Lowest Excited Electronic States Research Articles

Ground and Low-Lying Excited States of NaC Molecule

Potential energy curves (PECs) for the ground electronic state (X4Σ) and the three lowest excited electronic states (a2Π,b2Σ,A4Π) of NaC molecule were calculated using the multi-configuration reference single and double excited configuration interaction method,including Davidson's corrections for quadruple excitations (MRCI+Q).The equilibrium bond length Re and the vertical excited energy Te were determined directly and the PECs were fitted to an analytical Murrell-Sorbie (MS) potential function to determine the spectroscopic parameters,which were the rotation coupling constant ωe,dissociation energy De,the anharmonic constant ωe χe,the equilibrium rotation constant Be and Drot,and the vibration-rotation coupling constant αe.These values were also compared and were in agreement with other theoretical and experimental results currently available.It is evident that the X4Σ,a2Π,and b2Σ states are bound.We found that in the ground state X4Σ,Re was 0.2259 nm,ωe was 431 cm-1,and De was 1.92 eV,while in the excited states a2Π and b2Σ,Re and ωe were 0.2447,0.2369 nm and 329,335 cm-1,respectively.Te was found to be 1.58 and 1.75 eV and De was 0.71 and 0.42 eV.A4Π is a repulsive excited state when Te is 2.48 eV relative to the ground state.By solving the radial Schrdinger equation of nuclear motion the vibration levels and inertial rotation constant at rotational quantum number J=0 are reported for the X4Σ,a2Π,and b2Σ states.

Singlet oxygen: there is indeed something new under the sun

Singlet oxygen, O(2)(a(1)Delta(g)), the lowest excited electronic state of molecular oxygen, has been known to the scientific community for approximately 80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen, O(2)(X(3)Sigma), and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a "mature citizen", singlet oxygen nevertheless remains at the cutting-edge of modern science. In this critical review, recent work on singlet oxygen is summarized, focusing primarily on systems that involve light. It is clear that there is indeed still something new under the sun (243 references).

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part III: Theoretical investigation of oxalyl fluoride

We present a theoretical investigation of oxalyl fluoride (COF) 2 in the ground and the four lowest excited (two singlet and two triplet) electronic states of the n,π∗-type mainly with the CASPT2(8-6)/cc-pVTZ method. Geometries, vibrational frequencies, potential energy functions of internal rotation, and adiabatic electronic transition energies were obtained. The conformer energy difference and the barrier to internal rotation in the ground electronic state were extrapolated to the complete basis set limit. The planar trans and cis conformations were the most stable configurations for all five electronic states under study. We found that the allowed electronic transition of the cis conformer has a transition energy that is significantly higher than that predicted in previous studies. For the excited states, the internal rotation was found to be accompanied by significant non-planar distortion of both carbonyl fragments, indicating strong coupling between these molecular motions.

Nature of the lowest excited states of neutral polyenyl radicals and polyene radical cations

Due to the close relation of the polyenyl radicals C(2n+1)H(2n+3) () and polyene radical cations C(2n)H(2n+2) (+) to the neutral linear polyenes, one may suspect their excited states to possess substantial double excitation character, similar to the famous S(1) state of neutral polyenes and thus to be equally problematic for simple excited state theories. Using the recently developed unrestricted algebraic-diagrammatic construction scheme of second order perturbation theory and the equation-of-motion coupled-cluster method, the vertical excitation energies, their corresponding oscillator strengths, and the nature of the wave functions of the lowest excited electronic states of the radicals are calculated and analyzed in detail. For the polyenyl radicals two one-photon allowed states are found as D(1) and D(4) states, with two symmetry-forbidden D(2) and D(3) states in between, while in the polyene radical cations D(1) and D(2) are allowed and D(3) is forbidden. The order of the states is conserved with increasing chain length. It is found that all low-lying excited states exhibit a significant but similar amount of doubly excited configuration in their wave functions of 15%-20%. Using extrapolation, predictions for the excitation energies of the five lowest excited states of the polyene radical cations are made for longer chain lengths.

Photochemistry of the water dimer: Time-dependent quantum wave-packet description of the dynamics at the S1-S conical intersection

The photoinduced electron-driven proton-transfer dynamics of the water-dimer system has been investigated by time-dependent quantum wave-packet calculations. The main nuclear degrees of freedom driving the system from the Frank-Condon region to the S(0)-S(1) conical intersection are the distance between the oxygen atoms and the displacement of the hydrogen atom from the oxygen-oxygen bond center. Two important coupling modes have been investigated: Rotation of the H-donating water dangling proton and asymmetric stretching of the H-accepting water dangling protons' O(a)H bonds. Potential energy surfaces of the ground and lowest excited electronic states have been constructed on the basis of ab initio calculations. The time-dependent quantum wave-packet propagation has been employed within the (2 + 1)-dimensional systems for the description of the nonadiabatic dynamics of water dimer. The effects of the initial vibrational state of the system on the electronic population transfer and dissociation dynamics are presented. To approximate the photochemical behavior of water dimer in bulk water, we add a boundary condition into the (2 + 1)-dimensional systems to simulate the existence of water bulk. The results provide insight into the mechanisms of excited state deactivation of the water-dimer system in gas phase and in bulk water through the electron-driven proton-transfer process.

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part II: Theoretical investigation of oxalyl chloride

The structures of the oxalyl chloride molecule (COCl) 2 in the ground and the four lowest (two singlet and two triplet) excited electronic states were investigated by means of the CASPT2(8-6)/cc-pVTZ technique. The equilibrium geometric parameters, harmonic vibrational frequencies, and adiabatic energies of the electronic transitions were determined for all states under investigation. The calculations predicted the existence of the trans- and gauche- conformers in the ground state and the trans- and cis-conformers in all excited states. For the ground electronic state, the conformer energy difference and the barriers to conformational transitions were estimated using extrapolation to the complete basis set within a Valence Focal-Point Analysis procedure. The internal rotation in the excited electronic states was found to be strongly coupled with the non-planar symmetric CCOCl wagging. Two-dimensional potential energy surface sections along internal rotation and non-planar coordinates were constructed, and the corresponding anharmonic vibrational problems for the trans-conformer were solved.

Computational Study on the Photophysics of Protonated Benzene

The reaction paths in the lowest excited electronic states relevant for the photophysics of protonated benzene, C(6)H(7)(+), have been explored by ab initio techniques of electronic structure theory. For this purpose, the first four excited singlet electronic states of C(6)H(7)(+) have been calculated at the CC2/cc-pVTZ level of theory. The CC2 approach has been validated by CASPT2 and TD-DFT calculations. The calculated UV absorption spectrum is in good agreement with the experimental spectrum. It has been found that the out-of-plane and the in-plane ring deformation leads in the excited states in an essential barrierless manner to a low-lying conical intersection between the lowest excited states and with the ground state, providing a mechanism for efficient radiationless deactivation, which is expected to quench luminescence of the isolated molecular ion.

Charge Redistribution in Oxidized and Semiquinone E. coli DNA Photolyase upon Photoexcitation: Stark Spectroscopy Reveals a Rationale for the Position of Trp382

The electronic structure of the two lowest excited electronic states of FAD and FADH(*) in folate-depleted E. coli DNA photolyase (PL(OX) and PL(SQ), respectively) was measured using absorption Stark spectroscopy. The experimental analysis was supported by TDDFT calculations of both the charge redistribution and the difference dipole moments for the transitions of both oxidation states using lumiflavin as a model. The difference dipole moments and polarizabilities for PL(OX) are similar to those obtained in our previous work for flavins in simple solvents and in an FMN-containing flavoprotein. No such comparison can be made for PL(SQ), as we believe this to be the first experimental report of the direction and magnitude of excited-state charge redistribution in any flavosemiquinone. The picture that emerges from these studies is discussed in the context of electron transfer in photolyase, particularly for the semiquinone photoreduction process, which involves nearby tryptophan residues as electron donors. The direction of charge displacement derived from an analysis of the Stark spectra rationalizes the positioning of the critical Trp382 residue relative to the flavin for efficient vectorial electron transfer leading to photoreduction. The ramifications of vectorial charge redistribution are discussed in the context of the wider class of flavoprotein blue light photoreceptors.

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part I: Reanalysis of the 3680Å and 4100Å absorption systems of oxalyl chloride

The vapor-phase absorption spectrum of oxalyl chloride in the 3000–4180 Å region has been re-examined at high resolution. Singlet–singlet A ˜ 1 A u ← X ˜ 1 A g and singlet–triplet a ˜ 3 A u ← X ˜ 1 A g electronic transitions of the trans- conformer found in the spectrum are in agreement with earlier works [W.J. Balfour, G.W. King, J. Mol. Spectrosc. 26 (1968) 384–397; ibid. 27 (1968) 432–442]. Torsion levels of trans -oxalyl chloride in the ground X ˜ 1 A g and excited A ˜ 1 A u and a ˜ 3 A u states were found for the first time. Ab initio calculations of structures for conformers of oxalyl chloride in the ground and lowest excited electronic states explain the absence of second conformer transitions in the vibronic absorption spectrum.

Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

Tunneling electrons from a scanning tunneling microscope (STM) induce luminescence from C(60) and C(70) molecules forming fullerene nanocrystals grown on ultrathin NaCl films on Au(111). Intramolecular fluorescence and phosphorescence associated with the transitions between the lowest electronic excited state and ground state of C(70) molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover we demonstrate that the molecular luminescence is selectively enhanced by localized surface plasmons in the STM tip-sample gap.

Theoretical study of the spectroscopy and the metastability of the NS+ cation

The potential energy curves of the NS+ electronic states are computed at the aug-cc-pV5Z/CASSCF/MRCI level of theory. Using these highly correlated wavefunctions, we evaluated their mutual spin–orbit coupling terms and the transition moment evolutions. Then, we deduced an accurate set of spectroscopic constants and investigated the spin–orbit induced predissociation of the lowest electronic excited states of this cation. In particular, we identify a new well-bound quintet state, namely the NS+ (15Π). The NS+(15Π) v′+ ⩽ 5 levels decay radiatively to populate the NS+(1 5Σ+) state after emission of visible light, whereas the upper rovibrational levels undergo rapid spin–orbit induced predissociation processes forming S+(4S) and N(4S) fragments via the repulsive 17Σ+ state. The potential energy curve of NS (X2Π) is also computed and used together with those of NS+ electronic states for the prediction of the single ionization spectrum of NS.

Accelerated and Efficient Photochemistry from Higher Excited Electronic States in Fulgide Molecules

The photoinduced electrocyclic ring-opening of a fluorinated indolylfulgide is investigated by stationary and ultrafast spectroscopy in the UV/vis spectral range. Photoreactions, initiated by optical excitation into the S(1) (570 nm) and S(N) (340 nm) absorption band of the closed isomer, lead to considerable differences in reaction dynamics and quantum yields. Transient absorption studies point to different reaction pathways depending on the specific excitation wavelength: excitation into the S(1) state leads to the known reaction behavior with a picosecond decay to the ground state and a small quantum yield of 7% for the photoproduct. The S(N) state shows an unexpected long lifetime of 0.5 ps. The photoreaction starting from the S(N) state leads to a large extent directly to the product ground state and back to the educt ground state. This results in an increased reaction quantum yield of 28%. In contradiction to Kasha's rule, the S(1) state is only populated with an efficiency of 38%. The observed behavior strongly differs from the expected picture with fast relaxation into the S(1) state and a subsequent ring-opening reaction starting from the lowest excited electronic state. Quantum chemical calculations confirm and complement the experimental findings allowing a sound molecular interpretation to be obtained.

Schottky anomaly of linear birefringence of light in the antiferromagnetic crystal LiCoPO4

Anomalous behavior of the birefringence of linearly polarized light has been observed in the antiferromagnetic magnetoelectric crystal LiCoPO4 at temperatures approaching the Néel temperature. Arguments are present in support of the assumption that the observed nonmonotonic change of the birefringence of light is due to the interplay of two anomalies—a magnetic anomaly, associated with the intensification of the contribution of correlated spin fluctuations, and the Schottky anomaly, due to the changing occupations of the ground and lowest excited electronic states of Co2+ ions. The estimate of the energy splitting from the excited state (45–55K) agrees satisfactorily with the position of the weakly dispersing branch in the energy spectrum of this crystal, observed in an investigation of the inelastic scattering of neutrons, as well as with the energies of the low-energy bands in the Raman scattering spectrum.

Electronic relaxation and vibrational dynamics in a thiophene oligomer studied under the same experimental condition with a sub-5 fs laser

Using a sub-5 fs pulse laser, the pump–probe experiment was performed on a quinoid thiophene derivative and both electronic relaxation and vibrational dynamics were clarified with the same experimental data. From the data, two population decay times of the lowest electronic excited state were determined to be 200 fs and 1.8 ps, which can be explained by the assumption that the 21Ag state lies below the exciton 11Bu state together with the very low fluorescence quantum efficiency. The relaxation process after excitation by the pump pulse was studied to be 11Bu→11B*u→21Ag→11Ag. According to the experimental data, the location of the energy level was also discussed. The electronic dephasing time was determined to be 64 ± 4 fs by utilizing the data in the ‘negative time’ range. The vibrational dephasing time constant of the most strongly coupled mode with frequency of 1466 cm-1 was determined to be 520 ± 20 fs from the widths of the corresponding Fourier power spectra.

Absorption and emission lines of a mercury molecule in a mercury discharge lamp

Absorption lines are revealed in spectra of a mercury discharge lamp (Planck’s radiator). Some of the lines were identified by Lord Rayleigh as the absorption lines of a Hg2 molecule. New absorption lines at 253.29, 253.53, 253.76, and 254.20 nm, as well as emission lines at 264.80, 254.96, 265.11, 279.89, 288.82, 301.50, 301.74, and 301.95 nm are discovered, which enabled the construction of a diagram of the lower excited electronic states of a mercury molecule. It is demonstrated that the dips in the intensity profiles of the atomic mercury lines at 253.65, 435.83, 404.65, and 546.07 nm arise from the radiation absorption in vibronic transitions of diatomic Hg2 molecules. The concentration of mercury molecules in the plasma of mercury discharge lamps is determined to be of the order of 108 cm−3. However, their contribution to the formation of the emission line profiles is essential due to the multiple radiation passage through the emitting plasma.

Spectroscopy and metastability of BeO+

The potential energy curve of the ground electronic state of BeO and those of the lowest electronic states of the BeO+ cation are computed using the CASSCF/MRCI methods and a large basis set. For the cation, the spin–orbit coupling and the transition momentum integrals are also evaluated. These data are used later to deduce an accurate set of spectroscopic constants and to investigate the spin–orbit-induced predissociation of the lowest electronic excited states of BeO+. Our calculations show that the high-rovibrational levels of the BeO+ (12Σ+) electronic state exhibit rapid predissociation processes forming Be+(2S) + O(3P). Our curves are also used for predicting the single ionization spectrum of BeO.

Molecular parameters of tetraatomic carbonyls X2CO and XYCO (X, Y = H, F, Cl) in the ground and lowest excited electronic states, part 1: A test ofab initiomethods

AbstractGeometrical parameters of tetraatomic carbonyl molecules X2CO and XYCO (X, Y = H, F, Cl) in the ground (S0) and lowest excited singlet (S1) and triplet (T1) electronic states as well as values of barriers to inversion inS1andT1states andS1←S0andT1←S0adiabatic transition energies were systematically investigated by means of various quantum‐chemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOM‐CCSD, CCSD(T), CR‐EOM‐CCSD(T), CASSCF, MR‐MP2, CASPT2, CASPT3, NEVPT2, MR‐CISD, and MR‐AQCC within cc‐pVTZ and cc‐pVQZ basis sets. The accuracy of quantum‐chemical methods was estimated in comparison with experimental data and rather accurate structures of excited electronic states were obtained. MP2 and CASPT2 methods appeared to be the most efficient and CCSD(T), CR‐EOM‐CCSD(T), and MR‐AQCC the most accurate. It was found that at equilibrium all the molecules under study are nonplanar inS1andT1electronic states with CO out‐of‐plane angle ranging from 34° (H2CO,S1) to 52° (F2CO,T1), and height of barrier to inversion varying from 300 (H2CO,S1) to 11,000 (F2CO,T1) cm−1. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Ab initio study of the one- and two-photon circular dichroism of R-(+)-3-methyl-cyclopentanone

One- and two-photon circular dichroism spectra of R-(+)-3-methyl-cyclopentanone, a system that has been the subject of recent experimental studies of (2+1) resonance-enhanced multiphoton ionization circular dichroism, have been calculated with an origin-invariant density functional theory approximation in the region of the lowest electronic excited states, both for the gas phase and for a selection of solvents. A polarizable continuum model is used in the calculations performed on the solvated system. Two low-lying conformers are analyzed, and a comparison of the intensities and characteristic features is made with the corresponding two-photon absorption for each species, also for the Boltzmann-averaged spectra. The effect of the choice of geometry, basis set, and exchange-correlation functional is carefully analyzed. It is found that a density functional theory approach using the Coulomb attenuating method variant of Becke's three-parameter exchange and the Lee-Yang-Parr correlation functionals with correlation-consistent basis sets of double-zeta quality can reproduce the experimental electronic circular dichroism spectra very well. The features appearing in experiment are characterized in terms of molecular excitations, and the differences in the response of each state in the one- and two-photon processes are highlighted.

Electron impact excitations of S2 molecules

Low-energy electron impact excitations of S2 molecules are studied using the fixed-nuclei R-matrix method based on state-averaged complete active space SCF orbitals. Integral cross sections are calculated for elastic electron collision as well as impact excitation of the 7 lowest excited electronic states. Also, differential cross sections are obtained for elastic collision and excitation of the a1Δg,b1Σg+ and B3Σu- states. The integrated cross section of optically allowed excitation of the B3Σu- state agrees reasonably well with the available theoretical result.

The electronic delocalization in para‐substituted β‐nitrostyrenes probed by resonance Raman spectroscopy and quantum‐chemical calculations

AbstractThe electronic (UV‐vis) and resonance Raman (RR) spectra of a series of para‐substituted trans‐β‐nitrostyrenes were investigated to determine the influence of the electron donating properties of the substituent (X = H, NO2, COOH, Cl, OCH3, OH, N(CH3)2, and O−) on the extent of the charge transfer to the electron‐withdrawing NO2 group directly linked to the ethylenic (C = C) unit. The Raman spectra and quantum chemical calculations show clearly the correlation of the electron donating power of the X group with the wavenumbers of the νs(NO2) and ν (C = C)sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH3)2 that the symmetric stretching of the NO2, νs(NO2), is the most substantially enhanced mode, whereas for X = O−, the chromophore is extended over the whole molecule, with substantial enhancement of several carbon backbone modes. Copyright © 2008 John Wiley & Sons, Ltd.