Infrared and Raman spectra, conformational analysis, barriers to internal rotation, vibrational assignment, and ab initio calculations of trans-1,3-dichloropropene

Abstract

The infrared (3200-50 cm-') and Raman (3200-10 cm-I) spectra of gaseous and solid trans-l,3-dichloroprope.ne, CIHC-CH(CH2Cl), have been recorded. Additionally, the Raman spectrum of the liquid has been obtained and qualitative depolarization ratios have been measured. The fundamental CH2CI torsion for the more stable gauche conformer has been observed in the far-infrared spectrum of the gas and has a hybrid contour centered at 84.0 cm-I. The corresponding mode for the cis conformer (allylic chlorine atom oriented cis to the double bond) gives rise to a series of well defined C-type. Q branches beginning at 129.8 cm-' and proceeding to 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 obtained: VI = -558 f 7, V, = -38 f 10, V3 = 637 f 9, V, = 174 f 7, V, = -16 f 4 cm-'. This potential function has the gauche rotamer more stable by 318 f 26 cm-l (909 f 74 cal/mol). The gauche to gauche, gauche to cis, and cis to gauche barriers have been determined to be 397,957, and 639 cm-', respectively. From studies of the Raman and infrared spectra of the gas at variable temperatures, the conformational enthalpy difference has been determined to be 342 f 36 cm-l(978 f 103 cal/mol). A complete assignment of the vibrational fundamentals is proposed. All of these data are compared to the corresponding quantities obtained from ab initio HartretFock gradient calculations employing either the 3-21GS or 6-31G5 basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules. latroductionThe vibrational and rotational spectra of the allyl halide molecules have been of interest for a number of Additionally, the electron diffraction patternP and the NMRL7J8 shifts have been invdigated. Allyl fluoride, chloride, and bromide have been determined to exist tts a mixture of cis and gauche conformers. For the ~apeous state of the fluorideI3 and chloride14 the cis conformer is slightly more stable, but for the b r o m i d~'~ the gauche form is the thermodynamically preferred rotamer. For 3chloropropme, however, the value of the energy difference was inferred to be leas than 100 cm-' (286 cal/mol). The spectral data merely indicated a very slight preference for the cis conformer and the possibility that the gauche form could be preferred was not completely eliminated.14 The far-infrared and low-frequency Raman spectra of the gas phase of allyl fluoride1IJ3 and chloride14 have been reported from which the asymmetric torsional potential functions governing internal rotation were determined. The asymmetric torsional fundamental and associated upper level transitions for the cis conformer occur as well-defmed Q branches for both molecules in the far infrared spectrum of the gas. For 3-fluoropropene the Av = 2 transitions for the cis conformer were also observed in the Raman spectrum of the g...

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

“…For trnns-l,3-dichloropropene (33) we showed that both the syn and gauche conformers are present in the vibrational spectra and that the gauche conformer is thermodynamically preferred by 342 cm-', which is consistent with the previously reported NMR (34) and molecular mechanics (35) results.…”

“…dichloropropene (33) this vibration occurs near 150 cm-I. This variation in assignment is consistent with the change in potential energy distribution among a similar set of symmetry coordinates.…”

“…: or RHF/STO-3G"' basis sets. For example, the torsional mode for the gauche conformer of trans-l,3-dichloropropene (33) was predicted to occur at 87 cm-I (RHF/3-21G:':) and was observed in the farinfrared spectrum of the gas at 84.0 cm-'. For cis-1,3-dichloropropene this mode is calculated to occur at 45 cm-I, which seems too low when compared to the corresponding modes of some similar molecules.…”

“…Another interaction affecting the torsional frequency is the extent of mixing of the torsional mode with other skeletal bending modes, which is present for 3-chloropropene (27) and trans-l,3-dichloropropene (33), but absent for cis-1,3-dichloropropene. Additionally, hyperconjugative interactions similar to those found in acetaldehyde (43) and fluoroacetyl chloride (44), both of which have the internal rotor bonded to an sp7 carbon, may play a significant role in the ultimate value of this barrier.…”

“…Because an appreciable amount of a second higher energy conformer has been detected in the vibrational spectra of the 3-halopropenes (24-28, 3 1,32) and trans-1,3-dichloropropene (33), the possibility remains that a small amount of it may exist for cis-l,3-dichloropropene. Therefore, we have undertaken the present spectroscopic study to carry out a conformational analysis, based on the previously unreported vibrational spectra, and to perform ub initio calculations for comparison with the results obtained from molecular mechanics calculations (35).…”