C. R. Quade

Microwave Spectrum of Benzaldehyde

The microwave absorption spectrum of C6H5CHO has been investigated in the region 17.5–40.0 GHz and that for C6H5CDO in the region 26.5–40.0 GHz. Rotational transitions in the ground and first three excited torsional states in both molecular species have been identified. There is evidence for interaction of the second torsional state of C6H5CHO with another mode of vibration of the molecule. Accurate relative intensity measurements give the first torsional frequency of 113.8 ± 5.0 cm−1 for C6H5CHO and of 108.4 ± 4.25 cm−1 for C6H5CDO. The barriers to internal rotation of the aldehyde group calculated from these frequencies are 4.90 ± 0.43 and 5.28 ± 0.42 kcal/mole for C6H5CHO and C6H5CDO, respectively. The discrepancy between the observed and calculated values of inertial coefficients suggests the importance of vibration–rotation interaction in the theory of internal rotors with twofold potential barriers.

Microwave spectrum of ethyl mercaptan

The microwave spectrum of the trans and gauche isomers of the normal and two isotopic species (CH3CH2SD and CH2DCH2SH) of ethyl mercaptan has been studied. Identification of the spectrum of the trans isomer of the normal and sym‐CH2DCH2SH molecules has been extended up to J=19, enabling preliminary centrifugal distortion constants to be determined. From a and c dipole transitions full sets of rotational constants of the gauche isomer of all species have been determined. For the normal molecule they are A=28 746.37 MHz, B=5294.85 MHz, and C=4846.96 MHz. The tunneling frequency of the thiol top between its two equivalent gauche configurations has been measured directly for the normal species (1754.09 MHz), sym‐CH2DCH2SH (1790.05 MHz), and CH3CH2SD (70.40 MHz). Identification has been made of excited thiol torsion satellites in both isomers. Dipole moments of all configurations for each species have been determined using second‐order Stark effects. For the trans isomer of the normal molecule they are μa=1.06...

Internal Rotation in Two‐Top Molecules. I. A General Theory

The dynamics is developed for internal rotation in two‐top molecules where both internal rotors may, or may not, be asymmetric. A framework fixed axis method (FFAM) is used for the formulation. The explicit α1 and α2 dependence of the moments of inertia is obtained for over‐all rotation, internal rotations, and coupling between over‐all and internal rotations. The Hamiltonian in proper Hermitian form is derived. Inertial parameters and rotational coefficients are calculated for two representative molecules.

Some Effects of Vibration and Molecular Structure on Internal Rotation in CH2DSH and CHD2SH

The J = 1 ← 0, ΔK = 0 lines have been assigned in the microwave rotational spectra for the partially deuterated methyl mercaptans. The fine structure of the transitions for the torsional ground state has been analyzed on the basis of internal rotation and molecular structure. It is necessary to introduce a V1 cosα term into the effective potential energy for internal rotation to account for the relative splittings. Although it is not possible to determine a unique value for V1, the limits 1.5 cm−1 ≤ | V1 | ≤ 10 cm−1 are obtained. From the splittings it is possible to determine an improved value of 3°5′ for the so‐called C–S tilt with respect to the methyl axis. The staggered configuration for the thiol–methyl orientation is found to be the minimum in the threefold potential energy.

Interaction of Microwave Radiation with a Polar Gas under Conditions of Power Saturation

The propagation of microwave radiation through a gas‐phase dielectric under conditions of power saturation is considered. It is shown that the nonuniform polarization gives rise to additional modes of electromagnetic oscillation. The amplitudes of the induced electric field are estimated and shown to be small. The results of the calculation substantiate that Harringtons method of averaging the power distribution in determining the power density distinction function φwg is correct for application to microwave spectroscopy.