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

Abstract

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 cis to gauche, gauche to gauche, and gauche to cis barriers are 1201 cm−1 (3.43 kcal/mol), 498 cm−1 (1.42 kcal/mol) and 869 cm−1 (2.48 kcal/mol), respectively. An alternative potential function was also calculated utilizing the successive gauche asymmetric torsional transitions proceeding to higher frequencies. A second series of asymmetric torsional transitions were observed for the cis conformer which are believed to be due to ‘‘hot bands’’ associated with the first excited state of the CCC bend. Additionally, each of the torsional transitions for the cis conformer exhibits multiple splitting which is believed to be due to excited states of the CCF bend. These results are compared to similar quantities in some related molecules.

Solvent-induced conformational changes in benzyl fluoride. Long-range C,F; H,H; H,F spin–spin couplings

“…The preference of an in-plane C-F bond is possibly related to an electrostatic preference, the Ct--Fdipole lying near the ortho C-Ht dipole (calculated net charges on fluorine and the ortho hydrogen are +0.06 and -0.15 in units of electron charge for 1). An analogous eclipsing conformation is found to be most stable in 3-fluoropropene (8).…”

Raman and infrared spectra, conformational stability, vibrational assignment, and ab initio calculations of cis-1,3-dichloropropene

“…The force field in Cartesian coordinates was calculated by the GAUSSIAN-90 program (38) with the RHF/3-21G" basis set. From previous studies (24), it was found that the differences in the values of force constants and frequencies are not significant when the smaller, as opposed to the larger, RHF/ 6-31G* basis set is used. These calculations were performed by using the Wilson FG matrix method (40).…”

“…Our recent investigation~ have included the determination of the potential functions governing internal rotation about the ~( s p~) -~( s p ' ) bonds of the parent 3-halopropenes and some substituted derivatives (24)(25)(26)(27)(28)(29)(30)(31)(32). Ally1 fluoride (24)(25)(26), chloride (27), bromide (28,31), and iodide (32) have been determined to exist as a mixture of syn and gauche conformers, and for the fluoride we have shown that the conformer that has the halogen atom syn to the double bond is the more stable rotamer (24)(25)(26). For 3-chloropropene, the value of the energy difference (27) was inferred to be less than 100 cm-' (286 cal/mol) and the spectral data merely indicated a very slight preference for the syn conformer for the gaseous state.…”

Raman and infrared spectra, conformational stability, and ab initio calculations oftrans-1-fluoro-2-butene

“…These compounds have been of interest for a number of years, since allyl fluoride [1][2][3][4] has its more stable conformer as the cis (synclinal) rotamer, whereas the other allyl halides [5][6][7][8][9] have the gauche (anticlinal) form as the most stable conformer in the fluid phases. For allyl fluoride, the most stable conformer in the solid is the cis rotamer, whereas the other conformers have the gauche rotamer as the more stable conformer in the solid.…”

“…For some time we have been investigating the relative stability of the rotamers of the allyl halides [1][2][3][4][5][6][7][8][9], along with the methyl [10][11][12][13] and halo [14,15] substituted molecules. These compounds have been of interest for a number of years, since allyl fluoride [1][2][3][4] has its more stable conformer as the cis (synclinal) rotamer, whereas the other allyl halides [5][6][7][8][9] have the gauche (anticlinal) form as the most stable conformer in the fluid phases.…”