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Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Lin-(C2H4)m Complexes Laura F Pinelo, Elsbeth R. Klotz, William R. Wonderly, Leif Oscar Paulson, Sharon C Kettwich, Jan Kubelka, and David Todd Anderson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11223 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry

Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Lin-(C2H4)m Complexes Laura F. Pineloa, Elsbeth R. Klotza, William R. Wonderlya, Leif O. Paulson, Sharon C. Kettwich, Jan Kubelka, and David T. Andersonb Department of Chemistry, University of Wyoming, Laramie, WY 82071-3838, USA

ABSTRACT Complexes of lithium atoms with ethylene have been identified as potential hydrogen storage materials. As a Li atom approaches an ethylene molecule two distinct low lying electronic states are established; one is the 2A1 electronic state (for C2v geometries) that is repulsive but supports a shallow van der Waals well and correlates with the Li 2s atomic state and the second is a 2B2 electronic state that correlates with the Li 2p atomic orbital and is a strongly bound charge transfer state. Only the 2B2 charge transfer state would be advantageous for hydrogen storage because the strong electric dipole created in the Li-(C2H4) complex due to charge transfer can bind molecular hydrogen through dipole-induced dipole and dipole-quadrupole electrostatic interactions. Ab initio studies have produced conflicting results for which electronic state is the true ground state for the Li-(C2H4) complex. The most accurate ab initio calculations indicate that the 2A1 van der Waals state is slightly more stable. In contrast, argon matrix isolation experiments have clearly identified the Li-(C2H4) complex exists in the 2B2 state. Some have suggested that argon matrix effects shift the equilibrium towards the 2B2 state. We report the low temperature synthesis and IR characterization of Lin-(C2H4)m (n=1, m=1 and 2) complexes in solid parahydrogen which are observed using the C=C stretching vibration of ethylene in the complex. These results show that under cryogenic hydrogen storage conditions the Li-(C2H4) complex is more stable in the 2B2 electronic state and thus constitutes a potential hydrogen storage material with desirable characteristics. _______________________________ a)

NSF REU student

b)

Email: [email protected]

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1. Introduction The chemical bonding and infrared spectroscopy of 1:1 and 2:1 Lin-(C2H4)m complexes (labeled by the ratio of n:m) are of interest as model chemical systems for hydrogen storage.1 These recent computational studies predict that 0.47 of an electron is transferred from the Li atom to the Π* antibonding molecular orbital of C2H4 in the 1:1 complex, and this metal-toligand charge transfer bonding results in a complex that can bind H2 electrostatically through strong electric dipole-induced dipole and electric dipole-quadrupole intermolecular interactions.1 However, these and earlier ab initio calculations have produced conflicting results for what is the true ground electronic state of the complex in the gaseous state.1-6 As first pointed out by Manceron7 in 1993, there are two possibilities for the interaction of a Li atom with an ethylene molecule with a filled Π molecular orbital. In the first, the atomic orbitals on Li do not interact strongly with Π-system of C2H4 resulting in a 2A1 ground electronic state (in the C2v geometry) which is purely repulsive and the complex is only bound by weak van der Waals forces.7 In the second case, the 2p orbital on the Li atom interacts strongly with the Π* orbital on C2H4 resulting in a 2B2 ground electronic state. In this second case, there is significant electron donation from the Li atom to the Π* molecular orbital on C2H4 that results in an elongation of the C=C bond and bending of the C-H bonds away from planarity.7 Calculating which is the true ground electronic state of the complex is made more difficult because the weak van der Waals interaction correlates asymptotically with the lower energy Li(2s) atomic orbital while the stronger electron transfer interaction correlates with the Li(2p) orbital which is 42.6 kcal mol-1 (14,904 cm-1) higher in energy. The most recent ab initio studies6 predict in the gas phase, in the absence of solvent, the van der Waals 2A1 state is 1.4 kcal mol-1 lower than the 2B2 state. If this were the case then these 1:1 complexes would be weakly bound by only van der Waals forces

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and would therefore not make good hydrogen storage materials. From a more fundamental perspective, this is an example of a radical complex with two low lying electronic states that are structurally quite different and therefore interesting to study using IR spectroscopy. The 2A1 electronic state is repulsive and only supports a van der Waals well at a relatively long carbonmetal distance (4.7 Å) and the 2B2 state is a charge-transfer state with a much shorter carbonmetal distance (2.1 Å).5 Probing which state is accessed when a ground electronic state Li atom approaches a C2H4 molecule effectively reveals the relative energetics between these two very different electronic states. However, the relative energies of these two very different electronic states can be shifted by solvation effects that would typically favor the 2B2 state. Indeed, a variety of Lin-(C2H4)m complexes have been observed in Ar matrix isolation experiments which have shown through ESR, UV, and IR spectroscopy that the 1:1 complex is unequivocally in the 2B2 charge-transfer state.7-9 Of all the alkali atoms only Li is predicted to show this behavior while Na and K only form weak van der Waals complexes.4 Modern ab initio calculations with diffuse-augmented basis sets predict the ground state in vacuo is the 2A1 van der Waals state; however the 2B2 state is only predicted to be 1-3 kcal mol-1 higher in energy.5,6

Some have suggested that solvation of the complex even by the non-polar but

polarizable Ar matrix shifts this balance towards the 2B2 charge transfer state which will have the greater solvation energy.3,5 In computational studies that included the effects of the Ar solvent using a polarizable continuum model the 2B2 state was predicted to be lower in energy by roughly 3 kcal mol-1, but these differences are still rather small.5 We decided to study these Lin(C2H4)m complexes solvated in solid parahydrogen (pH2) to see if the 2B2 charge transfer state is formed similar to the Ar matrix results and to study the complexes under conditions that can directly probe their hydrogen storage capabilities.

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2. Experimental and Computational Section Solid parahydrogen (pH2) matrices are synthesized using rapid vapor deposition10,11 of H2 gas that is enriched in the para nuclear spin state to approximately 99.97% levels using a home built ortho/para converter and matrix isolation setup.12,13 For this specific application, we built a dual source Li rod holder and C2H4 bleed tube to allow simultaneous double doping of Li and C2H4 using our existing solid pH2 matrix apparatus.14 Figure 1 is a cartoon of a birds eye view of the deposition cryostat used in these studies. An aluminum Li rod holder was machined to house a 1″ diameter Li rod just inside a hole cut in our 2″ diameter radiation shield to allow 1064 nm laser ablation of Li during pH2 deposition. The Li rod is held at a 45° angle with respect to the BaF2 surface normal and the 1064 nm output of a 10 Hz Nd:YAG laser is loosely focused onto the Li rod with a 20″ EFL lens through a window on the opposite side of the octagonal vacuum shroud. A hole drilled through the center of the Li rod holder permits flow of C2H4 gas into the cryostat while the Li rod is being ablated; the C2H4 gas enters the cryostat from the base of the Li rod holder near where the Li rod bottomed out in the cup of the holder. The basic procedure is to cool the substrate to approximately 2.5 K, start the flow of pH2 gas and wait until pH2 is depositing onto the BaF2 substrate as evidenced by a substrate temperature increase, a simultaneous rise in the cryostat vacuum pressure, and by monitoring the IR spectrum during deposition using the characteristic solid pH2 absorption features.10,11 Once sure that a layer of pure pH2 is deposited, the ablation laser is started and the Li rod is constantly rotated by hand using the Li rod holder to ensure that the Li rod surface is consistently replenished. Typically, we used approximately 5.0 mJ pulse energies for the ablation laser as measured right before the ablation window on the cryostat. Shortly after the ablation laser is started the C2H4 gas flow is started. The extent of Li ablation is monitored visually by the

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characteristic red glow of the Li plume; rotated to keep a strong glow. The laser ablation part of the deposition process lasts approximately 45 minutes until growth of the pH2 crystal interferes with the beam path of the 1064 nm ablation laser. Then the laser is turned off and the C2H4 flow stopped. The pH2 gas deposition is continued for approximately 4 additional minutes and then stopped. By monitoring the integrated intensity of the ν12 feature of C2H4 we estimate the amount of C2H4 delivered to the pH2 matrix using the dual flow Li rod holder is significantly less than when using the typical ¼″ stainless steel dopant line.14 This indicates that a significant proportion of the C2H4 gas (~50%) misses the target and gets pumped away by the turbo molecular pump mounted to the base of the vacuum shroud. However, we can account for this by using larger C2H4 flow rates when using the dual flow Li rod holder. FTIR absorption spectra of each solid are recorded using two different configurations of the (Bruker IFS120HR) FTIR spectrometer; mid-IR spectra from 800 to 5000 cm-1 are recorded at 0.05 cm-1 resolution using 100 scans of the FTIR equipped with a globar source, KBr beamsplitter, and liquid nitrogen cooled HgCdTe detector. Near-IR spectra from 1800 to 5000 cm-1 at 0.01 cm-1 resolution are recorded using 49 averaged scans with the FTIR equipped with a tungsten source, KBr beamsplitter and InSb detector. The entire optical path outside of the spectrometer and cryostat vacuum shroud is purged using a dry air generator (Domnick Hunter CO2RP850) to reduce atmospheric absorptions. The interferograms of the mid-IR spectra were edited to remove interference glitches from “etaloning” caused by the BaF2 substrate and the spectral resolution of the near-IR spectra were degraded to 0.05 cm-1 resolution. So-called “asdeposited” spectra are recorded just after deposition is complete for samples that have never been exposed to temperatures greater than ~2.5 K. The as-deposited samples are annealed by raising the temperature of the solid to ~4.3 K for at least 10 minutes.

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The concentrations of C2H4, LiH, (LiH)2, 1:1 and 1:2 complexes are measured from the integrated intensity of various peaks using the integration protocols listed in Table S1 in the Supporting Information (SI). We calculate the mole fraction of dopant species in units of parts per million (ppm) using the following equation, [X] =

− 2.303∫ log10 ( I / I 0 ) dν~

εl

V0 (1x10 6 )

(1)

where X is the species of interest, ε is the integrated absorption coefficient for the transition used, l is the IR pathlength through the crystal, and V0 is the molar volume of solid pH2 at liquid helium temperatures (23.16 cm3 mol-1).15 The value of l is determined for each sample using the FTIR spectroscopic method developed by Fajardo.16,17 The IR spectrum of C2H4 isolated in solid pH2 has been reported previously in studies of the Cl + C2H4 reaction.18 In these previous studies, the peak wavenumbers for C2H4 isolated in solid pH2 were reported for spectra recorded at 3.2 K for C2H4 concentrations of around 1000 ppm. Our studies utilized C2H4 concentrations ranging from 30 – 500 ppm. The observed peak maxima are presented in Table S2 and show excellent agreement with the literature values.18 To aid in the interpretation of the experimental data, vibrational IR spectra for C2H4 and 1:1 and 1:2 lithium-ethylene complexes were modeled computationally. The computations were carried out at DFT and high-level ab initio CCSD(T) levels of theory using the Gaussian 09 quantum chemistry package.19 The DFT B3LYP/6-311++G(2df,2p) level20,21 was utilized for geometry optimizations, harmonic vibrational frequency and IR intensity calculations in both the gas phase and in the pH2 matrix environment that is approximated by the conductor-like polarized continuum model (CPCM)22,23 (dielectric constant ε = 1.294).24 Gas phase geometry optimizations and harmonic frequency calculations were also carried out at ab initio CCSD(T)/6311+G(d, p) level,21,25,26 with CCSD/6-311+G(d, p) used to obtain the atomic polar tensor (APT)

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for the IR spectral intensities. The 1:1 complex is calculated to form C2v structures where the Li atom sits atop the center of the C=C bond and the hydrogens bend away from planarity. The 1:2 complexes are predicted to adopt D2h structures where the Li atom is sandwiched between two ethylene molecules. The IR spectra were simulated from the computed vibrational frequencies and intensities by assigning a Lorentzian profile with a uniform width of 1 cm-1 to each transition.

The vibrational frequencies, intensities and mode assignments are summarized,

respectively, in Tables S3 – S7 in the SI. We also computed IR spectra for the cationic complexes Li+-(C2H4) and Li+-(C2H4)2 since it is likely that laser ablation produces ionized Li species (SI, Tables S6 and S7). To highlight the dominant spectral features, simulated IR spectra of C2H4 are overlaid in Fig. S1 and S2 for 1:1 and 1:2 neutral Lin-(C2H4)m and charged (Li+)n(C2H4)m complexes, respectively. Figure 2 shows DFT relaxed potential energy scans for the 2A1 and 2B2 electronic states of the 1:1 complex as a function of the distance (Å) between the Li atom and the center of the C=C bond in the gas phase and solid pH2 matrix approximated by the CPCM model. In Figure 2 the energy of the 2A1 state at large intermolecular distances was set to zero for comparison. A similar figure is presented in the SI for the same surfaces (Hartree) without setting the 2A1 state to zero. Note that at each step all other coordinates are optimized while keeping the C2v symmetry. Figure 2 shows how the Li-C2H4 interaction is large for the 2B2 charge transfer interaction and purely repulsive for the 2A1 state. This also illustrates why predicting the relative energies of the two electronic states is difficult because the stronger interaction correlates asymptotically with the Li(2p) excited state.

The PES comparison

highlights the relative stabilization of the 2B2 state with respect to 2A1 in the pH2 matrix.

3. Results

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A total of eight different experiments were conducted to identify and assign Lin-(C2H4)m IR absorptions in solid pH2, and the primary results are illustrated in Figure 3, which displays the IR spectrum from 800 to 1250 cm-1 for three different doped pH2 samples (a larger wavenumber region is shown in Figure S4 in the SI). Traces (a) and (b) in Figure 3 are spectra recorded of single doped pH2 matrices; trace (a) is for C2H4/pH2 and trace (b) is for Li ablated into solid pH2 with no C2H4 added. All the spectra presented in Figure 3 are for annealed samples recorded at low temperature (~1.9 K). Annealing involves raising the sample temperature to 4.4 K for a fixed period of time after deposition is complete and we will show in detail the effects of annealing shortly. In this spectral region there are C2H4 absorptions near 824 cm-1 (ν10) and 949 cm-1 (ν7). The peaks labeled (LiH)2 and (LiH)3 at 905.6 cm-1 and 1138.0 cm-1, respectively, are due to lithium hydride clusters that form during laser ablation of Li into the pH2 solid as shown previously by Wang and Andrews.27,28 The peak labeled pH2 at 1167.11 cm-1 in Figure 3 is the U0(0) pure rotational transition (J=4←0, v=0←0) of pH2 that is induced in the solid phase.29 Note that the concentration of Li atoms in the different samples cannot be monitored directly in the IR spectral region, but previous work suggests that nearly 40% of the Li atoms remain unreacted.30 Using this strategy of conducting separate single and double doping experiments, trace (c) can be searched for absorption features that are only present in Li/C2H4 double doping experiments. After careful analysis of the entire region from 600 to 5000 cm-1 for four different double doping experiments, the two peaks at 1179.8 and 1199.7 cm-1 shown in Figure 3 are observed only when both Li and C2H4 are present. Comparison of the measured transition frequencies of these two new peaks with the DFT and ab initio calculations of the vibrational spectra allow us to assign the peak at 1179.8 cm-1 to the ν3 C=C stretching mode of the 1:1 Lin-(C2H4)m complex in the 2B2 charge transfer electronic

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state and the peak at 1199.7 cm-1 to the ν31 asymmetric C=C stretching mode (with some CH2 scissor mode character as well) of 1:2 Lin-(C2H4)m complex in the corresponding 2B2U state. The vibrational frequencies and intensities calculated in this work are presented in Tables S3-S5 for C2H4, 1:1, and 1:2 Lin-(C2H4)m clusters, respectively, in the SI. Simulated IR spectra from the DFT level calculations both in the gas phase and with a pH2 environment incorporated via a CPCM model22,23 are displayed in Figure S1. Additionally, we computed IR spectra for the cationic complexes Li+-(C2H4) and Li+-(C2H4)2 since it is likely that laser ablation produces ionized Li species (SI, Tables S6, S7 and Figure S2). However, the strongest IR transitions from C=C stretching modes in the ionized clusters are predicted at much lower wavenumbers (~1050 cm-1), which is not consistent with the observed signals (see Fig. 3). Accordingly, the two peaks observed in Figure 3 at 1179.8 and 1199.7 cm-1 are assigned to the strongest IR absorptions of the neutral 1:1 and 1:2 complexes, respectively. The very large calculated intensity (1472.4 km mol-1) for the ν31 mode of the 1:2 complex helps explain why we are able to detect this species under such dilute C2H4 concentrations (e.g., [C2H4]=125 ppm). Using the calculated intensities and the measured path length, we estimate the concentrations of the 1:1 and 1:2 complexes for the sample shown in Figure 3 to be approximately 1.6 and 0.13 ppm, respectively. The agreement between the calculated and measured frequencies is very good and therefore consistent with a 2B2 charge-transfer state for the 1:1 complexes. The measured frequencies are also consistent with a strong Li-C2H4 interaction in both complexes because the observed vibrational modes are largely shifted from the vibrational absorptions of C2H4 in solid pH2. Indeed, the C=C stretching mode in C2H4 is calculated to be at 1687.9 cm-1 while in the 1:1 and 1:2 complexes it is predicted to shift by –510.5 cm-1 and –451.4 cm-1, respectively. Further, the C=C stretching mode is IR inactive in C2H4 and thereby only gains

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intensity by the interaction with Li. No new peaks are measured at smaller shifts with respect to the IR absorptions of C2H4 in the double doping experiments. These experimental findings are consistent with the complexes only being formed in the 2B2 and 2B2U charge transfer electronic states under these conditions. To further test this assignment we use changes in the intensities of these features upon annealing. Shown in Figure 4 are spectra in the region of the newly assigned features for a Li/C2H4 co-doped pH2 sample as a function of sample history (a larger wavenumber region is shown in Figure S5 in the SI). Trace (a) in Figure 4 is recorded at 1.9 K directly after deposition. During deposition the sample temperature varies between 2.1 and 2.4 K due to the thermal load on the cryostat caused by rapid vapor deposition. Previous studies31,32 have shown that mixed face centered cubic/hexagonal close packed (fcc/hcp) crystal structures are formed using rapid vapor deposition and that annealing at ~4.3 K converts the crystal domains almost completely to the lower energy hcp form. Trace (b) in Figure 4 is the spectrum recorded at 4.4 K while annealing the sample. As can be seen in Figure 4, the intensity of both complex features increase significantly the first time the sample is annealed at 4.4 K. Trace (c) in Figure 4 is recorded after lowering the temperature back to 1.9 K, and trace (d) is recorded after raising the temperature of the sample again to 4.4 K. The irreversible growth in the two features assigned to the 1:1 and 1:2 complexes upon the first annealing cycle of the sample is consistent with enhanced diffusion of impurities during this first annealing of the sample. Note that the (LiH)3 cluster intensity does not growth appreciably during this same time period presumably because LiH does not diffuse readily and these LiH clusters only form during laser ablation and deposition. We remark that irreversible growth in the intensities of the cluster peaks is consistent with previous studies of Br-HBr clusters that showed this same behavior; clusters were only readily formed after the first

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annealing cycle.33 We suspect that facile Li atom diffusion during the first annealing cycle of the sample permits the growth in the 1:1 and 1:2 clusters. We can use intensity-intensity correlations to test if the two newly assigned peaks are caused by the same species or different clusters as we propose. Clearly, in Figure 4 the intensity changes caused by annealing show that the relative intensities of these two features are not conserved implying that they correspond to different clusters. Using data from three separate double doping experiments, a plot of the 1:2 versus 1:1 intensity shows that the two peaks are not strongly linearly correlated which suggests they correspond to different species (see Figure S6 in the SI). To help confirm this assignment we searched for additional absorptions that could be assigned to the 1:1 and 1:2 complexes. The two assigned features in Figures 3 and 4 correspond to the greatest intensity absorptions for both the 1:1 and 1:2 complexes, but given the S/N additional features may be observed. Only two additional features could be assigned to the complexes. A feature at 3060 cm-1 could be assigned to the C-H stretching vibration of the 1:1 complex and representative spectra are presented in Figure S7 in the SI. Another feature at 1469.6 cm-1 could be assigned to the ν30 (C=C stretch and CH2 scissor) vibration of the 1:2 complex that is also predicted to have a large integrated absorption strength (see Figure S5). Similar intensity-intensity correlation plots (see Figure S8 in SI) show slightly better correlations using the present assignment instead of the reverse assignment (assigning the 1469.6 cm-1 feature to the 1:1 complex and the 3060 cm-1 feature to 1:2) and further lend support to the spectral assignment. Only this assignment of these new features makes sense based upon the calculated vibrational frequencies and intensities. In both cases, the analogous absorptions were also observed in the Ar matrix studies consistent with the idea that we are observing the analogous complexes except in an pH2 matrix.9

Finally, this assignment is also consistent with the

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measured C2H4 concentrations which show that as the C2H4 concentration is increased the relative intensity of the 1:2 complex increases compared to the 1:1 complex. We therefore feel confident in the present assignment and that both species correspond to the charge-transfer complexes.

However, we obviously cannot rule out that some weakly bound Lin-(C2H4)m

complexes are formed in the pH2 samples and just not detected. We also point out that in Figure S5 in the SI there is a peak that grows in upon annealing at 1248.2 cm-1. We tentatively assign this peak to the C=C stretching vibration of the 1:3 cluster which has a strong absorption at 1243 cm-1 in the Ar matrix studies.9

4. Discussion The recent theoretical studies on lithium-ethylene complexes aimed at hydrogen storage1 only considered the 1:1 and 2:1 complexes. Therefore, we will separate our discussion of the 1:1 and 1:2 complexes observed in this study as only the 1:1 complexes have been considered to have potential in hydrogen storage. Both vibrational frequencies assigned to the 1:1 complex are systematically higher in wavenumbers than the corresponding values from the Ar matrix results.9 As we show in the DFT vibrational calculations for the gas phase and pH2 matrix, the vibrational frequencies of both observed vibrational modes shift to lower wavenumbers in the CPCM calculations aimed at modeling the effects of the pH2 matrix.

We therefore interpret the

vibrational frequencies from these pH2 matrix studies to be closer to gas phase values than the Ar matrix results. As a result of large amplitude zero-point motion, the nearest neighbor distance15 in solid pH2 (3.79 Å) is very close to the value in solid Ar (3.76 Å) and yet the polarizability34 of Ar is 1.6411x10-24 cm3 while the polarizability of pH2 is 0.8023x10-24 cm3.

Thus, the

perturbations caused by trapping the 1:1 complex in solid pH2 are expected to be less than those produced in an Ar matrix. However, the fact that the pH2 matrix isolation measurements are

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very close to the Ar results indicate that even in the less polarizable pH2 solvent, the 2B2 charge transfer state is the global minimum. This also supports the claim that 1:1 complexes should make good hydrogen storage materials since the large dipole moment, or large partial positive charge on the Li atom in the complex, can electrostatically bind H2. As described elsewhere,1 the interaction between hydrogen molecules and the 1:1 complex lies between chemisorption and physisorption, with a binding energy of ~23 kJ mol-1 per H2 which is compatible with absorption and desorption at ambient conditions.1 Further, the absorption capacity for two Li atoms bound to a single ethylene molecule (2:1 complex) is predicted to be 16 wt %, a record high value.1 The stability of the 2:1 complex was tested by extensive molecular dynamics simulations1 and indicate these complexes should be stable up to 500 K. Current work in our lab is focused on trying to optimize the deposition conditions in order to synthesize the 2:1 complex and to increase the concentration of these Lin-(C2H4)m species to measure the IR absorptions of H2 molecules bound to the various Lin-(C2H4)m complexes. Studying H2 induced absorptions is not easily accomplished because laser ablation of Li into solid pH2 creates a number of lithium hydride clusters and charged species that produce induced H2 absorption features, but looking for H2 induced absorptions that scale with the C2H4 concentration we may be able to identify features. By measuring these induced-infrared H2 absorption peaks we can estimate the H2 binding energy to the complex from the shift in the H2 vibrational frequency.35-37 We can also examine the thermal stability of these complexes if sufficient amounts can be synthesized. Similar observations are made for the 1:2 complexes. This cluster species was not included in the hydrogen storage studies,1 but was studied in the earlier Ar matrix isolations studies.9 Similar to the 1:1 complexes, the vibrational frequencies observed in this study were systematically at higher wavenumbers than the corresponding frequencies measured in the Ar

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matrix studies.9 These matrix dependent shifts were larger for the 1:2 complexes compared to the 1:1 complex. The dominant shift in the ethylene C=C stretching vibration comes from the charge transfer reaction with the Li atom. As discussed in the original Ar matrix studies,9 the interaction with the Li atom weakens the C=C bond due to charge transfer into the Π* antibonding molecular orbital on the C2H4 and shifts the vibration to lower wavenumbers. Thus, in the 1:2 complexes where the Li atom interacts with two ethylene molecules, the shift is less than in the 1:1 complex and the C=C stretching vibration comes at a higher wavenumber. Now this is an over simplification since the Li atom both weakens the C=C bond and bends the methylene hydrogen atoms away from planarity, and thus the Li atom mixes in vibrational mode character from both the C=C stretching mode and methylene group scissoring modes, but the general picture is correct and consistent with our observations. In our computational studies, only the D2h structure was able to be optimized suggesting that the D2d structure considered in the Ar matrix studies does not exist.9 The ESR paper also concludes that only the D2h structure is present.7 These structures also support the mixed C=C stretch and CH2 scissoring modes with enhanced intensities such that the ν31 mode is predicted to have an integrated intensity of 1472.4 km mol-1. However, these intensity calculations are very sensitive to the level of theory used as shown in Table S3. The observed peaks for the 1:2 complexes do support this picture of enhanced IR absorption strength. Furthermore, the same basic charge-transfer interaction is responsible for the binding energy within the 1:2 complex although now it is distributed over two C2H4 molecules. The Lin-(C2H4)m complexes studied in this work are also interesting from the perspective of calculating from first principles the Li-C2H4 intermolecular bonding for the two separate minima of the 2B2 and 2A1 electronic states. Using the B3LYP density functional we never

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observed the van der Waals well on the 2A1 surface of the 1:1 complex. This is not surprising because this functional does not capture long-range correlation effects (dispersion forces) at a quantitative level.

Examination of Figure 2 shows this explicitly; the B3LYP functional

reproduces the large stabilization energy for the 2B2 electronic state but only shows the 2A1 state to be repulsive (no van der Waals well). The most recent ab initio calculations predict that for the relaxed potential energy curves (relaxed from planar C2H4 geometries) the 2A1 state is 0.06 eV lower in energy than the 2B2 state.6 It would be interesting if newer long-range corrected hybrid density functionals38 could be applied to this problem to more fully map out the longrange interaction of the 2A1 state. Further, if these same DFT calculations could be applied to the 1:2 complexes where the stabilization of the 2B2U state may be less because the single Li atom interacts with two C2H4 moieties.

5. Conclusions In conclusion, we have demonstrated through double doping experiments that 1:1 and 1:2 lithium-ethylene complexes can be synthesized and detected using IR spectroscopy in solid pH2. As observed previously in Ar matrix studies,9 the IR absorption with greatest intensity corresponds to the C=C stretching vibration of ethylene in the complex. In fact, the intensity of the ν31 vibration in the 1:2 complexes is so strong that this species can be detected with IR absorption spectroscopy at ppb concentrations. Due to the small observed matrix dependent shifts in the vibrational frequencies of the 1:1 complex, this study clearly demonstrates that the electron transfer complex is formed in solid pH2. This implies that the 2B2 electronic state is lower in energy than the 2A1 van der Waals well and that there is not a significant barrier separating these two wells. This is still an interesting question for ab initio theory in that the best published estimates indicates the 2A1 state is lower in energy. In our computational studies the

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2

B2 state was always lower than the 2A1 state but the employed DFT methods do not account

quantitatively for dispersion energies. These studies also suggest that under cryogenic hydrogen storage conditions the 1:1 complexes will exist in the 2B2 electron transfer state which is advantageous for hydrogen storage.

At the concentrations that the 1:1 complexes were

synthesized in this work the IR induced transitions of adsorbed H2 molecules were not detected. However, now that we have clear IR absorption peaks to work with we can try to better optimize the conditions to increase the concentration of these complexes in order to study the number and types of adsorbed H2 molecules.

The Lin-(C2H4)m complexes are also interesting from a

fundamental chemical structure point of view in that there are two low lying electronic states with very different electronic wave functions and the energy balance between these two states can be experimentally verified using IR spectroscopy.

Supporting Information Table of integration parameters used to calculate dopant mole fractions (ppm), table of C2H4 peak frequencies for comparison to previous work,

tables of the vibrational frequencies,

intensities, and mode assignments for C2H4, Li-(C2H4), Li-(C2H4)2, Li+-(C2H4), and Li+-(C2H4)2, spectral figures of the simulated IR spectra, potential energy scans for the 2B2 and 2A1 electronic states of the 1:1 complex, comparison spectra for single and double doped pH2 samples, IR spectra showing the effects of annealing, intensity-intensity correlation plot for the 1:1 and 1:2 complexes, spectra showing the 3060 cm-1 peak, and intensity-intensity correlation plots for the peaks assigned to the 1:1 and 1:2 complexes.

Acknowledgment

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This work was sponsored by the Chemistry Division of the US National Science Foundation (CHE 13-62497) and the Chemistry NSF REU program (CHE 13-58498) hosted at the University of Wyoming.

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Figures

Figure 1.

Experimental apparatus for growing Li/C2H4 co-doped pH2 solids.

Lithium is

introduced using 1064 nm laser ablation and gaseous C2H4 is deposited through a hole in the Li rod holder.

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Figure 2. Potential energy surfaces for the 1:1 lithium-ethylene complex. Relaxed potential energy scans computed at the DFT B3LYP/6-311++G(2df,2p) level are plotted as a function of Li distance from the center of C=C bond. Gas phase Li-C2H4 energies are shown in black (2B2) and blue (2A1). The surface for Li-C2H4 isolated in the pH2 matrix is approximated by the CPCM implicit solvent model and are plotted in green (2B2) and red (2A1). The horizontal lines on the 2B2 profiles correspond to the zero-point vibrational energy levels.

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Figure 3. FTIR spectra of different pH2 solids in the region of the two strongest peaks assigned to Lin-(C2H4)m complexes. All the spectra are for annealed samples recorded at ~1.9 K and are offset and have a maximum intensity (

Solid Parahydrogen Infrared Matrix Isolation and ... - ACS Publications

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