Theoretical study of the dimerization of rhodanine in various tautomeric forms

The density functional theory (DFT) was used to calculate the relative stability of rhodanine dimers and the energy of intermolecular interaction in them. Analysis of the electron density showed hydrogen bonding in the dimers. The energies of individual hydrogen bonds were determined for the symmetrical dimers. The polarizable continuum model was used to calculate the solvation (hydration) energies of the structures studied. The effect of dimerization on the position of infrared absorption bands was demonstrated.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Density functional study of neutral allopurinol tautomeric forms

Optimized structures and total molecular energy density functional theory calculations for the 14 possible neutral allopurinol tautomers give the decreasing stability order for the three most stable forms. Several molecular and electronic structure properties, the stabilities in oxidation and reduction processes, the tautomeric equilibrium constants, and the IR vibrational spectra were obtained for these. Those properties corresponding to the ketonic forms are discussed and compared with the theoretical ones for the two most stable species of the isomer hypoxanthine. A noticeable agreement is found between the predicted properties and the experimental behavior known up to date for both isomers. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 200–206, 1999     

Ab initio study of the tautomeric forms of some quinolinediones

7,8-dihydroquinoline-4,5 (1H,6H)-dione (1) and 7,8-dihydroquinoline-2,5-(1H,6H)-dione (2) in their tautomeric oxo and hydroxy forms have been studied by ab initio Hartree-Fock calculations; tautomerization energies predict a more stable hydroxy structure having an intramolecular hydrogen bond for compound 1, whereas the oxo form is slightly-preferred for compound 2. Fourier Transform-Infra Red (FT-IR) spectra in CHCl₃ solution indicate that the predicted most stable tautomers in the vapour phase remain as such.     

MRD-CI studies of vertical excitation energies of unsaturated hydrocarbon molecules

Systematic MRD-CI calculations using the AM1 Hamiltonian have been carried out for two polyenes and eight aromatic hydrocarbons ranging from benzene to ovalene (C₃₂H₁₄). Twenty singlet–singlet excitation energies in these compounds were calculated and compared with experimental data and ab initio STO-3G results. On an absolute scale, the AM1/MRD-CI approach underestimates the excitation energies to states with dominant covalent character by an average of 1.1 eV, whereas the errors for ionic states are between ?1.0 and 1.0 eV. The STO-3G calculated data are much too high by ≈ 1 eV and ≈ 5 eV, respectively. The inclusion of σπ-correlation effects through second-order Epstein–Nesbet perturbation theory combined with the use of localized orbitals leads to a significant improvement of the ab initio calculated state energies. In an analogous AM1 treatment, negligible corrections for the σπ correlations are found, which is attributed to the implicit account in the parameters and approximation of the semiempirical Hamiltonian. The possible error sources of the calculational methods are discussed. © 1994 by John Wiley & Sons, Inc.     

Calculation of vertical excitation energies of closed-shell molecules in the CNDO and INDO approximations

The Green's function method for the calculation of vertical excitation energies is adapted to the CNDO and INDO approximations by introducing an effective interaction into the irreducible vertex part. The computational scheme is explicitly developed for closed-shell molecules and applied to H₂O, H₂CO, HCOOH, HCONH₂.     

Theoretical study of the lowest pi-->pi* excitation energies for neutral and doped polyenes

In earlier theoretical studies, it has been widely noticed that the electron correlation effect played an important role in determining the excitation energies of low-lying pi-->pi(*) excited states for neutral polyenes and their radical cations and dications. In this paper, neutral and doped polyene oligomers of medium to large sizes are investigated with the Pariser-Parr-Pople model, and the pi-electron correlation effect is fully taken into consideration by virtue of the density-matrix renormalization group method. The excitation properties in the polymer limit are also obtained by exponential extrapolation from the finite oligomers. The reasonable agreement of our results with the available experimental observations and advanced ab initio calculations is witnessed. It is also observed that while charge doping can significantly lower the exciting energy, the odd-charged oligomers show lower excitation energies than the even-charged ones.     

Mass spectral study of the occurrence of tautomeric forms of thiohydantoins

Mass spectrometry is used to evaluate the occurrence of thio-enol structures among the several possible tautomers of thiohydantoins and dithiohydantoins. Mass spectra of differently substituted thiohydantoins are examined looking for common mass spectral behaviors. Ion fragmentations from specific tautomers allow to predict the most stable thio-enol structure for both type of compounds. The mass spectrum of the alkylation product of 5,5- dimethyldithiohydantoin and the nuclear magnetic resonance spectra of the alkylation products of both 2- thiohydantoin and dithiohydantoin support the fact that the most likely thio-enolstructure is determined by the presence of one or two thio-carbonyl groups in the hydantoin molecule.     

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, He _n ⁺ ,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He ₄ ⁺ and He ₅ ⁺ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He ₂ ⁺ or He ₃ ⁺ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He ₂ ⁺ and He ₃ ⁺ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He ₂ ⁺ or He ₃ ⁺ are placed as “chromophores” into the centre of a larger He _n ⁺ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He ₂ ⁺ or He ₃ ⁺ cores.     

Theoretical prediction of vertical transition energies of diaminosilylenes and aminosubstituted disilenes

Vertical electronic transition energies of diaminosilylenes and their dimers (disilenes and nitrogen‐bridged) were investigated by ab initio and density functional calculations. A good linear correlation was found between the observed UV transition energies of various silylenes and disilenes and those of model compounds calculated using the CIS and TD–DFT methods. On the basis of these computations the experimental UV absorption maximum observed for the dimer of (i‐Pr₂N)₂Si: (λ_max 439 nm at 77 K), could be assigned to an Si? Si bonded dimer with an unusually long Si? Si distance of 2.472 Å, and the isomeric amino‐bridged cyclic dimer could be discarded. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1536–1541, 2001     

Time-dependent density functional theory predictions of the vertical excitation energies of silanones as models for the excitation process in porous silicon

Time-dependent density functional theory calculations with a proper treatment of the asymptotic form of the exchange-correlation potential have been performed on R(R')Si=O to predict vertical excitation energies. The species R(R')Si=O is used as a model for the binding of the -(R)Si=O chromophore to a porous silicon surface. The calculated vertical excitation energies are substantially lower than those determined previously and show that vertical excitation of the lone chromophore is possible for all types of substituents including electronegative ones with KrF laser excitation in contrast to other predictions. If the substituents are electropositive, the chromophore can also be excited by a nitrogen laser. These results, in concert with the effect of the porous silicon surface on the R(R')Si=O excited states, confirm our previous explanation of the photoluminescence of porous silicon as being due to the presence of Si=O chromophores and provide new insights into the photoexcitation process. The results show that the differences in the vertical and adiabatic excitation energies are strongly dependent on whether the substituents are electronegative or electropositive with the former leading to larger differences and the latter leading to smaller differences. The results for the energy differences are explained in terms of the changes in the Si=O bond length on vertical excitation and on the changes in bond angles, which are related to the ability of the Si center in the excited state to undergo an inversion process.     

Configuration interaction and density functional study of the influence of lithium cation complexation on vertical and adiabatic excitation energies of enones

The changes in the excited state energies of representative cyclic enones (cyclopentenone and cyclohexenone) induced by lithium ion coordination have been examined using ab initio and DFT methods. Quantitative estimates of the vertical triplet state energies were obtained using configuration interaction calculations at the CIS and CIS(D) levels with the 6‐31+G(d) basis. Inclusion of perturbative doubles corrections has a marked effect on the relative energies of the n–π* and π–π* triplet states. At both CI and CIS(D) levels, lithium complexation is predicted to raise the energy of the n–π* triplet state much more than the π–π* triplet. The trends obtained at the CIS(D) level are reproduced using B3LYP/6‐31+G(d) calculations. Adiabatic excitation energies were also computed by carrying out geometry optimization of the triplet states at the B3LYP level. While the separation between the geometry optimized n–π* and π–π* triplet states is very small for the parent enones, the π–π* triplet is clearly favored in the lithium complexes. These results suggest the possibility of reversing the reactive photoexcited state in enones through cation complexation. The conclusions provide a rationale for interesting variations in product distributions observed for enones in cation exchanged zeolites. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1598–1604, 2001     

Influence of excitation and solvation on the shift of tautomeric equilibrium. A quantum-mechanical study

Two quantum-mechanical models are proposed to described a shift of tautomeric equilibrium as a result of electronic excitation and change of environment. According to the first n PD MEP model which is used to estimate the relative solvation effect on the stability of tautomers in an excited state, the calculation of the interaction energy between a solvent (simulated by a set of n point dipoles, n PD) and an excited solute molecule is based on the molecular electrostatic potential (MEP) of the corresponding excited state. In the second n PDQ model, a solvent represented by a set of n point dipoles and quadrupoles (n PDQ) modifies the solute's hamiltonian via an electrostatic interaction contribution. Comparing the results of the calculation for isolated and solvated tautomers, the n PDQ model is used to estimate the influence of electronic excitation on the change of relative stability of tautomers existing in a solution. An application of both models to 2- and 4-oxopyridine predicts a shift of the tautomeric equilibria in their excited states in accordance with experimental evidence.     

Theoretical study of the mechanism of proton transfer in tautomeric systems: Alloxan

Semiempirical SCF-MO studies of tautomerism in alloxan preclude the possibility of direct proton transfer in the gas phase due to the strain in the four-centred transition state, in which the proton being transferred is forced to come close to the positively charged carbon atom at the opposite corner of the four-membered ring. However, in aqueous solution, the activation barrier reduces appreciably, not only due to reduction in strain, but also due to charge separation in the transition state, which is stabilized due to ionic resonance. The N-H bond is almost broken, while the O-H bond is only partially formed in the transition state. The other stabilizing effect in aqueous solution is due to bulk solvent dielectric effects, which stabilize the transition state to a greater extent due to its higher dipole moment. Although the transition states for proton transfer to the neighbouring oxygen atoms on either side have comparable energies, as the mechanisms of proton transfer leading to the formation of the 2-hydroxy and 4-hydroxy tautomers are similar, bulk solvent effects are larger in the latter due to the higher dipole moment of the transition state. The reason is the almost complete separation of the two entities, i.e. the alloxan anion and the hydronium ion in the latter case, indicating that in this case a dissociative mechanism of the kind encountered in acid-base equilibria is operating.     

Tautomeric forms of azolide anions: vertical electron detachment energies and Dyson orbitals

Several decouplings of the electron propagator, including the relatively new P3+ approximation for the self-energy, have been used to calculate vertical electron detachment energies of tautomeric forms of closed-shell, pentagonal, aromatic anions in which ring carbons without bonds to hydrogens appear. This study extends previous work in which the most stable forms of anionic, five-member rings with one to five nitrogens were considered. Whereas the lowest electron detachment energies sometimes are assigned by Koopmans's theorem results to pi orbital vacancies, electron propagator calculations always obtain sigma orbital vacancies for the ground states of the doublet radicals. Higher electron detachment energies that correspond to excited doublets with pi vacancies also are presented. The predicted transition energies are in good agreement with low-intensity peaks in recent anion photoelectron spectra that have been assigned to less stable, tautomeric forms of these anions.     

Theoretical study on the excitation energies of six tautomers of guanine: evidence for the assignment of the rare tautomers

The CASPT2//CASSCF method with the 6-31G basis set and an active space up to (16,12) was used to calculate the excitation energies for six tautomers of guanine. Our calculations provide further support on the recent reassignment of the near-UV resonant two-photon ionization (R2PI) spectrum, in which two rare tautomers of the 7H-oxo-imino form were proposed to replace the previously assigned 7H/9H-oxo-amino tautomers. The adiabatic excitation energies of the 7H-oxo-imino tautomers are calculated to be 0.3-0.5 eV higher than those of the 7H/9H-oxo-amino tautomers. Our calculations also indicate that the missing most stable tautomers (7H/9H-oxo-amino tautomers) in the R2PI experiment is possibly due to the existence of an ultrafast nonradiative deactivation process in the excited-state of these two tautomers.     

Gas-phase tautomeric equilibrium of 4-hydroxypyrimidine with its ketonic forms: a free jet millimeterwave spectroscopy study

4-Hydroxypyrimidine (4HP) has two conformational forms (the hydroxyl hydrogen cis or trans with respect to the adjacent nitrogen), which are in tautomeric equilibrium with two ketonic forms, 4-pyrimidinone (4PO) and 6-pyrimidinone (6PO). We have investigated the free jet absorption millimeterwave spectrum of this system, assigning the rotational spectra of 4HPcis and 4PO; the latter species is more stable by 2.0(9) kJ/mol. No lines corresponding to the trans isomer of 4-hydroxypyridine and to 6PO have been observed.