5614
J. Am. Chem. SOC.1987, 109, 5614-5620
Gas-Phase Infrared Spectra of the Unstable Phosphaalkenes CF2=PH, CF2=PCF3,and CH2=PC1: The C=P Stretching Vibration and Force Constant Keiichi Ohno,* Eiichi Kurita, Masanobu Kawamura, and Hiroatsu Matsuura Contribution from the Department of Chemistry, Faculty of Science, Hiroshima University, Higashisenda-machi, Naka- ku, Hiroshima 730, Japan. Received August 25, 1986
Abstract: Gas-phase infrared spectra of the unstable phosphaalkenes CF2=PH, CF2=PCF3, and CH,=PCI and their deuteriated species have been measured and vibrational assignments have been made. The C=P stretching bands are observed at 1349.5, 1350.2, and 1365.3 cm-' for CF2=PH, CF2=PD, and CF2=PCF3, respectively, and a t 979.7 and 847.9 cm-I for CH,=PCI and CD,=PCI, respectively. Normal coordinates were treated for these molecules. For the fluorophosphaalkenes, the C=P stretching mode is highly coupled with the CF, symmetric stretching mode; one of the coupled vibrations is shifted to higher wavenumber of about 1350 cm-l and the other to lower wavenumber of about 730 cm-I. For CH2=PC1, the C=P stretching vibration of 979.7 cm-' is almost negligibly perturbed by other vibrational modes, so that it gives an almost intrinsic C=P stretching wavenumber. For CD,=PCl, the C=P stretching mode is coupled largely with the CD2 scissoring mode. The band intensities of the C=P stretching vibration have also been discussed. The C=P stretching force constant was determined to be 562-668 N m-l. The values for the carbon-phosphorus force constants of the double and triple bonds are roughly two and three times, respectively, as large as the value for the single bond, and the values for the carbon-phosphorus bonds are about half the values for the corresponding carbon-nitrogen bonds.
In recent years, there has been a great interest in the multiple bonding that involves elements in the third and higher rows in the periodic table. A new class of transient molecules containing a C=P bond was first reported by Kroto et a1.I in 1976; these molecules were produced by thermal decomposition and were detected by microwave spectroscopy. At the same time, Becker2 found by N M R spectroscopy some phosphaalkenes that were kinetically stabilized by bulky substituents. Since then many phosphaalkenes have been prepared by a large variety of metho d ~ . ~However, .~ owing to their instability, gas-phase infrared spectra of simple phosphaalkenes have been scarcely obtained, and therefore the C=P stretching vibration has not been definitively assigned. In the present paper, we report the gas-phase infrared spectra of several unstable phosphaalkenes, namely difluorophosphaethene (CF2=PH and CF,=PD), perfluoro-2-phosphapropene (CF2= PCF3), and 1-chlorophosphaethene (CH,=PCI and CD2=PC1). W e have derived unambiguous wavenumbers for the C=P stretching vibration and the force constant associated with this vibration. Difluorophosphaethene CF,=PH was first detected in 1976l by microwave spectroscopy in the products of low-pressure flow pyrolysis of CF3PH2and was later identified by photoelectron and N M R spectroscopy in the products of a reaction of CF3PH2with KOH.5 Recently we have obtained the infrared spectrum of CF2=PH in part in the course of the preparation of FC=P.6 Perfluoro-2-phosphapropene CF2=PCF3 was first trapped in 1979 in base hydrolysis of (CF3),PH with K O H by Nixon et al.,' who identified it by N M R spectroscopy. Later Burg8 reported the infrared spectrum of this molecule in a pyrolysis study of its cyclic dimer and trimer species. Very recently, Steger et aL9 have (1) Hopkinson, M. J.; Kroto, H. W.; Nixon, J. F.; Simmons, N. P. C. J . Chem. SOC.,Chem. Commun. 1976, 513. (2) Becker, G. Z . Anorg. Allg. Chem. 1976, 423, 242. (3) Kroto, H. W. Chem. SOC.Rev. 1982, 11, 435. (4) Appel, R.; Knoll, F.; Ruppert, I. Angew. Chem., Int. E d . Engl. 1981, 20,
731.
(5) (a) Kroto, H. W.; Nixon, J . F.; Simmons, N. P. C.; Westwood, N. P. C. J . A m . Chem. SOC.1978, 100, 446. (b) Eshtiagh-Hosseini, H.; Kroto, H. W.; Nixon, J. F. J . Chem. SOC.,Chem. Commun. 1979, 653. (6) Ohno, K.; Matsuura, H.; Kroto, H. W.; Murata, H. Chem. Lett. 1982, 981. (7) Eshtiagh-Hosseini, H.; Kroto, H. W.; Nixon, J. F.; Ohashi, 0. J . Organomet. Chem. 1979, 181, C1. (8) Burg, A. B. Inorg. Chem. 1983, 22, 2573. (9) Steger, B.; Oberhammer, H.; Grobe, J.; Le Van, D. Inorg. Chem. 1986, 25,
3177.
0002-7863/87/ 1509-5614$01.50/0
determined the ra structure of CF2=PCF3 by using electron diffraction. 1-Chlorophosphaethene CH,=PCl was first characterized by microwave spectroscopy in 1976' through high-temperature pyrolysis of CH3PCI2and later identified by N M R spectroscopy in 1980 through a reaction of CH3PC12with N(C2H4),N.Io Among the several preparations an efficient route to CH2=PC1 is to utilize pyrolysis of (CHJ3SiCH2PC12 a t about 680 O C . Recently, the r, structure of this molecule has been determined by Kroto et al." and Bak et al.I3 by using a microwave technique. Experimental Section The samples of CF2=PH, CF2=PD, and CF2=PCF3 were prepared by passing CF3PH,, CF,PD,, and (CF,),PH vapors, respectively, at room temperature and low pressure (ca. 20 Pa) through a glass tube, 1 cm i.d. and 70 cm long, filled with KOH The products were passed through a trap cooled at -120 "C so that the generated water was condensed and were stored at -196 'C. No exchange reaction from CF2= PD to CF2=PH occurred in this preparation process. The precursors were prepared by reactions of the corresponding iodo compounds with PH, or PD,.I CH2=PCI was prepared by pyrolysis of (CH3),SiCH2PCI2 at about 680 "C," and CD2=PC1 was prepared, less efficiently than CH2=PC1, by pyrolysis of CD,PC12 at about 1000 'C.' The pyrolysis products were passed through two traps successively, one cooled at -1 20 'C and the other cooled at -196 'C. The greater part of the CH,=PCI or CD2=PC1 product was, however, polymerized to yield white solid on a glass wall inside the second trap. Vacuum distillation was used to separate CH,=PCI or CD,=PCI from a large amount of byproducts. The infrared spectra were recorded on a JE,OL JIR-40X Fourier transform spectrometer at a resolution of 0.12 or 0.25 cm-' with a globar source and a TGS or an MCT detector. Mylar and KBr/Ge beam splitters were used for the measurements in the regions 50-500 and 400-4000 cm-I, respectively. Dry nitrogen gas was continuously passed through the instrument during the measurement to get rid of water vapor. Sample pressures were 1-5 kPa in a 12-cm glass cell fitted with KBr or polyethylene windows. Wavenumbers of absorption peaks were calibrated against the standard lines of CO,, HzO, HC1, and DC1.I4 (10) Appel, R.; Westerhaus, A. Angew. Chem., Int. Ed. Engl. 1980, 19, 556
(11) Kroto, H.
W.; Nixon, J. F.; Ohashi, 0.;Ohno, K.; Simmons, N. P.
C. J . Mol. Spectrosc. 1984, 103, 113. (12) Bock, H.; Bankmann, M. Annew. Chem., Inr. Ed. Engl. 1986,25, 265
and references therein. (13) Bak, B.; Kristiansen, N. A,; Svanholf, H. Acta Chem. Scand., Ser. A 1982, 36, 1 . (14) Cole, A. R. H. Tables of Wavenumbers f o r the Calibration of Infrared Spectrometers, 2nd ed.; Pergamon: New York, 1977.
0 1987 American Chemical Society
J . Am. Chem. SOC.,Vol. 109, No. 19, 1987 5615
Gas-Phase Infrared Spectra of Unstable Phosphaalkenes
Table I. Rotational Constants and PR Separations of Band Envelopes for Phosphaalkenes rotational constant (cm-I) B 0.158 990 0.155 868 0.034 84 0.155685 0.144 181
A 0.370493 0.356 114 0.1 I 3 43 0.757 607 0.655 267
C 0.111 103 0.108 282 0.030 90 0.128 940 0.1 18 003
CF,=PH“ CF,=PD“ CF2=PCF3b CH2=P35CIc CD2=P35C1c Microwave data.3 Calculated from the electron-diffraction data.9 Microwave data.”
PR separation (cm-I) A type 18.4 18.1 9.0 18.3 17.5
B type
c type
14.7 14.5 7.4 15.2 14.6
27.6 27.2 13.5 27.4 26.3
int, intensity; s, strong; m, medium; w, weak; v, very; and sh, shoulder. *PR separation of band envelope. CP.E.D.,potential-energy distribution. For the description of the symmetry coordinates si, see Table VIII. For the underlined P.E.D., see text.
Table 111. Observed and Calculated Wavenumbers for CF,=PCF? this work Burg“ u(obsd) (cm-I) intb u(obsd) (cm-I) *I
1365.3 1248.9 1149.1 1095 746 sh 737 484 sh 470 432
vs
1363 1248 1148
int
Results and Discussion The planar molecules of CF,=PH and CH2=PC1 have C, symmetry and nine fundamentals are divided into Seven of the a’ species and two of the a’’ species. The molecule of C F ~ = P C F ~ also belongs to the C, point group. O f eighteen fundamentals, twelve a r e b f the a’ species and six are of the a” species. The a’ vibrations are expected to give AB-type bands, whereas the a” vibrations a r e expected t o g i v e C-type bands. The rotational constants and the P R separations of the band envelopes calculated by the Seth-paul formulasis are given in Table I, The observed (15) Seth-Paul, W. A,; Dijkstra, G . Spectrochim. Acta 1967, 2 3 ~2861. ,
64 33 80
u(calcd) (cm-’)
P.E.D.‘ (%)
~ , ( 6 1 ) ,~ < ( 5 0 )s6(13) , Y> S s3(87), s7(17) u3 vs s9(99), s11(13) y4 W ~ ~ ( 6 s1(42), 7 ) ~ s10(29) US vw 742 2.3 ~ < ( 5 1 ) ,s,O 2.3 O6 W 737 s10(16),3 ~ ( 3 5 ) ~, ~ ( 2 5 ) Vl VW ~ ~ ( 2 8sl1(22), 1, s10(20),~ ~ ( 8 ) us W 458 0.6 s11(58),37(15), J I O ( ~ ~ ) u9 vw ~ , ( 8 2 ) ,s2(8) VI0 31(24), sA331, 310(32) VI 1 312(76), 3.49) VI2 ~ 4 ( 8 3 ) s, d 7 ) a” u13 1134.5 m 1132 86 s14(102),s d 1 4 ) . sI6(7) u14 551 VW 541 0.5 s13(97) VIS 475 W 458 0.6 31d87) u16 s1d7) VI7 s17(78), sis(19) VI8 S18(79), s,,(20) “Reference 8. bSee footnote a of Table 11. ‘P.E.D., potential-energy distribution. For the a’ species, $1 = C-P str, 52 = C=P str, 3 3 = CF2 a-str, s4 = CPC def, s5 = CF, s-str, s6 = CF, sci, s, = CF, rock, s8 = CF3 s-str, s9 = CF3 a-str, sIo= CF3 s-def, s I 1= CF, a-def, and s 1 2= CF, rock; and for the a” species, s I 3= CF, wag, s i 4= CF3 a-str, sI5= CF3 a-def, SI6 = CF3 rock, sl, = C=P tor, and s18 = C-P tor. a‘
1363 1247 1145 1094 763 736 488 470 424 327 27 1 97 1138 551 472 296 76 55
infrared data for these molecules are given in Tables 11-V. Difluorophosphaethene. The preliminary infrared spectrum of difluorophosphaethene CF2=PH has been obtained in the course o f t h e Preparation of fluorophosphaethyne F C E P by treatment Of CF3PH2 with KOH*6The may be described as follows KOH
CF3PH2 E CF2=PH
KOH
-KF. -H,O
FCGP
As the new bands appeared treatment proceeded$ in the infrared spectrum. These bands a r e assigned to the vibrations of CF,=PH or FCGP on the basis of the rate of the relative intensiiy change with the KOH contact time or elapse time; the bands due to the same species change with the same
5616 J . Am. Chem. SOC.,Vol. 109, No. 19, 1987
Ohno et al.
Table IV. Observed and Calculated Wavenumbers for CH,=PC1 and CD,=PCl v(obsd) (cm-I) int' type AY(PR)~ (cm-I) CH,=PCl a' VI 3095.6 vw AB u2 2974.3 vw y3 1372.3 m AB 17 *A 979.7 W AB 18 u3 792.4 S -B vs AB Y6 499.7 15 y7 340.2 W a" us 804.7 vs C y9 609.4 vw C
u(calcd) (cm-I)
P.E.D.' ('3%)
3087.0 2982.6 1373.0 980.2 794.9 505.3 339.8 806.2 609.4
Sd100) Sd99) S6(95), $2(11)9 SS(l) ~ g ( 6 ) , Sq(1) ~7(93),~4(6)1~ ~ ( 2 1 ~ ~ ( 8~4(12), 9 ) ~ 37(4) $4(82), ~ 1 ( 1 0 ~) ,7 ( 4 ) %(101), Ss(1) Sa(100)
2301.0 2165.7 1120.1 847.6 641.6 483.9 309.0 633.4 438.7
$498). S2(2), & S,(55), Ss(2) s6(48), s4(l) S7(72), ~4(18),~ i ( 1 2 ) ~ 1 ( 8 3 4) ,1 7 L ~4(2) Sd79), S7(12). S,(5) SdlOl), % ( I ) Sd100)
CD,=PCl a'
VI
Y2
1120.8 847.9 642.9 490.0
u3 y4
Y5
Y6
m
AB AB -B AB
W
S
vs
17 15
Y7
a"
634.5 43 1.6 a'bicSeefootnotes a, b, c of Table 11.
C C
S
vw
u9
Table V. Combination and Overtone Bands Observed for CF*=PH, CF,=PD, CH,=PCI, and CD,=PCl Y (obsd) u(obsd) (cm-I) into assignment (cm-I) into assignment CF2=PH 2699.1 vw 2 ~ 2 1370.1 w Y4 + v6 2229.0 vvw u2 + u4 1347.8 sh vs ~2 + UT - Y, 1346.2 sh vs Y, + 2u7 - 2u7 2111.4 vw ~ 3 + ~ 4 us + Y6, Fermi 2104.8 vw u3 + u4 + ul - u7 1211.3 sh s 1132.4 vw 2u9 ~2 + U S 2075.6 w 2069.6 vw ~2 + U S + U T - ~7 727.8 sh w US + ~7 - ~7 2700.2 2083.2 2079.1 2008.4 2744.2 2160.0 1951.5 1618.4 1597.7
CFI=PD 1878.9 vvw 2 ~ 2 Y* + US 1348.5 sh w 1347.1 sh vw U, + us + u7 - u1 1215.8 sh vw u2 + u4 vvw 2u3 vvw ~j +
US
vvw 2u4 vw 2 ~ 8 vvw U S U S
+
+ ~2 + ~7 - ~7
vvw
~3
vs vs s
u2
~4
0,
0
C 0
c
t .-
+ 2u1 - 2u7 us + v6, Fermi
E
CH,=PCl 1401.9 vvw ~g + ~9 996.7 w 2u6, Fermi 496.6 sh VS Yg + U T - UT 493.6 sh VS Yg + 2U7 - 2Y7
2 I-
CD,=PCI 1122.9 w us + u6, Fermi 486.8 sh vs 'See footnote a of Table 11.
v6
+ u7 - u7
intensity ratio. We have attempted to prepare a pure sample of CF2=PH, but actually we obtained a mixture of CF2=PH, FC=P, and CH3PH2. The spectrum of the mixture containing mainly CF2=PH and CF3PH2 (Figure l b ) was obtained by a single passing of CF3PH2vapor through a glass tube filled with K O H pellets. Thus, we had the spectrum of CF2=PH (Figure IC) by subtracting the spectrum of the precursor CF3PH2(Figure la) from the spectrum of Figure lb; the bands of CF2=PH are denoted by arrows in the spectrum. Similarly, the spectrum of CF2=PD (Figure Id) was obtained by the K O H treatment of CF3PD2. The observation of various combination bands is helpful for the spectral interpretation so that the bands which are pertinent to the component fundamentals of the combination are unequivocally assigned to the same molecular species. Table V lists the observed combination bands of CF2=PH and CF,=PD, together with the overtone bands. The observed PR separations of the bands for CF2=PH and CF2=PD (Table 11) are also in good agreement with the estimated values (Table I). Vibration-rotation analyses of the strong bands at about 1350 and 1220 cm-l for CF2=PH and CF2=PD gave reasonable values of the rotational constants in comparison with the reported microwave values (Table I), namely, the successive spacings of the bands are
v)
C
4 100)
a
+ CF, s-str.
y
C=P str.
1
'
\i
t CF, s - s t r .
1
J . Am. Chem. Soc.. Vol. 109, No. 19, 1987 5617
Gas- Phase Infrared Spectra of Unstable Phosphaalkenes
3
I
,
3000
I ! 1500
2000
3l I
3
lo-oo -
-
~
~
W a venum b e r / crn-l
I 1000
500 1500 Wavenumber/cm-'
IO00
500
Figure 2. (a) Infrared spectrum of precursor (CF3),PH; (b) spectrum of a mixture containing mainly (CF,),PH and CF,=PCF3, prepared by passing (CF,),PH vapor through a glass tube filled with KOH pellets; and (c) difference spectrum (spectrum b - spectrum a) of CF,=PCF,. The bands of CF,=PCF, are denoted by arrows.
Figure 3. Infrared spectra for pyrolysis products of (CH,),SiCH,PCI,: (a) immediately after the pyrolysis; (b) after elimination of HCI, HC=P, and CH,=CH, from the products; and (c) 12 h after spectrum b. The bands of CH,=PCI are denoted by arrows. Other bands are identified as (CH,),SiCI (l), HCI (2), CH,PCI, (3), CH2=CH2 (4), and HC=P (5).
(106.68°)26and PX, (X = H, 93.345°;24and X = C1, 100.2702') to CH,=NH (1 10.4")28and CH2=PX (X = H, 97.4°;25and X = C1, 1O3.Oo1') have a good correlation with each other. at 2326.9 (pseudo-B-type) and 884.4 cm-I (AB-type), which are Perfluoro-2-phosphapropene. Burg8 has obtained the infrared shifted to a lower wavenumber region on deuteriation, are readily spectrum of perfluoro-2-phosphapropene CF2=PCF3 by lowassigned to the P-H stretching and C P H in-plane deformation pressure pyrolysis of its cyclic dimer and trimer species. We have vibrations, respectively, and the weak Q branch at 1088.8 cm-', independently investigated the infrared spectrum of CF2=PCF3 which is overlapped with unidentified bands, is assigned tentatively by a base reaction of (CF,),PH with KOH. In this reaction, a to the C P H out-of-plane deformation vibration. The pseudo-Atrace of water played an important role in promoting the formation type band at 1349.5 cm-l, the pseudo-B-type band at 1228.5 cm-', of CF2=PCF3. In the absence of water, only the precursor was and the AB-type band at 729.3 cm-I are assigned respectively to recovered, whereas in the presence of a considerable amount of the C=P stretching, CF, antisymmetric stretching, and CF, water, most of the low-boiling products were CF,H and CF2H2. symmetric stretching vibrations. These bands are correlated well Spectra a and b of Figure 2 are the infrared spectra of the prewith the A-, B-, and A-type bands of CF2=S at 1368, 1189, and cursor (CF,),PH and the mixture containing mainly CF,=PCF, 787 ~ m - ' . ~For ' CF,=S, the CF2 wagging, scissoring, and rocking and (CF3),PH. The difference spectrum (Figure 2, spectrum vibrations have been assigned to the C-type band a t 622 cm-I, b-spectrum a) is shown in Figure 2c, where the bands of CF2= the A-type band at 526 cm-I, and the B-type band at 417 cm-I, PCF3 are denoted by arrows. The observed wavenumbers for respectively. Thus, for CF,=PH, the C-type band at 568.0 cm-' CF,=PCF, are listed in Table 111, together with Burg's r e s u k R and the AB-type band at 485.5 cm-I are safely assigned to the The strong bands observed above 700 cm-' are in good agreemelit CF2 wagging and scissoring vibrations, respectively, but the weak with Burg's observation. The PR separations of 8 cm-' for the band a t 418.3 cm-' is tentatively assigned to the CF, rocking 1365.3-cm-' band and 7 cm-' for the 1248.9-cm-' band compare vibration. The vibrational assignments of CF,=PH and CF,=PD well with the expected values given in Table I. The weak bands are summarized in Table 11, where some of the previous assignin the region below 700 cm-I, whose wavenumbers are somewhat ments for CF2=PH6 have been interchanged. The C=P different from Burg's results, have been assigned tentatively. The stretching vibration is assigned to the very strong bands at 1349.5 band assignable to the C=P stretching vibration is observed at and 1350.2 cm-' for CF,=PH and CF2=PD, respectively, whose 1365.3 cm-I; this assignment is reasonable in comparison with the wavenumbers are, however, considerably higher than the C=P assignment for CF2=PH and CF2=PD. stretching wavenumber (1278.3 cm-I) for HC=P.,O 1-Chlorophosphaethene. CH2=PC1 was prepared by the pyThe P-H and P-D stretching modes are highly localized in the rolysis of (CH3),SiCH2PC1, at about 680 'C under a pressure normal vibrations of 2326.9 and 1690.6 cm-', respectively, these of 5-8 Pa. The pyrolysis products were passed through two traps wavenumbers being almost coincident with those for P H 3 (av. cooled at -120 and -196 'C. The gas-phase infrared spectra of 2325.0 cm-1)21and PD, (av. 1689.6 cm-1).22 If the r e l a t i ~ n s h i p ~ ~ the mixture collected in the second trap are shown in Figure 3 , between the bond length and the stretching wavenumber is aswhere the bands assignable to CH,=PCl are denoted by arrows sumed, the P-H bond length of CF2=PH is expected to be very and other bands are identified as those of (CH3),SiCI, HCI, close to 1.420 A for PH3.24 The P-H bond length for CH2=PH CH3PC12,CH2=CH2, and H C e P . Spectra b and c of Figure has also been obtained to be 1.420 A.25 The C P H angle of 3 indicate that the bands due to CH2=PC1 decrease in intensity CF2=PH may also be similar to the corresponding angle for in the presence of HC1, while the bands due to CH3PC12gradually CH2=PH, because the changes of the bond angles from N H 3 increase. The spectral observation in Figure 3 suggests the following reaction process (20) Garneau, J. M.; Cabana, A. J . Mol. Spectrosc. 1981, 87, 490. A(680 "C) (21) Baldacci, A,; Devi, V.M.; Rao, K. N.; Tarrago, G. J . Mol. Spectrosc. (CH,),SiCH2PCl2 CH,=PCl HC=P -(CH3)3S~CI -HCI 1980, 81, 179. (22) Kijima, K.; Tanaka, T. J . Mol. Spectrosc. 1981, 89, 62. Byproducts such as CH2=CH2 and HCECH may be formed (23) Duncan, J. L.; Harvie. J. L.; McKean, D. C.; Cradock, S. J. Mol. ~~~
~~
~
-
~~
Struct. 1986, 145, 225. (24) Maki, A. G.;Sam, R. L.; Olson, W. B. J . Chem. Phys. 1973, 58, 4502. (25) Kroto, H. W.; Nixon, J . F.: Ohno, K. J . Mo/. Spectrosc. 1981, 90, 367.
__ (26) (27) (28)
Benedict, W . S.; Plyler, E. K. Can. J . Phys. 1957, 35, 1235. Hedberg, K.; Iwasaki, M. J . Chem. Phys. 1962, 36, 589. Pearson, R . ; Lovas, F. J. J . Chern. Phvs. 1977, 66, 4149.
5618
J. Am. Chem. SOC.,Vol. 109, No. 19, 1987
Ohno et al.
ti
1500
1000 Wavenumber/cm-#
500
Figure 4. Infrared spectrum for pyrolysis products of (CH3)3SiCH2PC12 containing mainly CH2=PCI. Fundamentals of CH2=PC1 are denoted by arrows. through a C-P bond cleavage which occurs more easily at higher temperatures. The sample of CH,=PCI readily reacts with HCl to yield CH3PC12in an infrared cell a t room temperature. The identification of CH2=PCI is also supported by the observation of the relevant combination bands (Table V) and the P R separations of the bands assignable to CH,=PCl (Tables I and IV). The vibration-rotation analysis of the AB-type band a t 1372.3 cm-' has also identified CHz=PC1.16 Figure 4 shows a typical spectrum of the products containing mainly CH2=PCI. The weak Q branch a t 3095.6 cm-' is reasonably assigned to the vinylic CH, antisymmetric stretching vibration, and the very weak band a t 2974.3 cm-' is assigned tentatively to the C H 2 symmetric stretching vibration. The ABtype bands a t 1372.3 and 979.7 cm-' may be attributed to the CH, scissoring and C=P stretching vibrations, respectively. The band contours around 800 cm-' are interpreted to be composites of the C-type band centered a t 804.7 cm-' (CH, wagging) and the pseudo-B-type band centered a t 792.4 cm-' (CHI rocking). The weak C-type band a t 609.4 cm-' is assigned to the C H 2 twisting vibration, the strong band a t 499.7 cm-' to the P-C1 stretching vibration, and the very weak band at 340.2 cm-' to the CPCl bending vibration. In this way, all of the fundamentals of CH2=PCI have been detected. The C H 2 scissoring, rocking, and wagging wavenumbers of CH,=PCl (1372.3, 792.4, and 804.7 cm-I, respectively) are similar to those of CH2=SiH2 (1350, 741, and 817 ~ m - ' ) ,but ~ ~ they are considerably smaller than the wavenumbers for the corresponding vibrations of CH2=S (1447.0, 991.0, and 990.2 c ~ - ' ) . ~ OSimilar wavenumber differences have been noted for the CF2vibrations between CFt=PH and CF2= S I 7 The C=P stretching wavenumber of CH2=PCl, 979.7 cm-', differs significantly from the wavenumbers of fluorophosphaalkenes (ca. 1350 cm-I). In order to confirm the band assignments of CH2=PC1 mentioned above, the deuteriated derivative CD2=PCl was prepared by the pyrolysis of CD3PC1,. The spectra of the pyrolysis products are shown in Figure 5; Figure 5a is the spectrum of the products immediately after the pyrolysis, Figure 5b is the spectrum of the products kept in an infrared cell for 15 h, and Figure 5c is the difference spectrum (spectrum a-spectrum b). More enhanced bands in Figure 5b than in Figure Sa are due to CD3PCI2and CD2HPC12. Thus, the following reaction process is suggested to occur CD,=PCI
-
CD3PC12 + CD2HPC12
The spectral pattern in the region 500-1500 cm-' of CD,=PCI (Figure 5c) resembles very much that of CH2=PCI (Figure 4); on deuteriation, the bands of CH2=PCI a t 1372.3, 979.7, 792.4, and 804.7 cm-' are shifted to 1120.8 (AB-type), 847.9 (AB-type), (29) (a) Reisenauer, H. P.; Mihm, G.; Maier, G. Angew. Chem., In?. Ed. End. 1982, 21, 854. (b) Maier, G.; Mihm, G.:Reisenauer. H. P. Chem. Ber. 1984, 117, 2351. (30) Turner, P. H.; Halonen, L.; Mills, I. M. J . Mol. Spectrosc. 1981, 88, 402.
i'l! I
I500
I
I
I
I
I
1000 Wovenumber/crn-'
1
*I
1
500
Figure 5. Infrared spectra for pyrolysis products of CD3PC1,: (a) immediately after the pyrolysis; (b) after 15 h of pyrolysis; and (c) after subtracting spectrum b from spectrum a. The bands of CD2=PCI are denoted by arrows. Other bands are identified as CD3PC12(1) and CD2HPCI2 (2). 642.9 (pseudo-B-type), and 634.5 cm-I (C-type), respectively. The Q branch a t 490.0 cm-', which is heavily overlapped with the absorptions of CD3PC12and CD2HPC12,is assigned to the P-C1 stretching vibration, and the very weak Q branch a t 431.6 cm-' is assigned tentatively to the CD2 twisting vibration. The large wavenumber shift of the 1372.3-cm-' band ( C H 2 scissoring) of CH2=PC1 to 1120.8 cm-I on deuteriation establishes the assignment of the fairly weak bands a t 979.7 cm-' for CH2=PC1 and 847.9 cm-' for CD2=PCl to the C=P stretching vibration. The weak intensities of these bands are in contrast with the strong intensities of the C=P stretching bands a t about 1350 cm-' for fluorophosphaalkenes. Other fundamentals of CD2=PC1 were not detected in the present study due possibly to their weak intensities. The observed wavenumbers and assignments of the infrared bands of CH,=PCl and CD2=PCl are listed in Table IV. The weak C-type bands a t 754.8 and 747.9 cm-' in Figure 5c are assigned to the C P H out-of-plane deformation of the CHD=PCl species with the cis and trans dispositions, respectively, of the H and C1 atoms. Normal Coordinate Treatment. Normal coordinates were treated for CF,=PH, CF2=PCF3, and CH2=PC1 and their deuteriated species in order to elucidate the large difference in the C-P stretching wavenumber between CF2=PH (1 349.5 cm-') and CH2=PCl (979.7 cm-') and to derive the force constant associated with this vibration, The molecular structures of CF2=PH and CH2=PCl were taken from the microwave results,'*" and those of CF2=PCF, were transferred from the results of CF,=PH' and CF3PH2.'* The Urey-Bradley force field with moderate constraints was employed for the normal coordinate treatment, since only the limited vibrational data were available in the present study. At first, the Urey-Bradley force constants were determined for CF3PHz and (CF3)2PH on the basis of Burger's vibrational a~signrnents.'~J~ Secondly, the force constants for CF2=S,17 CF3PH2, and (CF3),PH were transferred to CF2=PH as initial values. The force constants, except for the repulsion constants, F(FCF) and F(CPH), were then adjusted to fit the observed wavenumbers for CF2=PH and CF2=PD. Finally, the force constants for CF,=PH and (CF3)2PH were transferred to CF2=PCF3, and some of the force constants were adjusted. A similar procedure was utilized for the normal coordinate treatment of CH,=PCI. Namely, the Urey-Bradley force constants were first determined for CHz=S,30 CH3PC1,,31 (31) Durig, J. R.; Block, F.; Levin, I. W. Spectrochim. Acta 1965, 21, 1105.
J . A m . Chem. SOC.,Vol. 109, No. 19, 1987 5619
Gas- Phase Infrared Spectra of Unstable Phosphaalkenes Table VI. Urey-Bradley Force Constants for CFz=PH, CF2=PCF3, and CH,=PCI"
force constant
value
force constant
value
H(CPH) F(FCF) F(FCP) F(CPH)
0.322 1.103' 0.461 0.1 49'
H(CPCI) F(HCH) F(HCP) F(CPC1)
0.447 0.0' 0.285 0.162'
CFZ=PH K(P-H) K(C=P) K(C-F) H(FCF) H(FCP)
3.081 5.565 4.527 0.291 0.337
K(P-Cl) K(C=P) K(C-H) H(HCH) H(HCP)
CH,=PCI 2.331 5.068 4.879 0.348 0.102
K(C=P) K(C-P) K(C-F), K(C-F), H(FCF), H(FCP),
CF2=PCFjC H(CPC) 5.895 F(FCP), CF, 3.078 F(CPC) 4.885 F(CH, wag) 4.130 4CFd 0.202' r(CP) 0.242'
CF, CF3 CF, CF3
0.198 0.207' 0.041' 0.548 0.023 0.100'
uUnits of the force constants are 10, N m-I for stretching, K bending, H; and repulsion, F and N m for intramolecular tension, K ; torsion, T ; and F(CH2 wag). bValues fixed in the least-squares fit. Other constants than those listed here are transferred from CF,=PH.
and (CH3),PC132and were then transferred to CH2=PC1. The force constants, except for F ( H C H ) and F(CPCl), were varied to reproduce the observed wavenumbers of CH2=PCl and CD,=PCI. The observed and calculated wavenumbers for CF2=PH and CF2=PD are given in Table 11, those for CF2= PCF, in Table 111, and those for CH2=PCI and CD,=PCl in Table IV, where the calculated potential-energy distributions are also included. These results indicate that the vibrational coupling among the symmetry coordinates is appreciable in most of the normal vibrations. Accordingly, the most dominant mode in the potential-energy distribution is adopted, in general, for describing the vibrational assignment. The Urey-Bradley force constants obtained are listed in Table VI. The force constants for CF,=PH and CH,=PCI in terms of the symmetry coordinates are given in Table VII. The C=P stretching force constant F(C=P str), as a diagonal element of the F, matrix, is 642 N m-I for CF2=PH and CF2=PD, 668 N m-l for CF,=PCF,, and 562 N m-I for CH,=PCl and CD2=PCl; these values may be compared with those for the C=S stretching force constant, 764 N m-' for CF2=S and 655 N m-I for CH2=S. The C-P stretching force constant F(C-P str) was determined to be 328 N m-' for CF3PH2,329/324 N m-l for (CF3)2PH, 295 N m-' for CH,PCI,, and 267/262 N m-I for (CH3)2PC1.'6 These values are compared with 258, 302/250, and 308 N m-' reported previously for CF3PH2,'*(CF3),PH,I9 and CH3PC12,31respectively. Band Intensity of the C=P Stretching Vibration. Although the u2 band of CF,=PH (1349.5 cm-I) and the v4 band of CH2=PCl (979.7 cm-I) are assigned to the C=P stretching vibration, their intensities are remarkably different from one another (see Figures I C and 4). This difference in intensity may be qualitatively explained as follows. The band intensity ri is expressed as33 Ti = (NAndi/3c2wj)( C ~ P / ~ Q ~ ) ~ where ( a P / d Q i )is the derivative of the dipole moment P with respect to the ith normal coordinate Qi and dj is the degeneracy of the ith normal mode with wavenumber w j . N A is Avogadro's number and c is the velocity of light. The quantity (aP/aQj)may be transformed as
(aP/aQi) =
Xljj J
(aP/as,)
(32) (a) Goubeau, J.; Baurngartner, R.; Koch, W.; Muller, U. Z . Anorg. Allg. Chem. 1965, 337, 174. (b) Durig, J. R.; Saunders, J. E. J . Mol. Srruct. 1975, 27, 403. (33) Hopper, M . J.; Russell, J. W.; Overend, J. J . Chem. Phys. 1968, 48,
3165.
Table VII. Force Constants in Terms of the Symmetry Coordinates for CF2=PH and CH,=PCI" CF,=PH C H,=PC I force constant value force constant value a'
F,,(P-H
2.439 0.104 F14 0.143 Fl 4 FZ2(C=P str) FZ2(C=P str) 5.620 F23 F23 -0.0 12 F24 0.115 F24 F25 0.307 F25 F26 -0.130 F26 F2 7 0.01 1 F27 F,,(CF2 a-str) F3,(CH, a-str) 5.052 F 35 0.0 F 35 -0.014 F36 0.0 F36 -0.00 1 F 37 0.228 F 37 0.121 F,,(CPH def) 0.928 F4,(CPC1 def) 1.772 F,,(CF, s-str) 6.285 F,,(CH, s-str) 5.052 F56 -0.070 F56 0.589 F 57 0.002 F5 7 0.0 F66(CH2 SCi) 0.386 F66(CF2sci) 1.208 F6 7 -0.006 F67 0.0 F77(CF2 rock) 1.022 F77(CH2rock) 0.341 a" FB,(CPH def) 1.199 a" FB8(CH2twist) 0.368 F89 0.028 F8 9 -0.204 Fg9(CH2wag) 0.237 F,,(CF, wag) 0.666 "Units of the force constants are 10, N m-I for str-str, IO-* N for str-bend. and N m for bend-bend. Fl2
str)
3.151 0.093 0.092 6.420 0.0 0.110 0.530 -0.21 1 0.0 4.781
a'
F,,(P-CI
str)
Fl2
where sj is t h e j t h symmetry coordinate and lJ,is the element of the L matrix. The L matrix elements calculated for CF,=PH and CH2=PC1 are given in Table VIII. For CF,=PH, the C=P stretching (s2),CF, symmetric stretching (sj), and CF, scissoring modes (sg) contribute for the most part to the normal vibration v2.
If the derivatives of the dipole moment for CF2=PH are assumed to be equal to those for CF,=S, namely (aP/as2)= -3.07 D A-', (aP/as,) = 4.83 D A-I, and (dP/ds,) = -1.81 D rad-',I7 then the intensity r2is estimated to be much larger than rj,being consistent with the experimental observation as shown in Figure IC. For CH2=PC1, the calculated L matrix suggests that the normal vibration u4 is associated primarily with the C=P stretching ( s 2 )and C H 2 scissoring modes (sg), whereas the potential-energy distribution defined by (P.E.D.),, = (lJa2FJJ/Aj) 100 shows that the C=P stretching mode ( s 2 )contributes almost exclusively to v4. The coupling between vibrational modes, as evaluated through the potential-energy distribution, is reflected by the band shift from their intrinsic wavenumbers. These considerations indicate that the observed intensity of the v4 band of CH,=PCl does not necessarily represent the intrinsic C=P stretching intensity, although this vibration gives the intrinsic C=P stretching wavenumber. The values of (dP/as2)and (dP/ds,) are likely to be of the same sign, as suggested by the facts that the v3 band is stronger than the v4 band (Figures 4 and 5c) and that the L matrix elements associated with s2 and $6 are of the same sign for u3. The C=P Stretching Wavenumber and Force Constant. The calculated potential-energy distributions for CF2=PH, CF2=PD, and CF2=PCF3 (Tables I1 and 111) indicate that the C=P stretching mode (s2)is highly coupled with the CF, symmetric stretching mode (s5). This vibrational perturbation makes one of the coupled vibrations shift to higher wavenumber of about 1350 cm-I and makes the other shift to lower wavenumber of about 730 cm-*. For CH,=PCl (Table IV), on the other hand, the C=P stretching vibration of 979.7 cm-l is almost negligibly perturbed by other vibrational modes, so that it gives an almost intrinsic C=P stretching wavenumber. This wavenumber compares well with the C=S stretching wavenumber 1059.2 cm-' for CH2=Sl7 and the C=Si stretching wavenumbers 985 and 984 cm-I for CH2=SiH2 and CH2=SiHCI, r e ~ p e c t i v e l y . ~For ~ CD,=PCI,
5620 J. Am. Chem. Soc., Vol. 109, No. 19, 1987
Ohno et al.
Table VIII. L Matrix Elements for CF2=PH and CH,=PCI" CF,=PH
PI s,(P-H str) s2(C=P str) s3(CF2a-str) s,(CPH def) sS(CF2s-str) s,(CF, sci) s7(CF2rock)
1.0120 -0.0032 -0.0009 -0.0047 -0.0021 0.0035 -0.02 55
s8(CPH def) S9(CF2 wag)
0.7286 -0.1549
ea
Q3
Q4
Qs
Q6
Q7
-0.0073 -0.0039 -0.38 19 0.3375 -0.0049 0.0062 0.3905
0.0177 0.0272 -0.1106 -0.6580 0.0318 -0.0172 0.0452
-0.0038 0.1132 0.0108 0.0682 0.1650 0.1176 0.0025
-0.0012 0.0577 0.0052 0.0172 0.0157 -0.305 8 0.01 70
0.0034 -0.0031 0.0460 -0.0307 -0,001 5 0.01 25 0.2866
Q2
-0.007 6 0.3142 0.0005
-0.0382 -0.28 8 3 0.4127 -0.0006 Q9
0.1509 0.5426 CH2=PCI
QI s,(P-CI str) s2(C=P str) s3(CH, a-str) s,(CPCI def) s,(CH2 s-str) s,(CH, sci) s,(CH, rock)
-0.0002 0.0096 1.0554 -0.0573 -0.0377 0.007 1 -0.1313
s8(CH2twist) ~ C H wag) Z
-0.0787 1.2754
Qa
Q3
Q2
0.0000
-0.0628 0.0392 -0.0048 1.0159 -0.1884 -0.007 6
-0.0034 0.1449 0.0002 -0.0060 0.0442
1.6506 -0.021 1
Q4
-0.0235
o.3008
-0.0020 -0.0526 -0.0070 -0.2973 -0.0196
Q5
-0.0545 0.0057 -0.0137 0.1148 0.0004 0.0189 1.0074
Q6
0.2337 0.001 1
-0.0037 -0.0994 0 0001 0.0009 0.1370
Q7
0.0524 0.0064 0.0048 0.1771 0.0003 -0.0 107 -0.0858
Q9
0.7721 -0.0173 -
"Definition of the symmetry coordinates si is the same as that of CH2=NH3' except for the coordinates s3, s5, and s7;s3 = (Ar3 - Ar2)/d2, ss = ( A r , + A r , ) / d j , and s, = (A(.?, - Ap2)/d\/2. For the underlined L matrix elements. see text. fable IX. Stretching Force Constants of Carbon-Phosphorus and Carbon-Nitrogen Bonds force constant value' force constant value' F(C-N) 5.1-7.2' F(C-P) 2.5-3.3 F(C=P) 5.6-6.7 F(C=N) 13.4-1 4.3' F(CSP) F(CSN) 16.9-18.8* 8.0-9.2* 'In units of I O 2 N m-I. Reference 36. Reference 37.
constants of the carbon-nitrogen multiple bonds are also included. It is interesting to note that the values for the double and triple bonds are roughly two and three times, respectively, as large as the value for the single bond and that the values for the carbon-phosphorus bonds are about half the values for the corresponding carbon-nitrogen bonds.
Conclusions
(34) (a) Gusel'nikov, L. E.; Volkova, V. V.; Avakyan, V. G.; Nametkin, N. S. J . Organomet. Chem. 1980, 201, 137. (b) Nefedov, 0. M.; Mal'tsev, A. K.; Khabashesku, V. N.; Korolev, V. A. J . Organomet. Chem. 1980, 201,
The present infrared study of fluorophosphaalkenes and 1chlorophosphaethene has given useful information for the C=P stretching vibration. The C=P stretching bands are observed in a wide range of 840-1370 cm-I, in contrast with the characteristic C=N stretching bands that are observed a t 1600-1700 cm-' well outside the fingerprint region. On the other hand, the determined values of the C=P stretching force constant are in a narrow range of 562-668 N m-I and are roughly two times as large as the value of the C-P stretching force constant. Thus, the observed C=P stretching wavenumbers are greatly dependent not only on the value of the C=P stretching force constant, which gives an intrinsic wavenumber of approximately 980 cm-', but also on the magnitude of the coupling with nearby vibrations of the same symmetry. The intensity of the C=P stretching vibration also varies with the magnitude of the vibrational coupling. The effect of the coupling on the intensity is different from that on the band shift from the intrinsic wavenumber. The isoelectronic C=Si, C=P, and C=S bonds have been shown to have similar values for the stretching force constants and accordingly similar intrinsic stretching wavenumbers.
(35) Hamada, Y . , ;Hashiguchi, K.; Tsuboi, M.; Koga, Y . ; Kondo, S. J . Mol. Spectrosc. 1984, 105, 70. (36) Ohno, K.; Matsuura, H.; Murata, H. J . Phys. Chem. 1984, 88, 342 and references therein. (37) (a) Amatatsu, Y . ; Hamada, Y . ; Tsuboi, M.; Sugie, M. J. Mol. Specrrosc. 1985, 111, 29 and references therein. (b) Inamori, T.; Hamada, Y . ;Tsuboi, M.; Koga, Y . ;Kondo, S. J . Mol. Spectrosc. 1985, 109, 256 and references therein.
Acknowledgment. The present work was partially supported by Grants-in-Aid for Scientific Research Nos. 59540290 and 60303003 from the Ministry of Education, Science, and Culture, Japan. The calculations in this work were performed on a HITAC M-200H computer of the Information Processing Center, Hiroshima University.
however, large vibrational coupling occurs between the C=P stretching (s2)and C D 2 scissoring modes (s6) with their intrinsic vibrational energies almost coincident with one another, resulting in large shifts of their vibrational levels. Thus, the observed wavenumbers of these vibrations are shifted by about 130 cm-' to higher and lower wavenumbers from the unperturbed wavenumber of approximately 980 cm-'. This is the same situation as observed for fluorophosphaalkenes. The C=P stretching force constant F(C=P str) was determined to be 562-668 N m-l for CF2=PH, CF2=PCF3, and CH2=PC1. Similar force constant values, 635.7 and 560-577 N m-I, have been reported respectively for the C=S stretching vibration (1059.2 cm-I) of CH2=S30 and the C=Si stretching vibration (1003.5 cm-I) of CH2=Si(CH3)2.34 T h e stretching force constants of the carbon-phosphorus multiple bonds are summarized in Table IX, where the force
123.
