Low-energy Electronic Transitions Research Articles

The observation of surface optical phonons and low-energy electronic transitions in NiO single crystals by electron energy loss spectroscopy

A surface optical phonon energy close to that predicted by theory has been observed by high-resolution electron energy loss spectroscopy on NiO single crystals of varying surface orientation and composition. At higher loss energies, the ligand field transitions of the localised Ni 3d levels are observed below the absorption edge which lies at about 4 eV.

Optical activity of lactones and lactams—IV : Chiroptical properties of 4-imidazolidinones

The chiroptical properties of 4-imidazolidinones are reviewed. Several optically active compounds were obtained from amides and N-methylamides of corresponding N-acylamino acids. The origin of the two lowest energy electronic transitions is considered. The equilibrium between the two envelope conformations of heterocyclic ring was observed by means of NMR and CD spectra. The spectra of the compounds with non-protected amino group are affected by n → σ * transition involving nitrogen lone pair.

Theoretical investigations of the electronic states of porphyrins. II. Normal and hyper phosphorus porphyrins

AbstractAb initio methods have been used to calculate the ground and excited states of “normal” and “hyper” porphyrins. Perturbation theory and CI methods were used to determine differential ground and excited‐state correlation effects for [Pv(P)F2]+ and [PIII(P)]+. A comparison is made to the INDO/S/CI predicted wavefunctions and spectra and to the experimental spectra of closely related molecules. The “hyper” [PIII(P)]+ calculations show some very low energy electronic transitions which provide an explanation for an anomalous “red” band in the spectrum and for the lack of fluorescence. Ab initio calculations also predict that (1) the lowest energy 1A1 state is a two‐configuration wavefunction which can be described as a diradical, (2) the two lowest‐energy singlet excited states are double excitations from the closed shell SCF configuration, and (3) a 3B2 state is very close in energy to the lowest 1A1 state.

The Spectroscopy of Formaldehyde and Thioformaldehyde

Formaldehyde has played a key role in many aspects of our current understanding of the spectroscopy of poly atomic molecules. The analysis of the rotational subband structure of several vibronic transitions in the A I A2X I A1 band system of H2CO, in 1934, by Dieke & Kistiakowsky (1) is recognized as the first interpretation of the electronic spectrum of a poly atomic molecule (2). During the intervening five decades, formaldehyde has been thoroughly studied by both experimental and theoretical tech­ niques, and continues to be a prototype system for the investigation of the spectroscopy, photochemistry, and photophysics of poly atomic molecules. In sharp contrast, thioformaldehyde is a relative newcomer to the scene. This somewhat unstable species was first unambiguously identified in the laboratory little more than a decade ago (3) by observation of its microwave spectrum. Since then, rapid progress has been made in characterizing the ground and excited electronic states of HzCS. Thioformaldehyde and formaldehyde are spectroscopically similar in many aspects. For example, in both molecules the lowest energy electronic transitions may be characterized as forbidden electron promotions to n,1C* states, giving rise to ij3 A2-X I A l and A I A2-X I Al band systems. However, in formaldehyde these transitions occur in the near ultraviolet, whereas in thioformaldehyde the analogous systems are in the visible. One of the major

ChemInform Abstract: SINGLE‐CRYSTAL CIRCULAR DICHROISM SPECTRA OF TRIS(PENTANE‐2,4‐DIONATO)COBALT(III) AND TRIS(PENTANE‐2,4‐DIONATO)CHROMIUM(III)

AbstractDie Messung erfolgte an A‐Verbindungen, die im Einkristall von rac. Tris‐[pentan‐2,4‐dionato]‐rhodium gedopt sind.

Magnetic Circular Dichroism Studies. 62.1Sign Variation in the Magnetic Circular Dichroism Spectra of Some Perimeter Symmetric Free-Base Porphyrins

Abstract The MCD bands associated with the four lowest energy electronic transitions of perimeter symmetric porphyrin free-bases have commonly exhibited the “normal” sign pattern of -+-+ in the order of increasing energy. The first known exceptions to this rule, found in a series of tetrakis(o,o′-dihalophenyl)porphyrins, are presented in this communication. The sign inversions in the Q bands as well as the normal sign pattern in the Soret bands (overall pattern: +—+) can be understood on the basis of Michl's perimeter model.

The resonance Raman profile of a nickel Schiff base complex at 10 K

The resonance Raman profile for the Schiff-base complex N,N′-bis(salicyladehyde)- o-diaminobenzene nickel (II) has been measured at 290 and 10 K and the main peaks assigned. A broad fluorescence with a maximum at ca. 672 nm reduced the sensitivity of some measurements at room temperature but at 10 K this band had shifted sufficiently to enable definitive Raman spectra to be recorded over the entire range of exciting lines used. A second sharper fluorescence was observed and in this case, the band position was dependent on the exciting line, occurring about 7.4 nm from it. More bands were observed in the resonance profile at 10 K than at room temperature, and two electronic transitions which were not resolved in the electronic spectra at either temperature were characterized. The lowest-energy electronic transitions are charge-transfer transitions between a ground state, which is largely composed of phenyl ring and oxygen orbitals and an excited state, composed mainly of oxygen and nickel orbitals. The electronic spectrum in the visible region is assigned on the basis of the resonance profile.

Single-crystal circular dichroism spectra of tris(pentane-2,4-dionato)cobalt(III) and tris(pentane-2,4-dionato)chromium(III)

The axial circular dichroism spectrum of the lowest energy electronic transition has been measured for both Λ-[Co(pd)3](pd = pentane-2,4-dionate) and Λ-[Cr(pd)3] doped in a single crystal of rac-[Rh(pd)3]·nCHCl3(n=ca. 20). The rotational strengths so obtained are compared with those of the corresponding oxalato and 1,2-diaminoethane complexes and an explanation is offered for their relative magnitudes.

Spectroscopic and theoretical studies of metal cluster complexes. Part 2. X.alpha. calculations and spectroscopic studies of triruthenium and triosmium dodecacarbonyls

Non-empirical calculations of the X..cap alpha.. type are repoted for Ru/sub 3/(CO)/sub 12/ and Os/sub 3/(CO)/sub 12/. Two procedures were used to model the molecular charge density for calculating the self-consistent potential: an approximate self-consistent charge scheme and the more rigorous superposition of multipolar densities. This latter type of procedure accurately calculates the splitting of occupied d orbitals in Ru/sub 3/(CO)/sub 12/ and Os/sub 3/(CO)/sub 12/. Centrally directed metal-metal sigma and ..pi..-bonding orbitals are occupied as well as their antibonding counterparts. Stability of the latter levels results from metal-carbon monoxide ..pi.. bonding. A filled degenerate cyclopropane-like edge-bounding orbital furnishes four bonding electrons to the cluster framework. Difference electron density maps aid the analysis the two lowest energy electronic transitions. The dipole-allowed transition (15e' ..-->.. 6a/sub 2/') should be strongly metal-metal antibonding, and the dipole-forbidden HOMO ..-->.. LUMO excitation (10a/sub 1/' ..-->.. 6a/sub 2/') is predominantly transition-metal-CO antibonding. Raman spectra for RU/sub 3/(CO)/sub 12/ exhibit resonance enhancement of both the 185 (a/sub 1/') and 130-cm/sup -1/ (e') Ru-Ru stretching vibrations. This behavior confirms the predicted antibonding nature of the lowest allowed transition and suggests a Jahn-Teller distortion in the excited state or B-term scattering. Spectrophotometric titrations show the metal center inmore » Os/sub 3/(CO)/sub 12/ to be approximately 5 times more basic toward the proton than that in Ru/sub 3/(CO)/sub 12/.« less

ChemInform Abstract: LOW‐ENERGY ELECTRONIC TRANSITIONS OF ALKYLAMINES

Chemischer InformationsdienstVolume 12, Issue 45 Physical Organic Chemistry ChemInform Abstract: LOW-ENERGY ELECTRONIC TRANSITIONS OF ALKYLAMINES P. AVOURIS, P. AVOURISSearch for more papers by this authorA. R. ROSSI, A. R. ROSSISearch for more papers by this author P. AVOURIS, P. AVOURISSearch for more papers by this authorA. R. ROSSI, A. R. ROSSISearch for more papers by this author First published: November 10, 1981 https://doi.org/10.1002/chin.198145046AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume12, Issue45November 10, 1981 RelatedInformation

Low-energy electronic transitions and phase transition of the biphenyl crystal

The temperature change of the fluorescence and its excitation spectra of the biphenyl crystal between 100 and 5.2 K is interpreted by the phase transition. The absorption peak at 302.5 nm which appears below the transition temperature is assigned to the transition 1B3 ← 1A1. The origin of the fluorescence is observed around 303 nm.

Low-energy electronic transitions of alkylamines

Low-energy electronic transitions of alkylamines

MAGNETIC CIRCULAR DICHROISM OF BACTERIOCHLOROPHYLL a IN SOLUTION AND IN A PROTEIN

Abstract— The magnetic circular dichroism (MCD) (300–850nm) of the bacteriochlorophyll (Bchl) a‐protein from the green photosynthetic bacterium Prosthecochloris aestuarii 2 K is qualitatively similar to the MCD of Bchl a in methanol and ether solution. This result implies that the transition dipole of the lowest energy electronic transition (near 800 nm) is roughly perpendicular to the transition dipole of the next higher electronic band (near 600nm) for Bchl a molecules in the protein just as it is for molecules in solution. This result provides no support for the recent proposal that interactions with the protein rotates the direction of the transition dipoles of the 800nm band of all the Bchl a molecules in the protein by 90°. While a rotation of the 800nm transition dipoles cannot be rigorously excluded, it would be necessary for the postulated perturbation to rotate the transition dipoles of both the 800 and 600nm bands by 90°. In a broader sense, any postulated perturbation would have to be shown to leave both the absorption spectrum and the MCD largely unaffected. MCD is a more sensitive test than absorption spectroscopy for perturbations of electronic states and changes in the relative orientation of transitions, because it depends on both the magnitudes and directions of at least two electric and one magnetic transition dipole.

Electronic structure of the self-trapped exciton in the potassium halides

We present pseudopotential calculations of the energy levels and wave-functions of the electron associated with the self-trapped exciton in KCl, KBr and KI. Accurate values are obtained for the separation of the singlet and triplet luminescent bands, for the electronic transitions, and for the spin resonance linewidths. Lattice distortion is shown to be particularly important in KCl. Additional low-energy electronic transitions, at 0.7 eV in KCl and 0.94 eV in KBr, are predicted, but have not been seen experimentally.

A kinetic study of the hydrolysis of acetonitrile co-ordinated to platinum, yielding platinblau

Molecular orbital and crystal field calculations suggest that for iron phthalocyanine the 3A 2gground state corresponds to the electronic configuration e 2 g b 2 2g a 2 1g, while for cobalt phthalocyanine the ground state is 2A 1g-e 4 gb 2 2ga 1 1g. The MO results imply a differential covalency within the set of orbitals d xy, d xz, d yz and d z 2 and it is necessary to invoke this concept in the crystal field calculations in order to reproduce the ground state of iron phthalocyanine and the low energy electronic transitions of both molecules. The calculations indicate that, contrary to a recent suggestion, the medium intensity bands in the near infra-red region of both these complexes are due to d → d electronic transitions. The bands are assigned 2 A 1g → 2E g and 2B 2g for cobalt phthalocyanine and 3 A 2g → either both 3 E g and 3B 2g for iron phthalocyanine.

Conformation of histidine model peptides. II. Spectroscopic properties of the imidazole chromophore

AbstractSemi‐empirical molecular orbital calculations have been carried out for the free base and cationic forms of imidazole so as to obtain data which are required for the calculation of the chiroptical properties of molecules that contain this chromophoric group. The polarization, energy, and monopolar charge distribution are reported for the lowest energy electronic transitions. The absorption spectra for imidazole have been determined to 180 nm and circular dichroism spectra for L‐histidinol and L‐2‐amino‐1‐butanol have been measured.

Sensitization of Photoconduction in Polymers

Abstract Photoconductivity can be induced in most materials by employing radiation in an appropriate region of the electromagnetic spectrum. The radiation has to be sufficiently energetic to promote electronic transitions in the material, this being the first step in photoconduction phenomena. Thus, high-energy radiation, such as x rays and gamma rays, produces “photoconductivity,” i.e., conductivity induced by absorbed photons, in most materials. As the photon energy decreases through the UV, visible, and IR regions, many materials fail to respond when the photon energy becomes too small to excite the lowest lying electronic transitions in the material. As a class, many easily fabricated polymers have their lowest energy electronic transitions in the UV and vacuum-UV region of the electromagnetic spectrum. Polymers having electronic transitions in the visible region are often intractable and not readily fabricated into films and other useful forms. Consequently, much attention has been given to “sensitiz...

Calculations on the electronic spectrum of water

Ab initio SCF and CI calculations are reported for excitation energies, oscillator strengths and geometry of upper states in the low-energy electronic transitions of water. Good agreement is obtained with experimental data where available, both with regard to the location and intensity of the various spectral bands for this system. In addition several transitions of either weakly allowed or forbidden character respectively are predicted which have not yet been unambiguously identified via experimental techniques.

The intensities of low-energy electronic absorption transitions of some carbonyls and thiocarbonyls

The intensities of low-energy electronic transitions for some carbonyls and thiocarbonyls have been calculated from CNDO wavefunctions. Quite good agreement with experimental results has been obtained, where the latter are available. A satisfactory approximation for calculating intensities employs only one-center integrals. From the calculated trends in oscillator strengths, the absorption of thiophosgene at 4.46 eV can be identified as the π→π* 1A 1←X̃ 1A 1 system. Another, very weak, system of thiophosgene at ≈ 3.9 eV is tentatively assigned to an n→π* 1A 1 ← X̃ 1 A 1 transition, with the n orbital localized on the chlorine atoms.

Hydrogen bonding in crystalline arylazonaphthols

The electronic and vibrational spectra of trans-hydroxyazoaromatics show profound differences between the preferred species in solution and those prevalent in the cyrstalline solids. The hydrazone ⇌ azo tautomers dominate the respective electronic spectra of neutral solutions of these compounds. On the other hand, the lowest energy electronic transition of crystalline hydroxyazo compounds is strongly red-shifted (≈ 1000 Å) relative to that for the dissolved species. Support for the thesis that this anomalous absorption characteristics of crystalline azo compounds has its origin in intermolecularly hydrogen-bonded hydrazone-like aggregates is found in infrared and Raman spectroscopic studies of the solids, in electronic spectroscopy of solutions in acidic media, and comparison spectra of the amorphous solid compounds.