Ab initio calculation of carbon clusters. II. Relative stabilities of fullerene and nonfullerene C24

Chemical stabilities of six low-energy isomers of C24 derived from global-minimum search are investigated. The six isomers include one classical fullerene (isomer 1) whose cage is composed of only five- and six-membered rings (56-MRs), three nonclassical fullerene structures whose cages contain at least one four-membered ring (4-MR), one plate, and one monocyclic ring. Chemical and electronic properties of the six C24 isomers are calculated based on a density-functional theory method (hybrid PBE1PBE functional and cc-pVTZ basis set). The properties include the nucleus-independent chemical shifts (NICS), singlet-triplet splitting, electron affinity, ionization potential, and gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) gap. The calculation suggests that the neutral isomer 2, a nonclassical fullerene with two 4-MRs, may be more chemically stable than the classical fullerene (isomer 1). Analyses of molecular orbital NICS show that the incorporations of 4-MRs into the cage considerably reduce paratropic contributions from HOMO, HOMO-1, and HOMO-2, which are mainly responsible for the sign change in NICS from positive for isomer 1 (42) to negative (-19) for isomer 2, although C24 clusters satisfy neither 4N+2 nor 2(N+1)2 aromaticity rule. Anion photoelectron spectra of four cage isomers, one plate, one monocyclic ring, and one tadpole isomer, as well as three bicyclic ring isomers are calculated. The simulated photoelectron spectra of mono- and bicyclic rings (with C1 symmetry) appear to match the measured HOMO-LUMO gap (between the first and second band in the experimental spectra) [S. Yang et al., Chem. Phys. Lett. 144, 431 (1988)]. Nevertheless, the nonclassical fullerene isomers 3 and 4 apparently also match the measured vertical detachment energy (2.90 eV) reasonably well. These results suggest possible coexistence of nonclassical fullerene isomers with the mono- and bicyclic ring isomers of C24(-) under the experimental conditions.     

Rank-1 lattice rules for multivariate integration in spaces of permutation-invariant functions

We study multivariate integration of functions that are invariant under permutations (of subsets) of their arguments. We find an upper bound for the nth minimal worst case error and show that under certain conditions, it can be bounded independent of the number of dimensions. In particular, we study the application of unshifted and randomly shifted rank-1 lattice rules in such a problem setting. We derive conditions under which multivariate integration is polynomially or strongly polynomially tractable with the Monte Carlo rate of convergence \(\mathcal {O}(n^{-1/2})\). Furthermore, we prove that those tractability results can be achieved with shifted lattice rules and that the shifts are indeed necessary. Finally, we show the existence of rank-1 lattice rules whose worst case error on the permutation- and shift-invariant spaces converge with (almost) optimal rate. That is, we derive error bounds of the form \(\mathcal {O}(n^{-\lambda /2})\) for all 1≤λ<2α, where α denotes the smoothness of the spaces.     

Measurement of(n,α)and(n,2n)reaction cross sections at a neutron energy of 14.92±0.02 MeV for potassium and copper with uncertainty propagation

Experimentally measured neutron activation cross sections are presented for the ⁶⁵Cu(n,0)^62mCu,⁴¹ K(n,a)^38Cl,and ⁶⁵Cu(n.2n)⁶⁴Cu reactions with detailed uncertainty propagation.The neutron cross secions were measured at an incident energy of 14.92±0.02 MeV,and the neutrons were based on the(d,n)a fusion reaction.The ²⁷ Al(n,a)²⁴Na reaction was used as a reference reaction for the normalization of the neutron flux.The pre-calib-rated lead-shielded HPGe detector was used to detect the residues'γ-ray spetra.The data from the measured cross sections are compared to the previously measured cross sections from the EXFOR database,theoretically calculated cross sections using the TALYS and EMPIRE codes,and evaluated nuclear data.     

Differentiating the gum resins of two closely related Indian Gardenia species, G. gummifera and G. lucida, and establishing the source of dikamali gum resin using high-performance thin-layer chromatography and ultra-performance liquid chromatography-UV/MS

Dikamali is a gum resin obtained from the leaf buds of Gardenia lucida or G. gummifera. There is controversy regarding the botanical source of this gum resin with some stating it to be from G. lucida while others claim it to be from G. gummifera. Analytical methods including UPLC and HPTLC were developed for the qualitative analysis of Gardenia species and various commercial samples. The separation using a UPLC method was achieved within 12.0 min by using C18 column material, a water/acetonitrile mobile phase, both containing formic acid, a gradient system, and a temperature of 40 degrees C. Extensive studies of dikamali collected from various parts of India in comparison with the gum resins collected from G. lucida and G. gummifera clearly indicated that the botanical source of commercially available dikamali is G. lucida, not G. gummifera. The marker compounds isolated from a market sample of dikamali were present only in the gum resin of G. lucida and the compounds isolated from G. gummifera were not present in any of the dikamali samples, confirming the botanical source of dikamali. This work is of utmost importance, given the ambiguity regarding the botanical source of the gum resin dikamali. LC/MS coupled with electrospray ionization is described for the identification and confirmation of nine compounds from various samples of the gum resin. An HPTLC method was also developed for the fast chemical fingerprint analysis of Gardenia samples.     

Measurement of reaction cross-sections for 64Ni(n, γ) 65Ni at E n = 0.025 eV and 58Ni(n, p) 58Co at E n = 3.7 MeV

The reaction cross-sections for ⁶⁴Ni(n, γ) ⁶⁵Ni at E _n  = 0.025 eV and ⁵⁸Ni (n, p) ⁵⁸Co at E _n  = 3.7 MeV have been experimentally determined using activation and off-line γ-ray spectrometric technique. The thermal neutron flux used is from the thermal Column of the reactor APSARA at BARC, Mumbai, whereas the neutron energy of 3.7 MeV is from the ⁷Li(p, n) reaction at Pelletron facility, TIFR, Mumbai. The ⁶⁴Ni(n, γ) ⁶⁵Ni and ⁵⁸Ni(n, p) ⁵⁸Co reactions cross-sections from present work are compared with the available literature data and found to be in good agreement. The ⁵⁸Ni(n, p) ⁵⁸Co reaction as a function of neutron energy is also calculated theoretically using TALYS computer code version 1.2 and found to be higher than the experimental data.     

Near Ultraviolet Absorption Spectrum of Ethylpyrazine

Abstract

Vibrational analysis of the electronic absorption spectrum of ethylpyrazine observed around 320 nm is carried out. The bands are ascribed to the longest wavelength one of the four possible π? ← n systems. A “vibronic” interaction between the excited state of this system and another excited state of the molecule is observed. A π? ← π system with practically no vibrational structure is also observed and discussed.     

Electron spin resonance studies of chlorine centres in irradiated chlorine-doped Na2SO4 single crystals

The nature of paramagnetic centres trapped in X-irradiated Na₂SO₄ single crystals containing chlorine as impurity has been determined from E.S.R. measurements both at room and liquid nitrogen temperatures. ClO₃, ClO₂, SO₄ ^- and O₃ ^- are the paramagnetic centres identified. The g and A parameters of ClO₃ change considerably between 298 K and 77 K. Thus the parameters of ClO₃ obtained at 298 K are g_xx = 2·0123, g_yy = 2·0143, g_zz = 2·0206 G and those of the A-tensor are A_xx = 97, A_yy = 100 and A_zz = 112 G. The parameters obtained for the same centre at 77 K are g_xx = 2·0132, g_yy = 2·0173, g_zz = 2·0073 G and those of the A-tensor are A_xx = 130·6, A_yy = 146·3 and A_zz = 181·2 G. The temperature variation of the principal values are determined in the temperature range between 298 K and 77 K. These data can be interpreted on the assumption that ClO₃ undergoes libration at room temperature, which is frozen out reversibly on cooling. The structure and mechanism of these defects are discussed further.