Structural and Conformational Properties of 1,2-Propadienylphosphine (Allenylphosphine) Studied by Microwave Spectroscopy and Quantum Chemical Calculations

Microwave and Quantum Chemical Study of the Hydrazino Group as Proton Donor in Intramolecular Hydrogen Bonding of (2-Fluoroethyl)hydrazine (FCH2CH2NHNH2)

The microwave spectrum of (2-fluoroethyl)hydrazine (FCH2CH2NHNH2) was studied in the 11-123 GHz spectral region to investigate the ability of the hydrazino group to form intramolecular hydrogen bonds acting as a proton donor. This group can participate both in five-member and in six-member internal hydrogen bonds with the fluorine atom. The spectra of four conformers were assigned, and the rotational and centrifugal distortion constants of these rotameric forms were determined. Two of these conformers have five-member intramolecular hydrogen bonds, while the two other forms are without this interaction. The internal hydrogen bonds in the two hydrogen-bonded forms are assumed to be mainly electrostatic in origin because the N-H and C-F bonds are nearly parallel and the associated bond moments are antiparallel. This is the first example of a gas-phase study of a hydrazine where the hydrazino functional group acts as a proton donor in weak intramolecular hydrogen bonds. Extensive quantum chemical calculations at the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD/cc-pVQZ levels of theory accompanied and guided the experimental work. These calculations predict the existence of no less than 18 conformers, spanning a CCSD internal energy range of 15.4 kJ/mol. Intramolecular hydrogen bonds are predicted to be present in seven of these conformers. Three of these forms have six-member hydrogen bonds, while four have five-member hydrogen bonds. The three lowest-energy conformers have five-member internal hydrogen bonds. The spectrum of the conformer with the lowest energy was not assigned because it has a very small dipole moment. The CCSD relative energies of the two hydrogen-bonded rotamers whose spectra were assigned are 1.04 and 1.62 kJ/mol, respectively, whereas the relative energies of the two conformers with assigned spectra and no hydrogen bonds have relative energies of 6.46 and 4.89 kJ/mol.

Microwave spectrum, and conformational composition of (chloromethyl)phosphine (ClCH2PH2)

(Chloromethyl)phosphine, (ClCH(2)PH(2)) has been studied by microwave spectroscopy at -30 °C in the 22-80 GHz spectral interval. The experimental study has been augmented by quantum chemical calculations at the MP2/aug-cc-pVQZ and B3LYP/aug-cc-pVTZ levels of theory. The spectra of the ground as well as of several vibrationally excited states of the (35)ClCH(2)PH(2) and (37)ClCH(2)PH(2) isotopologues of two rotameric forms, denoted I and II, have been assigned. These conformers have different orientations of the phosphino group. I has a symmetry plane, consisting of the Cl-C-P link of atoms, whereas the phosphino group is rotated out of this symmetry plane in II. Conformer I was found to be 4.3(5) kJ/mol more stable than II by relative intensity measurements. The rotational and quartic centrifugal distortion constants calculated using the MP2/aug-cc-pVQZ procedure are in very good agreement with their experimental counterparts. Less good agreement is found in the B3LYP/aug-cc-pVTZ calculations. Both computational procedures predict energy differences between I and II that are close to the experimental energy difference. It is suggested that I is the preferred form of this molecule because it is stabilized by weak intramolecular hydrogen bonding between the chlorine atom and the hydrogen atoms of the phosphino group. Repulsion between the lone electron pair of the phosphorus atom and the chlorine atom also stabilizes I relative to II.

Synthesis, microwave spectrum, and conformational equilibrium of propa-1,2-dienethiol (H(2)=C=CHSH)

The first synthesis of the kinetically unstable compound propa-1,2-dienethiol (allenethiol; H(2)CCCHSH) is reported. Its microwave spectrum has been studied in the 41.5-80 GHz spectral range. The spectra of two rotameric forms have been assigned. The C-C-S-H chain of atoms is synperiplanar (0 degrees ) in one of the conformers. This dihedral angle is anticlinal in the second rotamer forming an angle of 140(5) degrees in the second form. The synperiplanar conformer is found to be 1.0(6) kJ/mol more stable than the anticlinal rotamer. The microwave study has been augmented by quantum chemical calculations at the MP2/aug-cc-pVTZ and B3LYP/6-311++G** levels of theory. The predictions of these two theoretical methods are in excellent agreement with the experimental findings.

Conformational Composition of Cyclopentadienylphosphine Investigated by Microwave Spectroscopy and Quantum Chemical Calculations

The properties of cyclopentadienylphosphine have been investigated by means of Stark-modulation microwave spectroscopy and quantum chemical calculations at the MP2/aug-cc-pVTZ, B3LYP/6-311++G(d,p), and G3 levels of theory. Spectra attributable to two rotamers denoted conformers I and II have been assigned. Conformer I has a symmetry plane (Cs symmetry) consisting of the bisectors of the cyclopentadiene ring and of the phosphino group with the lone electron pair of phosphorus pointing toward the carbon ring. In conformer II, the phosphino group is rotated approximately 120 degrees out of this plane. Relative intensity measurements have been made, and it was found that conformer II is more stable than I by 1.3(4) kJ/mol. The preferred conformer represents a borderline case of intramolecular hydrogen bond stabilization. The experimental and MP2/ aug-cc-pVTZ rotational constants differ by several percent, which indicates that the aug-cc-pVTZ basis set is not large enough to be able to predict an accurate structure for the two conformers that are close to the equilibrium geometries. 5-Substituted 1,3-cyclopentadienyl derivatives may undergo circumambulatory rearrangements. However, there is no manifestation of this effect in the microwave spectrum of cyclopentadienylphosphine.