Structural and conformational properties of 2-propynylphosphine (propargylphosphine) as studied by microwave spectroscopy supplemented by quantum chemical calculations

The microwave spectrum of 2-propynylphosphine (propargylphosphine); H-C dro pC-CH2-PH2, has been investigated in the 18-26.5 and 32-48 GHz spectral reg ions sit about -50 degreesC. Two conformers with different orientation:of t he phosphino-group, denoted conformer I-and conformer Il, respectively, wer e: assigned. Conformer I has a symmetry plane (C-s symmetry) with both hydr ogen atoms of the phosphino group pointing toward the triple bond (C-C-P-H dihedral angles approximately 47 degrees from syn-periplanar (0 degrees)). The C-C-P-H dihedral angles are 73 and 167 degrees, respectively, from syn- periplanar in,conformer II.,Only one of the hydrogen atoms of the phosphino group points toward the triple bond in this rotamer. Conformer I is 1.5(20 ) kJ/mol more stable than II. The dipole moment of LI was determined to be tin units of 10(-30) C m) mu (a) = 0 (assumed), mu (b) = 3.05(7), mu (c) = 1.60(9), and mu (tot) = 3.44(9) [mu (tot) = 1.03(3) D]. Two vibrationally e xcited states were assigned for each of the two rotamers I and II. Their fr equencies were determined by relative intensity measurements. Many of the t ransitions of conformer II were split into two components presumably becaus e of tunneling of the phosphino group. The tunneling frequency was determin ed to-be 0.814(42) MHz for the ground vibrational state and 11.49(18) MHz f or the first excited state of the C-P torsional vibration. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311++G(3df, 2pd) basis set reproduced experimental rotational constants, quartic centri fugal distortion constants, and dipole moment components within a few perce nt. The energy difference between the two conformers was calculated using t he Gaussian-2 theory, and conformer I was found to be more stable than conf ormer II by 2.1 kJ/mol.