Structural features of some small carbonyls in the 1,3nπ states: An mcscf study

Structural features of a series of tetraatomic carbonyls in the ^1,3nπ* states are studied by using a recently proposed MCSCF algorithm at the INDO/2 level. The computed geometrical parameters and the inversion barrier heights agree quite well with the available experimental or ab initio theoretical data. The calculated transition energies turn out to be somewhat lower than the experimental counterparts. The specific MCSCF algorithm used is found to have excellent convergence properties.     

A theoretical study of molecular structure in the excited state and molecular luminescence : III. Types of molecular structure in the excited ππ singlet states

The molecular geometry (bond lengths) in the S₁, and S₂ excited states of a large number of conjugated organic molecules is calculated with the help of the SCF-CI-SC procedure, already described in previous communications. The analysis shows that all excited states studied can be classified in three basic types of structure: (1) with strong conjugation over the whole molecule or along its periphery; (2) with a long main conjugated framework and other shorter conjugated fragments moderately or slightly bonded with the framework and (3) structures consisting of two or more fragments slightly conjugated to each other. An estimation of the role of the possible intramolecular motions of low frequency is done, as well as some other factors contributing to the radiationless deactivation of a given excited state. The presence of structures with both lowest excited singlets of type 1 is shown.     

A directing effect of n–π* transitions on electronic charges

Changes of electronic charge distribution following the lowest singlet n–π* transitions of benzaldehyde, benzamide, benzoic acid, the phthalaldehydes, pyridine, and the diazines have been examined using the CNDO -S /CI method. A Singlet n–π* transition is found as a general rule to direct electronic charges to the para position of the atom in the ring that carries the lone pair or to which the substituent carrying the lone pair is attached.     

Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance‐dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short‐range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O???C=O interactions.     

n→π*-Übergänge in Dicarbonylverbindungen Der Einfluss der n,n- und π*, π*-Wechselwirkung

By CNDO-CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO-models. The dominant features of the long-wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases:

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Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

We have studied the charge‐transfer‐induced deactivation of nπ* excited triplet states of benzophenone derivatives by O₂(³Σ), and the charge‐transfer‐induced deactivation of O₂(¹Δ_g) by ground‐state benzophenone derivatives in CH₂Cl₂ and CCl₄. The rate constants for both processes are described by Marcus electron‐transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge‐transfer and non‐charge‐transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck‐Condon factors for the deactivation of ketone‐oxygen complexes to the ground‐state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O₂(¹Δ_g) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non‐charge‐transfer process is largely enhanced, and charge‐transfer interactions are less important. The deactivation of singlet oxygen by ground‐state benzophenone derivatives proceeds via interactions of O₂(¹Δ_g) with the Ph rings.     

The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD‐DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, ¹A_u, which is conventionally regarded dark due to the dipole‐forbidden ¹A_u←¹A_g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* ¹B_3u and ππ* ¹B_2u states through vibronic coupling. The scans on potential‐energy surfaces further indicated that the ¹A_u state intersects with the ¹B_2u states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.     

Simulated T ← S spectra of benzaldehydes as a function of the energy gap between the ππ and nπ levels

The T_1,2 ← S₀ spectra of benzaldehydes have been studied as a function of the energy separation between the vibrationless levels. It is shown that the spectra are very complicated in the region of ΔE[T₂⁰(nπ^*)-T₁⁰(ππ^*)] = 250–400 cm⁻¹, reflecting effective vibronic interferences between T₂⁰(0-0) and each of the ν₃₆-ν₃₃ out-of-plane vibrational levels of T₁⁰(ππ^*). The simulated spectra correspond to the observed spectra. In the case of T₁⁰ = ³nπ^* and T₂⁰ = ³ππ^* the spectral change is not so drastic as in the reverse case loc. cit. because the optical intensity generally concentrates in the longest wavelength band, i.e., the origin band of the T₁(nπ^*) ← S₀ transition. The simulation spectra are useful for interpretation of the absorption spectra in similar electronic structure systems of substituted benzaldehydes.     

Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.     

Substituent effect on ionisation potential in a series of related molecules: A theoretical study in a molecular orbital framework

The effect of replacing the hydrogen atoms in thioformaldehyde by halogen atoms (F, Cl) on the ionisation potential of the non-bonding electron is analysed by using the Hellman-Feynman theorem, regarding the nuclear charge of the substituent as a parameter in the many-electron Hamiltonian. The trends predicted by our theory nicely agree with the relevant ionisation potentials computed either by applying Koopmans’ theorem or by the ΔE _SCF method. For the carbonyls, avaible experimental data indicate the reliability of our prediction.     

UV photodissociation of N-methylpyrrole: The role of πσ states in non-hydride heteroaromatic systems

The photophysics of jet-cooled N-methylpyrrole molecules following excitation to their first excited singlet state (the ¹A₂ state, arising from a 3s/σ^*←π electron promotion) has been investigated by resonance enhanced multiphoton ionisation spectroscopy, by measurements of wavelength resolved ‘action’ spectra for forming CH₃ photoproducts, and by velocity map imaging studies of these CH₃ products (in their v = 0 and v₂ = 1 vibrational levels). CH₃ products are observed at all excitation wavelengths within the NMP absorption band. Direct dissociation on the ¹A₂ potential energy surface (PES) yields ‘fast’ CH₃ fragments, with an average total kinetic energy release (TKER) of 6500 cm⁻¹, but this product channel is only observed in a narrow wavelength range near the absorption band origin. All of the measured CH₃ images also show a broad component, peaking at lower TKER (1700 cm⁻¹); this component extends beneath the ‘fast’ feature in images recorded at wavelengths near the origin, and accounts for all of the CH₃ products observed at shorter photolysis wavelengths. These products are attributed to decay of highly vibrationally excited ground state molecules formed by radiationless transfer from the ¹A₂ state. Similarities and differences with the results of previous studies of the H + pyrrolyl products arising in the UV photodissociation of pyrrole are discussed in terms of the likely nuclear motions on the relevant ground and excited PESs (along R_N–CH3/R_N–H), and the possible couplings between these surfaces. The present study confirms that the proposed model of ¹πσ^* state induced bond fission in heteroaromatic molecules [A.L. Sobolewski, W. Domcke, Chem. Phys. 259 (2000) 181] is also applicable to non-hydride substituted heteroaromatics, but that mass effects can have an important influence on the subsequent nuclear dynamics.     

Interpreting geometric preferences in π‐stacking interactions through molecular orbital analysis

The preference of π‐stacking interactions for parallel‐displaced (PD) and twisted (TW) conformations over the fully eclipsed sandwich (S) in small π‐stacked dimers of benzene, pyridine, pyrimidine, 1,3,5‐trifluorobenzene, and hexafluorobenzene are examined in terms of enhancement of the inter‐ring density through mixing of the monomer orbitals (MOs). PD and/or TW conformations are consistent with a non‐zero “stack bond order” (SBO), defined in analogy to the bond order of conventional MO theory, as the difference in the occupation of bonding and antibonding π‐type dimer MOs. In the S conformation, the equal number of bonding and antibonding MOs cancel overall stack bonding character between the monomers for an SBO of zero and an overall repulsive interaction. PD from the S shifts the character of at least one antibonding combination of monomer π‐type MOs with nodes perpendicular to the coordinate for PD to bonding, leading to an attractive nonzero SBO. The inter‐ring density measured through the Wiberg bond index analysis shows an enhancement at the PD conformations consistent with greater interpenetration of the monomer densities. This intuitive bonding model for π‐stacking interactions is complementary to highly accurate calculations of π‐stacking energies and allows a predictive understanding of relative stability using cheaper quantum chemical methods.     

Semiempirical determination of Dyson's states in conjugated systems within a full-CI π-electron scheme

The Dyson states as orbitals and geminals describing one- and two-electron detachment processes in molecules are treated within a spin-free formulation of the full configuration interaction (CI) theory. The ensuing computational scheme is presented and some semiempirical calculations on π-shells are given for unsaturated hydrocarbons such as medium-sized polyenes and aromatics. It is shown that in these systems the correlation effects turn out to be more important than the relaxation effects, leading to a marked reduction in the calculated full-CI photoionization crosss ections relative to the Hartree-Fock ones.     

A CASSCF study on the lowest π→π* excitation of urocanic acid

The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T₁ and S₁ excited states of the neutral UCA and in the T₁ but not in the S₁ excited state of the anionic UCA, as compared to the S₀ state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S₀, T₁, and S₁ states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S₀ state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S₀ state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999