The millimetre-wave rotational spectra of the excited vibrational state v10=2 of the symmetric top molecule, CF3CCD, have been recorded for J′′=12 up to J′′=25. The l=±2 and l=0 series have been assigned and the spectra analysed to give rotational parameters including xll=7716.975MHz. The main interactions between states of different l are the rt(2,−1)=0.158MHz and qt+(2,2)=3.308MHz. Two type of l-resonance are identified, one of which is due to an avoided crossing between the l=0 and l=+2 series. The spectra are qualitatively similar to the corresponding ones of CF3CCH.

The millimeter-wave rotational spectra of the excited vibrational states ν10 = 3 and ν10 = 4 of the symmetric top species 3,3,3-trifluoropropyne (trifluoromethylacetylene), CF3CCH, have been recorded for J″ = 16 up to J″ = 21. The various l branches have been assigned and the spectra analyzed to give rotational parameters and the vibrational anharmonicity, xll = 8135.83 MHz for ν10 = 3 and 8056.1 MHz for ν10 = 4. The main interactions between states of different l are due to the rt(2, −1) and q+t(2,2) interaction terms. Two types of l-resonance are identified, one of which is due to an avoided crossing between the l = 0 and l = 2 series which is similar to that found for the ν10 = 2 state previously investigated. Both states have been successfully analyzed as isolated systems.

The millimeter-wave rotational spectra of the ground state of the asymmetric-top species CF3C35Cl237CI have been recorded and assigned for J″ = 30 up to J″ = 55 and analyzed to yield the following rotational and centrifugal distortion constants: A = 1304.712(1.408) MHz, B = 1106.599(24) MHz, C = 1093.877(25) MHz, DJ = 0.05530(4) kHz, and DJK = 0.02417(44) kHz. The results have been used to examine the force field of trifluorotrichloroethane.

The millimeter-wave rotational spectra of the molecules (CH3)3C79Br and (CH3)3C81Br in an excited vibrational state have been recorded and analyzed. The spectra have been assigned to a low lying doubly degenerate vibrational mode. Moderate l-resonance interactions are present in both molecules, but at one particular J an accidental resonance is found which enables a determination of the axial rotational constant Av = 4519.4 MHz for (CH3)3C79Br and 4524.3 MHz for (CH3)3C81Br.

Millimetre-wave rotational spectra have been recorded for the molecules (CH3)(3CBr)-Br-79, (CH3)(3CBr)-Br-81 and (CH3)(3CI)-I-127. In addition, we have recorded low-J spectra at much greater precision using a Fourier-transform spectrometer. The combined results yield rotational, centrifugal distortion, quadrupole and spin-rotation constants which are more accurate than any determined independently.

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The millimeter-wave rotational spectrum of CF3C35Cl3 has been recorded. For the ground state, the results are in agreement with previous work; the very small value of DJK results in no measurable k-splitting. The rotational spectra of the torsional states have been recorded for v6 = 1 to v6 = 5. In addition, the rotational spectrum in the lowest doubly degenerate excited state has been analysed to yield values of the l-resonance parameters qi+ = 0.7474(6) MHz and Aζt = 283.2(2) MHz.

The splittings due to the three chlorine quadrupoles, observed in the millimeter-wave rotational spectra of Si35Cl3H in the ground and ν6 = 1 excited vibrational states, have been analyzed and the quadrupole parameters determined. In particular, splittings observed for the kl − 1 = −1 lines in ν6 = 1 are explained in terms of the asymmetry parameter ηQ. The value obtained is consistent with the electric field gradient at the 35Cl nuclei having cylindrical symmetry about the SiCl bond.

Millimeter wave spectra have been recorded for the excited vibrational state v6 = 2 of OPF3. A full analysis of these spectra yields new rovibrational parameters and also the vibrational separation of the l = 0 and l = 2 levels, given by xll = 16 134 ± 390 MHz. The results are compared with the v6 = 1 state.

The theory for radio frequency-millimetre wave double resonance for a four level system is presented and discussed with reference to the v 6 = 1 excited vibrational state of the molecule NSF3. The rf transitions are formally Δk = 3 and thus give information on the A rotational constant of this molecule.

The millimeter wave rotational spectra of P35Cl3 have been recorded for the excited vibrational states v2 = 1 and v4 = 1. The analysis of the latter yields ξ44c = −0.738(12) and resolves a dilemma in fitting the harmonic force field.

The rm method of determining molecular structures from isotopic zero-point rotational constants is tested by applying it to synthetic data for OCS calculated from an assumed equilibrium structure and force field. The results show that the rm structure is significantly different from the re structure, unless the substitution coordinates used in the rm calculation are corrected for the use of finite changes of mass Δmi by extrapolation to Δmi = 0. The latter procedure is not usually feasible experimentally. In the ordinary rm method the errors are generally comparable in magnitude to those in Costain's rg method. An exception arises when the number of structural parameters equals the number of independent moments of inertia of a single isotope (e.g., a diatomic molecule or a symmetrical XY2 molecule), in which case the rm structure is in good agreement wich the re structure.

The millimeter rotational spectra of PF3 have been recorded for all four excited fundamental vibrational states. A coriolis resonance between ν1 and ν3 results in anomalous distortion constants for these states and can be analyzed to yield |ζ13y| = 0.392. The value of ζ33c is found to be 0.4194 which does not fit the coriolis sum rule due to anharmonic resonance. Equilibrium values for the centrifugal distortion constants are determined and have been used to find the harmonic force field. All available data is used to determine some of the anharmonic force constants.

The millimetre wave rotational spectra of AsF3 have been recorded for all four fundamental vibrationally excited states. The analysis of these states has yielded values for the coriolis constants, ζ33 c = 0·2434(11), ζ44 c = -0·5308(12). Extrapolation has also yielded equilibrium values for the centrifugal distortion constants, DJ e = 4·554(6) kHz, DJK e = -5·861(24) kHz. These data have been used to obtain the harmonic force field. From the four observed α s B and the qt + constants, some estimate has been made of the anharmonic force field. It has been used to obtain C e from the observed C 3. This, together with the observed B e, yields the equilibrium structure r e = r e = 1·7041(10) Å, αe = 95° 46′ (7′).