Density functional theory study on the structure and vibrational frequencies of glycylglycine

Eleven possible conformers of glycylglycine have been studied by using the BLYP, B3LYP methods of density functional theory and the HF method at the basis set of 6-311++G**. BLYP (using Becke's and Lee-Yang-Parr's correlation functionals), ab initio Hartree-Fock (HF) and hybrid DFT/HF B3LYP calculations have been carried out to study the structure and vibrational spectra of glycylglycine. Glycylglycine crystal structure has been determined by X-ray diffraction analysis. The title compound has been crystallizes in the orthorhombic space group C1, with Z=4. And the unit cell parameters are: a=8.1184(12)A, b=9.5542(14)A, c=7.8192(11)A and V=577.95(15)A(3). Molecular conformation calculations have got 11 possible conformers. In these possible conformers, the most stable one has been selected. The BLYP/6-311++G** and scaled HF/6-311++G** frequencies correspond well with available experimental assignments of the normal vibrational modes. Comparison of the observed fundamental vibrational frequencies of glycylglycine and calculated results by density functional B3LYP and Hartree-Fock (HF) methods indicates that B3LYP is superior to the scaled Hartree-Fock (HF) for molecular vibrational issues.     

Density functional calculations for predicting structures and vibrational frequencies of aluminum chloride-fluoride complexes

Quantum chemical calculations are used to study AlCl_y−xF_x^3−y (y = 5 or 6, x = 0,…,y) species that can occur in aluminum electrorefining melts. These theoretical studies are included in a wider research program concerning the chemical instabilities in the bulk of molten salts during the refinement process. Stabilization energies, equilibrium geometries and vibrational frequencies of the complexes are calculated using the Delley functional methodology described in Ref. [1] (B. Delley, J. Chem. Phys., 92 (1990) 508). These computational simulations, discussed and compared with the experimental results demonstrate that density functional calculations can be reliably used in the study of complexes existing in molten salts. Quantum chemical calculations are accurate tools for theoretically predicting structures, physical and chemical properties and vibrational frequencies of known entities as well as unknown compounds.     

Density functional theory calculations of pressure effects on the vibrational structure of alpha-RDX

Pressure effects on the vibrational structure of alpha-RDX were examined using density functional theory (DFT) up to 4 GPa. The calculated vibrational frequencies at ambient conditions are in better agreement with experimental data than are previous single molecule calculations. The calculations showed the following pressure-induced changes: (i) larger shifts for lattice modes and for internal modes associated with the CH(2) and NO(2) groups as compared to the pressure shifts for modes associated with the triazine ring, (ii) enhancement of mixing between different vibrations, for example, between NN stretching and CH(2) scissor, wagging, twisting vibrations, and (iii) increase in mixing between translational lattice vibrations and the NO(2) wagging vibrations, reducing the distinction between internal and lattice modes. The calculated volume and lattice constants at ambient pressure are larger than the experimental values, due to the inability of the present density functional approach to correctly account for van der Waals forces. Consequently, the pressure-induced frequency shifts of many modes deviate substantially from experimental data for pressures below 1 GPa. With increasing pressure, both the lattice constants and the frequency shifts agree more closely with experimental values.     

First principles calculations on structure, bonding, and vibrational frequencies of SiP2

Pyrite type SiP(2) is reinvestigated by first principles calculations on various levels of functionals including local density approximation, generalized gradient approximation, Becke-Lee-Yang-Parr hybrid functional, and the Hartree-Fock method. SiP(2) is seen as a model compound with molecular [P-P] entities and [SiP(6)] octahedra. Structure and bonding are addressed by electronic structure calculations. Special attention is spent on P-P and Si-P bonds in terms of bond lengths and respective stretching modes from simulated Raman spectra. The electronic structure is analyzed in both direct and momentum space by the electron localization function and site projected density of states. The main goals of this work are to understand the nature of chemical bonding in SiP(2) and to compare and contrast the different methods of calculation.     

Structure and vibrational frequencies of 1-naphthaldehyde based on density functional theory calculations

The mid and far FTIR and Raman spectra were measured in the liquid state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) and standard B3LYP/6-311+G** basis set combination. The vibrational spectra were interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical (SQM) force field. The infrared and Raman spectra were also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. Unambiguous vibrational assignment of all the fundamentals was made using the total energy distribution (TED).     

Density functional theory calculations and vibrational circular dichroism of aromatic foldamers

Ab initio calculations together with vibrational circular dichroism (VCD) have been used for studying the conformations of a quinoline-derived oligoamide bearing a terminal chiral residue. Three helically folded conformers of the dimer, trimer, and tetramer forms of the oligomer were optimized at the density functional theory (DFT) level using the B3LYP functional and the 6-31G* basis set. For each form, the three conformers differ in their helical handedness and in the conformation of the chiral end group. The calculated structures of the tetramer and also the proportions predicted between them based on their calculated Gibbs free energies differences match remarkably well with experimental data collected on an octamer. Specifically, a R-phenethyl terminal group gives rise to a 91:9 ratio between left handed and right handed helices. The predicted VCD spectrum calculated from the Boltzmann population of the individual conformer reproduces very well the experimental VCD spectrum of the tetramer in CDCl3 solution. The DFT calculations performed for the trimer also allow one to assess the preferred handedness of the helix and the conformation of the chiral end group, but the calculated relative populations differ slightly from experimental data. Finally, this study shows that the dimer fragment is not sufficient to obtain valuable information on the conformation of this aromatic oligoamide foldamer.     

Density functional theory study of vibrational spectra, and assignment of fundamental vibrational modes of succinimide and N-bromosuccinimide

This work deals with the vibrational spectroscopy of succinimide and N-bromosuccinimide. The mid and far FTIR and FT-Raman spectra were measured in the condensed state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) using standard B3LYP/6-31G(*) and B3LYP/6-311+G(**) methods and basis set combinations. The vibrational spectra were interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical force field. The infrared and Raman spectra were also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. Unambiguous vibrational assignment of all the fundamentals were made using the total energy distribution (TED).     

Density functional theory calculations and vibrational spectra of 6-methyl 1,2,3,4-tetrahyroquinoline

The solid phase FTIR and Raman spectra of 6-methyl 1,2,3,4-tetrahydroquinoline (MTHQ) have been recorded in the regions 4000-100 and 3500-100 cm(-1), respectively. The spectra were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculations based on the density functional theory (DFT) using the standard B3LYP/6-311+G(**) basis set combination. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors.     

儿茶素结构和振动光谱的密度泛函理论研究

本文利用密度泛函理论方法B3LYP在6-311 G(d,p)水平上研究了儿茶素及其异构体表儿茶素分子的几何构型,计算结果与实验所得结构参数一致.计算了儿茶素分子平衡构型下的力常数,使用Wilson的GF矩阵方法计算了振动基频以及相应的势能分布,据此结合理论计算的光谱强度,对儿茶素分子的振动基频进行了完善合理的理论归属.     

Density functional theory study of vibrational spectra, and assignment of fundamental vibrational modes of 1-bromo 4-fluoronaphthalene

The FTIR and FT-Raman spectra of 1-bromo 4-fluoronaphthalene have been recorded in the regions 4000-100cm(-1) and 3500-100cm(-1), respectively. The spectra were interpreted with the aid of normal coordinate analysis based on DFT (density functional theory) using standard B3LYP/6-311+G** basis set combination for the most optimized geometry. Normal coordinate calculations performed with the DFT force field and subsequently corrected by a recommended set of scale factors, yielded fairly good agreement between observed and calculated frequencies. On the basis of the comparison between calculated and experimental results, assignments of fundamental modes were examined.     

Density functional theory study on the structure and vibrational spectra for cyanuric chloride

The molecular structure and vibrational spectra of cyanuric chloride have been investigated by density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compound. Both the calculated structural parameters and vibrational frequencies are in good agreement with the available experimental data.     

Density functional calculations on cyclodextrins

Conformations of α-, β-, and γ-CDs under anhydrous conditions in the gas phase were investigated by a density functional method, B3LYP/6-31G(d,p). These calculations resulted in several symmetric conformations with different energies. The lowest energy conformations contain two rings of homodromic hydrogen bonds and are referred to “one-gate-closed” conformations. Different orientations of hydrogen bonds lead to four minima. Other conformational minima were found for “open” conformations which correspond to some extent to experimentally determined structures.     

Density functional theory study of vibrational spectra and assignment of fundamental vibrational modes of 1-methyl-4-piperidone

The FT-IR and FT-Raman spectra of 1-methyl-4-piperidone was recorded and the observed bands were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculation based on the density functional theory (DFT) using the standard B3LYP/6-311G** method and basis set combinations. A very good agreement obtained between the simulated and experimental spectra was established and unambiguous vibrational assignments of various modes were proposed based on the results of potential energy distribution (PED) calculations.     

Density functional theory study on molecular structure and vibrational IR spectra of isocytosine

Density functional theory with the combined Becke3-LYP exchange-correlation energy functional [DFT(B3-LYP) method] using the 6-31G(d, p) basis set is applied to predict molecular parameters (geometries, rotational constants, dipole moments) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of six tautomers of the isocytosine molecule. The results are compared with the corresponding data calculated at the conventional ab initio Hartree-Fock (HF) level using the same basis set and with available experimental data. Calculations show that (a) three amino tautomers are slightly nonplanar species with, evidently, a distorted amino group, (b) the DFT (B3-LYP)/6-31G(d, p) method predicts better molecular parameters, than do the HF calculations, and (c) the DFT(B3-LYP)-calculated vibrational IR spectra of isocytosine agree well with the available recorded IR spectra, and they show marked improvement over the IR spectra predicted at the HF/6-31G(d, p) level. Tautomeric stabilities of isocytosine are discussed on the basis of computed electronic energies by the DFT(B3-LYP) and ab initio approaches [including the MP2 and MP4(SDQ) calculations of electronic energies] and predicted zero-point vibrational energies by DFT(B3-LYP) and HF methods. This relative energies at 0 K of the tautomeric forms of isocytosine predicted by both conventional ab initio and DFT(B3-LYP) methods correlate well with the experimental data, showing the predominance of the aminohydroxy tautomer of isocytosine for an isolated molecule. © 1997 John Wiley & Sons, Inc.     

Density functional theory calculations and vibrational spectra of 3,5-dibromopyridine and 3,5-dichloro-2,4,6-trifluoropyridine

The solid phase mid FTIR and FT-Raman spectra of 3,5-dibromopyridine (3,5-DBP) and 3,5-dichloro-2,4,6-trifluoropyridine (3,5-DCTFP) have been recorded in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. The spectra were interpreted with the help of normal coordinate analysis (NCA) following full structure optimisation and force field calculations based on the density functional theory (DFT) using the standard B3LYP/6-31G( *) method and basis set combination. The results of the calculations are applied to stimulate infrared and Raman spectra of the title compounds which showed excellent agreement with the observed spectra.     

Molecular structure, vibrational spectral studies of pyrazole and 3,5-dimethyl pyrazole based on density functional calculations

In this work, the experimental and theoretical vibrational spectra of pyrazole (PZ) and 3,5-dimethyl pyrazole (DMP) have been studied. FTIR and FT-Raman spectra of the title compounds in the solid phase are recorded in the region 4000-400 cm(-1) and 4000-50 cm(-1), respectively. The structural and spectroscopic data of the molecules in the ground state are calculated using density functional methods (B3LYP) with 6-311+G** basis set. The vibrational frequencies are calculated and scaled values are compared with experimental FTIR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete vibrational assignments are performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanical (SM) method. 13C and 1H NMR chemical shifts results are compared with the experimental values.     

Inclusion of non-bonded interactions in calculations of vibrational frequencies

The consequences of including non-bonded interactions in the calculation of vibrational frequencies are examined by comparing the results of the consistent force field method (CFF)and a conventional spectroscopic force field calculation method. Sample calculations are given, using ethane as a test case. It is shown that the CFF method is more practical for the calculation of non-bonded interaction parameters for large molecules, provided that the molecular potential energy is first minimized.