Enthalpies of formation and bond energies in lithium, beryllium, and boron derivatives. 2. Dative, single, and triple bonds

Abstract

For a set of small XYH, singlet-state molecules (where X stands for Li, Be, or B and Y is one of the second-row atoms), we have calculated the enthalpies of formation at the MP4(sdtq)/6-31l++G(3df,Zp) level by using MPP(full)/6-31G(d,p) fully optimized structures. At this level of theory, the heats of formation are expected to be in the range of the so-called "chemical accuracy" (fl kcal/mol). Some alkyl derivatives of the previous XYH, compounds have been studied at the RHF/6-31G(d) level (with fully optimized geometries). The theoretical enthalpies of formation reproduce the available experimental results quite satisfactorily. In addition to compounds with usual dative single bonds, we also describe molecules with covalent (nondative) single B-N and B-0 bonds. We use all the available (experimental and theoretical) data to calculate new bond energies for lithium, beryllium, and boron derivatives. IntroductionAlthough many features of electron-deficient compounds have been established by a number of experimental and theoretical investigations, accurate thermochemical properties remain undetermined, at least for the simplest derivatives. Except for LiOH, BeHF, and BH20H, very few enthalpies are known for lithium-, beryllium-, and boronbased materials (XYHJ. Most often, those species cannot be handled easily. For example, BH3*NH3 is stable but not easily vaporized2 and it rapidly decomposes when the temperature increases. BH2NH2 is un~table;~ its lifetime was reported to be several minutes, and it prefers to exist aa cyclic oligomers or polymers: BHNH may have a fairly long lifetime under arc discharge5 but is difficult to observe under other conditions. Therefore, direct determination of the energy content for such species is not easy. Nevertheless, a few alkyl and fluoro derivatives of the previously mentioned XYH, molecules have been investigated with success.From a theoretical point of view, difficulties arise because correlation effects account for a significant part of the energy content and can become a dominant part of reaction heats? Therefore, it is not surprising to observe large discrepancies in the calculated dissociation energies depending on the theoretical method in use. This has specially been pointed out for dib0rane(6)~J and for molecules with weak dative bonds. For example, the dissociation energy of ammonia-borane is predicted to be 46 kcal/mol at the HF/6-31G level, 21 kcal/mol at the HF/6-31G* level,s and 35 kcal/mol when the correlation energy is taken into a c~o u n t .~The aim of this work is to calculate accurate ( f l kcal/mol) heats of formation for the smallest lithium-,

Designing kinetically stable aryltrifluoroborates as 18F‐capture agents for PET imaging

“…For most of the 18 F-ArBF 3 tracers we imaged, bone uptake was approximately 1% ID/g or lower over an imaging window of 60-120 min. Such values are consistent with the rate of solvolytic defluoridation determined by 19 F-NMR spectroscopy and not dissimilar to values obtained with standard radiotracers labeled by multi-step C-18 F bond forming reactions. Because defluoridation in vivo parallels that seen by 19 F-NMR spectroscopy, it is unlikely that the ArBF 3 is metabolized.…”

“…With all rate constants expected to be much greater than k 1 , intermediates 2-5 will not accumulate and will go undetected by 19 F-NMR spectroscopy, in which case the solvolytic reaction reduces to a first-order process where k obs~k1 . [20] As the 19 F-fluorine nucleus is very sensitive to nuclear magnetic excitation, 19 F-NMR spectroscopy provided a sensitive means for assaying solvolytic stability aryltrifluoroborates and possibly detecting low levels of steady state intermediates. Indeed use of 19 F-NMR spectroscopy confirmed the ephemeral nature of intermediates 2-5 as only starting material and fluoride ion were observed.…”

“…[20] As the 19 F-fluorine nucleus is very sensitive to nuclear magnetic excitation, 19 F-NMR spectroscopy provided a sensitive means for assaying solvolytic stability aryltrifluoroborates and possibly detecting low levels of steady state intermediates. Indeed use of 19 F-NMR spectroscopy confirmed the ephemeral nature of intermediates 2-5 as only starting material and fluoride ion were observed. This finding has important implications in terms of producing tracers at high radiochemical purity: once the labeling reaction is quenched in buffer, any mono-and difluoroborate intermediates that may form under acidic conditions are destroyed.…”

“…Consistent with this model, EWG substituents at all positions slow the rate of solvolysis. A Hammett plot of ArBF 3 À solvolysis rates, obtained by 19 F-NMR, gave log(k/k H ) = -σ (ρ~À1). [20] Furthermore, the σ (or similarly the σ + -values) of all EWG substituents could be summed to give a predicted solvolysis rate, which followed the same overall Hammett relationship.…”

On the effect of water and oxygen in chemical vapor deposition of boron nitride

“…5, the presence of water leads rapidly to the formation of species with B-O bonds in the gas phase. The higher covalent bond strength for B-O bonds, compared to B-N bonds [23] suggests that once the B-O bonds are formed, it is unlikely that the oxygen will be replaced by nitrogen. It is further well known that boron has a strong affinity towards oxygen, which gives rise to the rich chemistry of the borates [24].…”