Ab initio derivative calculation of vibrational circular dichroism

The recent formalism of Stephens for the calculation of rotational strength in vibrational circular dichroism has been implemented, exploiting the analytical derivative technique for ab initio Hartree-Fock wavefunctions.     

Ab initio prediction of vibrational spectra: A database approach

Efforts to develop a database of quadratic force fields for organic molecules are described. The database is based on systematic ab initio calculations, scaled to reproduce the experimentally observed frequencies. The choice of the theoretical method, the basis sets, geometries, internal coordinates and the scaling procedure are discussed. A key point in the procedure is the automatic generation of the internal valence coordinates. This is also very advantageous for geometry optimization. The database should permit the prediction of vibrational frequencies for most organic molecules to 10–20 cm⁻¹, together with semiquantitative intensities. The accuracy is sufficient to identify unknown compounds from a list of reasonable candidates.     

DNA oligonucleotide-cis-platin binding: Ab initio interpretation of the vibrational spectra

The cis-platin binding to the d(CCTGGTCC)*d(GGACCAGG) model DNA octamer was monitored with infrared absorption (IR) and vibrational circular dichroism (VCD) spectroscopies. The spectra were modeled with the aid of density functional computations and a Cartesian coordinate-based transfer of molecular property tensors from smaller DNA fragments. Because of the fragmentation, the tensors could be calculated with a higher precision. Environmental effects, such as the presence of the solvent or the cis-platin ligand, could be included in the modeling. The solvent was modeled by an explicit inclusion of hydrogen-bound water molecules, positions of which were estimated from a molecular dynamics simulation, or by the polarized continuum COSMO model. The B3LYP and BPW91 functionals used for the calculations of the spectral parameters were combined with the relativistic LANL2DZ platinum pseudo-potentials. The simulations reproduced the main IR and VCD DNA spectral features and explained most of the changes observed experimentally upon metal binding. The results confirmed that the influence of the ligand on DNA vibrational properties is quite complex; it originates in the geometry deformation and normal mode coupling pattern changes of the platinated octamer, as well as in local perturbations of the electronic structure and force field of the GC base pairs to which the platinum is bound. Many of the local effects could be accounted for by a point charge used in place of the metal in the GC complex.     

Ab initio calculation of the rotational spectrum of methane vibrational ground state

In a previous article we have introduced an alternative perturbation scheme to the traditional one starting from the harmonic oscillator, rigid rotator Hamiltonian, to find approximate solutions of the spectral problem for rotation-vibration molecular Hamiltonians. The convergence of our method for the methane vibrational ground state rotational energy levels was quicker than that of the traditional method, as expected, and our predictions were quantitative. In this second article, we study the convergence of the ab initio calculation of effective dipole moments for methane within the same theoretical frame. The first order of perturbation when applied to the electric dipole moment operator of a spherical top gives the expression used in previous spectroscopic studies. Higher orders of perturbation give corrections corresponding to higher centrifugal distortion contributions and are calculated accurately for the first time. Two potential energy surfaces of the literature have been used for solving the anharmonic vibrational problem by means of the vibrational mean field configuration interaction approach. Two corresponding dipole moment surfaces were calculated in this work at a high level of theory. The predicted intensities agree better with recent experimental values than their empirical fit. This suggests that our ab initio dipole moment surface and effective dipole moment operator are both highly accurate.     

Ab initio calculation of the electronic structure of TCNQ and its ions

Ab initio SCF-LCAO-MO calculations have been performed for TCNQ and its positive and negative ions in various electronic states. A basis set consisting of 412 primitive Gaussian type orbitals contracted to 180 was used in the investigation. The electron density distribution in TCNQ and the negative ions, and the redistribution upon ionization has been illustrated by plotting difference density contour maps. The quinone structure of the neutral TCNQ system undergoes a transformation to a benzenoid structure when electrons are added. Electronic transitions, ionization potentials and electron affinities have also been calculated.     

Ab initio and DFT studies of the molecular structures and vibrational spectra of succinonitrile

The Molecular structure, conformational stability and vibrational frequencies of succinonitrile NCCH2CH2CN have been investigated with ab initio and density functional theory (DFT) methods implementing the standard 6-311++G* basis set. The potential energy surfaces (PES) have been explored at DFT-B3LYP, HF and MP2 levels of theory. In agreements with previous experimental results, the molecule was predicted to exist in equilibrium mixture of trans and gauche conforms with the trans form being slightly lower in energy. The vibrational frequencies and the corresponding vibrational assignments of succinonitrile in both C2h and C2 symmetry were examined theoretically and the calculated Infrared and Raman spectra of the molecule were plotted. Observed frequencies for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311++G* basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.     

Ab initio study of the vibrational spectra of the hydrogen-bonded nitric acid dimer.

The changes in the vibrational characteristics characterizing the dimerization of nitric acid have been investigated by ab initio calculations at the MP2 level, with 6-31G(d,p) and 6-31 + G(d,p) basis sets, and B3LYP/6-31G(d,p) calculations. The most consistent agreement between the computed values of the frequency shifts for the planar fully symmetric structure (2A) and those experimentally observed suggests that this structure is preferred. It was established that the most sensitive to the complexation is the stretching O-H vibration. The values of the frequency shift (-306 cm(-1)) is indicative for the formation of the relatively strong hydrogen bonds. The calculations predict an increase of the infrared intensity of the stretching O-H vibration in the nitric acid dimer more than 26 times.     

Ab initio calculations of the vibrational spectra of the ammonia and fluoramide complexes with HF

The structures, energetics, vibrational frequencies and IR intensities of the H₃N HF, H₃N F₂ and NH₂FHF (three isomers) complexes were examined using the self-consistent field method within the 6-311G^** basis set. The interaction energies were calculated using the MP2 approach. The results are compared with monomer calculations and experimental data. The complex NH₂FHF was found to exist in three forms: one with the HF molecule hydrogen bonded to the nitrogen lone pair of NH₂F (D₀ =7.403 kcal mol⁻¹), another a complex formed through the F atom lone pair (D₀=4.698 kcal mol⁻¹) and third a cyclic structure (D₀=5.644 kcal mol⁻¹).     

Ab initio calculation of some low-lying electronic excited states of methane

The energies of some low-lying electronic excited states of methane are calculated by using wave functions built up in terms of plane waves modulated by multicenter Gaussian factors. The wave functions of the various states are evaluated by a two steps iterative process. In the first step, each excited orbital is determined while keeping all other rigid; in the second, rearrangement effects are introduced. Final results are in good agreement with experimental data and allow to enhance an assignement hypothesis for the first electronic transitions.     

Ab initio study of electronic spectra of merocyanine 540 and its photoproducts

Molecular structures of three derivatives of merocyanine 540 (MC 540) were studied using the density functional method in conjunction with the 6‐31G*, 6‐31G**, and 6‐311G** basis sets. The excited states were calculated using the configuration interaction method involving singly excited configurations (CIS). The predicted transition energies and oscillator strength agree well with the experimental UV adsorption spectra of the studied systems. The existence of two stable conformers of merocyanine explain the experimentally observed dependence of the UV spectra upon the change of concentration of added salts. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Ab initio studies on the vibrational spectra and conformational isomerism of 2,4-dimethyl-1,3-pentadiene

Ab initio was used to study the structure of various conformational isomers and their vibrational spectra of 2,4-dimethyl-1,3-pentadiene (2,4-PD) in detail. Two stable conformations, s-trans and s-cis, were found in which s-trans is more stable. The geometry of stable conformations and charge distributions were studied, and the effect of different basis sets on geometry optimization is discussed. The results of complete optimization indicate that molecular skeleton is nearly in a plane, its largest deviation is only 0.3 degrees. Therefore, it is reasonable and available to hypothesis that the molecule has Cs symmetry. The thermal dynamics conformations were calculated and compared with experimental values. DeltaH(o) between two conformations of 2,4-PD measured from experiment is 4.36 kJ/mol, deltaS(o) is 2.56 J/mol K., calculated results are slightly different from experimental ones. Vibrational frequencies of 2,4-PD conformers have been studied by ab initio molecular orbital calculations using different basis set. The calculated vibrational frequencies are analyzed and compared with the experimental spectra.     

Ab initio studies of solvent effect on molecular conformation and vibrational spectra of diacetamide

The conformational equilibria and vibrational spectra of diacetamide have been investigated by ab initio molecular orbital studies using the basis sets 6-31g(d,p) and 6-31++g(d,p) at Hartree-Fock and MP2 levels. The vibrational spectra of diacetamide have been satisfactorily interpreted taking into consideration the agreement between the calculated harmonic vibrational frequencies, infrared and Raman band intensities and shifts in deuterated molecules with those observed. The solvent effects were investigated by the self-consistent reaction field (SCRF) theory. The effect of solvent on the conformational equilibria and vibrational spectra is discussed. The calculated changes in the geometry and vibrational spectra on going from the gas phase to the solvent medium are in accord with the increasing weight of the dipolar resonance structure of the amide group in more polar solvents.     

Octa-1,3,5,7-tetraene: Ab initio geometries and vibrational spectra of some stable structures

Complete geometry optimizations of trans, Trans, trans, Trans, trans-, gauche, Trans, trans, Trans, trans-, trans, Trans, trans, Cis, trans-, and gauche, Trans, trans, Cis, trans-octa-1,3,5,7-te-traenes were carried out at the RHF/6-31G level. Characteristic changes in the geometry are found in going from the planar conformers of octatetraene to the corresponding higher-energy stable forms. The harmonic force constants were computed for the above conformers at the RHF/6-31G//RHF/6-31G level using analytical second derivatives. The computed force fields of these four molecules were then corrected using empirical scale factors transferred fromtrans-buta-1,3-diene. To account for the vibronic coupling effect, which may be a characteristic of oligoenes, a special scale factor was introduced for the two internal coordinates which correspond to stretching the central C=C double bonds. A complete assignment of the experimental spectra oft, T, t, T, t-octatetraene is also given.On leave from Laboratory of Molecular Spectroscopy, Department of Chemistry, Moscow State University, Moscow 119899, U.S.S.R.Preliminary results were reported at the Thirteenth Austin Symposium on Molecular Structure, Austin, TX, USA March 12–14, 1990, S 5, p. 91. and at the Second World Congress of the Theoretical Organic Chemists, Toronto, Canada July 8–14, 1990, BP-35 (Canada).     

Ab initio and DFT studies of the vibrational spectra of benzofuran and some of its derivatives

The vibrational spectra of benzofuran and some of its derivatives have been systematically investigated by ab initio and density functional B3LYP methods. The harmonic vibrational wavenumbers and intensity of vibrational bands were calculated at ab initio and DFT levels invoking different basis sets up to 6-311++g**. Vibrational assignments have been made and it has been found that the calculated DFT frequencies agree well in most cases with the observed frequencies for each molecule. Conformational studies have also been carried out and it is evident from ab initio calculations that 2(3H) benzofuranone is more stable than 3(2H) benzofuranone in support to our earlier semiempirical results.     

Ab initio calculation of electronic circular dichroism fortrans-cyclooctene using London atomic orbitals

Summary The second-quantization magnetic dipole operator that arises when London atomic orbitals are used as basis functions is derived. In atomic units, the magnetic dipole operator is defined as the negative of the first derivative of the electronic Hamiltonian containing the interaction with the external magnetic field. It is shown that for finite basis sets, the gauge origin dependence of the resulting magnetic dipole operator is analogous to that of the exact operator, and that the derived operator converges to the exact operator in the limit of a complete basis set. It is also demonstrated that the length expression for the rotatory strength in linear response calculations gives gauge-origin-independent results. Sample calculations ontrans-cyclooctene and its fragments are presented. Compared to conventional orbitals, the basis set convergence of the rotatory strengths calculated in the length form using London atomic orbitals is favourable. The rotatory strength calculated fortrans-cyclooctene agrees nicely with the corresponding experimental circular dichroism spectrum, but the spectra for the fragment molecules show little resemblance with that oftrans-cyclooctene.Dedicated to Prof. Jan Linderberg     

Ab initio spectrograms of the molecular vibrational spectrum

Theoretical spectrograms of the vibrational spectrum of 3,3-dimethylcyclopropene were constructed and juxtaposed with the experimental Raman and IR spectrograms. The theoretical spectrograms are represented as sets of vertical lines starting from the points corresponding to the values of the vibrational frequencies calculated from the scaled quantum-mechanical (QM) force field obtained at the HF/6-31G*//HF/6-31G* level. Two theoretical Raman spectrograms were constructed. In the first case, the heights of the vertical lines correspond to the QM values of the Raman scattering activities. In the second case they represent the relative differential Raman cross-sections calculated using the QM values of Raman scattering activities. The initial vibrational mode matrix remains virtually unchanged upon scaling of the QM force constant matrix because the dispersion of the scale factor values is low. Therefore, the heights of the theoretical lines for the IR spectrogram represent the QM intensities directly. The theoretical spectrogram based on the relative differential Raman cross-sections was shown to depict the experimental Raman spectrum more adequately. This makes it possible to use the results of the corresponding QM calculations more completely and obtain well-substantiated assignments of the vibrational frequencies.     

Ab initio investigation of the vibrational hyper-Raman spectra of ethylene, ethane, and dimethyl ether

The time-dependent Hartree–Fock approach is employed with the aug-cc-pVDZ or 6-311++G** basis set to simulate, in the harmonic approximation, the hyper-Raman spectra of ethylene, ethane, and dimethyl ether. Comparison with the experiment of Verdieck etal. [(1970) Chem. Phys. Lett. 7:219] is performed for ethylene and ethane. Effects of the polarization of the incident light and the detected light are studied for ethylene. Special focus is given on these vibrational normal modes that cannot be detected in IR and Raman spectroscopies.Contribution to the Jacopo Tomasi Honorary Issue     

Ab initio vibrational spectra and dielectric properties of carbonates: magnesite, calcite and dolomite

The equilibrium geometry, the Raman and IR vibrational spectra at the Γ point, TO–LO splitting, IR intensities, Born and dielectric tensors of magnesite MgCO₃, dolomite MgCa(CO₃)₂ and calcite CaCO₃ have been calculated with the periodic ab initio program CRYSTAL, by using an all-electron gaussian type basis set and the B3LYP hamiltonian. LO (longitudinal-optical) modes are computed by correcting the dynamical matrix through Born charges and high frequency dielectric tensors obtained from well localized Wannier functions and a saw-tooth computational scheme. The mean absolute difference between calculated and experimental frequencies (IR TO and LO and RAMAN) is as small as 6.9 cm⁻¹ for magnesite, 7.7 cm⁻¹ for dolomite and 8.5 cm⁻¹ for calcite. Calculated IR intensities are in semiquantitative agreement with experiment. The modes of the three compounds are compared through graphical animation available on the CRYSTAL web-site.     

Ab initio SCF Calculation of the Fluoronium Ion: Geometry,electronic structure and vibrational constants

An ab initio SCF calculation of 42 points of the energy hypersurface of the fluoronium ion is presented using a contracted F(5s/3p), H(2s) gaussian basis set. In its equilibrium structure a bond length of 1.812 a.u. and a HFH bond angle of 127.2° are predicted. The calculated vibrational frequencies for H₂F⁺, HDF⁺, and D₂F⁺ are in good agreement with the experimental data.