Low-frequency vibrational modes in G-quadruplexes reveal the mechanical properties of nucleic acids

et al. 2022

Preprint

Self Cite

One of the common features of a glass is the so-called “boson peak”, observed as an excess in the heat capacity over the crystal or as an additional peak in the terahertz vibrational spectrum. The microscopic origins of the boson peak are controversial, albeit the emergence of locally ordered structures inhibiting crystallisation has been put forward as a possible culprit. Unfortunately, the terahertz spectral range is often congested with many other contributions complicating observation of the boson feature. Here, we show that depolarised Raman scattering—obtained using femtosecond optical Kerr-effect spectroscopy—in liquids consisting of highly symmetric molecules can be used to isolate the boson peak, allowing its detailed observation over a wide range of temperatures from the (supercooled) liquid into the glass. The boson peak in the vibrational spectrum matches the excess heat capacity observed in low-temperature heat capacity measurements. As the boson peak intensifies on cooling towards the glass transition, wide-angle x-ray scattering shows the simultaneous appearance of a pre-peak due to molecular clusters consisting of circa 20 molecules. Atomistic molecular dynamics simulations—reproducing both boson and pre peaks—indicate that these clusters are caused by over-coordinated molecules tending towards but prevented from attaining crystalline structural order. These findings represent an essential step toward our understanding of the physics of vitrification.

Section: Resultsmentioning

confidence: 99%

Understanding the emergence of the boson peak in molecular glasses

et al. 2022

Preprint

Self Cite

“…1 Femtosecond optical Kerr-effect (OKE) spectroscopy 36,37 was used to measure the Bose-Einstein corrected depolarised Raman spectrum using a time-resolved pump-probe technique and numerical Fourier deconvolution. In our set-up, 22,23,38,39 which has a time resolution of about 20 fs, the pump-probe delay can be as large as 4 ns resulting in spectra with a maximum spectral range from 125 MHz to 50 THz but is limited here to the range 10 GHz to ~10 THz to maximise the signal to noise in the data.…”

Section: Resultsmentioning

confidence: 99%

Understanding the emergence of the boson peak in molecular glasses

et al. 2022

Preprint

Self Cite

One of the common features of a glass is the so-called “boson peak”, observed as an excess in the heat capacity over the crystal or as an additional peak in the terahertz vibrational spectrum. The microscopic origins of the boson peak are controversial, albeit the emergence of locally ordered structures inhibiting crystallisation has been put forward as a possible culprit. Unfortunately, the terahertz spectral range is often congested with many other contributions complicating observation of the boson feature. Here, we show that depolarised Raman scattering—obtained using femtosecond optical Kerr-effect spectroscopy—in liquids consisting of highly symmetric molecules can be used to isolate the boson peak, allowing its detailed observation over a wide range of temperatures from the (supercooled) liquid into the glass. The boson peak in the vibrational spectrum matches the excess heat capacity observed in low-temperature heat capacity measurements. As the boson peak intensifies on cooling towards the glass transition, wide-angle x-ray scattering shows the simultaneous appearance of a pre-peak due to molecular clusters consisting of circa 20 molecules. Atomistic molecular dynamics simulations—reproducing both boson and pre peaks—indicate that these clusters are caused by over-coordinated molecules. These findings represent an essential step toward our understanding of the physics of vitrification.

“…Tetrabutyl orthosilicate (TBOS) is a viscous liquid that does not crystallise and has a glass transition temperature Tg as measured by calorimetry (see Supplementary Figure 1) of 120 K. The experimentally determined shear viscosity of TBOS is shown in Supplementary Figure 2 together with a fit to the Vogel-Fulcher-Tammann equation with T0 = 105.4±0.5 K. Femtosecond optical Kerr-effect (OKE) spectroscopy 37,38 was used to measure the Bose-Einstein corrected depolarised Raman spectrum using a time-resolved pump-probe technique and numerical Fourier deconvolution. In our set-up, 23,24,39,40 which has a time resolution of about 20 fs, the pump-probe delay can be as large as 4 ns resulting in spectra with a maximum spectral range from 125 MHz to 50 THz but is limited here to the range 10 GHz to ~10 THz to maximise the signal to noise in the data.…”

Section: Resultsmentioning

confidence: 99%

Understanding the emergence of the boson peak in molecular glasses

et al. 2022

Preprint

A common feature of glasses is the “boson peak”, observed as an excess in the heat capacity over the crystal or as an additional peak in the terahertz vibrational spectrum. The microscopic origins of this peak are not well understood; the emergence of locally ordered structures has been put forward as a possible candidate. Here, we show that depolarised Raman scattering in liquids consisting of highly symmetric molecules can be used to isolate the boson peak, allowing its detailed observation from the liquid into the glass. The boson peak in the vibrational spectrum matches the excess heat capacity. As the boson peak intensifies on cooling, wide-angle x-ray scattering shows the simultaneous appearance of a pre-peak due to molecular clusters consisting of circa 20 molecules. Atomistic molecular dynamics simulations indicate that these are caused by over-coordinated molecules. These findings represent an essential step toward our understanding of the physics of vitrification.