Microwave Spectrum, Structure, Barrier to Internal Rotation, and Dipole Moment of the Aziridine−Borane Complex (C₂H₅N−BH₃)

Abstract

The microwave spectrum of the aziridine-borane complex (C(2)H(5)N-BH(3)) has been investigated by microwave spectroscopy in the 18-80 GHz spectral region. The spectra of the ground vibrational state and three vibrationally excited states have been assigned, and the vibrational frequencies of these states have been determined. The complex was found to have a symmetry plane (C(s) symmetry) formed by the N-B bond and the bisector of the aziridine ring. The dative N-B bond is found to be as short as 161.5 pm in MP2/aug-cc-pVTZ calculations. The MP2 structure of the aziridine ring of this complex is nearly the same as that in the substitution structure of aziridine. Complex formation therefore appears to have little influence on this moiety. The dipole moment was determined to be mu(a) = 17.72(11), mu(b) = 0.0 (by symmetry), mu(c) = 5.70(8), and mu(tot) = 18.62(11) x 10(-30) C m [5.581(34) D]. The barrier to internal rotation of the borane group was determined to be 12.1(3) kJ/mol using the microwave splitting method. The microwave investigation has been augmented by high-level quantum chemical calculations at the MP2/aug-cc-pVTZ and B3LYP/6-311++G** levels of theory. There is generally good agreement between the experimental results and quantum chemical predictions.

High Resolution Millimeter-Wave Spectroscopy of Cyclopropylphosphine–Borane

“…For example, in the case of H 3 N−BH 3 , the barrier height was determined to be 8.565(38) kJ/mol, 6 whereas for C 2 H 5 N−BH 3 , the barrier height of 12.1(3) kJ/mol has been reported. 7 As it was found by Nemeth et al, on the basis of quantum chemical study, two stable conformations may exist for C 3 H 5 PH 2 −BH 3 . Their rotational isomerism is shown in Figure 1.…”

“…This value is comparable with the value of 4.13 uA 2 found for aziridine-borane. 7 The inverse value of inertia moment F 0 was therefore fixed to 122.576 GHz. The angle between the principal axis z and the internal rotation (angle δ) is fixed to the calculated value of 113.4°.…”

Direct Experimental Observation of in situ Dehydrogenation of an Amine–Borane System Using Gas Electron Diffraction

“…A was synthesised using the following procedure adapted from the literature. 44 In a two-necked roundbottomed flask equipped with a stirring bar and a nitrogen inlet, borane-dimethylsulfur complex solution (5 mL of 1 M sol., 5.0 mmol) was slowly added to a cooled (-30 °C) solution of the cyclic amine (0.215 g, 5.0 mmol) in dry dichloromethane (5 mL). The reaction mixture was allowed to warm to room temperature and was stirred for 5 min at this temperature.…”

“…32,41 Literature on the molecular structures of the cyclic amine boranes is scant as only aziridine-borane has been investigated as part of an earlier XRD 42 and theoretical 43 study, and in a later MWS investigation. 44 Cyclic amine boranes should be able to act as electron donors in a push-pull interactions, leading to the release of a hydrogen molecule. 45 Properties associated with the hydrogen storage capabilities of ammonia borane and its cyclic analogues such as aziridine-borane and azetidineborane were investigated by means of Fourier transform ion cyclotron resonance (FTICR) spectrometry.…”

“…The literature on the molecular structures of the cyclic amine boranes is scant as only aziridine–borane has been investigated as part of an earlier XRD and theoretical study, and in a later MWS investigation . Cyclic amine boranes should be able to act as electron donors in push–pull interactions, leading to the release of a hydrogen molecule .…”