Rotation of Water in Solid Parahydrogen and Orthodeuterium

Nov 24, 2010 - the corresponding spectra of solid parahydrogen and solid normal hydrogen. .... in a normal matrix experiment generally contains a mixt...
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J. Phys. Chem. A 2010, 114, 12979–12985

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Rotation of Water in Solid Parahydrogen and Orthodeuterium J. Ceponkus,†,§ P. Uvdal,†,‡ and B. Nelander*,† Max-Lab and Department of Chemical Physics, Chemical Center, Lund UniVersity, S-22100 Lund, Sweden ReceiVed: June 9, 2010; ReVised Manuscript ReceiVed: October 22, 2010

The far-infrared spectra of solid orthodeuterium and solid normal deuterium are presented and compared to the corresponding spectra of solid parahydrogen and solid normal hydrogen. Spectra of water in orthodeuterium are compared to spectra of water in parahydrogen. The water rotation constants in orthodeuterium are approximately 80% of the rotation constants of water in parahydrogen. The S0(0) band of orthodeuterium gets a strong satellite in the presence of water. The position and width of the satellite depends on the isotopic composition of the water present. If there is a corresponding satellite in parahydrogen it is weak and closer to the S0(0) band of the matrix. The conclusion of the paper is that interaction between guest rotation and the rotation of matrix molecules must be taken into account to explain the reduction of the rotation constants in orthodeuterium. Introduction Spectroscopic studies of weakly bound aggregates of small molecules can be carried out in molecular beams and give very detailed information about aggregate structure and intermolecular potential function.1-3 Unfortunately such studies are difficult and time-consuming. It is therefore desirable to have access to methods that make it possible to make systematic studies, for instance of a number of isotopomers. The matrix isolation technique may be used for this purpose. In many cases the species of interest can be trapped in a matrix of an inert material and studied by some kind of spectroscopy.4 As a matrix one may for instance use a solid noble gas or solid nitrogen. The matrix is chosen to minimize the interaction with the trapped species and allow relevant spectroscopic measurements. Most studies have been carried out with infrared spectroscopy and with argon as matrix material. Such studies are considerably faster than molecular beam studies but open to questions about the importance of matrix guest interactions. The collective evidence is that matrices of solid noble gases interact weakly with guest molecules, for instance, gas-to-matrix shifts of infrared bands are in general small.5 There are exceptions, where the matrix participates in photochemical reactions with excited guest molecules,6 but in general it is just a passive host. To understand the effects of the matrix on the intermolecular potential of trapped molecular complexes, it is clearly important to have a more detailed description of the guest matrix interaction. The gas-phase-to-matrix shifts of infrared bands of stable molecules result from interactions with the matrix, but it is difficult to identify the mechanisms behind the shifts, mostly since the structures of the trapping sites are unknown. Hole burning experiments in the 1970s and early 1980s suggested that most infrared bands of matrix isolated molecules are inhomogeneously broadened.7,8 One observes a distribution of different trapping sites. The observation of extremely sharp infrared absorption bands of molecules trapped in solid parahy* To whom correspondence should be addressed. E-mail: bengt. [email protected]. † Max-Lab, Lund University. § Permanent address Department of Spectroscopy and General Physics, Vilnius University, Vilnius, Lithuania. ‡ Department of Chemical Physics, Chemical Center, Lund University.

drogen (p-H2)9-11 may indicate that for this material the host guest interactions can be calculated from a molecular model. For this reason, the past decade has seen a large increase in the use of solid p-H2 as an inert matrix.12,13 The review of Silvera14 gives a detailed description of the hydrogen solids; a brief summary is given here. The spacing between the rotation levels of hydrogen and deuterium is large compared to the intermolecular interaction energy between pairs of molecules at distances found in the solids. This means that molecular rotation goes on almost unhindered in these solids. The Pauli principle forces hydrogen molecules in rotation states with an even rotation quantum number J, to have a singlet proton spin function and molecules with an odd J to have a triplet spin function. In the absence of a catalyst, the spin conversion in hydrogen is very slow. This means that solid hydrogen, which is condensed with a room temperature spin population, normal hydrogen, n-H2, will have 75% of its molecules in J ) 1, (oH2) and the remaining in J ) 0 (p-H2). By bringing low temperature hydrogen gas in contact with a catalyst, one can convert almost all molecules to J ) 0. The final o-H2 concentration depends on the temperature at the catalyst. At 14 K the remaining o-H2 concentration is 45 ppm.15 When this gas is condensed slowly, it forms a hexagonal close-packed solid,14 which appears to be an ideal matrix for the isolation of small molecules. There is one drawback with p-H2 as a matrix, the presence of o-H2. The nuclear spin diffuses in the matrix14 and tends to be trapped next to guest molecules. Since o-H2 has a nonzero quadrupole moment, it perturbs the guest and gives satellite bands, close to the normal guest bands.16 By using p-H2 matrices with different o-H2 concentrations one can assign the disturbing bands. A deuteron has spin one and therefore is a Boson in contrast to a proton, which is a Fermion. The total wave function of molecular deuterium must be symmetric under exchange of the nuclei. Molecules with even rotation quantum number have a symmetric nuclear spin function, and molecules with an odd rotation quantum number have an odd spin function. Here orthodeuterium (o-D2) with rotation quantum number J ) 0 is the low temperature form. Normal deuterium (n-D2) can be converted to o-D2 using a low temperature catalyst, in the same

10.1021/jp105303z  2010 American Chemical Society Published on Web 11/24/2010

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way as n-H2 is converted to p-H2. The fact that the rotation constant of deuterium is half that of hydrogen and the lower vapor pressure of deuterium compared to hydrogen means that the concentration of J ) 1 molecules generally higher in o-D2 matrices than in p-H2 matrices. Hydrogen and deuterium form homonuclear diatomic molecules, which are not expected to have infrared absorption bands. However, interactions in the solids induce infrared absorption in solid hydrogen and solid deuterium. The equilibrium structure of solid p-H2 (and o-D2) is a hexagonal close packed crystal,14 but solid p-H2 formed by condensation from the gas phase as in a normal matrix experiment generally contains a mixture of hexagonal and cubic close packed regions.17,18 By annealing for a short time at 4.8 K, the cubic close-packed regions are eliminated, and an almost perfect hexagonal close packed matrix is obtained. Mid infrared spectra of solid n-H2 and solid p-H2 have been measured by the Toronto school.19,20 Mid infrared spectra of solid n-D2 and solid o-D2 have been measured by Crane and Gush.21 Andrews and Wang have published a simplified method to produce o-D2, and they also give spectra of o-D2 with varying p-D2 concentrations, which make it possible to estimate the p-D2 concentrations in o-D2 matrices.22 Far infrared spectra of p-H2 have been published by Trefler et al23 and by Buontempo et al.24 Collins et al has published a Raman spectrum of the rotation transitions in o-D2.17 The theoretical basis for the interpretation of the hydrogen spectra has been provided by van Kranendonk and co-workers.25-27 For p-H2 the mid infrared spectrum is outside the regions of interest for most matrix isolation experiments. Its mid infrared bands make it possible to measure the matrix thickness28 and to monitor the o-H2 content.19 Matrix isolated molecules induce satellite bands to the Q1(0) band of solid p-H2,29 which may give useful information about the host-guest interaction.30,31 The mid infrared spectrum of o-D2 blocks the CH stretching region but it gives similar possibilities to monitor the matrix as the corresponding p-H2 bands. The far-infrared absorptions of p-H2 and o-D2 make measurements difficult in two relatively narrow regions. In addition to gas to matrix shifts and the shape of infrared and Raman bands, molecular rotation fine structure may give information on the guest-host interaction. Small molecules have been found to rotate when isolated in low-temperature solids. Since the rotation is sensitive to the structure of the trapping site, one may hope to be able to draw structural conclusions from rotation spectra. Water,32 ammonia,33 and methane34 rotate almost freely when isolated in noble gas solids. In solid nitrogen, only methane carries out a three-dimensional rotation35 and ammonia rotates around its C3-axis.36 Water does not rotate in solid nitrogen.37 The same is true for water in solid n-D238,39 and probably also for water in n-H2. Water,40 methane,41 hydrogen chloride,42 nitrogen,29 carbon monoxide,43 and methyl radicals44 have been found to rotate with rotation constants slightly smaller than in the gas phase, in solid p-H2. Methane has also been found to rotate in solid o-D2 but with a rotation constant that is only 80% of the gas phase value;44 the same is true for methyl radicals.44 The reduction of the rotation constants has been ascribed to an interaction between the translational vibrations of the host solid and the rotation of the molecular guest.44 One may ask why the rotation in neon, argon, and krypton appears to change very little with the mass of the host atoms, while the mass difference between hydrogen and deuterium has a very substantial effect. The large amplitude of the host atom vibrations in the case of hydrogen and deuterium compared to the noble gases seems to

Ceponkus et al. offer a very reasonable explanation. However we wish to report that also the rotation of the host molecules in p-H2 and o-D2 may play a role in the decrease of the rotation constants. We have studied water (normal and deuterated) in p-H2 and o-D2 as well as in the n-H2 and n-D2 solids. As has been observed previously, water rotates in p-H2 with rotation constants close to their gas-phase values.40 The J ) 1 state is split as expected for a substitutional site in a hexagonal close packed solid.45 We have found that in o-D2 water rotates with rotation constants which are approximately 80% of their gas phase values, completely analogous to what has been observed for methane in p-H2 and in o-D2. We have measured the far-infrared spectra of the pure matrices and water-doped matrices in the region of the S0(0) band of the solids. The results indicate a significantly stronger matrix perturbation in o-D2 than in p-H2. The rotation of the matrix host molecules appears to be involved in the interaction. Experimental Section The cryostat used in this work is a small immersion helium cryostat (IHC-3) from the Estonian Academy of Sciences (Dr. Ants Loˆmus), modified for matrix work. The cryostat can operate from approximately 2.5 to 300 K. The matrix is deposited on a gold-plated ocygen-free high-conductivity copper mirror. To allow the study of thick matrices, a 3 mm deep, 10 mm diameter cavity with a flat bottom is drilled in the center of the mirror. The mirror temperature is measured with a Lake Shore silicon diode. The temperature of the matrix mirror is stable within less than 0.1 K using feed-back electronics. The outer shroud has a valve, through which the depositions are performed. To reduce the heat load on the cryostat, the matrix gas is precooled with liquid nitrogen before entering the cryostat. The water is deposited from a separate volume, kept at 0 °C with an ice-water bath, through a needle valve and a separate stainless steel tube, parallel to the matrix gas inlet, without passing the liquid nitrogen trap. Before deposition, the valve on the shroud is opened, and the deposition tubes are slided into the cryostat to a well-defined position ∼10 mm from the cavity. After deposition the tubes are withdrawn and the valve is closed. The cryostat is used here to study almost three mm thick o-D2 matrices with no particular difficulties. This set up makes it possible to record spectra over the entire infrared region for one deposition using interchangeable CsI and TPX windows on the outer shroud. Water is doubly distilled and degassed, and D2O (Norsk hydro 99.5%D) is degassed. Hydrogen (AGA) is used as received. Deuterium (L’Air Liquide N27) is obtained from the nuclear physics group at Max-lab. p-D2/o-D2 conversion is performed in a stainless steel tube. The bottom of the tube is filled with a paramagnetic catalyst (FeO3). The tube inlet is connected to a 10 L volume, which is filled with a desired amount of n-D2. The inlet and outlet of the tube are connected in such a way that the gas coming from inlet has to pass the catalyst to reach the outlet. The tube is immersed in the liquid helium dewar, and the deuterium from the inlet volume is condensed on the catalyst. Deuterium is kept condensed on the catalyst for c.a. 20 min. Then the catalyst is warmed to approximately 15 K by taking the tube just above the liquid He level. The outlet of the converter tube is connected directly to the deposition system, and converted o-D2 is sublimated from the catalyst on to the cooled copper mirror in the cryostat. p-H2 is prepared in the similar way. The difference is that converted p-H2 is collected into separate volume and the

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J. Phys. Chem. A, Vol. 114, No. 50, 2010 12981 TABLE 1: Mid IR Bands of o-D2 and Water-Doped o-D2 (cm-1) assignment

Figure 1. The mid infrared spectra of o-D2 (upper curve, black) and n-D2 (lower curve, blue). (Absorbance in cm-1.)

deposition performed from that volume instead of connecting converter directly to the deposition system as in deuterium case. The matrices are deposited at 3.6 K. The deposition speed is kept constant by adjustment of the matrix gas flow to keep the temperature constant. Approximately 50 mbar of hydrogen or deuterium from 10 L volume is deposited in ca. 1 hour. The same deposition geometry is used for all experiments. Spectra are recorded with a Bruker HR120 Fourier transform infrared (FTIR) spectrometer at 0.1 and 1 cm-1 resolution in the mid infrared spectrum and at 1 cm-1 resolution below 650 cm-1. A Ge/KBr beamsplitter and an MCT detector operating above 650 cm-1 (Judson) is used in the mid infrared region, and a specially coated mylar beamsplitter and a helium-cooled Si bolometer (Infrared Laboratories) with cut off filters at 700 cm-1 (used for the CsI region) and at 350 cm-1 (used for the TPX region) in the far-infrared region are used. Spectra are recorded at temperatures from 2.8 up to 5 K (4.2K in p-H2 experiments). Attempts to increase the temperature higher lead to an immediate loss of the matrix. We were not able to anneal the matrices sufficiently to completely eliminate cubic close packed regions. Results and Discussion The observed spectra of o-D2 and n-D2 are given in Figure 1. They are in good agreement with refs 21 and 22. We could therefore use the mid infrared spectra to evaluate the o-D2 enrichment as described in ref 22. In the spectrum of n-D2 a broad band is observed near 2986 cm-1. It is attributed to the overlapping Q1(0) and Q1(1) transitions. We use the standard notation of ref 21, the change in J is indicated with a letter, Q for J unchanged, S for an increase in J of 2. The rotation quantum number, J, of the initial state is given in parentheses after the letter and the final vibration state is given by a subscript. The observed mid infrared transitions of o-D2 are summarized in Table 1. The pure vibrational transition band of an o-D2 molecule in the o-D2 solid, Q1(0), becomes very narrow and decreases in intensity with decreasing p-D2 concentration. At the same time the Q1(1) band vanishes. A very sharp band at 3151.7 cm-1 appears in the spectra of highly o-D2-enriched samples. This is attributed to the vibrational rotational transition of o-D2, S1(0), where one deuterium molecule is vibrationally excited and simultaneously changes J from 0 to 2. The sharpness and the intensity relative to the 2986.9 cm-1 band can be used to

Q1(1), H2O-induced Q1(0), H2O-induced Q1(0), H2O-induced Q1(1) Q1(0) Q1(0)+110r101 (H2O) Q1(0)+111r000 (H2O) Q1(0)+202r000 (H2O) QR QR QR S1(0) Q1(1)+S0(0) Q1(1)+S0(0) Q1(1)+S0(0) water-induced SR SR SR Q1(0)+S0(1) S1(0)+S0(0) S1(0)+S0(0)

this work

ref 21

ref 22

2984.6 2986.9

2986.77

2978.6 2982.9 2983.5 2986.9 2998.0 3011 3042 3046 3056 3151.72 3160.8 3170.0 3173.7(sh) 3191 3195.6(sh) 3206 3218 3285.4 3326 3338

3029 3043 3055 3151.9 3161.5 3166.5 3171.0 3194 3206 3215.5 3285.7 3326 3337.5

3151.74 3165

3285.4

estimate the o-D2 enrichment of the sample. Andrews and Wang.22 concluded that a 3151.7/2986.9 cm-1 band absorbance intensity ratio of 34 in their experiment was indication that sample is >99% o-D2. In the present work this ratio is 5.52. After comparison with refs 21 and 22, it is possible to conclude that we obtain an o-D2 concentration close to 99% in our samples. Far-IR spectra of n-D2 and o-D2 are presented in Figure 2. In the same way as in solid hydrogen, S0(0) and S0(1) transitions are infrared active due to interactions in the crystal. They are seen as relatively weak bands in n-D2 at 180 and 300 cm-1. When comparing with the far-IR spectrum of n-H2,24 note that in n-D2, one-third of the molecules are in the J ) 1 state, while in n-H2 three-quarters are in the J ) 1 state. The pure rotation transitions are followed by broad and strong phonon wings around 220 cm-1 and around 350 cm-1. In addition there are double transitions S0(0)+S0(1) at 480 cm-1 and S0(1)+S0(1) at 600 cm-1, where two neighboring D2 molecules are simultaneously rotationally excited. The S0(0)+S0(0) transition is hidden under the phonon wing of S0(1).

Figure 2. The far-infrared spectra of o-D2 (upper curve, black) and n-D2 (lower curve, blue). (Absorbance in cm-1.)

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Ceponkus et al. TABLE 3: Rotation Constants of Water in Different Matrices (cm-1)

Figure 3. Far-IR spectra of p-H2 (upper curve, red) and o-D2 (lower curve, blue). (Absorbance in cm-1.)

TABLE 2: Far-IR Bands of Hydrogen Matrices (cm-1) assignment

n-H2

p-H2

n-D2

o-D2

S0(0) S0(0)+phonon S0(1) S0(1)+phonon S0(0)+S0(0) S0(0)+S0(1) S0(1)+S0(1)

354 390-440 589 610-730

355.62 380-500

181 205-270 299 330-375

181.7 205-270

696,708 940 1176

342,362 480 600

In o-D2, S0(0) becomes a sharp spectral line at 181.7 cm-1. A broad and structured band around 230 cm-1 is associated with S0(0) plus crystal phonons. The S0(1) peaks of n-D2 vanish, and the double transition S0(0)+S0(0) becomes visible at 342 and 362 cm-1.27 The increased widths of bands in n-D2 may be in part due to electrostatic perturbations. D2 molecules in the J ) 1 state have a nonspherical charge distribution and have permanent quadrupole moments; therefore they can interact with ortho species thus somewhat perturbing the rotation of the ortho molecules. The FIR spectra of p-H2 and o-D2 are compared in Figure 3. The spectra look very similar except that deuterium bands are red-shifted. The peaks due to the double transitions S0(0)+S0(0) of p-H2 are outside the figure. The pure rotational band of o-D2 is found at half the frequency of the corresponding p-H2 band. The zero phonon line of o-D2 at 181.7 cm-1 is wider than the corresponding p-H2 band and has a shoulder at 180.5 cm-1. This may be due to a relatively higher concentration of p-D2 in the o-D2 matrix. The far-infrared bands of o-D2 and n-D2 are compared to the corresponding p-H2 and n-H2 bands in Table 2. Figure 4 compares the rotation fine structure of H2O in p-H2 and in o-D2. It can be seen from the figure that the rotation

Figure 4. The bending region of H2O in p-H2 (upper curve, red) and o-D2 (lower curve, blue). nr H2O H2, the nonrotation water line in p-H2. nr H2O D2, the corresponding line in o-D2. H2, 111r0, the indicated VR component in p-H2. D2 111r0, the corresponding line in o-D2. (Absorbance in cm-1.)

Kra

Ara

Ae Be Ce

23.34 15.90 7.78

Ae Be Ce Ae Be Ce a

Nea

p-H2a

o-D2a

gas phaseb

23.72 14.78 8.34

H 2O 23.63 25.41 15.69 13.11 8.44 10.16

19.13 10.12 8.44

27.38 14.58 9.53

14.55 7.19 4.88

14.72 7.16 4.91

D 2O 14.67 15.00 7.28 6.98 4.70 5.01

12.53 6.14 4.39

15.22 7.29 4.94

20.97 9.23 4.88

21.23 9.23 4.91

HDO 21.22 23.03 9.04 (9.13) 4.70 5.01

15.91 7.16 4.39

22.69 9.13 4.94

References 46 and 49. b Reference 47.

spacings are smaller in o-D2 than in p-H2. Note also the much larger widths of the bands in o-D2. Other experiments show that the same is true for all vibration rotation bands of H2O, HDO, and D2O; the rotation spacings are smaller and the band widths larger in o-D2. Table 3 summarizes the rotation constants, which were obtained from the observed vibration rotation lines in p-H2 and o-D2, and compares with rotation constants from noble gas matrices and gas phase values. Rotation line positions for water in noble gas matrices were taken from earlier work in this laboratory.46 To use the available data as much as possible, we assumed that the vibration rotation interaction constants are the same in the matrices as in the gas phase. This seems to be a reasonable assumption, since these constants give the changes in the rotation constants with vibrational excitation, and the vibration frequencies change by relatively small amounts in the matrices. The rotation vibration interaction constants were taken from ref 47. This assumption made it possible to combine data from all three water fundamentals in the determination of the rotation constants. The strong decrease of the rotation constants of water in o-D2 compared to in p-H2 is clearly seen. When one substitutes a water molecule for a hydrogen molecule in p-H2 or o-D2, one breaks the crystal symmetry around each of the 12 nearest-neighbor molecules to the water. This is expected to produce shifts of allowed lines and make some forbidden transitions allowed,25-27,42,45 in particular Q1(0). In o-D2, one observes the H2O-induced Q1(0) as a double peak at 2982.97 and 2983.53 cm-1. These bands shift to 2982.79 and 2983.38 cm-1 with D2O. The corresponding lines in p-H2 were first observed by Fajardo et al.40 The H2O-induced Q1(0) of p-H2 is marked p-H2 in Figure 5, and the corresponding line in o-D2 is marked o-D2 in Figure 6. Fajardo et al40 found that one can observe simultaneous transitions of p-H2 and water in water-doped p-H2 matrices. Here one hydrogen molecule is vibrationally excited at the same time as a water molecule is rotationally excited. We have observed analogous transitions in o-D2. The positions of these bands can be obtained from the water-induced Q1(0) line of hydrogen and the water rotation constants of the vibrational ground state, obtained from the equilibrium values of Table 3 and the vibration rotation interaction constants of ref 47. Figures 5 and 6 show the hydrogen and deuterium vibration regions in p-H2 and o-D2, and Table 4 compares observed and calculated line positions for the combination bands in solid p-H2 and o-D2. The entry, vibrational origin, gives the two H2O-induced, sharp peaks due to Q1(0) of o-D2 or p-H2. The low frequency component of these was used together with rotation constants of Table 1 and the vibration-rotation interaction constants of ref 47 to calculate line positions (see text).

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Figure 5. Water-induced bands in the H2 stretching region of a p-H2 matrix. Lower curve (blue), p-H2 matrix. Upper curve (red), H2O-doped p-H2 matrix. (Absorbance in cm-1.) p-H2, water-induced Q1(0). o-H2, water-induced Q1(1). H2+a, Q1(0)+111r000 (H2O). H2+b, Q1(0)+110r101 (H2O). H2+c, Q1(0)+202r000 (H2O). H2+d, Q1(0)+220r000 (H2O).

Figure 7. The S0(0) region of p-H2. Lower curve (blue), pure p-H2. Middle curve (green), D2O added. Upper curve (red), H2O added. (Absorbance in cm-1.)

Figure 8. The S0(0) region of o-D2. Lower curve (blue), pure o-D2. Middle curve (green), D2O added. Upper curve (red), H2O added. (Absorbance, cm-1.) Interaction between water rotation and o-D2 rotation in water-doped, solid o-D2. Figure 6. Water-induced bands in the D2 stretching region of an o-D2 matrix. Lower curve (blue), o-D2 matrix. Upper curve (red), waterdoped o-D2 matrix. (Absorbance in cm-1). p-D2: water-induced Q1(1). o-D2, water-induced Q1(0). Q1(0):Q1(0), induced by p-D2 impurities. D2+a:Q1(0)+111r000 (H2O). D2+b:Q1(0)+110r101 (H2O).

TABLE 4: Combination Bands Q1(0)(o-D2 or p-H2)+H2Orot (cm-1) transition vibrational origin 111r000 110r101 212r101 202r000 220r0000 a

o-D2 calculated

measured

3010.8 2994.4 3027.2 3037.5

2982.97 2983.53 3010.6 2998.2 3021a 3042

p-H2 calculated

measured

4184.8 4165.0 4204.7 4217.4 4276.0

4149.03 4149.66 4185.1 4164.6 4194.1 4217.4 4281.4

Visible as a shoulder after subtraction of o-D2 absorption.

Figures 7 and 8 show the S0(0) regions of p-H2 and o-D2 of the pure solids and in the presence of H2O or D2O, respectively. The very strong high wavenumber satellite of the zero phonon line in o-D2 is clearly seen. In this matrix there is also a weak low wavenumber satellite, whose position also shifts between H2O and D2O. In p-H2, there is a weak satellite on the low wavenumber side, whose position depends on the isotopic composition of water, similar to what is seen in o-D2. In addition, there appears to be a shoulder in p-H2 at approximately 360 cm-1, which is induced by H2O. The shoulder is not present with D2O instead of H2O in the matrix. This may suggest that the high wavenumber satellite is present also in p-H2, but much weaker than in o-D2 and that the shift is smaller in p-H2. Note that the D2O satellite in o-D2 is broader and weaker than the corresponding H2O satellite. If the same is true in p-H2, the D2O satellite may be very difficult to observe in this matrix.

To shed light on the nature of the interaction between water rotation and host molecule rotation, we have calculated the quadrupole-dipole interaction for a model system consisting of a hydrogen molecule and a hypothetical diatomic molecule at a distance equal to the nearest neighbor distance between the hydrogen molecules in solid p-H2. We used the quadrupole moments14 and rotation constants14 of hydrogen and deuterium, respectively. The model diatomic had a rotation constant of 10 cm-1 and a dipole moment of 1.85 D. For a pair of molecules, one H2 (D2) and one dipolar molecule with the dipole moment equal to that of H2O at the nearest neighbor distance of p-H2 (o-D2), the dipole-quadrupole moment interaction element between J ) 2, M ) 0 (H2 or D2), J ) 0 (dipole), and a J ) 0 (H2 or D2) and J ) 1, M ) 0 (dipole), 〈2,0(H2) |0,0(Aq)|Ω|1,0(Aq)|0,0(H2)〉 (Ω is the quadrupole-dipole interaction48) is 16.1 cm-1 for H2 and 19.4 cm-1 for D2. The distance between nearest neighbors is 3.789 Å in H214 and 3.605 Å14 in D2. Similar results are obtained for the other matrix elements needed in the second-order calculation of the energy shifts. The calculated second-order shift of the first rotation transition of the dipole is -0.80 cm-1 for H2 and -2.35 cm-1 for D2, of the right order of magnitude to produce the observed shifts. Note that there is no direct interaction between the two dipole rotor levels via the quadrupole dipole interaction potential. Both levels interact with higher rotation levels, involving the J ) 2 state of the quadrupole, and are therefore shifted to lower energies. The upper dipole rotor state is closer to the perturbing levels and is therefore more strongly shifted. The dipole rotation excitation energy of the J ) 0 to J ) 1 transition is therefore lowered. A more quantitative calculation will involve the interaction between the rotor and symmetry-adapted quadrupole rotor states of the twelve nearest neighbors. This will probably increase the shift somewhat. The main lesson from the calculation is that even a rather weak coupling between the rotor states

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of the trapped molecule and the rotation of the matrix molecules produces a significantly slower rotation of the trapped molecule in o-D2 compared to p-H2. We do not claim that interactions between the rotation of the trapped molecule and the matrix molecule rotation is the only factor giving the low effective rotation constants of o-D2, but the presence of the rotation satellite on S0(0) in o-D2 suggests that it is important. In the case of methane, the electrostatic interaction is hardly involved, but perhaps repulsive interactions between the methane hydrogens and the rotating host molecules may contribute. It has been shown that o-H2 is enriched next to guest molecules in p-H2,15and it seems likely that p-D2 molecules tend to be trapped next to guest molecules in o-D2. We have found that water no longer rotates freely in o-D2, (p-H2), when it has a p-D2, (oH2), as nearest neighbor.49 The presence of p-D2 is therefore not the reason for the change in the rotation constants in o-D2. Also, the S0(1) line of p-D2 is found close to 300 cm-1, far from the observed satellite band in water-doped o-D2. We therefore are convinced that the observed reduction in water rotation constants and the water-induced S0(0) satellite in o-D2 are due to an interaction between host and guest rotation. Conclusion Both the reduced rotation constants and the broad rotation vibration lines of water in o-D2 compared to p-H2 point to a stronger host-guest interaction in o-D2. The presence of a strong, water-induced satellite on S0(0) in o-D2 suggests that interactions with matrix molecule rotation is at least partly responsible. At present, it is impossible to estimate the relative contributions from the matrix phonon band and proton band, but it seems clear that the proton band is involved. Acknowledgment. This work was made possible by a grant from the Crafoord foundation. The living costs of J. Ceponkus were paid by a grant from SI. This work was carried out at the IR beamline at Max I. The running cost of the beamline was paid for by a grant from VR. The generous help from the Max lab staff is gratefully acknowledged. References and Notes (1) Munther, J. S. Magnetic and electric resonance spectroscopy. In Atomic and molecular beam methods; Scoles, G., Ed.; Oxford university press: Oxford, 1992. (2) Miller, R. E. Infrared laser spectroscopy. In Atomic and molecular beam methods; Scoles, G., Ed.; Oxford university press: Oxford, 1992. (3) Legon, A. C. Fourier Transform microwave spectroscopy. In Atomic and molecular beam methods Scoles, G., Ed.; Oxford university press: Oxford, 1992. (4) Andrews, L.; Moskowits, M. Chemistry and Physics of Matrix Isolated Species; North-Holland Amsterdam, 1989. (5) Jacox, M. E. Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules. J. Phys. Chem. Ref. Data Monograph No 3 1994. (6) Khriachtchev, L.; Ra¨senen, M.; Gerber, R. B. Noble gas hydrids: New chemistry at low temperatures. Acc. Chem. Res. 2009, 42, 183. (7) Dubs, M.; Gu¨nthard, Hs. H. IR hole burning and line shape of matrix isolated 1,2-difluoroethane with tunable diode lasers. Chem. Phys. Lett. 1979, 64, 105. (8) Dubs, M.; Ermanni, L.; Gu¨nthard, Hs. H. Infrared hole burning in matrix isolated 1,2-difluoroethane with a tunable diode laser. J. Mol. Spectrosc. 1982, 91, 458. (9) Oka, T. High-resolution spectroscopy of solid hydrogen. Annu. ReV. Phys. Chem. 1993, 44, 299. (10) Katsuki, H.; Momose, T. Observation of rovibrational dephasing in molecules in parahydrogen crystals by frequency domain spectroscopy. Phys. ReV. Lett. 2000, 84, 3286. (11) Momose, T.; Hoshina, H.; Fushitano, M.; Katsuki, H. High resolution spectroscopy and the analysis of ro-vibrational transitions of molecules in solid parahydrogen. Vibr. Spectrosc. 2004, 34, 95.

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