Heats of Formation for Tin-Containing Compounds

Representative heats of formation predicted by the ECP/BAC-MP4 method are given in Table 1 (the complete published [88] thermodynamic data set used in the analyses below is available online [91]). Data are shown for a range of compoimds, including tetravalent, trivalent, and divalent coordination at tin. Values for the reference compounds SnCU, SnH4, and Sn(CH3)4 are also given. Finally, heats of formation for atoms and groups needed to calculate reaction enthalpies are given. These results are used in the analysis below to identify potential reaction pathways for MBTC and its decomposition products. 

Note that the error estimates obtained from Eq. 19 are, in the majority of cases, less than 2kcalmol This indicates that the computational method is converging well from the lowest level of perturbation theory through to the highest. The only exceptions to this are the divalent singlet radicals (compounds of the form XSnY), in which there is a nonzero correction for UHF instability. In these cases, the ad hoc error estimation method indicates a higher level of uncertainty because the presence of UHF instability is an indication that the MP4(SDTQ) level of theory is not fully adequate to describe the electronic ground state of the molecule (the same is true of a nonzero spin contamination correction). 

In contrast, methyl-for-chlorine substitution is decidedly nonlinear, a feature also displayed by the lighter Group 14 compounds. This curvature is not an artifact of the BAC-MP4 predictions, since it is observable in the (admittedly limited) experimental data for these compounds (Fig. 10). In fact, the deviations from linearity are even greater in the experimental data. Such behavior is also observed in the analogous Si compounds and is related to the negative hypercongugation (anomeric) effect, in which electron density from 

Results of applying the CCSD(T) method to selected tin compounds are also given in Table 1. Again, there are almost no data available in the literature for comparison. However, the predicted heat of formation for SnO is in reasonable agreement with experiment. Since data for Sn - O species are so limited, it is difficult to fully validate this model chemistry. Thus, we placed relatively high uncertainties on the calculated values. Nevertheless, we are sufficiently confident of the results to use them to establish BAG parameters for Sn - OH bonds. The resulting BAC-MP4 predictions as well as the 

CCSD(T) results should be sufficiently accurate to allow qualitative evaluation of reaction pathways involving them. 

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