formamide:acetonitrile system

Journal of Molecular Structure 829 (2007) 37–43 www.elsevier.com/locate/molstruc

Vibrational spectroscopic characterization of stable solvates in the LiClO4/formamide:acetonitrile system Wagner A. Alves

*

Departamento de Quı´mica, Instituto de Cieˆncias Exatas, Universidade Federal Rural do Rio de Janeiro, Rodovia BR 465/Km 7, Serope´dica, RJ 23890-000, Brazil Received 26 April 2006; received in revised form 25 May 2006; accepted 5 June 2006 Available online 1 August 2006

Abstract Raman and infrared (IR) experiments of extremely high concentrated solutions (1.0–5.0 M) of lithium perchlorate in equimolar formamide (FA) and acetonitrile (ACN) mixture were carried out. Raman quantitative analyses performed in the C@O and C„N bands of FA and ACN, respectively, the appearance of a new IR band in the N–H stretching region and the presence of the band at 939 cm1 were interpreted in terms of a solvent separated ion pairs model. In salt concentrations higher than 3.0 M, a weak band at 945 cm1 (contact ion pairs) can also be seen. Indeed, the mixture of solvents with considerable differences in the acid-base characters allows to prepare electrolytic solutions where the contact ion pairs formation is low. A coordination number of 4 for the lithium cation is suggested and this value is in full agreement with other authors. Fundamental aspects and the importance of the present system for the development of new lithium-based rechargeable batteries are also discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: IR and Raman spectra; LiClO4; Formamide; Acetonitrile

1. Introduction Binary solvent mixtures and their solvation structures around lithium ion are of special importance to applications in technology and science. From the technological point of view, solvents with very different physical and chemical properties are often used together to perform various functions, simultaneously, in electrochemical devices. For example, binary solutions consisting of a high permittivity solvent and another of low viscosity are commonly used in lithium-based rechargeable batteries [1]. From the fundamental point of view, the different interactions which take place in an electrolytic solution often determine the battery performance [2]. In general, the solvents used in lithium-based rechargeable batteries are Lewis bases (aprotic solvents) that preferentially interact with cations [2,3]. On the other hand, anions are poorly solvated [4] and as *

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so a high degree of ion pairing in concentrated solutions is observed. This decreases the conductivity and the lithium ion transference in electrochemical systems. In order to increase the ion dissociation, McBreen et al. [5] have synthesized anion complexing agents based on Lewis acid centers. Their results show that the addition of aza-crown ether enhances the conductivity of lithium salt electrolytes in THF. Therefore, detailed information about the ion–ion, ion–solvent and solvent–solvent interactions would enable us to design advanced battery systems. Indeed, despite the impressive growth in sales of batteries worldwide, the fundamental science is still today often criticized for its slow advancement [6]. In recent paper [7], protic–aprotic solvent mixtures such as formamide (FA)–acetonitrile (ACN) allow one to observe by Raman spectroscopy the formation of a Lewis acid-base adduct. In these mixtures, ACN acts rather like small particles which disrupt the chainlike structure of FA, resulting in a new structural arrangement. The m2 (C„N) fundamental was then typically resolved and used

38

W.A. Alves / Journal of Molecular Structure 829 (2007) 37–43

to quantify the stoichiometry of the FA–ACN adduct in the limit of infinite dilution. Owing to the considerable differences in the acid-base characters of this mixture, selective interactions can be observed when LiClO4 is added [8]. Moreover, the conductivity is not reduced in the more concentrated solutions of this salt, suggesting that a high degree of ion pairing must not occur. Indeed, FA is an excellent solvent because it can coordinate with both cations and anions. The basic site (CO) can accommodate cations whereas the acid site (NH) can bond to anions. At the same time, the C„N group of ACN is a very sensitive probe for interactions of this solvent with cations [9] and other acidic centers [7,10]. In addition, the physical properties of both FA and ACN lead to an attractive system for electrolyte research, where the former acts as solvent and the latter as cosolvent. For example, in primary Li/SO2 cells, the need to maintain high solution conductivities at very low temperatures (40 °C) is accomplished by adding ACN to the electrolyte [11]. In this paper, extensive Raman and infrared studies of the LiClO4/formamide:acetonitrile system will be presented in order to ratify the results of a recent communication [8], where stable solvates were proposed. Here, the experiments are designed to quantify the individual solvation numbers around lithium ion and to discuss the different interactions present in extremely high concentrated solutions of the salt. 2. Experimental Raman spectra were obtained with Nicolet FT-Raman 950 using the 1064 nm line of the Nd:YAG laser at 1300 mW of power and InGaAs detector operating at room temperature. The samples were inserted in 5 mm diameter tubes. The infrared spectra were recorded on a Nicolet FT-IR Magna 760 spectrophotometer using NaCl windows. Both infrared and Raman spectra were obtained with 1 cm1 resolution at the temperature of 24 ± 2 °C. Formamide (Merck) was distilled at reduced pressure, discarding the first and last portions of distillate and maintained under a N2 atmosphere. Acetonitrile (Vetec) was ´˚ dried using thermally activated 3 A molecular sieves. Lithium and sodium perchlorates (Vetec) were dried by heating under vacuum at 150 and 100 °C, respectively. All concentrations are expressed as molarities (mol L1) and the spectral components were obtained by fitting Lorentzian profiles using ORIGIN MICROCAL 6.1 software [12]. To the C„N and C@O stretching regions, the result of the curve fit does not show well-pronounced components, however, the individual components are very well supported by analogy with the pure ACN and FA spectra with regard to the number, half height band widths, relative positions and intensities. Each component was fitted by a systematic series of manual adjustment of band parameters until contours in spectra of several different solutions were matched by changing only the height of the components and ending with computer refinement within the constraints of the established information.

Before calculation of the integrated intensities, each spectrum was normalized by dividing the experimental data points by the peak height value of the 459 cm1 line of CCl4, which was used as an external standard [7,13]. 3. Results 3.1. Cation–solvent interactions and determination of the solvation number of Li+ The characteristic bands of both FA and ACN are located at wavenumbers well distinct. Thus it is possible to observe separately the participation of both vibration modes in the interactions with the salt as well as to estimate the solvation number with accuracy. Raman spectra of the equimolar FA and ACN mixture containing variable lithium perchlorate concentrations (1.0–5.0 M) are illustrated in Figs. 1–3. The addition of LiClO4 to the present binary mixture gave significant spectral changes in the C@O stretching region (Fig. 1). In this region, the component at 1685 cm1 is assigned to hydrogen-bonded species forming a chain of FA molecules whereas the component at 1700 cm1 corresponds to FA molecules which have no hydrogen bonds to their carbonyl groups [14,15]. As the concentration of LiClO4 is increased, a new band at 1714 cm1 increases in intensity at the expense of the 1685 cm1 component. In the C–N stretching region, a similar behavior can also be seen for the bands at 1330 and 1311 cm1 (Fig. 2). These spectral variations are consistent with the assignment of the new bands to FA coordinated to the Li+ ions [16]. Significant spectral changes with increasing concentration of lithium cations can also be observed in the C„N region of ACN (Fig. 3). The band envelope in the region between 2270 and 2240 cm1 was discussed in details in a previous work [7]. The band at 2253 cm1 corresponds to non-hydrogen-associated ACN molecules whereas the 2257 cm1 band is assigned to a FA–ACN adduct, via hydrogen bonding. These two components are also accompanied by other two bands at 2250 and 2246 cm1, which are assigned to hot bands. With the addition of LiClO4, a new band at 2275 cm1 appears and its intensity increases with increasing salt concentration. A similar change is observed for the band at 2293 cm1. Another new band at 2306 cm1 increases in intensity at the expense of the original. The new bands at 2275 and 2306 cm1 are assigned to the C„N stretching and combination mode, (m3 + m4), of ACN complexed to the Li+ ions, respectively [9]. The resolved Raman spectra, in the C@O stretching region, were then used to calculate the concentrations of the different forms of FA in the binary mixture in different compositions of salt. Here, during the course of a quantitative analyses, the solvent molecules will be classified into two groups, non-coordinated and coordinated, corresponding to Raman bands observed at the lower and higher frequencies, respectively. By identical calculations to those described previously [7,17] the specific intensities

W.A. Alves / Journal of Molecular Structure 829 (2007) 37–43

a

39

a

0.05

0.00 1380

0,00 1760

1740

1720

1700

1680

1660

1640

1620

1360

1340

1320

1300

1280

1260

1240

1360

1340

1320

1300

1280

1260

1240

b

0,030 b 0,025

0.05

0,020

0,015

0,010

0,005 0.00 1380

0,000 1760

1740

1720

1700

1680

1660

1640

1620

c 0,03

Intensity

c

Intensity

0,02

0,01

0.0

1380 1360 1340 1320 1300 1280 1260 1240 0,00

1760

1740

1720

1700

1680

1660

1640

Wavenumber/cm-1

1620

Wavenumber/cm-1 Fig. 1. Raman spectra of LiClO4/FA:ACN solutions in the C@O stretching region. Molar concentrations of LiClO4 are: (a) 1.0; (b) 3.0; (c) 5.0. The deconvolution results using Lorentzian profiles are also shown.

of the bands from the non-coordinated (JNC) and coordinated (JC) forms of FA, were measured from a plot of IT/cT (total relative integrated intensity ratio for the two chemical species and the total FA concentration) against IC/cT (where IC corresponds to the relative integrated intensity for the coordinated species). The data fit the equation I T =cT ¼ ð1  J NC =J C ÞI C =cT þ J NC

ð1Þ

giving JNC = 0.189 and JC = 0.043. From these values the concentrations from the non-coordinated (cNC) and coordinated (cC) forms of FA were obtained. To the C„N

Fig. 2. Raman spectra in the C–N stretching region. (a) FA:ACN mixture; (b) 3.0 mol L1 of LiClO4/FA:ACN; (c) 5.0 mol L1 of LiClO4/ FA:ACN. The deconvolution results using Lorentzian profiles are also shown and the band located at 1375 cm1 is assigned to the CH3 bending of ACN.

stretching region, the concentration of ACN coordinated to the Li+ ions has been determined subtracting from the total ACN concentration the concentrations of non-hydrogen-associated ACN and of the FA–ACN adduct. To obtain the concentrations of these two latter species it was necessary to use the specific intensity values reported in a recent work [7]. In this way, the solvation number of the lithium cation, equivalent to the average ligand number of lithium, nS, can easily be derived nS ¼ cC =cSalt ¼ I C =ðcSalt J C Þ

ð2Þ

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W.A. Alves / Journal of Molecular Structure 829 (2007) 37–43

, is enhanced so that the total solvation number, nACN s nsFAþACN , remains unaltered. A detailed explanation on this subject will be presented in Section 4.

a 0, 5

0, 2 0

3.2. Anion–solvent interactions

0, 1 5

0, 1 0

0, 0 5

0, 0 0 234 0

2 32 0

2 300

22 80

22 60

224 0

2 22 0

2 200

234 0

2 32 0

2 300

22 80

22 60

224 0

2 22 0

2 200

2340

2320

2300

2280

2260

2240

2220

2200

b 0, 1 5

0, 1 0

0, 0 5

0, 0 0

Intensity

c

0. 0

-1

Wavenumber/cm

Spectral variations with increasing concentration of lithium perchlorate were also observed in the N–H stretching region (Fig. 4). In this region, the IR spectrum of neat FA shows a broad and very intense absorption with four bands centered at 3190, 3270, 3325 and 3410 cm1. The values in wavenumbers and bands shape are in agreement with the literature [18]. The appearance of the 3568 cm1 band suggests anion–solvent interactions, as adopted in a previous work [19]. However, to remove possible contradictions in the interpretation of the spectra, the cation effect on the N–H stretching region was also investigated (Fig. 5). As can be seen, there is no cation effect on the frequency of the new component, since the behavior observed for both lithium and sodium perchlorate is the same. Thus, the band at 3568 cm1 has been assigned to FA–ClO4  interactions via hydrogen bonding, similar to the work of Bukowska [20] where the new band was also seen on the high frequency side, using partly deuterated formamide, relative to the neat FA bands. The interpretation to the spectral changes in the N–H stretching region of FA is consistent with FA–ClO4  interactions via hydrogen bonding. At the same time, the quantitative treatment performed for the C@O and C„N stretching bands of FA and ACN, respectively, shows that a competition between the two solvents by Li+ occurs (Table 1). Thus, it would be interesting to investigate the region of the anion since concentration-dependent variations in the spectra of oxyanions are usually interpreted in terms of symmetry perturbations due to ion pairs formation [21]. 3.3. Cation–anion interactions

Fig. 3. Raman spectra of LiClO4/FA:ACN solutions in the C„N stretching region. Molar concentrations of LiClO4 are: (a) 1.0; (b) 3.0; (c) 5.0. The deconvolution results using Lorentzian profiles are also shown.

These values are presented in Table 1 for each lithium perchlorate concentration and as can be seen, the solvation number with respect to the FA molecules, nFA s , decreases as the salt concentration is increased. On the other hand, the solvation number with respect to the ACN molecules,

The non-coordinated ClO4  has Td symmetry and thus its nine vibrational degrees of freedom are divided into four modes: Cvib = A1 (m1) + E (m2) + 2F2 (m3; m4). All modes are Raman active, but only F2 modes are IR active. The weakly coordinating character of the perchlorate anion, in solution, gives Raman spectral variations limited to m1 and m2 regions alone [22]. In the present system, the m2 mode is not affected by increasing the salt concentration, but the m1 band presents a slight asymmetry. Thus, special atten-

Table 1 Concentration, relative integrated intensities and average solvation number of Li+ from Raman spectra [LiClO4]/M

[FA] = [ACN]/M

IC

IT

[FA-Li+]/M

nFA s

[ACN-Li+]/M

[FA–ACN]/M

nACN s

nFAþACN s

1.0 2.0 3.0 4.0 5.0

10.3 9.9 9.1 8.7 8.3

0.09 0.14 0.16 0.16 0.22

1.46 1.48 1.22 0.98 0.83

2.1 3.2 3.6 3.8 5.2

2.1 1.6 1.2 1.0 1.0

0 0.2 1.4 2.8 4.2

6.7 6.3 5.9 4.2 3.1

0 0.1 0.5 0.7 0.8

2.1 1.7 1.7 1.7 1.8

W.A. Alves / Journal of Molecular Structure 829 (2007) 37–43

a

41

1,0

1,6 1,4 0,8

a

Absorbance

1,2

0,6

1,0 0,8

b

0,6 0,4

0,4

0,2 3700

3600

0,2

3400

3300

3200

3100

3000

-1

Wavenumber/cm 3600

b

3500

3400

3200

3000

1,2

1,0

Fig. 5. IR spectra in the N–H stretching region. (a) 2.0 mol L1 of LiClO4/FA:ACN; (b) 2.0 mol L1 of NaClO4/FA:ACN.

a

35

0.30

0,8

0.25 0.20

0,6

0.15 0.10

0,4

0.05 0.00

0,2

975 3700

3600

3500

3400

3300

3200

3100

3000

c

925

950

925

900

b 0.4

0,8

0.3

Intensity

Absorbance

950

0,6

0.2

0.1

0.0

0,4

975

Wavenumber/cm

900 -1

0,2

3700 3600 3500 3400 3300 3200 3100 3000

Fig. 6. Raman m1 band of perchlorate in mixture of FA:ACN. Molar concentrations of LiClO4 are: (a) 3.0; (b) 5.0. Results of the curve-fitting are also shown.

-1

Wavenumber/cm

Fig. 4. IR spectra in the N–H stretching region. (a) Neat FA; (b) 1.0 mol L1 of LiClO4/FA:ACN; (c) 5.0 mol L1 of LiClO4/FA:ACN. The arrow indicates the position of the new component.

former is assigned for the C–C stretching of ACN and the second to the overtone of the m2 band of non-coordinated ClO4  [27].

tion has been paid to this band and as can be seen in the Raman spectra (Fig. 6), the m1 fundamental at 934 cm1 (non-coordinated ClO4  ) is accompanied by another band at 939 cm1 in all concentrations. In salt concentrations higher than 3.0 M, a weak band at 945 cm1 can also be observed. The positions of these later bands are in agreement with solvent separated ion pairs and contact ion pairs, respectively [23–26]. On the low frequency side two other features at 922 and 911 cm1 may still be seen. The

4. Discussion The spectral changes observed when LiClO4 is added to the equimolar FA and ACN mixture were interpreted in terms of ion solvation and association. The carbonyl band was extensively studied by Mortensen et al. [14,15]. Their results, using a dipolar aprotic organic solvent, show that the bands at 1685 and 1700 cm1 can be explained in terms of two sites. One belonging to a hydrogen-bonded species

42

W.A. Alves / Journal of Molecular Structure 829 (2007) 37–43

and the other to a ‘‘free’’ species without hydrogen bonding to the carbonyl group. In addition, the 1714 cm1 band is correctly assigned to FA coordinated to the Li+ ions, because it is not present in the Raman spectra of binary mixtures of FA and ACN in different compositions [7] and its intensity is dependent on the salt concentration. Although the spectral variations observed in the C–N stretching band of FA are lower than that exhibited to the carbonyl band, the behavior in both regions allows us to assume that Li+ is being coordinated by O and N atoms [16,28]. In the C„N stretching region of ACN, the intensity of the 2253 cm1 band is strongly reduced as the salt concentration is increased. On the other hand, the decrease in the intensity of the band at 2257 cm1 occurs smoothly. These discrepancies can be explained based on the liquid structure of the binary mixture. For example, when Li+ is inserted into such mixture, the non-hydrogen-associated ACN (dipole–dipole forces) needs to break weak intermolecular bonds, unlike the FA–ACN adduct whose interactions are of the hydrogen bonding type. This might be the reason for these discrepancies since the specific intensities of the bands at 2253 (JNA = 0.193) and 2257 cm1 (JA = 0.277) are not roughly different [7]. The new bands at 2275 and 2306 cm1 can then be assigned to ACN coordinated to the Li+ ions, because their intensities are clearly dependent on salt concentration. A competition between FA and ACN molecules by Li+ is observed and this is owing to their electron-pair donating abilities. However, based on Table 1, it is possible to observe a preferential solvation of Li+ by FA molecules, where in a salt concentration equal at 1.0 mol L1 ACN molecules are not present in the inner-sphere of the lithium cation. Indeed, the Gutmann donor numbers of FA (DN = 24.0 kcal mol1) and of ACN (DN = 14.1 kcal mol1) are well different [29]. At higher salt concentrations there are insufficient FA molecules to allow for a complete solvation sphere and thus ACN molecules begin to interact with Li+. It is worth noting that in a salt concentration equal to 5.0 mol L1, the total FA concentration is identical to the sum from the coordinated to Li+ and of the acid-base adduct forms of FA, showing that there are no available FA molecules to interact with lithium ions. The latter column of the Table 1 is also worthy of special attention. Although the total solvation number of the lithium cation, nFAþACN , remains unaltered, when s one ACN molecule penetrates in the inner-sphere of Li+ and releases one FA molecule, the coordination number changes from 4 to 3. This is due to the replacement of a bidentate ligand (FA) by another monodentade ligand (ACN). Thus, the fourth coordination site could be occupied by one of the O atoms of ClO4  and such interpretation is confirmed by the Raman spectra in the region of this anion (Fig. 6). Indeed, a value of 4 is usually found for the coordination number of the lithium cation, although it is obtained in the limit of infinite dilution [9,30]. The assignment for the 3568 cm1 band as belonging to FA–ClO4  interactions via hydrogen bonding is in agree-

ment with the electron-pair accepting ability of FA. It is worth noting that in the FA–ACN adduct the formamide hydrogen-to-CN p interactions are also present [7,29] and this enforce such interpretation. Indeed, the high Gutmann acceptor number (AN = 39.8 kcal mol1) of FA [29] might be the main cause for its interaction with anions and other molecules containing a donor atom. In a recent work [19], the appearance of the band at 3580 cm1, whose intensity increases with increasing concentration of sodium chlorate, was also interpreted taking into account the FA–ClO3  interactions. As can be seen, the magnitude of the frequency shift increases in the order ClO4  < ClO3  , the order of decrease of the anion radius. A similar behavior was also observed by Ishiguro et al. [31] with respect to the Mn(II), Zn(II) and Ni(II) ions. For end, the lack of a cation effect on the N–H stretching region discards any doubt about the assignment of the 3568 cm1 band. Unfortunately, the solvation number of the perchlorate anion has not been reported because most of the N–H bonds involve H-bonded oscillators and the spectral changes in this region were too small to be analytically useful. Ion pairs can be present in solution under several forms. The presence of these different structural arrangements is regarded one of the reasons for the decrease of conductivity in electrolytic systems. From the vibrational point of view, a cation–anion interaction induces measurable wavenumber shifts on the anion bands. The lowering of the local symmetry around ClO4  causes band splitting of the degenerate vibrations, except m1. Chabanel et al. [32] have affirmed that solvent separated ion pairs cannot be distinguished from free ions by vibrational spectroscopy, because the perturbation induced by them is smaller than that provoked by contact ion pairs. Thus, solvent separated ion pairs should yield bands at same wavenumber as for non-coordinated ClO4  . However, the affirmative of those authors is questionable since the 939 cm1 band has its intensity increased with increasing salt concentration. Moreover, the FA–ClO4  interactions observed in the N– H stretching region as well as ab initio molecular orbital calculations [33] show that the stabilization energy values for the solvent separated ion pair-FA complex are much higher than that determined for the ion-FA and contact ion pair-FA complexes. Therefore, the assignment for the band at 939 cm1 as belonging to solvent separated ion pairs seems consistent. In the solutions with salt concentrations higher than 3.0 M, a new structural arrangement such as contact ion pair is present. However, the concentration of this chemical species seems to be low (Fig. 6). Moreover, the absence of this later arrangement in solutions with salt concentrations 63.0 M makes the LiClO4/FA:ACN system a strong candidate for electrolytic research, since the concentration of most electrolytes for Li-based systems is almost always at least 1.0 M and, more commonly, between 1.5 and 3.0 M [11]. Finally, a recent work of Brouillette et al. [34] shows that systems where the participation of free ions and solvent separated ion pairs is majority have relatively high conductivities.

W.A. Alves / Journal of Molecular Structure 829 (2007) 37–43

Acknowledgments The author thanks Dr. C.A. Te´llez and Departamento de Quı´mica of PUC-RJ for the Raman facilities. This work was sponsored by Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico-CNPq, Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio de Janeiro-FAPERJ and Financiadora de Estudos e Projetos-FINEP (Project No. 65.953810). References [1] K. Xu, Chem. Rev. 104 (2004) 4303. [2] M. Morita, Y. Asai, N. Yoshimoto, M. Ishikawa, J. Chem. Soc. Faraday Trans. 94 (1998) 3451. [3] B. Klassen, R. Aroca, M. Nazri, G.A. Nazri, J. Phys. Chem. B 102 (1998) 4795. [4] W.A. Alves, R.B. Faria, Spectrochim. Acta A 58 (2002) 1395. [5] J. McBreen, H.S. Lee, X.Q. Yang, X. Sun, J. Power Sources 89 (2000) 163. [6] J.-M. Tarascon, M. Armand, Nature 414 (2001) 359. [7] W.A. Alves, O.A.C. Antunes, E. Hollauer, Vibrat. Spectrosc. 40 (2006) 257. [8] W.A. Alves, Vibrat. Spectrosc. Communication submitted. [9] X. Xuan, H. Zhang, J. Wang, H. Wang, J. Phys. Chem. A 108 (2004) 7513. [10] D. Jamroz, J. Stangret, J. Lindgren, J. Am. Chem. Soc. 115 (1993) 6165. [11] M. Salomon, Pure Appl. Chem. 70 (1998) 1905. [12] Origin Microcal, Version 6.1, Microcal Software, Northampton, USA, 2000.

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[13] W.A. Alves, C.A. Te´llez, O. Sala, P.S. Santos, R.B. Faria, J. Raman Spectrosc. 32 (2001) 1032. [14] A. Mortensen, O.F. Nielsen, J. Yarwood, V. Shelley, Vibrat. Spectrosc. 8 (1994) 37. [15] A. Mortensen, O.F. Nielsen, J. Yarwood, V. Shelley, J. Phys. Chem. 98 (1994) 5221. [16] J. Bukowska, J. Mol. Struct. 98 (1983) 1. [17] W.A. Alves, C.A. Te´llez, E. Hollauer, R.B. Faria, J. Raman Spectrosc. 35 (2004) 854. [18] J. Bukowska, Spectrochim. Acta A 35 (1979) 985. [19] W.A. Alves, C.A. Te´llez, E. Hollauer, R.B. Faria, Spectrochim. Acta A 62 (2005) 755. [20] J. Bukowska, Chem. Phys. Letters 57 (1978) 624. [21] W.A. Alves, R.B. Faria, Vibrat. Spectrosc. 31 (2003) 25. [22] A.G. Miller, J.W. Macklin, J. Phys. Chem. 89 (1985) 1193. [23] D.W. James, R.E. Mayes, Aust. J. Chem. 35 (1982) 1775. [24] R.L. Frost, D.W. James, R. Appleby, R.E. Mayes, J. Phys. Chem. 86 (1982) 3840. [25] D. Battisti, G.A. Nazri, B. Klassen, R. Aroca, J. Phys. Chem. 97 (1993) 5826. [26] X. Xuan, J. Wang, J. Lu, N. Pei, Y. Mo, Spectrochim. Acta A 57 (2001) 1555. [27] Y. Umebayashi, K. Matsumoto, M. Watanabe, K. Katoh, S. Ishiguro, Anal. Sci. 17 (2001) 323. [28] B.M. Rode, Chem. Phys. Lett. 35 (1975) 517. [29] J. Reimers, E.L. Hall, J. Am. Chem. Soc. 121 (1999) 3730. [30] J.M. Alı´a, H.G.M. Edwards, Vibrat. Spectrosc. 24 (2000) 185. [31] K. Fujii, T. Kumai, T. Takamuku, Y. Umebayashi, S. Ishiguro, J. Phys. Chem. A 110 (2006) 1798. [32] M. Chabanel, D. Legoff, K. Touaj, J. Chem. Soc. Faraday Trans. 92 (1996) 4199. [33] P. Mohandas, S. Singh, J. Mol. Struct. (Theochem) 361 (1996) 229. [34] D. Brouillette, D.E. Irish, N.J. Taylor, G. Perron, M. Odziemkowski, J.E. Desnoyers, Phys. Chem. Chem. Phys. 4 (2002) 6063.