Divalent N(I) compounds with two lone pairs on nitrogen

Carbon with the C(0) state has been reported recently, examples of which were known for the past decades. Silicon in the Si(0) state and phosphorus in the P(I) state are also known experimentally. This prompted us to search for divalent N(I) compounds, which resulted in the identification of ::N(←L)(2)(⊕) systems with bicoordinated nitrogen in the N(I) formal oxidation state. It was found that several biguanide derivatives (especially in their protonated state) belong to this class. Quantum chemical analysis provided the structural details, molecular orbitals, charge localization (vs delocalization) trends, etc. This class of compounds has been found to be characterized by two lone pairs on the central nitrogen, very similar to the central carbon in divalent C(0) compounds (::C(←L)(2)). The new bonding environment for nitrogen reported in this article, divalent nitrogen N(I), is clearly different from the nitrenium ions NR(2)(⊕). The electronic structure and reactivity of representative examples of this novel class of divalent nitrogen N(I) systems (::N(←L)(2)(⊕)) have been analyzed in detail, in terms of molecular orbitals, atomic charges, protonation energies, complexation energies with Lewis acids like BH(3), AlCl(3), and AuCl and compared with those of divalent C(0) systems.     

A single chiroptical spectroscopic method may not be able to establish the absolute configurations of diastereomers: dimethylesters of hibiscus and garcinia acids

Electronic circular dichroism (ECD), optical rotatory dispersion (ORD), and vibrational circular dichroism (VCD) spectra of hibiscus acid dimethyl ester have been measured and analyzed in combination with quantum chemical calculations of corresponding spectra. These results, along with those reported previously for garcinia acid dimethyl ester, reveal that none of these three (ECD, ORD, or VCD) spectroscopic methods, in isolation, can unequivocally establish the absolute configurations of diastereomers. This deficiency is eliminated when a combined spectral analysis of either ECD and VCD or ORD and VCD methods is used. It is also found that the ambiguities in the assignment of absolute configurations of diastereomers may also be overcome when unpolarized vibrational absorption is included in the spectral analysis.     

Synthesis and characterization of dithienylbenzobis(thiadiazole)-based low band-gap polymers for organic electronics

New donor-acceptor alternating conjugated polymers were synthesized and characterized. Among them, PCPBBT exhibited a band-gap of 1.01 eV and ambipolar characteristics with μ(h) = 7.1 × 10(-4) cm(2) V(-1) s(-1) and μ(e) = 3.3 × 10(-3) cm(2) V(-1) s(-1).     

Control of spins by ring deformation in a diiron(III)bisporphyrin: reversal of ClO4⁻ and CF3SO3⁻ ligand field strength in the magnetochemical series

The complete reversal of the ligand field strength of ClO(4)(-) and CF(3)SO(3)(-) in the magnetochemical series is observed in a diiron(III)bisporphyrin. While ClO(4)(-), as the axial ligand, gives a typical high-spin complex, just twisting the conformation of the porphyrin macrocycle gives properties of a pure intermediate-spin state with CF(3)SO(3)(-), although the axial ligand strengths suggest the reverse order of spin stabilization.     

Planar triazinium cations from VO(2+)-assisted ring cyclizations: a remarkably efficient thiazole species for nuclear staining, PDT and anaerobic photocleavage of DNA

Planar triazinium cationic species, from VO(2+)-assisted cyclization of 1-(2-thiazolylazo)-2-naphthol, shows efficient DNA intercalative binding, visible light-induced anaerobic plasmid DNA photocleavage activity and photocytotoxicity in HeLa and MCF-7 cancer cells by an apoptotic pathway with selective localization of the compound in the nucleus as evidenced from the nuclear staining and confocal imaging.     

Tunable emission of static excimer in a pyrene-modified polyamidoamine dendrimer aggregate through positive solvatochromism

Significant aggregation is observed in pyrene-modified zero- and first-generation polyamidoamine (PAMAM) dendrimers above their critical aggregation concentration (CAC, >10(-6) M). The pyrene units are attached to the dendrimer skeleton through imine bonds, which play a pivotal role in enhancing the aggregation propensity of the PAMAM dendrimers. Scanning electron microscopy studies suggest that pyrene-modified PAMAM dendrimers aggregate into doughnut-shaped assemblies. As a result of aggregation, the pyrene chromophores are pre-arranged in a face-to-face geometry in the ground state, and readily generate pyrene "static excimer" on photoexcitation. The static pyrene excimer emits with an unprecedented quantum yield of 0.62 ± 0.01 in dichloromethane, and also exhibits remarkable positive solvatochromism from 498 to 638 nm, which leads to the highest bathochromic shift for pyrene excimer emission in solution reported so far. Lippert-Mataga analysis of the system suggests that general and specific solvent effects play a crucial role in the positive solvatochromism exhibited by the system. Luminescence quenching studies on both monomer and aggregate systems were carried out in the presence of various metal ions, and the results imply that pyrene-modified PAMAM dendrimer can be utilized for selective detection of Hg ions in the presence of a wide variety of transition, alkali, and alkaline earth metal ions. This report presents the first dendrimer-based chromophoric system exhibiting positive solvatochromism over a range of 140 nm, and shows that pyrene-modified PAMAM dendrimers can be effectively utilized to generate wavelength-tunable emitting systems displaying bluish green, greenish yellow, and orange-red colors at room temperatures.     

Post-irradiation identification of papaya (Carica papaya L.) fruit

Impact of radiation processing on the volatile essential oil profile of papaya (Carica papaya) was investigated. Gamma-radiation processing resulted in the appearance of a new peak in the GLC profile that was identified as phenol. The observed dose dependent increase in phenol content suggested possible use of this compound as a marker for radiation processed papaya.     

Conformational polymorphism in bicalutamide: a quantum chemical study

Bicalutamide is an anti-neoplastic drug widely used for the treatment of prostate cancer and it exhibits conformational polymorphism. Three crystal structures of bicalutamide are reported as racemic mixtures, two of which are polymorphs. In addition, three co-crystals are also reported—two with organic coformers and one with adrenoreceptor (the macromolecular target). All the reported structures show significant conformational differences. Quantum chemical B3LYP/6-31+G(d,p) analysis has been carried out to understand the interplay of intra- and intermolecular interactions leading to the conformational preferences in this molecule. The difference between the two polymorphic forms has been traced to the C5–S8–C11–C12 torsional angle. Inside the cavity of androgen receptor, a completely different conformation is found but it does not correspond to any local minima on the potential energy surface of the drug. A relatively rigid torsional angle C11–C12–C15–N17 is also expected due to a strong five-membered ring intramolecular hydrogen bond (H–O13–C12–C15–O16), which has been reported to be desirable; quantum chemical analysis revealed that this rigidity is of the order of 11 kcal/mol. Ab initio calculations demonstrate that polymorphs and polymorphic co-crystals differ in the extent of intra- and intermolecular hydrogen bonding interactions. The strength of the intermolecular interactions associated with these structures is analyzed in terms of energy release due to dimerization.     

Energy absorption build-up factors in teeth

Geometric progression fitting method has been used to compute energy absorption build-up factor of teeth [enamel outer surface, enamel middle, enamel dentin junction towards enamel, enamel dentin junction towards dentin, dentin middle and dentin inner surface] for wide energy range (0.015?C15?MeV) up to the penetration depth of 40 mean free path. The dependence of energy absorption build-up factor on incident photon energy, penetration depth, electron density and effective atomic number has also been studied. The energy absorption build-up factors increases with the penetration depth and electron density of teeth. So that the degree of violation of Lambert?CBeer (I?=?I ₀e^???t ) law is less for least penetration depth and electron density. The energy absorption build-up factors for different regions of teeth are not same hence the energy absorbed by the different regions of teeth is not uniform which depends on the composition of the medium. The relative dose of gamma in different regions of teeth is also estimated. Dosimetric implication of energy absorption build-up factor in teeth has also been discussed. The estimated absorption build up factors in different regions of teeth may be useful in the electron spin resonance dosimetry.     

Influence of water on anharmonicity, stability, and vibrational energy distribution of hydrogen-bonded adducts in atmospheric reactions: case study of the OH + isoprene reaction intermediate using ab initio molecular dynamics

The effect of water on the stability and vibrational states of a hydroxy-isoprene adduct is probed through the introduction of 1-15 water molecules. It is found that when a static nuclear harmonic approximation is invoked there is a substantial red-shift of the alcohol O-H stretch (of the order of 800 cm(-1)) as a result of introduction of water. When potential energy surface sampling and associated anharmonicities are introduced through finite temperature ab initio dynamics, this hydroxy-isoprene OH stretch strongly couples with all the water vibrational modes as well as the hydroxy-isoprene OH bend modes. A new computational technique is introduced to probe the coupling between these modes. The method involves a two-dimensional, time-frequency analysis of the finite temperature vibrational properties. Such an analysis not only provides information about the modes that are coupled as a result of finite-temperature analysis, but also the temporal evolution of such coupling.