V. F. Kalasinsky

Spectra and structure of small ring compounds. XLVIII. Conformational stability of methylcyclobutane from low frequency Raman data of the gas

The low frequency (500–80 cm−1) Raman spectra of gaseous methylcyclobutane, c‐C4H7CH3, and methyl‐d3‐cyclobutane have been recorded. A series of Q branches beginning at 161 cm−1 for the light molecule and 154 cm−1 for the d3 compound with successive transitions falling to lower frequencies have been assigned to the ring puckering vibrations of both the low energy equatorial and high energy axial conformers. These data have been fitted to an asymmetric potential function of the form: V(cm−1)=(4.78±0.10)×105X4−(3.08±0.04)×104X2 +(2.18±0.1)×104X3 with an assumed reduced mass of 160 amu for the light compound. Utilizing this potential the difference between the puckering angles for the two conformers was calculated to be 4.3° with the equatorial conformer having the larger value of 20.7°. A similar potential was obtained for the d3 molecule. The energy difference between the equatorial and axial forms was found to be 247±20 cm−1 (706 cal/mol) and a barrier of 641±20 cm−1 (1.83 kcal/mol) was found for the inte...

Far-infrared spectra and ab initio calculations of nitrosyl chloride

Abstract The far-infrared spectra of ON 35 Cl and ON 37 Cl were recorded from 20 to 100 cm −1 at a resolution of 0.004 cm −1 . Thirteen pure rotational Q branches were assigned for each molecule in the ground vibrational state from which the rotational constants A and the distortion constants φ K and φ K were obtained utilizing previously reported values of the constants B and C from microwave studies. Rotational Q branches were also observed for both molecules in two vibrationally excited states. Ab initio calculations up to those using fourth-order perturbation (MP4) with configuration interaction utilizing the 6-31G* basis set were carried out. Optimized geometries obtained at this level are compared with those obtained with smaller basis sets without configuration interaction and with the experimentally determined values. Force constants were calculated which are compared with the values previously reported. The potential energy distributions and quartic distortion constants were obtained at all levels of ab initio calculations up to MP2/6-31G*.

Vibrational spectra and conformations of 2-fluoroethylamine☆

Abstract The i.r. (4000-200 cm −1 ) and Raman (4000-50 cm −1 ) spectra of gaseous and solid 2-fluoroethylamine have been recorded along with the Raman spectra of the neat liquid and a TMS solution. These data have been interpreted on the basis of at least four conformations in the gas phase and one conformation in the crystalline solid. Conformers identified as Gg ′ and Gt which exhibit intramolecular hydrogen bonds are the most stable forms in the gas phase and coexist with the Gg and Tg conformers. In the condensed phases, intermolecular hydrogen bonding becomes a dominant factor, and only the Gg conformer remains in the solid. The conformer stabilities determined here are discussed in terms of theoretical calculations and experimental data for similar molecules.

High-resolution infrared spectra of the ν1, ν2, and ν3 fundamentals of ON35Cl

Abstract The ν 1 , ν 2 , and ν 3 fundamentals of nitrosyl chloride, ON 35 Cl, have been recorded with a Fourier transform interferometer at an apodized resolution of 0.004 cm −1 . The data have been analyzed together with previous microwave data for the ground vibrational state to yield improved molecular parameters for the (100), (010), and (001) vibrational states. The main results (in cm −1 ) are: v 1 = 1 v 2 = 1 v 3 = 1 A 2.892513(18) 2.941109(9) 2.931514(12) B 0.1922820(7) 0.1906560(8) 0.1904847(9) C 0.1800373(7) 0.1783946(8) 0.1784287(8) ν i 1799.7316(2) 595.8518(2) 331.9707(2) Additionally, the values for a number of the centrifugal distortion constants have been obtained for each of these normal modes.

Infrared, Raman, and microwave spectra and conformations of dl-bisoxirane

Abstract The Raman spectra of liquid and solid dl-bisoxirane and the infrared spectra of all three phases have been recorded. The microwave spectrum has been studied in the X- and R-band frequency regions. The vibrational data indicate that the molecule exists as a mixture of at least two conformations in the fluid state. The more stable conformer, which has a trans orientation of the two oxirane rings and persists as the only conformer in the crystalline solid state, has also been observed in the microwave spectrum of the vapor. The rotational constants of the ground vibrational state are (in MHz) A = 10998.186, B = 2340.413, and C = 2126.233. From Stark effect measurements, the dipole moment component μb (along the symmetry axis) is found to be 3.43 ± 0.01 D. Rotational transitions and the Stark effect for two excited vibrational states have also been measured and analyzed. A second conformer, which is 0.23 ± 0.08 kcal/mole (0.96 ± 0.33 kJ/mole) less stable than the trans conformer in the liquid state, is identified as a near-gauche conformer whose dihedral angle is probably between 40° and 50°. The possibility of an additional conformer is considered, and the structure of dl-bisoxirane is discussed in terms of similar molecules.

On the existence of multiple conformations in cyclopropylamine

Abstract The vibrational spectra of cyclopropylamine have been reconsidered; the results are consistent with our earlier studies describing a trans/gauche conformational equilibrium. The conformer having the amino protons trans to the ring CC bonds is more stable than the gauche conformer. The equilibrium in cyclopropylamine is discussed and contrasted with a recent report in which only one conformer was identified.

Vibrational spectra and conformational behavior of dicyclopropylmethane

Abstract Raman spectra of the condensed phases of dicyclopropylmethane have been recorded. In addition, i.r. spectra of this compound have been obtained for all three phases. From the appearance of spectral doublets in the liquid phase Raman (and i.r.) spectra, one member of each of which vanishes upon crystallization, it has been concluded that, in the liquid state, DCPM exists as an equilibrium mixture of at least two conformers. Based upon the spectral results for DCPM, and upon the conformational preferences of a number of related molecules, it has been concluded that these two conformers are the C 2 and C s gauche/gauche rotational isomers, with the C 2 form being the one which remains in the solid phase. From a variable temperature study of liquid phase Raman peaks, it has been determined that the C 2 conformer of DCPM is more stable than the C s conformer by 0.93 ± 0.10 kcal/mole. In addition, it appears that the C 2 rotamer of DCPM also predominates in the gaseous phase. Tentative assignments of the major spectral bands of DCPM have also been proposed.

Vibrational spectra and conformational behavior of 1,1-diethylcyclopropane

Abstract Infrared spectra (from 4000 to 400 cm−1) of solid, liquid and gaseous 1,1-diethylcyclopropane and Raman spectra of the condensed phases of this compound have been recorded. Evidence for two conformational isomers, one of which vanishes in the polycrystalline solid phase, is found in the spectral data for the liquid phase. It has been concluded that these two rotational isomers are the gauche/gauche conformers which arise when the two methyl groups are displaced in a conrotatory sense (C2 symmetry) and a disrotatory sense (Cs symmetry) from a hypothetical cis/cis (C2υ) structure. These conclusions are consistent with the conformational results previously obtained for ethylcyclopropane and ethyloxirane. In addition, from the variable temperature liquid phase Raman intensity measurements, the C2 rotamer of 1,1- diethylcyclopropane has been calculated to be 1.1 ± 0.2 kcal/mole more stable than the Cs form, and is the sole conformer remaining in the solid phase. Tentative vibrational assignments, in agreement with those for related molecules, are proposed for the major spectral bands of 1,1-diethylcyclopropane.

Vibrational-rotational analysis of some of the fundamentals of vinylsilane

Abstract The infrared spectrum at a resolution of 0.06 cm −1 has been recorded for vinylsilane. Vibrational-rotational analysis has been carried out on ν 6 , ν 13 , ν 15 , and the strongly coupled pair of ν 12 and ν 19 . The rotational constants and some of the distortion constants have been determined for ν 6 , ν 13 , and ν 15 , whereas higher resolution of 0.004 cm −1 was required to obtain similar data from the coupled pair of ν 12 and ν 19 . These data are compared to the ground state constants previously obtained from the microwave spectrum.