Calculation Of Vertical Excitation Energies Research Articles

Time-dependent approach to electronically excited states of molecules with the multiconfiguration time-dependent Hartree-Fock method

In this paper the authors show how the multiconfiguration time-dependent Hartree-Fock (MCTDHF) method can be used for the calculation of electronic properties of molecules associated with the population of excited states. In contrast to other methods for correlated electron dynamics, such as configuration interaction, MCTDHF does not rely on a solution of the electronic Schrodinger equation prior to the propagation. The authors apply this approach to the calculation of vertical excitation energies, transition dipole moments, and oscillator strengths for two test molecules, lithium hydride and methane.

Radiationless Decay Mechanism of Cytosine: An Ab Initio Study with Comparisons to the Fluorescent Analogue 5-Methyl-2-pyrimidinone

The ultrafast radiationless decay mechanism of photoexcited cytosine has been theoretically supported by exploring the important potential energy surfaces using multireference configuration-interaction ab initio methods for the gas-phase keto-tautomer free base. At vertical excitation, the bright state is S1 (pipi*) at 5.14 eV, with S2 (nNpi*) and S3 (nOpi*) being dark states at 5.29 and 5.93 eV, respectively. Minimum energy paths connect the Franck-Condon region to a shallow minimum on the pipi* surface at 4.31 eV. Two different energetically accessible conical intersections with the ground state surface are shown to be connected to this minimum. One pathway involves N3 distorting out of plane in a sofa conformation, and the other pathway involves a dihedral twist about the C5-C6 bond. Each of these pathways from the minimum contains a low barrier of 0.14 eV, easily accessed by low vibronic levels. The path involving the N3 sofa distortion leads to a conical intersection with the ground state at 4.27 eV. The other pathway leads to an intersection with the ground state at 3.98 eV, lower than the minimum by about 0.3 eV. Comparisons with our previously reported study of the fluorescent cytosine analogue 5-methyl-2-pyrimidinone (5M2P) reveal remarkably similar conformational distortions throughout the decay pathways of both bases. The different photophysical behavior between the two molecules is attributed to energetic differences. Vertical excitation in cytosine occurs at a much higher energy initially, creating more vibrational energy than 5M2P in the Franck-Condon region, and the minimum S1 energy for 5M2P is too low to access an intersection with the ground state, causing population trapping and fluorescence. Calculations of vertical excitation energies of 5-amino-2-pyrimidinone and 2-pyrimidinone reveal that the higher excitation energy of cytosine is likely due to the presence of the amino group at the 4-position.

Vibrational and Electronic Absorption Spectroscopy of Dibenzo[b,def]chrysene and Its Ions

The vibrational and electronic absorption spectra of dibenzo[b,def]chrysene (DBC) and its ions in argon matrixes have been recorded. Assignment of the observed infrared (IR) bands has been made by comparison with the density functional theory (DFT) computations of harmonic vibrational frequencies (with 6-31G(d,p) or 6-311+G(d,p) basis sets). Extensive time-dependent (TD) DFT calculations of vertical excitation energies have aided in the assignment of the experimental electronic absorption transitions. In general, the theoretical predictions are in good agreement with the observed ultraviolet and visible bands. By correlating IR and UV-visible band intensities (after UV photolysis), it has been shown that both DBC cations and anions are formed. The IR band intensity distributions of the DBC ions differ markedly from neutral DBC. A synthetic spectrum composed of neutral, cationic, and anionic DBC contributions compares reasonably well with the interstellar features of the "unidentified infrared" (UIR) bands from the reflection nebula NGC 7023. Finally, it is shown that the electronic absorption bands of the DBC ions lie in close proximity to several of the diffuse interstellar visible absorption bands (DIBs).

Simple Ethers as Models of Sugar Molecules in Calculations of Vertical Excitation Energies of DNA and RNA Nucleosides

The ribose and deoxyribose molecules of RNA and DNA nucleosides are substituted with simple model compounds 1-methoxy-2-ethanol and 1-methoxypropane to mimic the effect of binding to sugars on the vertical excitation energies of purine and pyrimidine bases. The (R)-1-methoxy-2-ethanol, CH(3)OC*HCH(2)OH, for model ribose nucleosides and (R)-1-methoxypropane, CH(3)OC*HC(2)H(5), for model deoxyribose nucleosides have minimal structural characteristics of ribose and deoxyribose molecules when attached to nucleic acid purine and pyrimidine bases. The bases are attached to the C1 carbon atom designated by the asterisk. The vertical excitation energies of these model nucleosides are calculated with the time-dependent density functional theory method at the B3LYP level with 6-311++G(d,p) and aug-cc-pVDZ basis sets. The attachment of the ether molecules qualitatively and quantitatively modifies the excited state energy levels of the model nucleosides when compared to the free bases. These changes can affect the deexcitation mechanisms for photoexcited nucleosides.

Photochemical and electronic properties of conjugated bis(azo) compounds: an experimental and computational study.

We have investigated the photophysical, photochemical and electrochemical properties of two bis(azo) derivatives, (E,E)-m-1 and (E,E)-p-1. The two compounds, which can be viewed as being composed of a pair of azobenzene units sharing one of their phenyl rings, differ only for the relative position of the two azo groups on the central phenyl ring-meta and para for m-1 and p-1, respectively. The UV-visible absorption spectra and photoisomerisation properties are noticeably different for the two structural isomers; (E,E)-m-1 behaves similarly to (E)-azobenzene, while (E,E)-p-1 exhibits a substantial red shift in the absorption bands and a decreased photoreactivity. The three geometric isomers of m-1, namely the E,E, E,Z and Z,Z isomers, cannot be resolved in a mixture by absorption spectroscopy, while the presence of three distinct species can be revealed by analysis of the absorption changes observed upon photoisomerisation of (E,E)-p-1. Quantum chemical ZINDO/1 calculations of vertical excitation energies nicely reproduce the observed absorption changes and support the idea that, while the absorption spectra of the geometrical isomers of m-1 are approximately given by the sum of the spectra of the constituting azobenzene units in their relevant isomeric form, this is not the case for p-1. From a detailed study on the E-->Z photoisomerisation reaction it was observed that the photoreactivity of an azo unit in m-1 is influenced by the isomeric state of the other one. Such observations indicate a different degree of electronic coupling and communication between the two azo units in m-1 and p-1, as confirmed by electrochemical experiments and quantum chemical calculations. The decreased photoisomerisation efficiency of (E,E)-p-1 compared to (E,E)-m-1 is rationalised by modelling the geometry relaxation of the lowest pi-pi* state. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, with predetermined photoreactivity.

Effect of Thermal Motions on the Structure and UV−Visible Electronic Spectra of Stilbene and Model Oligomers of Poly(p-Phenylene Vinylene)

In the present study, we investigate the influence of temperature on the width and location of bands in the UV−visible absorption spectra of oligomer chains of poly(p-phenylene vinylene) (PPV). These spectra have been computed by means of molecular dynamics (MD) simulations along with the classical MM3 force field, in conjunction with (Z)INDO/S-CIS calculations of vertical excitation energies and transition moments. In addition, the MD(MM3) computations enable consistent insights into the average structures of cis- and trans-stilbene, at temperatures ranging from 0 to 500 K. For trans-stilbene and larger PV-n oligomer chains converging to PPV, thermal motions at room temperature in the vacuum result in a constant broadening of bands by 24 nm (0.20 eV) at half the maximum. Compared with the trans-isomer, thermal broadening intensifies to 33 nm (0.44 eV) for cis-stilbene, due to enhanced steric effects. When accounting in addition for the outcome of vibronic broadening, the width of the first absorption band increases at 300 K to 33 nm (0.48 eV) for trans-stilbene and ∼46 nm (0.40 eV) for the largest PV-n chains. At last, upon studying a large molecular cluster reproducing the crystalline structure of trans-stilbene, it has been found that, although they strongly impede the rotations of phenyl rings and out-of-plane distortions, packing effects and steric hindrances in the solid phase lead to a limitation of the width of the first absorption band by 6 nm (0.07 eV) only at 300 K.

Comparison of Multireference Møller−Plesset Theory and Time-Dependent Methods for the Calculation of Vertical Excitation Energies of Molecules

The vertical singlet−singlet excitation energies for a benchmark set of 14 medium and large molecules have been investigated with three quantum chemical methods. Calculations for electronic states with very different character in organic and inorganic systems are used to assess the accuracy and applicability of a simplified multireference Møller−Plesset (MR-MP2) approach, time-dependent density functional theory (TDDFT), and an approximate coupled cluster method with single and double excitations (CC2). In the pure ab initio approaches the resolution of the identity (RI) method for the calculation of the two-electron integrals is used to improve computational efficiency. It is shown that independently of the complexity of the electronic states involved, only the MR-MP2 method yields high accuracy (mean absolute deviation of 0.14 eV for 22 states). This finding is of particular importance because our scheme avoids computationally demanding orbital optimization steps and employs very compact reference wave functions. The TDDFT results are significantly poorer (mean absolute deviation of 0.26 eV), and systematic deviations for some π → π* states, Rydberg states, and systems with unusual electronic structure are obtained. It is concluded that TDDFT has a potential for exploratory investigations or for very large molecules due to its computational efficiency. The CC2 method shows a tendency to overestimate excitation energies and is also limited to systems where the ground state is well described by a single determinant.

Excitation Energies and Molecular Quantum Defect Orbital Transition Intensities for Rydberg States of ArH

The size-consistent self-consistent complete active space singles and doubles configuration interaction ((SC)2-CAS-SDCI) procedure has been applied to the calculation of vertical excitation energies, as well as the ionization energy, of the Rydberg radical ArH. Absorption oscillator strengths and Einstein emission coefficients for a number of electronic transitions have been obtained with the molecular adapted quantum defect orbital (MQDO) method. The adequacy of the two theoretical procedures employed in the present work is discussed.

Temperature effects on the UV–Vis electronic spectrum of trans‐stilbene

AbstractThe ultraviolet (UV)–Visible absorption spectrum of trans‐stilbene (tS) is computed at different temperatures by coupling molecular dynamics (MD) simulations with the classical MM3 force field to ZINDO/S‐CIS calculations of vertical excitation energies and transition dipole moments. The selection of a large number of structures along the MD trajectories enables a consistent treatment of temperature effects in the vacuum, whereas the ZINDO/S‐CIS calculations permit a reliable treatment of electron correlation and relaxation, taking account of multistate interactions in the final state. Thermal motions are found to alter very differently the width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very strongly influence the appearance of the first valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO–LUMO) transition. At temperatures less than 400 K, these are found to yield a merely Gaussian and very pronounced thermal broadening of the related band (A), up to nearly 30 nm, together with a minor blue shift of its maximum λmax. In contrast, a red shift by several nanometers occurs due to thermal motions for the remaining three valence bands. As can be expected, the broadening intensifies at higher temperatures, and for the A‐band, becomes markedly asymmetric when T exceeds 400 K. The combination of MD(MM3) and ZINDO/S‐CIS computations enables also consistent calculations of hot bands, which are forbidden by symmetry at 0 K. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Spectroscopic and semi-empirical MO study of substituent effects on the intramolecular proton transfer in anils of 2-hydroxybenzaldehydes

Substituent effects in singly and doubly substituted anils of 2-hydroxybenzaldehyde showing strong intramolecular hydrogen bonding are studied on the basis of solution 13C NMR spectroscopic data and semiempirical MO (AM1) calculations of the relative stability of tautomers. Excited state proton transfer is also investigated using electronic absorption and fluorescence spectroscopies, and calculation of vertical excitation energies (INDO/S). The theoretical predictions are in agreement with the experimental observations.

Vacuum UV absorption and excitation spectra of N 2H 4

The vacuum UV (λ > 105 nm) absorption spectrum of N 2H 4 was reinvestigated in the light of recent ab initio calculations of vertical excitation energies and transition dipole moments. At the same time, fluorescence excitation spectra were measured to study the formation of excited radicals in this wavelength region. Weak formation of NH(A) at short wavelengths and, most probably, NH 2(Ã 2A 1) over a wide wavelength range were observed. Both excited species are believed to be generated by direct photodissociation.

MNDO-EHP method for the calculation of vertical excitation energies

Applicability of the unmodified MNDO theory to the low-lying excited singlet and triplet states is tested through an adaptation of the single excitation electron-hole potential (EHP) method to the theory. The method is both structurally and computationally very simple and has the useful feature of orbital picture for the excited states. The agreement, in the transition energies and also in the relative ordering of various excited states, obtained by our method and from the extended CI method, is very good. There is some problem in convergence arising out of symmetry breaking in cases of degeneracy in the ground state SCF wavefunction.

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.

Limited GSMO CI and ESMO CI study of electronic transitions in FNO

Limited CI calculations of vertical excitation energies and oscillator strengths have been performed in the ground state molecular orbital basis set (GSMO) and the excited state ones (ESMO). The absorption and emission spectrum of FNO is rediscussed on the basis of these calculations. The relaxation energy of excited states and the convergence of the CI expansion in the GSMO and ESMO basis sets is discussed.