The electronic states of but-2-yne studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio configuration interaction methods

The photoabsorption spectrum of but-2-yne in the range 5.5–11 eV (225–110 nm) has been recorded using a synchrotron radiation source. The spectrum is dominated by three d-type Rydberg series, converging to the first ionisation energy (IE) (π⁻¹, 9.562 eV). Origins of the π3d members are 7.841, 7.977 and 8.018 eV, respectively. Transitions of low intensity, arising from excitation of the π3s state (origin, 6.35 eV) and two π3p Rydberg states (7.38 and 7.51 eV, respectively) have also been identified in the spectrum. Near-threshold electron energy-loss spectra reveal valence excited triplet states at about 5.2 and 5.8 eV, respectively.Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for Rydberg states are close to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. It was possible to generate singlet Rydberg states which relate to the 5-lowest IEs of but-2-yne; furthermore, the separation of these sequences shows that the IE order (under D_3h symmetry) is: , also supported by direct calculation of the IEs by CI.The lowest valence singlet states are ππ^*, optically forbidden, and calculated to lie near 7.3 and 7.6 eV. The states which contribute strongly to the observed spectrum are πσ^* near 7.9 eV having excitation, followed by several ππ^* and πσ^* states between 10.0 and 10.5 eV; an ¹E′ antisymmetric combination(2e′2e″ − 2e′2e″) is by far the strongest in intensity. A further group of symmetry-allowed valence states are calculated to lie near 12.3 and 12.9 eV. The two lowest triplet states, both of E′ symmetry (ππ^*), have vertical excitation energies of 5.7 and 6.2 eV, but are strongly bent with a trans-CCCC unit (C_S and C_2h). The theoretical work confirms that, on intensity grounds, valence excited states do not contribute significantly to the spectrum. CI calculations of the ionic states give the ionisation energy sequence (D_3h): . Adiabatic structures for the first cation, two triplets, and a singlet (C_2h) were obtained; these show shortening of C–C, and lengthening of CC, in a trans-CCCC, as is found with ethyne.     

The electronic states of buta-1,3-diene studied by ab initio configuration interaction and DFT methods,and electron energy loss spectroscopy

The electronic vertical excitation energies for singlet and triplet valence, and Rydberg states of trans-buta-1,3-diene have been computed using ab initio multi-reference multi-root CI procedures with a [4s3p3d3f] set of Rydberg functions. Close numerical agreement between theory and experiment was found for a number of low-lying electronic states.     

Excited electronic states and photophysics of uracil–water complexes

The electronically excited singlet states of complexes of uracil with one water molecule have been studied theoretically using ab initio multireference configuration interaction methods. In agreement with previous theoretical and experimental results, four cyclic isomers of uracil forming hydrogen bonds with the water molecule have been located with energies within 0.2 eV from the lowest energy isomer. Focus has been given on the mechanism for radiationless decay to the ground state after initial UV absorption and on the effect of complexation with water on previously reported radiationless decay pathways. Features on the excited state potential energy surfaces, such as minima, transition states and conical intersections, have been located for all isomers and compared with those of free uracil. The hydrogen-bonded water molecule changes the relative energies of these features and may lead to different excited state dynamics and lifetimes, in agreement with experimental observations.     

Ab Initio Studies on the Structure and Binding Interaction of M~ CO_2(M=Sc,Ti…Zn)

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Anharmonic vibrational spectroscopy calculations with electronic structure potentials: comparison of MP2 and DFT for organic molecules

Density functional theory (DFT) technique is the most commonly used approach when it comes to computation of vibrational spectra of molecular species. In this study, we compare anharmonic spectra of several organic molecules such as allene, propyne, glycine, and imidazole, computed from ab initio MP2 potentials and DFT potentials based on commonly used BLYP and B3LYP functionals. Anharmonic spectra are obtained using the direct vibrational self-consistent field (VSCF) method and its correlation-corrected extension (CC-VSCF). The results of computations are compared with available experimental data. It is shown that the most accurate vibrational frequencies are obtained with the MP2 method, followed by the DFT/B3LYP method, while DFT/BLYP results are often unsatisfactory. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

The X˜1A1, a˜3B1, a˜1B˜1, and B˜1A1 electronic states of SiH2

Four electronically low-lying states of silylene (SiH₂) have been studied systematically using high level ab initio electronic structure theory. Self-consistent field (SCF), two-configuration (TC) SCF, complete active space (CAS) SCF, configuration interaction with single and double excitations (CISD), and CASSCF second-order (SO) CI levels of theory were employed with eight distinct basis sets. The zeroth-order wave functions of the ground (X˜ ¹A₁ or 1 ¹A₁) and B˜ ¹A₁ (or 2 ¹A₁) excited states are appropriately described by the first and second eigenvectors of the TCSCF secular equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI wave functions for the B˜ ¹A₁ (or 2 ¹A₁) state were obtained by following the second root of the CISD, CASSCF, and SOCI Hamiltonian matrices. At the highest level of theory, the CASSCF-SOCI method with the triple zeta plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions basis set [TZ3P(2f,2d)+2diff], the energy separation (T₀) between the ground (X˜ ¹A₁) and first excited (a˜ ³B₁) states is determined to be 20.5 kcal/mol (0.890eV,7180cm⁻¹), which is in excellent agreement with the experimental T₀ value of 21.0 kcal/mol (0.910eV,7340cm⁻¹). With the same method the T₀ value for the a˜ ¹B₁−X˜ ¹A₁ separation is predicted to be 45.1 kcal/mol (1.957 eV,15780 cm⁻¹), which is also in fine agreement with the experimental value of 44.4 kcal/mol (1.925 eV,15530 cm⁻¹). The T₀ value for the B˜ ¹A₁−X˜ ¹A₁ separation is determined to be 79.6 kcal/mol (3.452 eV,27 840 cm⁻¹). After comparison of theoretical and experimental T₀ values for the a˜ ³B₁ and a˜ ¹B₁ states and previous studies, error bars for the B˜ ¹A₁ state are estimated to be ±1.5 kcal/mol (±525 cm⁻¹). The predicted geometry of the B˜ ¹A₁ state is r_e(SiH)=1.458 and θ_e=162.3^∘. The physical properties including harmonic vibrational frequencies of the B˜ ¹A₁ state are newly determined. Received: 10 March 1997 / Accepted: 2 April 1997     

Electronic and cationic states of 2-methylpropene (isobutene) studied by VUV absorption,scattered electron spectroscopy and AB initio multireference configuration interaction calculations

VUV (6.2–9 eV) and electron scattering spectra (1–9 eV) have been recorded for 2-methylpropene (isobutene). Also, electronic states of the molecule, including the ground state and cationic states, have been investigated using ab initio multi-reference configuration interaction calculations. Some Koopmans-type in the UV photoelectron spectrum are reassigned and a number of shake-up states computed. In the electronic spectrum, Rydberg excited have been assigned and a second valence excited state (σ π*) located within about 1 eV of the V(ππ*) state. The experiments show, and theory confirms, that the Rydberg R(π3s) state has a positive electron affinity. Some interesting correlations between ionisation energies, energies of shake-up state electronic excitation energies are identified.     

Polymorphism of N,N'-diacetylbiuret studied by solid-state 13C and 15N NMR spectroscopy, DFT calculations, and X-ray diffraction

The molecular configuration and crystal structure of solid polycrystalline N,N′′‐diacetylbiuret (DAB), a potential nitrogen‐rich fertilizer, have been analyzed by a combination of solid‐ and liquid‐state NMR spectroscopy, X‐ray diffraction, and DFT calculations. Initially a pure NMR study (“NMR crystallography”) was performed as available single crystals of DAB were not suitable for X‐ray diffraction. Solid‐state ¹³C NMR spectra revealed the unexpected existence of two polymorphic modifications (α‐ and β‐DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X‐ray diffraction. The X‐ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two‐fold screw in the case of α‐DAB and a c‐glide plane in the case of β‐DAB, lead to a more symmetric (α‐DAB) or asymmetric (β‐DAB) intermolecular hydrogen‐bonding pattern for each molecule.     

Photo-oxidation and ozonization of poly(3-hexylthiophene) thin films as studied by UV/VIS and photoelectron spectroscopy

The kinetics and the mechanisms of degradation of thin P3HT layers have been investigated quantitatively for ozonization and photo-oxidation. Both, decay kinetics and product evolution of the polymer degradation are monitored by in situ UV/VIS and X-ray photoelectron spectroscopy (XPS). The degradation pathways of ozonization and photo-oxidation of P3HT turn out to be significantly different. Ozone attacks the thiophene units mainly by direct addition to the double bonds, leading to the loss of UV/VIS absorption, while the aliphatic side chains [1] are hardly affected. During photo-oxidation, the polymer is primarily attacked at the alkyl side chain which leads to the formation of reactive peroxide species. These subsequently cause the oxidation of sulfur and concomitantly the destruction of the thiophene ring, resulting in the loss of absorption. From the kinetics of the blue shift of the optical absorption it is concluded that the polymer is mainly attacked at the terminal thiophene rings the case of photo-oxidation whereas ozone attacks positions more or less randomly distributed along the chain. The rate of photo-oxidation under AM 1.5 conditions is at least one order of magnitude faster than the decomposition of P3HT by ozone.     

Al(III) complexation by alizarin studied by electronic spectroscopy and quantum chemical calculations

The complexation reaction of Al(III) by alizarin (Az), in methanol solution, has been followed by electronic absorption spectroscopy. Chemometric methods applied to the spectra set have shown the formation of two complexes of stoichiometry 1:1 and 2:1, with stability constants of 6.44 and 11.61, respectively. In the alizarin ligand, the fixation of Al(III) can occur either with the hydroxy-keto site or the catechol site. The comparison between the experimental spectrum of the 1:1 complex and those calculated with time dependent density functional theory, from different hypothetical complex structures, has shown that the first site involved in the Al(III) fixation is the catecholate function. Quantum chemical calculations have also allowed a complete assignment of Az and its 1:1 complex electronic spectra. For both, the observed transitions have essentially a π → π^∗ character. For the complexed form, only intra-ligand charge transfers are observed. The chelation of Al(III) engendered some conformational modifications of the ligand, notably at the complexation site level but also at the level of the intermediate ring of Az.     

The role of Ag(I) ions in the electronic spectroscopy of adenine–cytosine mispairs: A MS-CASPT2 theoretical study

Adenine–cytosine (AC) mispairs have been theoretically studied with MS-CASPT2//CASSCF methods in the presence and absence of Ag ions. The electronically excited states of the most stable AC mispair in the reverse-Wobble (RW) conformation have been compared with those of different Ag(I)–AC complexes, including (i) metalated RW conformations, and (ii) the most stable structures in gas phase which contain the Ag ion bridging A and C. The spectra of these complexes are characterized by charge-transfer (CT) and strong locally excited (LE) states. The metal-to-metal, metal-to-ligand, and Rydberg transitions are very weak in comparison to the nucleobase transitions. Attending to the LE and CT states, and except for the shifts induced by the presence of the Ag, the electronic spectrum of metalated AC mispairs resembles the one of the RW, showing two intense LE bands around 4.5 and 5.5 eV, corresponding to transitions within the adenine and cytosine π-system, respectively. Additionally TD-DFT results obtained with the B3LYP functional are compared with MS-CASPT2//CASSCF calculations. The results clearly evidence the weakness of TD-DFT to describe long range exchange interactions leading to strongly underestimated CT states.     

The structures,stability, and photoelectron spectroscopy of GaPX (X = C,Si, Ge; O,S; P and Ga) clusters

The equilibrium geometries and vibrational frequencies of GaPX⁻ and GaPX (X = C, Si, Ge; O, S; P and Ga) have been studied by hybrid B3LYP functional at cc-PVTZ and aug-cc-PVTZ levels. The results predict that the most stable structure of GaPC⁻ is linear while the others are trigonal. As for GaPX (X = C, Si, Ge; O, S; P and Ga), the ground structures of GaPC and GaPO are linear while the others are trigonal. The adiabatic electron affinities (AEAs) and vertical detachment energies (VDEs) of GaPX⁻ are calculated at B3LYP/aug-cc-PVTZ level. And the order of the AEAs and VDEs of GaPX⁻ are C < O < Ge ≈ Si < P < S < Ga and C < Ge ≈ Si < P < O < S < Ga, respectively. GaPC exhibits the lowest adiabatic electron affinities of all the clusters studied, indicating a particularly stable neutral species.     

(Trifluoromethyl)sulfanyl sulfinylimine, CF3SNSO. Structure and conformation in the electronic ground and excited states

The molecular structure of (trifluoromethyl)sulfanyl sulfinylimine, CF₃---S---N=S=O, was determined by gas electron diffraction (GED) and ab initio calculations (HF/3–21G*, HF/6–31+G* and MP2/6–31+G*). Experimental and theoretical methods result in a structure with planar C---S---N=S=O skeleton (C_s symmetry), anti orientation of the S---C bond relative to the N=S bond and syn orientation of the S---N bond relative to the S=O double bond (anti-syn structure). The following skeletal parameters (r_a values with 3σ uncertainties) were derived in the GED analysis: S---C, 1.831(4) Å; S---N, 1.684(5) Å; N=S, 1.538(6) Å; S=O, 1.453(6) Å; C---S---N, 94.6(8)°; S---N=S, 120.6(6)°; N=S=O: 116.5(8)°. A normal coordinate analysis based on FTIR (gas), FTIR (matrix) and Raman (liquid) spectra was performed. The UV (gas) spectrum was recorded and interpretation of the resonance Raman effect leads to the conclusion that the molecular symmetry (C_s) is retained upon electronic excitation.     

Dimesitylborane monomer-dimer equilibrium in solution, and the solid-state structure of the dimer by single crystal neutron and X-ray diffraction

Dimesitylborane dimer has been shown to exist in equilibrium with dimesitylborane monomer in solution. This equilibrium has been investigated by variable concentration and variable temperature multinuclear NMR spectroscopy and values for the dissociation constant, enthalpy and entropy of dissociation were found to be K_diss=(3.2±0.4)×10⁻³ M, ΔH=70 kJ mol⁻¹, and ΔS=212 J K⁻¹mol⁻¹, respectively. Ab initio methods have been used to investigate the gas-phase structures and energies of both monomer and dimer, and calculated ¹¹B-NMR shifts are also presented. The solid-state structure of dimesitylborane dimer has been investigated by single crystal X-ray diffraction at 100 K and the position of the bridging hydrogen atoms (B---H=1.340(2), 1.342(2) Å, H---B---H=92.46(14)°) has been determined accurately, for the first time, by single crystal neutron diffraction at 20 K.     

Unusual conformational properties of 1,3-dimethyl-1,1,3,3-tetrakis(trimethylsilyl)trisilane: A preparative, X-ray, Raman spectroscopic and ab initio study

The heptasilane Me(SiMe₃)₂SiSiH₂SiMe(SiMe₃)₂ was synthesized from Me(SiMe₃)₂SiK and H₂Si(OSO₂CF₃)₂. Crystals suitable for a X-ray single crystal analysis could be grown, with the somewhat surprising result that the two dihedral angles (H₃)CSiSi(H₂)Si are different in the crystal (24.58(10)° and 31.67(11)°). SiSiSi-bonds angles are widened, with values up to 117°. Ab initio calculations at the density functional B3LYP level employing 6-311G(d) basis sets predict minima for five conformers 1-5 with relative energies 0.0, 3.1, 8.2, 10.8 and 18.1 kJ/mol, respectively. Moreover, SiSiSiSi dihedral angles spanning the range 43.5-172.3° are predicted, reflecting the small forces which are required for distorting these angles.In the Raman spectrum of a solution in toluene, three lines at 350, 340 and 330 cm⁻¹ are observed in a wavenumber range which is typical for the SiSi-pulsation of methylated oligosilanes. The relative intensity ratio of the bands is temperature dependent, reflecting the changes in conformer concentrations that occur according to Boltzman’s law. Supported by the ab initio calculations, the Raman band at 350 cm⁻¹ is assigned to an ‘averaged’ conformer 1 and 2, because a rapid interconversion between 1 and 2 has to be assumed due to a small barrier separating them. The bands with wavenumbers 340 and 330 cm⁻¹ originate from conformers 3 and 4. From the Raman spectra, relative energies 0.0 (1 + 2), 2.2 (3) and 6.3 (4) kJ/mol are deduced, the presence of 5 is not observed. Caused by solvent effects, these values differ somewhat from the ab initio results.     

An Ab initio theoretical investigation on the geometrical and electronic structures of BAun−/0 (n = 1–4) clusters

An ab initio theoretical investigation on the geometrical and electronic structures and photoelectron spectroscopies (PES) of BAu_n^?/0 (n = 1–4) auroboranes has been performed in this work. Density functional theory and coupled cluster method (CCSD(T)) calculations indicate that BAu (n = 1–4) clusters with n‐Au terminals possess similar geometrical structures and bonding patterns with the corresponding boron hydrides BH. The PES spectra of BAu (n = 1–4) anions have been simulated computationally to facilitate their future experimental characterizations. In this series, the T_d BAu anion appears to be unique and particularly interesting: it possesses a perfect tetrahedral geometry and has the highest vertical electron detachment energy (VDE = 3.69 eV), largest HOMO‐LUMO gap (ΔE_gap = 3.0 eV), and the highest first excitation energy (E_ex = 2.18 eV). The possibility to use the tetrahedral BAu unit as the building block of Li⁺[BAu₄]^? ion‐pair and other [BAu₄]^?‐containing inorganic solids is discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

X-ray transient absorption and picosecond IR spectroscopy of fulvalene(tetracarbonyl)diruthenium on photoexcitation

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn?configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti?biradical.     

Theoretical study of the structural,elastic, electronic and optical properties of XCaF3 (X = K and Rb)

The PLANE WAVE pseudo-potential method within density functional theory (DFT) has been used to investigate the structural, elastic, electronic and optical properties of XCaF₃ (X = K and Rb) insulating. The studied compounds show a weak resistance to shear deformation compared to the resistance to the unidirectional compression. KCaF₃ and RbCaF₃ are considered ductile. The elastic constants and related parameters were predicted. The stiffness is more important in KCaF₃, whereas, the lateral expansion is more important in RbCaF₃. KCaF₃ and RbCaF₃ have R- Г indirect band gap. The main peaks in the imaginary part of the dielectric function correspond to the transition from the occupied state F₋p to the unoccupied states Ca: s or K, Rb: p. At lower energies, KCaF₃ and RbCaF₃ show the same optical properties. Under pressure effect, the peaks of imaginary part of dielectric function were shifted toward high energy.