Absorption Spectra of Aspirin and Benzoic Acid

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Superficial and Fundamental Correspondences in the Terahertz/IR (6−15 THz) Absorption Spectra of Aspirin and Benzoic Acid L. M. Lepodise,†,‡ J. Horvat,*,‡ and R. A. Lewis‡ †

Botswana International University of Science and Technology, Palapye, Botswana School of Physics and Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia

‡

J. Phys. Chem. A Downloaded from pubs.acs.org by UNIV OF KENTUCKY on 08/23/18. For personal use only.

S Supporting Information *

ABSTRACT: The terahertz absorption spectra of aspirin and benzoic acid have been measured in the range 200−500 cm−1 (6−15 THz). Density-functional theory (DFT) modeling has assigned fundamental vibrational modes to the observed absorption bands. Hydrogen bonds between the crystalline planes of aspirin resulted in better agreement between the experimental and modeled spectra than for benzoic acid. The similar structure of these two molecules suggests a similar absorption spectrum, which indeed was obtained experimentally. However, the detailed crystal structure and molecular differences result in some of the apparently common absorption bands being assigned to different vibrational modes through the DFT modeling. Thus, our study importantly reveals that even though crystalline forms of two similar molecules may have similar experimental terahertz spectra, the resemblance may be superficial rather than fundamental. copy (TDS) region, and some in the infrared region.11−15 Studies in the higher-frequency terahertz region (6−15 THz), crossing over into mid-IR band, are scarce. Furthermore, few studies have investigated the materials at low temperature, even though cooling reduces both homogeneous and inhomogeneous broadening and thus reveals more and sharper spectral lines.16,17 The present study meets these deficiencies by reporting low-temperature measurements in the 6−15 THz range. Similar chemical compounds are expected to yield similar infrared spectra, as the common atomic configurations vibrate at almost the same frequencies. However, the crystalline structures of chemically similar compounds can be quite different, especially for molecular crystals, in which weak van der Waals bonds play a significant role. Substantial differences in these spectra can be obtained at low vibrational energies, namely, in the terahertz range. The inter- and intramolecular vibrations can couple, creating complex vibrational modes. Unfortunately, these spectral differences cannot be deduced on the basis of the structural differences in a straightforward way. The promise and challenge of this context motivates the present study. This paper deals with two similar compounds that have similar terahertz absorption spectra, but their crystalline planes

I. INTRODUCTION Terahertz spectroscopy, particularly terahertz pulsed spectroscopy, has been employed extensively in the pharmaceutical industry.1−7 Terahertz techniques improve on previously used methods. In the past, products were manufactured in batches and laboratory analysis was used to verify the quality. This turned out to be costly, because if the analyzed product did not meet the required specifications, the whole batch was destroyed.8 In contrast, using terahertz spectroscopy, products may be tested and monitored throughout the manufacturing process. Testing can be done at every step, thus saving product, money and time. Other optical techniques like Raman spectroscopy and nearinfrared and mid-infrared spectroscopy also have been used to monitor the quality of manufactured pharmaceuticals. Optical methods probe atomic bonds in molecules. The atoms vibrate in discrete modes, which may be used to characterize or fingerprint the material. Therefore, these optical methods, including terahertz spectroscopy, are used to distinguish different compounds. For this purpose, the characteristic absorption lines for each compound have to be unambiguously identified. Author: Terahertz spectroscopic techniques have proven to be more sensitive than other techniques in picking up small differences in molecules. Consequently, many drugs have been investigated using terahertz techniques.8−10 Most of the previous reports on these materials are concentrated at low frequencies, that is, in the Terahertz Time Domain Spectros© XXXX American Chemical Society

Received: June 5, 2018 Revised: July 29, 2018 Published: July 30, 2018 A

DOI: 10.1021/acs.jpca.8b05393 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Crystal structures of (a) aspirin, b-axis projection, and (b) benzoic acid, a-axis projection. These specific projections are chosen to clearly visualize the crystalline planes. Hydrogen is white, oxygen red, and carbon black. The dimers formed by hydrogen bonds between the −COOH groups of the two molecules define the crystalline planes, which are oriented differently for the two compounds. For simplicity, the intramolecular hydrogen bonds and interplanar hydrogen bonds for aspirin are not shown. Figures were generated using VESTA.18

that differences in bonding between the planes make some of the essentially similar vibrational modes of the two compounds different in detail, resulting in substantially different absorption energies for these modes for the two compounds. Nevertheless, the absorption spectra of the two materials remain superficially similar.

are bonded either by van der Waals bonds alone or by a combination of van der Waals and hydrogen bonds. Specifically, a comparison is made of the terahertz spectra of the common painkiller, aspirin, and one of the most studied aromatic compounds, benzoic acid. These two materials have in common a benzene ring and the −COOH group. However, aspirin in addition has a −OCOCH3 group attached to the benzene ring. Both compounds form molecular crystals, with hydrogen-bonding between −COOH groups creating dimers as the basic crystal building blocks (Figure 1). The dimers form crystalline planes for both systems. In benzoic acid, the dimers form mutually parallel planar structures within each plane and the planes are connected through van der Waals bonds, with no hydrogen bonds. Additional −OCOCH3 groups in aspirin result in a ribbon-like structure within each of the crystalline planes. Aspirin dimers are parallel to each other within each ribbon, but neighboring ribbons are tilted with respect to each other. The crystalline planes of aspirin are bonded through hydrogen bonds between −OCOCH3 groups. Density functional theory (DFT) calculations are employed to assign the experimental terahertz spectra to fundamental vibration modes. Contrary to expectation, our study shows that quite different vibrational modes can be obtained at the same frequencies for two similar compounds. The origin of this surprising result is

II. RESEARCH METHODS Samples were prepared by mixing 5% by weight of aspirin or benzoic acid powder with polyblend powder (a polymer mostly transparent in the terahertz region) and pressing into pellets of about 0.5 mm thickness. Pressing was into a die of 13 mm diameter at 1.5 bar for 2 min. The pellets were measured in transmission geometry in a Bomem DA8 FTIR spectrometer, within the 6−15 THz (200−500 cm−1) frequency range. The resolution at all frequencies was 0.015 THz (0.5 cm−1). The sample compartment was evacuated to reduce the effects of water vapor in the spectra. Cooling of the samples was achieved in an Oxford Instruments continuous-flow helium cryostat. A globar was used as the radiation source. A liquidhelium cooled Si bolometer was used as the detector. More details of the experimental method appear in our previous work.19 The spectrum of polyblend consists of a sharp band at 202 cm−1 (6.06 THz) and several broader bands at higher energies, 248 cm−1 (7.43 THz), 381 cm−1 (11.42 THz), 396 B


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The Journal of Physical Chemistry A cm−1 (11.87 THz), and 438 cm−1 (13.13 THz). To eliminate the contribution of the polyblend, all sample spectra were divided by the spectrum of polyblend. This successfully eliminated the polyblend bands from all measurements; even the sharp band at 202 cm−1 (6.06 THz) gave very little trace. Numerical modeling was performed using the QUANTUM ESPRESSO package.20 The PBEsol DFT functional was used.21 Norm-conserving scalar-relativistic PBE pseudopotentials were used for all three elements.22 The kinetic energy cutoff for wave functions was 1225 and was 4422 eV for charge density. In view of the importance of the bonds between the crystalline planes for the two systems, variable-cell geometry optimization was performed, with van der Waals correction turned on.23,24 The starting geometry was obtained from XRD data.25,26 The starting geometry for aspirin was form I,27 which was retained throughout the geometry optimization. Form I is the common, stable polymorph of aspirin, while the elusive form II is obtained under special conditions, mixed in with form I.28 Because the van der Waals interaction plays an important role in building the crystal structure with the dimers, the van der Waals interaction was used in the variable-cell optimization. However, the numerical module for phonon calculation in Quantum Espresso does not have van der Waals interactions incorporated. The unit cell was then frozen and further geometry optimization performed without van der Waals interactions. This moved the ions into their minimum energy positions when no van der Waals interaction is present in the calculus, while the unit cell parameters retained the characteristics determined by van der Waals interactions. The phonon modes were then calculated without van der Waals interactions. Stringent convergence criteria were applied: 10−16 eV/atom for electron convergence and 10−13 eV for geometry optimization, resulting in a total force smaller than 0.002 eV/Å. The phonon modes were calculated in the harmonic approximation. As thermal effects are not accounted for in the modeling, the modeled spectra were appropriately compared to low-temperature measurements.

Figure 2. Room temperature terahertz spectra of aspirin (top, black) and benzoic acid (bottom, red). Absorption bands for aspirin are identified by the numbers 1−6 and for benzoic acid by the letters A− E. The spectra are shifted vertically to facilitate comparison.

used to obtain the absorption spectra and corresponding normal modes of vibration. In this combined experimental and theoretical framework, we can accurately ascertain which experimental bands of aspirin and benzoic acid correspond to the same vibrational modes. Figure 3 presents the measured low-temperature (15 K) spectrum of aspirin and the modeled spectrum. Since thermal

III. RESULTS Figure 2 presents the room temperature spectra of aspirin and benzoic acid. Both exhibit broad absorption bands. The similarity of the spectra suggests that there are common bands between aspirin and benzoic acid. However, aspirin has six absorption bands (which we label 1−6) compared to benzoic acid with five (which we label A−E) and it is unclear which of the six aspirin bands are specific to aspirin only, although by observation it is likely to be 1, 2, or 3. From a theoretical perspective, the dimers of each compound have similar masses and similar bonds within each dimer. While there are differences in the formation of crystal bonds between these two systems, it is not immediately clear how these affect their terahertz absorption. The use of the absorption energy alone as a criterion to find the corresponding bands for the two compounds may be unreliable as vibrational energies can shift due to inter- and intramolecular interactions. Thus, the room temperature spectra alone provide only limited insight into the similarities and differences between the absorption features and their origins in these two related materials. We proceed to push further in both the experimental and theoretical spheres. Experimentally, we now collect lowtemperature spectra, which are expected to sharpen the observed absorptions. Numerically, DFT calculations are now

Figure 3. Low-temperature (T = 15 K) experimental and theoretical spectrum of aspirin. The plain numbers denote the experimental bands. The dashed numbers denote the corresponding assigned theoretical bands. The theoretical spectrum was obtained by plotting Gaussians of 5 cm−1 (0.15 THz) half-width at the energies of the calculated spectra.

excitations are not included in the modeling, the modeled spectrum is better compared with the low-temperature measurement in Figure 3 than with the room-temperature measurement of Figure 2. Our modeled absorption spectrum shown in Figure 3 was obtained by fitting a Gaussian form centered at each modeled vibrational energy. The Gaussian widths were taken to be the same for all absorption lines (5 C


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Table 1. Comparison between the Absorption Bands of Aspirin from Theoretical Calculations and from Experimenta mode label

experiment, cm−1 (THz) 300 K


1

262 (7.87)

270 (8.11)

2

289 (8.68)

291(8.74)

3

322 (9.67)

324 (9.73)

4

382 (11.47)

388 (11.65)

5

424 (12.73)

425 (12.76)

6

too noisy to fit

437 (13.12)

numerical, cm−1 (THz) 285 287 300 303 327 329 398 402 434 438 446 447

(8.56) (8.62) (9.01) (9.10) (9.82) (9.88) (11.95) (12.07) (13.03) (13.15) (13.39) (13.42)

description δ(COOH−C6 bond), in one-half of planes only δ(COOH−C6 bond), in the other half of the planes γ(C6−OCOCH3, C6−COOH bonds) symmetric γ(C6−OCOCH3, C6−COOH bonds) antisymmetric ν(C6−OCOCH3 bond), in one-half of planes only ν(C6−OCOCH3 bond), in the other half of the planes ν(C6, along the −COOH bond) + γ(−CH3) symmetric ν(C6, along the −COOH bond) + γ(−CH3) antisymmetric γ(C6) antisymmetric γ(C6) symmetric γ(C6), in one-half of the planes only γ(C6), in the other half of the planes

Corresponding visualization files are given in the Supporting Information. (δ denotes in plane bending, ν denotes stretching, and γ denotes out of plane bending.) The point group in all cases is Au. Estimated uncertainties for absorption band energies are 1.5 cm−1 for band 2 and less than 1 cm−1 for other bands. a

cm−1 or 0.15 THz) and they were chosen to best match the experimental spectrum. Two theoretical modes per experimental band appear because the unit cell contains two dimers. Each experimental band was therefore assigned two closely spaced modeled modes, with the same vibration type, but with different symmetry of vibration (Table 1). Very good agreement was obtained between the experimental and modeled spectra, both in terms of absorption energy and in terms of intensity. The modeled absorption energies are blue-shifted from the experimental energies by a factor of 1.028, which is a consequence of the numerical method used,29 arising from approximations in the DFT theory, harmonic phonon modes, imperfections in the geometry optimization, and neglecting van der Waals interactions in calculating the phonon modes. This blue shift aside, excellent agreement between the modeled and experimental energies of absorption bands is observed (Figure 4), implying there is a genuine identification between the modeled modes and the experimental bands. The intensity of absorption bands 5 and 6 is not reproduced as well in the modeling as for the other four bands. On the one hand, the modeled band corresponding to the experimental band 5, denoted as 5′, is relatively low in intensity. On the other hand, the modeled band corresponding to the experimental band 6, denoted as 6′, is relatively high in intensity. Since all the other numerical bands are obtained at higher energies than the experimental ones, we assign bands 5′ and 6′ on the basis of their respective energies. Table 1 compares the experimental and theoretical results, as well as gives the assignment of the absorption bands. Avogadro software30 was used in the visualization of the vibrational modes. The visualization files are provided in the Supporting Information. Boczar et al. modeled the infrared spectrum of the aspirin dimer using the B3LYP hybrid DFT method with a 6-31+ +G** basis set.31 Our vibrational modes 1 and 2 are the same as theirs. However, we obtained stretching modes 3 and 4, while they assigned in-plane bending modes for those same experimental bands. For bands 5 and 6, we obtained out-ofplane bending, while they assigned a mixture of in-plane and out-of-plane bending modes to those same experimental bands. We consider our modeling of the whole crystal better captures interactions between the atoms and so provides a better

Figure 4. Correspondence between the absorption band energies obtained from numerical calculations and experiment for aspirin (15 K). The solid line is the fit to the experimental points, crossing the origin and with a gradient of 1.028. The dotted line has gradient 1 and passes through the origin. It is a guide to the eye to indicate identical calculated and measured bands. Experimental uncertainties are less than the symbol size.

description of the vibrational modes than the dimer modeling of Boczar and colleagues. The agreement between our experimental and modeled spectra further supports this contention. It is of interest to note that Bozcar et al. obtained similarly good agreement between the modeled and experimental absorption energies as we have, with their modeled energies being 1.022 times higher than the experimental ones. Boczar et al. also report on the infrared spectra of benzoic acid, accompanied by numerical modeling of the dimer at B3LYP/6-31++G** and B3LYP/cc-pVTZ level.32 Their assignment of the vibrational modes B, D, and E is the same as ours. However, they do not report the experimental absorption bands A and C. These two bands are subtle and would be easy to miss, depending on the instrumental setup used. We note that the polyblend spectrum has two bands near the band C (see the Supporting Information). One may argue that the band C in our spectrum is actually a polyblend band at 381 D


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The Journal of Physical Chemistry A cm−1 (11.42 THz). We discarded this possibility, on the basis of the form of the obtained spectra. If the polyblend band at 381 cm−1 (11.42 THz) appeared in the ratioed spectrum of benzoic acid, other polyblend bands would be expected to appear, which was not the case. We note that even the sharp polyblend band at 202 cm−1 (6.60 THz) does not appear in the ratioed spectra of benzoic acid or aspirin and such sharp bands are most difficult to eliminate in ratio-ing. Figure 5 gives the experimental and modeled spectra of benzoic acid in the same energy range as for aspirin. Two

Figure 6. Correspondence between the absorption band energies obtained from numerical calculations and experiment (10 K) for benzoic acid. The dotted line has gradient 1 and passes through the origin. It is a guide to the eye to indicate identical calculated and measured bands. Experimental uncertainties are less than the size of the symbols, except for the band at 230 cm−1, where it is 5 cm−1.

mode IR inactive. A basic requirement for IR activity is that the dipolar moment of the unit cell changes as a consequence of the vibrations described by a particular phonon mode. In our case, molecular dimers are bonded into crystals through weaker van der Waals bonds: there are two dimers in a unit cell. The van der Waals bonds may produce a time dependence in the electric dipole of a unit cell, even if the dipole of each dimer does not change in time for this particular phonon mode. This is because van der Waals forces between the two dimers can facilitate asymmetric vibrations between the two vibrating dimers, resulting in a change of electric dipole of the whole unit cell. If the modeling of such a phonon mode is performed without any van der Waals interaction, the interplay between the two dimers in the unit cell will be lost. The noninteracting dimers will vibrate in almost the same way and at almost the same frequency as when the van der Waals interaction is present, because the bonds within each dimer are much stronger than the van der Waals interaction between the dimers. However, this phonon mode will now not be IR active, as the dipole of each dimer does not change in time for this phonon mode. Therefore, it is plausible that the IR inactivity of the modeled phonon mode at 381 cm−1 (11.42 THz) occurred because van der Waals interactions were not accounted for when the phonon modes were calculated. The modeled spectrum does not give as good agreement with the experimental spectrum for benzoic acid as it does in the case of aspirin. The reason may be either the differences in crystal structures themselves or shortcomings of the GGA-type DFT functionals employed here to reproduce the long-range forces, such as the van der Waals forces. The availability of numerical methods incorporating a van der Waals density functional in phonon calculation would be of great benefit.23,33 Figure 1 shows there are hydrogen and oxygen atoms at the interfaces between the aspirin crystal planes. The distance between these atoms is 2.75 Å in our optimized geometry and they form hydrogen bonds,34 but weaker than the hydrogen bonds creating the intraplane dimers (separated by 1.66 Å). However, there are no oxygens at the planar interfaces of

Figure 5. Low-temperature (T = 10 K) experimental and modeled spectra of benzoic acid. The plain letters denote the experimental bands. The dashed letters denote the corresponding assigned theoretical bands. The theoretical spectrum was obtained by plotting the Gaussians of 5 cm−1 or 0.15 THz half-width at energies of the calculated spectra. The sticks to zero with solid symbols give the energy and intensity of modeled absorption modes, including the modes that are not IR active in the modeling.

modes per experimental absorption band are expected, as the unit cell contains two dimers of benzoic acid. The modeled spectrum was obtained by plotting Gaussian forms at each of the numerically obtained absorption lines. The latter are shown by solid round symbols. There is a clear resemblance between the experimental and numerical spectra, with a shift of the numerical bands in regard to the experimental ones. This shift is not as systematic as for aspirin (Figure 6). There is a significant overestimate of the band B energy. The band E energy is underestimated even though the band C and D energies are reproduced quite well (Figure 6). Nevertheless, there is still a good resemblance to the experimental spectrum. The assignment of the vibrational modes is given in Table 2. The visualization files are provided in the Supporting Information. The spectrum is dominated by in-plane bending (band B), in which the bond between the rigid C6 rings and −COOH group bends. The relative intensities of the modeled bands are in good agreement with the experimental bands, except for the mode E’, which is of significantly lower intensity than the measured band E. The experimental band C does not have the corresponding mode in the modeled spectrum. However, our modeling gives a vibrational mode at 380 cm−1 (11.39 THz) that is not IRactive, marked as C′ in Figure 5. It is possible that the approximations of DFT modeling rendered this vibrational E


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Table 2. Comparison between the Experimental and Modeled Absorption Bands of Benzoic Acid, Together with the Mode Assignmenta mode label



A

230 (6.91)

∼235 (7.05)

B

288 (8.65)

294 (8.83)

C D

374 (11.23) 392 (11.77)

375 (11.26) 393 (11.80)

E

429 (12.88)

433 (13.00)

modeling, cm−1 (THz) 205 (6.15) 213 (6.40) 337 (10.12) 347 (10.42) 380 (11.41) (not IR active) 389 (11.68) 390 (11.71) 425 (12.76) (main) 416 (12.49), 434 (13.03), 435 (13.06)

description γ(C6−COOH bond) symmetric γ(C6−COOH bond) antisymmetric δ(C6−COOH bond) symmetric δ(C6−COOH bond) antisymmetric γ(C6), not along the C6−COOH bond ν(C6, along the C6−COOH bond) antisymmetric between planes ν(C6, along the C6−COOH bond) symmetric between planes γ(C6, along the C6−COOH bond) γ(C6) all similar modes, vibrations along different C6 axes, but not along the C6−COOH bond.

a Corresponding visualization files are given in the Supporting Information. (δ denotes in plane bending, ν denotes stretching, and γ denotes out of plane bending.) The point group in all cases is Au. Estimated uncertainties are 5 cm−1 for band A, 2 cm−1 for band B, and less than 1 cm−1 for the remaining bands.

between C6 rings and the −COOH group bend. Aspirin consists of two groups of crystalline planes, each containing the aspirin dimers oriented at different angles (Figure 1). Either one or the other group of crystalline planes vibrates in mode 1 (see the Supporting Information), giving the two modes obtained by the modeling for the experimental band 1 (Table 1). The matching band of benzoic acid would be band B, observed at 288 cm−1 (8.65 THz). However, all the planes of benzoic acid crystal vibrate for both the symmetric and antisymmetric vibration assigned to band B. Therefore, we cannot assert that there is a full matching of the modes here, but rather an approximate match. We denote this 1 ∼ B. The aspirin bands 3 and 6 do not seem to have matching benzoic acid bands. Likewise, band C of benzoic acid, observed at 374 cm−1 (11.23 THz) does not seem to have an exact aspirin counterpart. Band 6 of aspirin, observed at low temperatures at ∼437 cm−1 (13.12 THz) is assigned to an out-of-plane vibration of C6 rings that is somewhat similar to band C of benzoic acid. However, only half of the planes vibrate for aspirin, whereas all of the planes vibrate for benzoic acid. This is cognate to the relation of bands 1 and B. Therefore, there is a partial matching between the bands 1 ∼ B and 6 ∼ C, while there is no match for band 3. Thus, our comparison of aspirin and benzoic acid absorption bands demonstrates how the similarity of two chemical compounds does not necessarily mean there will be matching absorption spectra for the two compounds. Moreover, even if the spectra do have similar features, the absorption bands that correspond to the same vibrational modes may appear at entirely different energies.

benzoic acid. Therefore, weak van der Waals bonds are entirely responsible for bonding between the crystalline planes of benzoic acid. This is not reproduced by the PBEsol functional as well as the interplane hydrogen bonds of aspirin. The van der Waals corrections could not be used in calculations of the phonon modes in our modeling. Comparing the vibrational modes and spectra of aspirin and benzoic acid (Tables 1 and 2, Figures 3 and 5), we can identify the absorption bands for the two compounds that correspond to the same type of vibrational modes, as set out in Table 3. Table 3. Correspondence of Aspirin and Benzoic Acid Absorption Bands, As Obtained from the Type of the Vibration Modes aspirin mode

corresponding benzoic acid mode

1 2 3 4 5 6

∼B A none D E ∼C

Band 2 of aspirin, observed at 289 cm−1 (8.68 THz), corresponds to an out-of-plane bending vibrational mode, wherein the bond between the C6 ring and the −COOH group bends (Table 1). The same vibrational mode for benzoic acid is observed at 230 cm−1 (6.91 THz) and is denoted mode A in Table 2. We denote this 2 = A. Band 4 of aspirin was observed at 382 cm−1 (11.47 THz) and corresponds to stretching vibrations of the C6 ring, together with stretching of the bond between the C6 ring and the −COOH group. An out-of-plane bending of the −CH3 group also occurs for this band. The corresponding vibrational mode for benzoic acid is observed at 392 cm−1 (11.77 THz) and is denoted mode D in Table 2. The only difference is that benzoic acid does not contain the −CH3 group and consequently no vibration can be associated with it. Thus, 4 = D. Band 5 of aspirin was observed at 424 cm−1 (12.73 THz) and corresponds to out-of-plane vibrations of C6 rings. The matching vibrational mode of benzoic acid is mode E, observed at 429 cm−1 (12.89 THz). In our notation, 5 = E. Band 1 of aspirin was observed at 262 cm−1 (7.89 THz) and corresponds to in-plane vibrations of C6 rings, wherein bonds

IV. CONCLUSION The absorption spectra for polycrystalline aspirin and benzoic acid were measured between 200 and 500 cm−1 (6 and 15 THz). DFT modeling of aspirin produced a slightly blueshifted spectrum that was otherwise in excellent agreement with the experiment. The modeled spectrum of benzoic acid agreed with the experiment less well; however, the correspondence between the modeled and measured absorption bands was still good. The reason for this less successful agreement in the case of benzoic acid is that the crystalline planes of benzoic acid are connected by van der Waals bonds, which are not accounted for well in the DFT modeling. The dominant bonds between the crystal planes of aspirin are hydrogen bonds, which are handled better in the modeling. F


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Comparing the vibrational modes assigned to each experimental absorption band of aspirin and benzoic acid, we conclude that the similarity of their spectra is merely superficial. While some of the experimental bands that appear at the same energies (e.g., 4 = D and 5 = E) are produced by the same vibrational modes, the same type of the vibrational modes for these two crystals can give experimental bands at quite different energies (e.g., 2 = A). Further, some of the vibrational modes are similar but not exactly the same in origin (i.e., 1 ∼ B and C ∼ 6) and they also result in experimental bands at different energies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b05393. The theoretical spectra of aspirin (form I) and benzoic acid were computed and were used in the assignment of the observed experimental features. To view the vibrations for the energy range considered in this manuscript, the .mold files can be opened using the freely available Avogadro software. The .mold files contain vibrational modes corresponding to each of the numerical absorption frequencies, as presented in Tables 1 and 2 (rounded to three digits). The .mold files give the viewer liberty to rotate the crystals as needed, to help properly visualize each of the vibrational modes. The transmission spectrum of polyblend is also given. (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*J. Horvat. E-mail: [email protected]. ORCID

J. Horvat: 0000-0002-4008-9842 R. A. Lewis: 0000-0002-4598-7553 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council, the University of Wollongong, and the Botswana International University of Science and Technology. The computational work was undertaken with resources and services provided by the Australian Government through the National Computational Infrastructure (NCI) facility.

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Absorption Spectra of Aspirin and Benzoic Acid

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