J. Phys. Chem. 1989, 93, 7262-7265
7262
Vibrational Pressure Tuning Spectroscopy of the Polymethylene Chain. 2. Various Polyethylenest W. W. Ley and H . G . Drickamer* Materials Research Laboratory and School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: February 27, 1989; In Final Form: May 9, 1989)
In this paper, we present the effect of pressure on the vibrational spectra of a series of six polyethylenes of varying molecular weight and crystallinity. As in part 1 of this study, the focus is on the increase in intensity of the CH2 wagging and C-C stretching vibrations with pressure. It is demonstrated that the intensities of the CHI wag and C-C stretch increase with pressure and that the C-C stretch borrows intensity from the CH2 wag to the extent of overwhelming it. The results imply that the primary event is an increase in the wagging vibration intensity due to increased intermolecularcoupling. This intensity is redistributed to the C-C stretching vibration via some lattice mode (phonon-assisted coupling). This coupling is much more efficient for the polyethylenes than the linear hydrocarbons studied in part 1 primarily due to the very small difference in energy between the two peaks.
Introduction As we state in part 1 of this study,' the solid-state vibrational spectra of polyethylene is well-known and the theory of its normal modes of vibration has been worked out (see ref 1-11 in the preceding paper in this issue). Crystalline polyethylene has an orthorhombic structure like the odd straight-chain hydrocarbons but with 2 C2H4units/unit cell.* There have been previous high-pressure investigations of the vibrational spectra of p o l y e t h y l e ~ ~but e , ~the ~ effects observed and discussed in this paper were not as fully exploited due to the limitations in both pressure cells and spectrometers available at the time.
Experimental Procedure All spectra were recorded by a Nicolet Model 7199 FTIR fitted with a 4X Perkin-Elmer beam condenser to focus the IR radiation on the sample. The samples were held in a diamond anvil cell with type-I1 diamonds and confined by an Inconel gasket;6 pressure was measured by the ruby fluorescence method? All samples were loaded with mineral oil as a pressurizing fluid and run for 300-1000 scans, depending on the strength, and hence S/N ratio, of the interferogram. The polymers studied include the following: (1) A sample of Marlex (with no thermal pretreatment) was furnished by Dr. Bernhard Wunderlich of the University of Tennessee and Oak Ridge National Laboratory. This is the material he used as a starting point in many of his studies.'-I2 It has M , = 153 000 and M , = 8530. (2) A 99% crystalline polymer was prepared by Dr. Wunderlich from the above material. It has a density of 0.995 1. Four polyethylenes were obtained from Polysciences Inc., Warrington, PA. They include the following: (1) a sample with M , = 700 and density 0.96; (2) a sample with M , = 2000 and density 0.96; (3) a standard linear sample with M, = 52000, M, = 19 000, and density 0.978; and (4) a standard branched-chain sample with density 0.93. All spectra taken were found to be reversible upon release of pressure, and all pressure runs were reproducible.
Results Spectra are shown for the following polyethylenes: M , = 700, Figure 1; M , = 2000, Figure 2; 99% crystalline, Figure 3; and branched, Figure 4. 'This work was supported in part by the Materials Science Division, Department of Energy, under Contract DE-AC02-76ER01198.
0022-3654/89/2093-7262$01.50/0
In our discussion, we concentrate on three overall effects observed as a function of pressure: the methylene wagging mode, indicated by an arrow at higher energy, increases in intensity with pressure; the carbon-carbon stretch, indicated by an arrow at lower energy, also increases in intensity with pressure; and the separation in energy between these two peaks diminishes with increasing pressure. The CH2 wag and the C-C stretch referred to in this paper are both ~5 = 0. All of the intensity changes and peak shifts are shown quantitatively in Figures 5-10. The intensity data are expressed as the ratio of the integrated area of the peak of interest to the relatively unchanged area of the CHI bending mode. This intensity was normalized to unity at the first reliably measurable pressure. The shift data was taken as the simple position of the transmittance minimum of the peak. Figure 5 shows the intensity of the wag and C-C stretch peaks for polyethylene, M , = 700. The wag intensity increases to a maximum at about 70 kbar. It then decreases until it is overwhelmed by the increasing C-C stretch and the two become indistinguishable at about 107 kbar. At this molecular weight, about a 50-carbon-atom chain length, one can see clearly that the first-order effect is the intensity increase of the CH2 wag and that the C-C stretch borrows intensity from the wag and eventually overwhelms it. The shift data for polyethylene, M, = 700, are shown in Figure 6. The C - C stretch shifts to higher energy with pressure, a blue shift, and merges with the CH2 wag at an energy separation of about 12 cm-I at 107 kbar. The CH2 wag has a small red shift, similar to that exhibited by the higher n-paraffins as discussed in the preceding paper in this issue, which reverses a t 65 kbar. The CH2 wag shift exhibits a cusp where the two peaks become ~~
~~~
(1) Ley, W. W.; Drickamer, H. G. J. Phys. Chem., preceding paper in this
issue. (2) Painter, P. C.; Coleman, M. M.; Koenig, J. L. The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials; Wiley-Interscience: New York, 1982. (3) Wu, C.; Nicol, M. J . Chem. Phys. 1973,58, 5150-5162. (4) Wu, C.; Nicol, M. Chem. Phys. Lett. 1973, 18, 83-86. (5) Wu, C.; Jura, G.Spectrochim. Acta 1974, 30A, 797-812. (6) Sherman, W. F.; Stadtmuller, A. A. Experimental Techniques in High Pressure Research; Wiley: New York, 1987. (7) Wunderlich, B.; Davidson, T. J . Polym. Sei., Part A-2 1969, 7, 2043. (8) Wunderlich, B.; Davidson, T. J . Polym. Sci., Part A-2 1969, 7, 2051. (9) Prime, R. B.; Wunderlich, B. J. Polym. Sci., Part A-2 1969, 7, 2061. (10) Prime, R. B.; Wunderlich, B. J. Polym. Sci. Part A-2 1969,7,2073. (1 1) Prime, R. B.; Wunderlich, B.; Melillo, L. J . Polym. Sci., Part A-2 1969, 7, 2091. (1 2) Gruner, C. L.; Wunderlich, B.; Bopp R. C. J . Polym. Sci., Part A-2 1969, 7, 2099.
0 1989 American Chemical Society
Vibrational Spectroscopy of Various Polyethylenes
M,
Linear Polyethylene
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7263
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Figure 2. IR spectra of linear polyethylene, M, = 2000. The CH2 wag is indicated by the arrow.
Figure 4. IR spectra of branched polyethylene. The single observable peak is indicated by the heavy arrow.
approximately equal in intensity at 86 kbar. We propose that this cusp is a fingerprint of the CH2 wag and C-C stretch peaks merging, although we cannot eliminate the possibility of a phase transition. Figure 7 shows the intensity of the wag and C-C stretch peaks for polyethylene, M, = 2000. Both peaks increase in intensity but merge and become indistinguishable at about 57 kbar before significant intensity borrowing can be observed. The shift data for polyethylene, M, = 2000, are shown in Figure 8. The C-C stretch exhibits a blue shift and merges with the CH2 wag at an energy separation of about 15 cm-* a t 56 kbar. The wag exhibits a small red shift as noted above and, as in the M, = 700 polyethylene, exhibits a cusp a t 86 kbar. The intensity data for the single observable peak in the higher
molecular weight polyethylenes are shown in Figure 9. The three linear samples, M, = 52000, 99% crystalline, and Marlex 50, increase in intensity by about a factor of 8. The intensity of the branched sample grows by only half that amount. This verifies that the first-order effect of pressure is to cause an intermolecular coupling since the effect of the branches would be to disrupt the crystalline domain and hinder any such effect. The shift data for the M, = 52 000 and branched polyethylenes are shown in Figure 10. For the linear M, = 52 090 sample, the characteristic cusp occurs at 90 kbar. This is about the same cusp location as in the linear M, = 700 and 2000 polyethylenes and indicates that the two-peak process that occurs in the lower molecular weight polyethylenes is also occurring in the higher molecular weight materials. The only difference between the twp
7264 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
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Discussion The above observations indicate and help verify the 2-fold coupling process proposed in part 1 of this study. The data for the polyethylene, M , = 700, show conclusively that the first-order effect is the growth of wag intensity and that the C-C stretch borrows intensity from it. The data for the branched polyethylene show that the mechanism responsible for the CH2 wag intensity
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behavior of the single peak formed when the wag and stretch meld together and become indistinguishable. increase is intermolecular coupling that is hindered by disruption of the crystalline domain in this material. In polyethylene, the 4 = 0 C-C stretch is not IR-allowed under the D2* selection rules of the orthorhombic unit cell.4 At high pressure, however, the intermolecular distances are greatly compressed, which increases the strength of the crystal field. At this higher crystal field strength, the selection rules are then governed more strongly by the C, site symmetry of a single C2H, moiety in the unit cell; thus, the C-C stretch appears in the infrared spectrum.
Summary The effect of pressure upon the intensity and position of the CH2wag and C-C stretch vibrations in various polyethylenes has been interpreted in terms of two major phenomena: intermolecular coupling and phonon-assisted resonance between C H 2 wag and
J. Phys. Chem. 1989, 93,1265-1269
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C-C stretch vibrational coordinates. The intermolecular coupling arises from the compression of adjacent molecules and is a direct result of pressure. The resonance is apparently assisted by the decrease in energy separation of the two modes, which in turn arises from the inherent shift behavior of the polyethylenes. The primary effect of pressure is to increase the intensity of the CH2 wagging mode via intermolecular coupling. At sufficiently high pressures, the second-order effect of phonon-assisted resonance becomes an increasingly dominant factor in the behavior of the two vibrational modes studied. This is best demonstrated in the polyethylene M , = IO0 where the phonon-assisted resonance
Zinc Complexes of Water, Hydroxide, and Ammonia Douglas B. Kitchen and L. C. Allen* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: March 9, 1987)
Several complexes of chemical and biochemical interest were investigated by ab initio MO calculations using effective core potentials. The binding energies of water and hydroxide ion to (NH3)3Zn2+were determined to be 43 and 284 kcal/mol, respectively. The proton affinity of the hydroxide complex was determined to be 167 kcal/mol. The first two results are likely to be in error by less than 3% while the proton affinity may be in error by as muchas 20 kcal/mol. The effective core potentials of Hay and Wadt (J. Chem. Phys. 1985,82, 270) and Stephens, Basch, and Krauss (J. Chem. Phys. 1984, 81, 6026) have been tested against experimental results for some small zinc and copper systems and were found to be very accurate. Correlation effects on the reaction energies of closed-shell reactions were found to be in the range of 2-3% versus 50% for reactions to open-shell products. The appropriate degree of contraction for the zinc basis set that would provide accurate results and computational efficiency for larger systems is a 2s,lp,ld contraction on zinc with a 31G contraction on 0, N, and H. Added p and d functions were found to be unnecessary for the charge-transfer systems.
Introduction With the advent of effective core potentials (ECP's) for most of the periodic table, new possibilities exist for the study of transition-metal compounds using a b initio MO methods. Zinc chemistry is one area of great importance because of its ability to form alloys and amalgam' and because its occurrence in biology is second only to that of iron among transition elements. More than 80 enzymes containing zinc have been reported, the best known of which are carboxypeptidase A, liver alcohol de-
* To whom correspondence should be addressed.
hydrogenase, and carbonic anhydrase.* In most of these, zinc coordinates to water, hydroxide, or amines, thus providing a major motivation for the study reported here. We have investigated the Hay and Wadt ECP's3 (H&W) for zinc and the Stephens, Basch, (1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1980; p 589. (2) (a) Cook, C. M.; Haydock, K.;Lee,R. H.; Allen, L. C. J . Phys. Chem. 1984,88,4875. (b) Cook, C. M.;Allen, L. C. Ann. N.Y.Acad. Sci. 1984, 429, 84. (c) Cook, C. M.; Lee, R. H.; Allen, L. C. In?. J . Quantum Chem. Symp. 1983, 10, 263. (3) Hay, P. J.; Wadt, W. R. J . Chem. Phys. 1985, 82, 270.
0022-3654/89/2093-1265$01.50/00 1989 American Chemical Society
