Ab initio study of the decomposition of 2,5-dimethylfuran

The initial steps in the thermal decomposition of 2,5-dimethylfuran are identified as scission of the C-H bond in the methyl side chain and formation of β- and α-carbenes via 3,2-H and 2,3-methyl shifts, respectively. A variety of channels are explored which prise the aromatic ring open and lead to a number of intermediates whose basic properties are essentially unknown. Once the furan ring is opened demethylation to yield highly unsaturated species such as allenylketenes appears to be a feature of this chemistry. The energetics of H abstraction by the hydroxyl radical (and other abstracting species) from a number of mono- and disubstituted methyl furans has been studied. H-atom addition to 2,5-dimethylfuran followed by methyl elimination is shown to be the most important route to formation of the less reactive 2-methylfuran. Identification of 2-ethenylfuran as an C(6)H(6)O intermediate in 2,5-dimethylfuran flames is probably not correct and is more likely the isomeric 2,5-dimethylene-2,5-dihydrofuran for which credible formation channels exist.     

Ab initio study of the tautomerism of 2,5-substituted diazoles

Detailed investigation of the tautomerism of oxadiazoles, thiadiazoles, and selenadiazoles containing hydroxy, mercapto, or selenyl groups in position 2 and 5 of the five-membered ring was carried out at MP2, MP4, and CCSD(T) levels of theory. The relative stabilities of the tautomers of the heterocycles were investigated in gas phase. It is found that the dioxo form is the preferred tautomer if the exocyclic group is hydroxy. If the exocyclic group is mercapto or selenyl group the most stable tautomer is the mixed mercapto/thione or selenyl/selone form. The barriers of intramolecular proton transfer are too large and tautomerization should not occur.     

Ab initio rotation-vibration spectra of HCN and HNC

We have calculated an ab initio HCN/HNC linelist for all transitions up to J= 25 and 18000 cm(-1) above the zero point energy. This linelist contains more than 200 million lines each with frequencies and transition dipoles. The linelist has been calculated using our semi-global HCN/HNC VQZANO + PES and dipole moment surface, which were reported in van Mourik et al. (J. Chem. Phys. 115 (2001) 3706). With this linelist we synthesise absorption spectra of HCN and HNC at 298 K and we present the band centre and band transition dipoles for the bands which are major features in these spectra. Several of the HCN bands and many of the HNC bands have not been previously studied. Our line intensities reproduce via fully ab initio methods the unusual intensity structure of the HCN CN stretch fundamental (00(0)1) for the first time and also the forbidden (02(2)0) HCN bending overtone. We also compare the J = 1-->0 pure rotational transition dipole in the HCN/HNC ground and vibrationally excited states with experimental and existing ab initio results.     

Ab initio CI study and vibronic analysis of the photoelectron spectra of formaldoxime

The photoelectron spectrum of formaldoximie, CH₂NOH, has been re-investigated with higher resolution and interpreted by, means of ab initio SCF Cl calculations. Calculations have confirmed that the states increase in energy as π₁ < n₁ < π₂ < n₂ and have shown the existence of a shake-up peak at ≈15 eV. The calculation of Franck-Condon factors allowed the interpretation of the observed vibrational structure.     

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     

The vibrational structure of the oxygen K-shell spectra in acenaphthenequinones: an ab initio study

The vibrational structure of the K-shell O1s → π? of acenaphthenequinone C(12)H(6)O(2) and its halogenated compound C(12)H(2)Br(2)Cl(2)O(2) has been simulated using an entirely ab initio approach. For both molecules, analysis of the calculated Franck-Condon factors confirm without ambiguity that, contrary to initial claims, the C-H stretching modes are not modified in the core states and are not excited. For C(12)H(6)O(2), the vibrational fine structure appears to be mainly due to three modes, involving C=O? asymmetric stretch and in-plane ring deformation modes, due to the symmetry breaking of the core state. For C(12)H(2)Br(2)Cl(2)O(2), the vibrational excitation arises essentially from the C=O? asymmetric stretch, with numerous secondary peaks arising from hot and combination bands. For both molecules, these bands are probably responsible for the asymmetry deduced in the experimental fits using a unique Morse potential and initially assigned to anharmonic effects.     

Polymerization behaviour of 2,5-dimethylene- 2,5-dihydrofuran

Spontaneous homopolymerization of 2,5-dimethylene-2,5-dihydrofuran (DDF) was studied. The polymerization rates in two different initial monomer concentrations of DDF were analyzed with the first-order and second-order kinetics, and the homopolymerization of DDF was found to obey the first-order kinetics. The Arrhenius plot of the apparent rate constants at 30, 40, 50, and 60° gave an overall activation energy of 68.0 kJ/mol for the polymerization of DDF. From the comparison of the apparent rate constants at –78° and the time (the so-called half-life time) to decrease in half the monomer concentration for DDF with the corresponding values for p-xylylene (QM), DDF was found to be a less reactive monomer than QM. The copolymerizations of DDF with vinyl monomers such as acrylonitrile (AN), α-chloroacrylonitrile (CIAN), diethyl fumarate (DEF), and fumaronitrile (FN) were carried out in chloroform at 50° in the presence of AIBN to obtain the monomer reactivity ratios r₁(DDF) = 30.0 ± 3.0 and r₂ (AN) = 0 for the DDF-AN system, r₁ (DDF) = 1.55 ± 0.2 and r₂(CIAN) = 0 for the DDF-CIAN system, r₁(DDF) = 3.88 ± 0.2 and r₂(DEF) = 0 for the DDF-DEF system, and r₁(DDF) = 2.41 ± 0.1 and r₂ (FN) = 0 for the DDF-FN system, respectively. As the monomer reactivity ratios of r₂ for all systems were zero, Q and e values of DDF were calculated from the combination of two r₁ (DDF) values of any two copolymerization systems to be the 7.64 to 6.63 ×10²¹ range for Q and the –0.70 to –6.31 range for e, indicating that DDF is a highly conjugative and electron-donating monomer. © 1995 John Wiley & Sons, Inc.     

Vibrational assignment and analysis for 2,3-dihydrofuran and 2,5-dihydrofuran

The vibrational spectra of 2,3-dihydrofuran and 2,5-dihydrofuran have been recorded using IR and Raman spectroscopy for the gas, liquid and solid states. A vibrational assignment consisting of a nearly complete set of vapor phase wavenumbers is proposed for both molecules based on the observed spectra and normal coordinate analyses. The normal coordinate analyses have been made by scaling the AM1 force field for each molecule with scale factors transferred from an analysis of the cyclopentene fundamental vibrations. The predicted a priori vibrational frequencies justify one reassignment of the fundamentals for 2,5-dihydrofuran from that previously reported. The vibrational assignment for 2,3-dihydrofuran is reported for the first time. Thermodynamic functions are computed for each molecule using the experimentally determined vibrational frequencies.     

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.     

Protolytic cleavage of derivatives of 2,5-dimethoxy-2,5-dihydrofuran

By analysis of the products from the protolytic cleavage of methyl 3-(2,5-dialkoxy-2,5-dihydro-2-furyl)propionate (I) in the presence of mixed nucleophiles and investigation of their mutual transformations the main paths in the dissociation of the protonated molecule of (I), with the formation of both linear cleavage products (derivatives of 1,4-diketones and 4-ketopimelic acid) and furylcarbinol derivatives, were determined.Latvian Institute of Organic Synthesis. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 190–197, February, 1997.     

Ab initio and semiempirical study of structure and electronic spectra of hydroxy substituted naphthoquinones

The geometries of hydroxy derivatives of 1,4-naphthoquinone (NQ), viz., 2-hydroxy-1,4-naphthoquinone (2HNQ), 5-hydroxy-1,4-naphthoquinone (5HNQ), and 5,8-dihydroxy-1,4-naphthoquinone (DHNQ), have been optimized using the semiempirical and ab initio theoretical methods. Semiempirical methods used for the optimization are Austin Model 1 (AM1) and Zerner's Intermediate Neglect of Differential Overlap/1(ZINDO/1). For ab initio calculations the 6-31G* basis set is used. The electronic spectra of 1,4-naphthoquinone and its hydroxy derivatives are calculated using the semiempirical Zerner's Intermediate Neglect of Differential Overlap/Spectroscopy (ZINDO/S) method employing the geometries optimized at AM1, ZINDO/1 and ab initio levels and compared with their electronic absorption spectra measured by us. For hydroxy substituted systems, such calculations for spectral assignments are made for the first time. It is found that though the predictions of the three theoretical methods for the geometries are similar, the predictions of the ZINDO/S method using the ZINDO/1 optimized geometries, are better for the transition wavelengths in the visible region of the hydroxy substituted naphthoquinones, especially for 5HNQ and DHNQ.     

Ab initio calculations for the photoelectron spectra of vanadium clusters

We report ab initio calculations for the electronic and structural properties of V(n), V(n) (-), and V(n) (+) clusters up to n=8. We performed the calculations using a real-space pseudopotential method based on the local spin density approximation for exchange and correlation. This method assumes no explicit basis. Wave functions are evaluated on a uniform grid; only one parameter, the grid spacing, is used to control convergence of the electronic properties. Charged states are easily handled in real space, in contrast to methods based on supercells where Coulombic divergences require special handling. For each size and charge state, we find the lowest energy structure. Our results for the photoelectron spectra, using the optimized structure, agree well with those obtained by experiment. We also obtain satisfactory agreement with the measured ionization potential and electron affinity, and compare our results to calculations using an explicit basis.     

Ab initio study of KN

The potential energy curves for the lowest (3)Sigma(-), (3)Pi, and (5)Sigma(-) states of the KN molecule have been calculated by the multireference singles and doubles configuration interaction method, including Davidson's corrections for quadruple excitations [MRCI(+Q)]. It is shown that the former two are bound, while the last one is repulsive. The electronic ground state of KN is predicted as (3)Sigma(-) state, although the term energy of the (3)Pi state is very small, 177.3 cm(-1). The binding energy for the (3)Sigma(-) state is evaluated as 0.838 eV, the rotational constant B(0) as 0.250 63 cm(-1), and harmonic frequency as 324.4 cm(-1). The spin-orbit coupling effects between the (3)Sigma(-) and (3)Pi states of KN are evaluated and discussed. The same MRCI(+Q) computational procedures are applied to the isovalent LiN, KC, KO, and KCl to confirm the accuracy of present calculations. Theoretical spectroscopic constants presented here will inspire experimental studies of KN.     

Ab initio investigation on the structures and spectra of the firefly luciferin

The ground state (S0) geometry of the firefly luciferin (LH2) was optimized by both DFT B3LYP and CASSCF methods. The vertical excitation energies (Tv) of three low-lying states (S1, S2, and S3) were calculated by TD-DFT B3LYP//CASSCF method. The S1 geometry was optimized by CASSCF method. Its Tv and the transition energy (Te) were calculated by MS-CASPT2//CASSCF method. Both the TD-DFT and MS-CASPT2 calculated S1 state Tv values agree with the experimental one. The IPEA shift greatly affects the MS-CASPT2 calculated Tv values. Some important excited states of LH2 and oxyluciferin (oxyLH2) are charge-transfer states and have more than one dominant configuration, so for deeply researching the firefly bioluminescence, the multireference calculations are desired.     

Ab initio study of the cyclooctatetraenyl radical

Ab initio calculations have been carried out on the 1,3,5,7- and 1,2,4,7-tetraene configurations of the cyclooctatetraenyl radical at UHF, ROHF, MCSCF, ROCISD, QCISD, and CCSD(T) levels of theory with 6-311G(d,p) and cc-pVDZ basis sets. Although spin contamination is present, the ROCISD calculations support the energies obtained from less intensive, UHF-based coupled cluster calculations over the energies obtained from MCSCF analysis of the pi-electron orbitals. The 1,3,5,7-form is a local minimum at the coupled cluster levels, higher in energy than the resonance-stabilized 1,2,4,7-form by 10-13 kJ/mol, but bounded by a barrier of less than 0.5 kJ/mol. The isomerization surface connecting these two structures is described and results reported from integration of the vibrational Schr?dinger equation on that surface. Excited vibrational states at energies just above the isomerization barrier are dominated by the character of the 1,3,5,7-tetraenyl radical, which suggests that chemistry involving this intermediate at typical combustion temperatures may branch at this juncture.     

An Ab initio molecular orbital study of the systematics in the photoelectron spectra of halomethanes

Ab initio molecular orbital calculations on fluoro- and chloro-methanes, CH_4—nX_n, predict the correct trends in the photoelectron spectra except in the case of the C(2s) bands of fluoromethanes. Thus, the calculated energies corresponding to the lone pair, σ (CX) and C(1s) bands increase with increasing n as found experimentally; the C(2s) energy in chloromethanes decreases with increasing n, again, in agreement with experiment.     

Ab initio calculations of the ultraviolet resonance Raman spectra of uracil

An equation been derived to calculate, ab initio, the frequencies and intensities of a resonant Raman spectrum from the transform theory of resonance Raman scattering. This equation has been used to calculate the intensities of the ultraviolet resonance Raman spectra from the first π-π* excited state of uracil and 1,3-dideuterouracil. The protocol for this calculation is as follows: (1) The force constant matrix elements in Cartesian coordinate space, the vibrational frequencies, and the minimum energy ground and excited state geometries of the molecule are calculated ab initio using the molecular orbital program Gaussian 92, (2) the force constants in Cartesian coordinates are transformed into force constants in the space of a set of 3N – 6 nonredundant symmetrized internal coordinates, (3) the G matrix is constructed from the energy minimized ground state Cartesian coordinates and the GFL = LΛ eigenvalue equation is solved in internal coordinate space, (4) the elements of the L and L^?1 matrices are calculated, (5) the changes in all of the internal coordinates in going from the ground to the excited state are calculated, and (6) these results are used in combination with the transform theory of resonance Raman scattering to calculate the relative intensities of each of the 3N – 6 vibrations as a function of the exciting laser frequency. There are no adjustable parameters in this calculation, which reproduces the experimental frequencies and intensities with remarkable fidelity. This indicates that the Dushinsky rotation of the modes in the excited state of these molecules is not important and that the simplest form of the transform theory is adequate. © 1995 John Wiley & Sons, Inc.     

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 study of lithiated N-methylpyridones

Ab initio calculations were performed on the N-methylpyridones lithiated on the aromatic ring using a 6-31G* basis set. Whenever the lithium atom is on a carbon adjacent to the carbonylic group, a bridged structure is obtained where lithium is coordinated to both carbon and oxygen; these structures are the most stable isomers. © 1996 John Wiley & Sons, Inc.