The microwave spectra of five further isotopic species of nitric acid, H18ONO2, cis-HON18OO, trans-HONO18O, cis-HO15N18OO, and D15NO3, have been investigated. The data have been used to refine the previous structural determination of Millen and Morton. In the calculation of structure, particular consideration has been given to the use of ground-state moments of inertia in Kraitchman's equations for near-oblate planar molecules. The structure was calculated using several different methods, including the double-substitution method due to Pierce, in order to substantiate the following structural parameters: O–H=0.964, (H)O–N=1.406, cis–N–O=1.211, trans-N–O=1.199 Å, ∠HON=102°9′, ∠cis-ONO=115°53′, ∠trans-ONO=113°51′, and O···O (NO2 group)=2.184 Å. This determination clearly demonstrates a 2° tilt of the NO2 group away from the hydrogen atom. The NO2 parameters are close to those found in a number of related molecules, but there is some evidence that the cis-NO bond in HNO3 is slightly longer than the trans-NO bond. The NOH configuration is very similar to that found in formaldoxime. Further evidence for the exact planarity of nitric acid is given. Stark-effect measurements limit the out-of-plane component of the dipole moment μC to a value less than 1.1×10−3 D. The other components have been redetermined to have the values μA=1.99 D and μB=0.88 D, giving a total dipole moment μ=2.17±0.02 D.

New ENDOR measurements on the single substitutional nitrogen centre in diamond are reported. The CW-ENDOR mechanism utilizes cross relaxation, and measurements have been made on both 14N and 15N, as well as the first detailed 13C ENDOR study on the isotope at the natural abundance of 1.1%. The change in EPR intensity induced by driving the 13C nuclear transitions was approximately equal to 100% of the EPR intensity of the appropriate 13C hyperfine satellite. The assignment of 13C hyperfine coupling matrices with specific sites is considered, and predictions made about the signs of some of the 13C hyperfine interactions.

The microwave spectra of nitromethane and five isotopic species have been investigated Groundstate moments of inertia for the rn = 0 internal rotation state have been used to derive the following structure :Nuclear spin weightings for various I K--m I states, derived using non-rigid group theory, have been confirmed by relative intensity measurements in the microwave spectra. The same theory has been used to explain the unusually large increase (0.060 a.m.u. A") in the inertial defect of CD3N02.The structure and bonding of nitromethane is compared with some related molecules. The structure of the nitro group in nitromethane is shown to fit a series of NOa compounds, between NO; and NO;, where the parameters vary according to the electronegativity of the attached grouping.

The microwave spectra of phenylacetylene, C6H5CCH, the deuterated species, CGHSCCD, and carbon-13 species at positions C(2), C(3), C(4), C(7) and C(8) have been measured and analysed. Inertial defects confirm the expected planarity of the molecule, and pronounced nuclear spin effects in the ground-state and vibrationally excited spectra confirm the C2, symmetry. The detailed structure is determined to be: 208, and C(8) 3) C(4) = 119.9" and C(3)C(4)C(5) = 119.9'. The dipole moment has been accurately determined using a new Stark-effect technique to be p = pa = 0.656+0.005 D and is found to be smaller for the deuterated species by 0.010 k 0.003 D.