Experimental and modeling study of the ion-molecule association reaction H3O+ + H2O (+M) --> H5O2(+) (+M)

Experimental results for the rate of the association reaction H3O+ + H2O (+M) --> H5O2(+) (+M) obtained with the Cinetique de Reactions en Ecoulement Supersonique Uniforme flow technique are reported. The reaction was studied in the bath gases M=He and N2, over the temperature range of 23-170 K, and at pressures between 0.16 and 3.1 mbar. At the highest temperatures, the reaction was found to be close to the limiting low-pressure termolecular range, whereas the limiting high-pressure bimolecular range was approached at the lowest temperatures. Whereas the low-pressure rate coefficients can satisfactorily be reproduced by standard unimolecular rate theory, the derived high-pressure rate coefficients in the bath gas He at the lowest temperatures are found to be markedly smaller than given by simple ion-dipole capture theory. This result differs from previous observations on the related reaction NH4(+) + NH3 (+M) --> N2H7(+) (+M). This observation is tentatively attributed to more pronounced contributions of the valence part of the potential-energy surface to the reaction in H5O2(+) than in N2H7(+). Falloff curves of the reaction H3O+ + H2O (+M) --> H5O2(+) (+M) are constructed over wide ranges of conditions and represented in compact analytical form.     

A study of the reaction NO3 + NO2 + M → N2O5 + M(M = N2, O2)

The gas-phase reaction of the NO₃ radical with NO₂ was investigated, using a flash photolysis-visible absorption technique, over the total pressure range 25–400 Torr of nitrogen or oxygen diluent at 298 ± 2 K. The absolute rate constants determined (in units of 10^?13 cm³ molecule^?1 s^?1) at 25, 100, and 400 Torr total pressure were, respectively, (4.0 ± 0.5), (7.0 ± 0.7), and (10 ± 2) for M = N₂ and (4.5 ± 0.5), (8.0 ± 0.4), and (8.8 ± 2.0) for M = O₂. These data show that the third-body efficiencies of N₂ and O₂ are identical, within the error limits, and that previous evaluations for M = N₂ are applicable to the atmosphere. In addition, upper limits were determined for the rate constants of the reactions of the NO₃ radical with methanol, ethanol, and propan-2-ol of ?6 × 10^?16, ?9 × 10^?16, and ?2.3 × 10^?15 cm³ molecule^?1 s^?1, respectively, at 298 ± 2 K.     

Kinetics of the reaction HO2 + NO2(+M) = HO2NO2 using molecular modulation spectrometry

Rate constants for the reaction HO₂ + NO₂(+ M) = HO₂NO₂(+ M) have been obtained from direct observations of the HO₂ radical using the technique of molecular modulation ultraviolet spectrometry. HO₂ was generated by periodic photolysis of Cl₂ in the presence of excess H₂ and O₂, and k₁ was determined from the measured concentrations and lifetime of HO₂ with NO₂ present. k₁ increased with pressure in the range of 40–600 Torr, and a simple energy transfer model gave the following limiting second- and third-order rate constants at 283 K: k₁^∞ = 1.5 ± 0.5 × 10^?12 cm³/molec·sec and k₁^III = 2.5 ± 0.5 × 10^?31 cm⁶/molec·sec. The ultraviolet absorption spectrum of peroxynitric acid was also recorded in the range of 195–265 nm; it showed a broad feature with a maximum at 200 nm, σ_max = 4.4 × 10^?18 cm².     

Determination of the rate coefficients of the SO2 + O + M → SO3 + M reaction

Rate coefficients of the title reaction R₃₁ (SO₂ + O + M → SO₃ + M) and R₅₆ (SO₂ + HO₂→ SO₃ + OH), important in the conversion of S(IV) to S(VI), were obtained at T = 970–1150 K and ρ_ave = 16.2 μmol cm^?3 behind reflected shock waves by a perturbation method. Shock‐heated H₂/O₂/Ar mixtures were perturbed by adding small amounts of SO₂ (1%, 2%, and 3%) and the OH temporal profiles were then measured using laser absorption spectroscopy. Reaction rate coefficients were elucidated by matching the characteristic reaction times acquired from the individual experimental absorption profiles via simultaneous optimization of k₃₁ and k₅₆ values in the reaction modeling (for satisfactory matches to the observed characteristic times, it was necessary to take into account R₅₆). In the experimental conditions of this study, R₃₁ is in the low‐pressure limit. The rate coefficient expressions fitted using the combined data of this study and the previous experimental results are k_31,0/[Ar] = 2.9 × 10³⁵ T^?6.0 exp(?4780 K/T) + 6.1 × 10²⁴ T^?3.0 exp(?1980 K/T) cm⁶ mol^?2 s^?1 at T = 300–2500 K; k₅₆ = 1.36 × 10¹¹ exp(?3420 K/T) cm³ mol^?1 s^?1 at T = 970–1150 K. Computer simulations of typical aircraft engine environments, using the reaction mechanism with the above k_31,0 and k₅₆ expressions, gave the maximum S(IV) to S(VI) conversion yield of ca. 3.5% and 2.5% for the constant density and constant pressure flow condition, respectively. Moreover, maximum conversions occur at rather higher temperatures (～1200 K) than that where the maximum k_31,0 value is located (～800 K). This is because the conversion yield is dependent upon not only the k_31,0 and k₅₆ values (production flux) but also the availability of H, O, and HO₂ in the system (consumption flux). © 2010 Wiley Periodicals, Inc. *

1 This article is a U.S. Government work and, as such, is in the public domain of the United States of America.

Int J Chem Kinet 42: 168–180, 2010     

A spectroscopic study of the equilibrium NO2 + NO3 + M 2 N2O5 + M and the kinetics of the O3/N2O5/NO3/NO2/ air system

The equilibrium constant, K_eq of the reaction NO₂ + NO₃ + M 2 N₂O₅ + M has been determined for a small range of temperatures around room temperature in air at 740 torr by direct spectroscopical measurements of NO₂, NO₃, and N₂O₅. At 298 K, K_eq was determined as (3.73 ± 0.61) × 10⁻¹¹ cm³ molecule⁻¹. Averaging this and 11 other independent evaluations of K_eq yields K_eq = (3.31 ± 0.82) × 10⁻¹¹ cm³ molecule⁻¹, where the uncertainty is given as one standard deviation. The kinetics of the O₃/NO₂/N₂O₅/NO₃/ air system was studied in a static chamber at room temperature and 740 torr total pressure. Evidence of a unimolecular decay reaction of NO₃, NO₃ → NO + O₂, was found and its rate coefficient was estimated as (1.6 ± 0.7) × 10⁻³ s⁻¹ at 295 ± 2 K.     

Kinetic study of the recombination reaction OD + NO2 + M → DNO3 + M

Rate constants for the recombination reaction OD + NO₂ + M → DNO₃ + M have been determined in the falloff region (5–500 torr) and at 297 ± 2 K, in the presence of He, N₂, and SF₆ as third bodies, by using a pulsed laser photolysis-resonance absorption technique. Values of k₀, k_x and the falloff parameter F_c have been estimated. Our rate constants were, within the experimental uncertainty, the same as those reported for the reaction of OH radicals with NO₂.     

Analysis of the unimolecular reaction N2O5 + M ⇌ NO2 + NO3 + M

Recent experimental results on the thermal decomposition of N₂O₅ in N₂ are evaluated in terms of unimolecular rate theory. A theoretically consistent set of fall-off curves is constructed which allows to identify experimental errors or misinterpretations. Limiting rate constants k₀ = [N₂] 2.2 × 10^?3 (T/300)^?4.4 exp(?11,080/T) cm³/molec·s over the range of 220–300 K, k_∞ = 9.7 × 10¹⁴ (T/300)^+0.1 exp(?11,080/T) s^?1 over the range of 220–300 K, and broadening factors of the fall-off curve F_cent = exp(-T/250) + exp(?1050/T) over the range of 220–520 K have been derived. NO₂ + NO₃ recombination rate constants over the range of 200–300 K are k_rec,0 = [N₂] 3.7 × 10^?30 (T/300)^?4.1 cm⁶/molec²·s and k_rec,∞ = 1.6 × 10^?12 (T/300)^+0.2 cm³/molec·s.     

Comprehensive H2/O2 kinetic model for high‐pressure combustion

An updated H₂/O₂ kinetic model based on that of Li et al. (Int J Chem Kinet 36, 2004, 566–575) is presented and tested against a wide range of combustion targets. The primary motivations of the model revision are to incorporate recent improvements in rate constant treatment and resolve discrepancies between experimental data and predictions using recently published kinetic models in dilute, high‐pressure flames. Attempts are made to identify major remaining sources of uncertainties, in both the reaction rate parameters and the assumptions of the kinetic model, affecting predictions of relevant combustion behavior. With regard to model parameters, present uncertainties in the temperature and pressure dependence of rate constants for HO₂ formation and consumption reactions are demonstrated to substantially affect predictive capabilities at high‐pressure, low‐temperature conditions. With regard to model assumptions, calculations are performed to investigate several reactions/processes that have not received much attention previously. Results from ab initio calculations and modeling studies imply that inclusion of H + HO₂ = H₂O + O in the kinetic model might be warranted, though further studies are necessary to ascertain its role in combustion modeling. In addition, it appears that characterization of nonlinear bath‐gas mixture rule behavior for H + O₂(+ M) = HO₂(+ M) in multicomponent bath gases might be necessary to predict high‐pressure flame speeds within ～15%. The updated model is tested against all of the previous validation targets considered by Li et al. as well as new targets from a number of recent studies. Special attention is devoted to establishing a context for evaluating model performance against experimental data by careful consideration of uncertainties in measurements, initial conditions, and physical model assumptions. For example, ignition delay times in shock tubes are shown to be sensitive to potential impurity effects, which have been suggested to accelerate early radical pool growth in shock tube speciation studies. In addition, speciation predictions in burner‐stabilized flames are found to be more sensitive to uncertainties in experimental boundary conditions than to uncertainties in kinetics and transport. Predictions using the present model adequately reproduce previous validation targets and show substantially improved agreement against recent high‐pressure flame speed and shock tube speciation measurements. Comparisons of predictions of several other kinetic models with the experimental data for nearly the entire validation set used here are also provided in the Supporting Information. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 444–474, 2012     

Falloff curves for the Reaction CH3 + O2 (+ M) --> CH3O2 (+ M) in the pressure range 2-1000 bar and the temperature range 300-700 K

The reaction CH(3) + O(2) (+M) --> CH(3)O(2) (+M) was studied in the bath gases Ar and N(2) in a high-temperature/high-pressure flow cell at pressures ranging from 2 to 1000 bar and at temperatures between 300 and 700 K. Methyl radicals were generated by laser flash photolysis of azomethane or acetone. Methylperoxy radicals were monitored by UV absorption at 240 nm. The falloff curves of the rate constants are represented by the simplified expression k/k(infinity) approximately [x/(1 + x)]F(cent)(1/{1+[(log)(x)/)(N)(]2}) with x = k(0)/k(infinity) F(cent) approximately 0.33, and N approximately 1.47, where k(0) and k(infinity) denote the limiting low and high-pressure rate constants, respectively. At low temperatures, 300-400 K, and pressures >300 bar, a fairly abrupt increase of the rate constants beyond the values given by the falloff expressions was observed. This effect is attributed to a contribution from the radical complex mechanism as was also observed in other recombination reactions of larger radicals. Equal limiting low-pressure rate constants k(0) = [M]7 x 10(-31)(T/300 K)(-3.0) cm(6) molecule(-2) s(-1) were fitted for M = Ar and N(2) whereas limiting high-pressure rate constants k(infinity) = 2.2 x 10(-12)(T/300 K)(0.9) cm(3) molecule(-1) s(-1) were approached. These values are discussed in terms of unimolecular rate theory. It is concluded that a theoretical interpretation of the derived rate constants has to be postponed until better information of the potential energy surface is available. Preliminary theoretical evaluation suggests that there is an "anisotropy bottleneck" in the otherwise barrierless interaction potential between CH(3) and O(2).     

Thermal reduction of NO by H2: Kinetic measurement and computer modeling of the HNO + NO reaction

The kinetics and mechanism of the thermal reduction of NO by H₂ have been investigated by FTIR spectrometry in the temperature range of 900 to 1225 K at a constant pressure of 700 torr using mixtures of varying NO/H₂ ratios. In about half of our experimental runs, CO was introduced to capture the OH radical formed in the system with the well-known, fast reaction, OH + CO → H + CO₂. The rates of NO decay and CO₂ formation were kinetically modeled to extract the rate constant for the rate-controlling step, (2) HNO + NO → N₂O + OH. Combining the modeled values with those from the computer simulation of earlier kinetic data reported by Hinshelwood and co-workers (refs. [3] and [4]), Graven (ref.[5]), and Kaufman and Decker (ref. [6]) gives rise to the following expression: . This encompasses 45 data points and covers the temperature range of 900 to 1425 K. RRKM calculations based on the latest ab initio MO results indicate that the reaction is controlled by the addition/stabilization processes forming the HN(O)NO intermediate at low temperatures and by the addition/isomerization/decomposition processes producing N₂O + OH above 900 K. The calculated value of k₂ agrees satisfactorily with the experimental result. © 1995 John Wiley & Sons, Inc.     

Non-RRKM dynamics in the CH3O2 + NO reaction system

Quasi-classical trajectory (QCT) calculations on a model potential energy surface (PES) show strong deviations from statistical Rice-Ramsperger-Kassel-Marcus (RRKM) rate theory for the decomposition reaction (1) CH3OONO* --> CH3O + NO2, where the highly excited CH3OONO* was formed by (2) CH3O2 + NO --> CH3OONO*. The model PES accurately describes the vibrational frequencies, structures, and thermochemistry of the cis- and trans-CH3OONO isomers; it includes cis-trans isomerization in addition to reactions 1 and 2 but does not include nitrate formation, which is too slow to affect the decay rate of CH3OONO*. The QCT results give a strongly time-dependent rate constant for decomposition and damped oscillations in the decomposition rate, not predicted by statistical rate theory. Anharmonicity is shown to play an important role in reducing the rate constant by a factor of 10 smaller than predicted using classical harmonic RRKM theory (microcanonical variational transition state theory). Master equation simulations of organic nitrate yields published previously by two groups assumed that RRKM theory is accurate for reactions 1 and 2 but required surprising parametrizations to fit experimental nitrate yield data. In the present work, it is hypothesized that the non-RRKM rate of reaction (1) and vibrational anharmonicity are at least partly responsible for the surprising parameters.     

The importance of NO+ (H2O)4 in the conversion of NO+ (H2O)n to H3O+ (H2O)n: I. Kinetics measurements and statistical rate modeling

The kinetics for conversion of NO(+)(H(2)O)(n) to H(3)O(+)(H(2)O)(n) has been investigated as a function of temperature from 150 to 400 K. In contrast to previous studies, which show that the conversion goes completely through a reaction of NO(+)(H(2)O)(3), the present results show that NO(+)(H(2)O)(4) plays an increasing role in the conversion as the temperature is lowered. Rate constants are derived for the clustering of H(2)O to NO(+)(H(2)O)(1-3) and the reactions of NO(+)(H(2)O)(3,4) with H(2)O to form H(3)O(+)(H(2)O)(2,3), respectively. In addition, thermal dissociation of NO(+)(H(2)O)(4) to lose HNO(2) was also found to be important. The rate constants for the clustering increase substantially with the lowering of the temperature. Flux calculations show that NO(+)(H(2)O)(4) accounts for over 99% of the conversion at 150 K and even 20% at 300 K, although it is too small to be detectable. The experimental data are complimented by modeling of the falloff curves for the clustering reactions. The modeling shows that, for many of the conditions, the data correspond to the falloff regime of third body association.     

Selten‐Erd‐Metall‐Koordinationspolymere: Synthese und Kristallstrukturen von fünf neuen Adipinaten, [M2(Adi)3(H2O)4](AdiH2)(H2O)4 (M = La,Nd), [Er(Adi)(H2O)5]Cl(H2O) und [M(Adi)(H2O)5](NO3)(H2O) (M = Gd,Er)

Rare‐Earth‐Metal Coordination Polymers: Synthesis and Crystal Structures of Five New Adipinates, [M₂(Adi)₃(H₂O)₄](AdiH₂)(H₂O)₄ (M = La, Nd), [Er(Adi)(H₂O)₅]Cl(H₂O) and [M(Adi)(H₂O)₅](NO₃)(H₂O) (M = Gd, Er) The new rare‐earth compounds [M₂(Adi)₃(H₂O)₄](AdiH₂)(H₂O)₄ (M = La ( 1 ), Nd ( 2 )), [Er(Adi)(H₂O)₅]Cl(H₂O) ( 3 ) and [M(Adi)(H₂O)₅](NO₃)(H₂O) (M = Gd ( 4 ), Er ( 5 )) were obtained from the reaction of adipinic acid with La(OH)₃·xH₂O, Nd₂O₃, ErCl₃·6H₂O, Gd(NO)₃·xH₂O and Er₂O₃, respectively. Their crystal structures were determined by single‐crystal X‐ray diffraction. The coordination polymers [M₂(Adi)₃(H₂O)₄](AdiH₂)(H₂O)₄ crystallize in the triclinic space group (no. 2) with a = 875.4(1), b = 1000.4(2), c = 1179.0(2) pm, α = 74.70(1), β = 69.85(1), γ = 86.18(2)° and Z = 1 (crystal data for M = La, ( 1 )). The quasi‐isostructural compounds [Er(Adi)(H₂O)₅]Cl(H₂O) ( 3 ) and [M(Adi)(H₂O)₅](NO₃)(H₂O) (M = Gd ( 4 ), Er ( 5 )) crystallize with monoclinic symmetry, space group C2/c (no. 15) with lattice parameters of a = 1231.5(1), b = 1532.6(1), c = 895.4(1) pm, β = 123.44(1)° and Z = 4 (crystal data for ( 3 )). The rare‐earth cations have the coordination numbers 10 ( 1 , 2 ) and 9 ( 3 , 4 and 5 ), respectively. The compounds [M₂(Adi)₃(H₂O)₄](AdiH₂)(H₂O)₄ are constructed of infinite chains of edge‐sharig [MO₈(H₂O)₂] polyhedra that are cross‐linked by adipinic acid molecules to form framework structures. In [Er(Adi)(H₂O)₅]Cl(H₂O) ( 3 ) and [M(Adi)(H₂O)₅](NO₃)(H₂O) (M = Gd ( 4 ), Er ( 5 )) the central cations are bridged by adipinic acid molecules in a bidentate‐chelating manner to positively charged zigzag chains. Between these the counter ions and crystal water molecules are incorporated.     

Rate data for O + OCS → SO + CO and SO + O3 → SO2 + O2 by a new time-resolved technique

Pulsed laser photolysis of O₃ in a large excess of N₂ has been used to generate O(³P) atoms in the presence of OCS. By observing chemiluminescence from the small fraction of electronically excited SO₂ formed in the reaction of SO with O₃, rate constants of (1.7 ± 0.2) × 10^?14 and (8.7 ± 1.6) × 10^?14 cm³/molecule sec have been determined at 296 ± 4 K for the reactions and In addition, it has been shown that any reaction between SO and OCS has a rate constant 10^?14 cm³/molecule sec.     

Ab initio study of the mechanism of the atmospheric reaction: NO2 + O3 --> NO3 + O2

The atmospheric reaction NO2 + O3 --> NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single-reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5-6.1 kcal mol(-1), in reasonable agreement with experimental data. The calculated reaction enthalpy is -24.6 kcal mol(-1) and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 x 10(-18) cm(3) molecule(-1) s(-1). Both results can be regarded also as accurate predictions of the methodology employed in this article.     

Determination of the rate coefficients for the reactions IO + NO2 + M (air) --> IONO2 + M and O(3P) + NO2 --> O2 + NO using laser-induced fluorescence spectroscopy

Laser-induced fluorescence spectroscopy via excitation of the A2pi(3/2) <-- X2pi(3/2) (2,0) band at 445 nm was used to monitor IO in the presence of NO2 following its generation in the reactions O(3P) + CF3I and O(3P) + I2. Both photolysis of O3 (248 nm) and NO2 (351 nm) were used to initiate the production of IO. The rate coefficients for the thermolecular reaction IO + NO2 + M --> IONO2 + M were measured in air, N2, and O2 over the range P = 18-760 Torr, covering typical tropospheric conditions, and were found to be in the falloff region. No dependence of k1 upon bath gas identity was observed, and in general, the results are in good agreement with recent determinations. Using a Troe broadening factor of F(B) = 0.4, the falloff parameters k0(1) = (9.5 +/- 1.6) x 10(-31) cm6 molecule(-2) s(-1) and k(infinity)(1) = (1.7 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) were determined at 294 K. The temporal profile of IO at elevated temperatures was used to investigate the thermal stability of the product, IONO2, but no evidence was observed for the regeneration of IO, consistent with recent calculations for the IO-NO2 bond strength being approximately 100 kJ mol(-1). Previous modeling studies of iodine chemistry in the marine boundary layer that utilize values of k1 measured in N2 are hence validated by these results conducted in air. The rate coefficient for the reaction O(3P) + NO2 --> O2 + NO at 294 K and in 100 Torr of air was determined to be k2 = (9.3 +/- 0.9) x 10(-12) cm3 molecule(-1) s(-1), in good agreement with recommended values. All uncertainties are quoted at the 95% confidence limit.     

Kinetics of the reaction O(3P) + SO2 + M → SO3 + M over the temperature range of 299°–440°K

Rate constants for the reaction O(³P) + SO₂ + M have been determined over the temperature range of 299°–440°K, using a flash photolysis–NO₂ chemiluminescence technique. For M?Ar, the Arrhenius expression was obtained. At room temperature k₂^Ar = (1.05 ± 0.21) × 10^?33 cm⁶/molec²·sec. In addition, the rate constants k₂ = (1.37 + 0.27) × 10^?33 cm⁶/molec²·sec, k₂ = (9.5 ± 3.0) ± 10^?33 cm⁶/molec²·sec, k₃ = (1.1 ± 0.2) ± 10^?31 cm⁶/molec²·sec, and k₃ = (2.6 ? 0.9) ± 10^?31 cm⁶/molec²·sec were obtained at room temperature where k₃^M is the rate constant for the reaction O + NO + M → NO₂ + M. The rate data are compared and discussed with literature values.     

Kinetic modeling of the CH3 + C2H2 reaction data with sensitivity analyses

Kinetic modeling and sensitivity analyses for the reaction CH₃ + C₂H₂ → CH₃C₂H₂ (4) have been performed according to the experimental conditions and results of Mandelcorn and Steacie, Garcia Dominguez and Trotman–Dickenson, and Holt and Kerr. The kinetically modeled results show that Mandelcorn and Steacie overestimated the rate constant of reaction (4) whereas Garcia Dominguez and Trotman-Dickenson underestimated it, and that there could be significant uncertainty in the steady-state treated results of Holt and Kerr. Reanalysis of Garcia Dominguez and Trotman–Dickenson's experimental data by kinetic modeling with the proper mechanism gives a more reliable rate constant for reaction (4). The improved rate constant (k₄) is in good agreement with our theoretically predicted values. © 1995 John Wiley & Sons, Inc.     

Hexachloroplatinates of the Lanthanides: Syntheses and Thermal Decomposition of [M(NO3)2(H2O)6]2[PtCl6]·2H2O (M = La,Pr) and [M(NO3)(H2O)7][PtCl6]·4H2O (M = Gd,Dy)

The reaction of the nitrates M(NO₃)₃·6H₂O (M = La, Pr) and (H₃O)₂PtCl₆ led to yellow single crystals of [M(NO₃)₂(H₂O)₆]₂[PtCl₆]·2H₂O (M = La, Pr) (monoclinic, P2₁/c, Z = 2, La/Pr: a = 697.4(3)/695.5(1), b = 1654.5(1)/1652.5(2), c = 1317.7(6)/1318.5(3) pm, β = 93.97°(7)/93.93°(2), R_all = 0.0169/0.0659) while the reaction of M(NO₃)₃·5H₂O (M = Gd, Dy) and (H₃O)₂PtCl₆ yielded yellow single crystals of [M(NO₃)(H₂O)₇][PtCl₆]·4H₂O (monoclinic, P2₁/n, Z = 4, Gd/Dy: a = 838.72(3)/838.40(2), b = 2131.98(6)/2139.50(7), c = 1142.63(3)/1143.10(3) pm, β = 95.670(4)/95.698(3), R_all = 0.0475/0.0337). The crystal structures consist of octahedral [PtCl₆]^2? anions and complex [M(NO₃)₂(H₂O)₆]₂⁺ and [M(NO₃)(H₂O)₇]²⁺ cations, respectively. The thermal decomposition of both types of compounds leads via various steps to elemental platinum and the oxide chlorides MOCl (M = La, Pr, Gd, Dy).