Murthy S. Gudipati

Higher electronically excited states of phenanthrene, carbazole and fluorene

Abstract Polarized excitation spectra of matrix-isolated phenanthrene, carbazole and fluorene are measured up to excitation energies of about 70000 cm −1 using synchrotron radiation. The measurements render symmetry assignments for most of the experimentally observed transitions. For the low energy part of the excitation spectra, up to energies about 45000 cm −1 , the results are in complete agreement with earlier investigations. The results are compared with the theoretical predictions obtained from CNDO/S-SDCI calculations.

Temperature and Viscosity Dependence of the Spin‐Directed Stereoselectivity of the Carbonyl–Alkene Photocycloaddition

Table. Temperature dependence of the Paterno-Buchi reaktion of 2,3-dihydrofuran (2, 1M und 5M) mit 1a (1M) and 1b (1M and 5M) in n-hexane. temperature [°C] [K] 1/T [K] endo / exo for 2+1a endo / exo for 2+1b (1M) endo / exo for 2+1b (5M) +25 293 0.00341 88 : 12 45.3 : 54.7 47.6 : 52.4 0 273 0.00366 91 : 9 50.5 : 49.5 43.8 : 56.2 -10 263 0.00380 90 : 10 54.6 : 45.4 41.4 : 58.6 -15 258 0.00388 91 : 9 56.6 : 43.4 37.3 : 62.7 -25 248 0.00403 88 : 12 57.4 : 42.6 35.7 : 64.3 -32 241 0.00415 88 : 12 58.9 : 41.1 37.1 : 62.9 -42 231 0.00433 91 : 9 61.5 : 38.5 42.2 : 57.8 -52 221 0.00453 90 : 10 61.7 : 38.3 48.0 : 51.9 -61 212 0.00472 86 : 14 63.2 : 36.8 52.4 : 47.6 -72 201 0.00498 90 : 10 63.4 : 36.6 58.1 : 41.9 -78 195 0.00513 99 : 10 63.6 : 36.4 63.3 : 36.7

Higher excited states of aromatic hydrocarbons. III. Assigning the in-plane polarized transitions of low-symmetry molecules: chrysene and E-stilbene

Abstract The polarization of the UV and near VUV bands of chrysene and E-stilbene have been measured up to 70000 cm −1 . The high natural polarization of the synchrotron radiation was used in combination with matrix-isolation to perform photoselection experiments. The limits of the of photoselection technique in connection with low symmetry molecules are discussed. Comparison with existing data for the low energy region ( v ≤ 50000 cm −1 ) confirms the applicability of the photoselection technique to determine electronic transition moment directions in low-symmetry molecules. CNDO/S-SDCI results allow a consistent assignment of the whole energy region up to the experimental limit of about 70000 cm −1 .

Higher excited states of aromatic hydrocarbons: polarized VUV fluorescence-excitation spectra of anthracence and pyrene in argon matrices at 15 K using synchrotron radiation

Abstract Synchrotron radiation was applied to measure polarized fluorescence and fluorescence excitation spectra of anthracene and pyrene isolated in argon matrices up to an excitation energy of 75000 cm −1 . The polarization data allow an assignment of most bands that appear in the energy range of 45000 to 75000 cm −1 . The order of the observed transitions is in general agreement with predictions of semiemperical calculations.

Mixing of the Immiscible: Hydrocarbons in Water-Ice near the Ice Crystallization Temperature

Structural changes in hydrocarbon-doped water-ice during amorphous to crystalline phase conversion are investigated using polycyclic aromatic hydrocarbons (PAHs) as probes. We show that aggregation of impurity molecules occurs due to the amorphous-crystalline transition in ice, especially when they are hydrophobic molecules such as PAHs. Using ultraviolet-visible (UV-vis), Fourier-transform Infrared (FTIR), and laser-induced-fluorescence (LIF) spectroscopic techniques, we show that, although ice infrared absorption features change from a broad structureless band corresponding to amorphous ice to a sharp structured crystalline ice bands, simultaneously, sharper isolated PAH UV absorption features measured in the amorphous ice host turn broad upon ice crystallization. A simultaneous decrease in the monomer fluorescence and increase in the excimer emission band is observed, a clear indication for the formation of PAH molecular aggregates when amorphous ice is converted to crystalline ice at higher temperatures. Similar to the irreversible amorphous-crystalline phase transitions, the UV, fluorescence, and excimer emissions indicate that PAHs undergo irreversible aggregation. Our studies suggest that organic impurities exist as aggregates rather than monomers trapped in crystalline water-ice when cycled through temperatures that convert amorphous ice to crystalline ice, rendering a better insight into phenomena such as the formation of cometary crust. This aggregate formation also may significantly change the secondary reaction pathways and rates in impurity-doped ices in the lab, on Earth, in the solar system, and in the interstellar medium.

RYDBERG AND CHARGE-TRANSFER STATES OF ATOMIC OXYGEN IN AR AND KR MATRICES : IDENTIFICATION OF TWO DISTINCT SITES

Abstract The excitation spectra, between 190 and 100 nm, of atomic oxygen imbedded in Ar and Kr crystals are presented. The spectra were obtained by monitoring the 1 S → 1 D emission in the 560 nm region. Based on the spectral shifts going from Ar to Kr matrices, which are proportional to the difference between the ionization energies of Ar and Kr, the excitation maxima at 145 and 133 nm in Ar and 179 and 162 nm in Kr are assigned to transitions involving charge-transfer upper states. Transition into the (2s22p33s) 3 S Rydberg state from the (2s22p4) 3 P ground state of O are also observed with a blue-shift of 0.69 eV in Ar and 0.32 eV in Kr.

Schumann-Runge bands of O2 in Ar, Kr and Xe matrices revisited: potential curves of the B 3Σu− state

Abstract Absorption/excitation spectra of 16 O 2 isolated in Ar and Kr matrices and 18 O 2 isolated in Ar matrices at 15 K for the B 3 Σ u − ( ν ′)←X 3 Σ g − ( ν ″=0) transitions (Schumann-Runge bands) of O 2 are presented. The spectra in rare-gas matrices differ considerably from the gas-phase spectrum. The cage effect caused by the host lattice has been analyzed using van der Waals interactions between the oxygen atom and rare-gas atoms. Potential curves of the B 3 Σ u − state have been derived for D 3d and D 4h orientations of O 2 in a substitutional site in rare-gas lattices. It has been found that when O 2 occupies D 4h orientation, these potential curves compare closest with the potential curves obtained independently by using the experimental vibronic energies and the method of Oldenberg [O. Oldenberg, Z. Physik 56 (1929) 563]. An analysis of the potential curves indicates that at larger OO separation (> 3.2 A) in the B 3 Σ u − state, the rare-gas atoms start interacting electronically with the O 2 molecule.

PHOTOOXYGENATION OF 2,4-DIMETHYL-1,3-PENTADIENE : SOLVENT DEPENDENCE OF THE CHEMICAL (ENE REACTION AND 4 + 2 CYCLOADDITION) AND PHYSICAL QUENCHING OF SINGLET OXYGEN

The photooxygenation of 2,4-dimethyl-1,3-pentadiene (1) was investigated in seven polar and nonpolar solvents by oxygen-uptake measurements. The overall deactivation rate ko ( = kr + kq) was additionally measured in chloroform solutions by singlet-oxygen (1Δg) phosphorescence quenching which showed excellent agreement with the data from the detailed steady-state kinetics. The difference in solvent-polarity effects on the [4 + 2] cycloaddition (major path, leading to the endoperoxide 2) and ene reaction (minor path, leading to the allylic hydroperoxide 3) are explained by competition between a concerted and a perepoxide mechanism. In all solvents the physical quenching of singlet oxygen by 1 is at least as efficient as the chemical quenching. The reaction of the endoperoxide 2 and 3,3,6,6-tetramethyl-1,2-dioxene 9 with carbonyl compounds in the presence of TMSOTf resulting in the dihydrofuran 8 and the 1,2,4-trioxane 10, was also studied.

Photolysis of N2O at 125 nm in Ar matrices at 15 K: further evidence for the 120.7 nm band of O(3P)

Abstract VUV photolysis of 1% N2O has been carried out in Ar matrices at 125 nm. Excitation spectra obtained between 200 and 100 nm by monitoring the 1 S → 1 D emission of O at 560 nm, after constant intervals of photolysis, revealed a new band at 120.7 nm. This band has previously been observed during the photolysis of O2 in Ar matrices. The present investigation further confirms the earlier assignment of the 120.7 nm band to the Ar+O−(23Π) ← ArO(13Π) transition.

On the 1S → 1D emission of O by exciting O2 into and beyond the Schumann-Runge continuum in Ar matrices at 15 K

Abstract The excitation spectrum of a stable photoproduct of O2 is recorded by monitoring the 1 S → 1 D emission of O in Ar matrices at 15 K between 200 and 100 nm. The time evolution and changes in intensity of the 1 S → 1 D emission of O at different excitation energies indicate the presence of a stable species of O in Ar matrices. It is proposed that the stable species which disappears upon warming to 30 K is O(3P). When irradiated at 140 nm the 1 S → 1 D emission intensity of O increases and irradiation at 121 nm results in the bleaching of this emission. Based on the present study and theoretical data, the possibility that this species could be O2 in a metastable site or O(1D) in Ar matrices is ruled out.