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Infrared spectra of the H2O–Kr and H2O–Xe complexes in argon matrices
Chemical Physics Letters 418 (2006) 323–327 www.elsevier.com/locate/cplett
Infrared spectra of the H2O–Kr and H2O–Xe complexes in argon matrices Shinichi Hirabayashi *, Koichi M.T. Yamada National Institute of Advanced Industrial Science and Technology (AIST), EMTech, AIST Tsukuba-West, Onogawa 16-1, Tsukuba 305-8569, Japan Received 3 August 2005; in final form 2 November 2005 Available online 23 November 2005
Abstract We have identified the infrared spectra of the H2O–Kr and H2O–Xe van der Waals complexes trapped in argon matrices. For both the 1:1 complexes, the line positions for the antisymmetric OH stretching mode are blue shifted from the band origin for the H2O monomer, suggesting that H2O is bound to Kr and Xe with the non-hydrogen bonded geometry. In addition, the presence of Kr at nearest neighbor position induces changes on the line profile of two rovibrational transitions of the H2O monomer in the OH bending region, which is interpreted in terms of anisotropic potential perturbing the rotational levels. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Water–rare gas (H2O–Rg) complexes are most basic system for understanding the intermolecular and hydrogen bond interactions involving H2O molecule. These complexes have also attracted our interest from the viewpoint of the cryogenic matrix isolation studies. In the crystals of solid Rg such as Ne, Ar, Kr, and Xe, which have either face-centered cubic (fcc) or hexagonal close-packed (hcp) structure, the trapped H2O monomer is surrounded by twelve nearest neighbor Rg atoms. Therefore, the 1:1 H2O–Rg complexes serve as prototype for H2O molecules isolated in Rg matrices. The H2O–Ar complex has been extensively investigated in the gas phase using far-infrared [1–5], infrared [6–9], and microwave [10,11] spectroscopy. The intermolecular potential energy surface determined by Cohen and Saykally [12] revealed that the global minimum has a planar geometry, where the distance between Ar atom and center of mass ˚ and the angle between Rcm vector of H2O (Rcm) is 3.636 A and C2-axis of H2O (h) is 74.3° (see Fig. 1). This result is in
*
Corresponding author. Fax: +81 29 861 8264. E-mail address: [email protected] (S. Hirabayashi).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.11.011
good agreement with later ab initio calculations by Tao and Klemperer [13]. The rotational spectra of the larger H2O–Ar complexes, H2O–Ar2 [14], H2O–Ar3 [15], and (H2O)2–Ar [16] were also observed by Fourier transform microwave spectroscopy. For other H2O–Rg complexes, experimental information is limited to the microwave spectroscopic study of the H2O–Kr complex by van Wijngaarden and Ja¨ger [17], who suggested that the structure is similar to that of the H2O–Ar complex on the basis of the comparable D and 17O nuclear quadrupole coupling constants. Furthermore, ab initio potential energy surfaces [18] showed that the H2O–Kr complex has more hydrogen bonded geometry, compared to the H2O–Ar complex. To our knowledge, the H2O–Xe complex has been studied neither by quantum chemical calculation nor by gas phase spectroscopy. In recent studies of H2O monomer and clusters in Rg matrices [19], we have shown that the matrix-induced red shifts increase in the order of Ar, Kr, and Xe, indicating that H2O has stronger interaction energy with Kr and Xe than with Ar. The 1:1 complexes of H2O–Kr and H2O– Xe are thus expected to form in Ar matrices by doping a small amount of Kr and Xe. In this study, we present the infrared spectra of the H2O–Kr and H2O–Xe complexes
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S. Hirabayashi, K.M.T. Yamada / Chemical Physics Letters 418 (2006) 323–327
MCT detector at resolutions of 0.25 and 0.5 cm1, in the 600–4000 cm1 region.
C2
H2O
3. Results and discussion
Rcm=3.636A
Ar
Fig. 1. The structure of the H2O–Ar complex determined by Cohen and Saykally [12] is schematically shown.
trapped in Ar matrices; the line positions for the antisymmetric OH stretching mode are blue shifted from the band origin of the H2O monomer by 19.6 and 15.2 cm1 for the Kr and Xe complexes, respectively. 2. Experimental The experiments were carried out using a continuousflow liquid-helium cryostat (Oxford, Ultrastat). H2O was distilled and degassed in a vacuum line, and Ar (Nippon Sanso, 99.9995%), Kr (Nippon Sanso, 99.995%), and Xe (Spectra Gases, 99.999%) were used without further purification. The Ar/H2O/Rg (Rg = Kr or Xe) mixing ratios were varied between 1000/2/1 and 1000/2/20. The gas mixtures were prepared by standard manometric techniques and were deposited onto a gold-coated copper plate maintained at 25 K. The deposition rate was controlled by a needle valve and set to 2 mmol/h. Infrared spectra were measured in a reflection mode at 25 K using a JASCO 620 FTIR spectrometer equipped with a liquid-N2-cooled
The infrared spectrum of pure H2O in Ar matrices is shown in Fig. 2a, for reference. The rovibrational lines of H2O monomer are dominant in three fundamentals of H2O;m1 (symmetric OH stretching), m2 (OH bending), and m3 (antisymmetric OH stretching) modes, which are well characterized in the recent literatures [20,21]. Five of six infrared active modes of H2O dimer are also seen at 3708, 3633, 3574, 1611, and 1593 cm1. Although several weak lines of the water complexes with N2 impurity are also detected, the assignment has been reported in our previous study [22]. By adding a small amount of Kr or Xe to the sample, we have observed a few absorption lines arising from the complexes as follows. 3.1. H2O–Xe complex Fig. 2 shows the infrared spectra of two samples with different concentrations, Ar/H2O/Xe = 1000/2/1 (trace b) and 1000/2/5 (trace c). When Xe is added to the Ar/H2O samples, we have detected new absorption lines at 1589.3, 3635.0, and 3748.7 cm1. The intensities of these lines grow with increase of the Xe concentration in the sample. Therefore, they are assigned to the 1:1 H2O–Xe complex. The higher resolution (0.25 cm1) spectra of Xe-doped samples reveal a shoulder in the low wavenumber side of the 1:1 complex in the m2 region. Since the shoulder becomes more
Kr-H2O Kr-H2O
e
*
d
Absorbance
Xe-H2O
Xe-H2O Xe-H2O c b
*
* 3850
3800
3750
3700
3650
3600 1700
1650
1600
a
1550
1500
Wavenumber (cm -1) Fig. 2. Observed Ar matrix spectra in the OH stretching and bending regions of H2O are reproduced for five samples with different concentrations: (a) Ar/ H2O = 1000/2, (b) Ar/H2O/Xe = 1000/2/1, (c) Ar/H2O/Xe = 1000/2/5, (d) Ar/H2O/Kr = 1000/2/5, and (e) Ar/H2O/Kr = 1000/2/20. The spectra were recorded at 25 K with 0.5 cm1 resolution. The absorbance is normalized so that the peak intensities of the monomer lines at 3757 and 1608 cm1 are approximately same. Because of the N2 contamination, weak lines of H2O–N2 complex are detected as shown by the asterisks.
S. Hirabayashi, K.M.T. Yamada / Chemical Physics Letters 418 (2006) 323–327
325
Kr concentration in the sample, compared to the Xe-doped experiment. This is interpreted in terms of the difference of the binding energy between the complexes; the attractive force between H2O and Xe is larger than that between H2O and Kr.
Ar/H2O/Kr=1000/2/x
3.3. Line profile of H2O monomer
Absorbance
(c) x=20
(b) x=5
(a) x=0
1585
1580
1575
1570
1565
1560
Wavenumber (cm -1) Fig. 3. Effect of Kr concentration on the line profile is shown for the 110 ! 101 rovibrational transition of H2O monomer in the m2 region: (a) Ar/H2O = 1000/2, (b) Ar/H2O/Kr = 1000/2/5, and (c) Ar/H2O/ Kr = 1000/2/20. The spectra were recorded with 0.25 cm1 resolution. The absorbance is scaled for easy comparison.
discernible at high Xe concentration, this is likely due to the 1:n H2O–Xe complexes with n > 1. The study of H2O in Xe-doped Ar matrices was carried out more than a quarter of a century ago, in 1976, by Ayers and Pullin [23] who assigned the observed lines to the nonrotating H2O monomer. Since the lines were observed only by doping Xe, it is natural to assign those to the van der Waals complex H2O–Xe. We note that the reported frequencies [23] differ by a few cm1 from the value in this work. 3.2. H2O–Kr complex In contrast to the doped Xe experiments, no new absorption line has been detected with Ar/H2O/ Kr = 1000/2/1 sample. However, for high Kr concentration, we have observed new lines at 1589.1 cm1 in the m2 region and at 3553.1 cm1 in the m3 region. The infrared spectra of Ar/H2O/Kr = 1000/2/x samples with two different Kr concentration are shown in Fig. 2d (x = 5) and in Fig. 2e (x = 20). The observed lines are assigned to the 1:1 H2O–Kr complex on the basis of the concentration effects. Unfortunately, the m1 line of the 1:1 complex has not been seen even at the highest Kr concentration (x = 20) in the present study, presumably because of the low probability for complex formation. The observation of the 1:1 H2O–Kr complex lines requires a relatively high
In addition to the appearance of the 1:1 complex lines, at least two rovibrational transitions of the H2O monomer in the m2 region change their line profiles depending on the Kr concentration in the sample. Fig. 3 shows the profile of the 110 ! 101 rovibrational line near 1574 cm1; weak absorptions on both the high and low frequency sides appear when Kr was added to the sample. A weak shoulder is also observed on the low frequency side of the 101 ! 110 rovibrational transition near 1608 cm1, while no features can be identified on the high frequency side because of the overlap of the proton donor band of H2O dimer near 1611 cm1. Similar phenomenon in the rovibrational lines was found for the R(0) and P(1) lines of H35Cl in Kr-doped Ar matrices [24], which is interpreted as m degeneracy lifting of the J = 1 rotational level arising from the presence of one Kr atom in the first matrix shell surrounding H35Cl molecule. The line profile of the HCl–Rg complexes trapped in rare gas matrices was also theoretically investigated in detail [25,26]. We therefore propose that there are two kinds of matrix sites containing one Kr atom in the first Ar matrix shell; one results in the rotational quenching of H2O molecules by forming H2O–Kr complex, and the other results in the rotation of H2O in an anisotropic potential. In the Xe-doped Ar matrix, such changes in the spectral profiles have not been detected for the rotating monomer lines. Instead, the spectrum shows strong H2O–Xe complex lines. This indicates that the nearest neighbor Xe atoms mainly quench the rotation of H2O molecules and form complexes. 3.4. Structure of H2O–Rg complex Table 1 lists the line positions of three OH fundamental vibration modes within different H2O–X 1:1 complexes isolated in Ar matrices and their frequency shifts due to the cluster formation (cluster shifts) with respect to the band origin of the H2O monomer in the Ar matrix [27]. The structures of H2O complexes are classified into two categories: the hydrogen bonded and oxygen bonded geometries. In many cases, both the hydrogen and oxygen bonded complexes show red shifts in the two OH stretching modes, e.g. H2O–N2 [28], H2O–CO [29,30], H2O–NH3 [31], H2O– HCl [23,32], H2O–CO2 [21], and H2O–SO2 [33], except for the m1 mode of the H2O–N2. The m2 bending frequencies, on the other hand, show blue shifts, the magnitude of which seems to be slightly large in hydrogen bonded complexes compared to the oxygen bonded complexes. For the gas phase H2O–Ar complex, Nesbitt and Lascola
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Table 1 OH stretching (m1 and m3) and bending (m2) frequencies and their cluster shifts (Dm) for the H2O–X 1:1 complexes trapped in solid argon (in cm1) Species H2O, m0 (Ar) H2O–N2 H2O–CO H2O–NH3 H2O–HCl H2O–CO2 H2O–SO2 H2O–Kr H2O–Xe (H2O)2, PAa (H2O)2, PDb a b
Structure H-bonded H-bonded H-bonded O-bonded O-bonded O-bonded
O-bonded H-bonded
m3
Dm3
m1
Dm1
m2
3733.5 3729.6 3723.5 3702.2 3721 3732.46 3722.5 3753.1 3748.7 3737.8 3708.0
0.0 3.9 10.0 31.3 12.5 1.0 11.0 +19.6 +15.2 +4.3 25.5
3638.5 3640.2 3627.8 3434.9 3629 3638.0 3630.5
0.0 +1.7 10.7 203.6 9.5 0.5 8.0
1589 1593.1 1595
0.0 +4.1 +6.0
3635.0 3633.0 3573.6
3.5 5.5 64.9
1590.4 1589.48 1591 1589.1 1589.3 1593.1 1610.6
+1.4 +0.5 +2.0 +0.1 +0.3 +4.1 +21.6
Dm2
References [27] [28] [29,30] [31] [23,32] [21] [33] This work This work [20] [20]
Proton acceptor. Proton donor.
[7] derived the m3 origin to be 3754.4092 cm1, corresponding to a 1.72 cm1 red shift from the free monomer value. This result does not contradict with the hydrogen bonded like structure determined by Cohen and Saykally [12]. In contrast to these complexes, the m3 frequencies of the studied complexes are largely blue shifted by 19.6 cm1 for the H2O–Kr and 15.2 cm1 for the H2O–Xe with respect to the band origin of the H2O monomer in the Ar matrix. These blue shifts, considering the cluster shifts of various molecules listed in Table 1, suggest that H2O molecule is bound to Kr and Xe atoms with neither the hydrogen bonded nor oxygen bonded geometry. Since the previous studies of the H2O–Ar complex [12,13] indicated that the geometry with the planar structure is more favorable than out of plane structure, we assume that the Rg atom lies around intermediate position between the oxygen bonded geometry (h = 180°) and the linear hydrogen bonded geometry (h = 52°), which possibly strengthen one of the OH bonds upon complexation. The most stable structure of the H2O–Kr complex in the Ar matrix might not be identical to that in the gas phase proposed on the basis of the D and 17O nuclear quadrupole coupling constants [17]. We also note, on the other hand, that the angle h is estimated from the 83Kr nuclear quadrupole coupling constants to be 118.5° and 110.9° for the R000 and R101 states in the gas phase study [17]. Further gas phase experimental examination of the H2O–Kr and H2O–Xe complexes as well as theoretical study may be worthwhile to determine the structures uniquely. 4. Summary We have investigated the infrared spectra of the H2O– Kr and H2O–Xe complexes trapped in argon matrices. The absorption lines of the 1:1 complex are identified on the basis of the concentration effects. The line positions for the antisymmetric OH stretching mode in Ar matrices are blue shifted by complex formation. These blue shifts suggest that the non-hydrogen bonded structure is formed in argon matrices. We have also found that the presence of one Kr atom in the first matrix shell perturbs the rotational energy. In the Ar matrix, the weak interaction between
H2O and Kr results not only in the formation of the 1:1 complex but also in the changes of line profiles of rotating molecules, while the interaction between H2O and Xe causes only the 1:1 complex formation. Acknowledgments We thank Dr. F. Ito and Dr. H. Abe for helpful discussions. References [1] R.C. Cohen, K.L. Busarow, K.B. Laughlin, G.A. Blake, M. Havenith, Y.T. Lee, R.J. Saykally, J. Chem. Phys. 89 (1988) 4494. [2] R.C. Cohen, K.L. Busarow, Y.T. Lee, R.J. Saykally, J. Chem. Phys. 92 (1990) 169. [3] S. Suzuki, R.E. Bumgamer, P.A. Stockman, P.G. Green, G.A. Blake, J. Chem. Phys. 94 (1991) 824. [4] E. Zwart, W.L. Meerts, Chem. Phys. 151 (1991) 407. [5] R.C. Cohen, R.J. Saykally, J. Chem. Phys. 95 (1991) 7891. [6] R. Lascola, D.J. Nesbitt, J. Chem. Phys. 95 (1991) 7917. [7] D.J. Nesbitt, R. Lascola, J. Chem. Phys. 97 (1992) 8096. [8] M.J. Weida, D.J. Nesbitt, J. Chem. Phys. 106 (1997) 3078. [9] D. Verdes, H. Linnartz, Chem. Phys. Lett. 355 (2002) 538. [10] G.T. Fraser, F.J. Lovas, R.D. Suenram, K.T. Matsumura, J. Mol. Spectrosc. 144 (1990) 97. [11] T.C. Germann, H.S. Gutowsky, J. Chem. Phys. 98 (1993) 5235. [12] R.C. Cohen, R.J. Saykally, J. Chem. Phys. 98 (1993) 6007. [13] F.-M. Tao, W. Klemperer, J. Chem. Phys. 101 (1994) 1129. [14] E. Arunan, C.E. Dykstra, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 105 (1996) 8495. [15] E. Arunan, T. Emilsson, H.S. Gutowsky, C.E. Dykstra, J. Chem. Phys. 114 (2001) 1242. [16] E. Arunan, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 116 (2002) 4886. [17] J. van Wijngaarden, W. Ja¨ger, Mol. Phys. 98 (2000) 1575. [18] G. Chalasinski, M.M. Szczesniak, S. Scheiner, J. Chem. Phys. 97 (1992) 8181. [19] S. Hirabayashi, K.M.T. Yamada, J. Chem. Phys. 122 (2005) 244501. [20] J.P. Perchard, Chem. Phys. 273 (2001) 217. [21] X. Michaut, A.M. Vasserot, L. Abouaf-Marguin, J. Low Temp. Phys. 29 (2003) 852. [22] S. Hirabayashi, K. Ohno, H. Abe, K.M.T. Yamada, J. Chem. Phys. 122 (2005) 194506. [23] G.P. Ayers, A.D.E. Pullin, Spectrochim. Acta 32A (1976) 1641. [24] B. Laroui, J.P. Perchard, C. Girardet, J. Chem. Phys. 97 (1992) 2347.
S. Hirabayashi, K.M.T. Yamada / Chemical Physics Letters 418 (2006) 323–327 [25] B. Laroui, O. Damak, O. Maillard, C. Girardet, J. Chem. Phys. 97 (1992) 2359. [26] C. Girardet, A. Lakhlifi, B. Laroui, J. Chem. Phys. 97 (1992) 7955. [27] X. Michaut, A.-M. Vasserot, L. Abouaf-Marguin, Vib. Spectrosc. 34 (2004) 83. [28] S. Coussan, A. Loutellier, J.P. Perchard, S. Racine, Y. Bouteiller, J. Mol. Struct. 471 (1998) 37.
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[29] H. Abe, K.M.T. Yamada, J. Chem. Phys. 114 (2001) 6134. [30] A. Givan, A. Loewenschuss, C.J. Nielsen, J. Chem. Soc., Faraday Trans. 92 (1996) 4927. [31] A. Engdahl, B. Nelander, Chem. Phys. 91 (1989) 6604. [32] C. Amirand, D. Maillard, J. Mol. Struct. 176 (1988) 181. [33] A. Schriver, L. Schriver, J.P. Perchard, J. Mol. Spectrosc. 127 (1988) 125.
Infrared spectra of the H2O–Kr and H2O–Xe complexes in argon matrices Shinichi Hirabayashi *, Koichi M.T. Yamada National Institute of Advanced Industrial Science and Technology (AIST), EMTech, AIST Tsukuba-West, Onogawa 16-1, Tsukuba 305-8569, Japan Received 3 August 2005; in final form 2 November 2005 Available online 23 November 2005
Abstract We have identified the infrared spectra of the H2O–Kr and H2O–Xe van der Waals complexes trapped in argon matrices. For both the 1:1 complexes, the line positions for the antisymmetric OH stretching mode are blue shifted from the band origin for the H2O monomer, suggesting that H2O is bound to Kr and Xe with the non-hydrogen bonded geometry. In addition, the presence of Kr at nearest neighbor position induces changes on the line profile of two rovibrational transitions of the H2O monomer in the OH bending region, which is interpreted in terms of anisotropic potential perturbing the rotational levels. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Water–rare gas (H2O–Rg) complexes are most basic system for understanding the intermolecular and hydrogen bond interactions involving H2O molecule. These complexes have also attracted our interest from the viewpoint of the cryogenic matrix isolation studies. In the crystals of solid Rg such as Ne, Ar, Kr, and Xe, which have either face-centered cubic (fcc) or hexagonal close-packed (hcp) structure, the trapped H2O monomer is surrounded by twelve nearest neighbor Rg atoms. Therefore, the 1:1 H2O–Rg complexes serve as prototype for H2O molecules isolated in Rg matrices. The H2O–Ar complex has been extensively investigated in the gas phase using far-infrared [1–5], infrared [6–9], and microwave [10,11] spectroscopy. The intermolecular potential energy surface determined by Cohen and Saykally [12] revealed that the global minimum has a planar geometry, where the distance between Ar atom and center of mass ˚ and the angle between Rcm vector of H2O (Rcm) is 3.636 A and C2-axis of H2O (h) is 74.3° (see Fig. 1). This result is in
*
Corresponding author. Fax: +81 29 861 8264. E-mail address: [email protected] (S. Hirabayashi).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.11.011
good agreement with later ab initio calculations by Tao and Klemperer [13]. The rotational spectra of the larger H2O–Ar complexes, H2O–Ar2 [14], H2O–Ar3 [15], and (H2O)2–Ar [16] were also observed by Fourier transform microwave spectroscopy. For other H2O–Rg complexes, experimental information is limited to the microwave spectroscopic study of the H2O–Kr complex by van Wijngaarden and Ja¨ger [17], who suggested that the structure is similar to that of the H2O–Ar complex on the basis of the comparable D and 17O nuclear quadrupole coupling constants. Furthermore, ab initio potential energy surfaces [18] showed that the H2O–Kr complex has more hydrogen bonded geometry, compared to the H2O–Ar complex. To our knowledge, the H2O–Xe complex has been studied neither by quantum chemical calculation nor by gas phase spectroscopy. In recent studies of H2O monomer and clusters in Rg matrices [19], we have shown that the matrix-induced red shifts increase in the order of Ar, Kr, and Xe, indicating that H2O has stronger interaction energy with Kr and Xe than with Ar. The 1:1 complexes of H2O–Kr and H2O– Xe are thus expected to form in Ar matrices by doping a small amount of Kr and Xe. In this study, we present the infrared spectra of the H2O–Kr and H2O–Xe complexes
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MCT detector at resolutions of 0.25 and 0.5 cm1, in the 600–4000 cm1 region.
C2
H2O
3. Results and discussion
Rcm=3.636A
Ar
Fig. 1. The structure of the H2O–Ar complex determined by Cohen and Saykally [12] is schematically shown.
trapped in Ar matrices; the line positions for the antisymmetric OH stretching mode are blue shifted from the band origin of the H2O monomer by 19.6 and 15.2 cm1 for the Kr and Xe complexes, respectively. 2. Experimental The experiments were carried out using a continuousflow liquid-helium cryostat (Oxford, Ultrastat). H2O was distilled and degassed in a vacuum line, and Ar (Nippon Sanso, 99.9995%), Kr (Nippon Sanso, 99.995%), and Xe (Spectra Gases, 99.999%) were used without further purification. The Ar/H2O/Rg (Rg = Kr or Xe) mixing ratios were varied between 1000/2/1 and 1000/2/20. The gas mixtures were prepared by standard manometric techniques and were deposited onto a gold-coated copper plate maintained at 25 K. The deposition rate was controlled by a needle valve and set to 2 mmol/h. Infrared spectra were measured in a reflection mode at 25 K using a JASCO 620 FTIR spectrometer equipped with a liquid-N2-cooled
The infrared spectrum of pure H2O in Ar matrices is shown in Fig. 2a, for reference. The rovibrational lines of H2O monomer are dominant in three fundamentals of H2O;m1 (symmetric OH stretching), m2 (OH bending), and m3 (antisymmetric OH stretching) modes, which are well characterized in the recent literatures [20,21]. Five of six infrared active modes of H2O dimer are also seen at 3708, 3633, 3574, 1611, and 1593 cm1. Although several weak lines of the water complexes with N2 impurity are also detected, the assignment has been reported in our previous study [22]. By adding a small amount of Kr or Xe to the sample, we have observed a few absorption lines arising from the complexes as follows. 3.1. H2O–Xe complex Fig. 2 shows the infrared spectra of two samples with different concentrations, Ar/H2O/Xe = 1000/2/1 (trace b) and 1000/2/5 (trace c). When Xe is added to the Ar/H2O samples, we have detected new absorption lines at 1589.3, 3635.0, and 3748.7 cm1. The intensities of these lines grow with increase of the Xe concentration in the sample. Therefore, they are assigned to the 1:1 H2O–Xe complex. The higher resolution (0.25 cm1) spectra of Xe-doped samples reveal a shoulder in the low wavenumber side of the 1:1 complex in the m2 region. Since the shoulder becomes more
Kr-H2O Kr-H2O
e
*
d
Absorbance
Xe-H2O
Xe-H2O Xe-H2O c b
*
* 3850
3800
3750
3700
3650
3600 1700
1650
1600
a
1550
1500
Wavenumber (cm -1) Fig. 2. Observed Ar matrix spectra in the OH stretching and bending regions of H2O are reproduced for five samples with different concentrations: (a) Ar/ H2O = 1000/2, (b) Ar/H2O/Xe = 1000/2/1, (c) Ar/H2O/Xe = 1000/2/5, (d) Ar/H2O/Kr = 1000/2/5, and (e) Ar/H2O/Kr = 1000/2/20. The spectra were recorded at 25 K with 0.5 cm1 resolution. The absorbance is normalized so that the peak intensities of the monomer lines at 3757 and 1608 cm1 are approximately same. Because of the N2 contamination, weak lines of H2O–N2 complex are detected as shown by the asterisks.
S. Hirabayashi, K.M.T. Yamada / Chemical Physics Letters 418 (2006) 323–327
325
Kr concentration in the sample, compared to the Xe-doped experiment. This is interpreted in terms of the difference of the binding energy between the complexes; the attractive force between H2O and Xe is larger than that between H2O and Kr.
Ar/H2O/Kr=1000/2/x
3.3. Line profile of H2O monomer
Absorbance
(c) x=20
(b) x=5
(a) x=0
1585
1580
1575
1570
1565
1560
Wavenumber (cm -1) Fig. 3. Effect of Kr concentration on the line profile is shown for the 110 ! 101 rovibrational transition of H2O monomer in the m2 region: (a) Ar/H2O = 1000/2, (b) Ar/H2O/Kr = 1000/2/5, and (c) Ar/H2O/ Kr = 1000/2/20. The spectra were recorded with 0.25 cm1 resolution. The absorbance is scaled for easy comparison.
discernible at high Xe concentration, this is likely due to the 1:n H2O–Xe complexes with n > 1. The study of H2O in Xe-doped Ar matrices was carried out more than a quarter of a century ago, in 1976, by Ayers and Pullin [23] who assigned the observed lines to the nonrotating H2O monomer. Since the lines were observed only by doping Xe, it is natural to assign those to the van der Waals complex H2O–Xe. We note that the reported frequencies [23] differ by a few cm1 from the value in this work. 3.2. H2O–Kr complex In contrast to the doped Xe experiments, no new absorption line has been detected with Ar/H2O/ Kr = 1000/2/1 sample. However, for high Kr concentration, we have observed new lines at 1589.1 cm1 in the m2 region and at 3553.1 cm1 in the m3 region. The infrared spectra of Ar/H2O/Kr = 1000/2/x samples with two different Kr concentration are shown in Fig. 2d (x = 5) and in Fig. 2e (x = 20). The observed lines are assigned to the 1:1 H2O–Kr complex on the basis of the concentration effects. Unfortunately, the m1 line of the 1:1 complex has not been seen even at the highest Kr concentration (x = 20) in the present study, presumably because of the low probability for complex formation. The observation of the 1:1 H2O–Kr complex lines requires a relatively high
In addition to the appearance of the 1:1 complex lines, at least two rovibrational transitions of the H2O monomer in the m2 region change their line profiles depending on the Kr concentration in the sample. Fig. 3 shows the profile of the 110 ! 101 rovibrational line near 1574 cm1; weak absorptions on both the high and low frequency sides appear when Kr was added to the sample. A weak shoulder is also observed on the low frequency side of the 101 ! 110 rovibrational transition near 1608 cm1, while no features can be identified on the high frequency side because of the overlap of the proton donor band of H2O dimer near 1611 cm1. Similar phenomenon in the rovibrational lines was found for the R(0) and P(1) lines of H35Cl in Kr-doped Ar matrices [24], which is interpreted as m degeneracy lifting of the J = 1 rotational level arising from the presence of one Kr atom in the first matrix shell surrounding H35Cl molecule. The line profile of the HCl–Rg complexes trapped in rare gas matrices was also theoretically investigated in detail [25,26]. We therefore propose that there are two kinds of matrix sites containing one Kr atom in the first Ar matrix shell; one results in the rotational quenching of H2O molecules by forming H2O–Kr complex, and the other results in the rotation of H2O in an anisotropic potential. In the Xe-doped Ar matrix, such changes in the spectral profiles have not been detected for the rotating monomer lines. Instead, the spectrum shows strong H2O–Xe complex lines. This indicates that the nearest neighbor Xe atoms mainly quench the rotation of H2O molecules and form complexes. 3.4. Structure of H2O–Rg complex Table 1 lists the line positions of three OH fundamental vibration modes within different H2O–X 1:1 complexes isolated in Ar matrices and their frequency shifts due to the cluster formation (cluster shifts) with respect to the band origin of the H2O monomer in the Ar matrix [27]. The structures of H2O complexes are classified into two categories: the hydrogen bonded and oxygen bonded geometries. In many cases, both the hydrogen and oxygen bonded complexes show red shifts in the two OH stretching modes, e.g. H2O–N2 [28], H2O–CO [29,30], H2O–NH3 [31], H2O– HCl [23,32], H2O–CO2 [21], and H2O–SO2 [33], except for the m1 mode of the H2O–N2. The m2 bending frequencies, on the other hand, show blue shifts, the magnitude of which seems to be slightly large in hydrogen bonded complexes compared to the oxygen bonded complexes. For the gas phase H2O–Ar complex, Nesbitt and Lascola
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Table 1 OH stretching (m1 and m3) and bending (m2) frequencies and their cluster shifts (Dm) for the H2O–X 1:1 complexes trapped in solid argon (in cm1) Species H2O, m0 (Ar) H2O–N2 H2O–CO H2O–NH3 H2O–HCl H2O–CO2 H2O–SO2 H2O–Kr H2O–Xe (H2O)2, PAa (H2O)2, PDb a b
Structure H-bonded H-bonded H-bonded O-bonded O-bonded O-bonded
O-bonded H-bonded
m3
Dm3
m1
Dm1
m2
3733.5 3729.6 3723.5 3702.2 3721 3732.46 3722.5 3753.1 3748.7 3737.8 3708.0
0.0 3.9 10.0 31.3 12.5 1.0 11.0 +19.6 +15.2 +4.3 25.5
3638.5 3640.2 3627.8 3434.9 3629 3638.0 3630.5
0.0 +1.7 10.7 203.6 9.5 0.5 8.0
1589 1593.1 1595
0.0 +4.1 +6.0
3635.0 3633.0 3573.6
3.5 5.5 64.9
1590.4 1589.48 1591 1589.1 1589.3 1593.1 1610.6
+1.4 +0.5 +2.0 +0.1 +0.3 +4.1 +21.6
Dm2
References [27] [28] [29,30] [31] [23,32] [21] [33] This work This work [20] [20]
Proton acceptor. Proton donor.
[7] derived the m3 origin to be 3754.4092 cm1, corresponding to a 1.72 cm1 red shift from the free monomer value. This result does not contradict with the hydrogen bonded like structure determined by Cohen and Saykally [12]. In contrast to these complexes, the m3 frequencies of the studied complexes are largely blue shifted by 19.6 cm1 for the H2O–Kr and 15.2 cm1 for the H2O–Xe with respect to the band origin of the H2O monomer in the Ar matrix. These blue shifts, considering the cluster shifts of various molecules listed in Table 1, suggest that H2O molecule is bound to Kr and Xe atoms with neither the hydrogen bonded nor oxygen bonded geometry. Since the previous studies of the H2O–Ar complex [12,13] indicated that the geometry with the planar structure is more favorable than out of plane structure, we assume that the Rg atom lies around intermediate position between the oxygen bonded geometry (h = 180°) and the linear hydrogen bonded geometry (h = 52°), which possibly strengthen one of the OH bonds upon complexation. The most stable structure of the H2O–Kr complex in the Ar matrix might not be identical to that in the gas phase proposed on the basis of the D and 17O nuclear quadrupole coupling constants [17]. We also note, on the other hand, that the angle h is estimated from the 83Kr nuclear quadrupole coupling constants to be 118.5° and 110.9° for the R000 and R101 states in the gas phase study [17]. Further gas phase experimental examination of the H2O–Kr and H2O–Xe complexes as well as theoretical study may be worthwhile to determine the structures uniquely. 4. Summary We have investigated the infrared spectra of the H2O– Kr and H2O–Xe complexes trapped in argon matrices. The absorption lines of the 1:1 complex are identified on the basis of the concentration effects. The line positions for the antisymmetric OH stretching mode in Ar matrices are blue shifted by complex formation. These blue shifts suggest that the non-hydrogen bonded structure is formed in argon matrices. We have also found that the presence of one Kr atom in the first matrix shell perturbs the rotational energy. In the Ar matrix, the weak interaction between
H2O and Kr results not only in the formation of the 1:1 complex but also in the changes of line profiles of rotating molecules, while the interaction between H2O and Xe causes only the 1:1 complex formation. Acknowledgments We thank Dr. F. Ito and Dr. H. Abe for helpful discussions. References [1] R.C. Cohen, K.L. Busarow, K.B. Laughlin, G.A. Blake, M. Havenith, Y.T. Lee, R.J. Saykally, J. Chem. Phys. 89 (1988) 4494. [2] R.C. Cohen, K.L. Busarow, Y.T. Lee, R.J. Saykally, J. Chem. Phys. 92 (1990) 169. [3] S. Suzuki, R.E. Bumgamer, P.A. Stockman, P.G. Green, G.A. Blake, J. Chem. Phys. 94 (1991) 824. [4] E. Zwart, W.L. Meerts, Chem. Phys. 151 (1991) 407. [5] R.C. Cohen, R.J. Saykally, J. Chem. Phys. 95 (1991) 7891. [6] R. Lascola, D.J. Nesbitt, J. Chem. Phys. 95 (1991) 7917. [7] D.J. Nesbitt, R. Lascola, J. Chem. Phys. 97 (1992) 8096. [8] M.J. Weida, D.J. Nesbitt, J. Chem. Phys. 106 (1997) 3078. [9] D. Verdes, H. Linnartz, Chem. Phys. Lett. 355 (2002) 538. [10] G.T. Fraser, F.J. Lovas, R.D. Suenram, K.T. Matsumura, J. Mol. Spectrosc. 144 (1990) 97. [11] T.C. Germann, H.S. Gutowsky, J. Chem. Phys. 98 (1993) 5235. [12] R.C. Cohen, R.J. Saykally, J. Chem. Phys. 98 (1993) 6007. [13] F.-M. Tao, W. Klemperer, J. Chem. Phys. 101 (1994) 1129. [14] E. Arunan, C.E. Dykstra, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 105 (1996) 8495. [15] E. Arunan, T. Emilsson, H.S. Gutowsky, C.E. Dykstra, J. Chem. Phys. 114 (2001) 1242. [16] E. Arunan, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 116 (2002) 4886. [17] J. van Wijngaarden, W. Ja¨ger, Mol. Phys. 98 (2000) 1575. [18] G. Chalasinski, M.M. Szczesniak, S. Scheiner, J. Chem. Phys. 97 (1992) 8181. [19] S. Hirabayashi, K.M.T. Yamada, J. Chem. Phys. 122 (2005) 244501. [20] J.P. Perchard, Chem. Phys. 273 (2001) 217. [21] X. Michaut, A.M. Vasserot, L. Abouaf-Marguin, J. Low Temp. Phys. 29 (2003) 852. [22] S. Hirabayashi, K. Ohno, H. Abe, K.M.T. Yamada, J. Chem. Phys. 122 (2005) 194506. [23] G.P. Ayers, A.D.E. Pullin, Spectrochim. Acta 32A (1976) 1641. [24] B. Laroui, J.P. Perchard, C. Girardet, J. Chem. Phys. 97 (1992) 2347.
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