J. Phys. Chem. A 2010, 114, 8319–8322
8319
Effect of Cations on Absorption Bands of First Electronic Transition of Liquid Water Akifumi Ikehata,*,†,‡ Motoki Mitsuoka,† Yusuke Morisawa,† Naomi Kariyama,§ Noboru Higashi,§ and Yukihiro Ozaki*,† Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity, 2-1 Gakuen, Sanda 669-1337, Japan, National Food Research Institute, National Agriculture and Food Research Organization (NARO), 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan, and KURABO Industries Ltd., 14-5 Shimokida-cho, Neyagawa 572-0823, Japan ReceiVed: May 30, 2010; ReVised Manuscript ReceiVed: July 13, 2010
˜ rX ˜ ) of liquid water The effect of cations (Li+, Na+, K+, Rb+, and Cs+) on the first electronic transition (A was investigated by attenuated total reflection far ultraviolet spectroscopy. To negate the effect of anions, aqueous solutions of 1 M alkali metal nitrates and bromides were compared at a temperature of 25 °C. It is ˜ rX ˜ band of water, which shows a marked red shift with decreasing found that the peak energy of the A ˜ rX ˜ band can be hydrogen-bond strength, decreases with increasing cation size. The peak energies of the A approximated by a linear function of the inverse of the ionic radii of the alkali metal cations, which indicates (according to the Born equation) that the first electronic transition of water is characterized by the solvation energy of the cations. 1. Introduction The origin of the low-lying electronic transition band of water, ˜ r X ˜ , has still not been conclusively established. In the A ˜ rX ˜ band is significantly affected by condensed phase, the A molecular environments. It is empirically known that the peak ˜ rX ˜ band of water shows a marked shift from energy of the A 7.4 to 8.6 eV as a result of the phase transformation from vapor1 to ice.2 In the case of liquid water, the observed peak energy is approximately 8.3 eV, which decreases on heating3,4 or on addition of a salt; this is discussed in detail later. The decrease in the peak energy can be explained on the basis of the changes ˜ rX ˜ in the hydrogen bonds of water.5,6 The origin of the A band of water is attributed to the transition from the nonbonding orbital, 1b1 (closely related to the 2px orbital of the oxygen atom), to the valence (σ*) or Rydberg (3s) orbitals. The latter transition is caused by molecular orbital (MO) Rydbergization7 and has also been discussed on the basis of the analysis of water present in rare-gas solids.8 Following the early work of Williams et al.,9 liquid water has been accepted as an amorphous semiconductor possessing ˜ rX ˜ band can be regarded as an a very large band gap. The A excitonic transition from the highest valence band (originating from the nonbonding 1b1 orbital) to the conduction band. Coe et al. suggested that there is almost no probability of a direct vertical transition from the valence band to the bottom of the conduction band due to the ability of water molecules to ˜ rX ˜ transition of reorganize.10 The observed energy of the A water is characterized by thermodynamic cycles exhibiting adiabatic electron affinity and solvation energy of internal (OH-) and/or external anionic defects. The effect of anions on the long˜ rX ˜ band of water has been analyzed wavelength edge of the A from the viewpoint of the Urbach rule.9 However, the effect of cations has not been investigated extensively. This paper * Corresponding authors. E-mail address: A.I.,
[email protected]; Y.O.,
[email protected]. † Kwansei Gakuin University. ‡ National Agriculture and Food Research Organization (NARO). § KURABO Industries Ltd.
˜ rX ˜ band describes the effect of alkali metal cations on the A of water, investigated by attenuated total reflection far ultraviolet (ATR-FUV) spectroscopysa technique that enables spectral measurement up to 8.55 eV.11 2. Experimental Section ˜ rX ˜ band of To investigate the effect of cations on the A water, the ATR-FUV spectra of 1 M solutions of alkali metal salts (nitrates and bromides) were measured. Alkali metal (Li, Na, K, Rb, and Cs) nitrates and bromides were purchased from Wako Pure Chemical Industries Ltd. (Tokyo, Japan) and mixed with ultrapure water (Purelab Ultra, Analytic, Organo Co. Ltd., Tokyo, Japan) without further purification. FUV spectra were measured by using an FUV monochromator purged with nitrogen gas (based on KV-200, Bunkoh-Keiki Co. Ltd., Tokyo, Japan) and equipped with an internal reflection element (IRE).11 The IRE was a 6-mm-equilateral triangular prism made of sapphire (Kyocera Co., Kyoto, Japan), and the angle of incidence was set at 60° (the total reflection condition). The absorption ˜ rX ˜ band maximum are so large (ca. 0.3 coefficients at the A for water) that it is very difficult to measure them by using the transmittance method. The penetration depth of the evanescent waves on the ATR probe developed by us was less than 40 nm; this enabled the measurement of the whole spectral shapes ˜ r X ˜ bands of various liquids.4,12 ATR absorbance of the A was defined as follows:
A ) -log(I/I0)
(1)
where I and I0 represent detected light intensities for sample solution and air, respectively. The values of refractive indices, nD, of the salt solutions were measured by an Abbe refractometer (NAR-1T, Atago Co., Ltd., Tokyo) at 25 °C. 3. Results Figure 1 shows the ATR-FUV spectra of 1 M alkali metal (Li, Na, K, Rb, and Cs) nitrate solutions and pure water obtained
10.1021/jp104951m 2010 American Chemical Society Published on Web 07/28/2010
8320
J. Phys. Chem. A, Vol. 114, No. 32, 2010
Figure 1. ATR-FUV spectra of aqueous solutions of 1 M alkali metal nitrates measured at 25 °C.
Ikehata et al.
˜ rX ˜ band Figure 3. Plots of ATR absorbance, -log(I/I0), of the A maxima of water versus inverse of ionic radii of cations, Rc-1.
Figure 2. ATR-FUV spectra of aqueous solutions of 1 M alkali metal bromides measured at 25 °C.
at 25 °C. The absorption bands at around 6.1 eV (203 nm) and 8.0 eV (155 nm) are due to NO3- and water, respectively. The band at 6.1 eV is assigned to the π f π* transition of NO3(from D3h symmetric structure, 1A1′ to the low-lying electronic excited state, 1E′),13 whereas the high-absorption band at 8.0 ˜ r X ˜ ) of water. eV is the first electronic transition (A Significantly, the spectral shapes of the π f π* transition band are perfectly matched for all the cations (because they have the same concentration), even though the cations are of different ˜ rX ˜ band of water is different kinds. On the other hand, the A ˜ rX ˜ band decreases for each cation. The absorbance of the A with the addition of nitrate salts and with decreasing cation size. Moreover, there is a notable, albeit slight increase in the peak energy with decreasing cation size. Figure 2 shows the ATRFUV spectra of 1 M alkali metal bromide solutions and pure water. A broad peak centered at 6.5 eV (190 nm), which is composed of splitting two peaks at 6.3 eV and at 6.7 eV, is the charge-transfer-to-solvent (CTTS) band corresponding to the CT ˜ rX ˜ band of water present in from Br- ions to water.14 The A the bromide solutions also reveals this effect of cations. 4. Discussion ˜ r X ˜ Band. As Intensity of ATR Absorbance of the A ˜ rX ˜ band shown in Figures 1 and 2, the absorbance of the A ˜ decreases with decreasing cation size. The intensities of the A ˜ r X band maxima were plotted in Figure 3 as functions of the
Figure 4. Refractive indices, nD, of 1 M bromide and nitrate solutions versus inverse of ionic radii of cations, Rc-1.
inverse of Shannon’s ionic radii of the alkali metal cations, Rc-1. We adopted the ionic radii determined in the case where the coordination numbers of water are 4 for Li+ and 6 for the other cations.15 The plots of the ATR intensities seem to have no correlation with Rc-1; however, the data show similar profiles for the nitrate and bromide. Further, we found similar profiles in the plots of the refractive indices, nD, of 1 M solutions vs Rc-1 as shown in Figure 4. As compared with the values of nD, it is considered that the ˜ r X ˜ band is change in the absorption intensity of the A predominantly attributed to the refractive indices of the salt solutions. The effect of refractive index on an ATR spectrum can be easily confirmed by a simple optical simulation based on the Fresnel formulas. Figure 5 shows simulated ATR spectra of water and aqueous solutions having different refractive indices. Optical constants of water and sapphire (the internal reflection element) were modeled by suitable functions of wavelength comparable to the reported values.16,17 For these calculations, the refractive indices of the solutions, n(λ), were only increased by a step of 0.001 (n, n + 0.001, ..., n + 0.005). The real gap of the refractive indices of the solutions of LiNO3 and CsNO3 (or LiBr and CsBr) is about 0.003. The corresponding change in the ATR absorbance was 0.01. It is found in
First Electronic Transition of Liquid Water
J. Phys. Chem. A, Vol. 114, No. 32, 2010 8321
E ) aRc-1 + ∆E
(2)
There is a significant difference in the intercepts, ∆E, for the nitrate and bromide solutions (7.97 and 7.88 eV, respectively) because of the effect due to anions; however, this has not been addressed comprehensively in this study. The slopes, a, attributed to the cations are approximately 0.04 for both the nitrate and bromide series. The Gibbs energies of solvation, ∆G, are proportional to Rc-1 of ions that interact via a Coulomb force ˜ rX ˜ transition energy.10 On the and are also related to the A basis of an analogy between the linear relation in Figure 6 and the Born equation18 ˜ rX ˜ band of water for Figure 5. Simulated ATR spectra in the A samples with different refractive indices.
˜ rX ˜ band in ATR-FUV spectra of Figure 6. Peak energies of the A alkali metal nitrates and bromides versus inverse of ionic radii of cations.
Figure 4 that a difference for the increase of 0.003 in the refractive index produces an increase of the ATR absorbance ˜ rX ˜ band of approximately 0.005. For the at the peak of the A calculation, the real part of the refractive index was independently changed from the imaginary part of the complex refractive index. However, in reality, the imaginary part, that is, the absorption index, κ, should be changed in conjunction with the real part, n. The deviation between the observed and calculated values of the ATR absorbance may be caused by the effect of the absorption index itself. However, it is anyway ˜ rX ˜ clear that the differences of the ATR absorbance of the A band of water are predominantly due to refractive indices of solutions. ˜ rX ˜ Band. To confirm the effect of Energy Shift of the A ˜ rX ˜ band, the peak energies were determined cations on the A by cubic spline interpolation. The Kramers-Kronig transformation has not been used to calculate the proper peak energies4 because the peak energies are too close to the spectral cutoff at 8.55 eV (see Supporting Information). Figure 6 shows the ˜ rX ˜ band and the relations between the peak energy of the A inverse of the radii of the alkali metal cations, Rc-1. The circles and rectangles represent the peak energies for the nitrate and bromide solutions, respectively. The plots indicate the average and error resulting from multiple measurements. As seen in the figure, there are linear relations between the series of salts. The solid lines are linear fits to the data, obtained by
∆GBorn )
q2 1 1 - Rc-1 2 ε
(
)
(3)
˜ rX ˜ transition is characterized by the it is suggested that the A solvation energy of the cations. The nearly identical values of the slopes (∼0.04) indicate the validity of the Born equation for water as a continuum having a uniform dielectric constant, ε. More accurate modified Born equations (a distance-dependent dielectric field,19 a modified cavity radius,20 and an empirical coefficient based on electronegativity21) were also tested. Although the peak energies must be compared with the modified radii for these equations, linear relations were obtained in the error range. Since our purpose is not a verification of the Born equation, we emphasize here the relation between the peak ˜ rX ˜ transition and the solvation energies of energies of the A the cations. Further, from Figure 6, it can be seen that the energy ˜ rX ˜ band from Li+ to Cs+ are approximately shifts of the A -0.03 eV for both series. The corresponding shift of the O-H stretching vibration energy of water, observed in the IR region, is approximately +0.004 eV and is not proportional to Rc-1.22,23 ˜ rX ˜ transition for H2O and Given that the band gap of the A D2O and that of the stretching vibration are of the same order of magnitude,5 the shifts shown in Figure 6 can be attributed not to the vibrational state but, instead, to a nonvertical transition with the reorganization of water molecules around the cations. 5. Conclusions We found that the peak energy of the first electronic transition ˜ rX ˜ ) band of water decreases with increasing cation size (A ˜ by using ATR-FUV spectroscopy. The peak energies of the A ˜ band can be approximated by a linear function of the r X inverse of the ionic radii of the alkali metal cations. According to the Born equation, the result indicates that the first electronic transition of water is characterized by the solvation energy of ˜ the cations. We also found that the ATR absorbance of the A ˜ r X band maximum relates to the refractive index of the salt solution. Compared with the transmission spectrum, the ATR spectrum is shifted to lower energies, owing to the strength of absorption. Although it is not impossible to calculate the corresponding peak energies by the Kramers-Kronig transformation, the results ˜ r X ˜ peak have considerable uncertainties because the A energies are too close to the high-energy spectral cutoff at 8.55 eV (see Supporting Information). We would rather emphasize to the ATR-FUV spectroscopy successfully works as a down converter and allows us to measure entire band of the first electronic transition of water and will open up new areas of research on hydration nature of water as well as quantitative analysis of minor electrolytes.
8322
J. Phys. Chem. A, Vol. 114, No. 32, 2010
Acknowledgment. This work was supported by the System Development Program for Advanced Measurement and Analysis (Program-S) of the Japan Science and Technology Agency (JST). Supporting Information Available: Discussion of the results of the Kramers-Kronig transformation of the ATR-FUV spectra, including absorption spectra, refractive index spectra, and plots of the peak energies and intensities vs the inverse of the ionic radii. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gurtler, P.; Saile, V.; Koch, E. E. Chem. Phys. Lett. 1977, 51, 386. (2) Shibaguchi, T.; Onuki, H.; Onaka, R. J. Phys. Soc. Jpn. 1977, 42, 152. (3) Marin, T. W.; Takahashi, K.; Bartels, D. M. J. Chem. Phys. 2006, 125, 104314. (4) Ikehata, A.; Ozaki, Y.; Higashi, N. J. Chem. Phys. 2008, 129, 234510. (5) Bursulaya, B. D.; Jeon, J.; Yang, C.-N.; Kim, H. J. J. Phys. Chem. A 2000, 104, 45. (6) Chipman, D. M. J. Chem. Phys. 2005, 122, 044111.
Ikehata et al. (7) Rubio, M.; Serrano-Andres, L.; Merchan, M. J. Chem. Phys. 2008, 128, 104305. (8) Chergui, M.; Schwentner, N. Chem. Phys. Lett. 1994, 219, 237. (9) Williams, F.; Varma, S. P.; Hillenius, S. J. Chem. Phys. 1976, 64, 1549. (10) Coe, J. V.; Earhart, A. D.; Cohen, M. H.; Hoffman, G. J.; Sarkas, H. W.; Bowen, K. H. J. Chem. Phys. 1997, 107, 6023. (11) Higashi, N.; Ikehata, A.; Ozaki, Y. ReV. Sci. Instrum. 2007, 78, 103107. (12) Morisawa, Y.; Ikehata, A.; Higashi, N.; Ozaki, Y. Chem. Phys. Lett. 2009, 476, 205. (13) Maria, H. J.; McDonald, J. R.; McGlynn, S. P. J. Am. Chem. Soc. 1973, 95, 1050. (14) Majumdar, D.; Kim, J.; Kim, K. S. J. Chem. Phys. 2000, 112, 101. (15) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (16) Leviton, D. B.; Madison, T. J. Proc. SPIE 1998, 3425, 219. (17) Heller, J. M., Jr.; Hamm, R. N.; Birkhoff, R. D.; Painter, L. R. J. Chem. Phys. 1974, 60, 3483. (18) Born, B. Z. Phys. 1920, 1, 45. (19) Abe, T. J. Phys. Chem. 1986, 90, 713. (20) Latimer, W. M.; Pitzer, K. S.; Slansky, C. M. J. Chem. Phys. 1939, 7, 108. (21) Qureshi, P. M.; Varshney, R. K. J. Chem. Educ. 1989, 66, 641. (22) Max, J.-J.; de Blois, S.; Veilleux, A.; Chapados, C. Can. J. Chem. 2001, 79, 13. (23) Lee, H.-M.; Tarakeshwar, P.; Park, J.; Kolaski, M. R.; Yoon, Y. J.; Yi, H.-B.; Kim, W- Y.; Kim, K. S. J. Phys. Chem. A 2004, 108, 2949.
JP104951M
