Accurate Heats Of Formation Research Articles

Ab initio characterization of HBrO 2 isomers: implications for stratospheric bromine chemistry

Equilibrium geometries, dipole moments, harmonic vibrational frequencies, and IR intensities are determined for the BrOOH, HOBrO, and HBrO 2 molecules using the CCSD(T) correlation method in conjunction with a TZ2P one-particle basis set. Accurate heats of formation are evaluated using the CCSD(T) method together with spdfg one-particle basis sets. BrOOH is the lowest energy isomer, although HOBrO is predicted to be less than 5 kcal/mol higher in energy. The torsional barrier for trans-BrOOH is 2.9 kcal/mol (0 K), while for cis-BrOOH it is 5.4 kcal/mol. BrOOH is shown to be stable with respect to thermal dissociation and implications for stratospheric bromine chemistry are discussed.

An evaluation of the performance of G2, G2(MP2) and G2(MP2,SVP) theories for heats of formation and heats of reaction in the case of 'large' hydrocarbons

Heats of formation for benzene, cyclohexane and other selected hydrocarbons, along with the heats of several isodesmic and isogyric reactions involving these species, are reported at the G2,G2(MP2) and G2(MP2,SVP) levels of theory. All three models predict heats of isodesmic reactions very accurately. G2 heats of isogyric non-isodesmic reactions are also reliable, but G2(MP2,SVP) heats of such reactions (e.g. hydrogenation) can be significantly in error. In the case of 'large' hydrocarbons, an accumulation of small component errors can cause G2,G2(MP2) and (surprisingly to a lesser extent) G2(MP2,SVP) heats of formation evaluated from atomization energies to be unreliable. However, the accumulated error in the heat of formation can be estimated in advance. In cases where the predicted error is large, more accurate heats of formation can generally be obtained with the help of isodesmic (and to a lesser degree isogyric) reactions rather than by following the standard procedure based on heats of atomization.

Calculated structures and fluoride affinities for fluorides

It is shown that SCF-MO calculations provide good estimates of the energies of the processes MF/sub n/ ..-->.. M/sup n+/ + nF/sup -/ where M/sup n+/ is an ion of a first- or second-row element in a closed-shell or s/sup 2/ configuration. The fluoride ion affinities are then calculated for a number of molecules and ions. Where comparison with experiment is possible, the agreement is generally good when allowance is made for experimental uncertainties. In favorable cases, accurate heats of formation may be calculated from fluoride affinities.