Terahertz Spectroscopy and Solid-State Density Functional Theory Calculations of Cyanobenzaldehyde Isomers

Abstract

Spectral signatures in the terahertz (THz) frequency region are mainly due to bulk vibrations of the molecules. These resonances are highly sensitive to the relative position of atoms in a molecule as well as the crystal packing arrangement. To understand the variation of THz resonances, THz spectra (2-10 THz) of three structural isomers: 2-, 3-, and 4-cyanobenzaldehyde have been studied. THz spectra obtained from Fourier transform infrared (FTIR) spectrometry of these isomers show that the resonances are distinctly different especially below 5 THz. For understanding the intermolecular interactions due to hydrogen bonds, four molecule cluster simulations of each of the isomers have been carried out using the B3LYP density functional with the 6-31G(d,p) basis set in Gaussian09 software and the compliance constants are obtained. However, to understand the exact reason behind the observed resonances, simulation of each isomer considering the full crystal structure is essential. The crystal structure of each isomer has been determined using X-ray diffraction (XRD) analysis for carrying out crystal structure simulations. Density functional theory (DFT) simulations using CRYSTAL14 software, utilizing the hybrid density functional B3LYP, have been carried out to understand the vibrational modes. The bond lengths and bond angles from the optimized structures are compared with the XRD results in terms of root-mean-square-deviation (RMSD) values. Very low RMSD values confirm the overall accuracy of the results. The simulations are able to predict most of the spectral features exhibited by the isomers. The results show that low frequency modes (<3 THz) are mediated through hydrogen bonds and are dominated by intermolecular vibrations.

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…95,96 However, many recent studies have used ab initio gas-phase simulations to assign experimental condensed phase terahertz spectra, with varying levels of apparent success. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] It is important to note that in almost all of these cases the discussed 'agreement' with experiment is largely based upon the position of calculated vibrational transitions, with little focus placed upon the accuracy of the predicted mode-types -critical for gaining deeper insight into the way terahertz motions influence bulk material properties, for example. Therefore, this work aims to asses how gas-phase simulations reproduce terahertz spectra, as well as predicting the corresponding vibrational mode-types.…”

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…29 However, due to the complexity and (assumed) increased computational cost of periodic boundary condition simulations, gas-phase calculations involving clusters of molecules have been employed in studies describing terahertz vibrations, with varying levels of apparent success. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] It is important to note that, in general, such assessment is based solely on the position of the calculated transitions, with little focus on the actual motions involved or their accuracy. However, there is a lack of comparison in the literature between these methods, and therefore this work aims to assess how gas-phase simulations perform in regard to reproducing not only the experimental terahertz spectra, but also the prediction of the vibrational mode-types.…”

Distinguishing Quinacridone Pigments via Terahertz Spectroscopy: Absorption Experiments and Solid-State Density Functional Theory Simulations

“…The coupling of experimental terahertz spectroscopy with ss-DFT in recent years has proven to be a powerful combination, giving insight into many physical systems. For example, this approach has been applied fruitfully to sucrose, benzoic acid, and thymine, 23 crystalline pharmaceuticals, 24 collective librations of water molecules, 25 hydrophobic amino acid crystals, 26 anthracene, 27 2,4-dinitrotoluene, 28 cyanobenzaldehyde isomers, 29 L-glutamic acid polymorphs, 30 and CL-20/TNT energetic cocrystals. 31 ■ METHODS Experimental Section.…”