Terahertz and Mid-infrared Spectra of Cold Formic Acid Aerosol

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Terahertz and Mid Infrared Spectra of Cold Formic Acid Aerosol Particles Mahmut Ruzi, Rebecca Auchettl, Courtney Ennis, Dominique R.T. Appadoo, and Evan G. Robertson ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00085 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Earth and Space Chemistry

Terahertz and Mid Infrared Spectra of Cold Formic Acid Aerosol Particles M. Ruzi,1 R. Auchettl, 1 C. Ennis,1 D. R. T. Appadoo,2 and E. G. Robertson1,* 1

Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, 3086, Australia 2 Australian Synchrotron, 800 Blackburn Road, Clayton, Australia * Corresponding author: [email protected]

Abstract: Fourier transform infrared spectra of formic acid aerosol particles in situ generated in a collisional cooling cell at temperatures ranging between 90 K and 210 K are recorded in the mid infrared and THz/far infrared regions. Infrared spectroscopic features are used to identify the formic acid dimer above 200 K, the crystalline β1 phase in the 110 – 200 K temperature range, and amorphous solid formic acid at ~ 90 K. Density functional calculations of discrete clusters and the full periodic β1 crystalline structure help to assign the low wavenumber intermolecular modes by comparison with experimental far IR and Raman data from the literature. Refractive indices of the formic acid crystal in the mid IR range are retrieved. These data are essential for radiative transfer modelling of solid formic acid condensed in the terrestrial atmosphere. THz/far IR spectra, in addition to the well documented mid infrared spectra, may be useful for the identification of solid formic acid in astrophysical environments such as the young stellar object W33A.

Keywords: Infrared spectroscopy, interstellar ices, aerosols, formic acid, density function theory, optical constants – refractive indices.

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1. Introduction Formic acid (HCOOH) has been detected in the interstellar medium, comets, and star– forming regions.1-2 Laboratory simulation and theoretical work suggests a grain surface origin for HCOOH in these environments; the product of formyl (HCO) and hydroxyl (OH) radical recombination within irradiated water ice mantles. Furthermore, HCOOH could act as an intermediate in the extra-terrestrial production of important organic acids, such as acetic acid (CH3COOH) and glycine (NH2CH2COOH).3 It is the most abundant organic acid in the Earth’s atmosphere and contributes to rain acidity, especially in remote regions.4-5 Formic acid, along with other organic acids, plays an important role in cloud condensation nuclei formation.6 Formic acid in the atmosphere mainly exists as gas due to its relatively high vapour pressure but it has been detected both in clouds and aerosol particles.4 The cyclic formic acid dimer, as well as the solid phases of formic acid, are also of fundamental interest in studies of condensed phase interactions where the molecules are held together by strong hydrogen bonds.7-11

At room temperature formic acid gas has a considerable dimer concentration (~ 5%)12 due to its large binding energy (59.5 kJ/mol)7. The cyclic formic acid dimer has been extensively studied both experimentally13-14 7, 15-19 and theoretically20-22 to assign its IR spectra. However, the peak assignment around the OH/CH stretch region from 2400 – 3600 cm-1 and the two peaks around 1740 cm-1 are ambiguous.15, 21 The congested band structure around the OH/CH stretch region is attributed to anharmonic coupling and Fermi resonances20-21, and dynamical effects due to double proton transfer.22 Two peaks are observed at around 1735 and 1746 cm-1 in room temperature spectra,14-15 and at 1738.5 and 1741.5 cm-1 in a jet-cooled laser spectrum;23 it has been proposed that these doublet peaks arise from the antisymmetric C=O stretch fundamental interacting with a near-degenerate combination band.23-24

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Crystalline formic acid is also a well-studied system; it has been investigated using various spectroscopic techniques such as IR spectroscopy,2,

19, 25-29

Raman spectroscopy29-30 and

inelastic neutron scattering.31-32,32-33 The most stable formic acid crystalline form at ambient pressure is the orthorhombic β1 phase, where formic acid molecules are hydrogen bonded in a linear chain, with the network of chains held together by weak van der Waals forces.11, 33-34 In early spectroscopic studies peak splitting in the frequency region between 1000 - 1800 cm-1 led to debate surrounding the crystal structure of formic acid – as to whether the spectral features were of the pure β1 phase or a polymorph mixture of β1 phase and β2 phase.35 29-30 A recent theoretical study demonstrated that most vibrational features of solid formic acid (20 225 K) are consistent with the β1 phase and that the difference in spectral features between the two β phases is most clear in the far IR region.32-33 The most significant splitting in the mid IR region can be explained as in-phase and out-of-phase vibrations.33 It is also noteworthy that the splitting and the intensity ratio of the two components vary as a function of temperature.28 It has been postulated that formic acid is deposited as dimers that are mostly retained in amorphous material at very low temperature (18 K), while subsequent warming increases the proportion of crystalline material.26,

28

Another interesting yet complicated issue is the

assignment of bands lying above 2000 cm-1, especially the intense and broad bands around 2900 cm-1.33 Resonance coupling21, 36, dynamic coupling, and Herzberg-Teller type theoretical models have been invoked to explain the observed features.27, 37

In this paper, we present FTIR spectra of formic acid particles at temperature range of 90 – 250 K in the THz/Far IR region, as well as in the mid IR region where we are able to retrieve temperature dependant refractive indices. Our main goal is to extract refractive indices and also shed light on the size and phase dependence of particles on the growth conditions. The refractive indices are needed for the remote sensing of aerosols, and for the evaluation of their effect on the climate. 3 ACS Paragon Plus Environment


2. Methods and Materials 2.1 Experimental Methods The formic acid aerosol particles are generated in an Enclosive Flow Cooling (EFC) cell38 that is coupled to a Bruker IFS-125-HR FTIR spectrometer located at the THz/Far IR beamline of the Australian Synchrotron, details of which described in previous publications.39-42 The EFC cell can be cooled to 90 - 160 K using liquid N2, and from 160 K to room temperature using cold N2 gas as coolant; the temperature is maintained/controlled by a series of resistive heating elements. The cell is first filled with N2 buffer gas at a desired pressure and left to equilibrate at a set temperature monitored by type-K thermocouples. The ice aerosols are generated by injecting a pulse (300 ms for mid IR and 500 ms for far IR) of N2 gas that is bubbled through liquid formic acid (Merck Millipore, >98%) into the EFC cell. Although the exact amount of formic acid could not be determined, its mole fraction is estimated to be 2.6% from the formic acid vapour pressure (6.19 kPa at 300 K)43 and the total pressure (34.7 psi) in the bubbler.

The EFC cell has White type optics so that the beam path length can be adjusted up to 25 m. In this study, the path length is set to 2.5 m for mid-IR and 5.0 m for far-IR studies. The midinfrared spectra (650-4500 cm-1) are taken at 2 cm-1 resolution, using the internal source of the spectrometer (Globar), a Ge coated KBr beamsplitter, and liquid N2 cooled medium range MCT detector; the EFC cell is equipped with KBr windows. The far IR/THz spectra (40-450 cm-1) are taken at 2 cm-1 resolution using the edge radiation infrared output of the Australian Synchrotron, Mylar beamsplitter, and Si bolometer; here, the EFC cell is equipped with polyethylene windows. The scanner velocity for both the mid and far IR measurements is set to 40 KHz. Each interferogram is phase corrected (Mertz), and convolved with a BlackmanHarris 3-term apodization function and zero-filled (order 2) before Fourier transformation using the OPUS software. 4 ACS Paragon Plus Environment

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We start the measurements with the spectrometer running in repeated measurement mode where the spectrometer is set to record ~400 spectra without delay (each spectrum taking ~0.5 seconds acquisition time), and immediately inject a single pulse of sample into the EFC cell. The temporal evolution of spectra generally shows the peak absorbance increases to maximum within the time period of the first few scans and then slowly decreases. The time dependence of absorbance is due to nucleation/growth of particles which eventually leave the optical path due to evaporation, diffusion or settling to the cell walls. The half lifetime of aerosol particles, the time it take to decrease the absorbance to half its maximum value, varies with temperature and buffer gas pressure, but is generally a few minutes. To increase signalto-noise ratio, we co-added all spectra (generally more than 100) within the half lifetime and averaged. Thus the spectra presented in this study are averaged unless specified.

2.2 Computational Methods The stable conformers of formic acid dimer,44 trimer,44-45 tetramer,9,

46-47

pentamer,48

hexamer,49 and dodecamer50 were studied at the B3LYP hybrid functional level by geometry optimisation and frequency calculations using tight criteria on an ultrafine grid. All calculations include Grimme’s empirical dispersion correction51 with Becke-Johnson damping (D3BJ)52-53 and are performed on the Gaussian 09 package.54 Dunning’s55augmented correlation-consistent polarised valence basis sets of triple zeta quality (aug-cc-pVTZ) were used for dimer to tetramers, and double zeta quality (aug-cc-pVDZ) for pentamer and hexamers, respectively. The calculations on the formic acid dodecamers (FA)12, the largest cluster considered here, are performed using Pople’s56 contracted Gaussian type basis set 6311+G(2df,pd). The vibrational spectra are calculated to confirm true potential energy minimum and the reported vibrational wavenumber values are unscaled. We also performed geometry optimisation and harmonic frequency calculations for the β1 phase crystalline 5 ACS Paragon Plus Environment


formic acid unit cell using the CRYSTAL14 code57-58 at the B3LYP-D3/6-311G(d) and PBE0-D3/6-311G(d) levels. Atomic coordinates and lattice parameters required for the input file were obtained from X-ray diffraction studies on formic acid ice at 98 K.11

3. Refractive Indices from Spectra When the particle size is very small compared to the wavelength of light, and the complex refractive index is not too large (i.e. in the Rayleigh limit), we have the expression for absorbance Al (at wavenumber ) as:59

= 6 = 1

where N (particles / cm3) is total particle number density, P(r) dr is the population fraction of particles of radius between r and r + dr, L is the optical path, Vi is the volume (cm3) of a sphere of radius ri (cm), ‘g’ is the number of different particles sizes, and is the imaginary

part of polarizability at wavenumber . Thus K is a constant (units cm) incorporating the particle volume, number density and optical path length of the incident beam:

= 6 2

The relationship between average (but non-uniform) molecule number density in the column,

ρΝ (cm-3), is given by:

= . . . . 3

where ρ is density of the material (g cm-3), MW is the molecular weight (g mol-1), and NA is Avagadro’s number (mol-1). Some or all of these parameters may not be directly measured with certain experimental setups;29, 38, 60-63 this includes the present setup where the particle number density cannot be directly measured.39-42, 64 However, the transition dipole moment 6 ACS Paragon Plus Environment

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(∂µ/∂Q), which is an intrinsic property of any material, is proportional to the area Cj under the curve bounded by the product of polarizability (" ), wavenumber #; and the relation between Cj and the IR intensity Aj:65 $% = & # # ∝ ( % = 8 , $% /

)* , ( 4 )+

0, + 2 32 0

,

3 5

Note that Aj and Cj are both measures of spectral intensity, though expressed differently.64 From equation (1) and (2) we deduce $% =

1 & # 6

If we know the IR intensity Aj of a band, using an initial estimate value of 0 from Equation (5), we can calculate initial Cj values. Here, 0 is the average value of n through the band of

interest.66 We can see from Equation (6) that the integrated intensity of a spectral peak is proportional to Cj values, with K being the proportionality constant. Once the K value is determined, we can calculate the polarizability " from Equation (1), and thus obtain the

refractive indices. The calculated refractive index and the input value of 0 should converge at

the same values, which can be done in a self-consistent manner.

We extract the polarizabilities of formic acid ice from small aerosol spectra that do not show noticeable scattering, employing the above-mentioned procedures. However, we need reference IR band strength to extract refractive indices. There are no IR band strength data of formic acid within our experimental temperatures (90 K-250 K), however, data are available at 25 K67 and ~300 K

13

. A previous study on formic acid films found that the C=O stretch

band strength is constant from 15 K to 165 K temperature range.28 To extract the polarizabilities, we choose the C=O stretch band strength (3.83×107 cm/mol )13 for 7 ACS Paragon Plus Environment


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temperatures between 90 K to 200 K, and for the cluster spectra above 203 K, we choose the CH bending band (1367 cm-1) with IR band strength (2.73×106 cm/mol).13 Note that above 203 K the CH bend band is the only peak that is free from monomer spectral overlap and has considerable band strength. There is no information available on the band strength in the THz/far IR region so we cannot extract refractive indices in this region (below 650 cm-1).

We follow the above mentioned procedures to calculate the imaginary part of the polarizability. The real part can be calculated by performing a Kramer-Kronig transformation, but due to the limited spectral range of the data, such calculations introduce truncation errors.68-69 In a previous study we have shown that the errors can be minimised by fitting model curves to the optical constants.69 Here, we use the classical damped harmonic oscillator (CDHO) model.65,

69

In the CDHO model, if the jth oscillator intensity is Sj (cm-2), the

damping constant is γj (cm-1), and the band position is # j (cm-1) then the polarizability is " # = ∑%

=

:; 67 87 9 =

:7 >9 :; = ? @87 =9 :; = 9 :; = 67 9 =

:7 = >9 :; = ? @87 = 9 :; =

Terahertz and Mid-infrared Spectra of Cold Formic Acid Aerosol

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