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What is the Driving Force Behind the Adsorption of Hydrophobic Molecules on Hydrophilic Surfaces? Yuan Fang, Saleh Riahi, Andrew McDonald, Mona Shrestha, Douglas J. Tobias, and Vicki H. Grassian J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03484 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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The Journal of Physical Chemistry Letters

What is the Driving Force Behind the Adsorption of Hydrophobic Molecules on Hydrophilic Surfaces?

Yuan Fang1, Saleh Riahi2, Andrew T. McDonald1, Mona Shrestha1, Douglas J. Tobias2*, Vicki H. Grassian1,3*

1Department

of Chemistry and Biochemistry, University of California, San Diego, La Jolla, 92093, CA, USA

2Department 3Scripps

of Chemistry, University of California, Irvine, 92697, CA, USA

Institution of Oceanography and Department of Nanoengineering University of California, San Diego, La Jolla, 92093, CA, USA

*To whom correspondence should be addressed: Douglas J. Tobias ([email protected]) and Vicki H. Grassian ([email protected])

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract. The adsorption of limonene, a common organic compound found in indoor air, on hydrophilic surfaces such as glass (SiO2), a prevalent surface in the indoor environment, is poorly understood. In this study, we have investigated the interaction of limonene and three other cyclic hydrocarbons (cyclohexane, cyclohexene, and benzene) on hydroxylated SiO2 using infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations. Experimental results show that there is an interaction between these cyclic hydrocarbons and surface hydroxyl groups. AIMD simulations demonstrate that all of the cyclic molecules, except for cyclohexane, π-hydrogen bond with surface hydroxyl groups while cyclohexane interacts with the surface OH groups through dispersion forces. According to experiments and simulations, the intermolecular interaction between limonene and SiO2 is significantly stronger than other compounds explored. This study provides an understanding of some of the driving forces behind the formation of organic coatings on glass surfaces which is important in indoor air quality. TOC:

Keywords indoor air, limonene, silica, adsorption, glass surfaces, ab initio molecular dynamics, transmission IR spectroscopy


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People in industrialized nations spend approximately 90% of their total time indoors.1 The indoor environment consists of a myriad of surfaces including, but not limited to, walls, windows and floors.1,2 By simplifying a square room to only its walls, the surface area to volume (S/V) ratio roughly scales by 1/(2L) (where L is the length of a each face of the room). As the room gets smaller, the S/V increases resulting in elevated S/V indoors compared to outdoors. Surfaces tend to prolong indoor lifetimes by providing reservoirs for indoor pollutants to partition into.3,4 Additionally, formation as well as growth of organic films on surfaces have also been reported.5,6 Thus, interfacial chemistry can play a significant role in the indoor environment. Adsorption of organic compounds onto surfaces is one of the major surface processes that can affect the composition of the indoor air.1 A broad range of organic compounds is present indoors and these compounds come from a variety of sources including the presence of occupants, building furnishings, outdoor environment, and gases produced from the indoor microbiome.1 Limonene, a cyclic monoterpene, is commonly found indoors due to its presence in cleaning products and odorants.7,8 Limonene concentrations can reach up to 175 ppb or higher immediately following product use.8 Unsaturated volatile organic compounds such as limonene have the potential to form secondary organic aerosols when oxidized by oxidants such as ozone.8,9 As limonene is found indoors and can interact with indoor surfaces, the adsorption of limonene on hydroxylated SiO2 surfaces, representative of glass, has been investigated here using experimental and theoretical approaches to provide molecular level insights into these interactions. Other cyclic compounds – cyclohexane, benzene,



and cyclohexene - were selected as model adsorbates to further our understanding of the adsorption of hydrophobic molecules on SiO2 surfaces and to answer the fundamental question: What is the driving force behind the adsorption of a hydrophobic molecule on a hydrophilic surface? This integrated experimental and theoretical study seeks to provide a greater understanding of the interactions of cyclic hydrocarbons on hydrophilic surfaces and to improve the current knowledge on the forces that drive adsorption on glass as well as other indoor surfaces. In general, the interaction between surfaces and organic species can be due to: (1) chemical bonding; (2) hydrogen bonding; and (3) van der Waals forces. Hydrogen bonding has been observed previously on SiO2 surfaces for aromatic compounds and other compounds that have hydrogen bonding potential.10-12 Zhao et al. investigated the adsorption isotherms for aromatic molecules on SiO2 and proposed the possible formation of hydrogen bonds between the SiO2 surface and aromatic molecules including hydrogen bonding through the π-electron system of the benzene ring and the hydrogen atom from hydroxyl groups.13 The adsorption of limonene on SiO2 was reported to be rather insignificant by Diaz et al. because it was suggested that there is no interaction between limonene and silanol groups as limonene contains no electronegative atoms.14 However, herein we show that it is clear that limonene forms π hydrogen bonds with surface hydroxyl groups on SiO2 surfaces and this interaction is stronger than that of the other cyclic compounds explored in this study. AIMD simulations provide insights into these interactions and show that all the cyclic molecules except for cyclohexane πhydrogen bond with hydroxyl groups of SiO2 while cyclohexane interacts with the surface through dispersion forces. Overall, this study provides key insights into some of


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the driving forces behind the adsorption of hydrophobic molecules on hydrophilic surfaces and relates that to the formation of organic coatings on glass surfaces for the first time.

Experimental results for limonene and other cyclic hydrocarbons adsorbed on SiO2. Infrared spectra for SiO2 following exposure to gaseous limonene as a function of varying limonene pressure are shown in Figure 1a. A detailed peak assignment for the IR bands in the gas phase and due to adsorption is provided in Table S1. The vibrational peak frequencies for adsorbed limonene are within 5 to 10 cm-1 of the values for limonene in gas and liquid phase, indicating that the limonene molecularly adsorbs on the surface. Since we are most interested here in understanding the molecular level interaction of limonene adsorbed on hydroxylated SiO2, we focus on the O–H stretching region and the hydrogen bonding interactions. In particular, the negative peak at 3742 cm-1 for adsorbed limonene is attributed to the loss of isolated surface silanol groups due to the SiO2 surface interaction with limonene which results in the broad band near 3504 cm-1 that is assigned to the hydrogen bond between Si-OH groups and limonene. The 3742 cm-1 peak decreases with increasing gaseous limonene pressure and increasing surface coverage of limonene. These data show that limonene interacts with the SiO2 surface through a hydrogen bonding interaction with surface hydroxyl groups. This interaction is reversible and absorption bands due to adsorbed limonene decrease in intensity upon evacuation, while the negative peak attributed to isolated hydroxyl groups (at 3742 cm-1) reappears. Thus, limonene reversibly adsorbs on hydroxylated SiO2.



The adsorption of limonene on SiO2 can also be described by a Langmuir adsorption isotherm as shown in Figure 1b. The Langmuir saturation coverage and constant of limonene adsorption are determined to be 1.37±0.07 × 1014 molecules/cm2 and 0.032±0.002 mTorr-1, respectively. Diaz et al. reported a theoretical monolayer coverage of limonene on the SiO2 surface as 3.23 μmol m-2 (equivalent to 1.95 × 1014 molecules/cm2) obtained by molecular modelling.14 Our experimental data of Langmuir saturation coverage is in fairly good agreement with the calculated monolayer coverage. Limonene is not expected to tessellate perfectly on the SiO2 surface. Therefore, the experimental coverage is expected to be lower than the theoretical value based on perfect molecular packing.14 (b)

(a)

Figure 1. (a) Absorbance spectra of limonene adsorbed on SiO2 under dry conditions (RH < 1 %) as a function of limonene pressure (1, 5, 10, 25, 50, 100, 200, 500 and 1000 mTorr) in the 1280 ~ 4000 cm-1 spectral regions. Note that SiO2 is opaque below 1280 cm-1 due to lattice vibrations. Gas phase limonene has been subtracted from these spectra. The surface spectrum following overnight evacuation is shown as a dashed line. (b) [P/N] vs pressure for limonene adsorbed on SiO2 using integrated peak area from 2785 to 3115 cm-1 where P is pressure and N is surface coverage.


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Other cyclic compounds also form hydrogen bonds with the surface hydroxyl groups present on the SiO2 surface as suggested by the negative peak centered at 3745 cm-1 in Figures 2a, b and c, which is attributed to the loss of isolated surface hydroxyl groups. Adsorbed surface species appear in very low intensity for these three cyclic compounds in comparison to limonene. Figure 2d shows the normalized loss of the peak intensity of surface hydroxyl groups at ~3745 cm-1 as a function of pressure for each cyclic compound. It must be noted that the loss of the surface hydroxyl groups herein was obtained by referencing single-beam spectra of the surface to the gas phase single beam spectra collected prior to the introduction of gas-phase limonene; an example of the resulting spectra is shown in Figure S1. Baseline correction was applied to all absorbance spectra. It can be clearly seen in Figure 2d that limonene has a significantly greater surface interaction in comparison to other cyclic compounds since there is a much larger loss of isolated surface hydroxyl groups in the presence of limonene than is observed for these other cyclic hydrocarbons. Among the three compounds, the loss of surface hydroxyl groups is smallest when cyclohexane is introduced, indicative that cyclohexane is weakly adsorbed on the SiO2 surface. Benzene results in a much larger loss of surface hydroxyl groups, reaching a plateau at pressures greater than 100 mTorr. Meanwhile, cyclohexene adsorption results in a higher loss of isolated surface O-H groups compared with cyclohexane, suggesting that more cyclohexene is adsorbed than cyclohexane. The interaction strength of the three cyclic compounds at pressures below 1 Torr is as follows: benzene > cyclohexene > cyclohexane. At ~1 Torr, cyclohexene adsorption becomes more comparable to that of benzene. To better elucidate the molecular interactions



observed experimentally between these cyclic compounds and the SiO2 surface, we also performed AIMD simulations, which will be presented and discussed in the next section. (a)

(c)

(b)

(d)

Figure 2. FTIR spectra of: (a) cyclohexane (3000 to 4000 cm-1, 1 Torr); (b) cyclohexene (3080 to 4000 cm-1, 500 mTorr); and (c) benzene (3200 to 4000 cm-1, 1 Torr). (d) Normalized peak intensity of surface hydroxyl groups (using peak centered at 3745 cm-1) loss as a function of pressure for the adsorption of the cyclic molecules of interest on the SiO2 surface. The dotted lines are guides for the eyes. Structural Characterization of the Interaction of Cyclic Organic Compounds with SiO2 – AIMD Simulations. We observed that the cyclic part of all the organic compounds considered predominantly stays parallel to the SiO2 cluster during the AIMD simulation, allowing more favorable contacts/interactions with the SiO2 surface as shown in Figure


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3a. Our previous molecular dynamics simulation of limonene on SiO2 (manuscript in preparation) demonstrates that limonene engages in a π–hydrogen bonding interaction with SiO2. Configurations with the SiO2 O–limonene Csp2 distance < 3.4 Å and the SiO2 OH…limonene Csp2 angle between 135˚ and 165˚ were considered as H–bonding interactions, which were present in 30% of all configurations. Furthermore, the limonene molecule assumes two prevalent orientations on the surface that are related by a 180˚ rotation parallel to the surface. The chiral carbon C* is further from the surface in the C* up arrangement in comparison to the C* down arrangement (see Figure 3a). (a)

(b)

benzene

cyclohexane

cyclohexene

limonene (c* up) limonene (c* down) benzene cyclohexene cyclohexane

RDF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

limonene (C* down)

2

2.5

3 3.5 r(Å)

4

4.5

5

limonene (C* up)

Figure 3. (a) Snapshots from the AIMD simulations of the cyclic molecules adsorbed on the SiO2 surface. The sp2 carbon, chiral carbon, oxygen, hydrogen, and silicon atoms are colored blue, pink, red, white, and yellow, respectively. (b) Radial distribution functions calculated between the hydroxyl hydrogen atoms and sp2 carbon atoms of limonene and cyclohexene, center of mass of benzene, and carbon atoms in cyclohexane.

The radial distribution function (RDF) is used to assess the interaction of organic compounds with the surface. The RDF is calculated between the SiO2 hydroxyl hydrogen atoms and sp2 carbon atoms of limonene and cyclohexene, the center of mass of benzene, 9 ACS Paragon Plus Environment


and carbon atoms of cyclohexane. The positions of the peaks in RDF correspond to the most probable distance of hydrogen atoms of hydroxyl group near the selected carbon atoms. The location of first peak for each system is summarized in Table 1. As can be seen in Figure 3b and Table 1, compounds with C=C bonds (limonene, benzene, and cyclohexene) localize at shorter distance from the surface hydroxyl group than the saturated cyclohexane.

Table 1. Location of the first peak in the RDFs (Figure 3b) depicting the interactions of organic compounds with the H atoms of the SiO2 surface. Adsorbed Cyclic Organic Compound limonene (C* up) limonene (C* down) benzene cyclohexene cyclohexane

location of 1st peak (Å) 2.3 2.2 2.3 2.2 4.4

The hydroxyl H atoms stay within 2.2–2.3 Å from the unsaturated carbon bonds. This close contact is associated with the π–hydrogen-bonding interaction between the SiO2 and the unsaturated cyclic hydrocarbons. Although the first peak for the C* up configuration is at 0.1 Å greater distance from the SiO2 H atoms than that of the C* down configuration, the sp2 carbon atoms located at the propenyl group form significantly closer contact with hydroxyl groups in the C* up conformation than in the C* down conformation, as illustrated in Figure S2. The position of the first peak in the RDF for cyclohexane is indicative of a weak interaction with the SiO2 surface, dominated by dispersion interactions. The higher RDF peak of benzene is linked to the rigidity and more delocalized electron density of benzene compared to the other cyclic compounds. 10 ACS Paragon Plus Environment

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Binding energies between the organic molecules and SiO2 cluster quantify the strength of the interaction between SiO2 and the cyclic compounds. The M06-2X/6– 311G(d)15 density functional theory method was employed to obtain the optimized structure and energy of the individual fragments (organic molecule and SiO2 cluster) and complex (organic molecule adsorbed on the SiO2 cluster). The binding energies, calculated as the difference between the energy of the complex and that of the individual fragments, were corrected for basis set superposition error (BSSE).16 Subsequently, single-point energy calculations were performed to calculate the binding energies with the 6–311++G(d,p) basis set using the geometries optimized with the 6–311G(d) basis set. According to these calculations, and consistent with the FTIR data, the interaction between limonene and SiO2 is significantly stronger than the other compounds explored in this study. This is due presumably to the larger size of limonene (more dispersion interaction) and the existence of two unsaturated bonds that can participate in π–H bonding interactions. Although limonene has fewer unsaturated bonds compared to benzene, its more flexible structure leads and greater size enables stronger interaction between the C=C moieties and hydroxyl groups, in addition to enhanced dispersion interactions, compared to benzene.

Table 2. Binding energies in kJ mol-1 calculated using M06-2X method. The reported values are corrected for the basis set superposition error. The 6–311++G(d,p) binding energies were determined from single point energy evaluation on the M06-2X/6–311G(d) optimized structures. Adsorbed Cyclic Hydrocarbon limonene (C* up) limonene (C* down) benzene cyclohexene cyclohexane

6–311G(d)

6–311++G(d,p)

-42.19 -43.89 -28.33 -28.05 -20.04

-46.26 -41.57 -27.50 -29.15 -24.26



Frequency distributions amenable for qualitative comparison with the experimental FTIR spectra were calculated as Fourier transforms of the velocity autocorrelation functions of selected atoms. Note that only the positions and not the intensities of the peaks in the power spectra are comparable to the experimental data. Consistent with the experimental data, the calculated power spectra presented in Figure 4 for the H atoms of the hydroxylated SiO2 surface display a red shift in the hydroxyl frequency upon the addition of the cyclic organic compounds. Figure S3 shows the power spectra for all atoms over the complete vibrational frequency range. The shifts in the vibrational frequency in the presence of the organic compounds vs. bare SiO2 are attributed to the hydrogen bonding interaction between the SiO2 surface and sp2 carbon atoms of the organic compounds. The trend in the shifts in O–H stretching frequency is in agreement with the binding energies (Table 2) and peak locations in the radial distribution functions (Figure 3b). As is evident in Figure 4, the C* up conformation of limonene molecule induces the largest vibration shift, followed by the C* down conformation of limonene. Interestingly, cyclohexane also alters the O–H vibrations by 50 cm-1, which might be a signature of hydrogen bonding interactions, due to the intermolecular charge transfer between the C–H and O–H groups.17,18 However, the role of dispersion forces in these types of interactions cannot be neglected. The first peak of the RDF calculated between the silica O and the H atoms of cyclohexane occurs at 2.85 Å, which is significantly greater than the peak locations for the other compounds listed in Table 1. Thus, we do not characterize the interaction between cyclohexane and the silica surface as a hydrogen bonding interaction.


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no adsorbate limonene (C* up) limonene (C* down) benzene cyclohexene cyclohexane intensity

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4000

3800

3600 3400 ω (cm-1)

3200

3000

Figure 4. Power spectra calculated for the hydroxyl group in the vicinity of the organic compounds. The dashed line marks 3650 cm-1 the wavenumber. In summary, FTIR spectra of adsorbed limonene and AIMD simulations are combined to study the driving forces behind the adsorption of hydrophobic molecules (such as limonene) on hydrophilic surfaces (such as SiO2). Limonene is found to form hydrogen bonds with surface hydroxyl groups on SiO2 via sp2 carbons. Other cyclic molecules, such as cyclohexane, benzene and cyclohexene were studied to further understand these interactions. Limonene, with a cyclohexene structure and an additional C=C bond is found to form the strongest interaction amongst all four of these molecules. The detailed description provided herein of the interactions between limonene, as well as the three selected cyclic hydrocarbon molecules with SiO2 studied here, could help improve our understanding of the interaction of other relevant hydrophobic indoor molecules with hydrophilic surfaces such as glass to better understand how organic coatings form and ultimately how organic compounds are removed from indoor air.



Experimental and Theoretical Methods Transmission FTIR Experiments. The adsorption of limonene on SiO2 surfaces at 296 ± 1 K was studied using transmission Fourier transform infrared (FTIR) spectroscopy coupled with a modified Teflon coated infrared cell.19,20 In these experiments, approximately 5 mg of high surface area SiO2 particles (Degussa OX50, BET surface area of 230 m2 g-1), was pressed onto one-half of a tungsten grid held by two Teflon coated jaws in the FTIR cell compartment. The sample cell was then evacuated for 6 hours using a turbo-molecular pump to clean the cell and the sample surface. After evacuation, the sample was exposed to the desired pressures of dry, gaseous limonene for 20 minutes under dry conditions (RH < 1%). The gaseous limonene was produced from (+) - Limonene (>99%, Fisher Scientific) by degassing at least three times with consecutive freeze-pump-thaw cycles. Prior to and following the exposure of limonene, the single-beam spectra of surface- and gas- phases (300 scans) were acquired at 296 K. A resolution of 4 cm-1 was used over the spectral range of 800 to 4000 cm-1. As SiO2 is opaque below ~1280 cm-1, spectra are shown only above 1280 cm-1. The IR cell and sample surface were evacuated after adsorption had reached equilibrium. Absorbance spectra of limonene on the SiO2 surface are reported as the difference in the SiO2 spectra before and following exposure to limonene. Absorption bands attributed to gas-phase limonene (measured through the blank half of the tungsten grid) were subtracted from the surface absorbance spectra to obtain the FTIR spectra of the adsorbed particle species loaded on the tungsten grid. The


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adsorption of other three cyclic molecules (cyclohexane, benzene and cyclohexene) was studied using the same method above. Baseline correction was applied to all absorbance spectra. The vapor above the liquid for these experiments were taken from (+) - Limonene (>99%, Fisher Scientific), cyclohexane (99.9% Fisher Scientific), benzene (≥ 99.9%, Sigma Aldrich) and cyclohexene (99%, Sigma Aldrich), respectively, by degassing at least three times with consecutive freeze-pump-thaw cycles.

Ab Initio Molecular Dynamics Simulations. AIMD simulations were utilized to elucidate the origin of the change in the vibrational frequency of SiO2 upon adsorption of organic compounds. AIMD simulations of systems composed of limonene, cyclohexene, benzene, and cyclohexane adsorbed on a cluster of hydroxylated SiO2 were carried out using the CP2K program.21 The SiO2 cluster was obtained from a force field-based MD simulation of amorphous SiO2. A silica cluster with Si23O66H40 stoichiometry was chosen such that it can fully accommodate the adsorption of the organic compounds. For each system 40 ps of AIMD simulation at 295 K with time step of 0.5 fs was performed. The BLYP–D3 exchange–correlation functional with the DZVP–MOLOPT–SR–GTH basis set and the GTH pseudopotentials in the QUICKSTEP module of CP2K package were employed.22,23 A SCF convergence criterion of 10-8 (a. u.) with the orbital transformation24 scheme was applied during the simulation. Each system was placed in an orthorhombic box of 20×21×22 Å3 under periodic boundary conditions in the Y and Z dimensions (the X direction is along the normal to the SiO2 surface). The simulation temperature was maintained at 295 K using a Nosé–Hoover thermostat25 with a relaxation



time of 100 fs. The binding energy of organic compounds to SiO2 was calculated using the Gaussian 16 package.26 Optimized structures of organic compounds and SiO2 cluster in the isolated and complex forms were obtained with M06-2X/6–311G(d). Subsequently, counterpoise corrected binding energies were estimated by M06-2X/6–311++G(d,p). Vibrational power spectra calculated from the AIMD simulation were utilized to shed light on the interaction of organic compound with the SiO2. Power spectra are computed from the mass-weighted velocity autocorrelation function27,28: 𝑝(𝜔) = 𝑚∫〈𝑣(0) 𝑣(𝑡)〉𝜏 𝑒 ―𝑖𝜔𝑡d𝑡, where v (𝑡) is the velocity of the selected atoms with mass 𝑚 at time 𝑡, and 𝜔 is the frequency. The brackets represent an average over atoms and time origins in the trajectory. Subsequently, a 900 fs wide Gaussian windowing function is applied to the correlation function. The power spectra were obtained by discrete Fourier transformation of the smoothed correlation functions. The velocity autocorrelation function computed for all atoms present in each system was used to obtain the power spectra expanding the full range of frequency (Figure S3). Whereas, only the hydroxyl H atoms in the vicinity of sp2 carbon atoms included in the calculation of power spectra presented in Figure 4. Supporting Information. Supporting information contains one table and two figures. The table lists the vibrational mode assignment for gas phase limonene and limonene adsorbed on SiO2 particle surfaces. The first figure (Figure S1) shows the baselinecorrected FTIR spectra displaying the loss of surface hydroxyl groups (at 3742 cm-1) as a function of limonene coverage. The second figure (Figure S2) illustrates the RDF calculated between the sp2 carbon atoms located at the ring and propenyl group of limonene and hydroxyl hydrogen atoms for the C* up and C* donwn orientations. The 16 ACS Paragon Plus Environment

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third figure (Figure S3) shows the power spectra calculated for the complete range of frequency for all of the atoms in each system. Note. The authors declare no competing financial interest. Acknowledgement. This material is based on the work supported by the Alfred P. Sloan Foundation under grant number G‐2017‐9692 (VHG) and G-2017-9796 (DJT). The contents of this study do not necessarily reflect the official views of the Alfred P. Sloan Foundation. The Alfred P. Sloan Foundation does not endorse the purchase of the commercial products used in this report. We would also like to thank Prof. Manabu Shiraiwa and Dr. Pascale Lakey (UC Irvine) for helpful discussions. We are grateful to the administrators of the HPC and Greenplanet computing clusters for their excellent technical support.



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