Production and characterization of para-hydrogen gas for matrix isolation infrared spectroscopy

Accepted Manuscript Production and Characterization of Para-Hydrogen Gas for Matrix Isolation Infrared Spectroscopy K. Sundararajan, K. Sankaran, N. Ramanathan, R. Gopi PII:

S0022-2860(16)30266-6

DOI:

10.1016/j.molstruc.2016.03.068

Reference:

MOLSTR 22379

To appear in:

Journal of Molecular Structure

Received Date: 9 February 2016 Revised Date:

21 March 2016

Accepted Date: 21 March 2016

Please cite this article as: K. Sundararajan, K. Sankaran, N. Ramanathan, R. Gopi, Production and Characterization of Para-Hydrogen Gas for Matrix Isolation Infrared Spectroscopy, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.03.068. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Production and Characterization of Para-Hydrogen Gas for Matrix Isolation Infrared Spectroscopy K. Sundararajan1, K. Sankaran, N. Ramanathan and R. Gopi

Abstract

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Chemistry Group, Indira Gandhi Centre for Atomic Research Kalpkkam-603102

Normal hydrogen (n-H2) has 3:1 ortho/para ratio and the production of enriched para-

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hydrogen (p-H2) from normal hydrogen is useful for many applications including matrix isolation experiments. In this paper, we describe the design, development and fabrication of the ortho-para

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converter that is capable of producing enriched p-H2. The p-H2 thus produced was probed using infrared and Raman techniques. Using infrared measurement, the thickness and the purity of the p-H2 matrix were determined. The purity of p-H2 was determined to be > 99 %. Matrix isolation infrared spectra of trimethylphosphate (TMP) and acetylene (C2H2) were studied in p-H2 and

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n-H2 matrices and the results were compared with the conventional inert matrices.

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1

[email protected]

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1. Introduction The production of para hydrogen assumes significance (p-H2) due to its wide variety of uses in several experimental techniques. To name a few, p-H2 is used in nuclear magnetic

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resonance (NMR) technique to enhance the signal intensity, in matrix isolation spectroscopy as a matrix material and in the superfluidity studies [1-6]. Matrix isolation technique (MI) is a well known method of isolating the molecules of interest in a rare gas and probe them using a variety

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of techniques [7]. The use of solid molecular hydrogens (H2, D2 and HD) as matrix below 4 K is well known and it is being extensively investigated [8-11]. Solid p-H2 as a matrix host has

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several advantages over conventional rare gas solids [12-17]. The ground state of p-H2 molecule is spherically symmetric with all molecules in the J=0 rotational state. As a result, the interaction between the guest molecules and host matrix (p-H2) is greatly minimized and thus the spectra of the guest molecules are unusually sharp in this host. The crystal structure of solid p-H2 is a pure

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hexagonal-closed pack (hcp), which makes the optical spectra simple whereas the crystal structures of Ne and Ar matrices consist of both hcp and face-centered cubic (fcc) structures, which results in broadening of the spectra. Furthermore, p-H2 solid has large amplitude of zero-

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point lattice vibration, which is characteristic of a quantum crystal. The quantum nature of the solid hydrogen is well suited for matrix isolation spectroscopy as it provides more free space for

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guest molecules compared to other matrices. Because of the large amplitude of zero-point lattice vibration of the solid p-H2, multiple trapping sites and crystal defects around the guest molecules are expected to get repaired automatically. This self repairing nature of the solid p-H2 makes the environment around the guest molecule homogeneous. In addition, the large lattice constant of solid p-H2 makes the interaction between the guest and the host molecules weak and as a result, the life time of the excited states of the guest molecule in solid p-H2 becomes longer. This could

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be the main reason for the relatively sharper spectra of the guest molecules in solid p-H2 matrix when compared to other solid matrices. Eventhough there are multitudinous advantages that make p-H2 as an attractive and

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promising matrix material, it is a real challenge to prepare pure p-H2 gas from n-H2. Normal hydrogen contains 75% o-H2 and 25% p-H2. In order to prepare pure p-H2 > 99%, the o-H2 is to be converted to p-H2. There are several methods available for the preparation of pure p-H2

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[10,12,16,18-24]. Tam and Fajardo constructed and operated a catalyst based device, which they used for pre-cooling and equilibrating the o/p composition of a hydrogen gas flow. They used

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rapid vapor deposition technique (flow rate of ~ 290 mmol/hr) and could get millimeter thick transparent solid p-H2 with a residual o-H2 < 0.01% [16]. The enclosed cell method developed by Oka condensed the p-H2 gas prepared along with the guest molecules in an enclosed cell maintained at ~8K to form transparent crystalline p-H2 of length 3-12 cm within 2h [12].

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Andrews and Wang showed a simple method for condensing and producing pure p-H2 by converting the hydrogen gas over a catalyst in a dipstick tube immersed in the liquid helium [4]. Lee et al. used pulse-deposition method using closed-cycle cryostat to prepare p-H2 matrix [23].

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Momose et al. designed and constructed o/p converter, which is capable of producing a wide range of p-H2 enrichments at a flow rate of up to 0.4 SLM (standard liters per minute). They also

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discussed the storage of p-H2 and its back conversion rates and the various techniques involved in quantifying the enrichment [24]. In this work, the design, development and fabrication of the o-p converter (similar to the converter designed by Momose et al.)

in our laboratory is

described. Raman and Infrared techniques were used to characterize and quantify the p-H2 enrichment.

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Organic phosphates are used as an extractant in a number of solvent extraction processes and it also serves as a model system in understanding the biological processes. To understand the extraction chemistry in nuclear industry, it is essential to first understand the conformational

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preferences of organic phosphates. Earlier, the lower homologue of organic phosphate, trimethyl phosphate (TMP), was studied for its conformations. We have reported earlier that TMP exists in two different conformations, having C3 (G±G±G±) and C1 (G±G±T) symmetries; with the C3

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conformer being lower in energy relative to the C1 [25,26]. Reva et al. reported the conformer interconversion of higher energy C1 conformer to ground state C3 in xenon matrix [27].

spectra are extensively studied [28].

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Acetylene and its multimers are important species in interstellar medium and their gas phase

Quantum solid nature of p-H2 facilitates the rotation of the guest molecules more easily in this matrix than in other inert gas matrices. For example, Lee et al. observed internal rotation of

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methanol CH3OH in solid p-H2 solid but not in solid Ne and Ar [29]. It was thought interesting to study the TMP and C2H2 molecules in solid p-H2 to find out whether these molecules can rotate in the p-H2 solid and in case of TMP whether internal rotation in p-H2 matrix leads to

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2. Experimental

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conformational interconversion.

2.1 Design, Development and Fabrication of ortho/para (o/p) converter A closed cycle helium cryostat (RDK408D2, Sumitomo Industries) is suitably modified

to produce pure p-H2 gas. Figure.1 shows the cold head section of the cryostat which holds a copper bobbin. The copper bobbin with a spiral groove was fabricated from a solid piece of oxygen free high conductivity copper (OFHC) block and machined. The total length of the copper bobbin is 105 mm and the depth of the groove is 5.0 mm. Around six loops of ¼” copper 4

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tube was brazed to the bobbin using silver solder in order to have a good thermal contact and to achieve the desired temperature. Figure.2a shows the picture of the copper bobbin attached to the cold head.

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The copper tubing was filled with 14 grams of 30X50 mesh hydrated Iron (III) oxide catalyst (Aldrich, catalyst grade). The catalyst was filled and compactly packed into the entire length of the copper tube. The catalyst was held inside the tube using small discs cut from porous

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20-mesh stainless steel (SS) material placed at the opposite end of the tube and crimped at both ends.

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The catalyst inside the copper tube was activated to remove moisture and other adsorbed impurities on the surface of the catalyst before using it for generating the p-H2. After the activation of the catalyst, the copper bobbin with coil was fixed to the cold head using M5 bolts (Figure.1). The temperature of the cold head was monitored using Cernox resistor (CX-1050-

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CU-1.4L) connected to a temperature controller (Lakeshore 340 model).The ends of the copper tube were connected to the top flange through swagelok stainless steel union and tee (SS-400-6 and SS-400-3) as shown in Figure 2a –c. After the copper bobbin was connected to the cold head

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of the cryostat, a vacuum shroud was covered around the copper bobbin to give thermal insulation. Figure 2b-c shows the picture of the copper bobbin covered with vacuum shroud. The

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copper bobbin and vacuum shroud were covered with an outer enclosure which is connected to the top flange as shown in the Figure 3. Stainless steel (SS) tubing with an outer diameter of 6.35 mm (1/4”) was connected to the

inlet and outlet of the top flanges. Swagelok integral bonnet valves (SS-1RS4) were connected to the SS tubing, which controls the flow of hydrogen in and out of the converter.

2.2 Production of p-H2 5

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Figure. 4 shows the schematic flow sheet of the p-H2 generation set-up in our laboratory at IGCAR. In order to produce pure p-H2, the temperature controller of the cryostat was set to

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13.5 K, which corresponds thermodynamically to 99.999% pure p-H2. At this temperature the vapor pressure of p-H2 is ~ 70 torr [17]. To attain the desired temperature (13.5 K) the cryostat took around 1 hour 45 minutes followed by another half an hour for thermal equilibrium across

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the entire coil. Our IGCAR device has a temperature sensor which is at the downstream end of the coil. High purity normal hydrogen gas (99.999%, Chemtron) was initially filled into the

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mixing chamber, and the pressure was monitored using absolute capacitance manometer (MKS Baratron 627B). Once the desired temperature of the cold head has reached, the n-H2 from the mixing chamber with a backing pressure of ~600 torr was slowly allowed into the converter (upstream) through a liquid nitrogen bath and the flow of the gas was controlled by a dosing valve (Pfeiffer vacuum, EVN 116) and ¼” Swagelok integral bonnet valve (SS-1RS4). As the

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n-H2 enters through the coil it undergoes a phase change to liquid. It takes around ~30 minutes to fill the entire coil with liquid H2. During this phase transition, there is no increase in the

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downstream pressure, which is continuously monitored using an absolute capacitance manometer (Pfeffier vacuum, APR265). Once the phase change was complete, the pressure in the

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downstream slowly increased, indicating the production of p-H2 gas. In order to increase the p-H2 gas flow at the downstream, the n-H2 gas pressure in the upstream end was increased to ~1200 torr so that the flow of the n-H2 gas to the bobbin was increased, while monitoring the temperature of the cryostat. The temperature gave an indication of the thermal load on the converter. In our experiments, the temperature fluctuation of the copper bobbin was found to be 13.5±0.2K. 2.3 Deposition of p-H2 6

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The p-H2 gas produced using the o/p converter is filled into a glass bulb (Borosil) and then used for deposition onto the cold KBr substrate of a second cryostat. It took nearly 1 hour 30 minutes to get a pressure of ~1000 torr in a 1 litre glass bulb.

Furthermore, we also

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performed experiments wherein the p-H2 gas produced was allowed to deposit directly onto the cold KBr substrate. Solid p-H2 samples were prepared by depositing onto a 25mm diameter and 5 mm thick KBr substrate clamped onto to a gold plated OFHC copper plate through a gas

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dosing valve (Pfeiffer vacuum, EVN 116). The contact between the OFHC copper plate and the KBr window is critical to obtain the required temperature on the substrate and the solid p-H2

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spectrum. Indium wire was placed between the front and back surface of KBr window and pressed tightly so that good contact is made between the cold tip and the window. Infrared spectra (IR) of solid p-H2 were recorded using Fourier transform Infrared (vertex 70 Bruker FTIR) spectrometer at 0.5 cm-1 resolution with a mercury cadmium telluride detector (MCT).

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Raman microscope (Renninshaw Inviva) equipped with a 514 nm Argon ion laser beam as an excitation source and a CCD detector was used to record the gas phase Raman spectra of n-H2 and p-H2 gas. For recording the Raman spectra, ~1000 torr of hydrogen gas was filled to a

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specially made glass bulb. Around 1000 scans were co-added to obtain the Raman spectrum covering the range 100-4000 cm-1.

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Figure 5A and B compare the Raman spectra of n-H2 with p-H2 gas produced from the

o/p converter at 13.5 K and 30 K. Table 1a gives the transitions observed in the n-H2 gas with their assignments. From the figure, it is clear that the room temperature hydrogen gas has rotational Raman quadrupolar lines and they are observed to occur at 354.3, 587.7, 813.5 cm-1, which corresponds to the S0(0), S0(1) and S0(2) transitions, respectively. The Q branch lines were observed at 4164.2, 4158.7, 4147.0 and 4129.0 cm-1, which corresponds to Q1(0), Q1(1) Q1(2)

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and Q1(3) transitions, respectively [8]. The features observed at 4158.7, 4129.0 and 587.7 cm-1 are due to o-H2. At 13.5 K, the Raman spectrum indicated the absence of o-H2 features and enrichment of pure p-H2 . At 30 K, a weak feature at 587.7 cm-1 was observed indicating that a

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small percentage of o-H2 is present in the p-H2 gas. It should be mentioned that the gas phase Raman spectra is purely qualitative and it is not possible to quantify the o-H2 impurity in the pH2 gas.

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Figure.6a-d shows the IR absorption spectra of solid p-H2 (direct deposition) prepared by maintaining the copper bobbin at different temperatures. The infrared spectrum of solid p-H2

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revealed a strong S1(0) absorption line at 4485.7 cm-1,which shows some of the p-H2 molecule reside in hcp or random-stacked close packed (rcp) regions, which lacks a center of inversion. A very strong double transitions Q1(0) + S0(0) and Q1(1) + S0(0) line at 4510.0 and 4503.1cm-1 and S1(0)+S0(0) line at 4835.5 and 4843.7 cm-1 and a weak Q1(0) + S0(1) line at 4739.3 cm-1 features

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were observed in the p-H2 matrix. The broad phonon side bands QR(0) and SR(0) were observed at 4549.6 and 4227.6 cm-1, respectively. In the same figure, the IR spectra of n-H2 showed a broad feature at 4503.6 and 4737.2 cm-1. A broad feature at 4152.8 cm-1 corresponds to the Q1(0)

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o-H2 absorption. The IR features agree well with the data reported in the literature [8]. Table 1a and 1b show the observed transition and their assignments in the n-H2 gas and the solid p-H2,

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respectively.

Hydrogen is a homonuclear diatomic molecule and lacks permanent electric dipole

moment, so the isolated molecule has no electric dipole allowed rotational or vibrational transitions. The IR absorptions in the Figure 6b-d are not due to ~10-6 weaker electric quadrupole allowed transitions, but arise due to the intermolecular interactions in the condensed phase environment. The observation of Q1(0) absorption at 4152.8 cm-1in the Figure.6b-d shows that

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there is o-H2 impurity present in the p-H2 solid. The presence of o-H2 impurity alters the symmetry of p-H2 molecules, which further distorts their charge distribution [30]. The nearest pH2 molecules are polarized by the electric quadrupolar interaction of the o-H2 molecule. In solid

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samples, a small percentage of o-H2 concentration is sufficient enough to induce the IR activity

the figure. 6b-d. 2.4 Sample thickness and % o-H2 determination

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in the Q1(0) and Q1(0) + S0(1) absorption at 4152.8 and 4739.3 cm-1, respectively as shown in

The strongest feature in the IR spectrum (Figure 6b) is the zero-phonon “double

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transitions” in which the neighboring p-H2 molecules undergo simultaneous rovibrational transitions Q1(0) +S0(0) and Q1(1)+S0(0) observed at 4510.0 and 4503.1cm-1, respectively. The double transition lines are relatively insensitive to the crystal microstructure and can be exploited to determine the thickness of the p-H2 solid. The integrated area of the double transition line can

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be used to obtain the thickness of the p-H2 solid [17]. Table 2 shows the thickness of the p-H2 solid performed at three different temperatures. From the table, it is clear that the thickness of the matrix obtained at different temperature is nearly the same. Further, the

% o-H2 impurity in

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p-H2 solid can be obtained from the thickness of the matrix and the area of the Q1(0) transition at 4152.8 cm-1. The % o-H2 calculated at 13.5, 20 and 25 K was found to be 0.7, 2.6, and 4.3,

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respectively, clearly showing that the purity of p-H2 gas produced strongly depends on the temperature of the bobbin. It should be mentioned that when the p-H2 gas was collected in the glass bulb and then deposited the o-H2 concentration was found to be 2.3% at 13.5K. This increase could be due to the interaction of p-H2 gas with the surface of the bulb (Borosil glass). When the same mixture prepared at 13.5 K was kept for three days in the glass bulb and then deposited the o-H2 concentration was increased to 12 %.

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3. Infrared Spectra of Trimethylhphosphate (TMP) in p-H2 matrix Figure.7 shows the matrix isolated spectra in the P=O stretching region of TMP in Ar, Kr, Xe, N2, p-H2 and n-H2 matrices. TMP is known to exist in two conformers, C3 and C1, the

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former is the global minimum and the latter the local minimum [25]. Experimentally, the feature observed as a site split doublet at 1286.7/1284.0 cm-1 and 1306.1/1301.7 cm-1 are assigned to C3 and C1 conformers of TMP in N2 matrix, respectively [31]. Among the inert matrices, Xe matrix

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has a higher polarizability than Kr and Ar, hence the C3 and C1 features show larger red-shift [32]. It should be pointed out that Xe matrix shows a simpler spectrum with no site splitting

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corresponding to the C3 and C1 conformers due to its larger cavity size. Multiple trapping sites were observed for the C3 and C1 conformers in Ar and Kr matrices. Among the diatomic matrices, n-H2 shows the largest shift in the P=O stretching vibrational wavenumber for the C3 conformer, likely due to the presence of higher concentration of interacting o-H2 (75%).

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Furthermore, the C1 conformer shows the intensity reversal of site split features in n-H2 when compared to the N2 matrix. The site splitting for the C1 conformer in p-H2 matrix is relatively less when compared to n-H2 and N2 matrices probably due to the larger cavity size in p-H2

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matrix. No evidence of conformer interconversion of TMP molecule was observed in solid p-H2 on annealing the matrix to the highest possible temperature of 4.5 K. Table. 3a compares the

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computed P=O stretching vibrational wavenumbers of the C3 and C1 conformer of TMP performed at B3LYP/6-311++G(d,p) level of theory using G09 package with the experimental wavenumbers in different matrices [33]. The structure of the C3 and C1 conformers of TMP along with the cartesian co-ordinate is given in supplementary content S1. 4. Acetylene in p-H2 matrix 4.1 ν3 mode of C2H2

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Figure. 8 block A and B shows the as-deposited (i.e soon after deposition) and annealed spectra of C2H2 in Ar (35 K), N2 (30 K), p-H2 (5 K) and n-H2 (5 K) matrices, respectively. In Ar matrix, C2H2 shows two strong absorptions at 3288.9 and 3302.8 cm-1 (Figure. 8, trace a), which

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have been assigned to components of Fermi diad involving the ν3 mode and a combination band (ν2+ ν4+ ν5) [34,35]. The corresponding feature in the N2 matrix appear as a strong feature at 3282.6 cm-1 and a barely visible feature at 3311.0 cm-1 (Figure.8, trace b). In p-H2 matrix, the ν3

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mode of C2H2 was observed at 3279.2 cm-1, which agree well with the reported literature value [36] whereas the combination band was observed at 3294.6 cm-1. Lee et al. observed a weak 3300.9 cm-1 and tentatively assigned it to the combination band of C2H2 in p-H2

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feature at

matrix [36]. Furthermore, they also observed some unresolved weak features at 3278.0, 3276.6 and 3275.9 cm-1 when the C2H2/p-H2 ratio was varied. In our experiments, when the % o-H2 concentration in p-H2 was varied from ~0.5 to 5.0% no new features were observed in the ν3

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mode of C2H2. It should be mentioned that in the gas phase, the ν3 and (ν2+ ν4+ ν5) bands are separated by 13.0 cm-1 and has nearly identical intensities (~1.0) whereas in Ar, N2, p-H2 and nH2 matrices these modes were separated by ~13.9, ~28.4, ~15.4 and ~17.6 cm-1 respectively, and

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the intensity of the (ν2+ ν4+ ν5) combination band relative to the ν3 band was observed to be ~1.0(gas), ~0.5(Ar), ~0.03(N2), 0.14(p-H2) and ~0.07(n-H2). The splitting and the intensity ratio

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of the ν3 and the (ν2+ ν4+ ν5) combination band clearly reveal that N2 interacts strongly with C2H2 and Ar matrix interacts weakly. Among n-H2 and p-H2, the former interaction is the strongest with C2H2 compared to the latter which essentially arises due to the variation of % o-H2 in these two matrices.

4.2 ν5 mode of C2H2

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The spectrum in the ν5 region (800-720 cm-1) also shows some differences (Figure. 9 block A and B). In Ar matrix, the doubly degenerate ν5 mode of C2H2 shows a sharp peak at 736.8 cm-1 and a shoulder at 734.8 cm-1, whereas the same mode appear as a doublet at 742.0 and

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747.4 cm-1 in N2 matrix. In p-H2 and n-H2 matrixes the doubly degenerate ν5 mode appear as a single sharp peak at 734.8 and 737.7 cm-1 (Figure.9 c-d, block A), respectively, clearly showing the C2H2 is trapped in a single substitutional site, whereas in N2 matrix, C2H2 is trapped in a site

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of low symmetry, which causes the double degeneracy of the ν5 to be lifted, resulting in a doublet. Furthermore, the ν5 mode is blue shifted in Ar, p-H2 and n-H2 matrices from the gas

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phase value. It should be mentioned that Lee et al. observed two features at 733.7 and 738.5 cm-1 in the ν5 region. The 733.7 cm-1 feature was assigned to C2H2 with nearby o-H2 and the 738.5 cm-1 feature purely to vibrational transitions of ν5. In our C2H2/p-H2 experiments, no doublet feature was observed in the ν5 region. Furthermore, the ν3 and ν5 mode of C2H2 was not affected

4.3 ν4+ν5 mode of C2H2

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when the concentration of the o-H2 was increased.

In p-H2 and n-H2 matrices, the ν4+ν5 mode of C2H2 was observed at 1332.1 and 1334.3

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cm-1. The same mode was observed at 1334.6 and 1349.9 cm-1 in Ar and N2 matrices, respectively. Lee et al. has observed two broad features at 1331.6 and 1340.1 cm-1 in the ν4+ν5

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region of C2H2. Further, they observed the former feature increases relative to the latter as the concentration of o-H2 increases from 0.28% to 1.21%. Hence, they assign the feature observed at 1340.1 cm-1 to C2H2 and the feature at 1331.6 cm-1 to C2H2 with nearby o-H2. In our C2H2/p-H2 experiments as the concentration of o-H2 was increased, no change in the ν4+ν5 mode of C2H2 was observed.

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Table. 4a compares the observed wavenumbers of the various spectral features assigned to the C2H2 monomer absorption in different matrices with gas phase. In the same table, vibrational wavenumbers of C2H2 monomer computed at B3LYP/6-311++G(d,p) level of theory

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is given for comparsion. The structure and the cartesian coordinates of the C2H2 monomer are given in supplementary content S-II. 4.4 Higher complexes of C2H2

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On annealing, new features were observed in the ν3 and ν5 mode of C2H2 in all the matrices (Figure 9 block B). The feature observed at 3285.0 cm-1 and a site split feature at

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3269.6/3262.8 cm-1 and the corresponding feature in the ν5 mode at 745.0 and 762.3 cm-1 in an Ar matrix is assigned to the T-shaped C2H2 dimer [37]. In N2 matrix, the C2H2 dimer features were observed in the ν3 and ν5 mode at 3278.2, 3258.0 and 760.4 cm-1, respectively. In p-H2 matrix the C2H2 dimer features were observed in the ν3 and ν5 mode at 3276.3, 3263.7 cm-1 and

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741.5, 747.8 cm-1 whereas in n-H2 matrix, these modes were observed at 3275.1, 3262.1 cm-1 and 743.0,749.7 cm-1, respectively (Figure 8 and 9) . Table. 4b compares the observed wavenumber for the C2H2 dimer in different matrices with the computed wavenumbers performed at

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B3LYP/6-311++G(d,p) level of theory.

Since water is an inevitable impurity in any matrix isolation experiments, features due to

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C2H2-H2O complex was observed at 3240.0, 785.5 cm-1 in an Ar matrix and as a site split doublet at 3218.5/3225.7 cm-1 and at 793.2 cm-1 in N2 matrix [38]. In p-H2 matrix, the corresponding C2H2-H2O complex was observed at 3241.8 cm-1 and 777.2 cm-1 whereas in n-H2 matrix the complex features were observed at 3237.3 and 778.7/786.1 cm-1. Table 5a compares the experimental vibrational wavenumbers of C2H2-H2O complex in different matrices with computed wavenumbers performed at B3LYP/6-311++G(d,p) level of theory. Table 5b gives the

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interaction

energies

of

the

C2H2

dimer

and

C2H2-H2O

complex

computed

at

B3LYP/6-311++G(d,p) level of theory. The structure of the C2H2 dimer and C2H2-H2O complex along with cartesian co-ordinates are given in the supplementary S-II and S-III.

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5. Conclusion

In this report, we have described the design, development and fabrication of the p-H2 converter that is capable of producing p-H2 gas with purity > 99%. The o/p converter can be used

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to produce a wide range of p-H2 enrichments. The p-H2 gas produced was probed using Raman and FTIR technique. FTIR spectra on the solid p-H2 produced at different temperature were

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recorded and compared with the n-H2 matrix. The % o-H2 at 13.5, 20, and 25 K was found to be 0.7, 2.6, and 4.3 respectively. Infrared spectra of TMP and C2H2 were recorded in p-H2 and n-H2 matrices and compared with the inert matrices. As the molecules of TMP and C2H2 are reasonably large, we could not observe any rotation of these molecules in solid p-H2. No

Acknowledgement

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conformer interconversion of TMP molecule was observed in solid p-H2.

The authors thank Prof. D. Mohan, Department of Chemical Engineering, A.C. Tech Campus,

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Anna University, Chennai for providing the computational support. The authors thank Dr. R. Kumaresan for recording the Raman spectra of n-H2 and p-H2 gases. We acknowledge Chemical

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Facilities Division, Chemistry Group for the design and fabrication of the o/p converter. R.G. acknowledges the grant of a research fellowship from, IGCAR, Department of Atomic Energy, India.

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[2] T. Momose, T. Shida, Bull. Chem.Soc. Jpn. 71 (1998) 1.

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[6] A. C. Clark, X. Lin, M. H. W. Chan Phys.Rev.Lett. 97 (2006) 245301.

[7] E. Whittle, D. A. Dows, G. C. Pimentel J.Chem.Phys. 22 (1954) 1943.

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[8] H. P. Gush, W. F. J. Har, E. J. Allin, H. L. Welsh, Can. J. Phys. 38 (1960) 176. [9] A. Crane, H. P. Gush, Can. J. Phys. 44 (1966) 373. [10] I. F. Silvera, Rev. Mod. Phys. 52 (1980) 393.

[11] J. Van Kranendonk, Solid Hydrogen Plenum: New York, 1982.

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[12] T. Oka, Annu. Rev. Phys. Chem. 44 (1993) 299.

[13] T. Momose, M. Miki, M. Uchida, T. Shimizu, I. Yoshizawa, T.Shida, J. Chem. Phys. 103 (1995) 1400.

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[14] T. Momose, M. Miki, T. Wakabayashi, T. Shida, M. C. Chan, S. Lee, T. Oka, J. Chem. Phys. 107 (1997) 7707.

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[15] M. Mengel, B. P. Winnewisser, M. Winnewisser, J. Mol. Spectrosc. 188 (1998) 221. [16] S. Tam, M. E. Fajardo, Rev. Sci. Instrum. 70 (1999) 1926. [17] M. E. Fajardo M E Chapter 6 Matrix Isolation spectroscopy in Solid Para Hydrogen A primer in Physics and Chemistry at low temperatures edited by L. Khariachtchev, Pan Stanford : Singapore, 2011. [18] K. F. Bonhoeffer, P. Harteck, Z.Phys.Chem. 113 (1929 ) B4.

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[19] T. C. Nast, I. C. Hsu Adv. Cryog. Eng. 29 (1984) 723. [20] F. G. Brickwedde , R. B. Scott, H. S. Taylor, J. Chem. Phys. 3 (1935) 653.

M. Winnewisser, Can. J. Phys. 72 (1994) 1122.

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[21] R. A. Steinhoff , K. V. S. R. Apparao, D. W. Ferguso, K. N. Rao, B. P. Winnewisser,

[22] A. M. Jaurez, D. Cubric, G. C. King, Meas.Sci.Technol. 1 (2002) N52-N55.

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[23] Y. J. Wu, X. Yang, Y. P. Lee, J.Chem.Phys. 120 (2004) 1168.

[24] B. A Tom, S. Bhasker, Y. Miyamoto, T. Momose, B. J. McCall, Rev. Sci. Instrum. 80

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[25] V. Vidya, K. Sankaran, K. S. Viswanathan, Chem. Phys.Lett. 258 (1996)113. [26] L. George, K. S. Viswanathan, S. Singh. J. Phys. Chem A. 101 (1997) 2459. [27] I. Reva, O. A. Sima, R. Fausto, Chem.Phys. Lett. 406 (2005) 126.

32 (2003) 921.

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[28] M. Herman, A. Camparguem, M. I. El Idrissi, J. V. Auwera, J.Phys.Chem. Ref Data

[29] Y. -P. Lee, Y.-J. Wu, R. M. Lees, L. -H. Xu, J. Y. Hougen, Science 311 (2006) 365. [30] V. Kranendock, H.P. Gush, Phys.lett.1 (1962) 22.

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[31] L. George, K. Sankaran, K. S. Viswanathan, C. K. Mathews Appl.Spectrosc. 48 (1994) 7.

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[32] N. Ramanathan, Ph.D Thesis, University of Madras, 2013. [33]Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.

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E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J.

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B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [34] R. J. Bemish, P. A. Block, L. G. Pederson, W. Yang, R. E. Miller J.Chem. Phys .99 (1993) 8585.

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[35] K. Sundararajan, K. S. Viswanthan J.Mol. Struct. 798 (2006)109.

[36] Y.-C. Lee, V. Venkatesan,Y.-P. Lee, P. Macko, K. Didiriche, M. Herman, Chem. Phys.

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Lett. 435 (2007) 247.

[37]E. D. Jemmis, K. T. Giju, K. Sundararajan, K. Sankaran, V. Vidya, K. S. Viswanathan, J. Leszczynski 1999 J.Mol. Struct. 510 (1999) 59.

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[38] A. Engdahl, B. Nelander Chem. Phys. Lett. 100 (1983) 129.

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Table. 1a The spectral transitions of o-H2 and p-H2 in the gas phase.

4164.2 4147.0 4158.7 4129.0 4500.0 4715.0

J=0 J=2 J=1 J=3 J=0 J=1

Assignments S0(0) S0(1) S0(2)

J=0 J=2 J=1 J=3 J=2 J=3

Transition J=0 J=2 Double transition

J=1 J=0

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J=1 J=0

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Wavenumber cm-1 4485.7 4510.0 4503.1 4739.3 4146.8 4152.8 4549.6 4835.5 4843.7 4227.6

Vibrational transition ∆ν=±1 ∆J=0

Vibrational –rotational transition ∆ν=±1, ∆J=±2

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Tale.1b The spectral transitions of solid p-H2.

Q1(0) Q1(2) Q1(1) Q1(3) S1(0) S1(1)

1

Rotational Raman transition ∆J=±2

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Transition J=0 J=2 J=1 J=3 J=2 J=4

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Wavenumber cm-1 354.3 587.7 813.5

Assignments S1(0) Q1(0)+ S0(0) Q1(1)+ S0(0) Q1(0)+ S0(1) Q1(1) Q1(0) SR(0) S1(0)+ S0(0) QR(0)

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Table.2 Integrated infrared absorbances observed for para enriched solid H2 at different temperatures Q1(0)/d cm-2

foc

%O-H2

-

-

0.75

75

13.5

0.0190

7.502

0.08336

0.2279

0.0065

0.7

c

20.0

0.0699

6.785

0.07539

0.9268

0.0264

2.6

d

25.0

0.1103

6.566

0.07296

1.5119

0.0431

4.3

Q1(0) cm-1

a

298

b

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Temp (K)

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d (cm)b

-

Double transitiona cm-1 -

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a

double transition = ᶘAQ+S dῡ “Q+S” = Q1(0)+S0(0): 4495-4520 cm-1 90±2 cm-2 =ᶘAQ+S dῡ /d ref. 17 c fraction of ortho hydrogen concentration fo = (Q1(0)/(d*35)) ref. 17

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Table 3. Comparison of P=O stretching vibrational wavenumbers of C3 and C1 conformers of TMP in Ar, Kr, Xe, N2, p-H2 and n-H2 matrices with computed wavenumbers performed at B3LYP/6-311++G(d,p) level of theory. Experimental wavenumbers (cm-1) Ar

Kr

Xe

N2

p-H2

n-H2

Computed Wavenumbers (cm-1)

1281.3 1284.8 1292.1 1305.9 1309.9

1283.6 1288.4

1282.5

1284.0 1286.7

1283.9 1282.3

1282.7 1287.3

1261.2 (213)

1301.4 1305.9

1294.8 1301.5

1301.7 1306.1

1304.2 1308.5

1303.4 1306.8

C3(G±G±G±) conformer

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C1(G±G±T) conformerb

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Mode assignment

C1(G±G±T) is present at 0.83 kcal/mol higher in energy than C3(G±G±G±) conformer.

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3

1290.1 (265)

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Table. 4a Vibrational assignments of the C2H2 bands observed under supersonic jet conditions and in different matrices. The computed vibrational wavenumbers at B3LYP/6-311++G(d,p) level of theory is also given for comparison. Experimental wavenumbers (cm-1) p-H2a p-H2b n-H2b Ar c

Jet

729.163 3281.899 1328.081 3294.839 (~1.0)e

738.5 3279.2 1340.1 3300.0 (~0.14)

734.8 3279.2 1332.1 3294.6 (~0.04)

737.7 3278.0 1334.3 3295.6 (~0.07)

736.8/734.8 3288.9 1334.6 3302.8 (~0.5)

N2

d

Computed wavenumbers (cm-1) 769.4 (112) 3424.8 (94) -----

742.0/747.4 3282.6 1349.9 3311.0 (~0.03)

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ν5 ν3 ν4 + ν5 ν2+ν4+ ν5

a

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Band

a

values taken from ref.36 this work c ref 37 d ref 35 e The ratio of combination band ν2+ν4+ ν5 and the ν3band are given in parentheses.

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Table. 4b Vibrational assignments of the T-shaped C2H2 dimer bands observed in different matrices. The computed harmonic vibrational wavenumbers at B3LYP/6311++G(d,p) level of theory is also given for comparison.

ν5 ν3 a

Experimental wavenumbers (cm-1) p-H2a n-H2a Ar d N2e b b b 741.5 743.0 745.0 -f 747.8 c 749.7 c 762.3 c 760.4c b b b 3276.3 3275.1 3285.0 3279.2 b c c c 3263.7 3262.1 3264.0 3258.0 c

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Band

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this work C2H2 as a proton acceptor. c C2H2 as a proton donor. d ref. 37 e ref. 35 f Experimental feature not observed. b

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Computed wavenumbers (cm-1) 772.3 (45)/ 781.3 (123) 791.4 (155)/ 797.4 (85) 3516.1 (100) 3402.5 (172)

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Table. 5a Vibrational assignments of the C2H2-H2O linear complex bands observed in different matrices. The computed vibrational wavenumbers at B3LYP/6-311++G(d,p) level of theory is also given for comparison. Experimental wavenumbers (cm-1) Computed wavenumbers (cm-1) p-H2 n-H2a Ar b N2c 777.2 778.7/786.1 785.5 793.2 860.9 (92)/883.9(95) 3241.8 3237.3 3240.0 3218.5/3225.7 3362.7 (269)

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Band

a

ν5 ν3 a

this work ref. 37 c ref. 35

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Table 5b. Stabilization energy (ZPE corrected/BSSE corrected) of T-shaped C2H2 dimer and C2H2-H2O linear complexes computed at B3LYP/6-311++G(d,p) level of theory Complex

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C2H2 dimer C2H2-H2O

Stabilization energy (kcal/mol) -0.38 /-0.73 -2.01/-2.67

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Figure Captions Figure 1. Design of the copper bobbin.

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Figure 2. Photograph showing A) copper bobbin along with copper tube attached to the cold head of the cryostat B and C) Vacuum shroud covering the copper bobbin. Figure 3. A) Design of the outer chamber showing the copper bobbin and the vacuum shroud B) photograph of the outer chamber. Figure 4. Schematic of the para-hydrogen set-up.

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Figure 5. Raman spectra (Block A and B) of normal-H2 and p-H2 gas at different temperatures.

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Figure 6. Infrared absorption spectra spanning the region 4900-4100 cm-1 a) n-H2 ; p-H2 at different temperatures b) 13.5K c) 20K and d) 25K. Figure 7. Infrared absorption spectra covering the P=O stretching region 1330-1250 cm-1 of TMP in different matrices. Typical sample to matrix concentration was 1:1000.Spectra shown here was recorded soon after deposition.

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Figure 8. Infrared absorption spectra of C2H2 in different matrices a) Ar b) N2 c) p-H2 and d) n-H2 covering the spectral region 3320-3200 cm-1. Block A corresponds to the spectra recorded soon after deposition and Block B corresponds to the spectra annealed at 30K (N2), 35K(Ar), 5 K (p-H2 and n-H2)

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Figure 9. Infrared absorption spectra of C2H2 in different matrices a) Ar b) N2 c) p-H2 and d) n-H2 covering the spectral region 800-720 cm-1. Block A corresponds to the spectra recorded soon after deposition and and Block B corresponds to the spectra annealed at 30K (N2), 35K (Ar), 5 K (p-H2 and n-H2)

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Figure 1

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Top flange A

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Cernox resistor

B

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Copper Bobbin

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resistor

C

Vacuum Shroud

Figure 2

A

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B

Outer enclosure

Figure 3

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MC-Mixing Chamber

Figure 4

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60000

A

55000

30K

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45000

20000 15000 4200

4180

13.5 K

4160

4140

Q1(3)4129.0

25000

Q1(2) 4147.0

Q1(0) 4164.2

30000

Q1(1) 4158.7

35000

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40000

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Raman counts

50000

n-H2

4120

4100

-1

Raman Shift cm

24000 20000

30K

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16000

0

300

S0(2) 813.5

4000

13.5 K S0(1)587.7

8000

S0(0) 354.3

12000

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Raman counts

B

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28000

n-H2

600

900 -1

Raman Shift cm

Figure 5

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4900 4800 4739.3

4843.7 4835.5

4549.6

4600

4500

4400 4152.8 4146.8 4139.6

4300 4152.8

4227.6

4485.7

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4737.2

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4700 4200 b

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Absorbance

4503.1

4510.0

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d

c

Wavenumber (cm )

-1

Figure 6

b

a

4100

1330

1290

1281.3

1292.1

1283.6

1294.8 1282.5

Normal-H2

1303.4

1306.8

1287.3

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1283.9

1308.5

1282.3

1304.2

Para-H2

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1284.0

1286.7

1306.1 1301.7

Absorbance

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1288.4

1301.4

1305.9

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1284.8

1305.9

1309.9

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1282.7

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N2

Xe

Kr

Ar

Wavenumber, cm

-1

1250

Figure 7

3320

3280

3240

3320 3200 800

Wavenumber cm-1 3240.0

3258.0

3278.2

Absorbance

3241.8

3263.7

3280

3240

3225.7

3218.5

3241.8

3263.7

3276.3

3279.2

3262.1

3275.1

3295.6

3237.3

3295.6

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3279.2

SC 3294.6

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3282.6

3294.6

3278.0

3237.3

3278.0

B

3262.8

3288.9

3302.8

EP 3225.7 3218.5

3311.0

A

3269.6

3285.0

3240.0

3269.6 3266.2

3288.9

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3285.0

3302.8

Absorbance

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d

c

b

a

3200 800

Figure 8

Wavenumber

3200 800

780

760

740

720 800

Wavenumber cm-1

780

760

745.0

734.8

736.8

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760.4 747.4 742.0

747.8 741.5

737.7

737.7

B

734.8

749.7 743.0

778.7

786.1

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734.8

SC 777.2

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747.4 742.0

A

762.3

785.5 780.1

734.8

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736.8

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c

b

a

740

-1

Figure 9

720

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Highlights Design, development and fabrication of the ortho-para converter Para-H2 characterized using infrared and Raman spectroscopic techniques

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The purity of p-H2 found to be > 99 %

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Infrared spectra of trimethylphosphate and acetylene studied in p-H2 matrix