Molecular geometry and excited electronic states : Part VII. Theoretical study on conformational geometries and the dual fluorescence of styrene

Equilibrium molecular geometries of styrene in the electronic S₀, S₁, and S₂, states have been calculated within the scope of the adiabatic approximation by minimization of the total energy with respect to all molecular coordinates. Besides the most stable planar structures, some further conformers twisted about the single and/or double bonds have been found. An interpretation of the styrene dual fluorescence is given using these results.     

Molecular geometry and excited electronic states : Part XIII. Conformational geometries of 9,10-diphenyl anthracene in different electronic states and its electronic spectral behaviour

The conformational geometries of 9,10-diphenyl anthracene in the S₀, S₁, and T₁ electronic states determined by means of the Warshel—Karplus method are presented. Based on the theoretical equilibrium geometries of the con- and dis-twisted conformations the calculated electronic transition energies and oscillator strengths are used for an interpretation of the electronic spectral behaviour in absorption and fluorescence. The corresponding theory-experiment comparison performed is satisfactory.     

Molecular geometry and excited electronic states. Theoretic study on the molecular geometry and the fluorescence lifetime difference of isometric phenylnapkthalenes1

The equilibrium molecular geometries of 1-phenyl- and 2-phenyl-naphthalenes in the electronic S_o and S₁ states have been calculated by minimization of the total energy with respect to all molecular coordinates. The singlet term systems of both isomers have been determined using these results. Although the fluorescence transition energy is nearly the same in both cases it was found that the corresponding electronic transitions were allowed in 1-phenylnaphthalene but were forbidden in 2-phenylnaphthalene. The result explains the different fluorescence lifetimes observed.     

Theoretical study on geometry and spectroscopic properties of 1,1′-binaphthyl in the electronic ground and first excited singlet states

Equilibrium geometries for the electronic ground and first excited singlet states of 1,1'-binaphthyl have been calculated by minimization of the total energy with respect to all internal coordinates. Using these results, an interpretation of the fluorescence S₁→ S₀ and absorption spectra S_m ← S₀ and S_n ← S₁ in rigid and fluid solutions is given.For the first time the equilibrium geometry of the first excited singlet state of 1, 1′-binaphthyl has been calculated. On excitation to the S₁ state the dihedral angle θ between the two naphthalene moieties is de- creased from 61 ° to 41 °. A detailed survey of CH bond lengths in the S₀ and S₁ states has been given. This result should be of particular importance for the theoretical treatment of radiationless transitions.Using equilibrium geometries for the S₀ and S₁ states a satisfactory interpretation of the S_m ← S₀ and S_n ← S₁ absorption spectra as well as of the fluorescence spectra in fluid and rigid solutions can be given. Concerning the S_n ← S₁ absorption spectrum in fluid solution, the calculations predict a strong absorption (A ← B transition) in the still uninvestigated region of energies lower than 11000 cm^?1.From the results of this paper and of other calculations it can be concluded that the Warshel-Karplus method yields reliable equilibrium geometries for electronic ground and excited states of unsaturated hydrocarbons [22,23].     

Theoretical study of the electronic excited states of tetracyanoethylene and its radical anion.

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.     

Triplet excited states of cyclic disulfides and related compounds: electronic structures, geometries, energies, and decay

We have performed a computational study on the properties of a series of heterocycles bearing two adjacent heteroatoms, focusing on the structures and electronic properties of their first excited triplet states. If the heteroatoms are both heavy chalcogens (S, Se, or Te) or isoelectronic species, then the lowest excited triplet state usually has (π*, σ*) character. The triplet energies are fairly low (30-50 kcal mol(-1)). The (π*, σ*) triplet states are characterized by a significantly lengthened bond between the two heteroatoms. Thus, in 1,2-dithiolane (1b), the S-S bond length is calculated to be 2.088 ? in the singlet ground state and 2.568 ? in the first triplet excited state. The spin density is predicted to be localized almost exclusively on the sulfur atoms. Replacing one heavy chalcogen atom by an oxygen atom or an NR group results in a significant destabilization of the (π*, σ*) triplet excited state, which then no longer is lower in energy than an open-chain biradical. The size of the heterocyclic ring also contributes to the stability of the (π*, σ*) triplet state, with five-membered rings being more favorable than six-membered rings. Benzoannulation, finally, usually lowers the energy of the (π*, σ*) triplet excited states. If one of the heteroatoms is an oxygen or nitrogen atom, however, the corresponding lowest triplet states are better described as σ,π-biradicals.     

Theoretical study on potential energy curves and spectroscopy properties of ground and low-lying excited electronic states of BrCl~

The calculations on the potential energy curves and spectroscopic constants of the ground and low-lying excited states of BrCl ,one of the important molecular ions in environment science,have been performed by using the multireference configuration interaction method at high level of theory in quantum chemistry.Through analyses of the effects of the spin-orbit coupling interaction on the elec-tronic structures and spectroscopic properties,the multiconfiguration characteristic of the X2Π ground state and low-lying excited states was established.The spin-orbit coupling splitting energy of the X2 Π ground state was calculated to be 1814 cm-1,close to the experimental value 2070 cm-1.The spin-orbit coupling splitting energy of the 2Π(Ⅱ) exited state was predicted to be 766 cm-1.The transition dipole moments and Frank-Condon factors of the 3/2(Ⅲ)-X3/2 and 1/2(Ⅲ)-1/2(I) transitions were estimated,and the radiative lifetimes of the two transitions were briefly discussed.     

The structure and properties of flavins: Molecular orbital study based on totally optimized geometries. I. Molecular geometry investigations

Molecular orbital calculations (MINDO /3) using energy minimized molecular geometries were performed on oxidized and reduced lumiflavin and related methylated isoalloxazines, including cationic and anionic species. Close agreement with experimental geometry, photoelectron spectra, and NMR data supports the importance of optimized geometries for these molecular systems and provides the basis for interpretation of chemical and biological properties. Oxidized forms are shown to be most stable in the planar configuration but also highly flexible about the N(5)—N(10) axis; only 1 kcal/mol is required for a 10° bend. N(10) is generally out of the plane slightly; also, C(9)-methyl substitution introduces nonplanarity. The unsubstituted isoalloxazine is computed to be 0.76 kcal/mol (ΔH) less stable than its isomer, alloxazine. Calculations were also performed on enol as well as quinone-methide tautomeric forms. Reduced flavin geometry depends on methyl substitution pattern: N(10) substituted forms are bent with typical fold angles around 155°, whereas the unsubstituted reduced form is planar. Both oxidized and reduced forms are also flexible. Proton affinities were calculated for protonation and deprotonation of oxidized and reduced forms. Protonation of oxidized forms is favored at N(1) by 10–12 kcal/mol and produces somewhat nonplanar isoalloxazinium ions. In addition, ΔH for the two-electron reduction of lumiflavin is estimated to be ?19.7 kcal/mol. In this paper investigations of geometric aspects are presented along with introductory and background material. Orbital structure and electron distribution studies are presented in paper II.     

Ab initio study of cyclobutadiene in excited states: optimized geometries,electronic transitions and aromaticities

An Ab initio SCF-CI study of planar cyclobutadiene (CB) in ground and excited states has been carried out. The equilibrium geometries of some valence and Rydberg states have been calculated, as well as the energies of the vertical (absorption and emission) and non-vertical transitions. Using the optimized geometries, it is discussed how the aromaticity changes upon excitation of CB to the lowest-lying singlet and triplet states. The following conclusion is made: upon excitation to the fluorescent (S₁) or phosphorscent (T₁), states, the aromaticity of the anti-Hückel system cyclobutadiene increases significantly, whereas that of the Hückel system benzene descreases.     

Theoretical study of deactivation and isomerization pathways of 1,2-dithiete in excited electronic states

The potential energy surface crossings for 1,2-dithiete have been investigated using the complete active space self-consistent field(CASSCF) method and simple group theory.Using the full Pauli-Breit spin-orbit coupling(SOC) operator(■) SO) which consists of the one-electron(■) SO1) and two-electron(■) SO2) terms,we estimate the strengths of the SOC(198.37 cm-1 when symmetry is imposed,and 211.35 cm-1 with no symmetry constraints),which plays an essential role in the spin transitions between different spin s...     

Theoretical study on the excited states of HCN

In the flash-photolysis of oxazole, iso-oxazole, and thiozole a transient band system was observed in the region 2500-3050 angstroms. This band system was attributed to a meta-stable form of HCN, i.e., either HNC or triplet HCN. Theoretical investigations have been carried out on the ground and excited states of HCN to characterize this and other experimentally observed transitions. The predicted geometries are compared with the experiment and earlier theoretical calculations. The present calculations show that the band system in the region 2500-3050 A corresponds to the transition 4 3-A' <-- 1 3-A' of HCN.     

Theoretical study on the excited states of psoralen compounds bonded to a thymine residue

Time-dependent density functional theory calculations have been performed for the excited states of psoralen, 5-methoxypsoralen, and 8-methoxypsoralen in systems and furan and pyrone monoadducts bonded to a thymine residue. The theoretical assignments to ultraviolet (UV) absorption spectra of isolated systems have been performed. The present calculations have clarified that the excitation energies of the first singlet excited (S1) state of monoadducts are blue-shifted compared with the isolated systems. It is shown that, in particular, the S1 excitation energy of the pyrone monoadduct is significantly blue-shifted and, therefore, the pyrone monoadduct is not excited by UV-A light (300-400 nm), which is used in the photochemotherapy.     

Theoretical study of low-lying excited states of molecular aggregates. I. Development of linear-scaling TD-DFT

The project aims to develop an integrated linear-scaling time-dependent density functional theory (TD-DFT) for studying low-lying excited states of luminescent molecular materials, especially those fluorescence and phosphorescence co-emitting systems. The central idea will be "from fragments to molecule" (FF2M). That is, the fragmental information will be employed to synthesize the molecular wave function, such that the locality (transferability) of the fragments (functional groups) is directly built into the algorithms. Both relativistic and spin-adapted open-shell TD-DFT will be considered. Use of the renormalized exciton method will also be made to further enhance the efficiency and accuracy of TD-DFT. Solvent effects are to be targeted with the fragment-based solvent model. It is expected that the integrated TD-DFT and program will be of great value in rational design of luminescent molecular materials.     

Theoretical investigations of the electronic states of porphyrins. IV. Low-lying electronic states of bisammineporphinatoiron(II)

Ab initio CI calculations are reported on the lowest quintet, triplet, and singlet states of Fe^II(P) (NH₃)₂. The lowest singlet state has strong mixing between the configuration (d_xy² (d_π)⁴ and (d_xy)²(d_π)³e_gπ*. The lowest quintet is mixed between ⁶A_1g)d_π and (⁶A_1g)e_gπ*, where ⁶A_1g refers to the high-spin ferric configuration. We calculate many low-energy states as ³(π→π*) ring and metal triplet and quintet configurations [“triptriplets” and “tripquintets”]. The calculations also show low-energy charge-transfer configurations of ring anion excited quartets and ferric quartets and sextets [“quartquartets” and “quartsextets”]. The farthest red x,y-polarized bands of the experimental spectra of low-spin hemoproteins are identified as d_xy → e_gπ* or d_π → d mixed with d_π → d and the z-polarized bands are assigned as d_π → e_gπ*. The farthest red x,y-polarized bands of the high-spin hemoproteins are identified as excited quartsextet states. Picosecond transients observed in Fe^II(TPP) (pip)₂ are attributed to an initial ¹(d_π → e_gπ*) state, which inter-system crosses to high-spin states that lose one ligand.     

Molecular fragmentations: Part IV. The mass spectral fragmentation of carbon tetrachloride

MINDO/3 calculations have been made of the molecular structures and energies of seven isomeric forms of the molecular cation (CCl₄)⁺, of the mass spectral fragment pairs:

and also of a number of neutral fragment pairs. Reaction energy profiles have been calculated for two fragmentations of (CCl₄)⁺, into [(CCl₂)⁺ + Cl₂], and into [(CCl₃)⁺ + Cl^?], in the latter of which the reaction proceeds via a rather stable intermediate; for the fragmentation of three electronic states of (CCl₃)⁺ into [(CCl₂)⁺ + Cl^?], where the ground singlet state and first triplet state of (CCl₃⁺ yield the ground doublet state of (CCl₂)⁺, but the first excited singlet of (CCl₃)⁺ yields the first excited doublet of (CCl₂)⁺ ; and for the fragmentation of the ground state of (CCl₃)⁺ into [(CCl)⁺ + Cl₂].     

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.     

Theoretical study of spectroscopic and molecular properties of several low‐lying electronic states of CO molecule

The potential energy curves (PECs) of eight low‐lying electronic states (X¹Σ⁺, a³Π, a′³Σ⁺, d³Δ, e³Σ^?, A¹Π, I¹Σ^?, and D¹Δ) of the carbon monoxide molecule have been studied by an ab initio quantum chemical method. The calculations have been performed using the complete active space self‐consistent field method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with the correlation‐consistent aug‐cc‐pV5Z basis set. The effects on the PECs by the core‐valence correlation and relativistic corrections are included. The way to consider the relativistic corrections is to use the third‐order Douglas–Kroll Hamiltonian approximation at the level of a cc‐pV5Z basis set. Core‐valence correlation corrections are performed using the cc‐pCVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are corrected for size‐extensivity errors by means of the Davidson modification (MRCI+Q). The spectroscopic parameters (D_e, T_e, R_e, ω_e, ω_ex_e, ω_ey_e, B_e, α_e, and γ_e) of these electronic states are calculated using these PECs. The spectroscopic parameters are compared with those reported in the literature. Using the Breit–Pauli operator, the spin–orbit coupling effect on the spectroscopic parameters is discussed for the a³Π electronic state. With the PECs obtained by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations, the complete vibrational states of each electronic state have been determined. The vibrational manifolds have been calculated for each vibrational state of each electronic state. The vibrational level G(ν), inertial rotation constant B_ν, and centrifugal distortion constant D_ν of the first 20 vibrational states when the rotational quantum number J equals zero are reported and compared with the experimental data. Comparison with the measurements demonstrates that the present spectroscopic parameters and molecular constants determined by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations are both reliable and accurate. © 2012 Wiley Periodicals, Inc.