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The introduction of electron-withdrawing groups on 8(meso)-pyridyl-BODIPYs tends to increase the fluorescence quantum yields of this type of compound due to the decrease in electronic charge density on the BODIPY core. A new series of 8(meso)-pyridyl-BODIPYs bearing a 2-, 3-, or 4-pyridyl group was synthesized and functionalized with nitro and chlorine groups at the 2,6-positions. The 2,6-methoxycarbonyl-8-pyridyl-BODIPYs analogs were also synthesized by condensation of 2,4-dimethyl-3-methoxycarbonyl-pyrrole with 2-, 3-, or 4-formylpyridine followed by oxidation and boron complexation. The structures and spectroscopic properties of the new series of 8(meso)-pyridyl-BODIPYs were investigated both experimentally and computationally. The BODIPYs bearing 2,6-methoxycarbonyl groups showed enhanced relative fluorescence quantum yields in polar organic solvents due to their electron-withdrawing effect. However, the introduction of a single nitro group significantly quenched the fluorescence of the BODIPYs and caused hypsochromic shifts in the absorption and emission bands. The introduction of a chloro substituent partially restored the fluorescence of the mono-nitro-BODIPYs and induced significant bathochromic shifts. [ABSTRACT FROM AUTHOR]

Boron dipyrromethene (BODIPY) dyes bearing a pyridyl moiety have been used as metal ion sensors, pH sensors, fluorescence probes, and as sensitizers for phototherapy. A comparative study of the properties of the three structural isomers of meso-pyridyl-BODIPYs, their 2,6-dichloro derivatives, and their corresponding methylated cationic pyridinium-BODIPYs was conducted using spectroscopic and electrochemical methods, X-ray analyses, and TD-DFT calculations. Among the neutral derivatives, the 3Py and 4Py isomers showed the highest relative fluorescence quantum yields in organic solvents, which were further enhanced 2-4-fold via the introduction of two chlorines at the 2,6-positions. Among the cationic derivatives, the 2catPy showed the highest relative fluorescence quantum yield in organic solvents, which was further enhanced by the use of a bulky counter anion (PF6−). In water, the quantum yields were greatly reduced for all three isomers but were shown to be enhanced upon introduction of 2,6-dichloro groups. Our results indicate that 2,6-dichloro-meso-(2- and 3-pyridinium)-BODIPYs are the most promising for sensing applications. Furthermore, all pyridinium BODIPYs are highly water-soluble and display low cytotoxicity towards human HEp2 cells. [ABSTRACT FROM AUTHOR]

The potential energy surface of the reaction [[[([[eta].sup.5]-[C.sub.5][Me.sub.n][H.sub.5-n]).sub.2]M].sub.2]([[mu].sub.2], [[eta].sup.2], [[eta].sup.2]-[N.sub.2])+[H.sub.2]->[[[([[eta].sup.5]-[C.sub.5][Me.sub.n][H.sub.5-n]).sub.2]M].sub.2]([[mu].sub.2], [[eta].sup.2], [[eta].sup.2]-NNH) was investigated using density functional methods. The results indicate that [[[([[eta].sup.5]-[C.sub.5][Me.sub.n][H.sub.5-n]).sub.2]M].sub.2]([[mu].sub.2], [[eta].sup.2], [[eta].sup.2]-[N.sub.2]) complex cannot react with an [H.sub.2] molecule for M=Ti while those for M=Zr and Hf can.

The hydrogen bonding between hydrogen fluoride and a series of simple nitrile and carbonyl compounds was investigated using ab initio molecular calculations at the HF/6-31+G(d,p) level. The results can be explained in terms of the variation of the molecular electrostatic potential at the nitrile nitrogen for the series of nitrile compounds and at the carbonyl oxygen for the series of carbonyl compounds. A dependence between the calculated molecular electrostatic potential and interaction energy that unifies the two series was also observed.

A study was conducted relating to research on the intermolecular phenomenon of hydrogen bonding in terms of the structural and electronic properties of proton donor and acceptor molecules. The experiment aimed primarily to analyze the trends of the changes in the energy of hydrogen-bond formation in aliphatic carbonyl derivatives and to explain the results in terms of properties of either the complex or the isolated molecule. Results of the study are discussed.

Accurate thermochemical data of small molecules are invaluable to the progress of every aspect of chemistry, especially in the atmosphere, combustion and industry. In this work, photofragmentation translational spectroscopy and 1st principles electronic structure theory reveal the literature value of the heat of formation of chlorine-isocyanate to be in error by more than 40 kcal/mol. We report a revised experimental value for D0(Cl–NCO)=51±3 kcal/mol which leads to a ΔHf (ClNCO)=8.5±3 kcal/mol. High level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit give D0(Cl–NCO)=56.3 kcal/mol, in good agreement with experiment. In light of the present results, the destabilization of azides relative to isoelectronic isocyanates has been evaluated empirically for three pairs of related molecules. It is found to be 90–110 kcal/mol, and has been attributed mainly to the weakening of the N–NN bond relative to the N–CO bond. Electronic structure calculations employing decomposition analysis suggest that, compared to homopolar N2, the +δCO-δ π polarity provides better orbital interaction (charge transfer) and electrostatic attraction and results in a closer encounter and larger stabilization between the fragments and that this is the origin of isoelectronic destabilization of azides relative to the isocyanates. [ABSTRACT FROM AUTHOR]

The detailed computation study of the mechanism of dinitrogen hydrogenation in the Chirik-type complex [[[([eta.sup.5]-[C.sub.5]M[e.sub.4]H).sub.2]Zr].sub.2]([mu.sub.2],[eta.sup.2],[eta.sup.2]-[N.sub.2] and the Fryzuk-type complex [{[Php(C[H.sub.2]SiM[e.sub.2]NSiM[e.sub.2]C[H.sub.2])PPh]Zr}.sub.2]([mu.sub.2],[eta.sup.2],[eta.sup.2]-[N.sub.2] was carried out to observe difference in their reactivity. The observed difference in the reactivity of these dizirconium complexes is caused by the former ligands rigid environment than the latter which results in the addition of the first [H.sub.2] molecule to two different products: a non-H-bridged intermediate for the Chirik-type complex and a H-bridged intermediate for the Fryzuk-type complex.

A new class of phenanthrene-fused BF2 azadipyrromethene (azaBODIPY) dyes have been synthesized through a tandem Suzuki reaction and oxidative ring-fusion reaction, or a palladium-catalyzed intramolecular C−H activation reaction. These phenanthrene-fused azaBODIPY dyes are highly photostable and display markedly redshifted absorption (up to λ=771 nm) and emission bands ( λ≈800 nm) in the near-infrared region. DFT calculations and cyclic voltammetry studies indicate that, upon annulation, more pronounced stabilization of the LUMO is the origin of the bathochromic shift of the absorption and high photostability. [ABSTRACT FROM AUTHOR]

The potential energy surface of the reaction [(η5-C5MenH5-n)2M]2(μ2,η²,η²-N2) + H2 → [(η5-C5-MenH5-n)2M][(η5-C5-MenH5-n)2MH][μ2η²,η²-NNH) at low-lying singlet and triplet electronic states of the reactants was investigated using density functional methods, for n = 0 and 4, and M = Ti, Zr, and Hf. Ground electronic states of the Ti complexes are found to be triplet states, while that for the corresponding Zr and Hf complexes are singlet states. In their singlet state, all these complexes satisfy known necessary conditions (they have a side-on-coordinated N2 molecule and appropriate frontier orbitals) for successful addition of an H2 molecule to the coordinated N2, and consequently, add of an H2 molecule with a reasonable energy barrier. Hf complexes show slightly higher reactivity than corresponding Zr complexes, and in turn, both are more reactive than their singlet-state Ti counterparts. The calculated trend in reactivity of Zr and Hf complexes is consistent with the latest experimental data (see refs 13 and 16). However, Ti complexes have the ground triplet state that lacks in appropriate frontier orbitals. As a result, H2 addition to the Ti complexes at their triplet ground states requires a larger activation barrier than the singlet state and is endothermic (lacks of driven force for reaction). On the basis of these results, we predict that the [(η5-C5Me4H)2M]2(μ2,η²,η²-N2) and [(η5-C5H5)2M]2(ημ2,η²,η²-N2) complexes cannot react with an H2 molecule for M = Ti, while those for M = Zr and Hf can. It was shown that the difference in the B3LYP (hybrid) and PBE (nonhybrid) calculated energy gaps between the lowest closed-shell singlet and triplet states of the present complexes reduces via first- > second- > third-row transition metals; both hybrid and nonhybrid density functionals can be safely used to describe reactivity of the low-lying low-spin and high-spin states of second- and third-row transition metal complexes. [ABSTRACT FROM AUTHOR]

The mechanisms of dinitrogen hydrogenation by two different complexes[(ρ6-C5Me4H)2Zr]2-(μ2,ρ2,ρ2-N2), synthesized by Chirik and co-workers [Nature 2004, 427, 527], and {[P2N2]Zr}2(2μ2,ρ2,ρ2-N2), where P2N2 = PhP(CH2SiMe2NSiMe2CH2)2PPh, synthesized by Fryzuk and co-workers [Science 1997, 275, 1445]are compared with density functional theory calculations. The former complex is experimentally known to be capable of adding more than one H2 molecule to the side-on coordinated N2 molecule, while the latter does not add more than one H2. We have shown that the observed difference in the reactivity of these dizirconium complexes is caused by the fact that the former ligand environment is more rigid than the latter. As a result, the addition of the first H2 molecule leads to two different products: a non-H-bridged intermediate for the Chirik-type complex and a H-bridged intermediate for the Fryzuk-type complex. The non-H-bridged intermediate requires a smaller energy barrier for the second H2 addition than the H-bridged intermediate. We have also examined the effect of different numbers of methyl substituents in [(ρ5-C5-MenH5-n)2(μ2,ρ2,ρ2-N2) for n = 0, 4, and 5 (n = 5 is hypothetical) and [(ρ5-C5H2-1,2,4-Me3)(ρ5-C5Me5)2-Zr]2(μ2,ρ2,ρ2-N2) and have shown that all complexes of this type would follow a similar H2 addition mechanism. We have also performed an extensive analysis on the factors (side-on coordination of N2 to two Zr centers, availability of the frontier orbitals with appropriate symmetry, and inflexibility of the catalyst ligand environment) that are required for successful hydrogenation of the coordinated dinitrogen. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution synchrotron-radiation-based photoionization mass spectrometry was applied to study the dissociative photoionization of ClN3 under collision-free molecular beam conditions at ionization energies between 10 and 17eV. No parent ion (ClN3 +) could be detected under our experimental conditions. This suggests that the ground and excited states of ClN3 + are weakly bound or repulsive, a conclusion supported by electronic structure calculations also reported here. We recorded photoionization yield spectra at m/z =49, 42, 35 and 14 from which we extracted the appearance potentials for NCl+, N3 +, Cl+, and N+. The appearance potential of NCl+ (10.17±0.02eV) observed here is close to the previously reported ionization potential of ClN3 obtained form photoelectron spectroscopy. Using the theoretically calculated binding energy of ClN3 + (0.2eV), we derive an estimate of the adiabatic ionization potential of ClN3 =9.97±0.02eV. The measured appearance potentials for N3 +, Cl+, and N+ provide three independent determinations of the Cl–N bond energy in ClN3, which agree within their respective error limits. The observations of this work are consistent with a new value of the N–Cl bond energy in ClN3, D 0(Cl–N3)=1.86±0.05eV, 0.3eV lower than previously reported values, which are however experimentally derived upper limits. The bond energy reported here is consistent with high level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit, which yield a value: D 0(Cl–N3)=1.87eV. [Copyright &y& Elsevier]