INFRARED ABSORPTION OF SOLID PHOSPHINE

Transcription

1 NFRARED ABSORPTON OF SOLD PHOSPHNE A. H. HARDN AND K. B. HARVEY Department of Chemistry, University of British Columbia, Vancouver, British Columbia Received August 13, 1963 The infrared absorption of solid phosphine has been measured in the region of 4" K to 68" K. An hitherto unreported transition at about 10" K has been observed. Above this temperature the spectrum is characteristic of a disordered system; below 10" K the absorption arising from the symmetric vibrational modes exhibits a fine structure composed of sharp lines. The large number of lines attributed to v? are interpreted as arising from inversion doubling, indicating a residual disorder in the system. NTRODUCTOK The crystal structure of solid phosphine is as yet unknown but it has been shown that it undergoes a number of transitions at low temperatures (1). The present investigation was undertaken to study the changes in the infrared spectrum expected to accompany these transitions. The data of Stephenson and Giauque (1) are summarized in Fig. 1, reproduced from FG t Temperature OK Heat capacity of phosphine in cal/deg mole (from Stephenson and Giauque). their paper. Three first-order transitions were observed at 30.29,49.43, and 88.10" K and, in addition, they report a second-order transition in the region of 31-35" K. The transition from the a form to the,8 form (these designations are those of the present authors) at 88.10" K occurred rapidly and was readily observed. n contrast, the P to y transition was an extremely slow one and Stephenson and Giauque found it necessary to hold the sample at 40' K for several days to obtain complete conversion. As a result, the,8 form could be readily supercooled to 30.29' K where a transition took place to the 6 form. Stephenson and Giauque interpreted the heat capacity curves of the /3 and 6 modifications as characteristic of the type associated with molecular rotation in the solid. Presuinably the 6 form is a disordered form of the y phase. The infrared spectrum of gaseous phosphine has been the subject of a number of investigations (2-6). RcConaghie and Nielson (5) have partially analyzed the vibrationrotation spectrum in the region of the fundamentals and their structural data for the ground vibrational state are in good agreement with the microwave results (7, 8). The molecular parameters, as defined in Fig. 2, have the following values: r = 1.42 A, h = A, a: = 93050f, and P = 57'30'. Canadian Journal of Chemistry. Volume 42 (1964) 84

2 HARDN AND HARVEY: SOLD PHOSPHNE = FG. 2. i Structural parameters of the ph3 molecule. The principal moments of inertia are,, = 6.29 X g cm2 and,, = 7.23 x g cm2, so that phosphine is an oblate symmetric top. t is also worth mentioning that no evidence has been obtained for inversion doubling in either the ground or first excited vibrational states of ph3. This places upper limits on the splitting of 2X10-5 cm-l in the ground state (8) and about 0.2 cm- for the first excited vibrational levels. EXPERMENTAL The phosphine was prepared by adding PC13 dropwise to a vigorously stirred ethereal solution of Li.4lH4 under an inert atmosphere of nitrogen (9-11). A cold-finger at dry-ice temperature \\as used to prel-ent the ether from escaping the reaction vessel and the reaction products were trapped at 78' K. Separation of the PH3 from P2H4, the other principle product, was accomplished by double distillation at 117' K (n-pentane slush) and the PHs produced was stored in the gaseous form. A slow reaction with stopcock grease was observed in the storage bulb, but this did not appear to introduce volatile impurities. As an added precaution, however, the phosphine was freshly distilled before each run. Noimpurities could be detected in the infrared spectrum of the gaseous phosphine. The latter was observed to explode on contact with air. The low-temperature infrared absorption cell used in this work has been described prexiously (12). Temperatures intermediate between those attainable with liquid nitrogen and liquid helium \\ere obtained by transferring quantities of helium just sufficient to maintain the desired temperature. t was found that the temperature of the copper block, in which the cesium iodide deposition plate is mounted, could be held to within 13' K of the desired temperature by this method. Spectroscopic measurements were carried out on a Perkin-Elmer 112G spectrometer with a modified source unit. Calibration was effected through established grating spectra and the reported frequencies are estimated to be accurate to within A2 cm-1. Spectral slit widths are indicated on the individual spectra. RESULTS The phosphine films investigated were deposited on the cesium iodide plate at about 65" K by admitting phosphine gas in short bursts. Spectra were recorded in the region of the fundamentals at a number of temperatures between 4" K and 65" K (approximately every 10 degrees), both while cooling and afterward during the warm-up. Several runs were made with deposits of different thicknesses and the results are summarized in Table and Fig. 3. Attempts to deposit the PH3 at temperatures above 88.10" K were unsuccessful because of the high vapor pressure of the solid so that we were unable to observe the spectrum of the a form. The high vapor pressure also limited the maximum annealing temperature to 65" K or 70' K. At temperatures above 10" K the spectrum of each of the fundamentals consists of

3 86 CANADAN JOURKAL OF CHEMSTRY. VOL. 42, 1964 FREQUENCY cm-1 FG. 3..Absorption spectrum of PH3 in the 4.2 and micron regions at 4' K (solid line) and 26" K (broken line). TABLE nfrared absorption of phosphine (frequencies in cm-) This work Nielson, - Mode gas 66' K X0 K 4" K NOTE: s = strong, m = medium, w = weak, vw = very weak. only a single absorption -peak and, surprisingly, none of these peaks underwent any significant changes as the temperature was varied. t seems certain that under our conditions of deposition and rate of cooling we obtained the form between 65" K and 30" K and the 6 modification between 30" K and 10" K. Our results indicate therefore that no significant spectral changes accompany the first- and second-order transitions in this region. On the other hand, the spectrum of the fundamental vz underwent a marked change at about 10" K to give a series of 10, very sharp, closely spaced lines. The overlap of vl and v3 introduces some confusion, but it seems likely that vl also exhibits fine structure below 10" K, although not to the same extent as vz. Since the fine structure disappeared at 10' K on warm-up, it can be concluded that the transition producing these changes is a reversible one. - DSCUSSON Assignments The assignment of the absorption due to the symmetric bending mode v2 and the degenerate, asymmetric bending mode v4 is straightforward. Some confusion arises from

4 HARDN AND HARVEY: SOLD PHOSPHNE 87 overlap of the other two fundamentals but, in the case of ph3, intermolecular interactions in the solid would not be expected to result in a reversal of order of the frequencies of vl and v3 SO that we have assigned them in the same order as in the gas phase. At 4" K the problem is a more difficult one, but it is reasonable to assume that the fine structure in the 2300 cm- region is due to vl, which is of the same symmetry (al) as v2, the other inode giving rise to fine structure at this temperature. We have, therefore, assigned the single strong peak at 2312 cm- to v3. At the same time, the assumption that all of the remaining lines in this region are due to vl must be treated with some reservation. Similarly, the strong absorption due to v3 may mask some of the vl fine structure. Frequency Shifts and Force Constants All four fundamentals of phosphine shift downward in frequency as a result of condensation. This behavior is in contrast to that in crystalline ammonia (13) where the symmetric bending mode shifts upward in the crystal by over 100 cm-l. t is unlikely therefore that hydrogen bonding, in the usual sense, plays any significant role in deternii~ling the crystal structure of phosphine. This conclusion is in agreement with the observed melting and boiling points of PHH. These relatively small downward shifts in crystalline ph3 are reflected in the potential function, for internal vibrations. Assuming the coupling between the lattice vibrations and internal vibrations to be small, we may treat the internal problem separately and apply the same type of potential function as that used in the gas phase. We employed a simple valence-force potential function of the form Since no corrections were made for anharmonicity, a more general potential function was unnecessary. The equations for the valence-force treatment of pyramidal XY3 molecules have been derived by Lechner (14) and are tabulated by Herzberg (15). Since the frequencies of all four fundamentals are known in both the crystal and the gas it is possible to calculate two independent sets of values for k and ks in each case. Assuming the structural parameters to be the same as in the gas phase, the values listed in Table 1 are obtained. TABLE 1 Valence force constants for PHs (in units of From vl and vn dynes/cm) From v~ and vi Gas " ( n view of the approximations involved, the agreement between the independent pairs of values is reasonable. At any rate there is little doubt that the stretching force constant k is about lyo smaller in the crystal as a result of crystal field perturbations, while the bending constant ks is about 3.5y0 smaller. n the case of ammonia the bending force constant was estimated by Hornig (13) to be about 5% larger in the crystal so that again we are led to the conclusion that hydrogen bonding is absent in crystalline ph3. Crystal Structure and the 10" K Transition At temperatures above 10" K the lines assigned to vl, v2, v3, and v4 have band widths of approxinlately 7, 8, 15, and 17 cm-l respectively; all far in excess of the spectral slit

5 88 CANADAN JOURXAL OF CHEMSTRY. VOL width of the instrument. By absolute standards, these lines are not excessively broad for crystal spectra but the line widths are certainly far larger than the approximately 1 cm-1 line widths of the fine structure below 10" K. The latter are, in fact, of the same order as the resolution of the spectrometer. We are led to conclude, then, that there is some degree of disorder in the P and 6 forms right down to 10" K. This conclusion is in accord with the data of Stephenson and Giauque but it is surprising that no spectral changes accompany the transitions in the 30 to 35' K region. Furthermore, if as the calorimetric work indicated, the PH3 molecules are executing rotational motion in the /3 and 6 phases, a narrowing of the absorption peak would be expected as the temperature was lowered. We have been unable to detect any change in band width to confirm this. Such an effect might be masked, however, by the broadening due to additional disorder. The transition at 10' K and the spectrum of the low temperature phase exhibit some interesting features. Since the transition is a reversible one and the P form is obtained on warming, the crystalline modification we observed at 4" K must also be metastable with respect to the y form. Some degree of disorder might be expected then, even at 4" K. This is in accord with the broadness of the v3 and v4 absorption peaks (about 15 cm-) in the 4" K spectrum. On the other hand, the sharpness of the lines in the fine structure of the absorption due to vl and v2 would indicate an ordered structure. Apart from their sharpness, one must also account for the large number of lines in the v2 absorption band. Under a factor or unit cell group analysis, the number of absorption peaks arising from a single non-degenerate, internal vibrational mode may have a maximum value equal to the number of molecules in the unit cell. t is unlikely, however, that there are as many as 10 molecules in the unit cell of phosphine. A figure of two, four, or six seems more reasonable. Noting, in addition, that in the absorption assigned to v2 there are about twice as many components as there are in the vl fine structure, one infers that some effect is giving rise to a doubling in the case of v2. The effect observed is, in fact, consistent with inversion-doubling arising from the two equilibrium positions of the ph3 molecule. n gaseous ammonia, for example, a splitting of some 35 cm- is observed for the vz = 1 level and 1 cm- for the vl = 1 level. On the other hand, the inversion doubling in gaseous phosphine is too small to be measured. t is clear, however, that the height of the barrier to inversion will be related to the magnitude of the bending force constant so that an increase in ks will decrease the magnitude of the splitting. This is observed in crystalline ammonia where the doubling is too small to be observed. n the case of phosphine, however, ka decreases on condensation so that the barrier to inversion will be lower than in the gas phase. t is difficult to assign a specific value to the magnitude of the doubling in crystalline PH3. On the basis of the two strong doublets (975, 976 cm- and 981, 983 cm-l) the 10 peaks of the v2 absorption can be divided in two ways to give two sets of five lines differing by about 2 cm-l or 6 cm-. Without single crystal spectra there is no criterion to decide which of these values is correct but the smaller one seems more reasonable. The vl fine structure is of little value for comparison because of the overlap with v3. ACKNOWLEDGMENTS The authors are indebted to Mr. Rudolf h4uehlchen for his assistance in the construction and maintenance of the apparatus. We wish also to acknowledge the financial assistance of the National Research Council of Canada and the University of British Columbia in carrying out this work.

6 HARDK AND HARVEY: SOLD PHOSPHNE 89 RBSUME On a mesurk 'absorption infrarouge de la phosphine crystalline entre 4" K et 68' K. Une nouvelle transition dans la rkgion de 10" K a kt6 observke. Aux tempkratures plus 6levkes que 10' K, le spectre est caractkristique d'une syst&me dksordonnke; au-dessous de 10" K, 'absorption des vibrations symktriques a une structure composke de plusieurs lignes ktraites. Dans le cas de v2 la multiplicitk des lignes a btk interprktke comme le rksultat de deux positions d'bquilibre occasionnkes par l'inversion. REFEREXCES 1. C. C. STEPHEXSON and by. F. GAUQUE. J. Chem. Phqs. 5, 149 (1937). 2. L. tv. FLPG and E. F. BARKER. Phvs. Rev. 45, 238 (1934). 3. J. B. HOTT.~RD. J. Chem. Phys. 3, 207 (1935). 4. E. LEE and C. K. LVC. Trans. Faradav Soc ). 5. V. Al. RCCOSAGHE and H. H. XELSO;. J. Chem. Phys. 21, 1836 (1953). 6. D. C. RlcC~al* and P. N. SCHATZ. J. Chem. Phqs. 24, 316 (1956). 7. C. C. Loollrs and h4. 1V. P. STRANDBERG. Phys. Rev. 81, '798 (1951). 8. ill. H. SR~ETZ and R. E. \\ ESTOX. J. Chem. Phqs. 21, 898 (1953). 9. A. E. FSHOLT, -4. C. BOND, and H.. SCHLESNGER. J. Am. Chem. Soc. 69, 1199 (47). 10. A. E. FYHOLT, A. C. BOSD, K. E. ~VLZBACH, and H.. SCHLESSGER. J. Am. Chem. Soc. 69,2692 (47). 11. E ~~BERG and G. MULLER-SCHED&~A~ER. Chem. Ber (1969). 12. K. B. HARVEY and J. F. OGLVE. Can. J. Chem. 40, 83 (1'962). 13. F. P. RCD~G and D. F. HORLG. J. Chem. Phys. 19, 594 (1951). 14. F. LECHXER. 1%-ien. Ber. 141, 633 (1932). 15. G. HERZBERG. nfrared and Raman spectra of polyatomic molecules. Van Yostrand, New York