G. A. Guirgis

Analysis of torsional spectra of molecules with two internal C3v rotors. XXIV. High resolution far infrared spectra of acetone‐d0, ‐d3, and ‐d6

The high resolution (0.10 cm−1) far infrared spectra of gaseous acetone‐d0, ‐d3, and ‐d6 were measured between 370 and 40 cm−1. The region of the methyl torsional transitions below 130 cm−1 was analyzed in terms of the semirigid rotor model for molecules with two internal C3v rotors and the two‐dimensional torsional potential functions were determined. Effective barriers of 291 cm−1 (832 cal/mol) and sine–sine interaction terms of −l08 cm−1 (−309 cal/mol) were found for all three isotopic species. The results are compared with those from ab initio calculations and from the analysis of splittings in the microwave spectrum.

Spectra and structure of small ring compounds: Part XXXV. Vibrational spectra and ring-puckering and torsional potential functions of cyclobutylamine

Abstract The IR spectra (50–4000 cm −1 ) of gaseous and solid cyclobutylamine and cyclobutylamine-N- d 2 and the Raman spectra (25–4000 cm −1 ) of gaseous, liquid and solid cyclobutylamine and cyclobutylamine-N- d 2 have been recorded. Depolarization values were measured for both the gaseous and liquid states. Most of the thirty-six fundamental vibrations have been assigned and support for more than one molecular configuration is presented. In the low frequency region for the “light” compound, a series of four Q -branches have been assigned to transitions between energy levels of the ring-puckering vibration for the equatorial isomer. The transitional frequencies were fitted to an asymmetric single-minimum potential function of the form: V(X) = 0.474 × 10 6 X 4 - 0.204 × 10 5 X 2 + 0.993 × 10 5 X 3 with a reduced mass of 160 amu. The following torsional potential constants were determined for the “light” molecule- V 1 = 77.8 ± 17.0 cm −1 , V 3 = 784.0 ± 3.3 cm −1 . The trans conformation was found to be more stable than the gauche form by approximately 58 cm −1 (0.17 kcal mol −1 ). The barriers to trans-gauche , gauche-trans , and gauche-gauche interconversion are 803, 745 and 803 cm −1 , respectively.

Far infrared spectra, conformational potential function, and barrier to methyl rotation of propionyl fluoride

The far infrared spectra of propionyl fluoride CH3CH2CFO in the gaseous and solid states have been recorded from 500 to 40 cm−1. A rather complex spectrum of the gas was observed and a substantial number of bands have been assigned to the asymmetric torsional modes for both the s‐cis (oxygen atom eclipsing the methyl group) and the high energy gauche conformers. Analysis of these bands permitted the calculation of the torsional potential function present in this molecule. The potential coefficients for the asymmetric torsional mode were calculated to be: V1=341±24, V2=236±20, V3=390±3, and V4=21±6 cm−1, with an enthalpy difference between the more stable s‐cis and the high energy gauche conformers of 434±20 cm−1 (1.24±0.06 kcal/mol). This function gives a potential barrier of 692 cm−1 (1.98 kcal/mol) separating the s‐cis from the gauche form and 283 cm−1 (0.81 kcal/mol) separating the two equivalent gauche forms. From a temperature study of the Raman spectrum of the gas, the enthalpy difference between th...

Vibrational spectra, conformational stability, barriers to internal rotation, r0 structural parameters and ab initio calculations of fluoromethyl methyl ether

The far-infrared spectrum of gaseous fluoromethyl methyl ether, FCH2OCH3, along with three of the deuterium isotopes, has been recorded at a resolution of 0.10 cm−1 in the 350 to 50 cm−1 region. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 182 and 132 cm−1, respectively, for the stablegauche conformer with the lower frequency band having several excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 527±9 cm−1 (1.51±0.03 kcal/mol). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500 to 50 cm−1) spectra of the gas and solid and from the Raman (3200 to 10 cm−1) spectra of the gas, liquid, and solid is proposed. No evidence could be found in any of the spectra for the high-energytrans conformer. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and 6-31G* basis sets along with the 6-31G* basis set with electron correlation at the MP2 level. Additionally, completer0 geometries have been determined from the previously reported microwave data and carbon-hydrogen distances determined from infrared studies. The heavy-atom structural parameters (distances in Å, angles in degrees) arer(C1-F) = 1.395 ± 0.005;r(C1-O) = 1.368 ± 0.007;r(C2-O) = 1.426 ±0.003; ⦔FC1O = 111.33 ± 0.25; ⦔C1OC2 = 113.50 ± 0.18 and dih FC1OC2 = 69.12 ± 0.26. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Torsional transitions and barrier to internal rotation of 1-butyne

Abstract The far infrared spectrum of 1-butyne (ethyl acetylene) has been recorded in the gas phase from 370 to 40 cm −1 with a resolution of 0.1 cm −1 . The skeletal bending fundamental transitions ν 23 ( a ″) and ν 15 ( a ′) give strong bands at 343.6 and 196.5 cm −1 , respectively, showing detailed rotational structure. Transitions of the torsional mode, ν 24 ( a ″), are assigned to prominent Q spikes superimposed on the ν 15 band at 212.8 and 201.5 cm −1 . From these data the coefficients of the periodic potential function are calculated to be 1060 and −30 cm −1 for the V 3 and V 6 terms, respectively.

Spectra and structure of small ring compounds: Part XXXVIII. Cyclobutanol

Abstract The Raman spectrum of gaseous cyclobutanol has been recorded and the far infrared spectrum of the gas has been obtained at a resolution of 0.5 cm−1. At least six Q-branches arising from the low frequency ring-puckering motion have been observed and assigned on the basis of a potential of the form V(X) = (6.32 ± 0.21) × 105X4−(4.18 ± 0.04) × 104X2+ (8.81 ± 1.20) × 103X3 with a reduced mass of 170 amu. An energy difference between the equatorial and axial forms was found to be 50–150 cm−1 with the equatorial being more stable and a barrier of 700–900 cm−1 was found for the interconversion. Three O-H stretching modes were observed in the Raman spectrum. It is concluded that the O-H moiety has both the gauche and trans conformations present in the equatorial form but only the gauche conformer is present in the axial form of the ring. Three O-H torsional modes were observed at 244 (trans conformer), 226.5 and 181.5 cm−1 (gauche conformer) for the equatorial form and one O-H torsion at 237.5 cm−1 (gauche conformer) for the axial form. The potential function governing the O-H torsional motion for the equatorial form was found to be V1 = 280 ± 7 cm−1 (800 cal mole−1) and V3 = 425 ± 3 cm− (121.5 cal mole−1) with the trans conformer being more stable than the gauche by approximately 206 cm−1 (589 cal mole−). The barriers to trans-gauche and gauche-gauche interconversion have essentially the same values, 500 cm−1 (1430 cal mole−1).

Raman and infrared spectra and ab initio calculations for the determination of the conformational stability for fluorocarbonyl and acetyl isocyanate

Abstract The infrared and Raman spectra of gaseous, liquid and solid fluorocarbonyl isocyanate, FC(O)NCO, and acetyl isocyanate, CH 3 C(O)NCO, have been recorded. From these data the conformational stability has been determined for both molecules for all three physical states. The conformational energy differences, optimized geometries, and fundamental vibrational frequencies have also been obtained from ab initio calculations.

Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment for chlorodifluoroacetyl chloride

Abstract The Raman (1900 to 10 cm −1 ) and infrared (1900 to 30 cm −1 ) spectra of chlorodifluoroacetyl chloride, CClF 2 CClO, in the gas and solid phases have been recorded. The Raman spectrum of the liquid, and qualitative depolarization ratios, have also been obtained. These data have been interpreted on the basis of an equilibrium between the gauche and trans conformers (chlorine atom trans to the other chlorine atom) in the gas and liquid phases. From the study of the Raman spectrum of the liquid at different temperatures, a value of 361 ± 40 cm −1 (1.03 ± 0.11 kcal mol −1 ) was determined for Δ H with the gauche conformer being the more stable form. A similar study of the sample dissolved in liquid xenon gave a Δ H value of 249 ± 60 cm −1 (0.71 ± 0.17 kcal mol −1 ), again with the gauche rotamer the more stable conformer. Therefore, the gauche conformer is the predominate rotamer in the gas and liquid and the only conformer present in the annealed solid. A complete vibrational assignment is proposed for both conformers based on infrared band contours. Raman depolarization data, group frequencies and normal coordinate calculations. Optimized geometries, conformational stabilities, unscaled and scaled vibrational frequencies and harmonic force fields are reported for both conformers from ab initio calculations utilizing the RHF/3-21G*, RHF/6-31G* and/or MP2/6-31G* basis sets. These results are compared with the corresponding results for some similar molecules.

Microwave and far infrared spectra, r0 structure, barriers to internal rotation, and abinitio calculations for 2‐fluoropropane

The microwave spectra of five isotopic species of 2‐fluoropropane, (CH3)CH2DCFH, (CH3)2CFD, (CH3)CD3CFH, (CD3)2CFD, and (CH3)213CFH, have been recorded from 12.4 to 39.7 GHz. The b‐ and c‐type R‐branch transitions have been observed and assigned for the ground state. Utilizing the rotational constants for these five isotopic species along with those reported earlier for the normal species the following r0 structural parameters have been determined: r(C–C) =1.522±0.007 A, r(C–F)=1.398±0.013 A, CCC =113.37±0.79°, and CCF=108.19±0.41°. All of the carbon–hydrogen parameters have also been determined from the rotational constants except for the r(C–Hsec) which was obtained from its frequency in the infrared spectrum. The far infrared spectra of 2‐fluoropropane‐d0, ‐d3 and ‐d7 in the gas phase were recorded with a resolution of 0.10 cm−1. Both torsional fundamentals along with several hot transitions were assigned for the three isotopic species. The barrier to internal rotation of the methyl rotors has been det...

Far-infrared spectra and barriers to internal rotation for the CF3CXO molecules where X = H, F, Cl, Br, CH3 and CF3

Abstract The far-infrared spectra (350-30 cm−1) of CF3CHO,CF3CFO, CF3CClO, CF3CBrO, CF3CO(CH3) and (CF3)2CO have been recorded at a resolution of 0.10 cm−1. The torsional fundamentals for the CF3 rotors have been assigned at 66.2, 45.6, 41.8, 38.3, 36.1 and 53.8 (39.0) cm−1, respectively, where two frequencies are observed or the CF3 torsional modes of (CF3)2CO, and each fundamental for all of the molecules has several accompanying “hot bands”. Utilizing these torsional data, along with reasonable structural parameters to calculate the kinetic constants, the periodic barriers to internal rotation have been determined. The barrier values for the CF3 rotors with the statistical uncertainties are 292 ± 5, 381 ± 1, 434 ± 2, 429 ± 2, 295 ± 1, and 777 ± 5 cm−1 for these CF3CXO molecules, where X  H, F, Cl, Br, CH3 and CF3, respectively. These results are compared to the methyl barriers in the corresponding series CH3CXO.