Diborene: Generation and Photoelectron Spectroscopy of an Inorganic Biradical

Abstract

Diborenes, R-BB-R′, are of current interest in inorganic chemistry because they offer the opportunity to tune the properties of a biradical by modifying the substituents of the diborene parent, HBBH. Here we synthesize the elusive diborene by H atom abstraction from diborane, B2H6, using fluorine atoms and report a vibrationally resolved photoelectron spectrum of the HBBH biradical. The spectrum is interpreted by comparison with high-level ab initio computations, taking into account the Renner–Teller splitting in the X+ 2Π ionic ground state, which show an excellent agreement with the experimental spectrum. An adiabatic ionization energy of 9.080 ± 0.015 eV was determined, and a vibrational progression in the boron–boron stretching vibration of 0.14 eV is visible. This is due to the reduction of bond order upon ionization, accompanied by an increase of the computed boron–boron bond length, R BB, from 1.514 to 1.606 Å.

Photoelectron spectroscopy of boron-containing reactive intermediates using synchrotron radiation: BH2, BH, and BF

“…The detailed setup was described elsewhere. 24,64 In brief, commercially available diluted B 2 H 6 (0.5% in Ar) was introduced in a flow reactor through a quartz injector tube, which can slide into the reactor tube. The generated F atoms were fed through a side arm attached directly with the reactor.…”

“…The SPES spectrum of the most prominent one, the biradical diborene has been reported recently. 24 Due to its IE of 11.12 eV, BF is only present at the highest photon energies, cf. trace a).…”

“…Note that for HBBH a SPES has already been reported. 24 SPES of BH 2 . The slow photoelectron spectrum (SPES) recorded for 11 BH 2 is depicted in trace a) of Figure 2.…”

“…They are not perturbed by possible side products, in contrast to conventional photoelectron spectroscopy and permit to study reactive species ranging from carbon-containing free radicals [14][15][16][17][18][19][20] to carbenes, 21 biradicals [22][23][24][25] and reactive boron compounds. 24,26,27 In recent years PEPICO has also evolved as a reliable analytical tool to detect elusive intermediates in reactive environments such as model flames 28 , catalytic reactors 29 or in kinetic experiments. 30,31 In its ground electronic state (X 2 A 1 ) BH 2 is a bent, near prolate asymmetric top (C 2v ) with bond angle ~129⁰, while in the first excited state (A 2 Π u ) it is linear (D ∞h ) with slightly shorter B-H bonds.…”

Photoionization studies of reactive intermediates using synchrotron radiation

“…However, essential experimental and theoretical information will be given, as required, in the Case Studies considered. In section 3, possible directions of future work are discussed by summarising recent photoelectron-photoion coincidence (PEPICO) and threshold photoelectron-photoion coincidence (TPEPICO) studies on reactive intermediates from other groups, see Table 2 refs (44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)(58)(59)(60)(61). The information derived from the studies described in sections 2 and 3 is compared with a view to combining the advantages of both types of measurement in future studies.…”