J . Phys. Chem. 1990, 94, 2801-2806
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Conformational Analysis of the Polypropylene Homologue 2,CDimethylpentane: Vibrational Spectra and Calculated Energies S. P. Bohan,+ H. L. Strauss, and R. G. Snyder* Department of Chemistry, University of California, Berkeley, California 94720 (Received: June 27, 1989; In Final Form: October 12, 1989)
The Raman spectrum of a polypropylene prototype model molecule, 2,4-dimethylpentane (DMP), has been analyzed in the liquid state. As expected, the liquid consists mostly of the conformer that has C2symmetry. However, a small concentration of a second conformer, of symmetry CI, was detected. An energy difference of 3.1 f 0.8 kcal/mol between the CI and C2 forms was determined from the temperature dependence of the Raman spectrum. This value for the energy difference is generally higher than values inferred from energy parameters previously reported for polypropylene. However, these latter parameters were determined by less direct methods. Our Raman value is in keeping with the value for DMP that we obtained from a potential energy minimization calculation using currently standard methods. Our analysis indicates that one or two other conformers may be present in concentrations comparable to that of the C1conformer. However, we did not observe them in the Raman spectrum of the liquid, probably because their spectra are very similar to that of the C2 form and consequently were obscured by the spectrum of that conformer.
I. Introduction The work reported in this paper was prompted by our recent analysis of the low-frequency isotropic Raman spectrum of isotactic polypropylene (IPP) in its conformationally disordered state.] The latter involves a calculation of the spectrum of an ensemble of disordered IPP chains and a comparison of the calculated spectrum with the spectrum observed for the molten polymer. To calculate the IPP spectrum, it was necessary to generate an ensemble of disordered chains, and for this we used the rotational isomeric state model ( R E M ) . Several different sets of bond rotational states have been proposed for polypropylene (PP).2,3 However, in contrast to the situation for the polymethylene chain, we found that very little experimental work has been carried out to establish the relative conformational energies of model homologues in order to evaluate the various rotational-state models proposed as the basis for computing the conformational properties of PP. Also, the vibrational analysis of the PP chain is in a much less satisfactory state than that for the polymethylene chain. Normal-coordinate analyses have been reported for those ordered PP chains that exist in the crystalline isotactic4 and syndiotacticS polymer. However, the analysis has not been extended to conformationally disordered chains. This paper contains a conformational and vibrational analysis of the branched hydrocarbon molecule 2,4-dimethylpentane (DMP). This molecule is the smallest homologue in which the principal short-range steric interactions in the polypropylene chain are represented. While there have been earlier papers concerning the vibrational spectrum of DMP, they are unsatisfactory with respect to the identification of conformers. As we will see, there are four vibrationally distinguishable conformers of DMP, and, on the basis of interaction energies estimated for IPP,6 three of these should be present in sufficient concentrations to be detected. It is therefore disturbing that, in the previous vibrational spectroscopic studies, only one conformer was found. Thus, Benedetti et al.' concluded from the infrared spectra of DMP in the liquid and crystalline states that only the most stable conformer is present in the liquid at room temperature. In a later infrared study, Jaiswal and Crowdera interpreted their spectra of the liquid on the assumption that only the most stable conformer is present. In the present work, we have identified one of the higher energy conformers of DMP in the Raman spectrum of the liquid and have measured its enthalpy relative to that of the most stable conformer. A third conformer should be detectable, but, as we will show, its spectrum is probably masked in the spectrum by the most abundant conformer. First, our experimental results will be de-
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Present address: Analytical Research Department, Rohm and Haas, Bristol. PA.
scribed, and then these will be compared with the results of theoretical calculations on the conformational energies of DMP and polypropylene. 11. Experimental Procedures A sample of 2,4-dimethylpentane, reported to be 99.7% pure, was obtained from Wiley Organics and used without further purification. Raman spectra were measured at a resolution of 2 cm-l with a Spex Model 1403 double monochromator. The excitation used was the 514.5-nm line of an argon ion laser at a power of 200 mW at the sample. A standard right-angle scattering geometry was employed. The scattered radiation was collected and passed through a polarizer and a polarization scrambler. The temperature of the sample, which was maintained to within f0.5 "C of a set value, was controlled with a flow of cooled nitrogen and measured by a thermocouple fixed to the capillary containing the sample. Infrared spectra of DMP were obtained with an evacuated Nicolet Model 8000 FTIR. A resolution of I cm-' was used. For the liquid, a Csl cell was used. A path length of about 15 Fm yielded appropriate absorbances in the frequency regions of interest. For measuring infrared spectra of the cooled liquid, we used an evacuated CTI Cryogenics Model SC 21 closed-cycle helium refrigerator equipped with a Si diode thermometer. Spectra of thin films of the solid were obtained by condensing DMP vapor onto a cooled KBr plate held in the evacuated sample compartment of the refrigerator. During the deposition, the KBr plate was maintained near 8 K. 111. Theoretical Estimates of Conformer Energies
We will consider two sets of DMP conformers. The individual conformers that comprise these sets will be described in terms of the dihedral angles, I$23 and I$34, which are respectively defined by the atom sequences H-C2-C3-C, and C2-C3-C4-H as shown in Figure I . The dihedral angle for the sequence ABCD is positive if atom D is displaced clockwise relative to atom A for an observer looking down the B-C bond, atom A being nearest the observer. ( 1 ) Hallmark, V. M.; Bohan, S. P.; Strauss, H. L.; Snyder, R. G.Macromolecules, to be submitted for publication. (2) Flory, P. J. Statistical Mechanics of Chain Molecules; Interscience:
New York, 1969. ( 3 ) Suter, U. W.; Flory, P. J. Macromolecules 1975, 8, 765. (4) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1964, 20, 853. ( 5 ) Schachtscheider, J . H.; Snyder, R. G . Spectrochim. Acta 1965, 21, 1527. (6) Tonelli, A. E. Macromolecules 1972, 5 , 563. (7) Benedetti, E.; Pucci, S.; Pino, P.; Schettino, V.; Califano, S. Spectrochim. Acta 1967, 23A, 2371. (8) Jaiswal, R. M. P.; Crowder, G. A. Can. J . Specrrosc. 1983. 28, 160.
0022-3654/90/2094-280 1$02.50/0 0 - 1990 American Chemical Societv
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The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Bohan et al. TABLE I: Conformers (Set SI)of DMP Obtaimd by Assuming a 3-Fold CC Potential for the Two Central CC Bondso
conformer
c2 (2) CS (2)
c, (4)
c2u (1) Figure 1. Atom numbering used in the text to define the dihedral angles and 634of 2,3-dimethylpentane. The C, conformer is shown here, for which &3 = 634= 60'.
$23
description' 623
634
60 -60 180 180
60 60 180
60
relative energyd 0
E, E, + E, 2E, + 2E,
"These conformers are depicted in Figure 2. 'Statistical weight.
