J. H. Goldstein

Microwave Spectrum, Structure, and Dipole Moment of s‐trans Acrolein

The microwave spectrum of acrolein has been studied in the 19–37 kmc region. A series of R‐type transitions was identified for the ground and three excited torsional states of the s‐trans isomer and these spectra were successfully fitted in the rigid rotor approximation. Using the ground state moments and additional electron diffraction data a reasonable though not unique structure has been obtained. The torsional frequency was found to be 200±50 cm—1 from relative intensity measurements, and a lower limit of about 2300 cal was estimated for the barrier hindering trans‐cis conversion. The dipole moment for the ground state was found to be 3.11±0.04 d, oriented about 14° from the carbonyl group. The results obtained and the absence of a strong s‐cis spectrum establish the s‐trans isomer as the predominant form.

Quadrupole Coupling and Bond Character in the Vinyl Halides

Quadrupole coupling data obtained from microwave spectroscopy are utilized in assessing double‐bond character in planar molecules such as the vinyl halides. The approximations involved and the effect of structural uncertainties have been considered in detail. An uncertainty of 20% or less is estimated for the calculated values of δ, the number of halogen py electrons lost as a result of double bonding. Application of the method to vinyl chloride and vinyl iodide yields δ values of 0.06 and 0.03, respectively. A revised value of −72 Mc for (eQq)bond in vinyl chloride is reported. The present results are compared with conclusions based on other molecular properties.

Microwave Spectrum and Dipole Moment of Pyridine

The microwave spectrum of pyridine has been studied in the region from 20 000 to 40 000 Mc. Twelve low‐J R‐branch lines have been identified. Analysis of the spectrum requires that the dipole moment lie in the a axis, and leads to the following values of rotational constants: a=6039.436 Mc, b=5804.997 Mc, c=2959.210 Mc, and κ=+0.847781. The dipole moment of pyridine vapor was found to be 2.15±0.05 D from quantitative Stark effect studies.

The Microwave Spectrum of Vinyl Cyanide

The rotational spectrum of the slightly asymmetric top, vinyl cyanide, has been investigated in a microwave spectrograph employing Stark effect modulation and in a direct absorption cell. The rotational constants a = 49 076.2 Mc/sec, b = 4971.33, and c = 4514.05 were computed from the observed frequencies of the J = 2→3 transitions. From measurements of the Stark effect, the components of the dipole moment were found to be μa = 3.68D, μb = 1.25, μ = 3.89. Partial resolution of the hyperfine structure of certain lines gave the value —3.0 Mc/sec for the ξaa component of quadrupole coupling.

The Microwave Spectra of the Deuterated Methyl Halides

The hyperfine structure lines of the pure rotational absorption transitions J=0→1 for CD3Cl and J=1→2 for CD3Br and CD3I, all in the ground vibrational state, have been measured in frequency and yield the following values of ν0 in Mc/sec and IB in g cm2×10−40, respectively: for CD3Cl35, 21683.75 and 77.3764; for CD3Cl37, 21316.86 and 78.7081; for CD3Br79, 30858.29 and 108.743; for CD3Br81, 30724.93 and 109.215; for CD3I, 24161.13 and 138.885. The quadrupole coupling coefficient, eqQ, has been determined for each of these molecules and its variation from eqQ for the corresponding normal methyl halide compound is considered.

Microwave Spectrum of Propiolic Aldehyde

The microwave spectrum of propiolic aldehyde has been studied in the 18—37 Kmc region. Analysis of the ground state spectrum provides the following rotational constants: b=4826.31±0.03 Mc, c=4499.75 ±0.03 Mc, and δ=0.00594±0.0030 Mc, consistent with a planar model. Strong vibrational satellites were observed and fitted, and approximate values of the two lowest vibrational frequencies were found from intensity ratios to be roughly 150 cm—1 and 230 cm—1. The dipole moment of the ground vibrational state is 2.46±0.04D, as determined from quantitative Stark effect studies.

Simple Molecular Orbital Theory of Conjugation in Vinyl Bromide and Bromobenzene

Quadrupole coupling constants for vinyl bromide, supplemented by other data of spectroscopic origin, have been utilized to obtain a value for the resonance integral, βCBr, in simple molecular orbital theory with inclusion of overlap. Using this parameter, resonance dipole moments were calculated for vinyl bromide and bromobenzene, in reasonable agreement with experiment. Assumptions and approximations involved in the method are reviewed, and the properties of carbon‐halogen bonds are discussed in the light of the results of this and similar previous studies.

Molecular Orbital Treatment of Conjugation in Vinyl Chloride, with Inclusion of Overlap

Simple molecular orbital theory, with inclusion of all overlap effects, has been applied to the problem of conjugation in vinyl chloride. Using the value of —2.45 ev for the carbon‐chlorine exchange integral, —10.64 ev for αC1 as calculated from atomic ionization potentials, and other parameters assumed equal to the ethylene values, values of 9.99 ev and 0.063 are calculated for the ionization potential and chlorine py‐electron defect resulting from conjugation. These results are in good agreement with the experimental values, 9.95 ev and 0.06, respectively. A convenient distribution matrix treatment is described and some of the effects of neglect of overlap are discussed.

Microwave Spectrum and Properties of Vinylene Carbonate

The microwave spectrum of vinylene carbonate has been investigated in the region from 19 000 to 32 000 Mc/sec. Eight R‐branch transitions have been identified and fitted with the following spectroscopic parameters: a = 9346.79 Mc/sec, b = 4188.46 Mc/sec, c = 2891.54 Mc/sec, all ±0.10 Mc/sec; k = —0.59818. The dipole moment of the vapor, as determined from Stark effect studies, is 4.51±0.05D and lies in the a‐axis. The molecule has been shown to be planar. The significance of these results is discussed.

Simple Molecular Orbital Treatment of Conjugation in Chlorobenzene

Simple molecular orbital theory, with inclusion of overlap, has been used to calculate the electron distribution and energy levels in chlorobenzene. Spectroscopic parameters were used, supplemented by the value of βCCl previously obtained from quadrupole coupling data for vinyl chloride. Resonance dipole moments were also calculated for chlorobenzene and vinyl chloride. The results are in reasonable agreement with the observed dipole moment decreases and ionization potential shifts, assuming these to be chiefly brought about by conjugation.