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1. Synthesis, Spectroscopic Characterization and Applications of Tin Dioxide

   https://doi.org/https://doi.org/10.1007/978-3-030-62761-4_11  

   Alghamdi, Hawazin ; Concepcion, Benjamin ; Baliga, Shankar ; and Misra, Prabhakar ( December 2020 , Chapter in Book: Contemporary Nanomaterials in Material Engineering Applications)

   Mubarak, Nabisab M. ; Walvekar, Rashmi ; Arshid, Numan ; and Khalid, Mohammad (Ed.)

   Metal oxides are useful for the detection and sensing of combustible and toxic gases, and for use in lithium batteries and solar cells. The present study focuses on the spectroscopic investigation of commercial and in-house laboratory synthesized tetragonal tin dioxide (SnO2), aimed at studying its physical and chemical properties at nanoscale levels and in bulk. We have investigated the pure powder form and thin films prepared on two different types of substrate, silicon and UV-Quartz, each with five different thicknesses (i.e. 41, 78, 96.5, 373, and 908 nm). Raman spectroscopy with two different laser excitation wavelengths, namely 780 and 532 nm, has been used to investigate the various SnO2 vibrational modes. Thermal effects on the primary vibrational features in the Raman spectra have been studied in the range 30–170 °C. X-ray diffraction (XRD) spectra have been recorded to confirm the rutile structure of tin dioxide and to obtain information on the spherical grain particle size of SnO2 with EDS analysis for the thin film samples. Scanning Electron Microscope (SEM) images have been recorded in order to understand the morphology of the particles of SnO2 at the nanoscale level. In addition, FT-IR spectra have been obtained to study the IR-active vibrational modes for the bulk and thin film samples on the two substrates. Moreover, UV-VIS spectra have been employed to determine the energy band gap for the SnO2 film samples by an efficient process facilitated by a Tauc plot technique utilizing an in-house developed python script. 

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2. A polarizability driven ab initio molecular dynamics approach to stimulating Raman activity: Application to C 20

   https://doi.org/10.1080/00268976.2021.1939453  

   Pierre-Jacques, Dominick ; Tyler, Ciara ; Dyke, Jason ; Kaledin, Alexey L. ; Kaledin, Martina ( June 2021 , Molecular Physics)

   null (Ed.)

   We describe a novel variant of the driven molecular dynamics (DMD) method derived for probing Raman active vibrations. The method is an extension of the conventional alpha-DMD formulation for simulating IR activity by means of coupling an oscillating electric field to the molecule’s dipole moment, miu, and inducing absorption of energy via tuning the field to a resonant frequency. In the present work, we modify the above prescription to invoke Raman activity by coupling two electric fields, i.e., a “Pump” photon of frequency wP and a Stokes photon of frequency wS to the molecule’s polarizability tensor, alpha, with the difference in the frequencies of the two photons w = wP - wS corresponding to the Stokes Raman shift. If a particular w is close to a Raman active vibrational frequency, energy absorption by the molecule ensues. Varying w over the desired frequency range allows identifying and assigning all Raman active vibrational modes, including anharmonic corrections, in the range by means of trajectory analysis. We show that only one element of the full polarizability tensor, and its nuclear derivative, is needed for an alpha-DMD trajectory, making this method well suited for ab initio dynamics implementation. Numerical results using first-principles calculations are presented and discussed for the vibrational fundamentals, combination bands, overtones of H2O, CH4, and the C20 fullerene. 

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3. Searching for correlations between vibrational spectral features and structural parameters of silicate glass network

   https://doi.org/10.1111/jace.17036  

   Liu, Hongshen ; Hahn, Seung Ho ; Ren, Mengguo ; Thiruvillamalai, Mahadevan ; Gross, Timothy M. ; Du, Jincheng ; van Duin, Adri C. T. ; Kim, Seong H. ( February 2020 , Journal of the American Ceramic Society)

   Infrared (IR) and Raman spectroscopic features of silicate glasses are often interpreted based on the analogy with those of smaller molecules, molecular clusters, or crystalline counterparts; this study tests the accuracy and validity of these widely cited peak assignment schemes by comparing vibrational spectral features with bond parameters of the glass network created by molecular dynamics (MD) simulations. A series of sodium silicate glasses with compositions of [Na₂O]_x[Al₂O₃]₂[SiO₂]_98−xwithx = 7, 12, 17, and 22 were synthesized and analyzed with IR and Raman. A silica glass substrate and a crystalline quartz were also analyzed for comparison. Glass structures with the same compositions were generated with MD simulations using three types of potentials: fixed partial charge pairwise (Teter), partial diffuse charge potential (MGFF), and bond order‐based charge transfer potential (ReaxFF). The comparison of simulated and experimental IR spectra showed that, among these three potentials tested, ReaxFF reproduces the concentration dependence of spectral features closest to the experimentally observed trend. Thus, the bond length and angle distributions as well as Si–Qⁿspecies and ring size distributions of silica and sodium silicate glasses were obtained from ReaxFF‐MD simulations and further compared with the peak assignment or deconvolution schemes—which have been widely used since 1970s and 1980s—(a) correlation between the IR peak position in the Si–O stretch region (1050‐1120 cm⁻¹) and the Si–O–Si bond angle; (b) deconvolution of the Raman bands in the Si–O stretch region with theQⁿspeciation; and (c) assignment of the Raman bands in the 420‐600 cm⁻¹region to the bending modes of (SiO)_nrings with different sizes (typically, n = 3‐6). The comparisons showed that none of these widely used methods is congruent with the bond parameters or structures of silicate glass networks produced via ReaxFF‐MD simulations. This finding invokes that the adequacy of these spectral interpretation methods must be questioned. Alternative interpretations are proposed, which are to be tested independently in future studies.

    

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4. A combined experimental and computational study on the transition of the calcium isopropoxide radical as a candidate for direct laser cooling

   https://doi.org/10.1039/D1CP04107J  

   Telfah, Hamzeh ; Sharma, Ketan ; Paul, Anam C. ; Riyadh, S. M. ; Miller, Terry A. ; Liu, Jinjun ( April 2022 , Physical Chemistry Chemical Physics)

   Vibronically resolved laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down (CRD) spectra of the electronic transition of the calcium isopropoxide [CaOCH(CH 3 ) 2 ] radical have been obtained under jet-cooled conditions. An essentially constant energy separation of 68 cm −1 has been observed for the vibrational ground levels and all fundamental vibrational levels accessed in the LIF measurement. To simulate the experimental spectra and assign the recorded vibronic bands, Franck–Condon (FC) factors and vibrational branching ratios (VBRs) are predicted from vibrational modes and their frequencies calculated using the complete-active-space self-consistent field (CASSCF) and equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) methods. Combined with the calculated electronic transition energy, the computational results, especially those from the EOM-CCSD calculations, reproduced the experimental spectra with considerable accuracy. The experimental and computational results suggest that the FC matrix for the studied electronic transition is largely diagonal, but transitions from the vibrationless levels of the à state to the X̃-state levels of the CCC bending ( ν 14 and ν 15 ), CaO stretch ( ν 13 ), and CaOC asymmetric stretch ( ν 9 and ν 11 ) modes also have considerable intensities. Transitions to low-frequency in-plane [ ν 17 ( a ′)] and out-of-plane [ ν 30 ( a ′′)] CaOC bending modes were observed in the experimental LIF/DF spectra, the latter being FC-forbidden but induced by the pseudo-Jahn–Teller (pJT) effect. Both bending modes are coupled to the CaOC asymmetric stretch mode via the Duschinsky rotation, as demonstrated in the DF spectra obtained by pumping non-origin vibronic transitions. The pJT interaction also induces transitions to the ground-state vibrational level of the ν 10 ( a ′) mode, which has the CaOC bending character. Our combined experimental and computational results provide critical information for future direct laser cooling of the target molecule and other alkaline earth monoalkoxide radicals. 

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5. Analysis of the conformational properties of amine ligands at the gold/water interface with QM, MM and QM/MM simulations

   https://doi.org/10.1039/C7CP06709G  

   Liang, Dongyue ; Hong, Jiewei ; Fang, Dong ; Bennett, Joseph W. ; Mason, Sara E. ; Hamers, Robert J. ; Cui, Qiang ( January 2018 , Physical Chemistry Chemical Physics)

   We describe a strategy of integrating quantum mechanical (QM), hybrid quantum mechanical/molecular mechanical (QM/MM) and MM simulations to analyze the physical properties of a solid/water interface. This protocol involves using a correlated ab initio (CCSD(T)) method to first calibrate Density Functional Theory (DFT) as the QM approach, which is then used in QM/MM simulations to compute relevant free energy quantities at the solid/water interface using a mean-field approximation of Yang et al. that decouples QM and MM thermal fluctuations; gas-phase QM/MM and periodic DFT calculations are used to determine the proper QM size in the QM/MM simulations. Finally, the QM/MM free energy results are compared with those obtained from MM simulations to directly calibrate the force field model for the solid/water interface. This protocol is illustrated by examining the orientations of an alkyl amine ligand at the gold/water interface, since the ligand conformation is expected to impact the chemical properties ( e.g. , charge) of the solid surface. DFT/MM and MM simulations using the INTERFACE force field lead to consistent results, suggesting that the effective gold/ligand interactions can be adequately described by a van der Waals model, while electrostatic and induction effects are largely quenched by solvation. The observed differences among periodic DFT, QM/MM and MM simulations, nevertheless, suggest that explicitly including electronic polarization and potentially charge transfer in the MM model can be important to the quantitative accuracy. The strategy of integrating multiple computational methods to cross-validate each other for complex interfaces is applicable to many problems that involve both inorganic/metallic and organic/biomolecular components, such as functionalized nanoparticles. 

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