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 only from the rs coordinates and make no attempt to reproduce the I0 values, the structure is virtually independent of the isotopic species used in the determination. The variation in the structures obtained from different sets of isotopic species appears to depend only on the uncertainties in the rotational constants. In addition, the accuracy is independent of the mass of the substituted atom, and therefore H atoms are located just as accurately as the heavier atoms. The rs structures of HCN, N2O, OCS, CH3CCH, CH3CN, CH3NC, and the methyl halides have been determined by the isotopic substitution method. For the first three molecules comparisons can be made with equilibrium data, and it is found that the rs bond lengths are no more than 0.002 A greater than the re. The primary interest in the other molecules studied is in the structure of the methyl group. The anomalous separation of the H and D coordinates previously found is readily explained and does not appear in the rs structures. The consistency of the results is such that comparisons between molecules can be made with confidence.

Abstract. We present maps of the 22 MHz radio emission between declinations −28• and +80• , covering ∼73% of the sky, derived from observations with the 22 MHz radiotelescope at the Dominion Radio Astrophysical Observatory (DRAO). The resolution of the telescope (EW × NS) is 1.1• × 1.7• secant(zenith angle). The maps show the large scale features of the emission from the Galaxy including the thick non-thermal disk, the North Polar Spur (NPS) and absorption due to discrete H ii regions and to an extended band of thermal electrons within 40• of the Galactic centre. We give the flux densities of nine extended supernova remnants shown on the maps. A comparison of the maps with the 408-MHz survey of Haslam et al. (1982) shows a remarkable uniformity of spectral index (T ∝ ν −β ) of most of the Galactic emission, with β in the range 2.40 to 2.55. Emission from the outer rim of the NPS shows a slightly greater spectral index than the distributed emission on either side of the feature. The mean local synchrotron emissivity at 22 MHz deduced from the emission toward nearby extended opaque H ii regions is ∼1.5 10 −40 Wm −3Hz −1 sr −1 , somewhat greater than previous estimates.

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The microwave spectra of ten isotopic species of formamide (H2N–CHO) have been investigated. Type a and type b transitions have been identified and measured for all ten species. The inertial defect is found to decrease whenever a heavier isotope is substituted for any atom in the NH2 group, and in fact is negative for four of the species studied. It is concluded that the molecule is nonplanar with the H2N–C group forming a shallow pyramid. The structural parameters deduced from the rotational constants are: r(N–H′, where H′ is trans to the aldehyde hydrogen) = 1.014±0.005 A, r(N–H″, where H″ is cis to the aldehyde hydrogen) = 1.002±0.005 A, r(N–C) = 1.376±0.010 A, r(C–H) = 1.102±0.010 A, r(C=O) = 1.193±0.020 A, ∠H′NH″ = 118°53′ ±40′, ∠H″NC = 120°37′±40′, ∠H′NC = 117°9′±40′, ∠NCO = 123°48′±40′, ∠NCH = 113°14′±40′ and ∠OCH = 122°58′±40′. The dihedral angles between the H′NC plane and the NCO plane, and between the H″NC plane and the NCH plane, are 7°±5°, and 12°±5°, respectively. In all the spectra investigated each line was accompanied by a vibrational satellite line of anomalously high intensity. Measurements of relative intensities were carried out for H2N14–CHO, cis- and trans-HDN14–CHO, and D2N14–CHO. The energy levels (above the zero point) deduced from these measurements show a very large isotope shift, and are interpreted as being due to the first excited state of the NH2 wagging frequency. A pyramidal model for formamide will have two equilibrium configurations separated by a potential barrier. With a Manning type potential, a barrier of 370±50 cm−1, hindering the ``inversion-wagging'' type of motion, is determined. The equilibrium value of the normal coordinate calculated from the Manning potential is found to be in good agreement with that found in the structure determination.

The theory of internal rotation in molecules is extended to the case of two internal symmetric rotors attached to a molecule with C2v symmetry. The calculation of the internal rotational energy levels and of the interaction with the over-all rotational energy levels is discussed both for the high barrier and the low barrier approximation. The symmetry of the Hamiltonian for a molecule with the acetone structure is considered and the selection rules and relative intensities of the various fine structure components are derived. The microwave spectra of acetone and of completely deuterated acetone have been measured and assigned. From the rotational constants, the molecular structure has been partially determined. The most probable values of the structural parameters are r0(C=O) = 1.215 A (assumed), r0(C–C) = 1.515±0.005 A, r0(C–H) = 1.086±0.010 A, ∠CCC = 116°14′±1°, and ∠HCC = 110°16′±1°. The angle between the axes of the methyl groups is approximately 3° larger than the ∠CCC. The dipole moment has been determined to be 2.90 D by a measurement of the Stark effect on the microwave transitions. Each of the rotational transitions is split into a number of components by the internal rotation. From an analysis of these patterns the barrier height to internal rotation is found to be 274 cm—1 for (CH3)2CO and 258 cm—1 for (CD3)2CO. The difference in the two calculated barrier heights and the errors in the method are discussed.