Vibrational analysis of 2,3,4-trichloropentanes

One meso form and the racemic isomer of 2,3,4-trichloropentane have been synthesized, and their infrared and Raman spectra analysed on the basis of a general force field refined for multiply-chlorinated hydrocarbons. Previous results on 2,3-dichlorobutane have been incorporated in this study, thus providing a force field for detailed analyses of vicinally-chlorinated hydrocarbons. VIBRATIONAL spectra of chlorinated normal hydrocarbons have been studied extensively in order to elucidate their conformational structures and to interpret the spectra of polymers that contain chlorine [l-6]. The bands arising from the stretching of the carbon-chlorine bond are relatively intense and have been shown experimentally to be sensitive to the geometry of the carbon chain around the chlorine atom [7,8]. Normal mode analysis has convincingly established that the conformational structures of simple chlorinated hydrocarbons containing isolated secondary chlorines, such as 2chlorobutane or 2,4-dichloropentanes, can be correlated with the observed frequencies of the carbon-chlorine stretch modes in the region of 500-700 cm-’ [3,4]. However, carbon-chlorine stretch is highly delocalized in the range of 300-800 cm-i as chlorine atoms appear vicinally along the chain, as in 2,3-dichlorobutanes [6]. In highly chlorinated hydrocarbons, structure determination, either configurational or conformational, becomes more complicated because of the overlap of many bands. For such molecules, normal mode analysis is indispensable in helping to analyse the spectral complexity. Since the calculated frequencies depend on the force constants used, the refinement procedure is very important. For the general application of normal mode analysis to highly chlorinated hydrocarbons, such as chlorinated poly(viny1 chloride) (CPVC), force constants have to be refined for the particular types of structures present, such as: 1) the isolated Cl, as in 2-chlorobutane, 2,4-dicholoropentanes, and 2,4,6trichloroheptanes, which could be models for poly(viny1 chloride) (PVC); 2) the geminal Cl, as in 2,2-dichloropropane and 2,2-dichlorobutane, which could be models for poly(vinylidene chloride); 3) the vicinal Cl, as in 2,3-dichlorobutanes and 2,3,4trichloropentanes, which could be models for chlorinated polyethylenes and CPVC; and 4) mixtures of geminal and vicinal Cl, as in 2,2,3-trichlorobutane, which could also be a model for structures in CPVC. Force constants for the isolated type of secondary chlorine have been developed by OPASKAR and KRIMM [2] and MOORE and KRIMM [3]. Their force constants could successfully explain conformational structures of the monochlorohydrocarbons [3], 2,4dichloropentanes [3,4], 2,4,6,-trichloroheptanes [4], and crystalline syndiotactic PVC [5]. Force constants applicable to the geminal Cl structure have been refined by WV et al. [9]. Recently, JING and KRIMM [6] have refined force constants for the vicinal type of secondary chlorine, using 2,3-dichlorobutanes. However, for secondary chlorines appearing in triplets along the chain, as in CPVC, specific force constants should be refined for general applications involving these structures. In addition to these, the mixed types of geminal and vicinal Cl structures should be considered together in the refinement procedures. * Present address: Department of Chemical Engineering, Chonnam National University, Kwangju, Korea 500.