%)|
|11B2g||8.34||149||forbidden||HOMO–5 → LUMO||(43%)|
|HOMO–4 → LUMO+1||(43%)|
aWeight percentage of the given configuration in the CASSCF(12,12) wave function within parentheses.
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