Vibrational spectra and conformations of 1,4-cyclohexadiene and its oxygen analogues: ab initio and density functional calculations

The vibrational spectra and ring-puckering potential energy functions of 1,4-cyclohexadiene, 4H-pyran and 1,4-dioxin have been examined using a density functional theory (DFT) method as well as the Hartree–Fock (HF) and second-order Møller–Plesset (MP2) methods. The calculated vibrational frequencies and potential energy functions of those molecules have been compared with previously reported experimental data and MM3 results. For all three molecules, the DFT method using Becke's three-parameter functional (B3LYP) has led to the prediction of more accurate vibrational frequencies than the HF and MP2 methods. The enlargement of the basis set at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. In particular, the C–O–C=C torsional force field parameters obtained from the B3LYP method have correctly predicted the ring-puckering potential energy functions of the oxygen-containing analogues, 4H-pyran and 1,4-dioxin, which could not be done by the MM3 method.     

Basis-independent potential energy curves for the neutral diatomics of Li,Na and K evaluated by means of Hartree-Fock and different density functional potentials

Summary The solution of the Schrödinger equation for diatomic molecules when the finite element method is used gives the possibility to evaluate highly accurate basis-independent potential energy curves. In this work such types of numerically accurate potential energy curves on the HF level have been evaluated for Li₂, Na₂ and K₂ and could be used as benchmarks in the optimization of basis sets. A comparison between recent LCAO HF calculations in which extended basis sets are used and the accurate values determined in this work show that there is a difference in total energy of 4×10^–5 and 10^–3 a.u. for Li, Li₂, and Na, Na₂, respectively. Evaluated dissociation energies are, however, due to the cancellation of numerical errors in much better agreement. Further, it is found that different exchange correlation potentials for the heavier molecules such as those given by von Barth-Hedin and Vosko, Wilk and Nusair reproduce experimental properties such as dissociation energies, vibrational frequencies almost as well as those achieved with advanced CI methods. TheX potential gives accurate bond lengths for Na₂ and K₂, whereas the dissociation energies are too small.     

Theoretical studies of the N2O van der Waals dimer: ab initio potential energy surface, intermolecular vibrations and rotational transition frequencies

Theoretical studies of the potential energy surface and bound states were performed for the N(2)O dimer. A four-dimensional intermolecular potential energy surface (PES) was constructed at the CCSD(T) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to a nonpolar isomer with slipped-antiparallel planar structure and two equivalent polar isomers with slipped-parallel planar structures. The nonpolar isomer is energetically more stable than the polar ones by 162 cm(-1). To assign the fundamental vibrational frequencies for both isomers, more than 150 vibrational bound states were calculated based on this PES. The orientation of the nodal surface of the wave functions plays an important role in the assignment of disrotation and conrotation vibrational modes. The calculated vibrational frequencies are in good agreement with the available experimental data. We have also found a quantum tunneling effect between the two equivalent polar structures in the higher vibrational excited states. Rotational transition frequencies of the polar structure were also calculated. The accuracy of the PES is validated by the good agreement between theoretical and experimental results for the transition frequencies and spectroscopic parameters.     

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions

We report an experimental study of the rotovibrational fundamental PQR-band shapes in the IR absorption spectra of HCl dissolved in condensed rare gases in a wide range of temperatures. The effective vibrational frequencies are determined from analysis of the fine rotational structure partially resolved in the band wings. The central Q-branch components appear redshifted with respect to the effective vibrational frequencies, their shifts in different solvents found to match the HCl stretching mode shifts in binary Rg...HCl van der Waals heterodimers. Theoretical quasi-free rotor and modified rotor models are applied to describe evolution of the band profiles at changing thermodynamic conditions. Both models are shown to reproduce equally well the observed spectral density distributions in the band wings. However, the modified rotor formalism that accounts for depopulation of the lower-energy rotational solute states provides better agreement with the experiment in the range of the P- and R-branch maxima. We surmise that the Q branches separated from the measured spectral profiles are formed by transitions between rotationally hindered states of diatomic molecules coupled to the solvent by the local anisotropy of the interaction potential.     

Internal dynamics of a globular protein in water

The frequency distributions of internal dynamics of a protein are calculated in solution using normal mode analysis. Our test case is bovine pancreatic trypsin inhibitor, consisting of 58 amino acid residues. Each water molecule surrounding the protein is treated as an internally rigid body that can move with the vibrating protein. The water molecules are redistributed around the protein, as dictated by the potential energy. It is shown that water molecules around the protein are essential for the protein to keep its tertiary structure close to the X-ray structure. The density of states calculated in this model is shifted toward high frequencies when compared with results previously obtained with a model in which the water molecules were not allowed to move with the protein. This shift toward high-frequency states originates from the stronger interactions of water molecules with the sidechain atoms in the protein. The present model is computationally demanding. So the previous (frozen water) model is suggested to be a reasonable approximation for expressing internal dynamics of a protein in solution. © 1996 by John Wiley & Sons, Inc.     

Density functional theory investigations on the conformational stability and vibrational spectra of the model compound of vitamin K

Becke 3-Lee–Yang–Parr density functional theory (DFT/B3LYP) using 6-31G(d) and 6-311G(d) basis sets and the scaled quantum mechanics (SQM) force field method are used to study molecular conformations and vibrational spectra of a model compound of vitamin K (VK). In this molecule, there are six conformers on the torsional potential energy map of the dihedral angles C₈C₁₄C₁₅C₁₆ () and C₇C₈C₁₄C₁₅ (β). It is shown that the VK_1 conformer ( = 237.7° and β = 274.2°) is the most stable form. For this lowest energy conformer, the harmonic force fields calculated by B3LYP/6-311G(d) and B3LYP/6-31G(d) methods are scaled with one scale factor of 0.9623 and a set of different scale factors transferred from the previous studies for the other similar molecules, respectively. The vibrational frequencies, IR intensities, and Raman activities are obtained for the lowest energy conformer. On the basis of B3LYP calculations, the normal mode analysis is performed to assign the vibrational fundamental frequencies according to the potential energy distributions. The computational frequencies are in good agreement with the observed results.     

Effects of hydration on scale factors for ab initio force constants

Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To this end, we have calculated scale factors for ab initio force constants for formic acid, acetic acid, and acetone using a least squares fit of calculated and experimental frequencies. We compare these scale factors with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G basis. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. The results indicate that scale factors can simulate the effects of solvation on molecular force constants to yield accurate scaled ab initio force fields.     

Properties of variational transition states for association reactions

The relationships between the structures of microcanonical variational transition states for association reactions and potential energy surface properties are analyzed. The analysis is facilitated by the recent finding that in many cases the vibrational frequencies for the transitional modes in the association reaction vary exponentially with the reaction coordinate. An increase in the attenuation of the transitional mode frequencies results in a tighter transition state. Long-range minimum energy path potentials of the form −c/r″ have only one transition state at each energy. Certain types of association reactions may not have transition states.     

Anharmonic oscillator model of overtone spectra for D3h symmetry molecules

The bond stretching vibrations of XF₅ molecules with D_3h symmetry are treated computationally on the Morse oscillator model in which the bond oscillators are coupled harmonically. Each calculation involves four parameters for two types of Morse potential and three parameters for the kinetic-energy, potential coupling terms. The eigenvalue formula for overtone and combination states up to three are presented and can be used to predict all the vibrational energy levels from local mode molecules through normal mode molecules. For PF₅, AsF₅ and VF₅, the coupled Morse oscillator model gives a prediction in good agreement with the experimental data.     

On Feasibility of “Spectroscopic” Calculations of XH Bond Dissociation Energies for Polyatomic Molecules from Fundamental Vibration Frequencies Using the Anharmonic Molecular Model

This paper discusses the applicability of the variational technique using a minimal Morse-harmonic basis set to calculations of the fundamental spectrum and the potential function parameters for polyatomic molecules. The potential function is assumed to be the sum of the Morse function for XH bonds and the harmonic function for the skeletal and deformation vibrations. The initial approximation for the potential function is found by ab initio calculations and refined by solving the inverse mechanical problem (selecting the scaling indices). The thus selected harmonic part of the potential function gives equally good agreement between the experimental and calculated transition frequencies in both harmonic and anharmonic approximations. The anharmonic (Morse) term of the potential function (bond dissociation energy) is selected by solving the inverse mechanical problem until the best agreement between the experimental and calculated CH bond stretching frequencies has been achieved. Problem solving ends with the construction of a transmission curve in the IR spectrum. Variations of the dipole moment of the molecule induced by vibrations are found by ab initio calculations.     

Exploring the effect of anharmonicity of molecular vibrations on thermodynamic properties

Thermodynamic properties of selected small and medium size molecules were calculated using harmonic and anharmonic vibrational frequencies. Harmonic vibrational frequencies were obtained by normal mode analysis, whereas anharmonic ones were calculated using the vibrational self-consistent field (VSCF) method. The calculated and available experimental thermodynamic data for zero point energy, enthalpy, entropy, and heat capacity are compared. It is found that the anharmonicity and coupling of molecular vibrations can play a significant role in predicting accurate thermodynamic quantities. Limitations of the current VSCF method for low frequency modes have been partially removed by following normal mode displacements in internal, rather than Cartesian, coordinates.     

Theoretical Investigation of 5d-Metal Monocarbides

Bond distances, vibrational frequencies, electron affinities, ionization potentials and dissociation energies of the title molecules in neutral, positively and negatively charged ions were studied by use of density functional method. The calculated results were compared with previous theoretical and experimental studies. Ground states for each molecule were assigned. It was found that for some molecules, low-lying state, in which the energy is much close to the ground state, was obtained. In this case, further studies both experimentally and theoretically are necessary in order to find the true global minimum.     

Spectroscopy of complex molecules in the gas phase

A brief review is presented of theoretical and experimental research into the role of statistical factors in the formation of the characteristics of absorption and emission processes of light, observed for low-density and rarefied vapours of complex molecules. It is shown, in particular, that the average energy of molecules excited per unit time differs from the mean energy of molecules in the ground state, not by the energy of the exciting photon hv, but by the sum of hv and the selective energy which is the result of different absorption probabilities for molecules of different energy. This correction is the most important in rotational—vibrational absorption bands. This was established when the selective energy was calculated using the experimental data with the help of the formulae obtained. Average energies of the initial and final combining states reduced by the average energy of molecules in the ground state are calculated. Correlation curves similar to those of Condon are plotted according to the calculated data. The electron transition frequencies and the identities of absorption and emission transitions are determined through these curves; whereas for rotational—vibrational bands the value of the rotational constant and the variation of the latter upon excitation are estimated.     

Dynamics of small,floppy molecules studied by high-resolution spectroscopic techniques

In molecular spectroscopy one of the common interests is how to transform the information obtained by high-resolution spectroscopic techniques into some reliable approximation of the potential energy surface of a particular molecule. Traditionally vibrational spectroscopy has been used. Rotational spectroscopy can only probe, at least at room temperature, molecular transitions arising from excited vibrational states up to approximately 1000 cm^?1. This corresponds roughly to 10% of a typical bond dissociation energy. However, floppy molecules which exhibit a large-amplitude, low-lying vibrational mode can be studied to a large extent by rotational spectroscopy in the microwave, millimeter and submillimeter wave range. Quasilinearity is a special form of large-amplitude motion, which complicates the observed molecular spectra substantially and which presents a real challenge to theoretical spectroscopists. In this lecture the highlights of quasilinear behavior of the molecules HCNO, OCCCO, HNCS and HNCO will be discussed. Another form of large amplitude motion is the inversion exhibited primarily by molecules derived from NH₃. Isocyanamide will be discussed and its special spectroscopic features will be shown. Cyanamide and isocyanamide are potential prebiotic molecules: cyanamide has been detected as a constituent in the interstellar medium. The analysis of the molecular dynamics of these molecules is shown to be necessary for understanding the frequencies and intensities of the observed spectra in the laboratory and in interstellar space.     

Ne-HCl势能面和振转光谱的理论研究

利用量子化学计算方法CCSD(T)和大基组aug-cc-pVTZ加键函数3_s3_p2_d对Ne-HCl体系的分子间势能面进行了理论研究.结果表明,势能面上有两个势阱,分别对应于线性Ne-ClH和Ne-HCl构型.通过精确求解核运动方程发现,该从头算势能面分别支持5个(对Ne-HCl)和7个(Ne-DCl)振动束缚态.计算得到的振转跃迁频率与实值值吻合.     

Excited-state behavior of trans and cis isomers of stilbene and stiff stilbene: a TD-DFT study

The first part of the isomerization path on the two lowest excited states of trans and cis isomers of stilbene and stiff stilbene is investigated by means of TD-PBE0 calculations in the gas phase and in heptane solution. Solvent effects are taken into account by the PCM model. The excited-state optimized structures and the computed absorption and emission frequencies are in good agreement with the available experimental results. In all of the examined compounds, the isomerization process before barrier crossing occurs on the HOMO --> LUMO bright state, whereas the role played by other single-excitation states appears negligible. The relative energy barriers on the isomerization paths are consistent with the experimental excited-state lifetimes, suggesting a unifying picture of the isomerization process in stilbene-like molecules.     

Accurate calculation of anharmonic vibrational frequencies of medium sized molecules using local coupled cluster methods

Local coupled cluster methods were applied for the automated generation of accurate multidimensional potential energy surfaces for a set of test molecules ranging from six to nine atoms. Based on these surfaces anharmonic fundamental frequencies were computed using vibrational self-consistent field and configuration interaction methods. The computed vibrational frequencies are compared to those obtained from similar calculations using conventional coupled cluster methods and to experimental values. The results from local and conventional methods are found to be of similar accuracy and in close agreement with experimental values. In addition, an efficient parallelization of the fully automated surface generation code is presented.     

HCB和HCB-的激发态光谱与态-态转化的理论研究

HCB是最简单的有机硼化合物，理论上已有少量研究，报导了部分激发态及其谐振动频率．关于HCB-及其异构体HBC-的研究未见报导．本文采用精确的量子化学计算方法，对它们的激发态性质、光谱和态一态转化进行了系统的研究，为实验上对这些物质的研究提供了一些有用的信息．计算采用极化的三Zeta基组（TZP）进行基态和激发态的构型优化与振动性质调查各电子态相关能估算应用基于多组态自洽场分子轨道的一级组态相互作用（FOCI／MCSCF）方法．计算过程中，考虑到对称性破裂（symmetrybrealling）的影响同，对存在对称性破裂的体系采用…     

Langevin model of the temperature and hydration dependence of protein vibrational dynamics

The modification of internal vibrational modes in a protein due to intraprotein anharmonicity and solvation effects is determined by performing molecular dynamics (MD) simulations of myoglobin, analyzing them using a Langevin model of the vibrational dynamics and comparing the Langevin results to a harmonic, normal mode model of the protein in vacuum. The diagonal and off-diagonal Langevin friction matrix elements, which model the roughness of the vibrational potential energy surfaces, are determined together with the vibrational potentials of mean force from the MD trajectories at 120 K and 300 K in vacuum and in solution. The frictional properties are found to be describable using simple phenomenological functions of the mode frequency, the accessible surface area, and the intraprotein interaction (the displacement vector overlap of any given mode with the other modes in the protein). The frictional damping of a vibrational mode in vacuum is found to be directly proportional to the intraprotein interaction of the mode, whereas in solution, the friction is proportional to the accessible surface area of the mode. In vacuum, the MD frequencies are lower than those of the normal modes, indicating intramolecular anharmonic broadening of the associated potential energy surfaces. Solvation has the opposite effect, increasing the large-amplitude vibrational frequencies relative to in vacuum and thus vibrationally confining the protein atoms. Frictional damping of the low-frequency modes is highly frequency dependent. In contrast to the damping effect of the solvent, the vibrational frequency increase due to solvation is relatively temperature independent, indicating that it is primarily a structural effect. The MD-derived vibrational dynamic structure factor and density of states are well reproduced by a model in which the Langevin friction and potential of mean force parameters are applied to the harmonic normal modes.     

Theoretical investigations on electronic structures and photophysical properties of N-heteroaryl carbazole derivatives as host materials

The carbazole-endcapped host molecules with tailoring different heteroaryl core and meta-position linkage mode have great potential on phosphorescent organic light-emitting diodes. To provide a profound view on structure?Cproperty relationships, new linear-shaped counterparts have been designed based on the existing molecular composition and the linkage at para-position (p-type molecules). A series of studies about the influence of the linkage mode on optical and electronic properties of these carbazole derivatives have carried out via density functional theory and time-dependent density functional theory calculations. The geometric and the electronic structure of these molecules in the ground states, ions states, and lowest triplet states have been calculated especially focusing on the analysis of highest occupied molecular orbitals, lowest unoccupied molecular orbitals, energy gaps, triplet energies, ionization potentials, electron affinities, reorganization energies, triplet exciton-formation fraction, and absorption spectra. These optoelectronic properties can be effectively tuned by the chemical modifications of different linkage pattern. The good coordination between our calculated results and the available experimental data has been observed. The study reveals that the designed p-type molecules show great promise as new high-performance red host materials with large triplet energy, narrow energy gap, good electron and hole-transport properties, and high triplet exciton-formation fraction.