Comparison of geometric, electronic, and vibrational properties for isomers of small fullerenes C20-C36

We employ the self-consistent-charge density-functional tight-binding (SCC-DFTB) method for computing geometric, electronic, and vibrational properties for various topological isomers of small fullerenes. We consider all 35 five- and six-member rings containing isomers of small fullerenes, C20, C24, C26, C28, C30, C32, C34, and C36, as first part of a larger effort to catalog CC distance distributions, valence CCC angle distributions, electronic densities of states (DOSs), vibrational densities of states (VDOSs), and infrared (IR) and Raman spectra for fullerenes C20-C180. Common features among the fullerenes are identified and properties characteristic for each specific fullerene isomer are discussed.     

Modeling vibrational spectra using the self-consistent charge density-functional tight-binding method. I. Raman spectra

An extension of the self-consistent charge density-functional tight-binding (SCC-DFTB) method is presented that allows for calculating intensities of peaks in vibrational Raman spectra for very large molecules. The extension is based on a simple ansatz: an extra term, which describes interaction of an external electric field with induced atomic charges, is added to the SCC-DFTB energy expression. We apply the modified SCC-DFTB formalism for reproducing vibrational Raman spectra of 17 organic molecules. The calculated spectra are compared with experiment and with spectra obtained from density functional theory (DFT) calculations. We find that the SCC-DFTB method is capable of reproducing most of the features of experimental Raman spectra. Limitations and advantages of this approach are analyzed and suggestions for interpreting calculated SCC-DFTB Raman spectra are given.     

Energetics and structure of glycine and alanine based model peptides: Approximate SCC-DFTB,AM1 and PM3 methods in comparison with DFT,HF and MP2 calculations

We calculate relative energies and geometries of important secondary structural elements for small glycine and alanine based polypeptides containing up to eight residues. We compare the performance of the approximate methods AM1, PM3 and self-consistent charge, density-functional tight-binding (SCC-DFTB) to density-functional theory (DFT), Hartree–Fock (HF) and MP2. The SCC-DFTB is able to reproduce structures and relative energies of various peptide models reliably compared to DFT results. The AM1 and PM3 methods show deficiencies in describing important secondary structure elements like extended, helical or turn structures. The discrepancies between different ab initio (HF, MP2) and DFT (B3LYP) methods for medium sized basis sets (6-31G*) also show the need for higher level calculations, since systematic errors found for small molecules may add up when investigating longer polypeptides.     

Analytical second-order geometrical derivatives of energy for the self-consistent-charge density-functional tight-binding method

Analytical formulation of the second-order geometrical derivatives of energy for the self-consistent-charge density-functional tight-binding (SCC-DFTB) method is presented. To test its quality and numerical performance, the derived formalism has been coded and applied for calculation of harmonic vibrational frequencies for a set of 17 small and medium size molecules. For this set, the average absolute deviation from experiment is 99 cm(-1) for SCC-DFTB vs 62 cm(-1) for the M?ller-Plesset second-order perturbation theory with the cc-pVDZ basis set (MP2/cc-pVDZ) and 32 cm(-1) for the B3LYP density functional method with the same basis set (B3LYP/cc-pVDZ), while the maximal deviation is 465 cm(-1) vs 1,741 cm(-1) for MP2/cc-pVDZ and 112 cm(-1) for B3LYP/cc-pVDZ. The SCC-DFTB results are in reasonable agreement with experiments as well as with ab initio and density-functional results, and are better than other semiempirical methods. The SCC-DFTB method allows for considerable computational time saving when compared to other methods while retaining similar overall accuracy. Data for a series of conjugated polyenes show that an analytical formulation of SCC-DFTB is noticeably faster than its numerical formulation.     

A density-functional study of the structural, electronic, magnetic, and vibrational properties of Ti8C12 metallocarbohedrynes

Calculations are presented for the structural, electronic, and vibrational properties of the different Ti8C12 metallocarbohedrynes. (Please note that we adopt the name "metallocarbohedrynes" instead of "metallocarbohedrenes" to denote the acetylenic nature of C2 units in this class of clusters demonstrated by several contributions in literature.) The density-functional theory (DFT) calculations are performed with the all-electron projector augmented-wave method and generalized gradient approximation for the exchange-correlation functional. We study the seven low-energy isomers of the Ti8C12 metallocarbohedrynes using spin-polarized DFT, where we find a correlation between the number of rotated carbon dimers and the cohesive energy of the structure. The electronic density of states (eDOS) show that C3nu, D*3d, and D3d isomers are spin polarized. The partial eDOS shows that, depending on the dimer orientation, carbon atoms and a subgroup of the metal atoms form a covalent framework while other metal atoms are bonded to this framework more ionically. This picture is further supported by the charge density of the different structures, where we see that the Ti atoms with higher charge density show less contribution to the covalent bonding of the Ti-C framework. The vibrational spectra of the different structures are calculated using the frozen-vibration method. Also, we calculate the vibrational spectra of the C3nu and C2nu structures using molecular-dynamics simulations at two different temperatures. The results of the simulations demonstrate the local stability of the structures beyond the harmonic limit explored by the frozen-vibration method.     

Experimental (FT-IR and FT-Raman), electronic structure and DFT studies on 1-methoxynaphthalene

In this work, FT-IR and FT-Raman spectra of 1-methoxynapthalene (C(11)H(10)O) have been reported in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. Density functional method (DFT) has been used to calculate the optimized geometrical parameters, atomic charges, vibrational wavenumbers and intensity of the vibrational bands. The vibrational frequencies have been calculated and scaled values are compared with experimental FT-IR and FT-Raman spectra. The structure optimizations and normal coordinate force field calculations are based on density functional theory (DFT) method with B3LYP/3-21G, B3LYP/6-31G, B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p) basis sets. The complete vibrational assignments of wavenumbers are made on the basis of potential energy distribution (PED). The optimized geometric parameters are compared with experimental values of naphthoic acid. The results of the calculation shows excellent agreement between experimental and calculated frequencies in B3LYP/6-311++G(d,p) basis set. The effects due to the substitutions of methyl group and carbon-oxygen bond are also investigated. A study on the electronic properties, such as excitation energies and wavelengths, were performed by time-dependent DFT (TD-DFT) approach. HOMO and LUMO energies are calculated that these energies show charge transfer occurs within the molecule.     

Structural and spectroscopic characterization of 2,3-difluorobenzoic acid and 2,4-difluorobenzoic acid with experimental techniques and quantum chemical calculations

In this study, the molecular conformation, vibrational and electronic transition analysis of 2,3-difluorobenzoic acid and 2,4-difluorobenzoic acid (C7H4F2O2) were presented using experimental techniques (FT-IR, FT-Raman and UV) and quantum chemical calculations. FT-IR and FT-Raman spectra in solid state were recorded in the region 4000-400 cm(-1) and 4000-5 cm(-1), respectively. The UV absorption spectra of the compounds that dissolved in ethanol were recorded in the range of 200-800 nm. The structural properties of the molecules in the ground state were calculated using density functional theory (DFT) and second order M?ller-Plesset perturbation theory (MP2) employing 6-311++G(d,p) basis set. Optimized structure of compounds was interpreted and compared with the earlier reported experimental values. The scaled vibrational wavenumbers were compared with experimental results. The complete assignments were performed on the basis of the experimental data and total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. A study on the electronic properties, such as absorption wavelength, excitation energy, dipole moment and frontier molecular orbital energy, were performed by time dependent DFT (TD-DFT) approach. Based on the UV spectra and TD-DFT calculations, the electronic structure and the assignments of the absorption bands of steady compounds were discussed. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecules.     

Experimental (13C NMR, 1H NMR, FT-IR, single-crystal X-ray diffraction) and DFT studies on 3,4-bis(isoproylamino)cyclobut-3-ene-1,2-dione

In this work, 3,4-bis(isoproylamino)cyclobut-3-ene-1,2-dione C(10)H(16)N(2)O(2) (I), was synthesized and characterized by (13)C NMR, (1)H NMR, FT-IR, UV-vis spectroscopy and single-crystal X-ray diffraction. DFT method with 6-31G(d,p) basis set has been used to calculate the optimized geometrical parameters, atomic charges, vibrational frequencies and chemical shift values. The calculated vibrational frequencies and chemical shift values are compared with experimental FT-IR and NMR spectra. The results of the calculation shows good agreement between experimental and calculated values of the compound I. The existence of N-H?O type intermolecular ve C-H?O type intramolecular hydrogen bonds can be deduced from differences between experimental and calculated results of FT-IR and NMR. In addition, the molecular electrostatic potential map and frontier molecular orbitals and electronic absorption spectra were performed at B3LYP/6-31G(d,p) level of theory. HOMO-LUMO electronic transition of 4.90 eV are derived from the contribution of the bands π→π* and n→π* The spectral results obtained from FT-IR, NMR and X-ray of I revealed that the compound I is in predominantly enamine tautomeric form, which was supported by DFT calculations.     

DFT vibrational calculations of rhodamine 6G adsorbed on silver: analysis of tip-enhanced Raman spectroscopy

The tip-enhanced near-field Raman (TERS) bands of Rhodamine 6G (R6G), that we reported earlier [Chem. Phys. Lett. 2001, 335, 369.], are assigned on the basis of density-functional theory (DFT) calculations at the 6-311++G(d,p) level. The Raman and infrared intensities as well as frequencies of the vibrational modes are used for band assignments. These vibrational modes, in combination with characterization of resonant electronic transitions using time-dependent DFT calculations, predict spectral changes in resonant Raman and surface-enhanced resonant Raman scatterings of R6G. Moreover, the TERS spectra of R6G are analyzed in detail, where interactions between the tip and R6G molecules and their enhancement mechanisms are discussed. Finally, we propose a novel Raman spectroscopy technique capable of detecting molecular vibrations at sub-nanometer scale.     

Vibrational spectroscopy (FT-IR and FT-Raman) investigation, and hybrid computational (HF and DFT) analysis on the structure of 2,3-naphthalenediol

The FT-IR and FT-Raman vibrational spectra of 2,3-naphthalenediol (C(10)H(8)O(2)) have been recorded using Bruker IFS 66V spectrometer in the range of 4000-100 cm(-1) in solid phase. A detailed vibrational spectral analysis has been carried out and the assignments of the observed fundamental bands have been proposed on the basis of peak positions and relative intensities. The optimized molecular geometry and vibrational frequencies in the ground state are calculated by using the ab initio Hartree-Fock (HF) and DFT (LSDA and B3LYP) methods with 6-31+G(d,p) and 6-311+G(d,p) basis sets. There are three conformers, C1, C2 and C3 for this molecule. The computational results diagnose the most stable conformer of title molecule as the C1 form. The isotropic computational analysis showed good agreement with the experimental observations. Comparison of the fundamental vibrational frequencies with calculated results by HF and DFT methods. Comparison of the simulated spectra provides important information about the capability of computational method to describe the vibrational modes. A study on the electronic properties, such as absorption wavelengths, excitation energy, dipole moment and Frontier molecular orbital energies, are performed by time dependent DFT approach. The electronic structure and the assignment of the absorption bands in the electronic spectra of steady compounds are discussed. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. On the basis of the thermodynamic properties of the title compound at different temperatures have been calculated. The statistical thermodynamic properties (standard heat capacities, standard entropies, and standard enthalpy changes) and their correlations with temperature have been obtained from the theoretical vibrations.     

采用优化的DFTB参数对铜(111)表面碳二聚化的分子动力学研究

针对铜表面化学反应,我们发展了一套铜-碳体系的密度泛函紧束缚(DFTB)参数。测试结果表明这套参数可以很好的描述吸附铜或碳原子前后铜表面的几何结构和能量。基于这套参数,我们对Cu(111)表面的碳二聚化过程进行了分子模拟研究。即使在高温下,直接的分子动力学模拟也很难观察到碳二聚体的形成。这是因为高温下铜表面显著的结构弛豫一定程度上阻止了二聚化。为了研究高温下铜表面碳二聚化的机理,我们进行了赝动力学模拟。发现在二聚化的过程中,碳原子形成C-Cu-C桥状结构以后,会绕中间Cu原子转动,最后形成碳二聚体。1300 K下碳二聚化的自由能垒约0.9 eV。     

Density functional theory study on the identification of 3-[(2-morpholinoethylimino)methyl]benzene-1,2-diol

This study deals with the identification of a title compound, 3-[(2-morpholinoethylimino)methyl]benzene-1,2-diol by means of quantum chemical calculations. The optimized molecular structures, vibrational frequencies and corresponding vibrational assignments, thermodynamic properties, charge analyses, nuclear magnetic resonance (NMR) chemical shifts and ultraviolet-visible (UV-vis) spectra of the title molecule in the ground state were evaluated using density functional theory (DFT) with the standard B3LYP/6-311++G(d,p) method and basis set combination for the first time. Theoretical vibrational spectra of the title compound were interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results show that the obtained optimized geometric parameters (bond lengths, bond angles and bond dihedrals) and vibrational frequencies were observed to be in good agreement with the available experimental results. Moreover, the calculations of the electronic spectra, (13)C and (1)H chemical shifts were compared with the experimental ones. Furthermore, we not only simulated the frontier molecular orbitals (FMO) and molecular electrostatic potential (MEP) but also determined the transition states and energy band gaps, as well. It was found that charge analyses supported the evidences of MEP. Infrared intensities and Raman activities were also reported.     

FT-IR, UV-vis, 1H and 13C NMR spectra and the equilibrium structure of organic dye molecule disperse red 1 acrylate: a combined experimental and theoretical analysis

This study reports the characterization of disperse red 1 acrylate compound by spectral techniques and quantum chemical calculations. The spectroscopic properties were analyzed by FT-IR, UV-vis, (1)H NMR and (13)C NMR techniques. FT-IR spectrum in solid state was recorded in the region 4000-400 cm(-1). The UV-vis absorption spectrum of the compound that dissolved in methanol was recorded in the range of 200-800 nm. The (1)H and (13)C NMR spectra were recorded in CDCl(3) solution. The structural and spectroscopic data of the molecule in the ground state were calculated using density functional theory (DFT) employing B3LYP exchange correlation and the 6-311++G(d,p) basis set. The vibrational wavenumbers were calculated and scaled values were compared with experimental FT-IR spectrum. A satisfactory consistency between the experimental and theoretical spectra was obtained and it shows that the hybrid DFT method is very useful in predicting accurate vibrational structure, especially for high-frequency region. The complete assignments were performed on the basis of the experimental results and total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. Isotropic chemical shifts were calculated using the gauge-invariant atomic orbital (GIAO) method. A study on the electronic properties were performed by timedependent DFT (TD-DFT) and CIS(D) approach. To investigate non linear optical properties, the electric dipole moment μ, polarizability α, anisotropy of polarizability Δα and molecular first hyperpolarizability β were computed. The linear polarizabilities and first hyperpolarizabilities of the studied molecule indicate that the compound can be a good candidate of nonlinear optical materials.     

Calculation of nonadiabatic couplings in density-functional theory

This paper proposes methods for calculating the derivative couplings between adiabatic states in density-functional theory (DFT) and compares them with each other and with multiconfigurational self-consistent field calculations. They are shown to be accurate and, as expected, the costs of their calculation scale more favorably with system size than post-Hartree-Fock calculations. The proposed methods are based on single-particle excitations and the associated Slater transition-state densities to overcome the problem of the unavailability of multielectron states in DFT which precludes a straightforward calculation of the matrix elements of the nuclear gradient operator. An iterative scheme employing linear-response theory was found to offer the best trade-off between accuracy and efficiency. The algorithms presented here have been implemented for doublet-doublet excitations within a plane-wave-basis and pseudopotential framework but are easily generalizable to other excitations and basis sets. Owing to their fundamental importance in cases where the Born-Oppenheimer separation of motions is not valid, these derivative couplings can facilitate, for example, the treatment of nonadiabatic charge transfers, of electron-phonon couplings, and of radiationless electronic transitions in DFT.     

Electronic structure of a vapor-deposited metal-free phthalocyanine thin film

The electronic structure of a vapor-sublimated thin film of metal-free phthalocyanine (H2Pc) is studied experimentally and theoretically. An atom-specific picture of the occupied and unoccupied electronic states is obtained using x-ray-absorption spectroscopy (XAS), core- and valence-level x-ray photoelectron spectroscopy (XPS), and density-functional theory (DFT) calculations. The DFT calculations allow for an identification of the contributions from individual nitrogen atoms to the experimental N1s XAS and valence XPS spectra. This comprehensive study of metal-free phthalocyanine is relevant for the application of such molecules in molecular electronics and provides a solid foundation for identifying modifications in the electronic structure induced by various substituent groups.     

不饱和类硅烯H2C=SiNaF的DFT研究

用密度泛函理论方法, 在B3LYP/6-31+G(d, p)水平上研究了不饱和类硅烯H2C=SiNaF的结构. 结果表明, 不饱和类硅烯H2C=SiNaF共有四种平衡构型, 其中非平面的p-配合物型构型能量最低, 是不饱和类硅烯H2C=SiNaF存在的主要构型. 对平衡构型间异构化反应的过渡态进行了计算, 求得了转化势垒. 计算预言了最稳定构型的振动频率和红外强度.     

Theoretical study on gas-phase conformations and vibrational assignment of methylphenidate

The molecular geometry, the normal mode frequencies and corresponding vibrational assignments of methylphenidate in the ground state were performed by DFT/B3LYP level of theory using the 6-311++G(d, p) basis set. Harmonic vibrational frequencies were calculated. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The conformational stabilities and optimized geometrical parameters have been carried out with density functional theory with 6-311++G(d, p) basic set by the DFT/B3LYP method. The frequency calculations have been performed with DFT to study the vibrational properties and their dependence on the molecular conformation.     

不饱和类硅烯H2C＝SiMBr (M＝Li, Na)的DFT研究

采用密度泛函理论方法, 在B3LYP/6-311G (d,p)水平上研究了不饱和类硅烯H₂C＝SiMBr (M＝Li, Na)的结构. 结果表明, 不饱和类硅烯H₂C＝SiLiBr与H₂C＝SiNaBr各有三种平衡构型, 其中非平面的p-配合物型构型能量最低, 是这两种不饱和类硅烯存在的主要构型. 对平衡构型间异构化反应的过渡态进行了计算, 求得了转化势垒. 计算预言了最稳定构型的振动频率和红外强度.