The H4SiO4 monomer, H6Si2O7 dimer, and H6Si3O9 ring trimer. Figure was drawn with VMD.28
Three groups of quasi-normal modes (QNMs) used in this study (Table I). Arrows indicate the velocity component of interest, i.e., the first projection. Dashed lines in bending modes represent additional projection vectors where a second projection is necessary.44 Parts of the figure were drawn with VMD,28 modified from Spiekermann et al. 44
The ring trimer modes of ring breathing () and geminal non-bridging oxygen (NBO) symmetric () and antisymmetric () stretch.
Comparison of QNM, NMA, and experimental results for the H4SiO4 monomer. (a) Spectral density distributions of the four tetrahedral QNMs at 1000 K. (b) Frequency histogram of NMA frequencies from in situ NMA of the same monomer from 200 snapshots of the run. (c) NMA frequencies from the present study, and literature computational ab initio Raman frequencies from NMA below 1200 cm−1.20,24,26 Full symbols represent modes with Raman activity of at least 1% of that of the tetrahedral symmetric stretch near 770 cm−1. The dashed box indicates the wavenumber region where the monomer does not have a vibrational mode in any computational study. (d) Experimental band assignments from McIntosh et al. 20 Dashed vertical lines are a guide to the eye.
The δSiOH QNM spectrum of the H4SiO4 monomer at different conditions. The gas-phase spectrum has been downscaled by a factor of 4.0 for comparison. Spectra of 700 K and 1000 K are shifted upward for visualization.
Overview of the H4SiO4 peak frequencies of the two tetrahedral stretching modes and and the δSiOH/δSiOD, with changing temperature and environment. Lines are a guide to the eye.
(a) The stretching QNM spectra of the H6Si2O7 dimer at 1000 K. (b) NMA frequency histogram of the dimer. (c) Raman frequencies of the dimer above 500 cm−1 from literature ab initio calculations,4,24 full symbols represent Raman modes that have at least 1% intensity of the most intense band at about 600 to 620 cm−1. (d) Experimental band assignment by Zotov and Keppler25 of bands at 630 cm−1 and 1100 cm−1 to the dimer, and two bands in their spectra not assigned to any species (marked by question marks). Dashed vertical lines are a guide to the eye.
Overview of the effect of temperature and environment on the dimer frequencies of the tetrahedral and , the bridging oxygen and , and the ethane-like and QNMs. Lines are a guide to the eye.
Results for the H6Si3O9 ring trimer. (a) QNM spectra at 1000 K. (b) Frequency histogram of NMA frequencies from snapshots of the same ring trimer MD run. (c) NMA frequencies from literature computational ab initio Raman frequencies.21,68 Full symbols represent modes with Raman activity of at least 1% of that of the most intense Raman mode. (d) Experimentally derived band assignments from Dutta and Shieh.15 Dashed lines are a guide to the eye.
Quasi-normal modes (QNMs) discussed in this study (Figs. 2 and 3). See Spiekermann et al. 44 for other QNMs in higher polymerized silica species.
Overview of simulation runs.
Overview over tetrahedral QNMs and δSiOH/δSiOD peak frequencies from various monomer runs. Frequencies (cm−1) are scaled by 1.039. The uncertainty is about 10 cm−1.
Overview over the dimer stretching QNM peak frequencies for various run conditions. Frequencies (in cm−1) are scaled by 1.039. The frequency uncertainty is about 10 cm−1 for TET and DIM modes, and about 20 cm−1 for BO modes.
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