Excitation-energy dependence of the phosphorescence quantum yields of pyridinecarboxaldehyde vapors

Emission and excitation spectra of 3- and 4-pyridinecarboxaldehyde vapors have been measured at different pressures down to 10(-2)Torr. The phosphorescence quantum yield measured at low pressure as a function of excitation energy is nearly constant in the range of excitation energy corresponding to the S1(n, pi*) state, but it decreases abruptly at the S2(pi, pi*) threshold. The onset of the abrupt decrease of the yield corresponds to the location of the S2 absorption origin of each molecule, indicating that the nonradiative pathway depends on the type of the excited singlet state to which the molecule is initially excited. The relaxation processes are discussed based on the pressure and excitation-energy dependence of the phosphorescence quantum yield.     

Ground-state, excited-singlet-state, and excited-triplet-state energy levels and photophysics of the three rotational isomers of isophthalaldehyde vapor

Emission, excitation, and absorption spectra of isophthalaldehyde (benzene-1,3-dicarboxaldehyde) vapor have been measured at different temperatures, along with the UV-vis and IR absorption spectra in solution. Analyses of the temperature dependence of the phosphorescence and excitation spectra of isophthalaldehyde vapor revealed the energetic relationships among the three rotational isomers in the T1(n, pi*), S1(n, pi*), and ground states. This appears to be the first example of the system where the S0, T1, and S1 energy levels are determined for the three rotational isomers. The phosphorescence, fluorescence, and excitation origins of the three rotamers were assigned on the basis of the results of the density functional theory (DFT) and semiempirical SCF-MO calculations and infrared data as well as on the basis of the temperature dependence of the emission and excitation spectra.     

Electronic structures and electronic spectra of the linkage isomers NSO and SNO

The relative stabilities and electronic structures of the linkage isomers NSO⁻ and SNO⁻ have been determined by the MNDO and ab initio Hartree—Fock—Slater methods. Both approaches predict a higher stability for SNO⁻ by ca. 100 kcal mol⁻¹, but an overlap population analysis indicates substantially higher bond orders for NSO⁻ compared to SNO⁻. The calculations also reveal a low energy pathway with a barrier of ca. 6 kcal mol⁻¹ for the isomerization process NSO⁻ → SNO⁻. Good agreement was found between the observed UV-visible absorption bands for NSO⁻ (λ_max 379 nm) and SNO⁻ (λ_max 340 nm) and calculated values of the electronic transition energies.     

全反式球形烯的光敏异构化及异构体的光谱

从Rhodobacter sphaeroides 2.4.1提取了全反式球形烯, 用高压液相色谱(HPLC)分离了它的I2增感异构化产物: 9,13'-cis(峰A), 5,13-cis+13,9'-cis(峰B), 13-cis(峰C), 5,13'-cis(峰E1), 9-cis(峰E2), 13'-cis(峰F),5,9'-cis(峰G), 9'-cis(峰H), all-trans(峰I), 研究了异构体的结构对其电子吸收光谱及HPLC保留时间的影响, 从而确认峰E1为5,13'-cis。对全反式球形烯的光敏异构化机理也作了初步探讨。     

Microwave spectra and molecular structures of rotational isomers of fluoroacetic acid and fluoroacetyl fluoride

The molecular structures of both the trans- and the cis-conformations of fluoroacetic acid and fluoroacetyl fluoride have been redetermined from the microwave spectra of isotopically substituted species. For the fluoride the results of single and multiple D-substitutions are compared, and the effect of the torsional motion on substitution coordinates is discussed.From the fairly large geometry differences between rotational isomers the energy effect involved in geometry relaxation is estimated to be roughly 1–2 kcal mole^?1 , which is more than the cis-trans energy differences themselves.     

Ionization energy measurements and electronic spectra for ThO

The ionization energy (IE) for ThO has been determined using photoionization efficiency and mass-analyzed threshold ionization measurements. An IE of 6.6038(12) eV was obtained, which was appreciably higher than the result from previous appearance potential measurements [6.1(1) eV]. The revised IE is 0.3 eV greater than that of atomic Th, indicating that neutral ThO is more tightly bound than ThO(+). The one-color two-photon resonant ionization spectrum of ThO was examined in the range of 315-370 nm. Rotationally resolved bands were recorded for three new electronic states (designated as E('),F('), and G(')). In addition, transitions to the A(')(v=1,2,3) levels and the N(v=2) level were observed for the first time. Ligand field theory predictions [L. A. Kaledin, J. E. McCord, and M. C. Heaven, J. Mol. Spectrosc. 164, 27 (1994)] were used to propose configurational assignments for 20 electronically excited states.     

X-ray spectra and electronic energy structure of carboaluminides Ti-Al-C

The electronic energy structure, TiL_2,3 X-ray emission, and TiL2j XANES spectra of diamond and titanium carboaluminides Ti-Al-C are calculated by the local coherent potential method in terms of multiple scattering theory. The cluster version of the MT approximation is used to calculate the crystal potential. The electronic energy structures of diamond and the ternary and binary titanium carbide systems are compared in a single approximation. The broadening of the hybridized band in Ti-Al-C is discussed and compared with the experimental CK_α emission spectra. The specifics of chemical binding in the compounds is explained by delocalization of the carbon sp³ configuration and formation of a metal bond in carboaluminides. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 505-514, May–June, 2000.     

SCF CI calculation of the electronic spectra of cis and trans isomers of stilbene and azo-benzene

SCF CI calculation has been made on cis and trans isomers of stilbene and azo-benzene. It has been shown that the electronic spectra of trans isomer should appear at a shorter wave length than the corresponding cis isomers, although the resonance theory reverses their positions. Experimentally azobenzene agrees with the conclusions of SCF theory while stilbene is better represented by the resonance theory.

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Zusammenfassung Für die cis- und trans-Isomeren des Stilbens und Azobenzols wird eine Berechnung nach der SCF-CI-Methode durchgeführt. Es wird gezeigt, daß die Elektronenspectren der Trans-Isomeren bei einer kürzeren Wellenlänge als diejenigen der cis-Isomeren liegen sollten, obwohl nach der Resonanztheorie eine umgekehrte Lage zu erwarten wäre. Die experimentellen Ergebnisse stehen beim Azobenzol in Einklang mit den Resultaten der SCF-Methode während sie beim Stilben besser im Rahmen der Resonanztheorie zu deuten sind.

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Résumé Nous avons calculé les spectres électroniques des isomères cis et trans du stilbéne et de l'azobenzène. Les spectres des isomères trans devraient apparaître à des longueurs d'onde plus courtes que ceux des cis correspondants. La théorie de résonance donne des résultats inverses. Les spectres des azobenzènes s'accordent aux conclusions de la théorie SCF, tandis que les stilbènes se représentent mieux par la théorie de résonance.

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Sincere thanks are due to Prof. P. O. Löwdin for providing financial assistance and laboratory facilities at Uppsala where a part of the work was done and to Dr. Klaus Appel for machine computation on Alwac III.     

Microwave spectrum and rotational isomers of epifluorohydrin

The microwave spectra of three rotational isomers of epifluorohydrin are Presented. The ground and 3 excited torsional states of one of the two gauche rotamers (III), and the ground states of the other gauche and cis rotamers (I & II), have been observed. The values of the ground state rotational constants. A, B, and C for rotamers I, II and III are 14 662.8, 3129.4, 2863.3; 10 645.8, 4050.2, 3679.1; and 14 833.1, 3210.2, 2933.9 MHz. The gaseous phase dipole components for the most abundant rotamer (III) are μ_a = 0.99 ± .02, μ_b = 2.90 ± .01 and μ_c = 0.0 ± .3, giving μ = 3.08 ± .02 D.     

Vibrational frequencies,infrared absorption intensities and energy differences between rotational isomers of propyl halides

Infrared absorption intensities of fundamental bands of propyl halides n-C₃ H₇ Cl, n-C₃H₇ Br and n-C₃H₇I were measured in the pure liquid state. In order to obtain L matrix data necessary for the intensity computation, normal frequencies of the rotational isomers were calculated, and LSFF force constants were determined by the least squares method so as to attain the best fit between the observed and calculated frequencies. By applying the absolute intensity method, energy differences between the rotational isomers were evaluated, which are in quite good agreement with values obtained by the temperature variation method.     

Microwave spectroscopy measurements of rotational spectra and DFT calculations for two distinct structural isomers of 1,1'-dimethylferrocene

Microwave spectra were obtained for two distinct structural isomers of 1,1'-dimethylferrocene, an eclipsed synperiplanar isomer (phi = 0 degrees, the E0 isomer), with A = 1176.9003(2) MHz, B = 898.3343(2) MHz, C = 668.7469(2) MHz, and an eclipsed synclinal isomer (phi = 72 degrees, the E72 isomer) with A = 1208.7117(14) MHz, B = 806.4101(12) MHz, and C = 718.7179(8) MHz. The b-dipole, asymmetric-top spectra of both structural isomers were measured in the frequency range of 5-12 GHz using a Flygare-Balle type of spectrometer. A very good fit to observed transitions, with small distortion constants, was obtained for the E0 conformer, indicating that this conformer is nearly rigid. The deviations obtained in a similar least-squares fit for the E72 confomer are significantly larger, indicating possible fluxional behavior for this conformer. In addition, 7 out of the 26 transitions observed for the E72 isomer conformer clearly exhibit very small splittings, giving further evidence for internal motion. DFT calculations for the different possible conformations of 1,1'- dimethylferrocene arising from rotation of one methyl cyclopentadienyl ligand relative to the other about the nominal C5 axis by an angle phi (dihedral angle) were performed using the B3PW91 functional. The calculations converged and were optimized for five structures on this torsional potential energy surface corresponding to different dihedral angles phi; three yielded energy minima, and two gave energy maxima, corresponding to transition states. The experimental results are in very good agreement with the results of the DFT calculations.     

The energy and isotope dependence of electronic relaxation in dilute vapors of fluorene and β-naphthylamine

The excitation energy and isotope dependence of fluorescence lifetimes and quantum yields in dilute vapors of fluorene and β-naphthylamine are discussed in relation to the manner in which different channels of radiationless transitions are affected by the vibrational energy content of the molecule. Evidence is presented which shows that vibrational relaxation is slow compared with electronic relaxation for molecules with low excess energies and that the rate of S₁ → S₀ internal conversion is greater in the deuterated compound than in the corresponding protonated species for very large excess energies.     

Microwave spectrum and rotational isomers of ethyl fluoroformate

The microwave spectrum of ethyl fluoroformate displays strong a-type R branch transitions from two rotameric forms. One species (extended form) has rotational constants A₀ = 9191.3(9) MHz, B₀ = 2112.61(1) MHz, C₀ = 1756.73(1) MHz which are consistent with a syn-anti (τ₁(OCOC) = 0°, r₂(cocc) = 180°) planar heavy atom structure. The second species (compact form) has rotational constants A₀ = 7760(3) MHz, B₀ = 2388.38(4) MHz, C₀ = 2102.47(3) MHz which are consistent with a syn-gauche (τ₁(ococ) = 0°, τ₂(cocc) ˜ 90°) structure. The two conformational forms have approximately equal energy (0 ± 40 cm⁻¹). Four vibrational satellites of the extended species have been analyzed yielding a torsional frequency around the O-ethyl bond of 70(10) cm⁻¹. Three vibrational satellites attributed to the O-ethyl torsion of the compact species have been analyzed yielding a vibrational frequency of 90(10) cm⁻¹. Approximate Fourier coefficients of a three term potential function for internal rotation about the O-ethyl bond have been determined. Vibrational satellites attributed to the first excited states of the O-ester torsion have been analyzed for both conformers. The torsional vibrational frequency around the O-ester bond is 110(15) cm⁻¹ for the extended conformers and 120(20) cm⁻¹ for the compact.     

Vibrational frequency and intensity spectroscopy of rotational isomers

The state of the art in conformational analysis of large chain molecules based on the use of infrared and Raman spectra is presented and discussed. The combined use of frequency spectroscopy and intensity spectroscopy is discussed and examples given.     

Alignment of electronic energy levels at electrochemical interfaces

The position of electronic energy levels in a phase depends on the surface potentials at its boundaries. Bringing two phases in contact at an interface will alter the surface potentials shifting the energy levels relative to each other. Calculating such shifts for electrochemical interfaces requires a combination of methods from computational surface science and physical chemistry. The problem is closely related to the computation of potentials of electrochemically inactive electrodes. These so-called ideally polarizable interfaces are impossible to cross for electrons. In this perspective we review two density functional theory based methods that have been developed for this purpose, the workfunction method and the hydrogen insertion method. The key expressions of the two methods are derived from the formal theory of absolute electrode potentials. As an illustration of the workfunction method we review the computation of the potential of zero charge of the Pt(111)-water interface as recently published by a number of groups. The example of the hydrogen insertion method is from our own work on the rutile TiO(2)(110)-water interface at the point of zero proton charge. The calculations are summarized in level diagrams aligning the electronic energy levels of the solid electrode (Fermi level of the metal, valence band maximum and conduction band minimum of the semiconductor) to the band edges of liquid water and the standard potential for the reduction of the hydroxyl radical. All potentials are calculated at the same level of density functional theory using the standard hydrogen electrode as common energy reference. Comparison to experiment identifies the treatment of the valence band of water as a potentially dangerous source of error for application to electrocatalysis and photocatalysis.     

Three rotational isomers of se-methyl N,N-di-isopropyldiselenocarbamate and the sulphur analogue found in low temperature 1H NMR spectra

Variable temperature ¹H NMR spectra of Se-methyl N,N-di-isopropyldiselenocarbamate, CH₃SeC(Se)N (C₃H₇-i)₂, in CS₂ have indicated that internal rotations around both the carbamate C? N and isopropyl–nitrogen bonds are restricted below ?40°C; the compound exists as three rotational isomers with respect to the isopropyl-nitrogen bond with mol ratios of approximately 0·54:0·31 :0·15. The corresponding dithiocarbamate, CH₃SC(S)N(C₃H₇-i)₂, has also been found to exist as three isomers with ratios of about 0·71:0·27:0·02 under similar conditions. Possible conformations of both carbamate esters are proposed, together with the assignments of each proton signal.