Physical Chemistry Chemical Physics

doi: 10.1039/b803861apmid: N/A

Kaltsoyannis, Nikolas; Plane, John M. C.

doi: 10.1039/b715687cpmid: 18350176

The electronic and geometric structures of the title complexes are studied quantum chemically using ab initio and density functional approaches. Coupled cluster calculations at the scalar relativistic (basis set) level are performed, and the results are corrected for spin–orbit coupling using data from relativistic density functional theory studies. The heats of formation (kJ mol−1) at 298 K are found to be: IO3 147.8, INO3 33.1, OIO 110.1, I2O3 64.0, I2O4 111.3, I2O5 33.0, IOIO 141.3, IOOI 179.9 and OI(I)O 157.9. These data are used to draw a number of conclusions regarding three important aspects of iodine chemistry in the marine boundary layer. (i) Although the IO self reaction produces the asymmetric dimer, IOIO, it is unlikely that this species plays a further role in the atmosphere as it is short-lived. (ii) INO3 is sufficiently stable to explain the kinetics of the recombination reaction between IO and NO2, and the reaction between I2 and NO3 to produce I + INO3 is almost certainly the major source of iodine oxides at night. (iii) The higher iodine oxides I2O3 and I2O5 are very stable molecules, by contrast to the OIO dimer, I2O4, which is much less stable but which should still survive long enough in the marine boundary layer to provide a building block for iodine oxide particle formation.

Jayapal, Prabha; Robinson, David; Sundararajan, Mahesh; Hillier, Ian H.; McDouall, Joseph J. W.

doi: 10.1039/b719980epmid: 18350177

Multi-reference Møller–Plesset calculations of a model of the Ni–SI state of nickel–iron hydrogenase predict a singlet rather than a triplet state for this species, and show that it is better described with a BP86 rather than a B3LYP functional.

Lee, Junghan; Kim, Junwon; Park, Eunjung; Jo, Shineun; Song, Rita

doi: 10.1039/b801317apmid: 18350178

Quantum dots (QDs) have size-tunable optical properties, such as photostability, strong photoluminescence and a large Stokes-shift, which make it possible to adapt them to various biological applications. In many cases, surface modification of QDs by carboxylate ligands has been extensively studied. However, there have been few applications on QDs modified with a potential cationic ligand such as amine. In this study, we synthesized robust amine-functionalized QDs and modified their surface with poly(ethylene glycol) for long-term stability. These QDs showed an excellent stability over a broad pH-range and remarkable internalization efficiency into living cells.

Gahungu, Godefroid; Zhang, Jingping

doi: 10.1039/b800685gpmid: 18350179

A quantum chemical investigation is made on the recently synthesized octathio[8]circulene (C16S8), an exotic molecule, the first fully heterocyclic circulene, from the structural and electronic properties and some charge-transport parameters viewpoints. Since the molecule consists of eight thiophene rings fused together, we have chosen to compare it with the acyclic (octathienoacene) analogue and to some relatives thereof, in which the sulfur atoms are substituted by Se, NH, CH2 and O. C16S8, C16Se8 and C16S4Se4 are found to show a low reorganization energy comparable or lower than that for already well known field-effect transistor (FET) materials, a promising property which, combined to some others revealed by this study, makes these compounds potential candidates for FET use. In addition, the twist angle is found to be tightly linked to the peripheral bonds lengths, the least twisted structures showing the most interesting properties for organic FET use.

Armbruster, Markus K.; Weigend, Florian; van Wüllen, Christoph; Klopper, Wim

doi: 10.1039/b717719dpmid: 18350180

Efficient self-consistent field (SCF) schemes including both scalar relativistic effects and spin–orbit (SO) interactions at Hartree–Fock (HF) and density functional (DFT) levels are presented. SO interactions require the extension of standard procedures to two-component formalisms. Efficiency is achieved by using effective core potentials (ECPs) and by employing the resolution-of-the-identity approximation for the Coulomb part (RI-J) in pure DFT calculations as well as also for the HF-exchange part (RI-JK) in the case of HF or hybrid-DFT treatments. The procedures were implemented in the program system TURBOMOLE; efficiency is demonstrated for comparably large systems, such as Pb54. Relevance of SO effects for electronic structure and stability is illustrated by treatments of small Pb and Po clusters with and without accounting for SO effects.

McGillen, Max R.; Carey, Trevor J.; Archibald, Alex T.; Wenger, John C.; Shallcross, Dudley E.; Percival, Carl J.

doi: 10.1039/b715394epmid: 18350181

The configuration of alkyl substituents about carbon–carbon unsaturated bonds exerts a controlling influence on the rate of the ozonolysis reaction. Alkyl substituents can increase (via the inductive effect) and decrease (via the steric effect) the activity of unsaturated bonds, and an accurate description of this information ought to correlate with the ozonolysis rate coefficient. A strong linear relationship is observed (R2 = 0.94), providing the basis of our SAR method. SAR estimates were tested against literature measurements of ozonolysis rate coefficients for 48 aliphatic alkenes and dialkenes, and were found to be accurate to within a factor of 2.3 of the measured value for the entire dataset. This represents a significant improvement over methods reported in the literature, where quoted predictions are at best accurate to within a factor of 6.5. Rates of gas-phase ozonolysis of alkenes and dialkenes can now be predicted with unprecedented accuracy using a simple SAR. The SAR was then validated against new experimental data. Absolute rate coefficients for the gas-phase reaction of ozone with a series of alkenes were determined in a simulation chamber at 295 ± 2 K and atmospheric pressure by monitoring the loss of ozone in the presence of excess alkene. The rate coefficients (in units of 1 × 10−18 cm3 molecule−1 s−1) are: 5.12 ± 0.93 for 1-pentene, 2,3-dimethyl; 406 ± 49 for 2-pentene, 2-methyl; 151 ± 5 for (E)-2-hexene, 14.5 ± 1.0 for 1,5-hexadiene and 20.7 ± 3.1 for 1,5-hexadiene, 2-methyl. There is good agreement between the experimental and predicted values and the adjustable parameters of the SAR are shown to be insensitive to the inclusion of the new data. The use of the SAR in atmospheric chemical modelling is investigated. Ozonolysis and OH radical rate coefficients are estimated for each alkene and dialkene present in the MCM v3.1. The effects of error within predicted rate coefficients upon modelled concentrations of a number of key species, including O3, OH, HO2, NO and NO2 were rather small and is not in itself a major cause of uncertainty in modelled concentrations.

Asatryan, Rubik; Bozzelli, Joseph W.

doi: 10.1039/b716179dpmid: 18350182

Dimethyl sulfoxide (DMSO) is the major sulfur-containing constituent of the Marine Boundary Layer. It is a significant source of HSO aerosol/particles and methane sulfonic acid atmospheric oxidation processes, where the mechanism is not established. In this study, several new, low-temperature pathways are revealed in the oxidation of DMSO using CBS-QB3 and G3MP2 multilevel and B3LYP hybrid density functional quantum chemical methods. Unlike analogous hydrocarbon peroxy radicals the chemically activated DMSO peroxy radical, [CHS(O)CHOO˙]*, predominantly undergoes simple dissociation to a methylsulfinyl radical CHS˙(O) and a Criegee intermediate, CHOO, with the barrier to dissociation 11.3 kcal mol below the energy of the CHS(O)CH˙ + O reactants. The well depth for addition of O to the CHS(O)CH˙ precursor radical is 29.6 kcal mol at the CBS-QB3 level of theory. We believe that this reaction may serve an important role in atmospheric photochemical and irradiated biological (oxygen-rich) media where formation of initial radicals is facilitated even at lower temperatures. The Criegee intermediate (carbonyl oxide, peroxymethylene) and sulfinyl radical can further decompose, resulting in additional chain branching. A second reaction channel important for oxidation processes includes formation ( intramolecular H atom transfer) and further decomposition of hydroperoxide methylsulfoxide radical, ˙CHS(O)CHOOH over a low barrier of activation. The initial H-transfer reaction is similar and common in analogous hydrocarbon radical + O reactions; but the subsequent very low (3–6 kcal mol) barrier (14 kcal mol below the initial reagents) to β-scission products is not common in HC systems. The low energy reaction of the hydroperoxide radical is a β-scission elimination of ˙CHS(O)CHOOH into the CHSO + CHO + ˙OH product set. This β-scission barrier is low, because of the delocalization of the ˙CH radical center through the –S(O) group, to the –CHOOH fragment in the transition state structure. The hydroperoxide methylsulfoxide radical can also decompose a second reaction channel of intramolecular OH migration, yielding formaldehyde and a sulfur-centered hydroxymethylsulfinyl radical HOCHS˙(O). The barrier of activation relative to initial reagents is 4.2 kcal mol. Heats of formation for DMSO, DMSO carbon-centered radical and Criegee intermediate are evaluated at 298 K as −35.97 ± 0.05, 13.0 ± 0.2 and 25.3 ± 0.7 kcal mol respectively using isodesmic reaction analysis. The [CHS˙(O) + CHOO] product set is shown to form a van der Waals complex that results in O-atom transfer reaction and the formation of new products CHSO˙ radical and CHO. Proper orientation of the Criegee intermediate and methylsulfinyl radical, as a pre-stabilized pre-reaction complex, assist the process. The DMSO radical reaction is also compared to that of acetonyl radical.