Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

Theoretical results for the excited states of ozone

On the basis of ab initio calculations (configuration interaction and generalized valence bond), we find eight excited states of ozone with vertical excitation energies less than 7 eV. Using these results the various experimental transitions are assigned. One state has an equilateral triangle as the minimum energy geometry (1.5 eV above the ground state).     

The lowest electronic states of the benzoyl ion. A molecular orbital study with configuration interaction

A configuration interaction method of molecular orbital theory shown to be accurate for the calculation of excited state energies in several aromatic systems was applied to the problem of excited states of the benzoyl ion. The excited singlet and triplet states of the benzoyl ion lie at least 3.8 eV and 2.6 eV, respectively, above the ground state. These results are not in agreement with a postulated state 20 kcal above the ground state. On the other hand, the charge distribution in excited states does agree with that postulated for the 20 kcal state. The p-hydroxy and p-cyano substituents do not greatly influence the charge distribution between the ring and the carbonyl group in either the ground or lowest excited states.     

Investigation of the electronic spectra and excited-state geometries of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PP) by the symmetry-adapted cluster configuration interaction (SAC-CI) method

The symmetry-adapted cluster-configuration interaction (SAC-CI) method has been used to investigate the optical and geometric properties of the oligomers of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PP). Vertical singlet and triplet absorption spectra and emission spectra have been calculated accurately; the mean average deviation from available experimental results lies within 0.2 eV. The chain length dependence of the transition energies has been improved in comparison to earlier TDDFT and MRSDCI calculations. The present analysis suggests that conventional TDDFT with the B3LYP functional should be used carefully, as it can provide inaccurate estimates of the chain length dependence of the excitation energies of these molecules with long pi conjugation. The T1 state was predicted to be at a lower energy, by 1.0-1.5 eV for PPV and by 0.9-1.7 eV for PP, than the S1 state, which indicates a localized T1 state with large exchange energy. By calculating the SAC-CI electron density difference between the ground and excited states, the geometry relaxations due to excitations can be analyzed in detail using electrostatic force theory. For trans-stilbene, the doubly excited 21Ag state was studied, and the calculated transition energy of 4.99 eV agrees very well with the experimental value of 4.84 eV. In contrast to previous ab initio calculations, we predict this doubly excited 21Ag state to lie above the 11Bu state.     

Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.     

An AB initio SCF MO CI study of the n → π* transition of methylenimine

The electron correlation energies of both the ground and n → π* excited states of methylenimine (CH₂NH) are investigated by means of ab initio SCF MO CI calculations. Then n → π* singlet and triplet state energies of methylenimine are obtained through 3461-dimensional CI including the singly, doubly and triply excited configurations. the excitation energy from the ground state to the ¹(n → π*) state nearly coincides with that obtained in the framework of the singly excited configuration interaction (SECI) procedure. This result suggests that there is good cancellation of the correlation energy between the ground and the excited singlet sates, proving the usefulness of the SECI method for the excitation energies.     

The triplet state of cytosine and its derivatives: electron impact and quantum chemical study

The excitation of the lowest electronic states and vibrational excitation of cytosine (C) have been studied using electron energy loss spectroscopy (EELS, 0-100 eV) with angular analysis. The singlet states have been found to be in good agreement with UV-VIS absorption results on sublimed films, slightly blueshifted by about 0.1 eV. The EEL spectra recorded at residual energy below 2 eV show clear shoulders at energy losses of 3.50 and 4.25 eV (+/-0.1 eV). They are assigned to the lowest triplet electronic states of cytosine. Energies and molecular structures of the lowest-lying triplet state of C and its methylated and halogenated 5-X-C, 6-X-C, and 5-X, 6-X-C substituted derivatives (X=CH3, F, Cl, and Br) have been studied using quantum chemical calculations with both molecular orbital and density functional methods, in conjunction with the 6-311++G(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets. The triplet-singlet energy gap obtained using coupled-cluster theory [CCSD(T)] and density functional theory (DFT) methods agrees well with those derived from EELS study. The first C's vertical triplet state is located at 3.6 eV, in good agreement with experiment. The weak band observed at 4.25 eV is tentatively assigned to the second C's vertical triplet excitation. For the substituted cytosines considered, the vertical triplet state is consistently centered at 3.0-3.2 eV above the corresponding singlet ground state but about 1.0 eV below the first excited singlet state. Geometrical relaxation involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.6-1.0 eV in favor of the equilibrium triplet. The lowest-lying adiabatic triplet states are located at 2.3-3.0 eV. Halogen substitution at both C(5) and C(6) positions tends to reduce the triplet-singlet separations whereas methylation tends to enlarge it. The vibrational modes of triplet cytosine and the ionization energies of substituted derivatives were also evaluated.     

Computational analyses of singlet-singlet and singlet-triplet transitions in mononuclear gold-capped carbon-rich conjugated complexes

Density functional theory and CASSCF calculations have been used to determine equilibrium geometries and vibrational frequencies of metal-capped one-dimensional pi-conjugated complexes (H3P)Au(C[triple chemical bond]C)(n)(Ph) (n = 1-6), (H3P)Au(C[triple chemical bond]CC6H4)(C[triple chemical bond]CPh), and H3P--Au(C[triple chemical bond]CC6H4)C[triple chemical bond]CAu--PH3 in their ground states and selected low-lying pi(pi)* excited states. Vertical excitation energies for spin-allowed singlet-singlet and spin-forbidden singlet-triplet transitions determined by the time-dependent density functional theory show good agreement with available experimental observations. Calculations indicate that the lowest energy 3(pi(pi)*) excited state is unlikely populated by the direct electronic excitation, while the low-lying singlet and triplet states, slightly higher in energy than the lowest triplet state, are easily accessible by the excitation light used in experiments. A series of radiationless transitions among related excited states yield the lowest 3(pi(pi)*) state, which has enough long lifetimes to exhibit its photochemical reactivities.     

Density functional theory and multireference configuration interaction studies on low-lying excited states of TiO2

Using density functional theory at the BPW916-311+G(3df) level, optimized geometries and energies of the lowest singlet, triplet, and quintet A(1), A(2), B(1), B(2)(C(2v)) states of the TiO(2) molecule were obtained. TiO(2) has a (1)A(1) ground state in C(2v) symmetry. Adiabatic excitation energies of the low-lying singlet and triplet states range from 2.1 to 3.0 eV. The (1,3)A(2) states optimize at bond angles of about 140 degrees , lying only 0.06 eV below linear (1,3)Delta(u), whereas (1,3)B(1) and (1,3)B(2), with bond angles of 120 degrees and 96 degrees , respectively, lie 0.3-0.4 eV below the respective (1,3)Pi(u) or (1,3)Delta(u) states. Minima with short O-O distances of approximately 1.46 A, at energies of 4.2 and 4.7 eV, were found for (1)A(1) and (3)A(1). The C(2v) minima of the lowest (1)B(1) and (3)B(1) states are saddle points, suggesting lower-energy structures in C(s) symmetry. The C(2v) quintet states start at energies of 5.7 eV. Multireference configuration interaction (MRCI) methods, employing a polarized valence triple-zeta basis set, lead to similar geometries and energies. MRCI vertical excitation energies up to 4.6 eV and oscillator strengths are given. The calculated excitation energy of 2.2 eV for (1)B(2) agrees well with 2.3 eV from a fluorescence spectrum. The vertical electron detachment energy of TiO(2) (-) is 1.5 eV, in good agreement with 1.6 eV from anion photoelectron spectroscopy. An observed second photoelectron band corresponds to (1)B(2) and/or (3)B(2), but the assignment of a third band could not be verified. Vibrational frequencies, ionization energies, electron affinities, and dissociation energies are given.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

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.     

Phenyloxenium ions: more like phenylnitrenium ions than isoelectronic phenylnitrenes?

The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph-O(+)) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration ((1)A(1)) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state ((3)A(2)). Open-shell singlet configurations ((1)A(2), (1)B(1), (1)B(2), 2(1)A(1)) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet-triplet energy gap (ΔE(ST)) is found to have a positive linear free energy relationship with the Hammett-like σ(+)(R)/σ(+) substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and p-cyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have low-energy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar-NH(+)) than their isoelectronic arylnitrene (Ar-N) counterparts.     

CCSD(T) and MRCI studies on the ground and excited states of BrOClO and ClOBrO

In this work, three forms (cis, trans and nonplanar) of ClOBrO and BrOClO were optimized at CCSD(T)/cc‐pVTZ level of theory. At the most stable forms (nonplanar form) of ClOBrO and BrOClO, the vertical excitation energies for the lowest six singlet states and two triplet states were calculated at the multireference internally contracted configuration interaction (MRCI) level of theory using cc‐pVDZ, Aug‐cc‐pVDZ, cc‐pVTZ, and Aug‐cc‐pVTZ basis sets. The scalar relativistic effect on the excited states of BrOClO and ClOBrO were estimated. In addition, the potential energy curves of the lowest six singlet states and two triplet states of BrOClO and ClOBrO, as well as BrOOCl were calculated at both MCSCF (complete active space self‐consistent field) and MRCI levels of theory using Aug‐cc‐pVDZ basis set on the active space (18e,12o) along the distances of BrO? ClO, ClO? BrO, and BrO? OCl. The results were compared among BrOOCl, ClOBrO, and BrOClO. The first singlet excited state of BrOOCl is 1.12 eV higher than that of BrOClO and 1.36 eV higher than that of ClOBrO at MRCI/cc‐pVTZ level of theory. The first triplet excited state of BrOOCl is 0.77 eV higher than that of BrOClO and 0.86 eV higher than that of ClOBrO at MRCI/cc‐pVTZ level of theory. Most of the excited states of BrOClO studied in this work are unbound states; but most of the ClOBrO and BrOOCl excited states studied in this work are weakly bound states at MRCI level of theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Low-energy states of manganese-oxo corrole and corrolazine: multiconfiguration reference ab initio calculations

Manganese(V)-oxo corrole and corrolazine have been studied with ab initio multiconfiguration reference methods (CASPT2 and RASPT2) and large atomic natural orbital (ANO) basis sets. The calculations confirm the expected singlet d(δ)(2) ground states for both complexes and rule out excited states within 0.5 eV of the ground states. The lowest excited states are a pair of Mn(V) triplet states with d(δ)(1)d(π)(1) configurations 0.5-0.75 eV above the ground state. Manganese(IV)-oxo macrocycle radical states are much higher in energy, ≥1.0 eV relative to the ground state. The macrocyclic ligands in the ground states of the complexes are thus unambiguously 'innocent'. The approximate similarity of the spin state energetics of the corrole and corrolazine complexes suggests that the latter macrocycle on its own does not afford any special stabilization for the Mn(V)O center. The remarkable stability of an Mn(V)O octaarylcorrolazine thus appears to be ascribable to the steric protection afforded by the β-aryl groups.     

O(-) + acetaldehyde reaction products: search for singlet formylmethylene, a Wolff rearrangement intermediate

The mass-resolved anionic products of the reaction of O(?-) with acetaldehyde, H(3)CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H(2)CCHO(-), formylmethylene anion, HCCHO(?-), and ketenylidene anion, CCO(?-). From photoelectron spectra of HCCHO(?-), the electron affinity of triplet (ground state) formylmethylene (1.87 ± 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO(?-), whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H(4)(+) abstraction. A possible mechanism is proposed, involving an [H(2)CCO(?-)]* intermediate. From the measured electron affinities of HCCHO (above), H(2)CCHO (1.82 ± 0.01 eV), and CCO (2.31 ± 0.01 eV), several new thermochemical properties are determined, including the C-H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O(?-) with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process.     

The electronic states of 1,2,3-triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy, and a comparison with ab initio configuration interaction methods

The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (～5.5-6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series.     

Singlet-triplet energy splitting and excited states of phenylnitrene

The vertical and adiabatic singlet-triplet energy splittings (Delta E ST) of phenylnitrene were computed by a variety of multireference configuration interaction and perturbation theory methods employing basis sets of up to quadruple-xi quality and extrapolation to the complete basis set limit. The vertical and adiabatic energy gaps are 18.9 and 15.9 kcal mol (-1), respectively, the latter in reasonable agreement with the revised experimental value of 15.1 +/- 0.2 kcal mol (-1). The energy difference between both states at the geometry of the a (1)A 2 singlet state was also considered and amounts to 13.8 kcal mol (-1). In obtaining accurate state energy splittings, basis set completeness turns out to be a more important issue than the level of dynamical electron correlation treatment. Density functional theory that is frequently employed to investigate phenylnitrenes and their rearrangements yields varying results and, depending on the functional, gives adiabatic energy differences between 9 and 16 kcal mol (-1). The b (1)A 1 state has a similar geometry as the ground state of 1 and is 31 kcal mol (-1) higher in energy. According to best estimates, the next higher singlet states, c (1)A 1 and d (1)B 1, are 57 and 72 kcal mol (-1) above the ground state. In the triplet manifold, vertical excitation energies to the A (3)B 1 and B (3)A 2 states are 71 and 77 kcal mol (-1), respectively.     

Ab initio multireference investigation of disjoint diradicals: singlet versus triplet ground states

We present improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO‐CASCI) and IVO‐CASCI‐based multireference Møller–Plesset perturbation theory (MRMPPT) calculations with an aim to elucidate the electronic structure of tetramethyleneethane (TME) in its lowest singlet and triplet state and to quantify their order and extent of splitting. The potential surfaces of singlet and triplet states for the twisting of TME are also studied. We found that the triplet state is higher in energy than the singlet one in the whole range of twisting angles with the energy gap minimum at a twisting angle of about 45°. Harmonic vibrational frequencies of TME have also been calculated for both the states. We also report the ground to first excited triplet state transition energies. Our results are analyzed with respect to the results available in the literature to illustrate the efficacy of our methods employed. We also demonstrate that the spin character of the ground state of disjoint, TME‐like diradicals can be manipulated by using appropriate selection of annulenic spacer to separate the allyl groups of TME.     

Full configuration interaction calculation of singlet excited states of Be3

The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. (4ex6MO)CAS-SDCI values for the same energies and properties are also reported.