Synthesis, Microwave Spectrum, Quantum Chemical Calculations,
and Conformational Composition of a Novel Primary Phosphine, Cyclopropylethynylphosphine,
(C<sub>3</sub>H<sub>5</sub>CCPH<sub>2</sub>)
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Abstract
The microwave spectrum of cyclopropylethynylphosphine,
C<sub>3</sub>H<sub>5</sub>CCPH<sub>2</sub>, has been investigated
in the
26–120 GHz spectral region. The spectrum is dominated by very
rich and complex <i>a</i>-type <i>R</i>-branch
pile-ups. There must be insignificant steric interaction between the
phosphino group and the cyclopropyl ring due to the long distance
between these two groups. However, the phosphino group does not undergo
free or nearly free internal rotation. Instead, the spectra of two
distinct conformers were assigned. Both these two forms have <i>C</i><sub>S</sub> symmetry. The symmetry plane bisects the cyclopropyl
ring and the phosphino group in both conformers, and the lone electron
pair of the phosphino group points in opposite directions in the two
rotamers. The energy difference between the two forms was determined
to be 1.9(6) kJ/mol. A simple model that takes into consideration
the interaction of the lone electron pair of the phosphino group with
the π-electrons of the ethynyl group and the Walsh electrons
of the cyclopropyl ring is able to give a qualitative explanation
of the observation of two conformers and the nonexistence of free
rotation of the phosphino group. The MW work was augmented by quantum
chemical calculations using second-order Møller–Plesset
perturbation and coupled cluster theory with results that are in good
agreement with the experiments