The high‐pressure response of trans ‐cinnamic acid crystals studied by Raman spectroscopy

Abstract

The pressure response of crystalline trans‐cinnamic acid is studied by means of Raman spectroscopy up to 6 GPa. Pressure application causes the reversible shift of all the observed Raman peaks to higher frequencies and changes in their relative intensities, with the intermolecular vibrational modes being by far more sensitive to pressurization compared to the intramolecular ones. The present high pressure Raman data indicate the structural stability of the trans‐cinnamic acid crystal and molecular conformation up to the highest pressure attained in the experiments, the importance of the hydrogen bonding, as well as the considerable strengthening of the intermolecular interactions at elevated pressures.This article is protected by copyright. All rights reserved.

Pressure Response of Crystalline Fluoranthene Probed by Raman Spectroscopy

“…A linear fit was also applied to the weak ω 21 peak at ~204 cm -1 (1 bar) assigned to an intramolecular vibration. The strong sublinear pressure dependence of the Raman peak frequencies is compatible with what is expected for the external modes in molecular crystals, where the initially weak intermolecular interactions quickly strengthen with pressure [39]. The solid lines through the experimental ω vs. P data in Figure 2b are their linear or parabolic least-square fits, using the following functions accordingly:…”

“…It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space-and hence its compressibility-should be smaller since it is a spiro-compound [52]. Moreover, the B 0 ~7 GPa value for fluoranthene is even smaller than the one we have obtained by the same method for trans-cinnamic acid crystals (~11 GPa), where the intermolecular interactions also included hydrogen bonding, making them stiffer [39]. More importantly, the value B 0 ~7 GPa is in remarkably good agreement with the value of ~6 GPa that can be extracted from the volume contraction data with pressure for solid fluoranthene by Bridgman [78], using the Murnaghan equation of state [79,80].…”

“…Since the volume contraction upon pressure application is predominantly accommodated by the compression of the intermolecular voids, we can roughly estimate the bulk modulus of fluoranthene at ambient conditions from the averaged Γ i parameter value of 0.30 GPa −1 for the intermolecular modes by means of Equation (7). Thus, assuming a reasonable value of ~2 for the Grüneisen parameter γ [37,38,39,52], we obtain B 0 ~7 GPa for crystalline fluoranthene, which is compatible with its soft molecular solid nature. It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space—and hence its compressibility—should be smaller since it is a spiro-compound [52].…”

Pressure Response of Crystalline Fluoranthene Probed by Raman Spectroscopy

“…A linear fit was also applied to the weak ω 21 peak at ~204 cm -1 (1 bar) assigned to an intramolecular vibration. The strong sublinear pressure dependence of the Raman peak frequencies is compatible with what is expected for the external modes in molecular crystals, where the initially weak intermolecular interactions quickly strengthen with pressure [39]. The solid lines through the experimental ω vs. P data in Figure 2b are their linear or parabolic least-square fits, using the following functions accordingly:…”

“…It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space-and hence its compressibility-should be smaller since it is a spiro-compound [52]. Moreover, the B 0 ~7 GPa value for fluoranthene is even smaller than the one we have obtained by the same method for trans-cinnamic acid crystals (~11 GPa), where the intermolecular interactions also included hydrogen bonding, making them stiffer [39]. More importantly, the value B 0 ~7 GPa is in remarkably good agreement with the value of ~6 GPa that can be extracted from the volume contraction data with pressure for solid fluoranthene by Bridgman [78], using the Murnaghan equation of state [79,80].…”

“…Since the volume contraction upon pressure application is predominantly accommodated by the compression of the intermolecular voids, we can roughly estimate the bulk modulus of fluoranthene at ambient conditions from the averaged Γ i parameter value of 0.30 GPa −1 for the intermolecular modes by means of Equation (7). Thus, assuming a reasonable value of ~2 for the Grüneisen parameter γ [37,38,39,52], we obtain B 0 ~7 GPa for crystalline fluoranthene, which is compatible with its soft molecular solid nature. It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space—and hence its compressibility—should be smaller since it is a spiro-compound [52].…”

High Pressure Raman Study of Racemic Ibuprofen Crystals

“…The bulk modulus value for the RS ibuprofen crystals is somewhat larger than that estimated using the same experimental method for crystalline fluorene and fluoranthene with purely van der Waals intermolecular interactions between quasiplanar molecules (~7 GPa [44,66], consistent with the corresponding volume contraction data with pressure and the high pressure XRD data [67,68]). However, it is smaller than those for the van der Waals crystals of the spiro compound 9,9 ′ -spirobifluorene (~9 GPa [69]) with smaller intermolecular voids (larger bulk modulus) as well as the hydrogen-bonded α-trans-cinnamic acid crystals (~11 GPa [45]). In the latter case, the structural units also consist of hydrogenbonded molecular dimers formed between carboxyl groups across centers of symmetry, but in nearly planar configuration [70,71].…”

“…In molecular crystals, the Grüneisen approximation-nearly equal γ i values for the various modes in a network crystal (γ i ≈ γ) and thus a uniform spectral scaling with pressure (ω~V -γ ) [64]-is generally valid only for the intermolecular modes. However, it breaks down for intramolecular vibrations, for which an approximate γ i ~ω-2 i0 dependence is typically observed [42,43,45]. The logarithmic pressure coefficients Γ i for P < 2 GPa of the Raman peak frequencies of the RS ibuprofen with respect to their frequency ω i0 at ambient pressure, as obtained from the Run 1 and Run 2 data, are presented in Figure 5 and Supplementary Figure S12, respectively.…”

Pressure Response of Crystalline Fluoranthene Probed by Raman Spectroscopy

“…A linear fit was also applied to the weak ω 21 peak at ~204 cm -1 (1 bar) assigned to an intramolecular vibration. The strong sublinear pressure dependence of the Raman peak frequencies is compatible with what is expected for the external modes in molecular crystals, where the initially weak intermolecular interactions quickly strengthen with pressure [39]. The solid lines through the experimental ω vs. P data in Figure 2b are their linear or parabolic least-square fits, using the following functions accordingly:…”

“…It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space-and hence its compressibility-should be smaller since it is a spiro-compound [52]. Moreover, the B 0 ~7 GPa value for fluoranthene is even smaller than the one we have obtained by the same method for trans-cinnamic acid crystals (~11 GPa), where the intermolecular interactions also included hydrogen bonding, making them stiffer [39]. More importantly, the value B 0 ~7 GPa is in remarkably good agreement with the value of ~6 GPa that can be extracted from the volume contraction data with pressure for solid fluoranthene by Bridgman [78], using the Murnaghan equation of state [79,80].…”

“…Since the volume contraction upon pressure application is predominantly accommodated by the compression of the intermolecular voids, we can roughly estimate the bulk modulus of fluoranthene at ambient conditions from the averaged Γ i parameter value of 0.30 GPa −1 for the intermolecular modes by means of Equation (7). Thus, assuming a reasonable value of ~2 for the Grüneisen parameter γ [37,38,39,52], we obtain B 0 ~7 GPa for crystalline fluoranthene, which is compatible with its soft molecular solid nature. It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space—and hence its compressibility—should be smaller since it is a spiro-compound [52].…”

High Pressure Raman Study of Racemic Ibuprofen Crystals

“…The bulk modulus value for the RS ibuprofen crystals is somewhat larger than that estimated using the same experimental method for crystalline fluorene and fluoranthene with purely van der Waals intermolecular interactions between quasiplanar molecules (~7 GPa [44,66], consistent with the corresponding volume contraction data with pressure and the high pressure XRD data [67,68]). However, it is smaller than those for the van der Waals crystals of the spiro compound 9,9 ′ -spirobifluorene (~9 GPa [69]) with smaller intermolecular voids (larger bulk modulus) as well as the hydrogen-bonded α-trans-cinnamic acid crystals (~11 GPa [45]). In the latter case, the structural units also consist of hydrogenbonded molecular dimers formed between carboxyl groups across centers of symmetry, but in nearly planar configuration [70,71].…”

“…In molecular crystals, the Grüneisen approximation-nearly equal γ i values for the various modes in a network crystal (γ i ≈ γ) and thus a uniform spectral scaling with pressure (ω~V -γ ) [64]-is generally valid only for the intermolecular modes. However, it breaks down for intramolecular vibrations, for which an approximate γ i ~ω-2 i0 dependence is typically observed [42,43,45]. The logarithmic pressure coefficients Γ i for P < 2 GPa of the Raman peak frequencies of the RS ibuprofen with respect to their frequency ω i0 at ambient pressure, as obtained from the Run 1 and Run 2 data, are presented in Figure 5 and Supplementary Figure S12, respectively.…”

Pressure Response of Crystalline Fluoranthene Probed by Raman Spectroscopy

“…A linear fit was also applied to the weak ω 21 peak at ~204 cm -1 (1 bar) assigned to an intramolecular vibration. The strong sublinear pressure dependence of the Raman peak frequencies is compatible with what is expected for the external modes in molecular crystals, where the initially weak intermolecular interactions quickly strengthen with pressure [39]. The solid lines through the experimental ω vs. P data in Figure 2b are their linear or parabolic least-square fits, using the following functions accordingly:…”

“…It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space-and hence its compressibility-should be smaller since it is a spiro-compound [52]. Moreover, the B 0 ~7 GPa value for fluoranthene is even smaller than the one we have obtained by the same method for trans-cinnamic acid crystals (~11 GPa), where the intermolecular interactions also included hydrogen bonding, making them stiffer [39]. More importantly, the value B 0 ~7 GPa is in remarkably good agreement with the value of ~6 GPa that can be extracted from the volume contraction data with pressure for solid fluoranthene by Bridgman [78], using the Murnaghan equation of state [79,80].…”

“…Since the volume contraction upon pressure application is predominantly accommodated by the compression of the intermolecular voids, we can roughly estimate the bulk modulus of fluoranthene at ambient conditions from the averaged Γ i parameter value of 0.30 GPa −1 for the intermolecular modes by means of Equation (7). Thus, assuming a reasonable value of ~2 for the Grüneisen parameter γ [37,38,39,52], we obtain B 0 ~7 GPa for crystalline fluoranthene, which is compatible with its soft molecular solid nature. It is worth noting that this B 0 value is somewhat smaller than that we have recently obtained using the same method for crystalline SBF (~9 GPa), where the intermolecular space—and hence its compressibility—should be smaller since it is a spiro-compound [52].…”

High Pressure Raman Study of Racemic Ibuprofen Crystals

“…The bulk modulus value for the RS ibuprofen crystals is somewhat larger than that estimated using the same experimental method for crystalline fluorene and fluoranthene with purely van der Waals intermolecular interactions between quasiplanar molecules (~7 GPa [44,66], consistent with the corresponding volume contraction data with pressure and the high pressure XRD data [67,68]). However, it is smaller than those for the van der Waals crystals of the spiro compound 9,9 ′ -spirobifluorene (~9 GPa [69]) with smaller intermolecular voids (larger bulk modulus) as well as the hydrogen-bonded α-trans-cinnamic acid crystals (~11 GPa [45]). In the latter case, the structural units also consist of hydrogenbonded molecular dimers formed between carboxyl groups across centers of symmetry, but in nearly planar configuration [70,71].…”

“…In molecular crystals, the Grüneisen approximation-nearly equal γ i values for the various modes in a network crystal (γ i ≈ γ) and thus a uniform spectral scaling with pressure (ω~V -γ ) [64]-is generally valid only for the intermolecular modes. However, it breaks down for intramolecular vibrations, for which an approximate γ i ~ω-2 i0 dependence is typically observed [42,43,45]. The logarithmic pressure coefficients Γ i for P < 2 GPa of the Raman peak frequencies of the RS ibuprofen with respect to their frequency ω i0 at ambient pressure, as obtained from the Run 1 and Run 2 data, are presented in Figure 5 and Supplementary Figure S12, respectively.…”