J. Phys. Chem. A 2010, 114, 6829–6831
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Complex Formation of Small Molecules during Isolation in Low Temperature Matrices: Water Dimers in p-H2 and Ne Matrices J. Ceponkus,†,§ P. Uvdal,†,‡ and B. Nelander*,† MAX-lab, P.O. Box 118, and Chemical Physics, Department of Chemistry, P.O. Box 124, Lund UniVersity, SE-22100 Lund, Sweden ReceiVed: March 11, 2010; ReVised Manuscript ReceiVed: May 12, 2010
The concentrations of water dimer are compared in Ne and p-H2 matrices at low temperatures, using infrared spectroscopy. Additional data are given for o-D2 and Ar matrices. For a given monomer concentration, the dimer concentration is significantly higher in solid Ne (or Ar) than in solid p-H2. In p-H2, the dimer concentration is only slightly higher than expected for a random distribution of water in the matrix. The dimer concentration in o-D2 matrices is intermediate between p-H2 and noble gas matrices. This strongly suggests that most dimers form on the surface of the growing matrix, and not as the result of diffusion in the bulk of the matrix. Introduction
Experimental Section
For more than 50 years, inert and infrared transparent matrices have been used to study “free” molecules, radicals, and weak molecular complexes at low temperatures. In his pioneering work on the matrix isolation method, Pimentel used the polymerization of water to investigate the ability of matrices to control cluster formation.1 Since that time, numerous matrix isolation studies of different molecular aggregates have been published. To the first approximation, the growth of matrices occurs, by condensation of both the host material (e.g., p-H2, Ne, Ar, Kr) and the guest molecules without any differentiation of the two. That is, the guest molecules are randomly distributed in the matrix of the host. A more detailed analysis of experimental spectra shows, however, that the concentration of complexes, for example, dimers, generally is higher than expected from a completely random distribution of guest molecules in the matrix. Also, one often notices that the dimer/ monomer ratio increases with deposition temperature and deposition rate. While well known to most low temperature matrix spectroscopists, these observations have not been discussed in the literature. A mechanism rationalizing the experimental observations is still lacking. Two competing scenarios come to mind: (a) the diffusion of guest molecules in the matrix is far more efficient than previously thought, and (b) the initial sticking process and diffusion along the surface of the growing matrix surface is responsible for the observed anomaly. In the present work, we will show experimental results that strongly favor the latter mechanism by investigating water dimer formation in two different matrices, neon and parahydrogen (pH2). The results get additional support from experiments in orthodeuterium (o-D2) and argon matrices. We suggest that the anomalous high concentration of water dimers in neon matrices as compared to that in p-H2 matrices can be understood using a classical momentum transfer model, sometimes referred to as the Baule formula.2
The experimental setup has been described elsewhere.3 In short, the matrices were deposited during ∼1 h on a gold plated OFHC copper mirror. The cryostat was cooled to 2.8 K before the start of the deposition. During the deposition, the temperature rose to 3.6 K. The deposition rate was kept constant by regulating the matrix gas flow to keep the deposition temperature constant at 3.6 K. When the deposition was stopped, the temperature immediately dropped to 2.8 K. Spectra were obtained at 2.8 K, directly after deposition. Argon matrices were deposited at 17 K and spectra recorded at 4.4 K. The thickness of a p-H2 matrix can be obtained from its infrared spectrum by measuring the integrated intensity, between 4495 and 4520 cm-1, of the Q1(0) + S0(0) transition of the p-H2 matrix.4 The same deposition setup is used for p-H2, Ar, and Ne matrices. The deposition geometries are identical, and gas pressures and host to guest ratios vary in the same intervals for all three matrices. From the p-H2 matrix experiments, we obtain a relation between the amount of matrix gas deposited and matrix thickness. This relation allows us to calculate the thickness of Ar and Ne matrices from the amount of gas deposited and crystal data of p-H2, Ne, or Ar, and the assumption that the sticking coefficients are equal. o-D2 was obtained from normal D2, by condensation of the D2 gas on a catalyst as described for p-H2 in ref 3. The o-D2 matrices were then deposited by sublimation directly from the condensate,5 instead of first collecting the gas in a volume at room temperature, as described for the p-H2 matrices in ref 3. The thickness of the o-D2 matrices was estimated from the maximum absorbance of the QR line at 3055 cm-1, using the o-D2 spectrum of ref 6. By making the assumption that the band strengths of monomeric water are the same in the matrices as in the gas phase, we can measure the monomer concentrations in the matrices from integrated monomer band intensities. The refractive indices of the matrices are not far from one, and we do not apply any correction for the dielectric properties of the matrix. The dimer concentrations are obtained from the integrated intensities of the bending fundamental of the proton acceptor, under the assumption that the strength of this band is identical to the band strength of the monomer bending fundamental.3 This
* Corresponding author. E-mail:
[email protected]. † MAX-lab, Lund University. ‡ Department of Chemistry, Lund University. § Permanent address: Department of General Physics and Spectroscopy, Universiteto str 3 LT-01513, Vilnius, Lithuania.
10.1021/jp1022218 2010 American Chemical Society Published on Web 06/10/2010
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J. Phys. Chem. A, Vol. 114, No. 25, 2010
Figure 1. Spectra of H2O in p-H2 (p-H2/H2O ) 870), upper curve (blue), and H2O in neon (Ne/H2O ) 1060), lower curve (red). MH, monomer in p-H2. MN, monomer in Ne. DH, dimer in p-H2. DN, dimer in Ne. The strong sharp peaks in p-H2 around 3645 cm-1 are due to H2O-o-H2.12 The structured absorption on the low wavenumber side on the DH band close to 3580 cm-1 is due to o-H2 water dimer aggregates (see text). The weak sharp peaks in both spectra are due to uncompensated gas-phase water vapor lines. Spectra were obtained at 0.1 cm-1 resolution. The ratio between the monomer peaks (MH and MN) depends on the time after deposition. In both matrices, the low frequency component decays and the high frequency component increases, as ortho-water is converted into para-water. This process apears to be faster in p-H2 than in Ne (absorbance, cm-1).
assumption is supported by ab initio calculations, reported in ref 3. In o-D2, the monomer absorption in the bending region is broader and overlaps the dimer absorption. This made it difficult to obtain the intensity of the acceptor bend. Instead, we used the intensity of the free OH stretch of the proton donor (ν3D) and the band strength of this band in p-H2 given in ref 3, assuming that the band strengths in the two matrices are approximately equal. In Ar matrices, we used the integrated intensity of the symmetric stretch of the acceptor (ν1A) together with the band intensity of this band in Ne matrices3 to estimate the dimer concentration. As a result, we could determine the concentrations of monomer and dimer in the different matrices. Results and Discussion Water spectra have been studied and assigned in many different matrices.7-9 The spectra observed in this work agree with those previous works. The spectra in o-D2 were similar to those in p-H2 and assigned accordingly. Interestingly, we observe a clear and significant difference in the dimer/monomer ratio of the matrices. For a given monomer concentration, the concentration of dimer is significantly lower in the p-H2 matrix than in the other matrices. The dimer concentrations in o-D2 appear to be intermediate between those in p-H2 and Ne or Ar. Figure 1 illustrates the difference. (In p-H2 matrices, the water dimer acts as a trap for o-H2 impurities. Dimers with different number of o-H2 neighbors have slightly different bound OH stretches, and this is the reason for the structured absorption on the low wavenumber side of the strong dimer band in the p-H2 spectrum in Figure 1. Compare with the CH3F o-H2 spectra of ref 10.) The dimer concentration in p-H2 is in fact rather close to what is expected from a random distribution model. The model is based on the following assumptions. There are SM sites around a monomer. The probability that one of these sites will be occupied by another monomer is SM*[M]/NA, where [M] is the monomer concentration in molecules/cm3 and NA is the number of matrix molecules per cm3. We then expect a dimer concentration [D] ) SM*[M]2/2*NA. The concentration of dimers with respect to the concentration of monomers is shown for p-H2, o-D2, Ar, and Ne matrices in Figure 2 in terms of 2NA*[D]/ [M]2 ()SM). For the p-H2 matrices, we obtain SM values of
Ceponkus et al.
Figure 2. 2*NA*[D]/[M]2 in p-H2 (blue 0), in Ne (red O), in Ar (3), and in o-D2 (green 4) versus [M] (1018 cm-3). [M] is the water monomer concentration, and [D] is the water dimer concentration. NA is the number of matrix atoms (molecules) per cm3. NA/[M] is the number of matrix atoms (molecules) per water monomer. The number of p-H2 in solid p-H2: NA ) 2.60 × 1022 cm-3. The number of o-D2 in solid o-D2: NA ) 3.02 × 1022 cm-3. The number of Ne in solid neon: NA ) 4.58 × 1022 cm-3. The number of Ar in solid argon: NA ) 2.67 × 1022 cm-3.
around 20, relatively close to the expected value for a single substitutional site, 12 for a close-packed lattice. In contrast, the corresponding values are between 70 and 90 for the neon and argon matrices. The values in o-D2 are intermediate between these values. Note that SM does not seem to vary with the water concentration. This supports its usefulness as a measure of the dimer formation probability in different matrices. We believe that there is a rather straightforward explanation for this difference. A water molecule is 9 times (18/2) more heavy than a hydrogen molecule but has approximately the same mass as a neon atom (18/20). An incoming water molecule, with a velocity not too far from the thermal velocity at room temperature, requires approximately 10 collisions with hydrogen molecules, before it has lost its initial kinetic energy. A maximum of 36% of the kinetic energy is lost at each single collision event. This most likely means that it will penetrate into the bulk of the p-H2 matrix before all kinetic energy is lost. Once inside the matrix, it will immediately lose its excess energy and will not be able to diffuse significantly. As a result, the probability for trapping a water molecule on the p-H2 matrix surface is strongly reduced. It therefore has little opportunity to diffuse along the p-H2 matrix surface and form water dimers. In contrast, a water molecule may lose as much as 99% of its kinetic energy in one single collision event, when impinging on a Ne matrix surface. Consequently, the water molecule is much more likely to be trapped (that is, lose all of its perpendicular momentum with respect to the surface) at the surface of the Ne matrix. It can then use its remaining, parallel momentum to diffuse along the surface and find other water molecules to form dimers. The same is true for an argon matrix, where a water molecule is even less likely to penetrate the matrix surface. A water molecule may lose up to 60% of its kinetic energy in a collision with a D2 molecule; it is therefore expected to have a larger probability of being trapped on the matrix surface in o-D2 than in p-H2 but not comparable to the probability in Ne. As mentioned above, a different mobility of water in the bulk of the different matrices would also account for the observed
Complex Formation of Small Molecules matrix effects. This would require that the Ne matrix is substantially softer than the p-H2 matrix, resulting in a more efficient diffusion inside the Ne matrix. The triple points of p-H2 and Ne are 14.0 and 24.6 K, respectively. p-H2 is therefore the softer matrix, and diffusion is expected to be more efficient in p-H2. Therefore, if dimer formation is dominated by diffusion inside the matrix, dimers would be more probable in the p-H2 matrix than in the Ne matrix in contrast to experimental observations. We note that the higher triple point of Ne may indicate a higher sticking coefficient of Ne as compared to p-H2. If the difference in sticking coefficients is significant, the water concentrations and the dimer formation probability would therefore be greater in p-H2 matrices, again, in contrast to experimental observations, Figure 2. Importantly, Figure 2 also shows that the guest to host ratio is in the highly diluted limit (
