J . Phys. Chem. 1991, 95,2988-2994
2988
in interpreting the temperature dependence of a) Contrary to previous expectation, there is important independent information to be gained about the anisotropy of the intermoimlar potential from the temperature dependence of aJ, which information would have been redundant and uninteresting if m values were all equal to the classical limit of -1. Furthermore, it is now possible to discern a correlation between the temperature dependence of the collision cross section and the average isotropic well depth.
1.6
1.3
B
l.l
Classical
igl e
0.0
0.7
0.6
1 : : : ; : : : : ; : : : : : : : : :
average o/kg, K for colllrion pair Figure 6. Average temperature dependence of the collision cross sections
correlates with the average well depth. sufficiently rapidly varying potentials to produce transitions, and thus the a-matrix elements are smaller and depend primarily upon strong collisions dominated by the repulsive anisotropy. The overall temperature dependence described by aJT) = aJ300 K)(T/300)"' then depicts the change in relative contributions of large and small a-matrix elements as the relative weighting of these large-a and small-a elements change with temperature according to the populations of the rotational states and the initial rotational quantum numbers. The latter appear explicitly in the averaging since it is a spin-rotation interaction relaxation mechanism being observed. The correlation that we have found seems to indicate the importance of Gordon's large a-matrix elements which depend primarily on the attractive potential. It is not surprising that the well depth has a significant influence on the averaging, but Figure 6 clearly shows that it is not simply a monotonic correlation. This is encouraging and can be taken as an indication that not just the well depth associated with the isotropic potential but the details of the anisotropy of the intermolecular potential are important
Conclusions
We have measured the cross sections for changing the rotational angular momentum of SeF6 and TeF, in collisions with self and with a set of 10 other molecules. The temperature dependence of these cross sections deviate significantly from the classically expected T I behavior. Prior to our work, there has been very little information on the temperature dependence of the collision cross sections for relaxation of the angular momentum vector. We compared the uJ of SeF, and TeF, with the uJof other mo~ecules previously studied in this laboratory. Although each collision pair has its unique temperature dependence and there is a sizeable range of such among the 10 collision partners, we have found a trend in the temperature dependence that ranges from a power predominantly less than to predominantly greater than this expected behavior. The deviation from the classically expected T 1 behavior is roughly in the order of increasing well depths: N2 = co < CF4 5 CH4 5 "0 6 C o 2 < SF6 < SeF, < TeF, which is useful by itself for estimating rotational relaxation times and poses a theoretical challenge. With this study we have definitely established that, outside of experimental error, the temperature dependence of a, is generally different from TI,is uniquely different for each collision pair, and has a general trend in the order shown above for these linear molecules and spherical tops. Acknowledgment. This research has been supported by The National Science Foundation (Grants CHE85-05725 and CHE89-01426). R.J.T. acknowledges the support of the Illinois Minority Graduate Incentive Program. Angel de Dios measured the I9F chemical shifts of MoF, and WF6 in the vapor phase relative to SiF4. Registry No. SeF,, 7783-79-1; TeF,, 7783-80-4; Ar, 7440-37-1; Kr, 7439-90-9; Xe, 7440-63-3; N2, 7727-37-9; CO, 630-08-0; HC1, 764701-0; C02, 124-38-9; CH4, 74-82-8; CF4, 75-73-0; SF6, 2551-62-4.
Matrlx Infrared Spectra of the Products of the P2 and O3 Reaction Matthew McCluskey and Lester Andrews* Chemistry Department, University of Virginia, Charlottesuille, Virginia 22901 (Received: August 24, 1990) Molecular P2 evaporated from heated GaP was codeposited with an argon/ozone stream at 12 K. Strong infrared product absorptionsverify the spontaneousreaction of P2and 03.Isotopic substitution, concentration variation, photolysis, and annealing behavior provide the basis for identification of primary reaction products PO and PO2and the new molecule P 2 0 and secondary products PO3, P205,and cyclic P402. Photolysis produced further secondary reaction products oxo-bridged P2O4 and a new cyclic P40 isomer, and annealing gave still another oxo-bridged P2O4 isomer. Infrared bands for these products are in good agreement with predictions from ab initio calculations.
Introduction
In recent years, considerable work has been done in this laboratory to study . the . oxidation . . reactions . of phosphorus ~. using matrix isolation infrared and optical spectroscopy. The aim of these studies has been to produce and characterize less phosphorus oxides and to develop an understanding of the 0022-3654/91/2095-2988$02.50/0
mechanisms of these very complicated Most recently, a study of the reaction of p2 and 0 2 in argon matrices Yielded (1) Andrews, L.;Withnall, R. J Am. Chem. Soc. 1988, 110.5605. The band suggested for P 2 0 at 1197 cm-I is probably due to a (P,)(PO) complex. (2) Withnall, R.;Andrews, L. J . Phys. Chem. 1W8,92, 4610.
0 1991 American Chemical Society
Pi and 0,Reaction the interesting and unexpected result that the reaction involved large phosphorus-oxygen clusters of the form (Pz),(Oz), ( x = 1,2; y = 2-5) and was initiated by the ultraviolet photoexcitation of dipole-forbidden excited states of PZa6Several new molecules, oxo-bridged P203 and two isomers of oxo-bridged P204, were identified; however, no monophosphorus free radical species were detected. In contrast to the P2 and O2system, the results of the present study indicate that P2 and O3 react spontaneously during condensation to produce PO, PO2, and linear P20, the phosphorus analogue of N 2 0 . In addition to PO3,two isomers of oxo-bridged P204,the PzOs molecule, and two novel cyclic P40 and P4O2 species have been identified as secondary reaction products of P20. In general, the results of the primary reactions are consistent with earlier work on the N,-ozone system,' with the notable exception that photolyzing ozone was not required to initiate the P2reaction. Experimental Section
The cryogenic refrigeration system and the vacuum vessel have been described previously.1-6 Molecular P2 was produced by heating GaP (Alfa Products, 99.9% purity) in a sapphire tube to 700-750 OC,which gives a P2 vapor pressure of 1-10 pm.6*839 Under these conditions, P4 was not detected and the P2concentration in the matrix was 0.5-1%. Ozone was prepared by subjecting oxygen at a pressure of 25-100 Torr to a Tesla coil discharge, in a Pyrex finger held at 77 K.'O The ozone condensed as a dark blue liquid and was purified of remaining oxygen by several recondensation cycles in a stainless steel manifold. Normal isotopic O2 and 55% and 98% 180-enriched samples were used directly from lecture bottles to prepare isotopic ozone samples. Argon/ozone mixtures with 0.25, 0.5, 1, and 2% ozone were prepared in a passivated stainless steel manifold according to standard manometric methods. P2 was codeposited with argon/ozone mixtures for 8 h. After deposition samples were irradiated for 10 min, with a BH-6 high-pressure mercury arc lamp (lo00 W, Illumination Ind., Inc.) combined with a IO-cm water filter, which resulted in a 2201000-nmwavelength range. The light from the arc lamp was focused into a Supracil rod, 6 X '/2-in. diameter, which served as a light pipe to direct the beam onto the matrix. The temperature of the matrix during deposition or photolysis was 13 f 2 K. As a final step, the samples were annealed to 30 K. Infrared spectra were recorded on a Perkin-Elmer 983 spectrophotometer in the 1550-180-cm-' range with 1-2-cm-' resolution. Typically 4-6 scans were averaged for each spectrum to improve the signal-to-noise ratio. For the scrambled isotope experiments, spectra were recorded with 0.5-1 -cm-' resolution over smaller intervals. Typically 15-20 scans were averaged to produce a final spectrum. An ultraviolet spectrum of P2in argon and a visible-ultraviolet spectrum of P2 and 0.5% O3in argon were recorded on a Cary- 17 visible-ultraviolet spectrophotometer. Conditions were nearly identical with the infrared study, but lower concentrations and shorter deposition times were used to produce the matrix. The Ar/P2 spectra agree with an earlier report and confirm that GaP is a source of molecular P2.'l
Results In a series of 15 experiments the products of the reaction of P2and ozone, and the effects of O3concentration, photolysis, and annealing on the product distribution were examined with infrared (3) Withnall, R.; McCluskey, M.; Andrews, L. J . Phys. Chem. 1989,93, 126. (4) Mjelke, Z.; Andrews. L. J . Phys. Chem. 1989, 93, 2971. (5) Mielke, Z.; McCluskey, M.; Andrews, L. Chem. Phys. Lerr. 1990,165, 146. (6) McCluskey, M.; Andrews, L. J . Phys. Chem., in press. ( 7 ) DeMore, W. B.; Davidson, W. J . Chem. Phys. 1959, 81, 5869. (8) Heimbrook, L. A.; Rasanen, M.; Bondeybey, V. E. Chem. Phys. Lerr. 1985, 120, 233. (9) Thurmond, C. D. J. Phys. Chem. Solids 1W5,26, 785. (10) Andrews, L.; Spiker, R. C. J . Phys. Chem. 1972, 76, 3208. ( 1 1) Larzilliere, M.; Jacox, M. E. NBS Spec. Publ. No.561 1979, 529; J . Mol. Specrrosc. 1980, 79, 132.
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 2989 0.
Figure 1. Infrared spectra in the 1550-300-cm-' region for products of the reaction of P2and I6O, (0.5%) in an argon matrix: (a) after codeposition of P2vapor with 0.5% ozone for 6 h at 12 K, (b) after IO min of 220-1000-nm photolysis, and (c) after 30 K annealing.
and optical spectroscopy. Some of these results were included, with those from the reaction of discharged P4 and ozone, in preliminary p ~ b l i c a t i o n s . ~Additional ~~~ information will be presented and discussed in detail below. Infrared Spectra. The deposition of P2and 0.5% O3in argon produced the spectrum shown in Figure la. New absorptions due to phosphorus oxides were observed in the terminal -PO (phosphoryl) bond stretching region at 1270.4 cm-' (A), in the symmetric -PO2 stretching region at 1179.4 cm-I, and in the oxo-bridged P-0-P antisymmetric stretching region a t 1007.9 (X), 866.9 (D), 825.7 (E), and at 423.6 (E) and 405.7 cm-I. Bands due to PO2 at 1319.1 and 386.4 cm-I, PO at 1218.1 cm-I, P2OS at 1473.2, 1158.2,735.1, and 479.3 cm-I, and cis-HOPO at 1254.1 and 840.2 cm-I were also observed in the initial reaction spectrum.lt2J1 Photolyzing the matrix samples with the light from a highpressure mercury arc lamp (220-1000 nm) produced new product absorptions in the antisymmetric -PO2 stretching region at 1438.4 cm-' (Y), in the symmetric -POz stretching region at 1168.1 cm-I ( Y ) ,and in the antisymmetric P-0-P stretching region at 955.4 cm-l ( Y ) and produced a weak band a t 1240.2 cm-I identified earlier as the phosphoryl group stretch of the terminal oxygenbonded isomer of P 4 0 1 and another weak band at 859 cm-' previously identified as the P-0-P stretch of oxo-bridged P203.6 Also, photolysis increased the bands at 825.4 (E) and 423.6 cm-I (E) (Figure lb). Annealing the samples to 30 K produced new product absorptions characteristic of the phosphyl group in the antisymmetric -PO2 stretching region at 1450.0 cm-I (X), the -PO2 deformation region at 583 (X) and 506 cm-' (X), and growth in the 1007.9-~m-~ (X) band. A broad band near 840.2 cm-' also grew on annealing, while the bands at 825.4 (E) and 423.6 cm-I (E) decreased significantly (Figure IC). Isotopic Substitution. All of the observed product bands shifted with oxygen-18 substitution (Table I). In the antisymmetric -PO2 stretching region, new product bands at 1450.0 (X) and 1438.4 cm-I (Y) shifted to 1408.0 and 1398.8 cm-I, respectively. Their counterparts in the symmetric -PO2 stretching region shifted from 1168.1 (Y) to 1119.9 cm-I. In the phosphoryl stretching region, the new product absorption at 1270.4 cm-I (A) and the cis-HOPO band at 1254.1 cm-I shifted to 1233.9 and 1207.3 cm-I, respectively. The band at 1218.1 cm-t due to diatomic PO shifted to 1173.0 cm-I. In the antisymmetric P-0-P stretching region, the band at 1007.9 cm-I (X) shifted to 972.0 cm-I, and the band at 955.4 cm-I (Y) shifted to 907.6 cm-'. The bands at 866.9 (D), 825.4 (E), and 840.2 cm-l shifted to 829.6, 805.8, and 790.2 cm-I, respectively. A medium-resolution spectrum from the codepsition of P2 and 55% le03 is shown in Figure 2, and a high-resolution (12) Andrews, L.;McCluskey, M.; Mielke, Z.; Withnall, R. J . Mol. Struct. 1990,222,95.
McCluskey and Andrews
2990 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991
m
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TABLE I: Infrared Bands (cm-') Observed from the Codeposition of P, and 0.5%0, with Arnon 1535.8, 1493.2, 1450.5 1473.3, 1454.1, 1431.0 1449.1, 1430.6, 1408.0 1319.2, 1270.3, 1252.6, 1218.1,
,
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1301.4, 1280.1 1233.9 1207.9 1173.0
1168.0, 1165.5, 1146, 1143.6, 1142.0, 1140.1, 1125.9, 1119.8 1158.2 1158.1, 1155.4, 1136.8, 1135.2, 1133.0, 1109.1, 1105.5
1007.9 955.4 955.5, 951.0, 946.1, 916.0, 911.8, 907.8, 866.3, 829.9 866.9 840.2 825.7 825.6, 790.2 735.1 735.1, 733.1, 730.9, 728.9, 708.1, ?, ? 583.3 506.5 479.3 423.6 405.7 398. I 386.4 386.4, 379.5, 375.2 382.2 37 1.4
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Figure 2. Infrared spectra in the 1550-300-cm-' region for products of (1%) in an argon matrix: (a) after codethe reaction of P2 and 16J803 position for 20 h, (b) after IO min of 220-1000-nm photolysis, and (c) after 30 K annealing.
1536.2 1473.2 1450.0 1438.4 1319.2 1270.4 1254.1 1218.1 1216 1168.1
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