Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

The microwave spectra of seven isotopic species of propylene have been studied in order to obtain an accurate molecular structure. The complete rs (substitution) structure has been calculated. The more important parameters are: r(C=C)=1.336±0.004 A,r(C–C)=1.501±0.004 A, ≰CCC=124.3∘±0.3∘. The structure is compared with those of related molecules. It is concluded that no difference can be detected in the double‐bond lengths in ethylene, propylene, and the vinyl halides. The CC single‐bond length in propylene is indistinguishable from that in acetaldehyde and other acetyl compounds, and is 0.025 A shorter than the CC distance in saturated hydrocarbons. In the =CH2 group in propylene, the CH bond trans to the methyl group appears slightly shorter than the cis CH bond; a similar effect occurs in the vinyl halides.

Molecular Structure of Propylene

Structure of fluorobenzene

The normal propyl fluoride molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, trans and gauche. Rotational constants in the ground vibrational state are A = 26 986.73, B = 3748.32, C = 3509.88 Mc/sec for the trans form and A = 14 503.69, B = 5085.71, C = 4295.28 Mc/sec for the gauche form. The dihedral angle of the gauche form is about 63° from the cis position.The barrier height hindering methyl rotation has been determined, using lines in the first excited state of methyl rotation, to be 2.69 kcal/mole and 2.87 kcal/mole for the trans and the gauche forms, respectively. The dipole moment and its components along the principal axes have been determined by the Stark effect: μa = 1.970±0.026, μb = 0.566±0.064, μtotal = 2.050±0.043D for the trans form and μa = 1.137±0.008, μb = 1.450±0.033, μc = 0.472±0.137, μtotal = 1.902±0.064 D for the gauche form. The total dipole moment is nearly parallel to the C–F bond for both forms; the angle between them is 10.6° in the trans...

Rotational Isomerism and Microwave Spectroscopy. I. The Microwave Spectrum of Normal Propyl Fluoride

The microwave spectra of the C12 and C13 species of cis‐difluoroethylene‐d1 and cis‐difluoroethylene‐d2 have been investigated in the region of 8–36 kMc. Effective rotational constants for the ground vibrational state have been determined and are combined with previous microwave data to obtain the structural parameters: rCC=1.324 A, rCH=1.089 A, rCF=1.335 A, ∠FCC=122.1°, ∠HCC=124.0°.The microwave spectrum of vinylidene fluoride has been reinvestigated and the rotational constants for C13 species obtained. These data lead to a structure: rCC=1.315 A, rCH=1.079 A, rCF=1.323 A, ∠FCF=109.1°, ∠HCH=121.8°.A comparison of the structures with data for vinyl fluoride and ethylene shows that fluorine substitution produces a systematic shortening of both the CC and CF bonds.

David R. Lide

Papers

An improved structure determination for vinyl fluoride