Revised thermochemistry of gaseous ammonium nitrate, NH4NO3(g).

In an earlier paper (Hildenbrand, D. L., Lau, K. H., and Chandra, D. J. Phys. Chem. B 2010, 114, 330.), we detected the presence of NH4NO3(g) in the vapor over the solid nitrate and evaluated its partial pressure along with those of HNO3 and NH3. The molecular constants of the gaseous nitrate were estimated in the absence of experimental values in an attempt to derive its enthalpies of sublimation and formation. After publication, we became aware of the sought molecular data, evaluated primarily from high-level theoretical calculations, and revised the analysis to yield Δ(f)H298°(NH4NO3(g)) = −61.8 ± 5 kcal mol⁻¹.

Thermochemistry of gaseous ammonium nitrate, NH4NO3(g).

From the measured molecular weight of vapor over NH(4)NO(3)(c) by the simultaneous torsion-effusion/mass loss method, the partial pressure of NH(4)NO(3)(g) was evaluated over the range 321-360 K; in addition, the presence of the gaseous nitrate was verified by effusion-beam mass spectrometry. Third-law calculations of the sublimation enthalpy of NH(4)NO(3) were made with estimated molecular constants, yielding DeltaH(298)(o)(sub) = 20.9 +/- 2 kcal mol(-1), as compared to a less-reliable second-law value of 23.2 +/- 3 kcal mol(-1). We prefer the third-law value, which leads to the enthalpy of formation Delta(f)H(298)(o) (NH(4)NO(3)(g)) = -66.5 +/- 2.1 kcal mol(-1) when combined with the known Delta(f)H(298)(o) value of NH(4)NO(3)(c).

Low-lying electronic states and revised thermochemistry of TiCl, TiCl2, and TiCl3.

Thermochemical analyses of gaseous equilibrium data involving the species TiCl, TiCl(2), and TiCl(3) were revised with the aid of more recent information on the low-lying electronic states, yielding more reliable values for the enthalpies of formation, Delta(f)H(o)(298), of 40.9, -49.0, and -121.5 kcal mol(-1), all +/-2 kcal mol(-1), for gaseous TiCl, TiCl(2), and TiCl(3), respectively. The new thermochemical data are in good agreement with the results of recent theoretical calculations, but they differ from earlier experimental results involving the reactions of Ti (s), TiCl(2)(s, g), TiCl(3)(s, g) and TiCl(4)(g).

Thermochemistry of the gaseous vanadium chlorides VCl, VCl2, VCl3, and VCl4.

Gaseous equilibria in the V-Ag-Cl system were studied at elevated temperatures by effusion-beam mass spectrometry, where the pertinent species were generated by reaction of Cl 2(g) with V + Ag granules in the effusion cell source. Reaction enthalpies were derived from the equilibrium data, and the standard enthalpies of formation at 298 K of gaseous VCl, VCl2, and VCl3 were found to be +49.7, -34.8, and -85.6 kcal mol(-1), respectively. The corresponding bond dissociation energies at 298 K are D(V-Cl) = 102.9 kcal, D(ClV-Cl) = 113.5 kcal, D(Cl2V-Cl) = 79.8 kcal, and D(Cl3V-Cl) = 69.5 kcal. From these data, the dissociation energy D degrees 0(VCl) = 101.9 kcal mol(-1) or 4.42 eV is obtained. An alternate value, Delta(f)H(o)298(VCl 3,g) = -87.0 kcal mol (-1) was derived from third-law analysis of literature sublimation data for VCl3(s). In addition, literature thermochemical data on VCl4(g) were re-evaluated, leading to Delta(f)H(o)298 = -126.1 kcal mol (-1). The results are compared with various estimates in the literature.

Low-lying electronic states and dissociation energies of the monochlorides of Cr, Mn, Fe, Co, and Ni.

Published equilibrium data involving the gaseous monochlorides of Cr, Mn, Fe, Co, and Ni have been re-examined by thermochemical analysis, using more recent information on the low-lying electronic states, yielding D degrees 0 values in kcal mol-1 of CrCl (90.0), MnCl (79.8), FeCl (79.3), CoCl (81.3), and NiCl (88.1). Although this revised approach is believed to yield more reliable values of the FeCl, CoCl, and NiCl dissociation energies, results show that use of M+ electronic levels in place of the adopted MCl values leads to alternate D degrees 0(MCl) values agreeing within 1.6 kcal mol-1, providing a useful check on electronic-level contributions to the thermochemical calculations.

Dissociation energies of the Cu and Ag monohalides and of Ni monofluoride.

Gaseous isomolecular equilibria of the type CuX + Ag = Cu + AgX, where X = F, Cl, Br, and I, were studied by effusion-beam mass spectrometry at elevated temperatures, and the differences between the dissociation energies of the CuX and AgX molecular species were determined with relatively high accuracy from thermochemical analysis of the equilibrium data. Analysis of literature data, plus new information on AgBr, yielded accurate values of D degrees 0 in kcal mol(-1) for CuF (102.0), AgCl (74.4), AgBr (66.4), and AgI (59.7), from which values were derived for AgF (81.5), CuCl (89.6), CuBr (79.2), and CuI (69.4), all +/-1 kcal mol(-1). The result is a consistent set of dissociation energies for all eight of the Cu and Ag monohalides that will be useful in checking the reliability of quantum chemical calculations for these molecular species containing elements of increasing atomic number. Also, the isomolecular exchange equilibrium between CuF and NiF was studied in a similar fashion, leading to D degrees 0(NiF) = 104.4 +/- 1.4 kcal mol(-1).

Composition and thermochemistry of silver bromide vapor.

The mole fractions of AgBr and Ag3Br3 in the saturated vapor at 840 K have been evaluated from the vapor mass spectrum, by comparison with the corresponding spectrum of AgCl vapor, where the monomer/trimer ratio is known accurately from vapor molecular weight measurements. Combination of these results with new measurements of the vapor pressure of molten AgBr by the torsion-effusion method in the range 805-936 K yielded the third law enthalpies of vaporization and the standard enthalpies of formation DeltafH degrees 298(AgBr, g) = 27.8 +/- 0.3 kcal mol(-1) and DeltafH degrees 298(Ag3Br3, g) = -19.0 +/- 1 kcal mol(-1). The dissociation energy, D degrees 0(AgBr), is found to be 66.4 +/- 0.3 kcal mol(-1), or 2.88 +/- 0.01 eV, some 3.5-5 kcal mol(-1) lower than previous literature values. Approximate thermochemical stabilities of the dimer species Ag2Cl2 and Ag2Br2 have also been evaluated.

Thermochemical properties of the gaseous bromo-iodides of Dy and of the Na-Dy tetrahalo complexes.

The gaseous mixed dimers NaDyBr4 and NaDyI4 and the bromo-iodide species DyBrI, DyBr2I, DyBrI2, NaDyBr3I, NaDyBr2I2, and NaDyBrI3 were generated in an effusion cell reactor at elevated temperatures using several different source configurations and were identified by mass spectrometry. A number of gaseous equilibria involving these species were studied, and thermochemical properties were derived with the aid of thermal functions based on estimated molecular constants. Enthalpies of formation of NaDyBr4 and NaDyI4 are compared with values in the literature, while results for the bromo-iodides are in close accord with values interpolated from data on the pure metal bromides and iodides.