H. V. Phan

Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of fluoroacetyl fluoride

Abstract The far infrared spectrum (350 to 35 cm −1 ) of gaseous fluoroacetyl fluoride has been recorded at a resolution of 0.10 cm −1 . The fundamental asymmetric torsion of the more stable trans conformer where the two fluorine atoms are trans to one another has been observed at 118.90 cm −1 with twelve upper state transitions falling to lower frequency. Additionally, the fundamental torsional transition of the cis conformer has been observed at 52.06 cm −1 with five excited states falling to lower frequency. From these data, the asymmetric torsional potential coefficients have been determined to be: V 1 = 86 ± 11; V 2 = 946 ± 33; V 3 = 407 ± 4; V 4 = 138 ± 20 and V 6 = −14 ± 7 cm −1 . The trans to cis and cis to trans barriers were calculated to be 1297 ± 26 (3.71 ± 0.07 kcal mol −1 ) and 803 ± 26 cm −1 (2.30 ± 0.07 kcal mol −1 ), respectively, with the energy difference being 494 ± 48 cm −1 (1.41 ± 0.14 kcal mol −1 ). From the temperature dependence of the Raman spectra the enthalpy difference between the conformations has been determined to be 456 ± 32 cm −1 (1.30 ± 0.09 kcal mol −1 ) and 695 ± 8 cm −1 (1.99 ± 0.02 kcal mol −1 ) for the gas and liquid, respectively. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally, are compared to those obtained from ab initio Hartree—Fock gradient calculations employing both the 3-21G and 6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules.

Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of dichloroacetyl chloride

Abstract The Raman (3100−10 cm −1 ) and infrared (3100−30 cm −1 ) spectra of dichloroacetyl chloride, CHCl 2 CClO, in the gas and solid phases have been recorded. Additionally, the Raman spectrum of the liquid and the qualitative depolarization ratios have been obtained. From these data a gauche/trans equilibrium is proposed in the gas and liquid phases, with the gauche conformer being the more stable form in both physical states and the only form present in the annealed solid. From the study of the Raman spectra at different temperatures, values of 56±5 cm −1 (160±14 cal mol −1 ) and 131±32 cm −1 (376±90 cal mol −1 ) were determined in Δ H of the liquid and gas, respectively. A potential function for the conformational interchange is suggested. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies and normal coordinate calculations. The experimental structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies are compared with those obtained from ab initio Hartree—Fock gradient calculations employing both the RHF/3-21G* and RHF/6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules.

Conformational analysis, barriers to internal rotation, ab initio calculations and vibrational assignment of fluoroacetone

Abstract The infrared (3500-20 cm −1 ) and Raman (3200-10 cm −1 ) spectra have been recorded for gaseous and solid fluoroacetone (1-fluoro-2-propanone), CH 2 FC(O)CH 3 . Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in the cis (fluorine atom oriented cis to the methyl group) conformation in the vapor but for the liquid a second conformer having a trans orientation (fluorine atom oriented trans to the methyl group) is present. From a study of the Raman spectrum of the liquid at variable temperatures the trans conformation has been determined to be more stable than the cis form by 416 ± 54 cm −1 (1.19 ± 0.15 kcal mol −1 ) and is the only conformation present in the spectrum of the annealed solid. The asymmetric torsional fundamental for the more stable cis conformer has been observed in the far infrared spectrum of the gas at 69.6 cm −1 with six accompanying hot band transitions proceeding to lower frequency. The corresponding mode for the high energy trans conformer is extensively overlapped but is distinguishable at ∼65 cm −1 . From these data the asymmetric torsional potential function governing internal rotation about the CC bond has been determined and the potential coefficients are: V 1 = 675 ± 2, V 2 = 991 ± 5, V 3 = 74 ± 1 and V 4 = 54 ± 2 cm −1 . The cis to trans and trans to cis barriers are 1332 ± 5 and 731 ± 5 cm −1 , respectively, with an enthalpy difference of 601 ± 8 cm −1 (1.72 ± 0.02 kcal mol −1 ). From ab initio calculations at the 3-21G and 6-31G* basis set levels optimized geometries for both the cis and trans conformers have been obtained and the potential surface governing internal rotation of the asymmetric top determined. The observed vibrational frequencies with their assignments for both the cis and trans conformers are compared to those from the ab initio calculations. All of these results are compared to the corresponding quantities for some similar molecules.

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.

Far-infrared spectrum, conformational stability, barriers to internal rotation, ab initio calculations, and vibrational assignment of propionyl bromide

Abstract The Raman (3200 to 10 cm −1 ) and IR spectra (3200 to 30 cm −1 ) of propionyl bromide, CH 3 CH 2 CBrO, of the gas and solid have been recorded. Additionally, the Raman spectrum of the liquid has been recorded with qualitative depolarization measurements. All of these data indicate that propionyl bromide exists as a mixture of two conformers in the fluid phases with one being a high energy gauche form and the other the thermodynamically preferred s- trans rotamer (Br trans to CH 3 ) which is the only form remaining in the annealed solid. From the studies of the Raman spectra at different temperatures, the enthalpy differences have been determined to be 403 ± 28 cm −1 (1.15±0.08 kcal mol −1 ) and 221 ± 25 cm −1 (0.63 ± 0.07 kcal mol −1 ) for the gas and liquid, respectively, with the s- trans conformer being the more stable form in both phases. The fundamental asymmetric torsions for both the s- trans and gauche conformers have been observed in the far-IR spectrum of the gas at 75.68 and 71.38 cm −1 , respectively, with each having several excited state transitions falling to lower frequencies. A potential function has been calculated from the observed asymmetric torsional frequencies with the following potential coefficients: V 1 =627±46, V 2 = −239±39, V 3 =744±14, V 4 = −17±6, and V 6 = 14±4 cm −1 . From this potential function, values of 711 ± 8, 1119 ± 58, and 459 ± 9 cm −1 were obtained for the s- trans to gauche , gauche to gauche and gauche to s- trans barrier, respectively, with an enthalpy difference of 252 ± 99 cm −1 . A complete vibrational assignment supported by normal coordinate calculations has been proposed for both conformers based on the observed IR band contours, the depolarization data, and group frequencies. Additionally, ab initio calculations have been carried out with the STO-3G* basis set from which the fundamental vibrational frequencies, conformational stabilities, barriers to internal rotation, and structural parameters have been obtained. The results of this study are compared to similar ones for the corresponding propionyl fluoride and propionyl chloride molecules.

Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of ethyl methyl sulfide

Abstract The far-infrared spectrum of ethyl methyl sulfide in the gas phase has been recorded at a resolution of 0.10 cm −1 in the region 370-30 cm −1 . The fundamental asymmetric torsions of the gauche and trans conformations have been observed at 93.4 cm −1 and 90.8 cm −1 respectively, with the gauche form having several excited-state transitions. From these data, the asymmetric torsional potential function has been recalculated with the following coefficients: V 1 =310±10, V 1 −190±7, v 3 =997±17, V 4 =101±4 and V 6 = −94±7 cm −1 . From this potential function, the trans to gauche, gauche to gauche and gauche to trans barriers are determined to be 1012±17 cm −1 , 1184±9 cm −1 and 881±21 cm −1 respectively, with an energy difference of 131 ± 45 cm −1 , the trans conformer being the more stable form in the gas phase. The enthalpy difference has also been determined experimentally from the variable temperature studies of the Raman lines and a value of 133±34 cm −1 (380±97 cal mol −10 ) has been obtained for the gas with the trans form more stable, but for the liquid phase the value is too small to be measured. A complete vibrational assignment is proposed for a trans/gauche equilibrium in the gas and liquid phases from the Raman (3200-10 cm −1 ) and infrared (3200-35 cm −1 ) spectra obtained for the gas, liquid and solid phases, and in the solid only the gauche conformer remains. The structural parameters, conformational stability, barriers to internal rotation and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations employing both the 3-21 G* and 6-31G* basis sets. These results are compared to those obtained for some similar molecules.

Far-infrared spectrum, conformational stability, barriers to internal rotation, normal coordinate calculations, and vibrational assignment for chloroacetaldehyde

The far infrared spectrum [350 to 25 cm−1] of gaseous chloroacetaldehyde, ClCH2CHO, has been recorded at a resolution of 0.10 cm−1. The first excited-state transition of the asymmetric torsion of the more stable near s-cis [chlorine atom s-cis to the aldehyde hydrogen atom] conformer has been observed at 26.9 cm−1, with seven additional upper state transitions falling to higher frequency. Additionally, the fundamental torsional transition of the s-trans conformer has been observed at 58.9 cm−1 with two excited states also falling to higher frequency. From these data, the asymmetric torsional potential coefficients have been determined to be:V1=414±11;V2 = 191±3;V3=−203±5;V4=44±1 andV6=−26±1 cm−1. The s-cis to s-trans barrier is 500±5 cm−1 (1.43±0.01 kcal mol−1) with the s-cis conformer being more stable by 267±19 cm−1 (0.76±0.05 kcal mol−1) than the s-trans form. The Raman [4000 to 100 cm−1] and infrared (4000 to 400 cm−1] spectra of the gas have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Complete vibrational assignments are proposed for both conformers based on band contours, depolarization values, and group frequencies. The assignments are supported by ab initio Hartree-Fock gradient calculations employing the 3–21G* basis set to obtain the frequencies and the potential energy distributions for the normal vibrations for both rotamers. Additional ab initio calculations at the MP4/6-31G* level have been carried out to determine the structural parameters for both conformers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of methyl propargyl ether

Abstract The Raman spectra (3400 to 10 cm −1 ) of gaseous, liquid and solid and the infrared spectra (3500 to 35 cm −1) of gaseous and solid methyl propargyl ether, HCCCH 2 OCH 3 , have been recorded. Furthermore, the far-infrared spectrum of the gas has been recorded in the region of 370 to 30 cm −1 at a resolution of 0.10 cm −1 . From the vibrational spectrum two conformers have been identified in the fluid phases at ambient temperature with the gauche rotamer the more stable conformer and the only one present in the annealed solid. The fundamental asymmetric torsions of the gauche and trans conformations have been observed at 113.2 and 102.0 cm −1 , respectively, with the gauche form having three excited state transitions falling to lower frequency. From these data, the asymmetric torsional potential function has been obtained with the following coefficients: V 1 = 325 ± 24, V 2 = −662 ± 23, V 3 = 1149 ± 14, V 4 = −102 ± 6 and V 6 = −46 ± 6 cm −1 . From this potential function, the trans to gauche, gauche to gauche and gauche to trans barriers are determined to be 664, 1831 and 1021 cm −1 , respectively, with an enthalpy difference between the conformers of 357 ± 61 cm −1 (1021 ± 174 cal mol−1 and the gauche conformer the more stable form in the gas. The enthalpy difference has been determined experimentally for the liquid from the variable-temperature studies of the Raman spectrum and a value of 153 ± 10 cm−1 (437 ± 29 cal mol −1 ) has been obtained with the gauche form again the more stable rotamer. The methyl torsional transitions of the gauche and trans rotamers have been observed at 166.0 and 214.2 cm −1 , respectively, and on the basis of the one-dimensional model the barrier to internal rotation of the methyl group is determined to be 707 ± 45 cm −1 (2.02 kcal mol −1 ) and 991 ± 67 cm −1 (2.83 kcal mol −1 ) for the gauche and trans conformers, respectively. A complete vibrational assignment is proposed for a gauche/trans equilibrium in the gas and liquid phases. The structural parameters, conformational stability, barriers to internal rotation and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations employing the RHF/4–31G*, RHF/6–31G*, MP2/6–31G* and/or MP2/6–31++G** basis sets. These results are compared to those obtained for some similar molecules.

Vibrational spectra, conformational stabilities, and barriers to internal rotation of the difluoroacetyl halides

Abstract From a combination of infrared and Raman spectral data and ab initio calculations, the conformational stabilities have been determined for the difluoroacetyl halides in vapor, liquid and solid phases.

Raman and infrared spectra, conformational stability, and ab initio calculations for chloromethyl methyl sulfide

The Raman (3500–20 cm−1) and infrared (3500–40 cm−1) spectra of gaseous and solid chloromethyl methyl sulfide [chloro(methylthio)methane], CICH2SCH3, and the corresponding deuterated molecule, CICD2SCD3, were recorded. Additionally, the Raman spectra of the liquids were recorded from 3500 to 50 cm−1 and qualitative depolarization values were obtained. These data were interpreted on the basis that only the gauche conformer (methyl group gauche to the chlorine atom) is present in all three physical states. A complete vibrational assignment is given for the normal and d5-isotopomer. Ab initio calculations were carried out with the RHF/6–31G* basis set, and also with full electron correlation by the perturbation method to second order (MP2) with the 6–31G* and 6–311+G** basis sets, to obtain the structural parameters, relative conformational stabilities, harmonic force constants, Raman and infrared intensities and fundamental wavenumbers. The fundamental vibrational wavenumbers and barriers to internal rotation which were obtained experimentally are compared with those obtained from the MP2/6–31G* calculation. From the ab initio calculations the gauche conformer is estimated to be more than 10.22 kJ mol−1 more stable than the trans conformer. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules. Copyright