T.S. Little

Conformational barriers to internal rotation and vibrational assignment of methyl vinyl ketone

The infrared (3500 to 50 cm−1) and Raman (3200 to 10 cm−1) spectra have been recorded for the gaseous and solid states of methyl vinyl ketone. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. The asymmetric torsion for the s‐trans conformer was observed at 116 cm−1 in the infrared spectrum of the gas with two accompanying hot bands and the corresponding torsion of the s‐cis conformer was observed at 87 cm−1 with an additional hot band occurring at 84 cm−1. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V1 = 180±9, V2 = 827±107, V3 = 113±8, and V4 = 150±34 cm−1. From these data it has been determined that the s‐trans conformer is the predominant form at ambient temperature and the enthalpy difference between the s‐trans and s‐cis conformers is 280 cm−1 (800 cal/mol) for the vapor. The calculated trans–cis barrier is 827 ...

Determination of the conformational barriers to internal rotation of 3‐fluoropropene from far infrared and low frequency Raman spectra

The far infrared (200–40 cm−1) and low frequency Raman (1000–20 cm−1) spectra of gaseous 3‐fluoropropene have been recorded. The fundamental asymmetric torsion for the conformer which has the fluorine atom cis to the double bond has been observed at 164.62 cm−1 with four excited states falling at lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 108.00 cm−1 with three excited states observed at lower frequencies. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V2=459±29, V3=830±9, V4=18±8, and V6=−37±5 cm−1, with a ΔH of 304±20 cm−1 (869 cal/mol). It has been determined that the cis conformer is the predominant form at ambient temperature in the gas phase and, from a temperature study of the Raman spectrum in this phase, the enthalpy difference between the cis and gauche conformers was determined to be 263±25 cm−1 (752 cal/mol). The calculated...

Vibrational assignment and conformational equilibrium for 3-fluoropropene based on ab initio calculations and high resolution far-infrared spectroscopy

Abstract The far-IR spectrum of gaseous 3-fluoropropene, H 2 CCHCH 2 F, has been recorded from 200 to 80 cm −1 at a resolution of 0.008 cm −1 . Asymmetric torsional transitions are observed for both the cis and gauche conformers and the potential function governing this internal rotation is determined. The cis to gauche barrier is determined to be 1164 ± 67 cm −1 (3.33 ± 0.19 kcal mol −1 ) and the gauche to gauche is 573 ± 16 cm −1 (1.64 ± 0.05 kcal mol −1 ) with the cis conformer being more stable than the gauche by 290 ± 35 cm −1 (829 ± 100 cal mol −1 ). The normal vibrations for both the cis and gauche rotamers are calculated by ab initio Hartree—Fock gradient calculations employing the 3-21G basis set and the calculated frequencies compared to those previously reported. Potential surface calculations are carried out employing both the 3-21G and 6-31G* basis sets. The calculated barriers are compared to those obtained experimentally from the asymmetric torsional transitions observed in the far-IR spectrum and the agreement is very good. The force constants obtained from the ab initio calculations are compared to those previously reported from a modified valence force field.

Far infrared spectra and barriers to internal rotation of benzaldehyde, benzoyl fluoride, benzoyl chloride and acetophenone

Abstract The far infrared (250-40 cm−1) spectra of gaseous benzaldehyde, benzoyl fluoride, benzoyl chloride and acetophenone have been recorded. The fundametnal CHO torsion for benzaldehyde has been observed at 110.85 cm−1 with three excited states at 109.51, 106.52 and 104.17 cm−1 along with several “hot bands” arising from the low frequency bending modes. The corresponding fundamental for benzoyl fluoride has been observed at 63.36 cm−1 with one well defined excited state at 61.91 cm−1. Similarly, bands observed at 44.6 and 49.5 cm−1 in the spectra of benzoyl chloride and acetophenone, respectively, have been assigned to the fundamental CXO torsions of these molecules. These data have allowed for the determination of the twofold barrier which governs the internal rotation of the CXO mojety and have been found to be 1611 cm−1 (4.61 kcal mol−1), 1739 cm−1 (4.97 kcal mol−1), 1162 cm−1 (3.32 kcal mol−1) and 1103 cm−1 (3.15 kcal mol−1) for the aldehyde, fluoride, chloride and ketone, respectively. These results are compared to previously obtained values for two of the molecules and to some corresponding barriers for several related molecules.

Infrared and Raman spectra, conformational stability, vibrational assignment, and ab initio calculations of chloromethyl isocyanate

Abstract The infrared (3200 to 400 cm −1 ) and Raman (3200 to 20 cm −1 ) spectra of gaseous and solid chloromethyl isocyanate, CICH 2 NCO, have been recorded. Also, the Raman spectrum of the liquid and qualitative depolarization values have been obtained. Additionally, infrared spectra have been recorded at various temperatures with the sample dissolved in liquid xenon. An assignment of the fundamental vibrations based on the infrared band contours, depolarization values and group frequencies is given and discussed. This assignment is supported by a normal coordinate analysis utilizing a force field obtained from ab initio calculations with the RHF/6-31G ∗ basis set as well as with electron correlation, MP2/6-31G ∗ . A complete equilibrium geometry has been determined by ab initio gradient calculations employing the RHF/3-21G ∗ , RHF/6-31G ∗ and MP2/6-31G ∗ basis sets. The potential surface governing internal rotation about the C-N bond is calculated to be consistent with a single minimum corresponding to a structure having the chlorine atom cis or near-cis to the NCO moiety. These results are compared with the corresponding quantities for ethyl isocyanate, CH 3 CH 2 NCO, and related compounds.

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

The far infrared spectrum (375 to 35 cm−1) of gaseous fluoroacetyl chloride, CH2FC(O)C1, has been recorded at a resolution of 0.10 cm−1. The fundamental asymmetric torsions of the more stable trans (halogen atoms are trans) and the high energy cis conformations have been observed at 116.18 and 49.42 cm−1, respectively, each with several upper state transitions falling to lower frequency. From these spectral data, an asymmetric potential function has been calculated and the potential coefficients are: V1=43±6, V2=1039±36, V3=498±3, V4=149±21, and V6=−10±7 cm−1. The trans to cis and cis to trans barriers are 1455±25 cm−1 (4.16±0.07 kcal/mol) and 914±24 cm−1 (2.61±0.07 kcal/mol), respectively, with an enthalpy difference of 541±45 cm−1 (1.55±0.13 kcal/mol). From studies of the Raman spectra at variable temperatures, values of 509±37 cm−1 (1.46±0.10 kcal/mol) and 310±8 cm−1 (0.89±0.02 kcal/mol) have been determined for the enthalpy difference for the gas and liquid, respectively. The conformational stability,...

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

The far infrared spectrum of gaseous chloroacetyl fluoride, CH2ClC(O)F, has been recorded at a resolution of 0.10 cm−1 in the 350 to 35 cm−1 region. The fundamental asymmetric torsional frequencies of the more stable trans (two halogen atoms oriented trans to one another) and high energy gauche (Cl–C–C=O torsional dihedral angle of 122°) have been observed at 86.5 and 48.8 cm−1, respectively, each with excited states falling to lower frequency. From these data the asymmetric torsional potential function governing internal rotation about the C–C bond has been determined. This potential function is consistent with torsional potential coefficients of: V1=350±12, V2=306±6, V3=420±1, V4=44±1, and V6=2±1 cm−1. The trans to gauche, gauche to gauche, and gauche to trans barriers have been determined to be 796, 245, and 271 cm−1, respectively, with an energy difference between the conformations of 525±24 cm−1 (1.50±0.07 kcal/mol). From studies of the Raman spectrum at variable temperatures the conformational energ...

Conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 2-chloropropenoyl fluoride

The far‐infrared spectrum of gaseous 2‐chloropropenoyl fluoride, CH2 CClCFO, has been recorded at a resolution of 0.10 cm−1 in the region of 350–35 cm−1. The fundamental asymmetric torsional frequencies of the more stable s‐trans (two double bonds oriented trans to one another) and the high energy s‐cis conformations have been observed at 67.80 and 49.96 cm−1, respectively, each with several excited states falling to lower frequencies. From these data the asymmetric torsional potential function governing the internal rotation about the C–C bond has been determined. The potential coefficients are V1 =−125±1, V2 =1586±6, V3 =375±2, V4 =−36±2, and V5 =−65±1 cm−1. The s‐trans to s‐cis and s‐cis to s‐trans barriers have been determined to be 1755 and 1570 cm−1, respectively, with an energy difference between the conformations of 185±9 cm−1 (529±26 cal/mol). From studies of the Raman spectrum at variable temperatures, the conformational enthalpy difference has been determined to be 176±40 cm−1 (503±114 cal/mol)...

Conformational stability and barriers to internal rotation of methacrolein (CHO and CDO) from far infrared spectral data, ab initio calculations and the microwave spectrum of methacrolein-d1

Abstract The far i.r. spectra of gaseous methacrolein (2-methylpropenal), CH 2 C(CH 3 )CHO and methacrolein- d 1 (2-methylpropenal-1- d 1 ) have been recorded in the region 350-50 cm −1 at a resolution of 0.10 cm −1 . The fundamental asymmetric torsions of the d 0 and d 1 compounds for the more stable s-trans conformer have been observed at 169.82 and 158.83 cm −1 , respectively, with each band having at least three additional “hot bands” associated with it. The corresponding fundamentals for the s-cis conformers have been observed at 163.74 and 151.26 cm −1 for the d 0 and d 1 compounds, respectively, with one well defined “hot band” in each case. From these data the asymmetric torsional potential coefficients have been determined to be: V 1 = 1148 ± 27; V 2 = 3421 ± 232; V 3 = −89 ± 15; and V 4 = −127 ± 36 cm −1 . The s-trans to s-cis barrier was calculated to be 3950 ± 42 cm −1 with the s-trans being more stable than the s-cis conformer by 1057 ± 42 cm −1 (3.02 ± 0.12 kcal/mol). The barrier to internal rotation of the methyl group for the s-trans conformer is 444 ± 3 cm −1 (1.27 ± 0.01 kcal/mol) whereas the corresponding barrier for the s-cis conformer is 441 ± 2 cm −1 (1.26 ± 0.01 kcal/mol). The fact that both the methyl and asymmetric torsion shift with the 1- d 1 substitution indicates that these two tops are kinetically coupled. The presence of the second conformer was confirmed by a study of the i.r. (3500-50 cm −1 ) and Raman (3200-10 cm −1 ) spectra of gaseous and solid methacrolein. From these data, a reassignment of some of the fundamentals was necessary. The microwave spectrum of methacrolein- d 1 was recorded from 19.0 to 39.0 GHz and the a -type R -branches assigned. Utilizing the rotational constants for the d 0 and d 1 molecules, some structural information has been obtained for the heavy atom parameters. These data are compared to the corresponding quantities from ab initio calculations at the 6-31G* level. All of these results for methacrolein are compared to the corresponding quantities of acrolein.

Rotational and vibrational spectra, conformational stability, potential functions and ab initio calculations of methacryloyl chloride

Abstract The microwave spectrum of CH 2 C(CH 3 )C 35 ClO has been recorded from 12.4 to 39.0 GHz. Both a - and b -type transitions were observed and an R -branch assignment has been made for the s-trans conformer. The rotational constants were found to have the following values: A =4785.13±0.06, B =2248.74±0.01, and C =1551.54±0.01 MHz. From a diagnostic least-squares adjustment to fit the three rotational constants, and with reasonable carbon—hydrogen bond distances and angles, the following heavy atom skeletal structural parameters were obtained: r (CCl)=1.792±0.008 A, r (CO) = 1.191 (fixed), r (CCH 3 ) = 1.509 (fixed), r (CCClO) = 1.492±0.007 A, r (CC) = 1.342 (fixed), ∠ CCCl = 116.1±1.1°, ∠ CCCClO = 122.7 (fixed), 3 = 123.9 (fixed), and ∠ CCO = 123.9±1.5°. The quadrupole coupling constants for the s-trans conformer have the following values: χ aa = −44.9, χ bb = 20.4, and χ cc = 24.5 MHz. The infrared (IR) (3500-20 cm −1 ) and Raman spectra (3500-10 cm −1 ) have been recorded for both the gaseous and solid phases of methacryloyl chloride. 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 of a more stable s-trans and high-energy s-cis conformational equilibrium for the fluid phases. The potential functions governing internal rotation of both the CClO and CH 3 tops have been determined from the far-IR spectrum of the gas. The asymmetric potential function, conformational energy difference, and optimized geometries have also been obtained from ab initio calculations at both the 3–21G* and 6–31G* basis-set levels. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G* basis set. All of these results are compared to corresponding quantities for some similar molecules.