Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of trans-1-chloro-2-butene

Abstract

Abstract Raman (3200 to 10 cm −1 ) and infrared (3200 to 50 cm −1 ) spectra have been recorded for the fluid and solid phases of trans -1-chloro-2-butene (crotyl chloride, trans -CH 3 CHCHCH 2 Cl), and a complete vibrational assignment is proposed. The fundamental asymmetric 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 82.5 cm −1 . The corresponding mode for the syn conformer (allylic chlorine atom oriented syn to the double bond) gives rise to a series of well defined C -type Q -branches beginning at 110.7 cm −1 and proceeding to lower frequencies. From these data the potential function governing internal rotation of the asymmetric top has been determined and the following potential coefficients have been obtained: V 1 = −371 ± 18, V 2 = −103 ± 9, V 3 = 662 − 30, V 4 = 123 ± 5, V 6 = −20 ± 12 cm −1 . This potential function has the gauche rotamer more stable by 267 ± 57 cm −1 (763 ± 163 cal mol −1 ). The gauche to gauche, gauche to syn, and syn to gauche barriers have been determined to be 518, 839, and 572 cm −1 , respectively. Similarly, the barrier governing internal rotation of the CH 3 group has also been determined from the far-infrared spectrum of the gas. All of these data are compared with the corresponding quantities obtained from ab initio Hartree—Fock gradient calculations employing the RHF/3-21G * , RHF/6-31G * and MP2/6-31G * basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities for some similar molecules.