Thermodynamic Properties of the XO2, X2O, XYO, X2O2, and XYO2 (X, Y = Cl, Br, and I) Isomers

Bond Dissociation Energies in Heavy Element Chalcogen and Halogen Small Molecules

Thermodynamic properties including bond dissociation energies (BDEs), heats of formation, and gas-phase acidities for the hydrides and dimers of chalcogens and halogens, H₂Y, HX, Y₂, and X₂ for Y = Se, Te, and At and X = Br, I, and At, have been predicted using the Feller-Peterson-Dixon composite-correlated molecular orbital theory approach. A full four-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se₂, where the AoC-DHF value was used due to strong multireference effects in Se₂ for the SO calculations. The calculated results show that the At₂ BDE is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin-orbit effects. H₂Po is not predicted to be stable to dehydrogenation to Po + H₂ in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than H₂SO₄. The current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.

Structures and Energetics of NI3 and N2 I4

We have applied new methods for performing coupled-cluster calculations to small molecules containing iodine atoms; specifically, NI₃ and N₂ I₄ . Because NI₃ is known to be very reactive, attempts to measure its thermodynamic properties have been challenging at best. To date, N₂ I₄ has not been isolated, and our results suggest that its isolation will be just as challenging. We find that the ΔH_f (NI₃ )=+307.7 kJ mol^-1 and ΔH_f (N₂ I₄ )=+551.6 kJ mol^-1 , confirming that they are unstable with respect to their decomposition products N₂ and I₂ .

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

Structures, vibrational frequencies, and stabilities of halogen cluster anions and cations, X(n)(+/-), n = 3, 4, and 5

The structures, vibrational frequencies, and thermodynamic stabilities of the homonuclear polyhalogen ions, X3(+), X3(-), X4(+), X4(-), X5(+), and X5(-) (X = Cl, Br, I), have been calculated at the CCSD(T) level. The energetics were calculated using the Feller-Peterson-Dixon approach for the prediction of reliable enthalpies of formation. The calculations allow the following predictions where stabilities are defined in terms of thermodynamic quantities. (1) The X3(+) cations are stable toward loss of X2; (2) the X3(-) anions are marginally stable toward loss of X2 with Cl3(-) being the least stable; (3) the X4(+) cations and X4(-) anions are only weakly bound dimers of X2(+1/2) and X2(-1/2) units, respectively, but the cations are marginally stable toward decomposition to X3(+) and X, with I4(+) having the lowest dissociation energy, whereas the X4(-) anions decompose spontaneously to X3(-) and X; (4) the X5(+) cations are only marginally stable at low temperatures toward loss of X2, with Cl5(+) being the least stable; and (5) the X5(-) anions are also only stable at low temperatures toward loss of X2, with Cl5(-) being the least stable.

Atmospheric chemistry of CF3O2: a theoretical study on mechanisms and pathways of the CF3O2 + IO reaction

The mechanisms and reaction pathways for the CF3O2 + IO reaction have been investigated by quantum chemistry methods. It has been found that the title reaction takes place on both the singlet and triplet potential energy surfaces (PES). On the singlet PES, the most important products include CF3OOOI, CF3OOIO, CF3OIO2, and CF2O + FIO2, while other products such as CF2O + FOIO, CF2O + FOOI, CF3OOI + O((3)P), CF3OI + O2 ((1)Δ and (3)Σ), and CF3O + OIO are negligible due to high barriers or unstable formations. On the triplet PES, CF3O + OIO is the dominant product with a lower barrier. As for FIO2 and it isomers, the most stable one is FIO2. TDDFT (Time Dependent Density Functional Theory) calculation indicates that CF3OOOI, CF3OOIO and CF3OIO2 undergo photolysis easily under sunlight. Moreover, a minor contribution relative to hydrogen is found in the CX3O2 + IO (X = H and F) reactions.

Reactivity of BrCl, Br₂, BrOCl, Br₂O, and HOBr toward dimethenamid in solutions of bromide + aqueous free chlorine

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.