A theoretical study of the fine and hyperfine interactions in the NCO and CNO radicals

The geometries, the harmonic vibrational frequencies, and the Renner-Teller parameter have been reported for the NCO(+)(X (3)Sigma(-)), NCO(X (2)Pi,A (2)Sigma(+),B (2)Pi,2 (2)Sigma(+)), NCO(-)(X (1)Sigma(+)), CNO(+)(X), CNO(X (2)Pi,A (2)Sigma(+),B (2)Pi,2 (2)Sigma(+)), and CNO(-)(X (1)Sigma(+)) systems at the full valence-complete active space self-consistent-field (fv-CASSCF) level of theory. The (2)Pi electronic states of the NCO and CNO radicals have two distinct real vibrational frequencies for the bending modes and these states are subject to the type A Renner-Teller effect. The total energy of CNO(+) without zero point energy correction of the linear geometry is approximately 31 cm(-1) higher than the bent geometry at the fv-CASSCF level and the inversion barrier vanishes after the zero point energy correction; therefore, the ground state of the CNO(+) may possess a quasilinear geometry. The spin-orbit coupling constants estimated using atomic mean field Hamiltonian at the fv-CASSCF level of theory are in better agreement with the experimental values. The excitation energies, the electron affinity, and the ionization potential have been computed at the complete active space second order perturbation theory (CASPT2) and the multireference singles and doubles configuration (MRSD-CI) levels of theory. The computed values of the electric hyperfine coupling constants for the (14)N atom in the ground state of the NCO radical agree well with the experimental data. The magnetic hyperfine coupling constants (HFCC's) have been estimated employing the configuration selected MRSD-CI and the multireference singles configuration interaction (MRS-CI) methods using iterative natural orbitals (ino) as one particle basis. Sufficiently accurate value of the isotropic contribution to the HFCC's can be obtained using an MRS-CI-ino procedure.     

Computational study on the structures of the [H, Si, N, C, O] isomers: possible species of interstellar interest

Ab initio methods have been used to study the lowest lying [H, Si, N, C, O] isomers, which are of astrochemical interest. Over 20 [H, Si, N, C, O] isomers in the 1A' electronic state have been investigated at the MP2/aug-cc-pVTZ level of theory. Of these, the seven lowest isomers have been further investigated using different levels of theory, including B3LYP and QCISD(T). It has been found that the relative energies of the isomers in their ground electronic state (1A') are very dependent on the level of theory used with either the cis-HOSiCN or cis-HOSiNC isomers being the lowest in energy. Overall, the four lowest isomers are within 6 kcal/mol of each other, and a further three isomers are less than 15 kcal/mol higher in energy than the lowest lying isomer, including HSiNCO, which has recently been detected spectroscopically. Natural bond analysis has been carried out on the ground electronic states of the seven lowest lying isomers to examine their electronic structure. The enthalpies of formation of the seven lowest isomers have also been evaluated using the G3MP2 and G3B3 multilevel methods and show that the isomers are relatively thermodynamically stable. The structures and energies of lowest lying 1A' and 3A' electronic states of these isomers have also been investigated and show that for most of the isomers the optimized structures in these excited electronic states correspond to a transition state structure.     

Theoretical studies of the excited doublet states of SF and SCl and singlet states of SF2, SFCl, and SCl2

In previous work, we reported that the lowest-lying excited states of SF, SCl, SF(2), SFCl, and SCl(2) have recoupled pair bonds. In this study, we examine the analogous low-spin states--the (2)Σ(-) and (2)Δ states of SF and SCl and the excited singlet states of SF(2), SCl(2), and SFCl--which also possess recoupled pair bonds. In contrast to the excited states treated previously, the states studied in the present work have the same spin multiplicities as their respective ground states and are thus potentially observable via electronic excitation. Of particular interest are the minima on the (1)A″ potential energy surface of SFCl corresponding to bond-stretch isomers analogous to those found on the (3)A″ surface. In addition, we discovered that the first two excited states ((1)A″) accessible via vertical excitations from the ground state of SFCl have the electronic structure of the bond-stretch isomers. Thus, electronic excitation spectroscopic studies of SFCl could reveal a signature of the bond-stretch isomers. We will also present limited data on the lowest singlet Rydberg states of the triatomic species. Calculations were performed at the MRCI+Q/aug-cc-pV(Q+d,5+d)Z levels of theory.     

Gas phase electronic spectrum of linear AlCCH

The electronic spectrum of the aluminium containing species AlCCH has been detected in the gas phase in the region 315-355 nm. The experiment used a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Structures of the AlCCH isomers have been optimized using density functional theory (DFT) and the excitation energies to the low-lying electronic excited states calculated. Based on the analysis of the observed rotational structure and the theoretical data, the spectrum is assigned to the A (1)Pi<-- X (1)Sigma(+) electronic transition of linear AlCCH. The vibronic band system is complicated by the Renner-Teller effect in the excited state. The assignment yields nu(4)' = 516.4 cm(-1) for the stretching mode in the ground X (1)Sigma(+) state and nu(4)' = 654.5 cm(-1) for A (1)Pi excited state. Molecular constants determined from the rotational analysis are B(0)' = 0.16487(14), B(0)' = 0.17845(13) and T(0) = 28 755.04 cm(-1). The experimental and theoretical data indicate a shorter Al-C bond in the A (1)Pi excited than the X (1)Sigma(+) ground state.     

CASSCF and CASPT2 studies on the structures, transition energies, and dipole moments of ground and excited states for azulene

The electronic structure of azulene molecule has been studied. We have obtained the optimized structures of ground and singlet excited states by using the complete active space self-consistent-field (CASSCF) method, and calculated vertical and 0-0 transition energies between the ground and excited states with second-order M?ller-Plesset perturbation theory (CASPT2). The CASPT2 calculations indicate that the bond-equalized C(2v) structure is more stable than the bond-alternating C(s) structure in the ground state. For a physical understanding of electronic structure change from C(2v) to C(s), we have performed the CASSCF calculations of Duschinsky matrix describing mixing of the b(2) vibrational mode between the ground (1A(1)) and the first excited (1B(2)) states based on the Kekule-crossing model. The CASPT2 0-0 transition energies are in fairly good agreement with experimental results within 0.1-0.3 eV. The CASSCF oscillator strengths between the ground and excited states are calculated and compared with experimental data. Furthermore, we have calculated the CASPT2 dipole moments of ground and excited states, which show good agreement with experimental values.     

DFT Investigation on the Trans-cis Photoisomerization of Pentamethine Cyanine Dye Model Molecule

The trans-cis photoisomerization of pentamethine cyanine dye (Cy5) has been theoretically investigated by the analysis on an analogical molecule model. All possible isomers have been searched by rotating the different bridge C-C bonds of the model. The relative stability of the isomers for the ground and first excited states as well as the corresponding excitation mechanism has been envisaged by DFT and time-dependent DFT method. The results show different conjugation degree of the isomers resulting in different absorption energy upon the different C-C bridge bond rotation. From the analyses of electronic density distribution of the frontier orbitals, it is predicted that there is charge transfer besides the π-π* excitation for the cyanine dye model, which makes the mechanism and photo-properties different form those of other conjugated molecules.     

Electronic structure calculations on the C4 cluster

The ground and the electronically excited states of the C4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers [l-C4(X(3)Sigmag (-)) and r-C4(X(1)Ag)], it is found that the ground singlet surface has two other stationary points: s-C4(X(1)Ag) and d-C4(X(1)A1). The d-C4 form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0-2 eV energy ranges. The high rovibrational levels of l-C4((3)Sigmau (-)) undergo predissociation processes via spin-orbit interactions with the neighboring (5)Sigmag + state.     

Structure and properties of the low-lying electronic states of CeC(2) and CeC(2)(+)

Theoretical studies on the electronic and thermodynamic properties of several electronic states of CeC(2) and CeC(2)(+) have been carried out employing state-of-the-art single- and multireference techniques. The ground and the low-lying electronic states of these two species have been found to possess C(2v) triangular structures. A (3)B(2) state has been found to be the ground state of CeC(2) while for CeC(2)(+) (2)A(2) is the ground state. The computed electron ionization energy is in excellent agreement with experiment. The experimentally observed thermodynamic properties (dissociation and atomization energies) of reactions involving CeC(2) dissociation are corrected using the computed gas-phase properties of the molecule and the partition functions. The bent triplet and singlet state of CeC(2) exhibit large dipole moments (7.0-10.5 D) and it is consistent with the ionic character (through dative charge transfer) of the cluster in ground and excited states.     

Electronic structure and spectroscopic properties of the two structural isomers of donor-acceptor substituted sesquifulvalene in the gas and solution phases-a case study of sudden polarization

The ground state equilibrium structure and electric properties of two structural isomers of donor-acceptor substituted sesquifulvalene have been calculated at ab initio HF and MP2 levels for different conformations. The electronic properties of low lying excited singlets are calculated by using CI calculations including single excitations only. Isomer I in which the inter-ring charge transfer (CT) is reinforced in the presence of substituents shows sudden polarization in the ground and two lower lying excited states, while isomer II in which the longitudinal CT interaction is attenuated does not exhibit sudden polarization. The phenomenon of sudden polarization has been rationalized in terms of the easy polarization, smaller rotational barrier, and enhanced inter-ring CT on going from the planar to the orthogonal geometry. The appreciably large static second-order polarizability of I stems from its sudden polarized ground state. The solvent (using the conductor-like polarizable continuum model (CPCM)) plays a significant role on the modulation of ground and excited state electronic properties which, in general, predicts blue-shifts for I. However, for molecule II, the two lower energy transitions show a red-shift while the others show a weaker blue-shift at any conformation.     

Probing the acidity of p-substituted phenols in the excited state: electronic spectroscopy of the p-cyanophenol-water cluster.

The hydrogen bond structure of the p-cyanophenol-water cluster has been determined in the ground and first excited electronic state by rotationally resolved UV spectroscopy. The water molecule is trans-linearly bound to the hydroxy group of the p-cyanophenol moiety, with hydrogen bond distances considerably shorter in both electronic states than in the similar phenol-water cluster. The structure of the cluster has been elucidated by ab initio calculations at various levels of theory and compared to the experimental findings. The barriers to internal rotation of the water moiety were determined experimentally to be 275 and 183 cm(-1) for the ground and excited state, respectively. Hydrogen bond distances and the energy barrier to water torsion correlate with the pK(a) values of different substituted phenols for both electronic states.     

Infrared/ultraviolet quadruple resonance spectroscopy to investigate structures of electronically excited states

Molecular beam investigations in combination with IR/UV spectroscopy offer the possibility to obtain structural information on isolated molecules and clusters. One of the demanding tasks is the discrimination of different isomers, e.g., by the use of isomer specific UV excitations. If this discrimination fails due to overlaying UV spectra of different isomers, IR/IR methods offer another possibility. Here, we present a new IR/UV/IR/UV quadruple resonance technique to distinguish between different isomers especially in the electronically excited state. Due to the IR spectra, structural changes and photochemical pathways in excited states can be assigned and identified. The method is applied to the dihydrated cluster of 3-hydroxyflavone which has been investigated as photochemically relevant system and proton wire model in the S(1) state. By applying the new IR/UV/IR/UV technique, we are able to show experimentally that both in the electronic ground (S(0)) and the electronically excited state (S(1)) two isomers have to be assigned.     

Gas-phase infrared and ab initio study of the unstable CF3CNO molecule and its stable furoxan ring dimer

The unstable trifluoroacetonitrile N-oxide molecule, CF3CNO, has been generated in high yield in the gas phase from CF3BrC=NOH and studied for the first time by gas-phase mid-infrared spectroscopy. Cold trapping of this molecule followed by slow warming forms the stable ring dimer, bis(trifluoromethyl)furoxan, also investigated by gas-phase infrared spectroscopy. The spectroscopy provides an investigation into the vibrational character of the two molecules, the assignments supported by calculations of the harmonic vibrational frequencies using in the case of CF3CNO both ab initio (CCSD(T)) and density functional theory (B3LYP) and B3LYP for the ring dimer. The ground-state structures of both molecules were investigated at the B3LYP level of theory, with CF3CNO further investigated using coupled-cluster. The CCSD(T) method suggests a slightly bent (C(s)) structure for CF3CNO, while the B3LYP method (with basis sets ranging from 6-311G(d) to cc-pVTZ) suggests a close-to-linear or linear CCNO chain. The CCN bending potential in CF3CNO was explored at the CCSD(T)(fc)/cc-pVTZ level, with the results suggesting that CF3CNO exhibits strong quasi-symmetric top behavior with a barrier to linearity of 174 cm(-1). Since both isomerization and dimerization are feasible loss processes for this unstable molecule, the relative stability of CF3CNO with respect to the known cyanate (CF3OCN), isocyanate (CF3NCO), and fulminate (CF3ONC) isomers and the mechanism of the dimerization process to the ring furoxan and other isomers were studied with density functional theory.     

Theoretical investigation of the photophysics of methyl salicylate isomers

The photophysics of methyl salicylate (MS) isomers has been studied using time-dependent density functional theory and large basis sets. First electronic singlet and triplet excited states energies, structure, and vibrational analysis were calculated for the ketoB, enol, and ketoA isomers. It is demonstrated that the photochemical pathway involving excited state intramolecular proton transfer (ESIPT) from the ketoB to the enol tautomer agrees well with the dual fluorescence in near-UV (from ketoB) and blue (from enol) wavelengths obtained from experiments. Our calculation confirms the existence of a double minimum in the excited state pathway along the O-H-O coordinate corresponding to two preferred energy regions: (1) the hydrogen belongs to the OH moiety and the structure of methyl salicylate is ketoB; (2) the hydrogen flips to the closest carboxyl entailing electronic rearrangement and tautomerization to the enol structure. This double well in the excited state is highly asymmetric. The Franck-Condon vibrational overlap is calculated and accounts for the broadening of the two bands. It is suggested that forward and backward ESIPT through the barrier separating the two minima is temperature-dependent and affects the intensity of the fluorescence as seen in experiments. When the enol fluoresces and returns to its ground state, a barrier-less back proton transfer repopulates the ground state of methyl salicylate ketoB. It is also demonstrated that the rotamer ketoA is not stable in an excited state close to the desired emission wavelength. This observation eliminates the conjecture that the near-UV emission of the dual fluorescence originates from the ketoA rotamer. New experimental results for pure MS in the liquid state are reported and theoretical results compared to them.     

Characterizing the deformational isomers of bimetallic Ir2(dimen)4(2+) (dimen = 1,8-diisocyano-p-menthane) with vibrational wavepacket dynamics

We studied the Ir(2)(dimen)(4)(2+) complex with ultrafast transient absorption spectroscopy and density functional theory and concluded that it possesses two singlet ground state isomers in room temperature solution. The molecule can adopt either a paddle wheel or a propeller conformation in solution, where the paddle wheel structure possesses a metal-metal bond of 4.4 ? and a dihedral angle between the quasi-C(4v) planes of 0° and the propeller structure has a metal-metal bond of 3.6 ? and a dihedral angle of 17° when crystallized. Each conformation has a distinct absorption in the visible attributed to a (1)(dσ(z)* → pσ(z)) excitation, with the long eclipsed structure absorbing at 475 nm and the short twisted structure absorbing at 585 nm. We independently pumped at each of these visible transitions to form vibrational wavepackets on the ground and excited state potential energy surfaces, which modulated the ground state bleach and stimulated emission signals, respectively. We found that the ground state wavepacket oscillates with a frequency of 48 cm(-1) when pumping the red peak and 11 cm(-1) when pumping the blue peak. We assign these frequencies to the Ir-Ir symmetric stretch, with the variation in frequency reflecting the variation in metal-metal bond strength in support of our assignment of the blue peak to the longer Ir-Ir bond length conformer and the red peak to the shorter Ir-Ir bond length conformer. When pumping the red peak, we found two modes with frequencies of 80 and 119 cm(-1) in the stimulated emission and only one mode at 75 cm(-1) when pumping the blue peak. We assign the 75-80 cm(-1) frequency to the Ir-Ir stretch and the 119 cm(-1) vibration to the dihedral angle twist in the excited state. The variation in the excited state dynamics does not result from the excitation of different electronic states, but rather from excitation to different Franck-Condon regions of the same electronic excited state potential energy surface. This occurs because of the large difference in ground state molecular structure. DFT calculations support the existence of a single electronic excited state being accessed from two distinct structural isomers with conformations similar to those observed with X-ray crystallography.     

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.     

An ab initio study of the lowest electronic states of yttrium dicarbide, YC2

The low-lying electronic states of yttrium dicarbide have been calculated using highly correlated wave functions and systematic sequences of correlation consistent basis sets. For the (2)A(1) ground electronic state, the near-equilibrium potential energy surface (PES) has been calculated using the coupled cluster method in conjunction with basis sets ranging in size from double to quintuple zeta. The relativistic effects have been taken into account by using pseudopotentials for the Y atom. After extrapolation to the complete basis set limit, additional corrections due to core-valence correlation and spin-orbit effects have also been included. The same approach has been followed for the (2)B(1), (2)B(2), and (2)A(2) states but only the C(2V) PESs have been considered in these cases. For the two (2)A(1) electronic excited states and, for comparison purposes, for the ground state, the multireference configuration interaction (MRCI) approach has been used in conjunction with double-zeta and triple-zeta basis sets for the construction of the PES. The molecular and spectroscopic properties predicted for the ground and excited states investigated in this work compare well with the available experimental data, particularly for the ground electronic state. The 0 K dissociation enthalpy of YC(2), DeltaH(Y-C2)(0 K), and its atomization enthalpy, SigmaD(0), are predicted to be 148.4 and 291.5 kcal/mol, respectively.     

Double proton transfer and charge transfer transitions in hydrogen-bonded systems: formic acid dimer

The barriers for double proton transfer in the ground and lowest Π-Π^* and Π-Π^* excited states of the formic acid dimer have been calculated within a modified INDO scheme. Analysis of the nature of the excited electronic states, with emphasis on charge-transfer transitions, has been performed. The results indicate a lower barrier in the excited Π-Π^* states than in the ground state.     

Elastic and charge transfer processes in H+ + CO collisions

Proton and hydrogen atom time-of-flight spectra in collision energy range of E(trans) = 9.5-30 eV show that the endoergic charge transfer process in the H+ + CO system is almost an order of magnitude less probable than the elastic scattering [G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)]. Ab initio computations at the multireference configuration interaction level have been performed to obtain the ground- and several low-lying excited electronic state potential energy curves in three different molecular orientations namely, H+ approaching the O-end and the C-end (collinear), and H+ approaching the CO molecule in perpendicular configuration with fixed CO internuclear distance. Nonadiabatic coupling terms between the ground electronic state (H+ + CO) and the three low-lying excited electronic states (H + CO+) have been computed and the corresponding diabatic potentials have been obtained. A time-dependent wavepacket dynamics study is modeled first involving only the ground and the first excited states and then involving the ground and the three lowest excited states at the collision energy of 9.5 eV. The overall charge transfer probability have been found to be approximately 20%-30% which is in qualitative agreement with the experimental findings.     

Photodissociation studies of the electronic and vibrational spectroscopy of Ni(+)(H2O)

The electronic spectrum of Ni?(H?O) has been measured from 16200 to 18000 cm?1 using photofragment spectroscopy. Transitions to two excited electronic states are observed; they are sufficiently long-lived that the spectrum is vibrationally and partially rotationally resolved. An extended progression in the metal-ligand stretch is observed, and the absolute vibrational quantum numbering is assigned by comparing isotopic shifts between ??Ni?(H?O) and ??Ni?(H?O). Time-dependent density functional calculations aid in assigning the spectrum. Two electronic transitions are observed, from the 2A? ground state (which correlates to the 2D, 3d? ground state of Ni?) to the 32A? and 22A? excited states. These states are nearly degenerate and correlate to the 2F, 3d?4s excited state of Ni?. Both transitions are quite weak, but surprisingly, the transition to the 2A? state is stronger, although it is symmetry-forbidden. The 3d?4s states of Ni? interact less strongly with water than does the ground state; therefore, the excited states observed are less tightly bound and have a longer metal-ligand bond than the ground state. Calculations at the CCSD(T)/aug-cc-pVTZ level predict that binding to Ni? increases the H-O-H angle in water from 104.2 to 107.5° as the metal removes electron density from the oxygen lone pairs. The photodissociation spectrum shows well-resolved rotational structure due to rotation about the Ni-O axis. This permits determination of the spin rotation constants ε(αα)' = -12 cm?1 and ε(αα)' = -3 cm?1 and the excited state rotational constant A' = 14.5 cm?1. This implies a H-O-H angle of 104 ± 1° in the 22A? excited state. The O-H stretching frequencies of the ground state of Ni?(H?O) were measured by combining IR excitation with visible photodissociation in a double resonance experiment. The O-H symmetric stretch is ν?' = 3616.5 cm?1; the antisymmetric stretch is ν?' = 3688 cm?1. These values are 40 and 68 cm?1 lower, respectively, than those in bare H?O.     

Influence of geometry relaxation on the energies of the S1 and S2 states of violaxanthin, zeaxanthin, and lutein

Precise knowledge of the excitation energies of the lowest excited states S(1) and S(2) of the carotenoids violaxanthin, lutein, and zeaxanthin is a prerequisite for a fundamental understanding of their role in light harvesting and photoprotection during photosynthesis. By means of density functional theory (DFT) and time-dependent DFT (TDDFT), the electronic and structural properties of the ground and first and second excited states are studied in detail. According to our calculations, all-s-cis-zeaxanthin and s-cis-lutein conformers possess lower total ground-state energies than the corresponding s-trans conformers. Thus, only s-cis isomers are probably physiologically relevant. Furthermore, the influence of geometric relaxation on the energies of the ground state and S(1) and S(2) states has been studied in detail. It is demonstrated that the energies of these states change significantly if the carotenoid adopts the equilibrium geometry of the S(1) state. Considering these energetic effects in the interpretation of S(1) excitation energies obtained from fluorescence and transient absorption spectroscopy shifts the S(1) excitation energies about 0.2 eV to higher energy above the excitation energy of the chlorophyll a.