Lise Nygaard

Structure of fluorobenzene

Abstract The microwave spectra of four ring-substituted fluorobenzenes have been measured and the molecular structure of fluorobenzene has been determined by the substitution method from the data so obtained in combination with earlier published data for deuterated fluorobenzenes. The result shows a small shortening of the two C-C bonds nearest fluorine, while the rest of the molecule is practically like benzene. The C-F bond length is 1.35 A, as in vinyl fluoride.

Microwave spectra of the six monodeuteriophenols. Molecular structure, dipole moment, and barrier to internal rotation of phenol

Abstract The microwave spectra of the six monodeuteriated phenols have been measured and a partial substitution structure of phenol has been determined. The selection rules for Class III molecules 5 have been derived, and the Stark effect has been discussed. Stark coefficients for [2-D]- and [6-D]phenol have been measured and the dipole moment of phenol deduced, Splitting data for phenol, [ 18 O]phenol, [4-D]phenol, and [7-D]phenol were used to determine the barrier to internal rotation, V 2 = 1175 ± 20 cm −1 .

Microwave spectra of isotopic benzonitriles. Refined molecular structure of benzonitrile

Abstract The microwave spectra of benzonitrile and nine isotopic species have been reinvestigated including Q -branch transitions. The derived rotational constants, corrected for centrifugal distortion, are of higher accuracy than in earlier work. The resulting refined substitution structure of bsnzonitrile shows the same characteristic shortening of the ring bonds nearest to the substituent as in fluorobenzene.

Microwave spectrum and planarity of nitrobenzene

Abstract The microwave spectrum of nitrobenzene has been reinvestigated. Transitions in the ground state and three torsionally excited states have been assigned. It is shown that the nitrobenzene molecule is planar in the ground state. The torsional frequency (50 ± 15 cm −1 ) and the barrier to internal rotation (1000 ±500 cm −1 ) have been derived from the inertial defect and the relative intensity of transitions in torsionally excited states.

Microwave spectra of isotopic pyrazoles and molecular structure of pyrazole

Abstract Microwave spectra of the complete set of monosubstituted isotopic pyrazoles have been investigated and the complete r s structure of pyrazole determined. The deuterated and 15 N species of pyrazole were prepared, while the 13 C species were examined in natural abundance. The spectrum of the parent compound was remeasured yielding rotational and centrifugal distortion constants. The mutual position of the hydrogen atoms and the direction of the dipole moment in the pyrazole principal axis system has been unambiguously established. The 14 N quadrupole hyperfine structure was resolved and the quadrupole coupling constants derived by Blackman et al. [8] were confirmed. An estimated r z structure is compared to crystal structures derived by X-ray and neutron diffraction showing the N-H bond in the crystal to be prolonged. But, otherwise no significant differences seem to develop when bringing the molecule from its isolated state into the crystal with its strong hydrogen bonds.

Microwave spectra of isotopic cyclobutenes: Molecular structure of cyclobutene

Abstract Microwave spectra of cyclobutene and four monosubstituted isotopic species have been recorded and analyzed, yielding a conventional r s -structure ( C 2v symmetry). The carbon ring is planar with valence angles 94.2° (at the double bond) and 85.8°. The C(1)-C(2) distance is 1.342 A, the C(2)-C(3) distance 1.517 A, and the C(3)-C(4) distance 1.566 A, all of these longer than any acyclic normals.

A revised structure of ethyl fluoride

Abstract Microwave data on ethyl fluoride and isotopic species have been re-investigated in an attempt to find a more reliable substitution structure of ethyl fluoride. The result is a C,C distance of 1·505 ± 0·004 A i.e. 0·02 A shorter than in ethane.

Microwave spectra of isotopic thiazoles. Molecular structure and 14N quadrupole coupling constants of thiazole

Abstract Microwave spectra of thiazole and the eight monosubstituted isotopic species have been investigated. For the parent and four substituted species the 14 N hyperfine structure was analyzed and the nuclear quadrupole coupling constants determined. These coupling constants were used to derive center frequencies for all except the 15 N species. The rotational constants were computed by least squares in the rigid rotor approximation. The substitution structure of thiazole is given and discussed. Except for the N,C(4) bond and its nearest angles, the thiazole molecule is very well described as a 1:1 composition of thiophene and 1,3,4-thiadiazole.

Microwave spectra and dipole moments of 1,2-difluorobenzene and 1,3-difluorobenzene

Abstract The microwave spectra of 1,2- and 1,3-difluorobenzene (I and II) have been assigned by quantitative measurements of the Stark effect. I has an α-type spectrum, II a b -type spectrum, both showing the expected intensity alternations from the nuclear spin statistical weights, 28:36 and 10:6, respectively. Rotational constants were computed from the spectra by a least squares method including centrifugal distortion correction. The small inertial defects indicate that both molecules are planar. The dipole moments of I and II derived from measured Stark coefficients are 2·59 ± 0·02 D and 1·51 ± 0·02 D, respectively. The possible structure of the two molecules is discussed and compared to the structure of fluorobenzene.

Microwave Spectrum, Molecular Structure, Barrier to Internal Rotation, and Dipole Moment of Methylketene

The microwave spectra of the CD3 species and of seven monosubstituted isotopic species of CH3CH=C=O have been examined, and a complete structure of methylketene has been obtained by the substitution method. The structural parameters found are: C=O=1.171, C=C=1.306, C–C=1.518, C–H (vinyl)=1.083, and C–H (methyl)=1.083 or 1.11 A (see text). The ketene group is linear, and the methyl group is staggered, ∠CCC=122.6°, ∠C(1)C(2)H(2)=113.7°, and ∠HCH=109.9° or 108.8°.The barrier splittings were used to determine the value of the barrier to hindered rotation and the angle between the methyl‐axis and the principal‐axis system for each of the monosubstituted species; V3=1177±20 cal/mole. The dipole moment of methylketene was measured on the 21,2→31,3transition, yielding μa=1.755, μb=0.35, μ=1.79 D.