A mass spectrometric investigation of B-N bond association energies in the amine-borane system



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A four part study of boron-nitrogen bond dissociation energies in a series of borane adducts of trimethylamine is reported. The first part involves a study of the low temperature thermal properties (290[degrees]K to 320[degrees]K) of trimethylamine-borane. The quantities measured include enthalpy, free energy, entropy, and the dissociative equilibrium constant for the reaction 2(CH[lowered 3])[lowered 3]NBH[lowered 3] ---> 2(CH[lowered 3])[lowered 3]N + B[lowered 3]H[lowered 6]. The B-N bond dissociation energy was calculated and found to be in good agreement with measurements made by mass spectrometry. The 70, 15 and 10 eV mass spectra of (CH[lowered 3])[lowered 3]N, (CH[lowered 3])[lowered 3]NBH[lowered 3], (CH[lowered 3])[lowered 3]NBD[lowered 3], (CH[lowered 3])[lowered 3]NBH[lowered 2]F, (CH[lowered 3])[lowered 3]NBH[lowered 2]Cl, (CH[lowered 3])[lowered 3]NBH[lowered 2]Br, (CH[lowered 3])[lowered 3]NBH[lowered 2]I, (CH[lowered 3])[lowered 3]NBHBr[lowered 2], (CH[lowered 3])[lowered 3]NBHF[lowered 2], (CH[lowered 3])[lowered 3]NBF[lowered 3], (CH[lowered 3])[lowered 3]NBCl[lowered 3] and (CH[lowered 3])[lowered 3]NBBr[lowered 3] were recorded. Metastable peaks common to all spectra were attributed to transitions involving only the trimethylamine portion of the adducts. Fragmentation schemes were not proposed due to the lack of supportive evidence. Trends in ion abundances are discussed. A computer of average transients is developed, and discussed, for appearance potential measurements where sample pressure fluctuations are a problem. Its applicability is tested by measuring the molecular ionization potential for trimethylamine-borane and the B-N bond dissociation energy for trimethylamine-borane and trimethylamine-trifluoroborane. A comparison in experimental results is made between Morrison's second differential method and Warren's extrapolated voltage difference technique. A systematic study of boron-nitrogen bond dissociation energies in compounds of the form (CH[lowered 3])[lowered 3]NBH[lowered 3-y]X[lowered y] (X represents F, Cl, Br, I) was undertaken. The state of the BX[lowered 3] moiety (sp[raised 3] vs sp[raised 2]) at the completion of clevage was investigated and it appears that dissociation occurs without complete reorganization. Therefore, a term representing the reorganization energy was not included in calculation of bond dissociation energies A comparison of methyl proton ((CH[lowered 3])[lowered 3]NBH[lowered 3-y]X[lowered y]) chemical shifts and boron-nitrogen bond strengths were made. It showed that nmr chemical shifts cannot be depended on to reflect relative adduct strengths except where very large differences occur such as with (CH[lowered 3])[lowered 3]NBF[lowered 3] and (CH[lowered 3])[lowered 3]NBBr[lowered 3].