Spectra-structure relationships in the near ultraviolet optical activity of chiral barbituric acid derivatives

The chiroptical properties associated with then-π^* (singlet-singlet) transitions in dissymmetric barbituric acid derivatives are examined on the basis of two theoretical models. The lower singlet excited states of unsubstituted and alkyl substituted barbituric acids are calculated on the semi-empirical CNDO/S-CI molecular orbital model, and the spectroscopic properties associated with transitions to these states are computed. In the structures we examined, threen-π^* transitions are found at λ>220 nm, two of which are nearly degenerate. Each of these transitions is computed to be strongly magnetic dipole allowed and to be forbidden or very weak (depending upon the exact symmetry and geometry of the trioxopyrimidine moiety) in electric dipole radiation. Contributions from chiral distortions within the trioxopyrimidine chromophoric system to the rotatory strengths of the three lowest energyn-π^* transitions are calculated directly from wave functions obtained by the CNDO/S-CI method. Contributions to then-π^* rotatory strengths arising from “vicinal” interactions between the trioxopyrimidine chromophore and asymmetric substituent groups are calculated by a perturbation method based on an independent systems representation of the optically active compounds. Various spectra-structure relationships are considered and correlations between experimental data and theoretically calculated results are examined.     

Theoretical study of the valence π → π* excited states of polyacenes: anthracene and naphthacene

The valence π → π ^* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon spectra and T-T spectra of low-lying valence π → π ^* excited states of anthracene and naphthacene. The present theory predicts the valence π → π ^* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions, but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can be used as a model for understanding the excited states of larger polyacenes. Received: 22 April 1998 / Accepted: 6 July 1998 / Published online: 28 September 1998     

Calculation of the vibronic spectra of adenine

The vibrational structure of the absorption spectra of the first two π-π* singlet transitions of adenine is calculated in the Franck-Condon approximation including Herzberg-Teller interactions. The effect of excitation-induced changes in molecular angles on the intensities of the vibrational components is estimated. Structural models of the adenine molecule in the excited states are constructed. The theoretical and absorption spectra of the first π-π* transition are compared. The results of the electronic structure calculations of adenine by different CNDO/S methods are discussed. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 334–344, March–April, 1997.     

Triplet states of furan,thiophene, and pyrrole

Low-lying triplet electronic states have been detected in furan, thiophene, and pyrrole by the method of variable-angle, electron-impact spectroscopy. Singlet → triplet transitions occur with maximum intensity at 3.99 eV and 5.22 eV in furan, 3.75 eV and 4.62 eV in thiophene, and 4.21 eV in pyrrole. A weak transition at 5.22 eV in pyrrole is assigned as the lowest observed singlet → singlet excitation in that molecule.     

The Effect of cis-trans Isomerism on the Electronic-vibrational Structure of the Ground and Excited States and Reactivity of 2-(2-Furyl)- and 2-(2-Thienyl)oxazole

The fluorescent properties, structure, and electronic structure of the ground and excited singlet and triplet electronic states of the cis and trans forms of 4,5-dihydro-2-(2-furyl)oxazole, 4,4-dihydro-2-(2-thienyl)oxazole, 2-(2-furyl)oxazole (FO), and 2-(2-thienyl)oxazole (TO) have been studied. The orbital nature of the lower excited singlet and triplet states has been studied by the semiempirical INDO/S (valence approximation) and PPP/S ( approximation) methods. It was shown that for FO and TO molecules the lower triplet state is of the * type, for which delocalization of the electronic excitation on atoms is characteristic. In the singlet excitation state inversion was observed of the energy levels of the delocalized * states and n* states localized over several bonds (for the free TO and FO molecules the lower excited singlet states S₁* were assigned to * and n* types respectively). Owing to the low position of the T * and T _n* levels relative to the singlet level of * type, the rate constant for intercombination conversion is greater than the rate constant for radiative decay. Consequently an efficient population of the triplet states of the molecules occurs under conditions of electronic-vibrational excitation. The direction of reactions during synthesis was compared with the localization indices in the ground state for electrophilic, nucleophilic, and radical substitution, and also with the excitation localization numbers L for a wide selection of electronically excited states. It was concluded that the change in the structure of the azole molecule on replacing an O atom by an S atom, or on changing from a partially hydrogenated to a heteroaromatic system, was the main reason for the change of all the spectral parameters characterizing the electronic-vibrational or the spin-orbital interaction of the most reactive groups of atoms in the molecular structure.     

A non-empirical SCF and CI study of the electronic spectrum of pyrrole

Various electronically excited states of pyrrole have been studied by ab initio SCF and CI calculations including π → π^* and π → Rydberg excitations. Optically allowed valence type transitions are found at energies higher than 6.5 eV whereas all the lower singlet states are of Rydberg type. In addition to the experimentally known triplet states at 4.23 and 5.10 eV, several new triplet transitions with energies from 5.71 to 7.10 eV are predicted. In most cases good agreement with experimental data is found.     

Chemically Initiated Electron Exchange Luminescence of Silyloxyaryl-Substituted Spiroadamantyl Dioxetanes: Kinetics and Excited State Yields

Abstract— The kinetics of the fluoride-induced decomposition of the thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F^-] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen-olate 4 (CIEEL emitter) excitation yields. Chloro-substi-tution in the spiroadamantyl dioxetane does not affect the deprotection step k ₂ but leads to a ca five-fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self-consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π-type molecular orbitals and the large singlet-triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion-triggered CIEEL process.     

Singlet and Triplet Excited States and Intersystem Crossing in Free‐Base Porphyrin: TDDFT and DFT/MRCI Study

Extensive time-dependent DFT (TDDFT) and DFT/multireference configuration interaction (MRCI) calculations are performed on the singlet and triplet excited states of free-base porphyrin, with emphasis on intersystem crossing processes. The equilibrium geometries, as well as the vertical and adiabatic excitation energies of the lowest singlet and triplet excited states are determined. Single and double proton-transfer reactions in the first excited singlet state are explored. Harmonic vibrational frequencies are calculated at the equilibrium geometries of the ground state and of the lowest singlet and triplet excited states. Furthermore, spin–orbit coupling matrix elements of the lowest singlet and triplet states and their numerical derivatives with respect to nuclear displacements are computed. It is shown that opening of an unprotonated pyrrole ring as well as excited-state single and double proton transfer inside the porphyrin cavity lead to crossings of the potential energy curves of the lowest singlet and triplet excited states. It is also found that displacements along out-of-plane normal modes of the first excited singlet state cause a significant increase of the 2|H_so|S₁>, 1|H_so|S₁>, and 1|H_so|S₀> spin–orbit coupling matrix elements. These phenomena lead to efficient radiationless deactivation of the lowest excited states of free-base porphyrin via intercombination conversion. In particular, the S₁→T₁ population transfer is found to proceed at a rate of ≈10⁷ s⁻¹ in the isolated molecule.     

A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil

Molecular geometries of the nucleic acid bases thymine, cytosine and uracil in the ground and the lowest two singlet excited states were optimized using the ab initio approach employing the 4-31G basis set for all the atoms except the amino group of cytosine for which the 6-311+G^* basis set was used. The excited state calculations were performed employing configuration interaction involving singly excited configurations (CIS). Vibrational frequencies were computed in order to examine the nature of the stationary points on the potential energy surfaces obtained by geometry optimization. While the ground state geometries of uracil and thymine (except the methyl group hydrogens) are planar, the corresponding excited state geometries were found to be significantly nonplanar. In the case of cytosine, the amino group is pyramidal and the rest of the molecule is only slightly nonplanar in the ground state, but the excited state geometries are appreciably nonplanar. In particular, consequent to the S₂(n–π^*) excitation of cytosine, the amino group plane is strongly rotated. While thymine is stable in the S₂(π–π^*) excited state, uracil appears to be dissociative in the corresponding excited state.     

The photorearrangement of gaseous furan a thereotical approach

The photorearrangement of gaseous furan when photolysed at 254 nm in the presence of mercury was studied using both an ab initio and a semiempirical treatment of excited states.A reaction pathway towards the cyclization into cyclopropenylaldehyde is proposed. It is shown that the rearrangement of photosensitized furan involves a singlet or triplet excited state of the π, σ^* type.     

Density functional theory study of 1,2‐dioxetanone decomposition in condensed phase

The decomposition of 1,2‐dioxetanone into a CO₂ molecule and into an excited state formaldehyde molecule was studied in condensed phase, using a density functional theory approach. Singlet and triplet ground and excited states were all included in the calculations. The calculations revealed a novel mechanism for the chemiluminescence of this compound. The triplet excitation can be explained by two intersystem crossings (ISCs) with the ground state, while the singlet excitation can be accounted by an ISC with the triplet state. The experimentally verified small excitation yield can then be explained by the presence of an energy barrier present in the potential energy surface of the triplet excited state, which will govern both triplet and singlet excitation. It was also found that the triplet ground state interacts with both the triplet excited and singlet ground states. A MPWB1K/mPWKCIS approach provided results in agreement with the existent literature. © 2012 Wiley Periodicals, Inc.     

Theoretical studies on the properties of uracil and its dimer upon thioketo substitution

The influences of thioketo substitution on the properties of uracil monomer and dimer and their interactions with Zn²⁺ have been systematically investigated at the B3LYP/6-311+G^*level of theory. Those properties include the structural characteristics, acidities, ionization potentials, and singlet–triplet energy gaps of SU monomers and their dimers, where SU=2-thiouracil, 4-thiouracil, and 2,4-dithiouracil, respectively. Computational results suggest that thioketo substitution leads to an increase in the acidities of the N-H groups for both uracil and its dimer, where the N₁–H group is still the most acidic site relative to that of N₃–H group. However, the opposite behaviors are true for the ionization potentials and the singlet–triplet energy gaps of uracil monomer and its dimer, suggesting that thiouracils are more susceptible to radiation damage relative to the unsubstituted uracil. For uracil and 2-thiouracil, the corresponding triplet excited-state geometries are predicted to be highly nonplanar compared with the planar geometries of the ground state as well as 4-thiouracil and 2,4-dithiouracil upon triplet excitation. As a rule, the intermolecular H-bonds involving the sulfur atom directly have been influenced more significant than those the oxygen atom directly involved for U::U and SU::SU base pairs upon ionization and excitation. Additionally, Zn²⁺ binding is expected to lead to an increase in the stability of U::U and SU::SU base pairs.     

Excited state dipole moments and geometries of the bases of nucleic acids

CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μ_hyb^(σ), μ_ch and μ_π has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different.Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π^* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π^* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.     

Fluorescence and photochemical properties of crystalline boron difluorides β-diketonato

The fluorescence and photochemical properties of crystalline β-diketonatoboron difluorides (DBD) RCOCHCOR¹BF₂ were studied. These compounds are characterized by relatively high photochemical stability. The introduction of electron-donating groups into the aromatic α-substituent of the chelate ring increases and the introduction of electron-withdrawing groups decreases the fluorescence intensity of DBD. Anisoylbenzoylmethanotoboron difluoride was found to exhibit the highest fluorescence intensity. The substituents were shown to influence the relative arrangement of singlet and triplet ηπ^* and ππ^* levels and luminescence properties of compounds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1030–1033, June, 2000.     

C-Br Bond Dissociation Mechanisms of 2-Bromothiophene and 3-Bromothiophene at 267 nm

C-Br bond dissociation mechanisms of 2-bromothiophene and 3-bromothiophene at 267 nm were investigated using ion velocity imaging technique. Translational energy distributions and angular distributions of the photoproducts, Br(²P_3/2) and Br^*(²P_½), were obtained and the possible dissociation channels were analyzed. For these two bromothiophenes, the Br fragments were produced via three channels: (i) the fast predissociation following the intersystem crossing from the excited singlet state to repulsive triplet state; (ii) the hot dissociation on highly vibrational ground state following the internal conversion of the excited singlet state; and (iii) the dissociation following the multiphoton ionization of the parent molecules. Similar channels are involved for photoproduct Br^* of the 2-bromothiophene dissociation at 267 nm; whereas for the photoproduct Br^* of 3-bromothiophene, the dissociation channel via internal conversion from the excited singlet state to highly vibrational ground state became dominating and the fast predissociation channel via the excited triplet state almost disappeared. Informations about the relative contribution, energy disposal, and the anisotropy of each channel were quantitatively given. It was found that with the position of Br atom in thienyl being far from S atom, the relative ratios of products from channels (i) and (ii) decreased obviously and the anisotropies corresponding to each channel became weaker.     

Molecular orbital studies of hydrogen bonds

The ground state, the lowest singlet and triplet n-* states, and the lowest triplet -* state of the formic acid monomer and dimer are studied with the ab initio molecular orbital theory. The two-configuration electron-hole potential method is used for calculations of excited states of dimers. The potential energy curves for the symmetrical simultaneous movement of two bridging protons are studied for all of the states. The barrier of the proton transfer in the ground state is found to be the smallest of the states studied. The association energy is analyzed in terms of various components.A preliminary account has been presented at the First International Congress of Quantum Chemistry, Menton, France, 1973, and has appeared in Ref. [3].     

Orbital phase control of the conformations of α- and β-substituted enamines and vinyl ethers

The conformational stabilities of the α- and β-substituted enamines and vinyl ethers were predicted by orbital phase theory and confirmed by ab initio molecular orbital calculations. Cyclic interaction significantly occurs among the nonbonding orbital n _Y for the lone pair on the hetero atom Y (N in the enamines or O in the ethers), the π and π* orbitals of the CC bond, and the σ_C-H or σ*_C-X orbitals on the substituent CH₂X. The cyclic -n _Y-π-σ_C-H-π*- interaction is favored by the orbital phase continuity in the α-substituted molecules, while the cyclic -n _Y-π-σ*_C-X-π*- interaction is favored in the β-substituted molecules. The most stable conformation was then predicted to be synperiplanar or (pseudo)equatorial in the α-substituted molecules and anticlinical or (pseudo)axial in the β-substituted molecules. Received: 8 May 1998 / Accepted: 30 July 1998 / Published online: 16 November 1998     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Excitation Energies of Canonical Nucleobases Computed by Multiconfigurational Perturbation Theories

In this computational work, we assessed the performance of ab initio multireference (MR) methods for the calculation of vertical excitation energies of five nucleobases: adenine, guanine, cytosine, thymine and uracil. In total, we have studied 38 singlet and 30 triplet excited states. Where possible we used the multireference configuration interaction (MRCI) method as a reference for various flavors of multireference perturbation theory to second order. In particular, we have benchmarked CASPT2, NEVPT2 and XMCQDPT2. For CASPT2, we have analyzed the single‐state, multistate (MS) and extended MS variants. In addition, we have assessed the effect of the ionization potential electron affinity (IPEA) shift. For NEVPT2, we have used the partially and the strongly contracted variants. Further, we have tested the commonly used RI‐CC2, RI‐ADC2 and EOM‐CCSD methods. Generally, we observe the following trends for singlet excited states: NEVPT2 is the closest MR method to MRCISD+Q, closely followed by CASPT2 with the default IPEA shift. The same trend is observed for triplet states, although NEVPT2 and CASPT2‐IPEA are getting closer. Interestingly, the n, π* singlet excited states were described more accurately than π, π* excited states, while for triplet states the trend is inverted except for NEVPT2. This work is an important benchmark for future photochemical investigations.     

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.