Ground Singlet Electronic States Research Articles

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc, Y, La) and the 18‐Electron Rule

AbstractWe report the gas‐phase synthesis of stable 20‐electron carbonyl anion complexes of group 3 transition metals, TM(CO)8− (TM=Sc, Y, La), which are studied by mass‐selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (Oh) equilibrium geometry and a singlet (1A1g) electronic ground state. The 20‐electron systems TM(CO)8− are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)7− in the 1A1 electronic ground state; these exhibit a capped octahedral structure with C3v symmetry. Analysis of the electronic structure of TM(CO)8− reveals that there is one occupied valence molecular orbital with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)8− fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8 ]- (TM=Sc, Y, La) and the 18-Electron Rule.

We report the gas-phase synthesis of stable 20-electron carbonyl anion complexes of group 3 transition metals, TM(CO)8- (TM=Sc, Y, La), which are studied by mass-selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (Oh ) equilibrium geometry and a singlet (1 A1g ) electronic ground state. The 20-electron systems TM(CO)8- are energetically stable toward loss of one CO ligand, yielding the 18-electron complexes TM(CO)7- in the 1 A1 electronic ground state; these exhibit a capped octahedral structure with C3v symmetry. Analysis of the electronic structure of TM(CO)8- reveals that there is one occupied valence molecular orbital with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)8- fulfill the 18-electron rule when only those valence electrons that occupy metal-ligand bonding orbitals are considered.

Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium

Aromaticity is a fundamental chemical concept of ever-increasing diversity. According to Hückel’s and Baird’s rules, cyclic conjugated species with 4n+2 π-electrons are aromatic in the singlet electronic ground state (S0) and antiaromatic in the lowest triplet state (T1), and vice-versa. Thus, species with aromaticity in both states have not yet been reported. Here we carry out density functional theory calculations on recently synthesized organometallics, namely osmapentalyne and osmapentalenes, and demonstrate the first example (16-electron osmapentalene) of aromaticity in both S0 and T1 states, which we term adaptive aromaticity. Further electronic structure analysis reveals that the excitation pattern for the formation of the T1 state plays a crucial role in the achievement of adaptive aromaticity. Our findings highlight the role of a transition metal in unorthodox excitation behavior, and may aid the design of adaptive aromatics for photochemical and molecular magnetism applications.

The equilibrium geometries of heptatriynylidene, cyclohepta-1,2,3,4-tetraen-6-yne, and heptahexaenylidene

The equilibrium geometries of heptatriynylidene (1), cyclohepta-1,2,3,4-tetraen-6-yne (2), and heptahexaenylidene (3) have been theoretically investigated at various level of theories. The methods employed are Møller–Plesset perturbation level of theory until second-order, coupled-cluster singles and doubles (CCSD), and CCSD with perturbative triple excitations [CCSD(T)]. Correlation consistent polarized valence double and triple zeta (cc-pVXZ; X=D and T) basis sets are used in all calculations, which are compatible with the frozen-core approximation entreated in this study. The relative energies of the ground triplet electronic state (X∼3Σg-) of 1 – the most stable isomer of C7H2 – to the ground singlet electronic states of 2 (X∼1A1) and 3 (X∼1A1) have also been estimated at different levels. With zero-point vibrational energy corrections, 2 and 3 are found to lie ∼12.27 and ∼20.62kcalmol−1, respectively, above 1 at the highest level of theory (CCSD(T)/cc-pVTZ). While 1 and 3 are observed in the laboratory, 2 is a hypothetical molecule hitherto. Since the optimal geometry of 2 suggests the presence of biradical character, the singlet–triplet (X∼1A1–3B1) energy gap for isomer 2 was also calculated at various levels. The ab initio data presented here may be useful for the laboratory detection of 2 and astronomical detection of2and3.

Coherent quantum control of internal conversion: in pyrazine via weak field excitation

Coherent control of internal conversion (IC) between the first (S1) and second (S2) singlet excited electronic states in pyrazine, where the S2 state is populated from the ground singlet electronic state S0 by weak field excitation, is examined. Control is implemented by shaping the laser which excites S2. Excitation and IC are considered simultaneously, using the recently introduced resonance-based control approach. Highly successful control is achieved by optimizing both the amplitude and phase profiles of the laser spectrum. The dependence of control on the properties of resonances in S2 is demonstrated.

Universality of weakly bound dimers and Efimov trimers close to Li–Cs Feshbach resonances

We study the interspecies scattering properties of ultracold Li–Cs mixtures in their two energetically lowest spin channels in the magnetic field range between 800 and 1000 G. Close to two broad Feshbach resonances (FR) we create weakly bound LiCs dimers by radio-frequency association and measure the dependence of their binding energy on the external magnetic field strength. Based on the binding energies and complementary atom loss spectroscopy of three other Li–Cs s-wave FRs we construct precise molecular singlet and triplet electronic ground state potentials using a coupled-channels calculation. We extract the Li–Cs interspecies scattering length as a function of the external field and obtain almost a ten-fold improvement in the precision of the values for the pole positions and widths of the s-wave FRs as compared to our previous work (Pires et al 2014 Phys. Rev. Lett. 112 250404). We discuss implications on the Efimov scenario and the universal geometric scaling for LiCsCs trimers.

Cold interactions between anYb+ion and a Li atom: Prospects for sympathetic cooling, radiative association, and Feshbach resonances

The electronic structure of the (LiYb)$^+$ molecular ion is investigated with two variants of the coupled cluster method restricted to single, double, and noniterative or linear triple excitations. Potential energy curves for the ground and excited states, permanent and transition electric dipole movements, and long-range interaction coefficients $C_4$ and $C_6$ are reported. The data is subsequently employed in scattering calculations and photoassociation studies. Feshbach resonances are shown to be measurable despite the ion's micromotion in the Paul trap. Molecular ions can be formed in their singlet electronic ground state by one-photon photoassociation and in triplet states by two-photon photoassociation; and control of cold atom-ion chemistry based on Feshbach resonances should be feasible. Conditions for sympathetic cooling of an Yb$^+$ ion by an ultracold gas of Li atoms are found to be favorable in the temperature range of 10$\,$mK to 10$\,$nK; and further improvements using Feshbach resonances should be possible. Overall, these results suggest excellent prospects for building a quantum simulator with ultracold Yb$^+$ ions and Li atoms.

Theoretical Studies on the Mechanism of Homogeneous Catalytic Olefin Hydrogenation and Amine–Borane Dehydrogenation by a Versatile Boryl-Ligand-Based Cobalt Catalyst

We have conducted mechanistic investigations using dispersion-corrected hybrid density functional theory on three different homogeneous processes: (a) hydrogenation of styrene using H2, (b) dehydrogenation of amine–borane, and (c) transfer hydrogenation of styrene using amine–borane catalyzed by a boryl-ligated Co-based catalytic system, LCo(N2) (where L = meridional bis-phosphinoboryl (PBP) ligand), recently developed by Peters and co-workers (Lin, T.-P; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310–15313). Our studies reveal that all three catalytic processes are facilitated by the same active species, which is of the form LCo(H)2. The formation of the active catalytic species in turn determines the rate-determining barrier (RDB) for the hydrogenation reactions of the olefin and also for the dehydrogenation reaction of amine–borane. We predict that the RDB for hydrogenation of styrene under H2 atmosphere is 17.3 kcal/mol, which occurs through a channel that involves switching of a singlet electronic ground state (S0) of the organometallic catalytic species to its low-lying triplet electronic state (T1) and returning back to the singlet surface through minimum energy crossing points along the reaction coordinate. Alternatively, we estimate the RDB to be 19.4 kcal/mol, slightly higher than that of the previous channel, if only the singlet spin state surface is considered. We find that the associated RDB for both the dehydrogenation of amine–borane (NMe2H-BH3) and transfer hydrogenation of styrene by amine–borane are higher than the hydrogenation of olefin using H2(g) and is predicted to be 24.7 kcal/mol. In addition, we show that in the reaction involving amine–borane, the active catalytic species (LCo(H)2) can get deactivated by forming a hydridoborane cobalt tetrahydridoborate complex, which happens through an SN2 type nucleophilic attack by the LCo(H)2 on amine–borane.

Probing the spin multiplicity of gas-phase polycyclic aromatic hydrocarbons through their infrared emission spectrum: A theoretical study

The anharmonic infrared emission spectrum following an optical excitation has been calculated for a variety of polycyclic aromatic hydrocarbon molecules in their ground singlet electronic state or in their triplet state. The computational protocol relies on second-order perturbation theory and involves a quartic vibrational Hamiltonian, the vibrational quantum numbers being sampled according to a Monte Carlo procedure. In the case of neutral naphthalene, the IR spectrum obtained in the (ground) singlet state differs significantly from the spectrum in the triplet state, especially for out-of-plane CH bending modes. Although not as prominent, spectral differences in larger molecules are still observable.

Half-metallicity and spin-contamination of the electronic ground state of graphene nanoribbons and related systems: An impossible compromise?

An analysis using the formalism of crystalline orbitals for extended systems with periodicity in one dimension demonstrates that any antiferromagnetic and half-metallic spin-polarization of the edge states in n-acenes, and more generally in zigzag graphene nanoislands and nanoribbons of finite width, would imply a spin contamination <linear span>S(2)<linear span> that increases proportionally to system size, in sharp and clear contradiction with the implications of Lieb's theorem for compensated bipartite lattices and the expected value for a singlet (S = 0) electronic ground state. Verifications on naphthalene, larger n-acenes (n = 3-10) and rectangular nanographene islands of increasing size, as well as a comparison using unrestricted Hartree-Fock theory along with basis sets of improving quality against various many-body treatments demonstrate altogether that antiferromagnetism and half-metallicity in extended graphene nanoribbons will be quenched by an exact treatment of electron correlation, at the confines of non-relativistic many-body quantum mechanics. Indeed, for singlet states, symmetry-breakings in spin-densities are necessarily the outcome of a too approximate treatment of static and dynamic electron correlation in single-determinantal approaches, such as unrestricted Hartree-Fock or Density Functional Theory. In this context, such as the size-extensive spin-contamination to which it relates, half-metallicity is thus nothing else than a methodological artefact.

Dual photochemistry of anthracene-9,10-endoperoxide studied by femtosecond spectroscopy

The dual photochemistry of anthracene-9,10-endoperoxide (APO) was investigated in a fs UV pump-supercontinuum probe experiment, along with anthracene (AC) and anthraquinone (AQ) for comparison. Excitation of APO at 282 nm leads to 100% product formation by two competing photoreaction channels. Cycloreversion generates with a ∼25% quantum yield (QY) (1)O(2) and AC vibrationally excited in the singlet electronic ground state (hot AC). 1-2% of the AC is generated in the lowest triplet state, but no AC is generated in electronically excited singlet states. Generation and cooling of hot AC are modeled using solution phase and broadened gas-phase AC absorption spectra at various temperatures. Results indicate ultrafast generation of hot AC within 3 ps, much faster than reported before for derivatives of anthracene endoperoxide, and subsequent cooling with an 18 ps time constant. The homolytic O-O cleavage pathway generates a biradical, which converts into electronically excited diepoxide (DE). Our data indicate a 1.5 ps time constant that we tentatively assign to the biradical decay and DE formation. Cooling of DE in this electronically excited state takes place with a ∼21 ps time constant. Excitation of AQ at 266 nm is followed by an ultrafast population of the T(1)(nπ*) triplet state of AQ with a time constant of (160 ± 60) fs.

Ab initio characterization of C5

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X1Sigmag+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the nu7(piu) anharmonic fundamental has been calculated to be at 102 cm(-1), which validates the assignment to C5 of the pattern of bands centered at 102 cm(-1) observed with the ISO telescope.

Potential Energy Surface and Product Branching Ratios for the Reaction of Dicarbon, C2(XΣg+), with Methylacetylene, CH3CCH(X1A1): An Ab Initio/RRKM Study

Ab initio calculations of the potential energy surface for the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction have been carried at the G2M level of theory. The calculations show that the dicarbon molecule in the ground singlet electronic state can add to methylacetylene without a barrier producing a three-member or a four-member ring intermediate, which can rapidly rearrange to the most stable H(3)CCCCCH isomer on the C(5)H(4) singlet surface. This isomer can then lose a hydrogen atom (H) or molecular hydrogen (H(2)) from the CH(3) group with the formation of H(2)CCCCCH and HCCCCCH, respectively. Alternatively, H atom migrations and three-member-ring closure/opening rearrangements followed by H and H(2) losses can lead to other isomers of the C(5)H(3) and C(5)H(2) species. According to the calculated energetics, the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH reaction is likely to be a major source of the C(5)H(3) radicals (in particular, the most stable H(2)CCCCCH and HCCCHCCH isomers, which are relevant to the formation of benzene through the reactions with CH(3)). Among heavy-fragment product channels, only C(3)H(3) + C(2)H and c-C(3)H(2) + C(2)H(2) might compete with C(5)H(3) + H and C(5)H(2) + H(2). RRKM calculations of reaction rate constants and product branching ratios depending on the reactive collision energy showed that the major reaction products are expected to be H(2)CCCCCH + H (64-66%) and HCCCHCCH + H (34-30%), with minor contributions from HCCCCCH + H(2) (1-2%), HCCCHCC + H(2) (up to 1%), C(3)H(3) + C(2)H (up to 1%), and c-C(3)H(2) + C(2)H(2) (up to 0.1%) if the energy randomization is complete. The calculations also indicate that the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction can proceed by direct H-abstraction of a methyl hydrogen to form C(3)H(3) + C(2)H almost without a barrier.

The singlet electronic ground state isomers of dialuminum monoxide: AlOAl, AlAlO, and the transition state connecting them

The singlet electronic ground state isomers, X (1)Sigma(g) (+) (AlOAl D(infinityh)) and X (1)Sigma(+) (AlAlO C(infinitynu)), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol(-1) below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X (1)Sigma(g) (+) AlOAl molecule, omega(1)=517 cm(-1), omega(2)=95 cm(-1), and omega(3)=1014 cm(-1), are in good agreement with experimental values. The harmonic vibrational frequencies for the X (1)Sigma(+) AlAlO structure, omega(1)=1042 cm(-1), omega(2)=73 cm(-1), and omega(3)=253 cm(-1), presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl-->AlAlO and AlAlO-->AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol(-1), respectively. The dissociation energies D(0) for AlOAl (X (1)Sigma(g) (+)) and AlAlO (X (1)Sigma(+))-->AlO (X (2)Sigma(+))+Al ((2)P) were determined to be 130.8 and 48.9 kcal mol(-1), respectively. Thus, both symmetric AlOAl (X (1)Sigma(g) (+)) and asymmetric AlAlO (X (1)Sigma(+)) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X (2)Sigma(+)) + Al ((2)P) and kinetically stable for the isomerization reaction (AlAlO-->AlOAl) at sufficiently low temperatures.

Theoretical Prediction of Benzyne-Like Species in Pyrene Diradicals

The different products that can be obtained from pyrene dehydrogenation have been studied by means of hybrid density functional theory. Most of the didehydrogenated species exhibit a diradical character. This is supported by the closeness between the geometry and energy of singlet and triplet states and the open-shell nature of the singlet state. However, three didehydrogenated species have been found to exhibit a closed-shell singlet electronic ground state. The analysis of the electronic structure, the formation energy of these didehydrogenated species, and their geometrical structure reveals that these three didehydrogenated species are better described as benzyne-like moieties; two of them are of o- and one of m-benzyne character. The two o-benzyne species have lower energies than the m-benzyne and are favored upon diradical formation. This interpretation is fully supported by the analysis of multiconfigurational wave functions and the order of stabilities confirmed by second-order multireference perturbation theory calculations. A possible implication of the reactivity of these benzyne-like species in asphaltene formation is discussed.

HCNCC – the possible isomer of cyanoacetylene

HCNCC, the exotic linear rearrangement of cyanoacetylene, is a probable interstellar molecule. The Letter compares CCSD(T)/aug-cc-pVTZ energies of five cyanoacetylene isomers, and predicts properties of the HCNCC ground singlet electronic state: geometry, equilibrium rotational constant, electric dipole moment and vibrational frequencies – based on the coupled-clusters and density functional theory (B3LYP/aug-cc-pVTZ) studies.

Ab initio theory and rotational spectra of linear carbon chains SiCnS

On the basis of extensive coupled cluster calculations, the rotational spectra of the linear silicon- and sulfur-containing carbon chains SiC2nS (n=1–3) in their singlet electronic ground state and SiC3S in its triplet electronic ground state have been detected and characterized by means of molecular beam Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined to high accuracy as well as the spin-spin coupling constant for triplet SiC3S. In addition, the Si29, S34, and both C13 isotopic species of SiC2S have been detected, allowing the determination of both an effective r0 structure, as well as a mixed experimental–theoretical structure resulting from the combination of the measured rotational constants with the vibration–rotation coupling constants calculated ab initio. Several rotational satellite lines have also been observed for SiC2S and, on the basis of their predicted vibration–rotation and l-type doubling constants, were assigned to the two highest-frequency stretching modes v1 and v2, and to a progression in the second lowest-frequency bending mode v4 up to 3v4. Equilibrium structures and various spectroscopic properties are predicted for all SiCnS species with n=1–8.

Studies of Fe2(CO)9 Formation in the Thermolysis of Fe(CO)5 Using Ab Initio Calculations of Fe(CO)5 and Fe(CO)4

Ab initio calculations using the GAMESS program package in the atomic basis TZV (Fe: (14s, 11p, 6d)/[10s, 8p, 3d]; C, O: (11s, 6p)/[5s, 3p]) were performed with account taken of the correlation with the second-order Moller–Plesset (MP2) perturbation theory to predict a new conformer Fe(CO)4 (with D4h symmetry). This conformer has a square planar configuration in the ground singlet electronic state and is a mild electrophile produced by dissociation of Fe(CO)5 along the axial Fe–C bond. The process of nucleation of iron nanoparticles Fe(CO)5 + Fe(CO)4 → Fe2(CO)9 is supposed to occur in two stages. The first stage is an orbital-controlled reaction which should be monitored as an increase in medium polarity and temperature. It should proceed with participation of only one of the stable conformers of the nucleophile Fe(CO)5, namely, a mild conformer with square-pyramidal structure (C4v) rather than a hard but energetically more advantageous conformer with trigonal–bipyramidal structure (D3h). The structure of a prereaction complex was discussed.

The potential energy surface and vibrational structure of C3H−

A six-dimensional potential energy surface (PES) for the singlet electronic ground state of C3H− has been generated by electronic structure calculations using the coupled cluster CCSD(T) approach. Two potential minima are located: the global one, corresponding to an aromatic, cyclic structure (1A1), and a local one, lying about 0.56 eV higher in energy, with a trans chain structure (1A′). Both minima are found to be separated by a relatively high barrier on the singlet surface but the isomerization process can also proceed via a coupling between the singlet and triplet PESs with a lower barrier. Variational calculations for the vibrational levels (J=0) up to 3000 cm−1 were carried out for both isomers, taking into account the full dimensionality of the problem. The present results allow a clear distinction between the singlet isomers by infrared experiments. The structure of the vibrational and rotational stacks in both isomers is very different. For some vibrational overtones and combination levels inverse anharmonicity was found. The high density of the vibrational levels particularly in the chain structure suggests the existence of strong anharmonic resonances. In the bent isomer, the excited in-plane ν5 and out-of-plane ν6 modes fall into near-degenerate clusters of levels.

Low-temperature study of the molecular and crystal structures of 2-(2′-tosylamino-5′-nitrophenyl)-4H-3,1-benzoxazin-4-one, an organic luminophore

The crystal structure of 2-(2′-tosylamino-5′-nitrophenyl)-4H-3,1-benzoxazin-4-one (I) is studied by X-ray diffraction at 100 K (C21H15N3O6S, a = 20.899(2) A, b = 10.948(1) A, c = 8.260(1) A, V = 1889.3(1) A3, Z = 4, and space group Pbn21). The compound exhibits an anomalous Stokes shift. Upon cooling, the oxazineaminophenyl fragment of compound I acquires a quinoid structure and the linear parameters of the intramolecular N-H⋯N hydrogen bond increase (the distance between the heterocyclic nitrogen atom and the hydrogen atom of the tosylamino group becomes 1.92 A). The complete optimization of the geometry of molecules in compound I and unsubstituted 2-(2′-tosylaminophenyl)-4H-3,1-benzoxazin-4-one in the ground singlet electronic state is performed by the semiempirical method with the MOPAC program. It is shown that the oxygen atoms in the sulfo group of molecule I are nonequivalent, because one of them is involved in the intermolecular C-H⋯O hydrogen bond.