Abordagem contextual nos capítulos de estequiometria e de soluções dos livros didáticos de Química aprovados pelo PNLD (Programa Nacional do Livro Didático - Brasil) /2012

A contextualização tem sido proposta no currículo escolar com o propósito de promover mudanças de comportamentos, atitudes e valores. Nessa abordagem, entende-se que o ensino de química deve se desenvolver de forma ampla, envolvendo também aspectos sociais, políticos, econômicos,ambientais e culturais, e não somente aspectos científicos. Tendo em vista a grande importância dada ao livro didático (LD) na prática docente, buscamos analisar a abordagem contextual nos capítulos de estequiometria e soluções nos LD de química aprovados pelo PNLD/2012. Os resultados mostraram que os autores dos LD analisados reconhecem a contextualização como elemento central para a formação da cidadania, porém a abordagem é diferente em cada obra. As leituras dos LD possibilitaram a identificação de alguns "níveis" de contextualização que percorreram quatro categorias de análise

Unraveling Confined Dynamics of Guests Trapped in Self-Assembled Pd₆L₄ Nanocages by Ultrafast Mid-IR Polarization-Dependent Spectroscopy

Self-assembled coordination cages form host–guest complexes through weak noncovalent interactions. Knowledge of how these weak interactions affect the structure, reactivity, and dynamics of guest molecules is important to further the design principles of current systems and optimize their specific functions. We apply ultrafast mid-IR polarization-dependent pump–probe spectroscopy to probe the effects of two Pd6L4 self-assembled nanocages on the properties and dynamics of fluxional group-VIII metal carbonyl guest molecules. We find that the interactions between the Pd6L4 nanocages and guest molecules act to alter the ultrafast dynamics of the guests, restricting rotational diffusional motion and decreasing the vibrational lifetime

Multiple Structures and Dynamics of [CpRu(CO)₂]₂ and [CpFe(CO)₂]₂ in Solution Revealed with Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared (2DIR) spectroscopy is applied to both (Cp)2Fe2(CO)4 and its ruthenium analog (Cp)2Ru2(CO)4 in order to study the vibrational dynamics of these two systems. Combining the results of 2DIR spectroscopy and DFT calculations, the different structural forms of both the iron and the ruthenium complexes were characterized, furthering the previous assignment of the linear IR spectrum by determining the transition frequencies associated with the different isomeric forms. Monitoring the time-dependent amplitudes of the cross peaks enabled the observation of equilibrium energy transfer dynamics between different vibrational modes of the cis-B (Cp)2Fe2(CO)4 and the gauche-NB (Cp)2Ru2(CO)4 complexes. Treating the energy transfer as an equilibrium process, we extracted the rate constants associated with both the uphill and the downhill transfer of vibrational energy, finding that the difference in the rate constants of the two metal complexes maps to the difference in the energy gap between the two modes involved

Dissecting Enthalpic and Entropic Barriers to Ultrafast Equilibrium Isomerization of a Flexible Molecule Using 2DIR Chemical Exchange Spectroscopy

Ultrafast 2DIR chemical exchange spectroscopy was used to study the dynamic equilibrium between different isomers of dicobalt octacarbonyl. Exchange of population between bridged and unbridged isomers takes place on the time scale of a few picoseconds, corresponding to activation barriers of several kcal/mol. Despite overlapping spectral features in the 2DIR spectrum, the exchange component of the waiting time dependence was isolated by exploiting the well-characterized coherent modulation of nonexchange crosspeaks. The temperature dependence of the forward and reverse rate constants enabled extraction of isomerization energy barriers, where analysis using the Eyring equation indicated a substantial entropic contribution to the free energy barrier (ΔS‡exp > 0). Comparison to quantum chemical calculations showed reasonable enthalpy agreement, but qualitative disagreement for the entropy of the transition state relative to the isomers (ΔS‡comp < 0)

Halogen Bonding Facilitates Intersystem Crossing in Iodo-BODIPY Chromophores

BODIPY chromophores can serve as organic-based triplet photosensitizers for a wide range of applications. To perform this function, the formation of the triplet state is critical, and a better understanding of how to modulate the formation of the triplet state could lead to further advances in BODIPY-based sensitizers for solar energy conversion and photodynamic therapy. In this work we investigate the ability of halogen bonding, a noncovalent solvent interaction, to facilitate intersystem crossing in a diiodo-BODIPY. Ultrafast transient absorption spectroscopy is applied to diiodo-BODIPY in the presence of pyridine-based halogen bonding solvent molecules to determine the rate constants for intersystem crossing. We find that halogen bonding facilitates the formation of the triplet state by increasing the intersystem crossing rate constant of diiodo-BODIPY. The results are interpreted in terms of the Marcus expression for intersystem crossing. Quantum chemical calculations show that halogen bonding acts to alter both the spin–orbit coupling terms and the relative energetics of the singlet and triplet states

Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions

Luminescent Ce­(III) complexes, Ce­[N­(SiMe₃)₂]₃ (<b>1</b>) and [(Me₃Si)₂NC­(RN)₂]­Ce­[N­(SiMe₃)₂]₂ (R = ^<i>i</i>Pr, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>; R = Cy, <b>1-Cy</b>), with <i>C</i>_3<i>v</i> and <i>C</i>_2<i>v</i> solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on <b>1</b> and <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b> revealed the ²A₁ excited states corresponded to singly occupied 5d_<i>z</i>² orbitals. The strongly reducing metalloradical character of <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> in their ²A₁ excited states afforded photochemical halogen atom abstraction reactions from sp³ and sp² C–X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions

Resolving the Impact of Hydrogen Bonding on the Phylloquinone Cofactor through Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared spectroscopy (2DIR) was applied to phylloquinone (PhQ), an important biological cofactor, to elucidate the impact of hydrogen bonding on the ultrafast dynamics and energetics of the carbonyl stretching modes. 2DIR measurements were performed on PhQ dissolved in hexanol, which served as the hydrogen bonding solvent, and hexane, which served as a non-hydrogen bonding control. Molecular dynamics simulations and quantum chemical calculations were performed to aid in spectral assignment and interpretation. From the position of the peaks in the 2DIR spectra, we extracted the transition frequencies for the fundamental, overtone, and combination bands of hydrogen bonded and non-hydrogen bonded carbonyl groups of PhQ in the 1635–1680 cm–1 region. We find that hydrogen bonding to a single carbonyl group acts to decouple the two carbonyl units of PhQ. Through analysis of the time-resolved 2DIR data, we find that hydrogen bonding leads to faster vibrational relaxation as well as an increase in the inhomogeneous broadening of the carbonyl groups. Overall, this work demonstrates how hydrogen bonding to the carbonyl groups of PhQ presents in the 2DIR spectra, laying the groundwork to use PhQ as a 2DIR probe to characterize the ultrafast fluctuations in the local environment of natural photosynthetic complexes

Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions

Luminescent Ce­(III) complexes, Ce­[N­(SiMe₃)₂]₃ (<b>1</b>) and [(Me₃Si)₂NC­(RN)₂]­Ce­[N­(SiMe₃)₂]₂ (R = ^<i>i</i>Pr, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>; R = Cy, <b>1-Cy</b>), with <i>C</i>_3<i>v</i> and <i>C</i>_2<i>v</i> solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on <b>1</b> and <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b> revealed the ²A₁ excited states corresponded to singly occupied 5d_<i>z</i>² orbitals. The strongly reducing metalloradical character of <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> in their ²A₁ excited states afforded photochemical halogen atom abstraction reactions from sp³ and sp² C–X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions