Lowest-lying Excited States Research Articles

Complete basis set limit extrapolation calculations with PNOF3

Complete basis set (CBS) extrapolated total energies have been calculated for the ground state and lowest-lying excited state with different spin of the second-row atoms (Li–Ne) using the recently developed spin-conserving PNOF3 natural orbital functional. CBS results are obtained by extrapolating power and exponential functions to the results obtained using the correlation-consistent aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets. The extrapolated total energy values are very accurate as compared to the benchmark data, and reproduce near 100% of the total energy. Interestingly, the benchmark data are lower bounds for the PNOF3 results. Finally, we have found that the calculated PNOF3 excitation energies are very accurate as comparing with both their corresponding CCSD(T, full) values and their experimental marks.

Standard photoacoustic spectrometer: Model and validation using O2 A-band spectra

We model and measure the absolute response of an intensity-modulated photoacoustic spectrometer comprising a 10 cm long resonator and having a Q-factor of approximately 30. We present a detailed theoretical analysis of the system and predict its response as a function of gas properties, resonance frequency, and sample energy transfer relaxation rates. We use a low-power continuous wave laser to probe O(2) A-band absorption transitions using atmospheric, humidified air as the sample gas to calibrate the system. This approach provides a convenient and well-characterized method for calibrating the absolute response of the system provided that water-vapor-mediated relaxation effects are properly taken into account. We show that for photoacoustic spectroscopy (PAS) of the O(2) A-band, the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40% and is limited by finite collision-induced relaxation rates between the two lowest-lying excited electronic states of O(2). PAS also shows great potential for high-resolution line shape measurements: calculated and experimental values for the PAS system response differ by about 1%.

Electronically excited and ionized states of the CH2CH2OH radical: A theoretical study

The low lying excited electronic states of the 2-hydroxyethyl radical, CH(2)CH(2)OH, have been investigated theoretically in the range 5-7 eV by using coupled-cluster and equation-of-motion coupled-cluster methods. Both dissociation and isomerization pathways are identified. On the ground electronic potential energy surface, two stable conformers and six saddle points at energies below approximately 900 cm(-1) are characterized. Vertical excitation energies and oscillator strengths for the lowest-lying excited valence state and the 3s, 3p(x), 3p(y), and 3p(z) Rydberg states have been calculated and it is predicted that the absorption spectrum at approximately 270-200 nm should be featureless. The stable conformers and saddle points differ primarily in their two dihedral coordinates, labeled d(HOCC) (OH torsion around CO), and d(OCCH) (CH(2) torsion around CC). Vertical ionization from the ground-state conformers and saddle points leads to an unstable structure of the open-chain CH(2)CH(2)OH(+) cation. The ion isomerizes promptly either to the 1-hydroxyethyl ion, CH(3)CHOH(+), or to the cyclic oxirane ion, CH(2)(OH)CH(2) (+), and the Rydberg states are expected to display a similar behavior. The isomerization pathway depends on the d(OCCH) angle in the ground state. The lowest valence state is repulsive and its dissociation along the CC, CO, and CH bonds, which leads to CH(2)+CH(2)OH, CH(2)CH(2)+OH, and H+CH(2)CHOH, should be prompt. The branching ratio among these channels depends sensitively on the dihedral angles. Surface crossings among Rydberg and valence states and with the ground state are likely to affect dissociation as well. It is concluded that the proximity of several low-lying excited electronic states, which can either dissociate directly or via isomerization and predissociation pathways, would give rise to prompt dissociation leading to several simultaneous dissociation channels.

Communications: Accurate description of atoms and molecules by natural orbital functional theory

The spin-conserving density matrix functional theory is used to propose an improved natural orbital functional. The Piris reconstruction functional, PNOF, which is based on an explicit form of the two-particle cumulant lambda(Delta,Lambda) satisfying necessary positivity conditions for the two-particle reduced density matrix, is used to reconstruct the latter. A new approach Lambda((3)), as well as an extension of the known Delta(alphabeta) to spin-uncompensated systems lead to PNOF3. The theory is applied to the calculation of the total energies of the first- and second-row atoms (H-Ne) and a number of selected small molecules. The energy differences between the ground state and the lowest-lying excited state with different spin for these atoms, and the atomization energies of the considered molecules are also presented. The obtained values agree remarkably well with their corresponding both CCSD(T, full) and experimental values.

Mono- versus Dinuclear Pt(II) 6-(5-Trifluoromethyl-Pyrazol-3-yl)-2,2′-Bipyridine Complexes: Synthesis, Characterization, and Remarkable Difference in Luminescent Properties

A series of charge-neutral mononuclear Pt(II) complexes Pt(fpbpy)(pz) (3a), Pt(fpbpy)(dmpz) (4a), Pt(fpbpy)(dbpz) (5a), and Pt(fpbpy)(dtfpz) (6a), fpbpyH = 6-(5-trifluoromethyl-pyrazol-3-yl)-2,2'-bipyridine, pzH = pyrazole, dmpzH = 3,5-dimethylpyrazole, dbpzH = 3,5-di-tert-butylpyrazole, and dtfpzH = 3,5-bis(trifluoromethyl)pyrazole, and the cationic Pt(II) dimer [{Pt(fpbpy)}(2)(mu-pz)](+) (3b), [{Pt(fpbpy)}(2)(mu-dmpz)](+) (4b), and [{Pt(fpbpy)}(2)(mu-dbpz)](+) (5b) were synthesized. Series a mononuclear complexes reveal two distinctive ligand arrangements. As unveiled by X-ray crystallography, 3a exhibits a nearly perfect planar geometry, while structural determination on 6a shows a perpendicular arrangement of dbpz ligand due to steric congestion. In sharp contrast, the dinuclear complexes, exemplified by 4b and 5b, display an intramolecular Pt...Pt separation of 3.601 and 3.403 A, respectively. As for photophysical properties, the structural variation leads to a salient difference in emission features between 3a (580 nm) and 6a (510 nm). The results are rationalized by the contribution of ligand-to-ligand charge transfer and intraligand pi-pi* transition for 3a and 6a in the lowest-lying excited state, respectively. On the other hand, dinuclear complexes 3b and 4b reveal dual phosphorescence (denoted as P(1) and P(2) bands), for which the short wavelength emission (the P(1) band) is akin to that observed for the intraligand pi-pi* transition of 6a, while the much red-shifted, broad emission (the P(2) band) is attributed to the formation of intramolecular ligand-metal-to-metal charge transfer excimer transition. Further studies of relaxation dynamics on both 3b and 4b showed fast excited-state equilibrium between the P(1) and P(2) bands. In contrast, only the P(2) emission band was resolved for 5b, indicating its exergonic excimer formation. Supplementary support of the excited-state thermodynamics is also provided by time-dependent density functional theory calculations, incorporating both geometry optimized S(0) and T(1) states.

Using End Groups to Tune the Linear and Nonlinear Optical Properties of Bis(dioxaborine)‐Terminated Polymethine Dyes

Six anionic pentamethine dyes with different 2,2-difluoro-4-aryl-1,3,2(2 H)-dioxaborin-6-yl termini were synthesized and isolated as tetra-n-octylammonium salts with a variety of aryl groups appended to increase conjugation beyond the dioxaborine termini. The increased conjugation was expected to decrease the energy of the lowest-lying excited state, and increase the transition dipole moment linking this state to the ground state, which would be anticipated to result in an increase in the real part of the third-order polarizability, Re(gamma). UV/Vis-NIR absorption spectroscopy indicates that the absorption maxima in DMSO vary from 691 to 761 nm, with the longest wavelength transitions observed for a derivative where the aryl group is 4-nitrophenyl. Closed-aperture Z-scan measurements at 1.3 microm in DMSO indicate that Re(gamma) varies from -2.9x10(-33) to -5.4x10(-33) esu in these systems. The largest magnitude of Re(gamma) was observed for a dye with E-4-styrylphenyl aryl groups. This result can be rationalized using a two-state expression which relates Re(gamma) to the energy and transition dipole moment of the transition from the ground state to the lowest-lying excited state. A nonamethine analogue of this compound was also synthesized and exhibits a slightly larger Re(gamma) with respect to a previously reported bis(dioxaborine)-terminated nonamethine. The extension of conjugation beyond the dioxaborine termini seems to result in an overall increase in Re(gamma). However, the effects are smaller than those found by increasing conjugation in the polymethine bridge due to reduced participation of terminal groups in the HOMO.

Reinvestigation of electronic structure and electric properties of large betaine molecules: A combined long-range-corrected DFT and coupled-cluster study

In this study we discuss the electric properties of imidazole pyridinic betaine family of molecules. We present the results of calculations of dipole moments corresponding to the ground and to the lowest-lying electronic excited state as well as first hyperpolarizability. The two quantities have been determined using high-level ab initio quantum-chemical approaches including the second-order Møller–Plesset perturbation theory and coupled-cluster methods. It is shown that the Hartree–Fock approximation, employed quite recently to describe photophysical properties for this class of compounds [ J.M.F. Pinheiro, P.H.R. Peixoto, C.P. de Melo, Chem. Phys. Lett. 463 (2008) 172], yields qualitatively and quantitatively incorrect results of molecular first hyperpolarizability as well as excited state dipole moments. We also show that as far as static first hyperpolarizability is concerned the studied compounds are challenging for long-range-corrected density functional theory.

Structure of the Photochemical Reaction Path Populated via Promotion of CF2I2 into Its First Excited State

The photochemical reaction path following the promotion of CF(2)I(2) into its lowest-lying excited electronic singlet state has been modeled using ab initio multiconfigurational quantum chemical calculations. It is found that a conical intersection drives the electronically excited CF(2)I(2)* species either to the CF(2)I + I radical pair or back to the starting CF(2)I(2) structure. The structures of the computed relaxation pathways explain the photoproduct selectivity previously observed in the gas phase. Furthermore, the results provide the basis for explaining the condensed-phase photochemistry of CF(2)I(2).

Internal energy distribution change of sputtered Al atom under several hundreds of eV oxygen ion-beam bombardment by resonant laser ionization sputtered neutral mass spectroscopy

It is important to evaluate the internal energy of the sputtered neutral particles to optimize their resonant ionization efficiency. In this study, the internal energy of the sputtered Al atom, especially the ground state P1∕22 and the lowest-lying excited state P3∕22 (112cm−1), was monitored by using resonant laser postionization sputtered neutral mass spectrometry. The results of the authors showed the density of the ground state of the sputtered Al atom drastically decreased due to surface oxidation, as well as the enhancement of the secondary Al+ yield.

Measurement of the solar neutrino capture rate with gallium metal. III. Results for the 2002–2007 data-taking period

The Russian-American experiment SAGE began to measure the solar neutrino capture rate with a target of gallium metal in December 1989. Measurements have continued with only a few brief interruptions since that time. In this article we present the experimental improvements in SAGE since its last published data summary in December 2001. Assuming the solar neutrino production rate was constant during the period of data collection, combined analysis of 168 extractions through December 2007 gives a capture rate of solar neutrinos with energy more than 233 keV of $65.{4}_{\ensuremath{-}3.0}^{+3.1}$ (stat) ${}_{\ensuremath{-}2.8}^{+2.6}$ (syst) SNU. The weighted average of the results of all three Ga solar neutrino experiments, SAGE, Gallex, and GNO, is now $66.1\ifmmode\pm\else\textpm\fi{}3.1$ SNU, where statistical and systematic uncertainties have been combined in quadrature. During the recent period of data collection a new test of SAGE was made with a reactor-produced $^{37}\mathrm{Ar}$ neutrino source. The ratio of observed to calculated rates in this experiment, combined with the measured rates in the three prior $^{51}\mathrm{Cr}$ neutrino-source experiments with Ga, is $0.87\ifmmode\pm\else\textpm\fi{}0.05$. A probable explanation for this low result is that the cross section for neutrino capture by the two lowest-lying excited states in $^{71}\mathrm{Ge}$ has been overestimated. If we assume these cross sections are zero, then the standard solar model including neutrino oscillations predicts a total capture rate in Ga in the range of 63 SNU to 66 SNU with an uncertainty of about 4%, in good agreement with experiment. We derive the current value of the neutrino flux produced in the Sun by the proton-proton fusion reaction to be ${\ensuremath{\phi}}_{\mathit{pp}}^{\ensuremath{\bigodot}}=(6.0\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{10}/({\mathrm{cm}}^{2} \mathrm{s})$, which agrees well with the $\mathit{pp}$ flux predicted by the standard solar model. Finally, we make several tests and show that the data are consistent with the assumption that the solar neutrino production rate is constant in time.

Tuning the Intramolecular Charge Transfer of Alkynylpyrenes: Effect on Photophysical Properties and Its Application in Design of OFF−ON Fluorescent Thiol Probes

Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV-vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., -CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF-ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S(1)) for these thiol probes (e.g., oscillator strength f = 0.0086 for S(1)<--S(0) transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore as electron donor and 2,4-dinitrobenzenesulfonyl unit as electron acceptor. Cleavage of the 2,4-dinitrobenzenesulfonyl unit by thiol releases the free fluorophore, for which the lowest-lying excited state S(1) is no longer a dark state, but an emissive state (f = 0.9776 for S(1)<--S(0) transition). These theoretical predictions on the photophysical properties of the molecular probes were fully proved by experimental results. Our results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect (which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of ICT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore. These investigation on the pyrene derived green-emitting fluorophores and the DFT/TDDFT calculation aided probe design suggest that future application of these results may prove useful toward the rational design of fluorophores or fluorescent probes with predetermined photophysical properties.

Linear and nonlinear optical properties of azobenzene derivatives

The results of computations of spectroscopic parameters of lowest-lying electronic excited states of azobenezene derivatives are presented. The analysis of experimentally recorded spectra was supported by quantum chemical calculations using density functional theory. The theoretically determined resonant (two-photon absorption probabilities) and non-resonant (first-order hyperpolarisability) nonlinear optical properties are also discussed, with an eye towards the performance of recently proposed long-range corrected (LRC) schemes (LC-BLYP and CAM-B3LYP functionals).

Femtosecond pump-supercontinuum probe and transient lens spectroscopy of adonixanthin

The ultrafast internal conversion (IC) dynamics of adonixanthin in organic solvents were studied by pump-supercontinuum probe (PSCP) and transient lens (TL) spectroscopy after photoexcitation to the S 2 state. Transient PSCP spectra in the range 344–768 nm provided the spectral evolution of the S 0 → S 2 ground state bleach and S 1 → S n excited state absorption. Time constants were τ 2 = 115 and 111 fs for the S 2 → S 1 IC and τ 1 = 6.4 and 5.8 ps for the S 1 → S 0 IC in acetone and methanol, respectively. There was only an insignificant polarity dependence of τ 1, underlining the negligible importance of intramolecular charge transfer (ICT) in the lowest-lying excited state of C 40 carotenoids with carbonyl substitution on the β-ionone ring. A blueshift and a spectral narrowing of the S 1 → S n ESA band, likely due to solvation dynamics, and formation of the adonixanthin radial cation at high pump energies via resonant two-photon ionization were found.

A novel series of iridium complexes with alkenylquinoline ligands: Theoretical study on electronic structure and spectroscopic property

The ground and the lowest-lying triplet excited state geometries, electronic structures, and spectroscopic properties of a novel series of neutral iridium(III) complexes with cyclometalated alkenylquinoline ligands [(C^N) 2Ir(acac)] (acac = acetoylacetonate; C^N = 2-[( E)-2-phenyl-1-ethenyl]pyridine (pep) 1; 2-[( E)-2-phenyl-1-ethenyl]quinoline (peq) 2; 1-[( E)-2-phenyl-1-ethenyl]isoquinoline (peiq) 3; 2-[( E)-1-propenyl]pyridine (pp) 4; 2-[( E)-1-fluoro-1-ethenyl]pyridine (fpp) 5) were investigated by DFT and CIS methods. The highest occupied molecular orbital is composed of d(Ir) and π(C^N) orbital, while the lowest unoccupied molecular orbital is dominantly localized on C^N ligand. Under the TD-DFT with PCM model level, the absorption and phosphorescence in CH 2Cl 2 media were calculated based on the optimized ground and triplet excited state geometries, respectively. The calculated lowest-lying absorptions at 437 nm ( 1), 481 nm ( 2), 487 nm ( 3), 422 nm ( 4), and 389 nm ( 5) are attributed to a {[ d x 2 - y 2 (Ir) + d xz (Ir) + π(C^N)] → [π∗(C^N)]} transition with metal-to-ligand/intra-ligand charge transfer (MLCT/ILCT) characters, and the calculated phosphorescence at 582 nm ( 1), 607 nm ( 2), 634 nm ( 3), 515 nm ( 4), and 491 nm ( 5) can be described as originating from the 3{[ d x 2 - y 2 (Ir) + d xz (Ir) + π (C^N)] [π∗(C^N)]} excited state with the 3MLCT/ 3ILCT characters. The calculated results revealed that the phosphorescent color of these new Ir(III) complexes can be tuned by changing the π-conjugation effect strength of the C^N ligand.

Role of Electronic Structure on DNA Light-Switch Behavior of Ru(II) Intercalators

A series of ruthenium(II) complexes possessing ligands with an extended pi system were synthesized and characterized. The complexes are derived from [Ru(bpy)3](2+) (1, bpy = 2,2'-bipyridine) and include [Ru(bpy)2(tpphz)](2+) (2, tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine), [Ru(bpy)2(dppx)](2+) (3, dppx = 7,8-dimethyldipyrido[3,2-a:2',3'-c]phenazine), [Ru(bpy)2(dppm2)](2+) (4, dppm2 = 6-methyldipyrido[3,2-a:2',3'-c]phenazine), and [Ru(bpy)2(dppp2)](2+) (5, dppp2 = pyrido[2',3':5,6]pyrazino[2,3-f][1,10]phenanthroline). The excited-state properties of these complexes, including their DNA "light-switch" behavior, were compared to those of [Ru(bpy)2(dppz)](2+) (6, dppz = dipyrido[3,2-a:2',3'-c]phenazine). Whereas 2, 3, and 4 can be classified as DNA light-switch complexes, 5 exhibits negligible luminescence enhancement in the presence of DNA. Because relative viscosity experiments show that 2-6 bind to DNA by intercalation, their electronic absorption and emission spectra, electrochemistry, and temperature dependence of the luminescence were used to explain the observed differences. The small energy gap between the lowest-lying dark excited state and the bright state in 2-4 and 6 is related to the ability of these complexes to exhibit DNA light-switch behavior, whereas the large energy gap in 5 precludes the emission enhancement in the presence of DNA. The effect of the energy gap among low-lying states on the photophysical properties of 1-6 is discussed. In addition, DFT and TD-DFT calculations support the conclusions from the experiments.

Synthesis, Electrochemical and Optical Properties of RuII–Diphenylphenanthroline–Ethynylpyrenephenanthroline Systems

AbstractThe synthesis, electrochemical and optical spectroscopic properties of a RuII–diphenylphenanthroline complex decorated with ethynylpyrene (EP) appendages, 1, and those of its parent bromo‐substituted complex, 2, are reported; complex 1 is [Ru(dpp)2(phen–EP2)](PF6)2 and 2 is [Ru(dpp)2(phen–Br2)](PF6)2, where dpp is 4,7‐diphenyl‐1,10‐phenanthroline, phen–EP is 5,6‐(1‐ethynylpyrene)‐1,10‐phenanthroline and phen–Br2 is 5,6‐dibromo‐1,10‐phenanthroline. In CH3CN solvent, both bromo‐ and EP‐substituted complexes are redox active and metal‐centred and ligand‐centred processes are assigned with reference to the analogous steps for [Ru(phen)3]2+ and to steps for the EP appendages. The complexes exhibit strong absorption bands in the near‐UV and visible regions due to ligand‐centred (LC) and metal‐to‐ligand charge‐transfer (MLCT) transition, respectively. For 1, the lowest‐lying absorption band is slightly redshifted due to better delocalisation at the phen–EP ligand. At room temperature and in O2‐free solvent, complex 1 appeared to be practically nonluminescent, whereas 2 exhibited intense 3MLCT emission; quantum yields of fem = 9 × 10–4 and 6.1 × 10–2 were found for 1 and 2, respectively and λexc = 462 nm in both cases. The lowest‐lying excited states of 1 were assigned as 3LC states, which were localised on the EP moiety. These states are nonluminescent, which thus explains the weak room‐temperature luminescence for this complex. At 77 K, 2 is emissive, as expected for 3MLCT levels; however, a moderately intense 3MLCT emission was also observed for 1 at this temperature. For the latter case, this outcome is tentatively explained in terms of local trapping of 3MLCT states at the 4,7‐diphenylphenanthroline ligands free of EP appendages.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Intramolecular Optical Electron Transfer in Mixed-Valent Dinuclear Iron−Ruthenium Complexes Featuring a 1,4-Diethynylaryl Spacer

The ground-state electronic structure and the lowest-lying excited states of the cationic mixed-valent dinuclear complexes [(η2-dppe)(η5-C5Me5)Fe[C≡C-1,4-(C6H4)C≡C]Ru(η2-dppe)2(X)][PF6] (X = Cl, 2; X = C≡C(4-C6H4NO2), 5) are discussed, with particular emphasis on the photoinduced intramolecular electron transfer between the ruthenium and iron centers. The location and intensities of the low-lying absorptions exhibited in the near-infrared (near-IR) range by these heterodinuclear mixed-valent (MV) complexes correlate with predictions based on the Hush model, strongly suggesting that they correspond to intervalence charge-transfer (IVCT) bands.

Ultrafast formation of I2 following 350-nm photodissociation of CF2I2 in n-hexane

This study presents the photodissociation dynamics of difluorodiiodomethane (CF2I2) in n-hexane by femtosecond pump-probe spectroscopy. Following 350-nm excitation of CF2I2 into the lowest-lying excited electronic state, the transient absorption spectra are measured between 355 and 785nm up to a time delay of 1200ps. The results suggest that electronically excited CF2I2∗ molecules undergo a step-wise decay giving rise to two iodine fragments. The majority of the nascent iodine fragments combine on a 2-ps timescale to form molecular iodine. The spectral evolution is complete within 500ps where the quantum yield of I2 formation is measured to be 32%.

Interactions in Diatomic Dimers Involving Closed-Shell Metals

Interaction energies of dimers containing alkaline earth (Be, Mg, and Ca) metals have been investigated using symmetry-adapted perturbation theory (SAPT) and supermolecular (SM) methods. Also, to enable broader comparisons, some calculations have been performed on the Zn dimer and on the He-Mg dimer. Although all of the investigated metallic atoms have closed electronic shells, the quasidegeneracy of the ground states of these atoms with the lowest-lying excited states leads to convergence problems in theories based on a single-determinant reference state. The main goal of the present work was to establish how the quality of the interaction energies computed using various electronic-structure methods changes across the range of atoms. We show that although the convergence problems become somewhat less severe with the increase of the atomic number, single-determinant-based methods do not provide reliable interaction energies for any of the investigated metallic dimers even at the level of the coupled-cluster method with single, double, and noniterative triple excitations [CCSD(T)]. However, interaction energies accurate to within a few percent can be obtained if CCSD(T) calculations in large basis sets are extrapolated to the complete basis set limit and followed by full configuration interaction (FCI) calculations with a frozen-core (FC) approximation. Since the systems considered contain only two valence electrons, FCI/FC calculations have been feasible for all of them except for Zn2, providing the best theoretical estimates of the binding energies to date. We found that a large part of the error of the SAPT results originates from limiting some exchange components to terms proportional to the squares of the intermonomer orbital overlap integrals. When the neglected terms were approximately accounted for, the accuracy improved significantly and became comparable to that of CCSD(T), allowing us to obtain for the first time a physical interpretation of the interaction energies in metallic dimers.

Low-lying quartet electronic states of nitrogen dioxide

The environmentally active molecule nitrogen dioxide (NO2) has been systematically studied using high level theoretical methods. The electronic ground state and the low-lying quartet states of NO2 have been investigated. Single reference restricted open-shell self-consistent field (SCF), complete active space SCF (CASSCF), spin-restricted (R) and spin-unrestricted (U) configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and internally contracted multireference configuration interaction (ICMRCI) methods along with Dunning's correlation consistent polarized valence cc-pVXZ and augmented cc-pVXZ (where X=T,Q,5) basis sets were used in this research. At the aug-cc-pV5Z/UCCSD(T) level the classical adiabatic excitation energies (Te values) of the three lowest-lying quartet excited states were predicted to be 83.3 kcalmol (3.61 eV, 29 200 cm(-1)) for the ã 4A2 state, 93.3 kcalmol (4.05 eV, 32 600 cm(-1)) for the b 4B2 state, and 100.8 kcalmol (4.37 eV, 35 300 cm(-1)) for the c 4A1 state. The quantum mechanical excitation energies (T 0 values) were determined to be 81.6 kcalmol (3.54 eV, 28 500 cm(-1)) for the a 4A2 state and 90.7 kcalmol (3.93 eV, 31 700 cm(-1)) for the b 4B2 state. The lowest quartet linear Renner-Teller 4Pi state gives rise to the a 4A2 state with 112.8 degrees and the b 4B2 state with 124.4 degrees <(ONO) bond angles upon bending. The b state shows some peculiar behavior. Although CASSCF, RCISD, UCISD, RCCSD, UCCSD, and RCCSD(T) methods predicted the presence of a Cs equilibrium geometry (a double minimum 4A' state), SCF, UCCSD(T), and ICMRCI wave functions predicted the C2v structure for the b 4B2 state. The importance of both dynamical and nondynamical correlation treatments for the energy difference between C2v and Cs structures of b state is highlighted in this context. The c 4A1 state is predicted to have a very small bond angle of 85.8 degrees . Potential energy diagrams with respect to the bond angles of the ground state and four quartet states are presented.