ACKNOWLEDGMENTS This research was performed with the financial support of the National Science Foundation Grant DMR 79-10841, Polymers Program. The authors wish to thank Professor H. R. Allcock for kindly supplying the chlorotetramer used in this study. 1. P. C. Painter, J. Zarian, and M. M. Coleman, Appl. Spectros. 36, 265 (1982). 2. I. C. Hisatsune, Spectrochim Acta 25A, 301 (1969). 3. A. J. Wagner and A. Vos, Acta Cryst. 19, 603 (1965). 4. R. Hazekamp, T. Migchelson, and A. Vos, Acta Cryst. B, 707 (1968). 5. N. L. Paddock, Quart. Rev. Chem. Soc. (London) 18, 168 (1964). 6. J. A. A. Ketelaar and T. A. deVries, Rec. Tray. Chim. Pays-Bas 58, 1081 (1939). 7. L. W. Daasch, J. Am. Chem. Soc. 76, 3403 (1964). 8. U. Stahlberg and E. Steger, Spectrochim Acta 23A, 627 (1967). 9. J. A. Creighton and K. M. Thomas, Spectrochim Acta 29A, 1077 (1973). 10. T. R. Manley and D. A. Williams, Spectrochim Acta 23A, 149 (1967). 11. A. C. Chapman and N. L. Paddock, J. Chem. Soc. (London) A, 635 (1962). 12. N. Nero, PhD thesis, University of Naples, (1962). 13. E. Steger and U. Stahlberg, Z. Anorg. Allgem. Chem. 326, 243 (1964). Vibrational Spectra and Normal Coordinate Calculations of Chlorophosphazene Compounds. III. Polydichlorophosphazene M. M. COLEMAN, J. ZARIAN, and P. C. PAINTER Material Science and Engineering Department, Polymer Science Section, The Pennsylvania State University, University Park, Pennsylvania 16802 Vibrational spectra and normal coordinate calculations of po- lydichlorophosphazene (PDP) are presented. The valence force field derived previously from the two conformers of octachlo- rocyclotetraphosphazene was directly transferred to a distorted "cis-plan" helical model of PDP without refinement. A reason- able agreement between the observed and calculated frequen- cies was obtained and the assignment of the normal modes of PDP is discussed. Index Headings: Infrared; Normal coordinate analysis; Raman spectroscopy. INTRODUCTION In the previous two papers of this series 1' 2 we reported normal coordinate calculations of hexachlorocyclotri- phosphazene (HCTP) and the two conformational iso- mers of octachlorocyclotetraphosphazene (OCTP). To reiterate, the initial goal of our studies was to obtain a common valence force field from the above model com- pounds, using the overlay technique, which could then be transferred to molecular models of the linear polymer, polydichlorophosphazene (PDP). However, we found that it was not possible to obtain a common valence force field that adequately described the vibrational spectra of both HCTP and OCTP. Nonetheless, a force field, for which we have reasonable confidence, was derived for the two conformational isomers of OCTP. The molecular structure of OCTP, in terms of bond lengths and bond angles, more closely resembles the polymer (PDP) than does HCTP. Accordingly, in this paper we describe the results obtained from normal coordinate calculations of PDP obtained by directly transferring the valence force field derived from OCTP. Received 27 October 1981; revision received 30 November 1981. Volume 36, Number 3, 1982 I. EXPERIMENTAL RESULTS Samples of PDP used in these studies were kindly prepared by Paul E. Austin of the laboratories of Profes- sor Harry R. Allcock of the Department of Chemistry at The Pennsylvania State University. The polymers as received were soft, colorless, transparent elastomers which dissolved in benzene, toluene, or tetrahydrofuran to form highly viscous solutions. PDP has a reported glass transition temperature (Tg) of -63°C. For the stretched films or fibers of the polymer, a crystalline melting temperature (Tm) has been reported near -30°C, and the range of polymer elasticity extends from this temperature to more than 300°C. ~ The uncross- linked PDP has been shown to have a weight average molecular weight near 1.5 × 10G, 3 although the molecular weight distribution is usually very broad. Raman spectra of PDP in the solid state in the range 10 to 1300 cm -1 are shown in Fig. 1. The bottom spectrum was obtained from an "as polymerized" sample which was presumed to be completely amorphous. The top spectrum was obtained from an oriented fiber of PDP which was prepared in the following manner., A dilute solution of PDP in dry tetrahydrofuran (THF) was cast onto a Teflon substrate (in a glove box under a steady flow of dry nitrogen) to form a thin film. The film was then rolled into a fiber and stretched to approximately 500% of its original length. The fiber was maintained under tension during the data collection. Enormously intense Raman lines at 132, 151, and 457 cm -1 dominate the spectra for both the amorphous and oriented samples. This is possibly due to preresonance Raman intensity enhancement. It should be mentioned that good quality Raman spec- tra could only be obtained from a freshly polymerized sample of PDP. Attempts to obtain spectra from samples that have aged for indeterminant periods of time invari- APPLIED SPECTROSCOPY 277
Transcript
ACKNOWLEDGMENTS
This research was performed with the financial support of the National Science Foundation Grant DMR 79-10841, Polymers Program.
The authors wish to thank Professor H. R. Allcock for kindly supplying the chlorotetramer used in this study.
1. P. C. Painter, J. Zarian, and M. M. Coleman, Appl. Spectros. 36, 265 (1982). 2. I. C. Hisatsune, Spectrochim Acta 25A, 301 (1969). 3. A. J. Wagner and A. Vos, Acta Cryst. 19, 603 (1965).
4. R. Hazekamp, T. Migchelson, and A. Vos, Acta Cryst. B, 707 (1968). 5. N. L. Paddock, Quart. Rev. Chem. Soc. (London) 18, 168 (1964). 6. J. A. A. Ketelaar and T. A. deVries, Rec. Tray. Chim. Pays-Bas 58, 1081
(1939). 7. L. W. Daasch, J. Am. Chem. Soc. 76, 3403 (1964). 8. U. Stahlberg and E. Steger, Spectrochim Acta 23A, 627 (1967). 9. J. A. Creighton and K. M. Thomas, Spectrochim Acta 29A, 1077 (1973).
10. T. R. Manley and D. A. Williams, Spectrochim Acta 23A, 149 (1967). 11. A. C. Chapman and N. L. Paddock, J. Chem. Soc. (London) A, 635 (1962). 12. N. Nero, PhD thesis, University of Naples, (1962). 13. E. Steger and U. Stahlberg, Z. Anorg. Allgem. Chem. 326, 243 (1964).
Vibrational Spectra and Normal Coordinate Calculations of Chlorophosphazene Compounds. III. Polydichlorophosphazene
M. M. COLEMAN, J . ZARIAN, and P. C. PAINTER Material Science and Engineering Department, Polymer Science Section, The Pennsylvania State University, University Park, Pennsylvania 16802
Vibrat ional spectra and normal coordinate calculat ions of po- lydichlorophosphazene (PDP) are presented. T h e v a l e n c e f o r c e
field derived previous ly from the t w o c o n f o r m e r s o f octachlo- rocyc lotetraphosphazene w a s d i r e c t l y t r a n s f e r r e d t o a d i s t o r t e d
"cis-plan" hel ical model of P D P wi thout ref inement. A reason- able agreement b e t w e e n the observed and c a l c u l a t e d f r e q u e n - c i e s w a s obtained and the ass ignment of the normal modes of P D P is discussed.
Index Headings: I n f r a r e d ; Normal coordinate analys is ; R a m a n spectroscopy.
INTRODUCTION
In the previous two papers of this series 1' 2 we reported normal coordinate calculations of hexachlorocyclotri- phosphazene (HCTP) and the two conformational iso- mers of octachlorocyclotetraphosphazene (OCTP). To reiterate, the initial goal of our studies was to obtain a common valence force field from the above model com- pounds, using the overlay technique, which could then be transferred to molecular models of the linear polymer, polydichlorophosphazene (PDP). However, we found that it was not possible to obtain a common valence force field that adequately described the vibrational spectra of both HCTP and OCTP. Nonetheless, a force field, for which we have reasonable confidence, was derived for the two conformational isomers of OCTP. The molecular structure of OCTP, in terms of bond lengths and bond angles, more closely resembles the polymer (PDP) than does HCTP. Accordingly, in this paper we describe the results obtained from normal coordinate calculations of PDP obtained by directly transferring the valence force field derived from OCTP.
Received 27 October 1981; revision received 30 November 1981.
Volume 36, Number 3, 1982
I. EXPERIMENTAL RESULTS
Samples of PDP used in these studies were kindly prepared by Paul E. Austin of the laboratories of Profes- sor Harry R. Allcock of the Department of Chemistry at The Pennsylvania State University. The polymers as received were soft, colorless, transparent elastomers which dissolved in benzene, toluene, or tetrahydrofuran to form highly viscous solutions. PDP has a reported glass transition temperature (Tg) of -63°C. For the stretched films or fibers of the polymer, a crystalline melting temperature (Tm) has been reported near -30°C, and the range of polymer elasticity extends from this temperature to more than 300°C. ~ The uncross- linked PDP has been shown to have a weight average molecular weight near 1.5 × 10G, 3 although the molecular weight distribution is usually very broad.
Raman spectra of PDP in the solid state in the range 10 to 1300 cm -1 are shown in Fig. 1. The bottom spectrum was obtained from an "as polymerized" sample which was presumed to be completely amorphous. The top spectrum was obtained from an oriented fiber of PDP which was prepared in the following manner., A dilute solution of PDP in dry tetrahydrofuran (THF) was cast onto a Teflon substrate (in a glove box under a steady flow of dry nitrogen) to form a thin film. The film was then rolled into a fiber and stretched to approximately 500% of its original length. The fiber was maintained under tension during the data collection. Enormously intense Raman lines at 132, 151, and 457 cm -1 dominate the spectra for both the amorphous and oriented samples. This is possibly due to preresonance Raman intensity enhancement.
It should be mentioned that good quality Raman spec- tra could only be obtained from a freshly polymerized sample of PDP. Attempts to obtain spectra from samples that have aged for indeterminant periods of time invari-
APPLIED SPECTROSCOPY 277
ably failed. These samples either decomposed in the laser beam or there was an inordinantly large background (fluorescence?) contribution which swamped the Raman effect.
Far-infrared spectra of PDP are shown in Figs. 2 and 3. In Fig. 2, spectrum A is of an amorphous sample prepared by casting a film from a dilute solution of the polymer in tetrahydrofuran onto a polyethylene sub- strate. Spectrum B was obtained from an oriented film of PDP {stretched to approximately 300% of original length) which was maintained under tension during data collection. The difference spectrum (B - A) was obtained by subtracting spectrum A from B using an appropriate weighting factor based on the elimination of the broad amorphous band at -265 cm -1. This spectrum represents the polymer in its preferred ("crystalline") conformation. Fig. 3 shows the far-infrared spectra obtained from the oriented film of PDP using polarized infrared radiation. Spectra B and C were obtained with the electric vector parallel (~r) and perpendicular (o) to the orientation di- rection, respectively. Subtraction of the amorphous com- ponent (spectrum A) yields the polarized infrared spectra
457 132
,sc:~s s?o
361 29=
22O
/ / ' ;o l o ' 5 d ' ' ' ' ' ' ' ' ,m. l ' ' ' ' ' ' '
FIG. 1. Raman spectra of PDP in the range 10 to 1300 cm -]. Bottom, amorphous; top, semicrysta]line (stretched fiber ~500%).
410
479 A
A I t 344
A
6
A
i ' I ' I ' I . . . . . 500 cmi4 I I I 1 0
FFm. 2. Absorbance FT- IR spectra of PDP in the range 100 to 500 cm -]. A, Amorphous; B, semicrysta]line; B - A, crystalline, obtained by subtracting A from B.
2 7 8 V o l u m e 3 6 , N u m b e r 3 , 1 9 8 2
410
479 344
5;0 ,..1 zso
FIG. 3. Absorbance FT-IR spectra of PDP in the range 250 to 500 cm -1. A, Amorphous; B, polarized parallel to the orientation direction; C, polarized perpendicular to the orientation direction; B - A, differ- ence spectrum (B minus A); C - A, difference spectrum (C minus A).
1221
• i • ' • i ' ' •
1700
730
' * , . o " ' " " ' ' " " " ' "
580
5 3 0 510
479
l 45O
FIG. 4. Absorbance FT-IR spectra of PDP in the range 450 to 1700 cm -1. Bottom, amorphous; middle, semicrystalline; top, crystalline, obtained by subtracting bottom spectrum from middle spectrum.
representative of the preferred conformation of the pol- ymer shown in spectra B - A (~r) and C - A (o), respectively.
Mid-infrared spectra of PDP are shown in Figs. 4 and 5. In Fig. 4, the bottom spectrum is that of an amorphous film cast from THF onto a KBr disc. The middle spec- trum was obtained from the stretched oriented sample. The difference spectrum, shown at the top, was obtained by subtracting the amorphous spectrum from that of the spectrum of the oriented sample and represents the pol- ymer in its preferred (crystalline) conformation. Fig. 5 shows the spectra of the oriented sample of PDP re- corded with the electric vector ~r and o to the orientation direction. Also shown is a ratio of the two dichroic spectra. Positive bands are associated with o polarization, whereas negative bands correspond to 7r polarization.
H. PREVIOUS STUDIES
1 2 2 8
5110
7 7 0 7 3 0 4 7 9
,8'00' ' ' ' , ~ . , ' ' ' ~ o
FIG. 5. Absorbance FT-IR spectra of oriented PDP in the range 450 to 1800 cm-'. A, radiation with electric vector paral]el to orientation direction; B, radiation with electric vector perpendicular to orientation direction; C, ratio of B/A.
T A B L E I. Observed v ibrat ional f requenc ies and init ia l ass ign- m e n t s based on the point group s y m m e t r y C2."
Infrared R a m a n Initial a s s ignmen t
1281 (s-sh) Ir A 1288 (vs) a B
999 (vvw) 882 (vw) ~r 880 (vvw) A 770 (w-sh) ? 730 (m) ? 731 (w) 610 (w-sh) o 600 (vvw) B (comb. 132A + 479B = 611B?) 580 (s) Ir A 565 (sh) a? 565 (w) B (comb. 132A + 410B = 542B?) 530 (w-sh) o 530 (w) B 479 (m) o B 447 (w) 7r 457 (vvs) A 410 (m) o 410 (m-sh) B 344 (mw) o 342 (w) B (comb. 132A + 206B = 338B?)
319 (vw) 292 iw-br) Amorphous ? 260 (vvw)
206 (wm) ? 205 (w) 160 (w-sh)
150 (w-sh) ? 151 (s-sh) 132 (vs)
125 (w) ? 70 (w-sh)
a Abbreviat ions used are: s, strong; m, medium; w, weak; v, very; sh, shoulder; or, parallel polarization; a, perpendicular polarization.
Due to the superposi t ion of bands the in terpre ta t ion of the rat ioed spec t rum is not always straightforward.
Ins t rumenta l details have been described previously. ~ However, it is impor tan t to note tha t for spectral manip- ulation it is essential to ensure tha t the films were suffi- ciently thin to be within an absorbance range where the Bee r -Lamber t Law is obeyed. 4
The results of the exper imenta l R a m a n and infrared spectroscopic studies of P D P are summar ized in Tab le I. Also included in this table are initial ass ignments to the s y m m e t r y species based on the point group C2 (see later).
The molecular s t ructure of P D P has been invest igated by x-ray diffraction techniques, 3' 5-8 vibrat ional spectro- scopic studies, 9 and conformat ional analysis using non- bonding in t ramolecular interactions. 6' 10 Meyer e t al . 7
studied the x-ray diffraction of P D P and obtained typical ro ta t ion- type fiber pat terns . The layer line separa t ion corresponded to a fiber distance of 4.92 /k and these authors deduced an or thorhombic unit cell with dimen- sions of a = 11.07, b = 4.92, and c = 12.72 A. T h e y also proposed tha t two m o n o m e r units occupy the fiber repea t distance with four chains passing through the cell. More- over, two chain conformat ions i somorphous to C2v and "dis tor ted" C2v line group symmet r i e s were advanced with more confidence expressed in the C2~ conformation. 7 Huggins s subsequent ly used electron diffraction and pro- posed a uni form helical conformat ion with two mono- meric units per turn for the l inear polymer. In this study, equivalent bond lengths and bond angles of 1.65/k and 124 ° , respectively, were assumed for the chain backbone.
Giglio e t aL 5 used an optical t ransform me thod to s imulate the x-ray pa t te rns expected f rom different pol- ymer conformations. Three different conformat ional models (uniform helix, cis-trans planar, and distorted cis- t rans planar) were studied. F rom their studies the au- thors concluded tha t their results were consistent with a slightly dis torted cis-trans p lanar po lymer chain with torsional angles of 156 and 14 ° around the P - - N and N - - P bonds, respectively. Fur thermore , the best agree- men t was observed for this model using the following parameters : P - - N = 1.60 A, P - - N - - P = 127 °, and N - - P - - N = 119 °. I t should be noted tha t the mode of packing in the crystal lattice was not considered.
Allcock 3 also repor ted a fiber repea t distance of 4.92 /k, similar to tha t of Meyer e t al. 7 However , these authors proposed a smaller uni t cell (a = 5.50, b = 12.72, and c = 4.92 A). Additionally the authors repor ted t ha t the observed x-ray intensities could not be explained in t e rms of an exactly p lanar cis-trans chain, and tha t a slight helical distortion appeared to be necessary to explain the results. However , Arcus 6 using an improved x-ray crys- tal lographic technique in which he obta ined twice the number of previously observed reflections concluded tha t the po lymer conformat ion is cis-trans planar.
T h e only vibrat ional spectroscopic studies of P D P is a t t r ibuted to Manley and Williams. 9 These authors ob- tained the infared spec t rum of P D P in the region 5000 to 20 cm -1. No R a m a n data were reported. Two models for the linear po lymer were cons ide red- -a uniform helical model i somorphous to point group s y m m e t r y D2 and a distorted cis-plan helical model i somorphous to point group s y m m e t r y C2. T h e y concluded their infrared re- sults were more consistent with the C2 conformation. Band ass ignments were made based on this model using the group f requency approach.
The elucidation of the s t ructure of P D P has also been the subject of conformat ional analysis. 6' lO Allcock, Allen, and Meis ter 1° per formed a conformat ional analysis using nonbonding int ramolecular interact ions based on a 6:2 Lennard-Jones potent ia l and a coloumbic term. One of the deepest min ima on the calculated conformat ional potent ia l surface appeared at angles 156 and 14 ° which
APPLIED SPECTROSCOPY 279
was in agreement with x-ray diffraction data of Giglio e t
a l . 5 From these results Allcock e t a l . 1° concluded that the preferred chain conformation of PDP is a distorted cis- trans planar.
HI. NORMAL COORDINATE CALCULATIONS AND DISCUSSION
Three different conformational models have been pro- posed for PDP. These conformational models include (1) a cis-plan model 5-7 which has a chain symmetry isomor- phous to point Cev; (2) a normal helical model 5' s isomor- phous to point group De; and (3) a distorted cis-plan model3,5,9, 10 isomorphous to point group Ce. These models are shown schematically in Fig. 6. All the models considered have two monomer units per translational repeat unit. There are eight atoms involved and a total of 24 normal modes are predicted• However the 24 normal modes contain three translations and one rotation about the chain axis, which leaves 20 active vibrational modes for a single polymer chain.
For the cis-plan model with Cev symmetry, the normal modes of vibration are distributed between the four symmetry species in the following manner: six A1, four A2, six B1, and four B2. The A1, B1, and B2 species are both Raman- and infrared-active, whereas the Ae is in- active in the infrared. Furthermore, A1 modes exhibit parallel (~r) polarization in the infrared, whereas B1 and Be modes exhibit perpendicular (0) polarization.
The normal helical model has a line symmetry isomor- phous to the point group D2. This model predicts 20 active vibrational modes evenly distributed among four symmetry species as five A, five B1, five Be, five B3. All the B species are infrared- and Raman-active, whereas the A modes are infrared-inactive. Additionally, the B1 modes exhibit ~r polarization in the infrared, whereas the Be and B3 modes exhibit o polarization.
The distorted cis-plan model has only one symmetry element: a twofold rotational axis along the polymer chain. Consequently its line group symmetry is isomor- phous to the point group C2. The 20 active vibrational modes of the C2 helix are divided equally into 10A and 10B symmetry species which are both infrared- and Raman-active. The A modes exhibit ~r polarization and B modes have o polarization under infared radiation.
Previous experimental evidence from x-ray studies 3'5'7'8 and theoretical conformational calcula- tions 6,10 have predominantly favored a distorted cis-plan model for the preferred conformation of PDP. Manley and Williams, 9 from infrared spectroscopic studies, have
02
Ip
C2v
Fro. 6. Proposed conformational models of PDP. A, Uniform helix; B, cis-planar 21 helix; C, distorted cis-plan 21 helix.
280 Volume 36, Number 3, 1982
cI
FIG. 7. Schematic representation of PDP together with internal coor- dinate defmitions.
also supported the distorted cis-plan chain conformation although this conclusion was based upon the number of observable infrared bands and appears somewhat spec- ulative. Furthermore, more recent x-ray evidence appears to support an undistorted cis-plan model. 6 It was orgin- ally hoped that the valence force field derived from the trimer and tetramer would be sufficiently well defined to test the various models through symmetry analysis and normal coordinate calculations. However, our force field obtained from the two conformers of the tetramer (OCTP) 2 is frankly not adequate to use in such a study. Accordingly, in our normal coordinate calculations of PDP a distorted cis-plan model was chosen with bond angles and bond lengths assumed as follows: P - - N = 1.55 A, P--C1 = 1.99 A, N - - P - - N = 120 °, N - - P - - N -- 133.5 °, N--P--C1 = 108 °, and C1--P--C1 = 103 °. Furthermore, bond rotational angles of 14 ° and 156 ° were assumed around alternating P - - N bonds. The internal coordinates were defined as illustrated by a schematic representation in Fig. 7.
The force field obtained from the two different modi- fications of the model compound OCTP, given in Table IV of the previous paper in this series 2 with two major assumptions, was directly transferred to PDP. The two assumptions were as follows. First, the bond lengths and bond angles were defined the same as that of chlorote- trainer whereas the x-ray results had suggested that the values for the polymer are slightly different. 5 Second, the force field is relatively simple and might not have been adequately defined. However, the results suggest at least enough transferability to offer the basis for reasonable band and line assignments. Agreement between observed and calculated frequencies could be improved by addi- tional refinement, but this is a specious exercise. The number of observed frequencies assigned with good con- fidence is insufficient to define a complete force field adequately. Moreover, we are more interested in the order and form of the calculated frequencies than the exact matching of the numbers. A comparison of the observed and calculated frequencies together with ap- proximate potential energy distributions are fisted in Table II.
Assignraents of the observed and calculated frequen- cies were made predominantly on the basis of the sym- metry and dichroism results. There are, however, several complications due to the presence of overlapping fre- quencies, overtones, combinations, and Fermi resonance. Accordingly, a few modes are ambiguous and the assign- ment can only be considered tentative. For example, the 730 cm -1 infrared band appears to have both ~r and o dichroic components and the Raman counterpart ap-
T A B L E H. Observed and calculated frequencies and approxi- mate potent ia l energy distribution of PDP based on C2 l ine group s y m m e t r y .
Observed Calculated Mode frequencies frequencies Potent ia l energy dis t r ibut ion a'b
70 H~ (77%), H~ (36%) a Only force cons tan ts whose contr ibut ion are equal to or greater t h a n
10% are included. b Because of negat ive contr ibut ions f rom off-diagonal force constants ,
t he contr ibut ions f rom some diagonal force cons tan ts m a y appear larger t han 100%.
pears very broad. Therefore the possibility of overlap of two bands f rom the A and B species exist. Corresponding vibrat ions for bo th A and B species are calculated near this f requency and the ass ignment given in Tab le I appears sound. In the chlorote t ramer , a t least two modes were also observed near this frequency. 2
The number of observed o-polarized bands exceeds the number predicted f rom a s y m m e t r y analysis of any of the isolated chain models. We believe tha t this m a y be reasonably explained on the basis of combinat ion bands and Fermi resonance with fundamenta ls involving pre- dominant ly the very intense R a m a n line at 132 cm -1. Thus we have tenta t ively assigned the o-polarized bands at 610, approximate ly 540 and 344 cm -1 to combinat ion bands as shown in Table I.
The very strong 457 cm -~ R a m a n line (A mode) is the mos t difficult to proper ly assign, because it appears al- mos t exactly between two calculated modes a t 532 and 377 cm -~ (Table II). An ass ignment to the calculated 532
cm -1 mode is preferred for several reasons. First, a num- ber of I R bands and R a m a n lines appear near 500 cm -1, but all are weak and infrared dichroism studies demon- s t ra te tha t they are not A modes. Fur thermore , the potent ia l energy distr ibution of the calculated 532 cm -1 mode indicates tha t this is p redominant ly P- -C1 stretch. In po lymers such as poly(vinyl chloride) and poly(vinylidene chloride) s tretching modes involving chlorine a tom appear very strong, so tha t we would intuit ively ant icipate tha t a mode tha t is p redominant ly P--C1 s t re tch should also be strong. Finally, if we as- signed the calculated modes a t 532 and 377 cm -~ to observed frequencies near 500 and 457 cm -~, respectively, then this would leave a number of bands near 350 cm -1 unassigned. The next lowest calculated wave n u m b e r band af ter the 377 cm -~ band is a t 230 cm -1. T h e potent ia l energy distr ibution of this la t ter mode demons t ra tes t ha t it is a symmetr ic bend and on the basis of studies of the chloro- t r imer and the ch loro- te t ramer should be ob- served in the 200 to 250 cm -~ region.
Turning our a t tent ion to the calculated B modes, the only major discrepancy is a vibrat ion calculated a t 932 cm -1. A very weak R a m a n line is observed at 999 cm -1 (Fig. 1) but we do not believe it can be assigned to the calculated mode a t 932 cm -1. This mode is equivalent (in t e rms of potent ia l energy distribution) to those observed mode near 880 cm -1 in the te t ramer . One possibility is tha t the 882 cm -~ A mode has two components at a lmost the same frequency with the ~r-polarized band dominat- ing a o-polarized B mode.
In summary , the agreement between the calculated and observed frequencies of P D P can only be considered to be fair. However, considering tha t the relat ively simple valence force field derived f rom the two conformat ional isomers of the t e t r amer was t ransferred without modifi- cation and tha t no a t t e m p t was made to refine the force constants to the spectral da ta of the polymer, the results are certainly reasonable and allow m a n y of the observed infrared bands and R a m a n lines to be assigned with confidence.
ACKNOWLEDGMENTS
This research was performed with the financial support of the National Science Foundation Grant DMR 79-19841, Polymers Program.
The authors wish to thank Professor H. R. AUcock and Paul E. Austin for kindly synthesizing and supplying the polymers used in this study.
1. P. C. Painter, J. Zarian, and M. M. Coleman, Appl. Spectrosc. 36, 265 (1982). 2. J. Zarian, P. C. Painter, and M. M. Coleman, Appl. Spectrosc. 36, 272 (1982). 3. H. R. Allcock, Phosphorus Nitrogen Compounds, (Academic Press, New
York, 1972). 4. M. M. Coleman and P. C. Painter, J. Macromol. Sci., Revs. Macromol. Chem.
C16, 1975 (1978). 5. E. Giglio, F. Pompa, and A. Ripamonti, J. Polym. Sci. 59, 293 (1962). 6. R. A. Arcus, PhD thesis, The Pennsylvania State University, (1979). 7. K. H. Meyer, W. Lotmar, and G. W. Pankow, Helv. Chim. Acta 19, 930 (1936). 8. M. L. Huggins, J. Chem. Phys. 13, 37 (1945). 9. T. R. Manley and D. A. Williams, Polymer 19, 307 (1969).
10. H. R. Allcock, R. W. Alien, and J. J. Meister, Macromolecules 9, 950 (1976).
