Børge Bak

The structure of thiophene

Abstract The microwave spectra of 2- C 13 -thiophene and of 3- C 13 -thiophene have been recorded and analyzed. In addition, three lines from the S 34 -thiophene were found and identified. Taken together with earlier published data for thiophene, thiophene-2 d , and thiophene-3 d , an unambiguous calculation of the (‘ r 8 ’-) structure of thiophene is now possible, the structural parameters being: (distances) CS = 1.714 A; CC = 1.370A; CC = 1.423 A; C(2)H(2) = 1.078 A; C(3)H(3) = 1.081 A; (angles) C(5)SC(2) = 92°10′; SC(2)C(3) = 111°28′; C(2)C(3)C(4) = 112°27′; SC(2)H(2) = 119°51′; C(4)C(3)H(3) = 124°16′.

The complete structure of furan

Abstract Microwave spectra of [2- 13 C] furan, [3- 13 C] furan, and [ 18 O] furan are reported. Moments of inertia of these species are combined with earlier data on furan and deuterated furans to give the r s -structure of furan. The final structure is: OC(2) = 1.362 A, C(2)C(3) = 1.361 A, C(3)C(4) = 1.431 A, C(2)H(2) = 1.075 A, C(3)H(3) = 1.077 A, C(5)OC(2) = 106°33′, OC(2)C(3) = 110°41′, C(2)C(3)C(4) = 106°3′, OC(2)H(2) = 115°55′, C(4)C(3)H(3) = 127°57′.

Complete determination of the structure of pyridine by microwave spectra

Abstract Isotopic mixtures of ordinary pyridine and 2- 13 C- and 3- 13 C-pyridine have been prepared and the microwave spectra recorded. For 2- 13 C- and 3- 13 C-pyridine, respectively, 12 and 10 transitions ( Q - and R -branch lines) were localized, a number of which could be identified by their Stark-effect. For both 13 C-species rotational constants of high precision were calculated. These constants together with known rotational constants for ordinary pyridine, 2 D -, 3 D -, and 4 D -pyridine are sufficient for a complete determination of the ten geometrical parameters of the molecule. The interatomic distances in the model finally adopted are: d [NC(2)] = 1.3402, d [C(2)C(3)] = 1.3945, d [C(3)C(4)] = 1.3944, d [C(2)H(2)] = 1.0843, d [C(3)H(3)] = 1.0805, and d [C(4)H(4)] = 1.0773 A. The valence angles in the aromatic ring [starting with the C(6)NC(2) angle] are: 116°50′, 123°53′, 118°32′, and 118°20′. The distances are thought to be correct to ±0.001A.

Benzene Ring Distortion by One Substituent. Microwave Determination of the Complete Structure of Benzonitrile

Microwave spectra of benzonitrile, C6H5CN, and 9 isotopic species are reported. Moments of inertia of these 10 molecules are combined to give the rs structure of benzonitrile. The final structure is: C(1)C(2) = 1.391 A, C(2)C(3) = 1.393 A, C(3)C(4) = 1.400 A, C(1)C(7) = 1.455 A, C≡N = 1.159 A, C(2)H(2) = 1.069 A, C(3)H(3) = 1.082 A, C(4)H(4) = 1.081 A, C(6)C(1)C(2) = 122.5°, C(1)C(2)C(3) = 118.45°, C(2)C(3)C(4) = 120.3°, C(3)C(4)C(5) = 120.0°, C(1)C(2)H(2) = 121.8°, C(4)C(3)H(3) = 119.9°.

Results of Ab Initio Calculations on Formamide

Ab initio calculations on formamide, HCONH2, have been carried out for 44 nuclear configurations with a (s, p) = (11, 7) basis set for C, O, and N and a (s, p) = (5, 3) set for the hydrogens, and for three nuclear configurations with a (s, p, d) = (11, 7, 1) basis set for C, O, and N and a (s, p) = (5, 3) set for the hydrogens. Primarily, a structure from microwave work was assumed. Within the range of experimental errors of the C=O, the C–N, and the N–H distances the molecular energy was shown to pass through minima. The calculated dipole moment and the barrier to internal rotation around the C, N bond are in satisfactory agreement with experiments. Also, the changes of molecular geometry occurring if HCONH2 is twisted around its amide CN bond are consistent with qualitative expectations. The calculated barrier to NH2 inversion in HCONH2 of 30–40 cm−1 is too low by a factor of ∼ 10.

Proton magnetic resonance spectra at 220 MHz of amino acids, porcine and bovine insulin and the A and B chains of bovine insulin

Abstract Proton magnetic resonance (PMR) spectra of 21 commonly occurring amino acids dissolved in CF 3 COOH or CF 3 COOD were recorded and analyzed in terms of proton chemical shifts, relative to (CH 3 ) 4 Si (TMS) as internal standard, and proton spin-spin coupling constants. The results were used for an analysis of PMR spectra of insulin and the individual A and B chains (as S -sulfonates), all dissolved in CF 3 COOH or CF 3 COOD. In the unusually high magnetic field applied (ca. 50,000 oersteds) large chemical shifts are found. Resonances from specific protons in arginine, alanine, asparagine, glycine, histidine, lysine, phenylalanine, threonine, and tyrosine residues are observed separately in single recordings. The γ-CH 2 protons of glutamic acid and glutamine residues form one separate band at 2.70 ppm, the CH 3 protons of leucine, isoleucine, and valine form another separate band at 1.00 ppm. Only proton resonances from cysteine, proline, and serine residues give no easily accessible evidence of their presence but they contribute significantly to the intensity versus chemical shift integrals.

Nuclear spin-spin coupling between fluorine and hydrogen in fluorobenzene

Abstract High resolution proton and fluorine nuclear magnetic resonance (nmr) spectra of fluorobenzene, 4D-, 2,4,6-D 3− , and 2,3,5,6-D 4 -fluorobenzene have been obtained and analyzed. Double irradiation techniques were employed to eliminate the effects of the deuterium nuclear spin. The secular equation for 4D-fluorobenzene has been solved and the parameters adjusted for the best correspondence between the predicted and observed spectra. The nuclear spin couplings between the fluorine nucleus and the ortho, meta, and para protons are found to be, respectively, 9.4 ± 0.2 cps, 5.8 ± 0.2 cps, and 0.0 ± 0.5 cps. These couplings are found to have the same sign as the proton-proton couplings in the fluorobenzene molecule. This result is discussed and related to previous work on fluorobenzenes.

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.

Infrared absorption spectra of deuterated vinyl fluorides

Abstract Infrared absorption spectra of vinyl fluoride and its seven deuterated derivatives have been taken. The assignment of fundamental vibrational frequencies for vinyl fluoride originally given by T orkington and T hompson must be slightly modified. A final assignment for all eight isotopic species is suggested.

The structure of vinyl fluoride

Abstract By means of microwave spectra of vinyl fluoride and five of its deuterated species the most probable structure of vinyl fluoride has been evaluated. This structure has C(1), F = 1·348 A; C(I), H(1) = 1·073 A; C(2), H(2) = 1·080 A; C(2), H(3) = 1·080 A; C(1), C(2) (double bond distance) = 1·333 A.