Theoretical Study Of Spectroscopy Research Articles

A theoretical spectroscopy study of the photoluminescence properties of narrow band Eu2+-doped phosphors containing multiple candidate doping centers. Prediction of an unprecedented narrow band red phosphor.

We have previously presented a computational protocol that is based on an embedded cluster model and operates in the framework of TD-DFT in conjunction with the excited state dynamics (ESD) approach. The protocol is able to predict the experimental absorption and emission spectral shapes of Eu2+-doped phosphors. In this work, the applicability domain of the above protocol is expanded to Eu2+-doped phosphors bearing multiple candidate Eu doping centers. It will be demonstrated that this protocol provides full control of the parameter space that describes the emission process. The stability of Eu doping at various centers is explored through local energy decomposition (LED) analysis of DLPNO-CCSD(T) energies. This enables further development of the understanding of the electronic structure of the targeted phosphors, the diverse interactions between Eu and the local environment, and their impact on Eu doping probability, and control of the emission properties. Hence, it can be employed to systematically improve deficiencies of existing phosphor materials, defined by the presence of various intensity emission bands at undesired frequencies, towards classes of candidate Eu2+-doped phosphors with desired narrow band red emission. For this purpose, the chosen study set consists of three UCr4C4-based narrow-band phosphors, namely the known alkali lithosilicates RbNa[Li3SiO4]2:Eu2+ (RNLSO2), RbNa3[Li3SiO4]4:Eu2+ (RNLSO) and their isotypic nitridolithoaluminate phosphors consisting of CaBa[LiAl3N4]2:Eu2+ (CBLA2) and the proposed Ca3Ba[LiAl3N4]4:Eu2+ (CBLA), respectively. The theoretical analysis presented in this work led us to propose a modification of the CBLA2 phosphor that should have improved and unprecedented narrow band red emission properties. Finally, we believe that the analysis presented here is important for the future rational design of novel Eu2+-doped phosphor materials, with a wide range of applications in science and technology.

Raman and IR spectra of water under graphene nanoconfinement at ambient and extreme pressure-temperature conditions: a first-principles study.

The nanoconfinement of water can result in dramatic differences in its physical and chemical properties compared to bulk water. However, a detailed molecular-level understanding of these properties is still lacking. Vibrational spectroscopy, such as Raman and infrared, is a popular experimental tool for studying the structure and dynamics of water, and is often complemented by atomistic simulations to interpret experimental spectra, but there have been few theoretical spectroscopy studies of nanoconfined water using first-principles methods at ambient conditions, let alone under extreme pressure-temperature conditions. Here, we compute the Raman and IR spectra of water nanoconfined by graphene at ambient and extreme pressure-temperature conditions using ab initio simulations. Our results revealed alterations in the Raman stretching and low-frequency bands due to the graphene confinement. We also found spectroscopic evidence indicating that nanoconfinement considerably changes the tetrahedral hydrogen bond network, which is typically found in bulk water. Furthermore, we observed an unusual bending band in the Raman spectrum at ∼10 GPa and 1000 K, which is attributed to the unique molecular structure of confined ionic water. Additionally, we found that at ∼20 GPa and 1000 K, confined water transformed into a superionic fluid, making it challenging to identify the IR stretching band. Finally, we computed the ionic conductivity of confined water in the ionic and superionic phases. Our results highlight the efficacy of Raman and IR spectroscopy in studying the structure and dynamics of nanoconfined water in a large pressure-temperature range. Our predicted Raman and IR spectra can serve as a valuable guide for future experiments.

Structural, vibrational and electronic spectroscopic study of 6-hydroxycoumarin using experimental and theoretical methods.

Understanding the photochemical behavior of structural isomers of hydroxycoumarin (HC) having different properties of consequence in biological activities demand spectroscopic information of this class of compounds. Barring 6-hydroxycoumarin (6-HC), other isomers of HC's are well studied spectroscopically. To understand and compare the photochemical activity of 6-HC with other isomers, a detailed study of this molecule has been taken up. For this purpose, electronic, vibrational and structural properties of 6-HC have been studied using ultraviolet absorption and Infrared spectroscopy techniques. Quantum chemical calculations have been performed at DFT/B3LYP level of theory to get the optimized geometry and vibrational frequencies of normal modes to support and analyze experimental data. The detailed vibrational assignments were made on the basis of potential energy distributions. Chemical activity, molecular orbital energies, band gap and hyper-polarizability information have been computed from quantum chemical simulations. NBO analysis helped in understanding the stability of the molecule arising from hyper-conjugative interaction and charge delocalization. UV-Visible spectrum of the compound was recorded in the region 300-600nm helped in obtaining band gap data of the compound. Molecular Electrostatic Potentials (MESP) were plotted and the respective centers of electrophilic and nucleophilic attacks were predicted with the help of Fukui functions calculations. Further, it was observed that the negative electrostatic potential regions are mainly localized over the oxygen atoms and the positive regions are localized over the benzene ring. Details of the results and analysis of experimental and theoretical spectroscopy studies are presented in this paper.

AUTOSURF: A Freely Available Program To Construct Potential Energy Surfaces.

The potential energy surface (PES) of a molecular system constitutes a cornerstone for nearly every theoretical study of spectroscopy and dynamics. We present here AUTOSURF, our freely distributed code for the automated construction of PESs. This first release treats van der Waals systems composed of two rigid fragments. A version for reactive systems with up to five atoms is under development. The AUTOSURF suite is designed to completely automate all of the steps and procedures that go into fitting various classes of PESs and facilitates certain PES refinements aimed toward specific applications in spectroscopy and dynamics. The algorithms are based on a local interpolating moving least-squares methodology and have many advanced features such as iterative refinement and symmetry recognition. The code interfaces to popular electronic structure codes such as MOLPRO and GAUSSIAN to automatically generate ab initio PESs and is well-suited for treating highly anisotropic interactions which are challenging for traditional quadrature type expansions. The niche of these algorithms is to obtain an interpolative representation of high-level electronic energies with negligible (arbitrarily small) fitting error, requiring minimal human supervision in the entire process of selection, computation, and fitting of the ab initio data. The code is designed to run in parallel on Linux-based machines ranging from small workstations to large high-performance computing clusters.

Theoretical spectroscopy study of the low-lying electronic states of UX and UX(+), X = F and Cl.

Spectroscopic constants (Te, re, B0, ωe, and ωexe) have been calculated for the low-lying electronic states of UF, UF(+), UCl, and UCl(+) using complete active space 2nd-order perturbation theory (CASPT2), with a series of correlation consistent basis sets. The latter included those based on both pseudopotential (PP) and all-electron Douglas-Kroll-Hess Hamiltonians for the U atom. Spin orbit (SO) effects were included a posteriori using the state interacting method using both PP and Breit Pauli (BP) operators, as well as from exact two-component methods for U(+) and UF(+). Complete basis set (CBS) limits were obtained by extrapolation where possible and the PP and BP calculations were compared at their respective CBS limits. The PP-based method was shown to be reliable in calculating spectroscopic constants, in particular when using the state interacting method with CASPT2 energies (SO-CASPT2). The two component calculations were limited by computational resources and could not include electron correlation from the nominally closed shell 6s and 6p orbitals of U. UF and UCl were both calculated to have Ω = 9/2 ground states. The first excited state of UCl was calculated to be an Ω = 7/2 state at 78 cm(-1) as opposed to the same state at 435 cm(-1) in UF, and the other low-lying states of UCl showed a similar compression relative to UF. Likewise, UF(+) and UCl(+) both have Ω = 4 ground states and the manifold of low-lying excited Ω = 3, 2, 1, 0 states was energetically closer together in UCl(+) than in UF(+), ranging up to 776 cm(-1) in UF(+) and only 438 cm(-1) in UCl(+). As in previous studies, the final PP-based SO-CASPT2 results for UF(+) and UF agree well with experiment and are expected to be predictive for UCl and UCl(+), which are reported here for the first time.

Theoretical study of spectroscopy, interaction, and dissociation of linear and T-shaped isomers of RgClF (Rg = He, Ne, and Ar) van der Waals complexes

Spectroscopy, interaction energy, and dissociation of linear and T-shaped isomers of HeClF, NeClF, and ArClF van der Waals complexes in their ground state have been studied in detail using MP2 and CCSD(T) methods in conjunction with correlation consistent valence triple and quadruple zeta basis sets. A method, called potential method, has been developed to remove the discrepancy between theoretical and experimental values for the depth of the potential well and dissociation energies for these complexes. This is also supported by the supermolecular approach. Most of the structural and spectroscopic properties of these complexes are first reported and the rest agree very well with the experimental and theoretical values wherever available. Two local minima corresponding to linear and asymmetric T-shaped are found for RgClF complexes. For NeClF complex, the predicted values for the equilibrium bond length and well depth are R NeCl = 3.096 A and $$ D_{\text{e}}^{\text{p}} $$ = 161.50 cm−1 for the linear isomer and R NeCl = 3.503 A and $$ D_{\text{e}}^{\text{p}} $$ = 126.10 cm−1 for the T-shaped isomer. Various dissociation channels are also investigated in detail.

A theoretical study of spectroscopy and metastability of the CN 2+ dication

Potential energy curves of low-lying electronic states of the CN 2+ dication and of the electronic ground states of CN + and the neutral CN molecule were calculated using internally contracted multireference CI and the coupled cluster RCCSD(T) methods. Spectroscopic constants and adiabatic excitation energies of 13 quasibound electronic states of the dication were obtained and the energy of charge stripping of CN + and double ionization energy of CN were predicted. Tunneling and spin–orbit induced predissociation lifetimes for the vibrational levels in the low-lying electronic states are presented and the metastability of the dication is discussed.

Theoretical study of spectroscopy and dissociation of SO 2Br 2 and SO 2I 2

Spectroscopy and dissociation of the sulfuryl halides SO 2Br 2 and SO 2I 2 have been studied in detail using ab initio MP2 and CCSD(T) methods. The discrepancy among the recent theoretical works for the assignment of harmonic vibrational frequencies of SO 2Br 2 has been addressed. The possibility of various dissociation channels has been explored considering that the fragmented atoms and molecules can stay in their ground state. Only halide producing channels have been studied from thermodynamic point of view. Interesting pattern has been observed in their dissociation energy spectra for the dissociation channels. Finally, the enthalpy of formation and stability of these molecules have been discussed.

A theoretical spectroscopy study of the X 3Σ− and the A 3Π states of the C2S radical

The spin-rovibronic levels for the X 3Σ−,A 3Π electronic system of C2S are calculated variationally, using ab initio potential energy surfaces and taking into account the non-adiabatic coupling between the two states. The energies of selected levels with Σ and Π vibronic symmetry, up to ∼16500cm−1, are reported and compared with available experimental data.

A theoretical study of spectroscopy and predissociation dynamics in nitrosoalkanes

We have computed ab initio transition energies, equilibrium geometries, force constants and potential energy curves for the dissociation of S0, T1, and S1 of two nitrosoalkanes, CH3NO and t-BuNO. A normal coordinate analysis has been performed for the three states, and the harmonic wave function for the C–N bond torsional coordinate has been replaced by hindered rotor eigenfunctions. The n→π* absorption spectra have been simulated by computing the appropriate Franck–Condon factors in order to assign the vibrational sub-bands. The predissociation lifetimes of several vibrational states of S1 have been evaluated by computing nonadiabatic and spin-orbit couplings, which determine the Internal Conversion and Intersystem Crossing rates. For t-BuNO the computed lifetimes (10–160 ns) are in the same range as those measured by Noble et al. [J. Chem. Phys. 85, 5763 (1986)]. The lifetimes of CH3NO, for which no experimental data are available, are longer (50–330 ns). Both the IC to S0 and the ISC to T1 are important.

Optical spectroscopy of heterocycles based on pyrrole, furan and thiophene

Abstract Recently Joshi et al synthesised a serie of nine triheterocycles based on pirrole, furan and thiophene. All the systems were chemically polymerised and oxidised with NOPF6. The experimental results indicated the presence of polarons in three of the systems that cause appreciable electrical conductivity values. In this work we present a theoretical spectroscopy study for oligomers builded with these materials. In our calculations we utilised the semi empirical AM1 method for the geometric simulation of the oligomers. Optical absorption spectra for neutral and charged systems were done by using ZINDO/S method.