Spectral bandshape and intensity of the C-H chromophore in the infrared spectra of CF3H and C4F9H

The vapour phase IR spectra and integrated band intensities of the fundamental and first-overtone transition of the C-H chromophore in CF₃H and (CF₃)₃CH are reported. The vibrational coarse bandshapes for (CF₃)₃CH are consistent with lorentzians of width Γ = 3 cm^?1 (fundamental) and Γ = 12 cm^?1 (overtone). The results are discussed in connection with time-dependent molecular processes and IR photochemistry.     

The geometry of the propynol cation,HC3O+

The geometry of a potential interstellar molecule, the propynol cation, HC₃O⁺, is determined by split-valence plus polarization (6-31G*) SCF method with the inclusion of electron correlation effects by third order Møller—Plesset perturbation theory (MP3). The 6-31G* MP3 geometry is: r_e(CO) = 1.125, r_e(CC) = 1.351, r_e(CC) = 1.214, r_e(CH) = 1.078 A, and thus a B_e of 4.421 GHz.     

The electronic gas-phase spectrum of B3 radical revisited

The gas-phase electronic spectrum of cyclic-B₃ (D_3h) radical has been remeasured in a supersonic molecular beam using a mass-selective resonant 2-color 2-photon technique, leading to a revision of previously reported spectroscopic constants. The species was prepared by ablation of a boron nitride rod in the presence of helium. Ab intio calculations on the geometries and vertical electronic excitation energies, as well as mass identification, indicate that the detected band, centered at 21848.77(2) cm⁻¹, is the origin of the cyclic-¹¹B₃ system. A spectral fit yields the rotational constants as B″ = 1.2246(45) and C″ = 0.62131(72) cm⁻¹ in the ground state, and B′ = 1.1914(44) and C′ = 0.61173(69) cm⁻¹ in the excited 2 ²E′ state.     

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.     

Theoretical study of electronic spectra of linear carbon clusters HC2nS (n = 1–5)

In this work we report the structures and stabilities of linear carbon clusters HC_2nS (n = 1–5) in their ground states using the B3LYP density functional. The rotational constants at the optimized geometries give excellent agreement with the experimental and previous theoretical values. The vertical excitation energies of the 2²Π ← X²Π transitions at the CASPT2 level are 3.16, 2.66, 2.05, 1.78, and 1.55 eV, respectively, in good agreement with the corresponding observed values of 3.01, 2.48, 2.10, 1.84, and 1.65 eV. Also, the exponential-decay curves for these vertical excitation energies obtained from experiments and theoretical calculations are illuminated.     

A study of electronic structures of some C9H9- carboanions

The electronic structures of five C₉H₉^-, carboanions have been studied by ab initio STO-3G calculations, and some general conclusions on related C₉H₉^- and C₉H₉⁺ structures are presented. Large antibonding interactions in one occupied MO make barbaral-9-yl anion (2) unstable as its cationic counterpart (8). The proposed D_9h-symmetrical cation and D_3h-symmetrical anion (3) do not exist due to Jahn-Teller distortions. A study of the MO correlations confirms that the two tetracyclic anions with C_2v symmetry (5 and 6) are the results of the Jahn-Teller distortions of 3. Anion 5 is identified as the proper intermediate of the Cope rearrangement of anion 2.     

The role of Mn in the electronic structure of Ba3Ti2MnO9

The energy loss near edge structure (ELNES) of the O-K, Ti-L₂₃ and Mn-L₂₃ edges have been recorded in hexagonal Ba₃Ti₂MnO₉ with an energy resolution of 0.10-0.20 eV using a monochromator on a commercial transmission electron microscope (TEM) and compared with a tetragonal BaTiO₃ reference sample. The formal valency and symmetry of Mn have been determined using atomic multiplets calculations and its effect on the electronic structure of BaTiO₃ has been interpreted through a molecular-orbital model.     

Normal coordinate analysis of M-Si 2H 5-moieties in transition metal complexes and comparison to Br-Si 2H 5

In order to examine the influence of the transition metal on the metal-disilanyl-fragment and especially the M-Si and Si-Si-bond polarized Raman spectra of the complexes (C₅R₅)(CO)₂Fe-Si₂H₅ {R=H (1a); R=Me (1b)} and (C₅R₅)(CO)₂(PMe₃)M-Si₂H₅ {R=Me, M=Mo (2); R=H, M=W (3)} have been recorded. The spectral data have been evaluated and interpreted on the basis of a normal coordinate analysis of the M-SiH₂-Si-fragment including Br-Si₂H₅ (4) and comparison with examinations of the M-SiH₃-fragment.     

Réactivité par voie thermique de RFH

Hydroperfluoroalkanes are well known as being rather inert chemically. Hydroperfluoroalkanes react at high temperature with carbon tetrabromide and carbon tetrachloride to yield bromoperfluoroalkanes and chloroperfluoroalkanes.

Résumé

Les hydroperfluoroalcanes sont connus pour être stables chimiquement. Nous montrons qu'ils réagissent à haute température vis-à-vis de tétrabromure de carbone et de tétrachlorure de carbone pour conduire aux bromoperfluoroalcanes et chloroperfluoroalcanes correspondants.     

Ionic and electronic transport in La2Ti2SiO9-based materials

The total conductivity of monoclinic La₂Ti₂SiO₉ is mixed oxygen-ionic and n-type electronic, and increases on reduction of the oxygen partial pressure down to 10⁻²¹ atm at 973-1223 K. The substitution of Ti⁴⁺ with Nb⁵⁺ decreases both contributions to the conductivity, whilst Pr doping and reducing p(O₂) have opposite effects. The oxygen ion transference numbers of La₂Ti₂SiO_9−δ, LaPrTi₂SiO_9±δ and La₂Ti_1.8Nb_0.2SiO_9±δ ceramics, measured by the faradaic efficiency and e.m.f. methods, vary in the range 0.15-0.32, increasing when temperature decreases. In air, the activation energies for the ionic and electronic transport are 1.23-1.40 and 1.59-1.74 eV, respectively. Protonic contribution to the conductivity in wet atmospheres becomes significant at temperatures below 1000 K. The experimental data and the results of atomistic computer simulations suggest that the oxygen-ionic and electronic transport is primarily determined by processes involving TiO₆ octahedra. The ionic conduction may occur via both the vacancy and interstitial migration mechanisms, but the former is more favorable energetically and should dominate, at least, in reducing atmospheres. The average thermal expansion coefficients of La₂Ti₂SiO₉-based ceramics, calculated from dilatometric data in air, are (8.7−9.5)×10⁻⁶ K⁻¹ at 300-1373 K. The lattice of lanthanum titanate-silicate is almost intolerant with respect to A-site deficiency and to doping with lower-valence cations, such as Sr and Fe.     

A theoretical study of the electronic spectrum of CCl2

Ab into configuration interaction calculations for some low-lying electronic states of the dichlorocarbene radical (CCl₂) have been carried out. The UV absorption band at 330 nm (3.76 eV) obtained by the pulse radiolysis experiment is confirmed and assigned to the ¹A₁ – ¹B₁ transition. The calculated transition energy amounts to 318.4 nm (3.90 eV). The first triplet state (³B₁) is found to lie 0.83 eV above the ¹A₁ ground state.     

Vibrational study of hydrogen bonding and structural disorder in Na2H(SO4)2, K3H(SO4)2 and (NH4)3H(SO4)2 crystals

Infrared and Raman spectra on Na₃H(SO₄)₂, K₃ H(SO₄)₂ and (NH₄)₃ H(SO₄)₂ crystals have been investigated at 300 and 100 K in the 4000 to 30 cm⁻¹ region. An assignment of bands in terms of OH group frequencies and more or less distorted tetrahedra of ammonium and sulphate ions is given. The crystallographic and spectroscopic symmetry and/or dissymetry of OHO hydrogen bonds linking sulphate ions into dimers is discussed using OH group frequencies and the splitting of the v₁ (SO₄) Raman bands as criteria. In the particular case of (NH₄)₃H(SO₄)₁ compound containing several solid phases it can be shown that the room temperature phase (II) is strongly disordered, principally because of an orientational disorder of ammonium ions, and that a progressive ordering takes place with temperature lowering.     

Syn-stereoselective epoxidation of allylic ethers using CF3CO3H.

Good $\text{syn}$-stereoselectivity is observed in the epoxidation of allylic silyl ethers with CF₃CO₃H, indicating a new type of hydrogen-bonded transition state.     

Ab initio CCSD(T) and MRD-CI study of excited states and the electronic spectrum of linear C5

Large-scale ab initio coupled cluster and multi-reference configuration interaction calculations (MRD-CI) are carried out to determine the equilibrium geometry and the vertical electronic spectrum of linear C₅⁺. Contrary to prior theoretical estimates we find three low-lying states within an energy range of 0.3 eV: ²Σ_u⁺, ²Σ_g⁺ and ²Π_g and a symmetric arrangement of nuclei. Transitions from ²Σ_u⁺ to these low-lying states are dipole-allowed; sizeable oscillator strengths are computed for the ²Π⁺_g←X²Σ_u⁺ transition at 2.62 eV and the ²Σ_g←X²Σ_u⁺ transition at 3.36 eV and should give a guide to spectroscopic identification of linear C₅⁺.     

The electronic spectrum of acrylonitrile

The vapor absorption spectrum of acrylonitrile CH₂CHCN has been measured in the vacuum ultraviolet region. In addition, an all-valence-electron molecular orbital calculation has been used to calculate the electronic structure and spectrum of the molecule. On the basis of the MO calculation, as well as a vibrational analysis of the observed spectrum, several electronic transitions are assigned. The lowest energy absorption band (2107Å, = 150) is assigned as an n ^* transition. Absorption bands at 2030Å (=1600), 1725Å ( = 2100), and 1570Å ( = 1920) are assigned as 0–0 bands associated with transitions that are, respectively, ^*,^*, and ^* in character.

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Zusammenfassung Das UV-Absorptionsspektrum von dampfförmigen Acrylnitril wurde gemessen und eine CNDO/2-Rechnung für die Elektronenstruktur durchgeführt. Auf dieser Basis konnten unter Zuhilfenahme der Analyse der Schwingungsstruktur im beobachteten Spektrum mehrere Banden zugeordnet werden: die 2107-Å-Bande ( = 150) einem n^*-Übergang, die drei Banden bei 2030Å ( = 1600), 1725Å ( = 2100) und 1570 Å ( = 1920)0-0-Übergängen von ^*-, ^*- bzw. ^*-Banden.

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Résumé Mesure du spectre d'absorption en phase vapeur de l'acrilonitrile CH₂CHCN dans la région de l'ultraviolet. Par ailleurs, la structure électronique et le spectre de la molécule ont été calculés à l'aide d'une méthode des orbitales moléculaires semi-empirique pour tous les électrons de valence. Sur cette base, ainsi que sur une analyse vibrationnelle du spectre expérimental, on procède à l'attribution de plusieurs transitions électroniques. La bande d'absorption de plus basse énergie (2107 Å, = 150) est attribuée à une transition n ^*. Les bandes d'absorption à 2030Å ( = 1600), 1275Å ( = 2100) et 1570Å ( = 1920) sont considérées comme des bandes 0 - 0 associées à des transitions ^*, ^* et ^* respectivement.

     

The electronic spectrum of benzene

Assignment of the benzene spectrum is discussed using a new approach to the semi-empirical π-electron theory. The ordering ¹B_1u <¹E_1u <¹E_2g is shown to be theoretically preferred.     

The electronic spectrum of o-benzyne

Semiempirical valence electron calculations using the CNDO/S SECI, HAM/3 Cl, LNDO/S PERTCI, MNDOC PERTCI, and MNDO/S PERTCI methods have been applied to the interpretation of the recently reported UV/vis absorption spectrum of transiento-benzyne. Detailed calculations were performed for benzene,o-benzyne, cyclopentadienylidenecarbene, cyclopentadienylideneketene, and cyclopentadienylideneketone (6-fulvenone). From these calculations and experimental data for benzene,o-benzyne, and cyclopentadienylideneketone, an interpretation of the electronic absorption spectrum ofo-benzyne is proposed. The interpretation includes the proposal that the first band and a part of the third band originally assigned too-benzyne may be due to cyclopentadienylideneketene, a common byproduct (or intermediate) of the photolyses of benzocyclobutenedione and 3-diazobenzofuranone.     

The electronic spectrum of VCr

The electronic spectrum of VCr has been studied using the complete-active-space self-consistent field complete-active-space second-order perturbation theory approach. Potential-energy curves for 12 electronic states have been computed. Transition energies, with respect to the X² ground state, for some of the calculated electronic states are (with possible experimental values within parentheses) 0.53 eV (0.56) for A²⁺, 1.03 eV (1.14) for A⁴, 1.20 eV (1.14) for B², 1.45 eV (1.51) for B⁴, 1.60 eV (1.51, 1.78) for C², and 1.61 eV (1.63) for A^4–. AcknowledgementsThe research reported in this communication was supported by a grant from the Swedish Natural Science Research Council (NFR). S. Alex and D.G. Leopold are acknowledged for providing results from their negative ion photoelectron spectroscopy on VCr.Contribution to Björn Roos Honorary Issue     

The electronic spectrum of purine

Absorption, phosphorescence and phosphorescence excitation spectra are reported for matrix-isolated purine at 20 K. In general, the spectra show enhanced vibrational structure over gas phase or solution conditions. The ¹(π*←π) absorption closely resembles that in the gas phase. The phosphorescence is rich in structure with the best resolution in argon. Several ground state fundamentals involving ring deformations are observed. The most active vibrations are the a′ modes 649, 793, 1260, 1482 and 1563 cm⁻¹. The ¹(π*←n) transition appears with vibrational structure in the phosphorescence excitation spectrum; the vibrations are 426, 566 and 846 cm⁻¹. Mixing between ¹ππ* and ¹nπ* and methane-purine hydrogen bonding appear to contribute to the observed spectra.