Georg Ole Sørensen

Molecular structure and internal motion of formamide from microwave spectrum

Abstract The molecular structure of formamide has been redetermined by observing the microwave spectra of the 13 C and 18 O species of NH 2 CHO and ND 2 CHO. The r s structure calculated assuming the planarity of the molecule is NH c = 1.001 6 ± 0.003 A, NH t = 1.001 5 ± 0.003 A, CN = 1.35 2 ± 0.012 A, CO = 1.21 9 ± 0.012 A, CH a = 1.09 8 ± 0.01 A, ∠H c NH t = 121.6° ± 0.3°, ∠H c NC = 118.5° ± 0.5°, ∠H t NC = 120.0° ± 0.5°, ∠NCO = 124.7° ± 0.3°, ∠NCH a = 112.7° ± 2°, and ∠OCH a = 122.5° ± 2°. The strongest satellite of the rotational spectrum is assigned to the first excited state of the amino inversion. The far-infrared spectra observed by King are assigned to the v = 1 ← 0, 2 ← 1, and 3 ← 2 transitions of the inversion, and the v = 1 ← 0 and 2 ← 1 transitions are detected also for the ND 2 CHO species. The observed vibrational frequencies are used to determine the two potential constants V 2 = 156 ± 34 cm −1 /rad 2 and V 4 = 398 ± 29 cm −1 /rad 4 in the potential function to the inversion of the form, V = V 2 τ 2 + V 4 τ 4 where τ is the inversion angle of the amino group. The molecule is thus essentially planar without any potential hump at the planar configuration. The changes of the inertia defect by the excitation of the inversion have been calculated approximately for NH 2 CHO and ND 2 CHO in fair agreement with the observed.

Microwave spectra of [15n] and [13c] pyridines, quadrupole coupling constants, dipole moment and molecular structure of pyridine

Abstract The microwave spectra of two ring-substituted species of pyridine have been remeasured, and the spectra of the two remaining species investigated and assigned. 14 N quadrupole coupling splittings were analyzed and used to determine the quadrupole coupling constants, while the derived center frequencies were fitted to give improved values for the rotational constants. A complete substitution structure can now be presented. The dipole moment of pyridine was redetermined from Stark measurements in the spectrum of [ 15 N]pyridine.

Microwave spectra of pyridine and monodeuterated pyridines. Revised molecular structure of pyridine

Abstract The microwave spectra of pyridine and its 2-D, 3-D and 4-D substituted species were reinvestigated. Nuclear quadrupole coupling and centrifugal distortion effects were considered also for the deuterated species resulting in improved values for the rotational constants of these species. Hereby more reliable information about the hydrogen positions could be obtained.

Centrifugal distortion analysis of microwave spectra of asymmetric top molecules. The microwave spectrum of pyridine

Abstract Several microwave investigators have experienced the great difficulties involved in the determination of the six linearly independent constants appearing in the first-order treatment of the centrifugal distortion effect. Ambiguity specially arises when dealing with the A1-A6 constants introduced by Kivelson and Wilson. The problem can be partly solved by proper selection between the possible sets of constants, the choice depending on the degree of asymmetry and the collection of transitions present. The suggested methods are tested on the remeasured spectrum of pyridine, on the earlier measured lines of furan including some new high-J Q-branch transitions, and on the spectra of dimethyl sulfide and dimethyl sulfoxide reported in the literature. In an appendix is shown that the results obtained for the nonplanar molecules are in agreement with a recent theoretical development by Watson.

Microwave spectra of deuterated furans. Revised molecular structure of furan

Abstract The microwave spectra of furan, [2-D], [3-D] and [2,5-D 2 ] furans have been reinvestigated. Also the spectrum of [D 4 ]furan was assigned and measured. Determination of atomic coordinates by mono- and disubstitution is discussed. The complete substitution structure has been redetermined confirming results of earlier work.

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.

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.

Nuclear quadrupole coupling and centrifugal distortion in the microwave spectrum of 1,3,4-thiadiazole

Abstract The microwave spectrum of 1,3,4-thiadiazole has been reinvestigated. 14 N quadrupole coupling and centrifugal distortion has been analysed for lines with J ≦ 5 and J ≦ 50, respectively. The nuclear quadrupole coupling data are discussed and compared with data from 1,3,4-oxadiazole and pyridazine. The centrifugal distortion results disclosed an inconsistency between the vibrational assignment and the distortion effects in the rotational spectrum, since the use of the force field derived earlier by Cyvin et al. leads to centrifugal distortion constants widely different from the experimental values. The molecular structure of 1,3,4-thiadiazole was derived with the more accurate dataandthe estimated average (r z ) structure compared to the recent electron diffraction result.

Microwave spectra of [13C] and [15N] pyridine N-oxides and a preliminary ring structure

Abstract Pyridine N -oxide and the four ring substituted pyridine N -oxides were prepared. The microwave spectrum of the parent species was reinvestigated to high J -values ( J max = 40) and centrifugal distortion constants determined. The microwave spectra of the four isotopic species were investigated, and selected lines were measured and corrected for centrifugal distortion. A preliminary substitution structure of the ring is derived and compared to the ring structure in pyridine. Significant differences were found in the CNC part of the ring, whereas C(3)C(4)C(5) is almost unaffected by the N -oxide formation.