Ground State Of Cation Research Articles

Equilibrium geometries and hyperfine interactions in propane and cyclopropane cations

The equilibrium geometries of the propane cation in its three lowest electronic states, 2B1, 2B2 and 2A1(C2v symmetry assumed), have been calculated by the UHF method within the ab initio MO-LCAO-SCF approximation. The 2B1 state is predicted to be lowest in energy and the 2B2 and 2A1 states to be of almost the same energy, 12 kcal mol–1 above the 2B1 state. The isotropic hyperfine coupling constants have been calculated and compared with experimental data. The calculations confirm the e.s.r. identification of the 2B1 state, obtained in an SF6 matrix. The previous experimental assignment, 2B2, for the state obtained in Freon matrices was found to be ambiguous and an assignment to the 2A1 state can not be ruled out. Dipolar hyperfine coupling constants have been calculated for the 2B1, 2B2 and 2A1 states of the propane cation and for the 2A1 ground state of the cyclopropane cation. In the latter case, comparison with experimental data has been made with reasonable agreement. The equilibrium geometry agrees well with previous calculations. Complementary experimental data for the propane and cyclopropane cations are reported.

Ultraviolet two-photon spectroscopy of benzene: A new gerade Rydberg series and evidence for the 1 1E2g valence state

In the first application of continuously tunable ultraviolet two-photon absorption spectroscopy to a study of the higher excited states of a polyatomic molecule, we have obtained the two-photon absorption spectrum of jet-cooled benzene in the energy region from 55 000 to 75 000 cm−1. The strongest features of this spectrum, seen as two-photon resonance structure in three-photon ionization, are assigned to members of a new Rydberg series. Vibronic elements of these Rydberg states show evidence of dynamical effects associated with a Jahn–Teller instability in the 2E1g ionic core toward which the series converges. Notably, the magnitude of vibronic coupling terms in these states appears quite comparable to that present in C6H6+ and the sym-halobenzene cation ground states, while differing significantly from theoretical calculations predicting ν8 (ring stretch) as the major distorting mode. In addition, we observe two other new band systems: The first, with an origin at 60 800 cm−1, has vibronic structure, isotope shifts, and dramatically increased diffuseness upon deuteration, which taken together suggest its assignment as a highly valenized 3d Rydberg state or a degenerate valence state, most likely the lower of the two 1E2g states predicted by molecular orbital theory. The second, lying in the 55 000–58 000 cm−1 region, can be at least partially assigned to the two-photon forbidden 3px,y Rydberg states.

ChemInform Abstract: RESONANCE‐ENHANCED TWO‐PHOTON IONIZATION SPECTRA OF NAPHTHALENE

Two-color two-photon ionization experiments were performed in order to analyze the one-color resonance-enhanced two-photon ionization (R2PI) spectra of room-temperature naphthalene. The data provide Franck-Condon (FC) factors connecting the S/sub 1/ state to the ground-state cation, and vibrational frequencies for the cation. The geometry and frequencies of the cation are close to those in the S/sub 1/ state. The spectra suggest that intramolecular vibrational redistribution occurs on a time scale less than 5 ns for S/sub 1/ vibrational levels with 1800 cm/sup -1/ or more energy. The one-color R2PI spectra are consistent, within experimental error, with predictions based on an incoherent rate equation model.

The ionization energies of bent and twisted double bonds. Part II. Tricyclo[4.2.2.2 2,5] Dodecadiene-1 (2),5 (6)

The electronic structure of the title compound (1) has been investigated using He(Iα) PE spectroscopy and assuming the validity of Koopmans' theorem. It is found that: (a) in analogy to the situation prevailing in the cyclophanes, hyperconjugation of the out-of-phase linear combination of the π-orbitals with the ethano-bridge σ orbitals is symmetry-forbidden; (b) in agreement with the ideas developed in Part I of this series [1], the strong local B 1u-type bending of the ethylene moieties does not affect their basis energy significantly; and (c) the “through-space” coupling of the two double-bond π orbitals is of the same order as the benzene π-orbital coupling in [2 n ] cyclophanes, even if the net contribution to the repulsion of the two double bonds is probably small. Because of the near degeneracy of the HOMO and HOMO — 1 of 1, the ground state of the radical cation 1 + cannot be determined uniquely: it is either 2 A g or 2 B 1u.

Resonance-enhanced two-photon ionization spectra of naphthalene

Two-color two-photon ionization experiments were performed in order to analyze the one-color resonance-enhanced two-photon ionization (R2PI) spectra of room-temperature naphthalene. The data provide Franck-Condon (FC) factors connecting the S/sub 1/ state to the ground-state cation, and vibrational frequencies for the cation. The geometry and frequencies of the cation are close to those in the S/sub 1/ state. The spectra suggest that intramolecular vibrational redistribution occurs on a time scale less than 5 ns for S/sub 1/ vibrational levels with 1800 cm/sup -1/ or more energy. The one-color R2PI spectra are consistent, within experimental error, with predictions based on an incoherent rate equation model.

The calculation of ground and excited doublet state molecular polarizabilities by the CNDO/S-CI method

The calculation of ground and excited state molecular polarizabilities by the CNDO/S-CI method has been extended to include doublet states. As incorporated, the second order perturbation scheme usually predicts average state polarizabilities of molecular cation ground states to be lower than the corresponding singlet ground states. Average anion polarizabilities are generally found to be higher than those of the corresponding singlet states. Excited doublet state polarizabilities are presented for the molecular cation of formaldehyde, benzene, 1,3,5- trans-hexatriene, and naphthalene. Due to the unavailability of experimental data it is currently impossible to assess appropriately the quality of these results.

Halogenated benzene radical cations and ground state degeneracy splitting by asymmetric substitution

The absorption and laser induced fluorescence of several halogenated benzene radical cations were studied in solid Ne matrices. The spectra of 1,2,4-trifluorobenzene, 1,3-dichloro-5-fluorobenzene, and 1-chloro-3,5-difluorobenzene radical cations are observed and analyzed. Studies of fluorescence polarization and a photoselection technique were used to examine the splitting of the degeneracy of the benzene cation ground state by asymmetric subsitution.

Laser induced excitation spectra of the cations of 1,3-pentadiynes, R(CC) 2H +; R  CH 3, CD 3: ( [formula omitted]2E ← [formula omitted]2E) and of 2,4-hexadiynes, R (CC) 2R +; R  CH 3, CD 3; ( [formula omitted]2E u ← [formula omitted]2E g) in the gaseous phase

Laser induced excitation spectra, A (π −1) ← X (π −1), of methyl, and trideuteromethyl, substituted diacetylene cations are reported. The well resolved vibrational structure is assigned to yield the frequencies of several of the totally symmetric fundamentals for the excited electronic state. These complement the values obtained for the ground cationic states from the corresponding emission spectra. The excitation spectra show that for these cations the radiative decay is apparent up to ≈0.4 eV internal energy in the A state in spite of the fact that fragment ions are also detected.

The ?2Πu→?2Πg emission and the photoelectron spectrum of dicyanodiacetylene radical cation

Gas phase emission spectrum of dicyanodiacetylene radical cation has been detected. The band system which lies in the 630–770 nm wavelength region was separated from the overlapping emission bands of the cyanogen radical by recording time resolved spectra. The He(Iα) photoelectron spectrum of dicyanodiacetylene is also presented and the inferred ionization energies, vibrational frequencies and assignments are given. From the latter data the emission spectrum is assigned as the ?2Πu→?2Πg electronic transition of dicyanodiacetylene radical cation. The frequencies of the four Σ+g vibrational fundamentals for the cationic ground state have been deduced from the emission spectrum.

A comparison of the role of charge transfer, hyperconjugation, inductive and field interactions in substitutd methyl and silyl substituent effects on benzene π vertical ionization energies

The first two vertical π-ionization energies of MX3 substituted benzenes as determined by photoelectron spectroscopy are compared with the results of modified CNDO/2 calculations, where M is Si or C and X is H, F, Cl and OCH3. The major substituent interaction mechanisms of MX3 are described within the CNDO/2 model. The results for methylorthobenzoate support intramolecular charge transfer stabilization of the benzene radical cation ground state by oxygen lone pair electrons, whereas the dominant interaction of the Si(OCH3)3 group is a positive charge stabilising field effect. The first vertical ionization energies of PhSiCl3 and PhCCl3 are unexpectedly assigned to ionization from the a′ π orbital rather than the a″ orbital nodal at the substituent site. Carbon-chlorine hyperconjugation is a suggested stabilizing interaction in the PhCCl32A′ cation. The shape of the first band in the PE spectrum of phenylsilane is described as a possible example of a pseudo Jahn—Teller effect. Unlike CX3, SiX3 substituents except, where X is methoxy, exhibit calculated opposing field and σ-inductive effects.

The photoelectron spectrum of butatriene

Photoelectron spectra of butatriene (I), tetradeuterobutatriene (II), tetramethylbutatriene (III) and tetra- t-butyl- butatriene (IV) are reported and discussed. The four main bands of the spectra of I and II are assigned to ionization from the 1b 3g, 2b 3u, 1b 2u and 1b 2g molecular orbitals on the basis of molecular orbital calculations (ab initio with STO-3G and 4-31G bases and semi-empirical SPINDO) all of which lead to the electron configuration… (1b 2g) 2 (1b 2u) 2(2b 3u) 2(1b 3g) 2. The first three bands of III and the first two of IV correspond to 2b 3g, 5b 3u, 2b 2u and 7b 3g, 12b 3u molecular orbitals respectively. The spectra of I and II also show an additional band which cannot be assigned to direct ionization of an occupied molecular orbital in the ground state configuration. it is suggested that this may arise from a cation state… (1b 2g) 2(1b 2u) 2(2b 3u) 2(2b 2u) 2B 2u. Such a state is correlated with the 2B 3g cationic ground state by internal rotation but corresponds to a local maximum in the rotational potential. Some simple model calculations are presented of Franck-Condon factors for the internal motion corresponding to these states. These show a composite band of the general type observed. However, the observed vibrational spacings indicate that significant carbon skeletal as well as torsional changes take place on ionization.