Fourier Transform Raman Spectra Research Articles

Experimental and DFT Quantum Chemical Studies on Structural, Vibrational and Molecular Properties of Some Substituted 4-Phenylphenols

Fourier Transform infrared spectra (4000–400 cm−1) and Fourier Transform Raman spectra (4000–50 cm−1) were recorded for 2-chloro-4-phenylphenol (CP); 2-nitro-4-phenylphenol (NP); and 2-amino-4-phenylphenol (AP). 1H and 13C NMR spectra, along with UV-Vis spectra of the three samples were also measured. Quantum chemical calculations, at the level of DFT/B3LYP/6-311++G(d,p) theory were implemented to study their ground state geometry, vibrational wave numbers, infrared and Raman intensities, 1H and 13C NMR spectra, frontier molecular orbital parameters, NLO behavior, NBO properties, thermodynamic quantities, rotational constants and MESP behavior. TD-DFT variant was employed to simulate electronic transitions of these molecules. Observed and calculated vibrational frequencies agreed with an rms error 7.44, 8.98 and 6.97 cm−1, the corresponding RMSD values being 7.09, 9.39 and 6.59 cm−1 for CP, NP and AP, respectively. Experimental chemical shifts concurred, with their theoretical counterparts, with RMSD value, 0.19, 0.29 and 0.56 ppm for 1H NMR; and 6.34, 6.28 and 5.39 ppm for 13C NMR, respectively, in CP, NP and AP. This kind of agreement was also true for absorption maxima (λ max) of their electronic transitions in solution form. Frontier molecular orbitals were found useful to understand origin of electronic transition maxima and chemical reactivity of the three molecules which was supported by NBO analyses. The computations showed that the three molecules were potentially good for developing NLO materials. MESP investigations showed that the most reactive sites are at oxygen atoms in the three molecules. Moreover, we also made an attempt to understand the effect of deactivating (Cl, NO2) and activating (NH2) groups on certain properties of the three molecules. Presence of intra-molecular hydrogen bond was predicted in CP, NP and AP.

Solvent selection in membrane preparation from polyethylene terephthalate plastic waste: computational and experimental study

The selection of the solvent during the membrane preparation process significantly affects the characteristics of the resulting membrane. The large number of organic solvents available for dissolving polymers renders this experimental approach ineffective. A computational approach can select a solvent using the solvation energy value approach. In addition, no organic waste is generated from the computational approach, which is a distinct advantage. A computational approach using the DFT/B3LYP/def2-TZVP RIJCOSX method was used to optimize the structure of polyethylene terephthalate (PET). The PET for the experiment was obtained from the utilization of plastic bottle waste. In addition, a review of the thermodynamics, geometry, HOMO-LUMO orbitals, and vibrational frequencies was conducted to validate the PET molecule against the experimental results. A conductor-like polarizable continuum model was used to determine the best solvent for dissolving the PET plastic waste. The results demonstrated that the Fourier Transform Infra-Red and Fourier Transform Raman spectra obtained from computational calculations were not significantly different from the experimental results. Based on a thermodynamic approach, computationally the Gibbs free energy (−724.723), entropy (0.0428), and enthalpy (−724,723 Kjmol−1 ) values of the PET dimer molecule are not much different from the experimental values (−601, 0.042, and −488 Kjmol−1). The computational approach was successful in selecting solvents that can dissolve PET plastic bottle waste. Phenol solvent has the lowest solvation energy value (−101.879 Kjmol−1) and the highest binding energy (2.4 Kjmol−1) than other solvents. Computational and experimental results demonstrated that the phenol solvent was able to dissolve PET plastic bottle waste better than the other solvents.

Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Uncovering strong π-metal interactions on Ag and Au nanosurfaces under ambient conditions via in-situ surface-enhanced Raman spectroscopy

Experimental and density functional theory study on structure, vibrational and molecular characteristics of 2-chloro-5-methylpyrimidine and 2,4-dichloro-5-methylpyrimidine

ABSTRACT This study deals with the determination of torsional potentials, optimised geometry in monomer and dimer form for gas phase for the molecular ground state and vibrational assignments of 2-chloro-5-methyl pyrimidine; and 2,4-dichloro-5-methyl pyrimidine carried out by density functional theory employing B3LYP exchange and correlational functional in conjunction with 6-311++G(d,p) basis set using quantum chemical calculations. The existence of inter-molecular hydrogen bonds was also predicted. Fourier Transform infrared and Fourier Transform Raman spectra were recorded and vibrational analysis of the molecules was made using potential energy distribution and eigen vectors obtained in the computations. Observed and calculated frequencies agreed with an rms error 10.03 and 10.0 cm−1 for 2-chloro-5-methyl pyrimidine and 2,4-dichloro-5-methyl pyrimidine, respectively. Electronic transitions were analysed using experimental and simulated Ultraviolet–Visible spectra of the two molecules wherein polarisable, continuum model in solvent phase was used for simulations. Molecular characteristics like highest occupied molecular orbital, lowest unoccupied molecular orbital, molecular electrostatic surface potential and thermodynamic parameters were also investigated. Stability of the molecules arising from hyper conjugative interactions, charge delocalisation has been analysed using natural bond orbital analysis.

Approach with DFT for non-linear optical properties of organic molecular complexes

Studies of nonlinear optical phenomena are prominent in research and technology for their molecular and electronic properties. In particular, properties of organic molecules gain sufficient impact in transition arising between coherent states influencing intrinsic geometry that occur due to interactions with stability. In the present work, distinct benzaldehydes, the host molecules H1and H2 are influenced by guest molecule p-chloro-aniline (G1) that changes the geometry for molecular and electronic properties using density functional theory (DFT). The method is implemented using B3LYP hybrid method that focuses on nonlinear phenomena for molecular geometry, vibrational frequencies, and quantum descriptors with the basis set 6-311++** of Gaussian 16 package and Gaussview 6.1 for analysis as polarizing and diffusion functions. Studies of these optimized structures provide Fourier Transform Infrared Spectra (FTIR) and Fourier Transform Raman spectra. The Molecular mechanism is attributed to changes in wavenumbers between host and guest molecules indicating the formation of bonding. Studies with Raman spectra indicated the presence of a shift towards the lower side in complexes with increased intensities. Quantum mechanical descriptors influenced factors responsible for the transition between coherent states, influencing non-linear optical properties.

Theoretical (DFT) and experimental (FT-IR & FT Raman) approach to investigate the molecular geometry and vibrational properties of 2,5- and 2,6-dihydroxytoluenes

Fourier Transform infrared spectra were measured for 2, 5- dihydroxytoluene (5DHT) and 2, 6- dihydroxytoluene (6DHT), in the spectral range 4000–400 cm−1. Fourier Transform Raman spectra were also recorded for the same molecules, in the spectral region 3500–100 cm−1. Torsional potential energy scans were attempted around ring – ortho hydroxyl C-O bonds, and ring-methyl C-C bond, for the two molecules. Barrier heights were computed for the above bonds for both the target molecules. Optimized molecular geometry, general valence force constants, vibrational wave numbers (harmonic), infrared intensities, and Raman scattering intensities were calculated, employing density functional theory and using B3LYP/6-311++G(d,p) level of formalism. Observed and measured frequencies agreed with an rms error 8.6 and 8.3 cm−1, for 5DHT and 6DHT, respectively, on scaling. In addition, experimental infrared and Raman spectra agreed with their simulated counterparts with good accuracy. Using potential energy distribution (PED) and eigenvectors, all fundamentals were assigned unambiguously for both molecules. Geometry optimization was made for trimers of the two molecules at the same level of theory as used for the monomers. Existence of inter-molecular hydrogen bond was predicted.

Vibrational, electronic and reactivity insight on (5-chloro-benzofuran-3-yl)-acetic acid hydrazide: A Spectroscopic and DFT approach

Extensive quantum chemical calculation have been carried out to examine the Fourier Transform Infrared(FT-IR), Fourier Transform Raman(FT-Raman) and Nuclear magnetic resonance(NMR) spectra of (5-chloro-benzofuran-3-yl)-acetic acid hydrazide (5CBAH) molecule. The Gaussian computations have been carried out by DFT method using B3LYP/6-311++G (d, p) basis sets. The fundamental vibrational frequencies and intensities of the vibrational bands are analysed with the help of optimized structure. The theoretical NMR chemical shift values have been obtained using gauge independent atomic orbital (GIAO) method. The computed band gap energy of HOMO and LUMO depicts that charge transfer existed within the molecule. The internal molecular electronic interactions and their stabilizing energies are identified by the NBO analysis. The MEP map analysis is conducted to determine the chemical reactive site of the molecule. The thermodynamic properties of the title compound are studied for different temperatures. Non-linear optical properties and Fukui activity are also performed. The electron distribution and reactive site on the surface of the molecule are analyzed using ELF and LOL analysis. Atoms in Molecule (AIM) analysis is used to study the nature of interactions in molecular structure and to categorize and recognize the bonding interactions on the basis of quantum mechanical properties. The noncovalent interaction (NCI) analysis provides the most important interaction information's such as van der Waals interactions, hydrogen bonds and steric interactions present in the molecule.

Computational spectroscopic investigations on structural validation with IR and Raman experimental evidence, projection of ultraviolet-visible excitations, natural bond orbital interpretations, and molecular docking studies under the biological investigation on N-Benzyloxycarbonyl-L-Aspartic acid 1-Benzyl ester

Abstract Experimental Fourier Transform Infra-Red spectrum and Fourier Transform Raman spectrum with Micro Raman spectrum of the N-Benzyloxycarbonyl-L-Aspartic acid 1-Benzyl Ester have been analyzed for molecular structural determination. A hybrid Becke's three-parameter technique with the Lee-Yang-Par (B3LYP) correlation method of Density functional theory (DFT) with 6–311++G(d,p) basic set was used for the investigations on quantum computational spectroscopic calculations. Chemical stabilization energy associated with hyper-conjugative interactions with polarization coefficients and natural Lewis structures were determined by Natural bond orbital analysis. Molecular dynamic simulations with global reactive descriptors were obtained by calculated Frontier molecular orbital energies. The charge density distribution has been visualized by electrostatic potential surfaces. Local reactive behaviors of the chemical compound with the graphical representation of the Fukui functions were obtained. The UltraViolet-Visible light absorptions with different electronic excitations were constructed by the method of Time-dependent DFT. Molecular docking was introduced for N-Benzyloxycarbonyl-L-Aspartic acid 1-Benzyl Ester into different proteins (1ITO, 2ZCO, 1J1J, and COVID-19 proteins).

Spectroscopic, quantum chemical and molecular docking studies on 2,4-dimethoxy-1,3,5-triazine: a potent inhibitor of protein kinase CK2 for the development of breast cancer drug

ABSTRACT The theoretical calculations of the 2,4-dimethoxy-1,3,5-triazine (DMT) molecule were performed with the help of density functional theory (DFT) calculations using 6-311++G(d,p) basis set. The conformational analysis of the DMT molecule was carried out and the most stable molecular structure was predicted. The structural parameters and vibrational wavenumbers of the title molecule were also calculated. Fourier transform-infrared (FT-IR) and Fourier transform-Raman (FT-Raman) spectra of the molecule were observed and simulated. The ultraviolet-visible (UV-Vis) absorption spectra of the molecule were simulated in the gas phase as well as in the liquid phase. The frontier molecular orbitals of the molecule were simulated, which confirms the bio-active nature of the molecule. The molecular electrostatic potential surface was visualized to confirm the reactive regions of the molecule. The natural bond orbital analysis reveals the bio-activity of the molecule. The thermodynamic functions of the molecule were computed. The molecular docking analysis confirms that the title molecule could act as a potent inhibitor of protein kinase, casein kinase (CK2), which is a key protein for breast cancer. The obtained results show that the title molecule is a potential bio-active agent which may be valuable for the designing of drugs for the treatment of breast cancer.

DFT Studies of Distinct Anilines with p-Hydroxycinnamic Acids for Antioxidant Profile.

Life style and jobs in current situations have generated increased free radicals such as hydroxyl (OH•) and superoxide (O2•) radicals, thereby increasing stress in humans. Interest in search of antioxidants that trap these free radicals has increased to relieve stress. β-carotene (provitamin A), ascorbic acid (vitamin C), tocopherol or vitamin E, Trolox; butyl hydroxy toluene and phenolic compounds are the well-known antioxidants. Several methods evaluate the antioxidant property existing in natural substances (medicinal plants and agri-food products) and synthetic compounds (2-methyl-3- (pyrrolidin-2-ylideneamino) quinazolin-4 (3H) -one and 3,3'- (1,4- phenylenebis (methanylylidene)) bis (azanylylidene) (2-methyl-quinazolin-4 (3H) -one). The objective of this study is to focus on complexes with p-hydroxycinnamic acids to trap free radicals in a greener way. Spectroscopic shifts and structural studies were employed to attribute electronic properties responsible for antioxidant profile. Spectroscopic shifts in wavenumbers were attributed with Fourier Transform Infrared Spectra (FTIR) and Fourier Transform Raman spectra (FT Raman Spectra). Structural studies were performed with Gaussian package, electron density method the B3LYP method, basis set 6-31(d) for attributing electronic properties responsible for antioxidant profile. Interpretation of FTIR spectra revealed spectroscopic shifts in wavenumbers in all the complexes responsible for bonding. Further, studies confirmed the formation of complex with reduced intensities in Raman spectra. Computational studies revealed enhancement in molecular and electronic properties responsible for antioxidant power. Studies revealed that complex with p-nitroaniline contribute to greater acceptor and donor power responsible for antioxidant power. These higher powers suggest the best antiradicals to trap free radicals.

Crystal structure, FT-Raman and FTIR spectra and DFT calculations of chalcone (2E)-1-(4-aminophenyl)-3-(furan-2-yl)prop-2-en-1-one monohydrate

In this work we present a study of the structural and vibrational properties of the chalcone (2E)-1-(4-aminophenyl)-3-(furan-2-yl)prop-2-en-1-one monohydrate, C13H11O2N·H2O, a strong candidate for fluorescent probes, luminescent materials and optoelectronic devices. The crystalline structure of this chalcone monohydrate was determined at 273 K using single-crystal X-ray diffraction. The Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) and Fourier Transform-Raman (FT-Raman) spectra of the crystal were recorded at room temperature in the regions 600–4000 cm−1 and 40–4000 cm−1, respectively. Vibrational wavenumbers were predicted using Density Functional Theory (DFT) calculations with B3LYP exchange-correlation functional with the B3LYP functional and 6-31G(d) basis set. A comparison with experimental spectra using a hydrated model allowed us to assign all of the normal modes of this crystal. Additionally, we report the UV–Vis spectra and the Hirshfeld surface analysis of the AFPO crystal.

Effects of different thawing methods on the quality of largemouth bass (Micropterus salmonides)

This study investigated the effects of different thawing methods on the quality of largemouth bass. The viscoelastic properties of proteins in Ultrasonics combined with microwave thawing (UMT) and Ultrasonics combined with far-infrared thawing (UIT) samples were closer to FS, which was an indication of good gelation and gel quality. Zeta potential, Raman spectra before and after H/D isotope exchange were used to measure the protein aggregation and the secondary structure of protein. Fourier transform Raman spectra and Raman spectra have been used to investigate the chemical changes in fish lipid during lipid oxidation. Results indicated that both UMT and UIT showed protein stability and gelation property. Thus, it was proved that UMT and UIT could be potentially used to improve the fish quality during thawing process.

Structural, Optical and Dielectric Properties of Aluminoborosilicate Glasses

Glass samples with composition 50AO-5Al2O3-30B2O3-15SiO2 (A = Ba, Sr, Ca, Zn) have been synthesized by conventional melt quenching technique. The amorphous nature of glasses has been confirmed from the x-ray diffraction patterns. The structural behavior of the glasses investigated using Fourier transform infrared spectra show a number of absorption peaks due to the bending of Si-O-B linkages, stretching vibrations of Si-O-Si and B-O-B in SiO4 and BO4 structural units, respectively. The Fourier Transform Raman spectra of all glasses exhibit different spectral bands and intensities of all bands change with the doping of cations (Ba2+, Sr2+, Ca2+ and Zn2+).The optical properties studied using UV–visible absorption spectra, give Urbach energy values in the range 0.51–0.59 eV. The microhardness of the glass samples measured by indentation technique shows a higher value for all glasses (7.68–9.25) GPa, reflecting higher bond strength. The co-efficient of thermal expansion of the glasses lies within the range 8.5–11.5 × 10−6°C−1. Dielectric constant varies from 3.4 to 1.6 and dielectric loss from 0.0016 to 0.0342 at 1 MHz for 50AO-5Al2O3-30B2O3-15SiO2 (A = Ba, Sr, Ca, Zn) glasses. The good thermal, mechanical and dielectric properties show that these glasses can be considered as suitable candidates for interlayer dielectrics in microelectronic applications.

Predicting the degradation potential of Acid blue 113 by different oxidants using quantum chemical analysis

In this work, quantum chemical analysis was used to predict the degradation potential of a recalcitrant dye, Acid blue 113, by hydrogen peroxide, ozone, hydroxyl radical and sulfate radical. Geometry optimization and frequency calculations were performed at ‘Hartree Fock’, ‘Becke, 3-parameter, Lee–Yang–Parr’ and ‘Modified Perdew-Wang exchange combined with PW91 correlation’ levels of study using 6-31G* and 6-31G** basis sets. The Fourier Transform-Raman spectra of Acid blue 113 were recorded and a complete analysis on vibrational assignment and fundamental modes of model compound was performed. Natural bond orbital analysis revealed that Acid blue 113 has a highly stable structure due to strong intermolecular and intra-molecular interactions. Mulliken charge distribution and molecular electrostatic potential map of the dye also showed a strong influence of functional groups on the neighboring atoms. Subsequently, the reactivity of the dye towards the oxidants was compared based on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy values. The results showed that Acid blue 113 with a HOMO value -5.227 eV exhibits a nucleophilic characteristic, with a high propensity to be degraded by ozone and hydroxyl radical due to their lower HOMO-LUMO energy gaps of 4.99 and 4.22 eV respectively. On the other hand, sulfate radical and hydrogen peroxide exhibit higher HOMO-LUMO energy gaps of 7.92 eV and 8.10 eV respectively, indicating their lower reactivity towards Acid blue 113. We conclude that oxidation processes based on hydroxyl radical and ozone would offer a more viable option for the degradation of Acid blue 113. This study shows that quantum chemical analysis can assist in selecting appropriate advanced oxidation processes for the treatment of textile effluent.

Rapid microwave-assisted synthesis of Ag-doped PbS nanoparticles for optoelectronic applications

Herein, novel Ag-doped PbS (Ag:PbS) nanoparticles (NPs) were synthesized via a facile and rapid microwave route. Field emission scanning electron microscopy (FESEM)/energy-dispersive X-ray (EDX) spectroscopy were employed to confirm the homogeneous doping of Ag in PbS. The transmission electron microscopy (TEM) exposed that the PbS NPs were fully encapsulated with Ag. X-ray diffraction (XRD) analysis revealed that all the samples possessed cubic crystal structures whereas the peak positions for the Ag:PbS were slightly shifted towards lower angle compared to pure PbS, further confirm the Ag doping in NPs. Fourier-transform Raman (FT-Raman) spectra also confirmed the phase and doping of Ag in PbS NPs. The grain sizes of the synthesized NPs were found to be in the range of 15–40 nm while calculated crystallite sizes of these samples were in the range of 18–26 nm. The zeta potential for 5.0 wt% Ag:PbS NPs was found to be approximately 10 times higher than that for pure PbS NPs, signifying the high stability of the doped NPs in aqueous media. The optical band gap of Ag:PbS NPs were evaluated from diffused reflectance spectra and found to be in range of 1.95–2.51 eV; increases with Ag doping. The quenching in photoluminescence (PL) intensity was observed with the increase of Ag doping concentration attributed to the charge transfer between Ag and PbS. Moreover, Ag doping also improve the electrical and dielectric properties of PbS NPs. Finally, optoelectrical properties of synthesized materials were investigated under 633 nm laser-light illumination to determine their applicability as visible-light photodetectors. It was found that the Ag doped PbS NPs exhibited enhanced photodetector figures of merit compared to the pure PbS NPs.

Photoluminescence studies on nanocomposite graphene decorated with tantalum oxide.

Graphene-tantalum oxide (Ta2O5) hybrid material is synthesized using a simple hydrothermal method to elucidate its optical properties. The prepared sample is characterized by X-ray diffraction, scanning electron microscope (SEM), high-resolution-transmission electron microscope (HR-TEM), thermo-gravimetric and differential thermal analysis (TG-DTA), Fourier transform-Raman spectra, and photoluminescence (PL) studies. SEM and HR-TEM analysis revealed that the Ta2O5 particles are embedded on the surface of thin sheets of well-defined graphene structure. Thermogravimetric analysis has provided substantial evidence for the thermal stability of the material with minimal percentage of weight loss at 700°C. Further, the excitation of the nanocomposite at a wavelength of 280nm leading to emission spectra at 567nm using PL studies, which clearly indicates the emission of light, occurs in the visible green region.

Systematic error in conventionally measured Raman spectra of boron carbide-A general issue in solid state Raman spectroscopy.

Solid state Raman spectroscopy requires careful attention to the penetration depth of exciting laser radiation. In cases like semiconducting boron carbide and metallic hexaborides, high fundamental absorption above the bandgap and reflectivity R ≈ 1 beyond the plasma edge respectively prevent the excitation of bulk phonons largely. Thus, correspondingly measured spectra stem preferably from surface scattering. For decades, Raman spectra of boron carbide obtained with different equipment were controversially discussed. Now, common features of icosahedral phonons in Fourier transform-Raman spectra of B4.3C and Raman spectra of α-rhombohedral boron evidence their bulk character, while refuting conventionally measured Raman spectra.

Synthesis and characterization of cinnamylidene acetone – A study on tuning of band gap by vibrational spectroscopic tools

The title molecule Cinnamylidene acetone (CA) was synthesized through a single step base catalyzed aldol type condensation between acetone and cinnamaldehyde. The prepared molecule has been characterized using NMR (1H and 13C) spectral analysis, FTIR, UV–Vis and Elemental analysis. The complete vibrational characterization of the molecule was performed using experimental (FTIR and FT-Raman) spectra and calculations at DFT level. In order to carry out a detailed vibrational spectroscopic analysis of CA, Fourier Transform Infrared and Fourier Transform Raman spectra recorded in condensed phase were used. To determine the optimized geometry and vibrational wavenumbers, Density functional theory calculations in the B3LYP/6-31G** level have been carried out followed by scaling using the scaled quantum mechanical methodology. A root mean square deviation of 6.53 cm−1 has resulted from the scaled quantum mechanical treatment. The ultraviolet visible spectra of the title compound dissolved in various solvents were recorded in the range of 300–600 nm. The natural bond orbital and frontier molecular orbital analyses were also performed using B3LYP/6-31G** method. These studies revealed a reduction in the band gap of CA in comparison with that of cinnamaldehyde (CY).

Spectroscopic (FT-IR, FT-Raman, UV–Vis), quantum chemical calculation and molecular docking evaluation of liquiritigenin: an influenza A (H1N1) neuraminidase inhibitor

The vibrational spectroscopic analysis of anti-influenza agent liquiritigenin (LGN) was performed using Fourier-transform infrared (FT-IR) and Fourier-transform Raman (FT-Raman) spectra. The experimental values of the LGN molecule was compared with vibrational frequencies obtained from the quantum chemical calculations using density functional theory (DFT) method employing 6-31G, 6-31G(d,p) and 6-311G(d,p) basis sets with scaled frequency, and these values are in good agreement with the computational one. The time-dependent density functional theory method was employed to compute the HOMO–LUMO energy gap of the LGN molecule and their differences were compared with transitions of UV-absorption spectra. The reactivity and selectivity of LGN were analyzed using parameters such as molecular electrostatic potential, global reactivity descriptors, Fukui functions and natural bond orbitals. The molecular orbital contributions were considered using the total, partial and overlap population density of states. The suitability of a drug candidate for human intake can be evaluated by absorption, distribution, metabolism, excretion and toxicity (ADMET) properties. The drug likeness and toxicity properties of LGN were confirmed with Lipinski’s rule of five and ADMET properties, respectively. The LGN molecule exhibits good bioactive score and less toxicity. A molecular docking analysis of LGN was carried out with influenza neuraminidase enzyme, and these results show that LGN has lowest binding affinity with inhibition constant when present in the active site.

Modeling and first-principles calculation of low-frequency quasi-localized vibrations of soft and rigid As–S nanoclusters

Experimental and theoretical studies were performed on Boson peak of binary AsxS100−x glasses and As–S polycrystalline composites (mixtures of glass with polycrystallites) of different compositions. Low-frequency Raman spectra of six different compositions ranging from As6S94 to As60S40, including the stoichiometric As40S60 composition, were measured in the region of 5–100 cm−1. The Fourier-transform Raman spectra of the As–S samples in the range of 50–600 cm−1 were also measured to reveal the structure of the materials at nanoscale. In addition, density functional theory calculations were performed on different As–S nanoclusters for determination of their low-frequency vibrational modes. The effect of the structural interconnection of the clusters on their vibrational mode frequencies was modeled by attaching different numbers of heavy dummy hydrogen atoms to the dangling bonds of branchy—As2+4/3S5 and 12-membered ring-like As6S6+6/2 nanoclusters. It was found that the vibrational mode frequencies have a U-shaped dependence on the level of interconnection, which correlates with experimental findings on compositional dependence of the Boson peak position in AsxS100−x glasses. The composition dependence of spectral behavior and very low-frequency features detected at low-energy side of the Boson peak in the Raman spectra of As–S samples were also analyzed and their structural origin is discussed.