The infrared spectrum of acetonitrile confined in hydrophilic silica pores roughly cylindrical and 2.4 nm in diameter has been simulated using molecular dynamics. Hydrogen bonding interactions between acetonitrile and silanol groups on the pore wall involve charge transfer effects that have been incorporated through corrections based on electronic structure calculations on a dimer. The simulated spectrum of confined acetonitrile differs most prominently from that of the bulk liquid by the appearance of a blue-shifted shoulder, in agreement with previous experimental measurements. The dominant peak is little changed in position relative to the bulk liquid case, but broadened by approximately 40%. A detailed analysis of the structure and dynamics of the confined liquid acetonitrile is presented, and the spectral features are examined in this context. It is found that packing effects, hydrogen bonding, and electrostatic interactions all play important roles. Finally, the molecular-level information that can be obtained about the dynamics of the confined liquid from the infrared line shape is discussed.

Relative to the p-block of the periodic table, data for transition metal-ligand bond dissociation enthalpies are less comprehensive. Recent developments in computational methods make systematic assessment of trends in metal-ligand bond enthalpies across the transition series a relatively rapid and accurate exercise. We report a systematic study of metal-ligand bond enthalpies for saturated transition metal complexes that encompasses the entire d-block of the periodic table and a wide assortment of ligands. The saturated complexes have the form MH n -L such that closed-shell molecules are formed with the maximum number of two-center, two-electron (2c/2e) bonds under the constraint that the metal electron count does not exceed 12. Bond enthalpies for MH n -L molecules with higher electron counts (14 and 16 electrons) are assessed for some group 10 and 11 metals. The primary methods are density functional theory (DFT) using the hybrid B3LYP density functional and CCSD(T) ab initio computations. Bond enthalpies are reported as the first bond dissociation enthalpies for neutral and cationic complexes of the type MH n -R (R ) H, CH 3 , C 2 H 5 , CH(CH 3 ) 2 , C(CH 3 ) 3 , CH 2 F, C 2 H, C 2 H 3 , NH 2 , OH, F, and BH 2 ) for all transition elements. Electronic structure analysis of the complexes features natural bond orbital (NBO) analysis of bond polarity.

The reaction between dioxygen and palladium(0) is a key step in palladium-catalyzed aerobic oxidation reactions. The spin-forbidden reaction between (en)Pd and O2 has been analyzed by spin-unrestricted density functional methods and shown to proceed by a stepwise process involving (1) formation of a triplet eta1-superoxo-PdI adduct, (2) spin-crossover from the triplet to the singlet surface, and (3) collapse of the singlet eta1-superoxo-PdI adduct into an eta2-peroxo-PdII complex. Delocalization of spin density from triplet O2 onto the palladium center in the first step reduces the exchange interaction between the unpaired spins and facilitates crossover from the triplet to singlet surface.

The mechanism of olefin substitution at palladium(0) has been studied, and the results provide unique insights into the fundamental reactivity of electron-rich late transition metals. A systematic series of bathocuproine-palladium(0) complexes bearing trans-beta-nitrostyrene ligands (ns(X) = X-C(6)H(4)CH=CHNO(2); X = OCH(3), CH(3), H, Br, CF(3)), (bc)Pd(0)ns(X) (3(X)), was prepared and characterized, and olefin-substitution reactions of these complexes were found to proceed by an associative mechanism. In cross-reactions between (bc)Pd(ns(CH)()3) and ns(X) (X = OCH(3), H, Br, CF(3)), more-electron-deficient olefins react more rapidly (relative rate: ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()3). Density functional theory calculations of model alkene-substitution reactions at a diimine-palladium(0) center reveal that the palladium center reacts as a nucleophile via attack of a metal-based lone pair on the empty pi orbital of the incoming olefin. This orbital picture contrasts that of traditional ligand-substitution reactions, in which the incoming ligand donates electron density into an acceptor orbital on the metal. On the basis of these results, olefin substitution at palladium(0) is classified as an "inverse-electron-demand" ligand-substitution reaction.

The study and analysis of precipitation has become a crucial tool in understanding the temporal and spatial behavior of water resources, in terms of availability and impact on extreme events. The objective of this study was to evaluate different rainfall parameters (intensities for 1-h duration D = 1 h and return periods of T = 5 and 100 yr, and mean annual precipitation) for different latitudinal and climatic zones in Chile. We analyzed the information recorded on thousands of pluvial bands and rain gauges for 49 stations; this because it is unclear how rainfall intensities change along the country (though total amounts do), in addition to a lack of literature focused on ranges and amounts on the behavior of rainfall variables. The Gumbel probability distribution function (PDF) and mathematical rainfall intensity formulas were used to develop intensity-duration-frequency (IDF) curves for each station. Maximum and minimum rainfall intensity values for T = 100 yr ranged from 8.79 (hyperarid zone) to 40.17 mm h -1 (subhumid-humid zone). Total annual rainfall values ranged between 43.9 (hyperarid zone) and 3891.0 mm yr -1 (humid zone). Additionally, the real maximum intensity registered on each station was analyzed, determining its exceedance probability. Likewise, multiple comparisons were made to detect significant differences between the gauge stations and different climatic zones using the Kruskal Wallis test (alpha = 0.05). Differences between maximum and minimum values registered for all stations were as much as 80 times for total rainfall amounts and 4.5 times for rainfall intensities (T = 100 yr). However, maximum rainfall intensities values were similar at different latitudes, suggesting the absence of correlation between maximum rainfall intensity and annual rainfall amount, as the latter variable increased gradually with latitude.