Thermochemistry of molecules in the B/F/H/N system

Abstract

ABSTRACT:A self-consistent set of thermochemical data for 88 chemical species in the B/F/H/N system is obtained from ab initio electronic structure calculation. Calculations were performed for both stable and radical species. The quantities calculated include the atomization energy (¥ D 0 ), heat of formation (⌬H f ) at 0 K and 298.15 K, and bond dissociation energies (BDE) for all species. Good agreement is found between the calculation data and experimental or theoretical reference data for the quantities analyzed in this work for several species containing B, F, H, and N atoms. We also present a compilation of experimental and theoretical atomization energies (16 experimental ϫ 26 theoretical data), heat of formation at 0 K (25 experimental ϫ 26 theoretical data), heat of formation at 298 K (29 experimental ϫ 36 theoretical data), and bond dissociation energy for several species. Polynomial fits of the predicted thermodynamic data (heat capacity, entropy, and enthalpy) over the 200 -6000 K temperature range are also included, for the 88 species. The species analyzed in this study are important in a kinetic mechanism for growth boron nitride films in a Chemical Vapour Deposition (CVD) reactor. It is generally difficult to optimize conditions in a CVD reactor because films properties depend on complex interactions involving heat and mass transport, chemical kinetics, and thermochemistry. Developing a reliable set of thermodynamic data is a necessary first step for system optimization, since it provides important constraints on the possible reaction mechanism. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem 103: 659 -684, 2005

The structure and energetics of triplet [B, C, F, H2]

“…As it was for the analogous singlets [1], this preference can be rationalized primarily on the basis of the greater strength of the B-F bond compared to that of the B-H bond [9,[12][13][14][15]42]. The stabilities of 5 t-H(F)CBH (6 c-H(F)CBH) relative to 1 H 2 CBF and of S2 H(F)CBH relative to S1 H 2 CBF are not greatly different in magnitude (195 kJ/ mol vs. 250 kJ/mol, this work and Ref.…”

“…Most of the above articles include a discussion of trends in sequential bond dissociation enthalpies and/or trends in bond dissociation enthalpies along the periodic table. Comparison of B-X bond dissociation enthalpies, X = H, C, F, Cl, Br, I, in analogous compounds has shown that (1) BDHs decrease down the group from F to I [9,11], (2) B-H and B-C bond strengths are similar in magnitude but much smaller than B-F bond strengths [9,[12][13][14][15], (3) the strengths of B-H and B-C bonds tend to be less dependent on the other boron substituents than do boron-halogen bonds [9,13,14], and (4) replacing a halogen substituent with hydrogen generally increases the BDHs of boron-halogen bonds [9,11,14]. However, because sequential adiabatic BDHs often have large fluctuations resulting from reorganization energy in the product fragments, the authors of some of these articles suggest that intrinsic bond strengths should be compared by evaluating diabatic BDHs [9,12,17] or the electron density at the bond critical point [15].…”

“…Barreto et al. [14] have provided polynomial fits to the thermochemical data they computed for a series of chemical species, important in the growth of boron nitride thin films, containing B, H, N and F atoms. Bond dissociation enthalpies of a wider range of molecules, those involving all possible A-X single bonds between first-and second-row atoms, have been evaluated by Mó , et al [15].…”

“…More computational work has been carried out on some of the boron-containing fragments relevant to the [B, C, F, H 2 ] species. Much of the recent work has focused on the thermochemical properties of these fragment species, in particular their atomization energies, enthalpies of formation, bond dissociation enthalpies BDHs, and excitation energies [8][9][10][11][12][13][14][15][16]. Most recently, Grant and Dixon [9] have reported these thermochemical data for H (3Àn) BX n compounds for which X is F, Cl, Br, I, NH 2 , OH and SH (where 1 n 3), using the composite ab initio molecular orbital theory approach Feller et al are developing, which allows them to calculate thermochemical In the current paper, we discuss our high level quantum chemical results for the structure and energetics of triplet (and hence open-shell) isomers corresponding to the stoichiometry of one boron, carbon, and fluorine apiece, and two hydrogens.…”

Theoretical study of the reactions BF3 + BX, where X = H or N

“…This mechanism is constantly upgraded, including new reactions and new species. Necessary input to these complex models is the thermodynamic data [6] for the species in the kinetic mechanisms, and the elementary chemical reactions among all the species with the respective rate constant. These elementary chemical reactions have the rate constant determined through experimental work or transition state theory [7,8], as in the case of this work.…”