Ab-initio calculations on the 1-imidazolyl radical

Ab-initio calculations on the 1-imidazolyl radical with a minimal basis set were performed. The ground state symmetry is calculated to be ²B₁(π); the first excited state is ²A₂(π) and lies at 0.359 eV. The σ-state shows a double minimum potential having its well 0.626 eV above the C_2v ground state minimum. The height of the inner hill, peaking at C_2v symmetry, is 0.612 eV; the state is of B₂ character. The reason for the distortion is discussed.     

Influence of alkyl substituents on the π* negative ion states of benzene and its derivatives : Electron transmission spectroscopy of p-alkylanilines

Electron affinities and electron attachment energies associated with the π^* orbitals of several p-alkylanilines (alkyl = Me, Et, i-Pr, and t-Bu) have been determined in the gas phase by electron transmission spectroscopy and have been analyzed by ab initio molecular orbital calculations at the STO-3G and 6-31G levels of basis set. The results lead to the conclusions that (a) the ²A₂ negative ion state in toluene lies below the ²B₁ state, (b) the ²A₂ state is stabilized in toluene relative to benzene by bonding overlap between the ortho carbon (C₂(2p_π)) and the Me H(1s) orbitals in the LUMO, (c) the ²B₁ state is stabilized on adding Me groups to toluene to form t-butylbenzene through reduced antibonding interactions between C₁(2p_π) and Me C(2p_π) orbitals, and (d) the lowest resonance in t-butylbenzene is extensively overlapped with the second resonance and may involve strong mixing of the ²A₂ and ²B₁ states.     

Self-quenching and electronic energy transfer of triplet molecules. The benzaldehyde-biacetyl system in the vapor phase

The triplet-triplet energy transfer from benzaldehyde to biacetyl and the competing self-quenching between triplets and ground state molecules of benzaldehyde were investigated in the dilute vapor phase by monitoring the phosphorescence (T₁(nπ^*)S_o) decay of benzaldehyde. Following excitation into the S₁(nπ^*)S₀ absorption band, a triplet self-quenching rate constant of k_SQ=(2.4±0.1) × 10⁴ s^?1 Torr^?1, corresponding to a gas-kinetic cross section of σ_SQ=0.22 A², was measured. The collision-free lifetime of the benzaldehyde triplet was found to be 2.3 ± 0.4 ms. Substitution of the aldehydic proton by deuterium reduces k_SQ by a factor of two: complete deuteration of the molecule has no further effect. Under the same excitation conditions, the energy transfer rate to biacetyl is k_ET=(2.8 ± 0.1) × 10⁶ s^?1 Torr^?1, with σ_ET = 24 A². This process is not influenced by deuteration.     

Cluster expansion of the wavefunction: Ionizations and some low-energy excitations of naphthalene

Ionizations and some lower singlet valence and Rydberg excitations of naphthalene are studied by the SAC/SAC CI method with 100 (44 π + 56 σ) active MOs. A systematic assignment of the ionization spectra is given. In particular, assignments are given for the peaks in the 13–20 eV region. A large σ-electron correlation effect is found for the valence excited B_1u state, together with a large effect of the d_π polarization function. The controversy concerning the assignment of the Rydberg nd_π orbitals is resolved: Robin's assignment of the 3d_π orbital at 6.73 eV above the ground state is supported.     

All-valence-electron Cl calculations on the electronic spectrum of diborane

All-valence-electron Cl calculations have been carried out for diborane B₂H₆ and its positive ion employing a rather large double-zeta AO basis including polarization functions in order to study the electronic spectrum of this system. Transitions from four different valence MOs are found to lead to low-lying electronic transitions of both Rydberg and valence type in each case. Ad mixture of valence character in the otherwise Rydberg-like (nx, 3s), (ny, 3s) and (σ, 3pz) transitions calculated to lie between 11.0 and 11.6 eV is indicated as being primarily responsible for the highly intense shoulder found in this region of the B₂H₆ spectrum. The other strong feature with essentially continuous absorption peaking at 9.3 eV is suggested to result from superposition of several Rydberg-type transitions in the generally broad absorption pattern expected for the ¹(π,π^*) species at significantly higher vertical excitation energy. Quite good agreement is obtained between calculation and experiment for all of the six lowest IPs of diborane and also for the locations of the ¹(n, π^*) and ¹(σ, π^*) transitions previously assigned to the two weak features observed at 6.8 and 8.3 eV in this spectrum.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum of molecular fluorine

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X¹Σ_g⁺→¹Σ_u⁺ bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π_g→npπ_u Rydberg and 3σ_g→3σ_u intravalence excitations. A depleted X→V_σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F₂ below the 3σ_g^?1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X¹Σ_g⁺→¹Π_u excitations the origins of which are attributed to configuration mixing between 1π_g→npσ_u and 1π_u→nsσ_g Rydberg series.     

Tautomerism of anthraquinones: III. Tautomerization and rotational isomerization as processes responsible for the appearance of several π1,π*-bands in the absorption spectra of hydroxy-substituted quinones

Hydroxy-substituted anthraquinones, naphthoquinones, and naphthacenequinones exist as equilibrium mixtures of different tautomers and rotational isomers which give rise to several π₁,π* bands in their electronic absorption spectra. Each π₁,π* band can be assigned to a particular tautomer and conformer on the basis of the substituent constants σ^A.     

Theoretical study of the energetics of the O3(X̃ 1A1) → O2(X 3Σ−g) + O(3P) dissociation process

The dissociation energy (D_e) for the O₃(X̃ ¹A₁) → O₂(X ³Σ⁻_g) + O(³P) process is computed using MC SCF, CI, MBPT, and CCD methods. A full-valence MC SCF calculation utilizing a [9s5p3d1f/5s3p2d1f] basis set yields a D_e value of 0.43 eV, far below the experimental value of 1.13 eV, demonstrating the importance of correlation effects involving non-valence orbitals. A CI calculation in the same basis set allowing for all single and double excitations from three-reference configuration yields a D_e value of 0.72 eV. This value is increased to 1.06 eV when the Davidson correction is included. When the number of reference configurations is increased to eight, the resulting CI calculation gives a D_e value of 0.82 eV prior to the Davidson correction and 1.10 eV after this correction.     

The Ionization Potentials of Butadiene,Hexatriene, and their Methyl Derivatives: Evidence for through space interaction between double bond π-orbitals and non-bonded pseudo-π orbitals of methyl groups?

It is shown that the correlation of the π-ionization potentials of ethylene ( 1 ), butadiene ( 2 ) and trans-1,3,5-hexatriene ( 4 ) favours the orbital sequence π, π, σ in butadiene and π, π, σ, π in the hexatriene in excellent agreement with the results of SPINDO calculations. Within the experimental error the π-ionization potentials of cis-1,3,5-hexatriene ( 3 ) and trans-1,3,5-hexatriene ( 4 ) are the same. Methyl-substitution of 2 lowers the π-ionization potentials I₁(π) and I₂(π). For 1 and/or 4 substitution the difference I₂(π)?I₁(π) remains constant (≈ 2.5 eV). On the other hand 2 and/or 3 substitution leads to a smaller gap of I₂(π)–I₁(π) ≈ 1.6 to 2.0 eV without changing the mean π-ionization potential I (π) relative to the corresponding 1,4 substituted derivatives. This result is rationalized in terms of a through space interaction between double bond π-orbitals and non-bonded pseudo-π-orbitals of the substituting methyl groups. The reduced split I₂(π)–I₁(π) in cyclopentadiene is attributed to hyperconjugation across the methylene group.     

Determination of the absolute scattering cross section for the reaction D + H2(v=1) → HD+H at 0.33 eV

The reaction D+H₂(v=1) has been investigated in a crossed molecular beam experiment at the most probable collision energy of E₀=0.33 eV. Angular and time-of-flight distributions have been measured and the total absolute cross section has been determined to be σ_r(j_i=0, v = 1, E_c.m. = 0.33 eV) = 1.14 ± 0.50 Ų. This value, as well as the distributions, are in good agreement with the results of quasiclassical trajectory calculations (QCT) and the reactive infinite-order sudden approximation (RIOSA).     

Analytical ab initio potential-energy surfaces for the ground and the first singlet excited states of HeH2

Analytical potential-energy surfaces have been constructed for the ground and the first excited states of HeH₂. The functions fit ab initio MRD CI calculations with standard deviations of 0.05 and 0.13 eV for the ground and the excited surface respectively. Classical trajectory calculations for collisions of ⁴Hc with HD(B ¹Σ_u⁺, υ = 3, J = 2) at the temperature T = 297 K yields the electronic quenching cross section σ_Q = 6.5 A² and the vibrational cross section σ_3→2 = 3.8 A². The results are in qualitative agreement with the experimental values of Fink, Akins and Moore.     

Non-empirical SCF and Cl Study of the ground and excited states of thioformaldehyde

Ab initio SCF and Cl calculations are reported for ground and various low-lying Rydberg and valence excited states of thioformaldehyde H₂CS. A double-zeta basis of near Hartree-Fock quality is employed in this work and the importance of polarization functions is also assessed. The calculations indicate uniformly larger CX bond lengths in this system than for H₂CO in the corresponding electronic states; they also lind potential minima for H₂CS non-planar nuclear conformations in the (n,π^*) and (π,π^*) excited states but in each case the calculated inversion barriers are seen to be smaller than those encountered in formaldehyde. The vertical transition energies to the various excited states studied are also found to be significantly smaller in H₂CS than in H₂CO but the order of electronic states is concluded to be virtually identical for the two systems. The lowest-lying excited states are the ^3,1(n,π^*) species calculated at 1.84 and 2.17 eV respectively; the first two allowed transitions are indicated to be the Rydberg species (n,s_R) and (n,px_R) at 5.83 and 6.62 eV. These are followed by the two allowed transitions σ → π^* and π → π^* at 7.51 and 7.92 eV respectively, both well below the first ionization limit in H₂CS. The much smaller splitting between the ^3,1(π,π^*) species in H₂CS than in H₂CO is attributed to the relatively diffuse charge distribution of the sulfur atom compared to that of oxygen.     

Photoelectron Spectra of Unsaturated Oxides. I. 1,4-Dioxin and related systems

The He (Iα) photoelectron spectra of the four unsaturated oxides 3,4-dihydropyran ( 6 ), γ-pyran ( 7 ), 2, 3-dihydro-1, 4-dioxin ( 9 ) and 1, 4-dioxin ( 10 ) are reported and analysed. Band assignments are based on ab-initio calculations, using the STO-3G basis set. The proposed orbital sequences (with reference to the coordinate systems given in Table 1) are, for the top three orbitals: 6 , π, nσ, nπ; 7 , 3b₁(π), 1a₂(π), 11a₁(σ); 9 , 11b(π), 12a(σ), 11a(π); 10 , 2b_3u(π), 1b_1g(π), 6a_g(σ). Finally the (almost) localized π-orbitals have been computed by the Foster-Boys localization procedure.     

Electronic band structure of solid CO2 as determined from the hv-dependence of photoelectron emission

Photoelectron energy distribution curves from solid CO₂ have been determined for excitation energies from hv = 14 up to 40 eV using synchrotron radiation. A 1:1 correspondence to the gas-phase photoelectron spectrum is observed for the occupied molecular orbitals. The vertical binding energies E_B^v (E_VAC = 0) and widths (fwhm) of the valence bands of solid CO₂ are determined to be 13.0 and 0.95 eV (1π_g); 16.7 and 1.1 eV (1π_u); 17.6 and 0.85 eV (3σ_u) and 18.8 and 0.8 eV (4σ_g) for the individual bands respectively. The partial photoemission cross sections differ importantly from those of the gas phase in exhibiting pronounced maxima at 5.2 eV (1π_g), 4.4–5.3 eV (1π_u + 3σ_u) and 4.2 eV (4σ_g) above the vacuum level, which is attributed to effects of high density of final (conduction-band) states. Further weaker maxima are observed at higher photon energies. Contrary to the case for the gas phase, the resonances are unperturbed in the solid by degenerate autoionizing molecular Rydberg states. The molecular origin of the resonances in the continuum is discussed and related to X-ray absorption spectra, electron-scattering data and to theoretical cross-section calculations. It is shown that the same set of resonances is observed in the different experiments. The resonances occur however at different energies due to different Coulomb interactions. The photoemission results presented provide also a key to the hitherto unexplained optical spectrum of solid CO₂ in the VUV range, making possible an assignment of the structures observed to Frenkel-type excitons (hv ≤ 15 eV) and interband transitions (hv ? 15 eV).     

LIF measurement of the vibrational dependence of the N2(B 3Π g ) hyperfine splitting parameters

From very high resolution (8 MHz FWHM) LIF measurements, the hyperfine coupling constants of N₂(A ³Σ _u ⁺ ) and N₂(B ³Π _g ) have been obtained for three pairs of vibrational quantum numbersv′,v″. TheA-state constants are in very good agreement with accurate literature data. The vibrational dependence of some of theB-state constants is found to be much more pronounced. This is qualitatively explained in terms of the electronic structure of the two states.     

Crossed-beam investigations of the oh(a2σ+→x2πi) emission spectrum after dissociative excitation of water by electron impact

We measured excitation functions of the electronic OH(A²Σ⁺→X²π_I) transition in the vibrational 0-0 and 1-0 band systems as well as of the Q₁(3) and P₁(21) rotational lines of the 0-0 transition produced by electron impact on water vapour from 9.3 eV (threshold) up to 100 eV by means of the crossed-beam technique.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

A thermochemical study of dissociative ionization of 4,4-dimethyl-1-thia-4-silacyclohexane and 2,3,3-trimethyl-1-thia-3-silacyclopentane. Decomposition pathways to produce a dimethylsilanethione ion-radical [Me2SiS]+•

The dissociative ionization of 4,4-dimethyl-1-thia-4-silacyclohexane (I) and 2,3,3-trimethyl-1-thia-3-silacyclopentane(II) has been studied by electron photoionization (PI) mass spectrometric methods. The molecular ion fragmentation is mainly related to the loss of ethylene and results in a [Me₂SiSC₂H₄]^+? (m/z 118) ion-radical (A). Further loss of ethylene from A produces a dimethylsilanethione [Me₂SiS]^+? (m/z 90) ion-radical (B). The latter is the most abundant ion in the mass spectra of I and II at 70 eV.The ionization energies (IE) of I (8.22 ± 0.07 eV) and II (8.06 ± 0.03 eV) and the appearance energies (AE) of ion-radicals A and B have been determined. Also, the following heats of formation were calculated (kJ/mol): ΔH_f⁰(I) = ?31.1; ΔH_f⁰(II) = ?65.8; ΔH_f⁰(M_I^+?) = 762.0; ΔH_f⁰(M_II^+?)= 712.1; ΔH_f⁰(A)_aver = 780.2; ΔH_f⁰(B)_aver = 847.7.     

The importance of ground-state vibronic mixing in molecular Raman scattering

We consider Albrecht's theory for Raman scattering of fundamentals in the far and pre-resonance regions. Destructive interferences inherent to the A and B terms augment the conventionally suppressed C term dramatically. Raman excitation profiles for the ν₁(a_1g) and ν₆(e_2g fundamentals in benzene can be well fitted with theoretical C-term profiles involving the ¹E_1u(π-π^*) state at 1800 A.