Synthesis and Microwave Spectrum of (2-Chloroethyl)phosphine (ClCH₂CH₂PH₂)

The synthesis of a 2-halogenoethylphosphine, (2-chloroethyl)phosphine (ClCH₂CH₂PH₂), is reported for the first time. This compound was prepared by a chemoselective reduction of diethylchloroethylphosphonate with dichloroalane using a vacuum line. (2-Chloroethyl)phosphine has been studied by microwave spectroscopy at room temperature, or at −20 °C, in the 22−80 GHz spectral interval. The experimental study has been augmented by quantum chemical calculations at the MP2/6-311++(3df,3pd) and B3LYP/6-311++(3df,3pd) levels of theory. The spectra of two rotameric forms, denoted I and II have been assigned. These conformers both have an antiperiplanar arrangement for the Cl−C−C−P chain of atoms, but with different orientation of the phosphine group. Conformer I was found to be 5.2(6) kJ/mol more stable than II by relative intensity measurements. The spectra of the first excited states of the C−C torsional vibration of both I and II were assigned. The torsional frequency was determined to be 63(20) cm⁻¹ for conformer I by relative intensity measurements, and it has a similar value in II. It was found that this normal vibration deviates from a near-harmonic behavior. The quantum chemical calculations produce rotational and centrifugal distortion constants that are in satisfactory agreement with observations, but fail to predict correctly low-frequency fundamental frequencies. The quantum chemical calculations indicate that three additional conformers characterized by a synclinal orientation for the Cl−C−C−P link of atoms and with different orientation of the phosphine group are high-energy forms of ClCH₂CH₂PH₂. Searches for these forms were unsuccessful.