Experimental vibrational spectra of gas-phase tantalum cluster cations

We present gas-phase infrared spectra of tantalum cluster cations containing 6-20 atoms. Infrared multiple photon dissociation of their complexes with argon atoms is used to obtain vibrational spectra in the region between 90 and 305 cm(-1). Many spectra have features in common with the vibrational spectra of the lighter homologs, vanadium and niobium, pointing to a common cluster growth mechanism.     

Electronic spectra of gas-phase polycyclic aromatic nitrogen heterocycle cations: isoquinoline+ and quinoline+

Electronic spectra of the gas-phase isoquinoline(+)-Ar and quinoline(+)-Ar complexes are recorded using photodissociation spectroscopy by monitoring the Ar loss channel. The D(3)←D(0) and D(4)←D(0) band origins for isoquinoline(+)-Ar are observed at 15245 ± 15 cm(-1) and 21960 ± 15 cm(-1), respectively, whereas for quinoline(+)-Ar they appear at 16050 ± 15 cm(-1) and 21955 ± 15 cm(-1), respectively. Strong vibronic progressions for the D(3)←D(0) band systems of both isoquinoline(+)-Ar and quinoline(+)-Ar are modeled and assigned in terms of ring deformation and carbon-carbon stretch vibrational modes using time-dependent density functional theory calculations in conjunction with Franck-Condon simulations. The properties of the isoquinoline(+) and quinoline(+) molecules are compared with those of the isoelectronic naphthalene(+) molecule. The existence of strong progressions in the visible spectra of isoquinoline(+)-Ar and quinoline(+)-Ar suggests that the corresponding isoquinoline(+) and quinoline(+) molecular cations are unlikely to be responsible for diffuse interstellar bands.     

Electronic fluorescence spectra of gas-phase positive molecular ions

A new technique for observing electronic spectra of gas-phase positive molecular ions is described. The ions are produced by electron impact on a supersonic molecular beam, and the resulting fluorescent radiation is dispersed. Some results on N₂, N₂O and CO₂ are presented. They suggest that this technique could be most productive for studying large polyatomic cations.     

Electronic spectra of jet-cooled cations of hydrogen-bonded complexes of phenol

Electronic spectra of hydrogen-bonded complex cations of phenol with N(CH₃)₃, NH₃ and H₂O were investigated by photodissociation spectroscopy combined with a new method for the generation of jet-cooled molecular complex ions. The method involves the ionization of phenol followed by a jet expansion for the efficient formation of the complex ions as well as for the cooling. Well-resolved vibronic bands, including the bands due to intermolecular stretching vibrations, were obtained for the complex ions of phenol with N(CH₃)₃ and NH₃. Differences between the ionic and neutral forms of these complexes were discussed on the basis of the results.     

Electronic spectra of finite polyenes and polyacetylene obtained by electron and polarization propagator calculations

Summary Polyacetylene and the short polyenes are used as common test cases for demonstrating the capability of electron and polarization propagator theory to simulate various types of molecular electronic spectra.Dedicated to Jan Linderberg on his 60th birthday     

Electronic structure of polyenes and polyacetylene

The CNDO/S2 model is applied to evaluate the molecular orbitals and ultraviolet absorption spectra of the series of polyenes C_4nH_4n+2, n = 1, 2, 3, 4; extended to encompass the infinite polyene chain, i.e., polyacetylene; and utilized to interpret the photoemission and absorption spectra of polyacetylene.     

Peierls instability and vibrational spectra of polyacetylene

It is shown that the Peierls instability in conjugated polymers is modified by the interaction of π electrons. This modification gives rise to the absence of a soft mode of stretching C? C vibrations. Taking into account electron correlations we obtain reasonable agreement between the calculated and measured frequencies of the carbon skeleton of polyacetylene with all the C? C bonds being equal in length. It is shown that the conclusion for the bond length alternation in polyacetylene does not follow from the experimental data on the vibrations of polyacetylene.     

Vibrational spectra of highly oriented polyacetylene.

Infrared spectra of highly oriented polyacetylenes have been recorded and analysed in terms of their polarization properties. Polarized infrared data for the iodine doped polymer are also reported and discussed.     

Electronic absorption spectra of charge-transfer complexes and effects of substituents in radical cations

The effects of X substituents on the energies of charge-transfer bandshv _CT in electronic absorption spectra of charge-transfer complexes of π-, n-, or σ-donors (DX) with π- or σ-acceptors (A) as well as on the ionization potentialsI _D of individual DX molecules are described by the equationhv _CT(l _D)=a +bσ₁ +cσ _R ⁺ +dσ_α. When DX and A are fixed, the inductive (bσ₁), resonance (cσ _R ⁺ ), and polarization (dσ_α) contributions tohv _CT andI _D are virtually identical. The electronic structure of the D^.+X donor component of the compact [A^.−, D^.+X] radical-ionic pair in a solution is similar to that of the radical cation generated upon photoionization of the individual DX molecule in the gaseous phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1002–1006, June, 2000.     

Electronic spectra of 1-methyl and 2-methyl phenanthrenes and their radical cations

Electronic spectra of phenanthrene (P), 1-methyl phenanthrene (1-MeP), 2-methyl phenanthrene (2-MeP) and their monopositive ions are investigated experimentally as well as theoretically. The ions were produced by photo-oxidation of the hydrocarbons in boric acid matrix. The electronic absorption spectrum of 2-methyl phenanthrene cation (2-MeP+) is entirely new. For the interpretation of the electronic spectra of neutral and ionized MePs, semi-empirical AM1 (Austin Model 1) calculations are carried out for the first time. The bathochromic shifts in the spectral bands of the neutral and ionized MePs are attributed to 'conjugative' effect. The present experiments reveal that the 448 nm band of 1-methyl phenanthrene cation (1-MeP+) and the 486 nm band of 2-MeP+ show close matching with the respective 450 nm and 488 nm 'diffuse interstellar bands'. This suggests the possibility of the existence of such ionic species in the interstellar matter.     

Metal-mediated formation of gas-phase amino acid radical cations

The results from an investigation of the collision-induced dissociation (CID) of the ternary complexes [Cu(II)(terpy)(AA)](2+) are presented (terpy = 2,2':6',2' '-terpyridine; AA = one of the twenty common amino acids). These complexes show a rich gas-phase chemistry, which depends on the identity of the amino acid. For the histidine-, lysine- and tryptophan-containing complexes, oxidative dissociation of the amino acid is observed, yielding the amino acid radical cation. The results of further mass selection and CID of these amino acid radical cations are presented. The CID of the series [Fe(III)(salen)(AA)](+) (where salen = N,N'-ethylenebis(salicylideneaminato)) is also examined. These complexes undergo loss of the neutral amino acid in all cases, although the radical cation of arginine is also produced and its subsequent fragmentation examined. B3-LYP/6-31G(d) computations were carried out to test aspects of the proposed fragmentation mechanism of the histidine and arginine radical cations.     

Interpreting doped polyacetylene13C NMR spectra

Karpov Physicochemical Research Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 3, pp. 142–144, May–June, 1989.     

Ab initio calculations of electronically excited states of cyano-substituted polyacetylene cations

The transition energies for the lowest energy pi --> pi* electronic excitations are calculated with the complete active space self-consistent field method (CASSCF) and with the complete active space second-order perturbation theory method (CASPT2) for the linear cyano-substituted polyacetylene cations, H-Cn-CN+, n = 4-11, and NC-Cn-CN+, n = 2-10. These systems are models for an important class of interstellar species. We demonstrate the utility of the theoretical calculations in assigning the experimental spectra.     

Electronic absorption spectra of the cations and anions formed from pyrrole analogs of the chalcones

The electronic absorption spectra of 29 pyrrole analogs of the chalcones have been studied in solutions of strong acids and alkalies, in which the corresponding cations and anions are formed. It has been shown by means of the IR spectra that the protonation of the pyrrole chalcones takes place at the carbonyl group. Considerations are given of the structure of the organic cations and anions studied and of the characteristic features in the change in their coloration as a function of their chemical structure. Satisfactory correlations have been found between the shift of the absorption frequency of the long-wave band which takes place at the transition from a neutral solution (molecules) to an acid solution (cations) or to an alkaline solution (anions) and Hammett's o constants. Three 3-(4-halophenyl)-1-(2-pyrryl)propenones not previously reported in the literature have been synthesized.     

Electronic absorption spectra of the cations and anions formed from pyrrole analogs of the chalcones

The electronic absorption spectra of 29 pyrrole analogs of the chalcones have been studied in solutions of strong acids and alkalies, in which the corresponding cations and anions are formed. It has been shown by means of the IR spectra that the protonation of the pyrrole chalcones takes place at the carbonyl group. Considerations are given of the structure of the organic cations and anions studied and of the characteristic features in the change in their coloration as a function of their chemical structure. Satisfactory correlations have been found between the shift of the absorption frequency of the long-wave band which takes place at the transition from a neutral solution (molecules) to an acid solution (cations) or to an alkaline solution (anions) and Hammett's o constants. Three 3-(4-halophenyl)-1-(2-pyrryl)propenones not previously reported in the literature have been synthesized.     

Electronic spectra and ionization potentials of a stable class of closed shell polycyclic aromatic hydrocarbon cations

Due to their stability, closed shell polycyclic aromatic hydrocarbon (PAH) cations are possible candidates as carriers for some of the diffuse interstellar bands (DIBs). The electronic absorption spectra and ionization potentials of several closed shell PAH cations are determined in this study. We use density functional theory (DFT) at the BLYP/6-31G* level to determine the ionization potentials and thus confirm the stability of the PAH cations of interest. We use time-dependent density functional theory (TDDFT), again at the BLYP/6-31G* level, to calculate the vertical excitation energies and oscillator strengths of the PAH cations. We observe dominant single absorptions within the DIB spectral region of interest in all of the PAH cation spectra except for the smallest member of the series.     

Stable gas-phase radical cations of dimeric tryptophan and tyrosine derivatives

Stable radical cations of dimeric amino acid derivatives of tryptophan and tyrosine were generated by collision-induced dissociation of [Cu(II)(diethylenetriamine)(amino acid derivative)2]*2+. The yields of the dimer radical cations were dependent on both the auxiliary ligand and the tryptophan or tyrosine derivatives used. Amino acid derivatives with an unmodified carboxylic acid group did not generate dimer radical cations. For the amino acid derivatives Ac-Trp-OMe and Ac-Trp-NH2 (Ac is N-acetyl; OMe and NH2 are the methyl ester and amide modifications of the C-terminal carboxylic group), no auxiliary ligand was required for generating the dimer radical cations. Collision-induced dissociation of the [Cu(II)(amino acid derivative)4]*2+ precursor generated the dimer radical cation [(amino acid derivative)2]*+. Stabilizing interactions, most likely involving hydrogen bonding, between the two amino acid derivatives are proposed to account for observation of the dimer radical cations. Dissociation of these ions yields protonated or radical cationic amino acid derivatives; these observations are consistent with the expectation of proton competition between monomeric units, whose proton affinities were calculated using density functional theory.     

Electronic structure and absorption spectra of verdazyl radicals,the corresponding cations,and leuco compounds

The wavelengths of transitions in electronic absorption spectra of verdazyl radicals, the corresponding cations, and leuco compounds were calculated in valence and -electron approximations with allowance for configuration interaction. It was shown that the longest-wave bands in the investigated compounds are due to ^* electron transitions. The effect of the nature of the substituents on the position of the bands in the spectrum is discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 4, pp. 473–477, July–August, 1986.     

Resonance Raman spectra of polyacetylene on zeolite and alumina surfaces

Raman spectra are reported for polyacetylene on exchanged zeolites and on alumina. The polymerization of acetylene on zeolites is found to depend on the zeolitic cation size and on surface coverage. Mean chain lengths of about fifteen CC bonds are formed on alumina.     

Electronic states of di-t-butylpolyacetylene radical cations

The π-ionization energies of the di-t-butylpolyacetylenes with two, three, four and five conjugated triple bonds have been determined by He (Iα) photoelectron spectroscopy. The assignment of the bands to the Π-states of the corresponding radical cations follows from simple correlations in agreement with previous experience. The influence of the t-butyl groups on the ionization energies is rationalized in terms of traditional, qualitative arguments assuming an inductive and/or hyperconjugative mechanism. However, a more careful analysis shows that the ‘The -higher-the-ionization-energy-the-higher-the-alkyl-induced-shift’ rule is not always true.