1843
J. Phys. Chem. 1980,84, 1843-1848
DMPP (Table 11). The long lifetime, together with the high fluorescence efficiency of DMP, implies a relatively small rate constant for radioationless deactivation of the singlet excited state. Huber and co-workersgfound that only intersystem crossing was competitive with fluores1)and in fact determined kiscto be just cence (q$ + &e 5 X lo6 s-l, relatively small in comparison to kiscfor other aromatic amines in s o l ~ t i o n . ~In J ~view of the similar natural radiative lifetimes, the shortened fluorescence lifetimes of DMAC (andDMPP imply relatively larger rate constants for radiationless deactivation of the excited singlet state, presumably by intersystem crossing, for DMAC and DMPP than for DMP. In sum, we have observed a reordering of electronic states with dibenzo[ b,i]annelation of N,N-dimethyldihydrophenazines, a reordering which does not occur with dibenzo[a,c]annelation. The different character of the lowest excited singlet state is reflected experimentally in markedly different radiative lifetimes and in the separation of emission and absorption maxima.
and Professor Kenneth Kaufmann and Mr. Roger Pearson of this department for measuring the lifetimes. This research was supported generously by the Office of Naval Research and the National Science Foundation.
-
References and Notes (1) Fellow of the Alfred P. Sloan Foundation, 1977-79. (2) N. Winograd and T. Kuwana, J. Am. Chem. Soc., 93,4343 (1971). (3) R. F. Nelson, D. W. Leedy, E. T. Seo, and R. N. Adams, Z. Anal. Chem., 224, 184 (1967). (4) J. W. Clark-Lewis and K. Moody, Aust. J. Chem., 24,2593 (1971). (5) U. Bruhlmann and J. R. Huber, J . Phys. Chem., 81,386 (1977). (6) G. B. Schuster, Acc. Chem. Res., 12,366 (1979); J. -y. Koo and G. B. Schuster, J . Am. Chem. SOC., 100,4496 (1978). (7) S.P. Schmidt and G. B. Schuster, J . Am. Chem. Soc., 102,306 (1980). (8) B. 0.Dixon and G. B. Schuster, J. Am. Chem. Soc., 101,3116 (1979). (9) J. V. Morris, U. Bruhlmann, 0. Serafimov, and J. R. Huber, Ber. Bunsenges. Phys. Chem., 78, 1348 (1974). (10) H. Gllman and J. J. Dletrich, J. Am. Chem. Soc., 79,6178 (1957). (11) J. G. Smith and E. M. Levi, J. Organomet. Chem., 36,215 (1972). (12) W. H. Melhuish, J. Phys. Chem., 65, 229 (1961). (13) D. R. Maulding and B. 0. Roberts, J. Org. Chem., 34,1734 (1969). f14) . . J. B. Birks. "PhOtoDhvsic.9 of Aromatic Molecules", Wilev-Interscience, New York, 1970,' p 8 8 . (15) J. E. Adams, W. W. Mantulin, and J. R. Huber, J . Am. Chem. Soc., 95,5477 (1973).
Acknowleciggment.We thank Dr. Tada Fukunaga Of E* I. du Pont de Nemours for carrying out the calculations
Spectra and Structure of Gallium Compounds. 1. Vibrational Studies of Trimethylphosphine-Gallane and Trimethylphosphine-Gallane-d3 J. D. Odom," K. K. Chatterjee, and J.
R. Durlg"
Depiattment of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received June 11, 1979) Publ!cation costs assisted by the University of South Carolina
The infrared (200-4000 cm-l) and Raman (50-3500 cm-l) spectra of (CH3)3P*GaH3 and (CH3)3P-GaD3 have been recorded for the solid state at low temperature. The spectra have been interpreted on the basis of CBv molecular symmetry. None of the Az vibrations has been observed although a lower site symmetry in the solid state is indicated by the splitting of the bands assigned to the antisymmetric GaH3 stretching and bending modes. The valence force field model has been utilized in calculating the frequencies and the potential energy distribution. The calculated potential constants for the adducts are compared to those previously reported for the Lewis base moiety, and the differences are shown to be consistent with structural changes upon adduct formation. Some coupling has been observed between the Ga--Pstretch and the symmetric P-C3 stretch. The Ga--P stretching force constant has been found to have a value of 2.0 mdyn/A, which is much smaller than the value of the Ga-N force constant (2.43mdyn/A) in (CH3)3N.GaH3.The magnitudes of the Ga-P and Ga-N force constants do nut appear to adequately reflect the relative stabilities of the adducts as determined by other methods.
Introduction The relative donor strengths of nitrogen and phosphorus toward a given acceptor have been a subject of continuing interest in coordination chemistry. During the past two decades spectroscopic methods have been widely applied in determining the relative stabilities of the adducts. Odom et a1.lY2investigated the vibrational spectra of trimethylphosphine and trimethylamine complexes of borane in order to estimate the relative strengths of the B-P and B-N dative bonds. The values of the force constants for the B-N and B-P stretching modes, obtained from normal coordinate calculaitions, were found to be 2.59 and 2.37 mdyn/A, respectively, which suggest that trimethylamine is a stronger donor toward borane than trimethylphosphine. However, a previous study3 of the gas-phase ligand displacement reaction (CH3),N.BH3+ (CH3)3P+ (CH3),P.BH3 + (CH3),N at 120 "C showed that 80% of the N(CH3):,is displaced by P(CH3)3. From this datum 0022-3654/80/2084-1843$01 .OO/O
it was inferred that trimethylphosphine forms a stronger bond with borane than trimethylamine. In view of the conflicting information provided by the aforesaid methods of study, we considered it necessary to extend similar investigations to the gallium hydride complexes of trimethylphosphine and trimethylamine. In addition to making band assignments of the vibrational spectra, one of our basic aims has been to examine to what extent the frequencies and the magnitudes of the force constants associated with the stretching of the dative bonds, Ga-P and Ga-N, could be related to the stabilities of the adducts. Although stable complexes of gallium hydride with trimethylphosphine and trimethylamine have been known for several years, little vibrational data are available for these compounds. The vapor-phase spectra of the compounds, measured by previous investigator^^*^ over 15 years ago, were relatively weak and unresolved, as were those 0 1980 American
Chemical Society
1844
The Journal of Physical Chemistty, Vol. 84, No. 14, 1980
measured in s ~ l u t i o n .These ~ are attributable to the low vapor pressures (1-2 mmHg at room temperature) and the highly reactive nature of these compounds. To overcome these difficulties we have measured the infrared and Raman spectra of the compounds in the solid state at liquid-nitrogen temperature. We have also carried out normal coordinate analyses for the light and heavy molecules to aid in the vibrational assignments and to obtain values of the force constants associated with the various vibrational modes of the molecules. In this report we present the results of our study of trimethylphosphine-gallane and trimethylphosphine-gallane-d,. The investigation of the corresponding trimethylamine complexes has been published6 elsewhere.
Experimental Section Because of the air-sensitive nature of the gallium complexes all preparative manipulations were carried out in a conventional high-vacuum system fitted with greaseless stopcocks or in an inert atmosphere. Anhydrous gallium chloride (Alfa) was purified by vacuum sublimation. Trimethylphosphine was obtained from Strem Chemicals, Inc. and was purified on a variable-temperature vacuum fractionation column (vapor pressure 158 torr a t 0 "C; literature' 158 torr). Trimethylphosphine-gallane was synthesized by mixing stoichiometric amounts of an ether solution of LiGaH4, HC1, and P(CH,), in a modification of the procedure described by Greenwood et al.5 The pure product was isolated by vacuum sublimation and was stored a t -196 "C in evacuated sample tubes when not being used. The deuterium compound was prepared in a similar manner except that LiGaD4 and DC1 were used. A Perkin-Elmer Model 621 mid-infrared grating spectrophotometer, equipped with an extended source compartment and purged with dry nitrogen, was used to record the spectra of the solid compounds from 4000 to 200 cm-'. The instrument was calibrated with standard gases in the high-frequency region and with atmospheric water vapor in the low-frequency r e g i ~ n .The ~ samples were slowly deposited on a CsI substrate which was mounted on a copper block in thermal contact with a reservoir of boiling liquid nitrogen. The samples were annealed until no change in the spectra was observed. The Raman spectra were recorded from 50 to 3500 cm-l by using a Cary Model 82 Raman spectrophotometer which was calibrated with neon emission lines. A Spectra Physics Model 171 laser operated a t 5145 was used as a source to record all spectra. A low-temperature cell analogous to the mid-infrared low-temperature cell, except that the sample substrate consisted of a blackened brass block at an angle of 75" from the normal, was used to contain the samples. Vibrational Assignments and Normal Coordinate Calculations The infrared and Raman spectra of (CH3),P.GaH3and (CH3),P.GaD3 are shown in Figures 1 and 2. Assuming C, molecular symmetry, it follows from group theoretical considerations that each molecule will have 10 Al, 5 Az, and 15 E normal modes of vibration and for this symmetry all the Az modes will be inactive in both the infrared and the Raman spectra. In actuality, no vibrations ascribable to the A2 modes were observed in the spectra of the compounds in the solid state. CH, Modes. The methyl stretching and bending modes are found to appear approximately in the same region as those of the free base. As observed in the case of (CH3),P.BH3,1 two antisymmetric C-H stretching modes, vI(AI) and vI6(E), form a single intense Raman band at
Odom et al.
4000
'3000
'2000
iooo
11500
'500
WAVENUMBER (CM.') Flgure 1. Infrared spectra of (CH,),P.GaH, recorded at liquid-nltrogen temperature.
(A) and (CH,),P.GaD,
(B)
-
3000
2000
1000
0
WAVENUMBER (CMW1) Figure 2. Raman spectra of (CH,),P.GaH, recorded at liquid-nitrogen temperature.
(A) and (CH,),P-GaD,
(B)
2983 cm-l, and the corresponding band in the infrared is weak. However, unlike the borane complex the remaining antisymmetric C-H stretch vI7(E)is also observed at 2971 cm-l as a strong band in the Raman spectra of (CH,),P.GaH3, and it appears as a weak band in the infrared spectra. Analogous to the borane complex, the two symmetric C-H stretches of (CH3),P.GaH3,v2(A1) and vls(E), are accidentally degenerate a t 2904 cm-l. None of the methyl stretches is appreciably affected by the deuteration of the GaH3 group. All five methyl deformational modes are clearly resolved in the Raman spectrum of (CH3),P.GaH3 although only three-vzl(E), ~4(A1),and ~5(A1),which have frequencies 1422, 1412, and 1286 cm-l, respectively-appear in the infrared spectrum. All of the Raman lines are relatively weak, but the corresponding infrared bands have varying intensities, with those of the deuterated species ranging from medium to strong. In the d3 compound the symmetric deformation vz2(E)overlaps the strong GaD, antisymmetric stretching mode at 1306 cm-l. The infrared band of medium intensity at 970 cm-l and the weak Raman band at 966 cm-l are each assigned to the v6(A1) and ~23(E)methyl rocking modes. The only other methyl rocking mode ~24(E)appears at 952 cm-l in the infrared and at 954 cm-l in the Raman spectra of the light compound. Upon deuteration the Raman band at 966 cm-l shifts to 962 cm-l although no appreciable change in the frequency of the corresponding band is observed in the infrared spectrum. Ga-P Stretching and Ga-P-C Bending Modes. As stated in the Introduction, the assignment of the Ga-P
Spectra and Structure of Gallium Compounds
stretching frequency is of prime importance as its magnitude could reflect to some extent the strength of the Ga-P dative bond. Greenwood et ala6reported a band at 326 cm-' in the infrared spectrum of (CH3)3P-GaH3measured in a benzene solution, and they assigned it to the Ga-P stretching mode. This band does not appear in the infrared spectrum of the solid state. However, a weak but sharp band occurs at 341 cm-l (vS) in the Raman spectrum of the light as well aLs the deuterated compound. The value of the Ga-P stretching frequency, calculated from the Ga-N stretching frequency (511 cm-l) in trimethylamine-gallane: is approximately 364 cm-l neglecting any difference in the magnitudes of the force constants of the Ga-N and Ga-P bonds. It is, therefore, reasonable to assign the Runan band at 341 cm-l to the Ga-P stretching motion. This band is not observed in the infrared spectrum, It is notewcirthy that several previous ~ t u d i e s l ~ ~ J ~ on phosphorus-boron adducts have shown the Raman technique to be more sensitive to the observation of the P-B stretching mode than the infrared technique, and this was also found to he true for the N-B stretch in (CHJ3NSBH~.~ The weak Raman band at 150 cm-' in the light compound is assigned to the Ga-PC3 rocking mode ~30(E). Upon deuteration it shifts to 148 cm-l. The frequencies of the Ga-PC3 rocking and the Ga-I? stretching modes are practically insensitive to the deuteration of the GaH, group. This is consistent with the relatively small difference in the masses of GaH3 and GaD3. C-P Skeletal Modes. The P-C3 antisymmetric stretching mode, v2&E),is observed at 753 cm-l as a strong band in the infrared spectrum of (CH3),P.GaH3,and its position is unaltered by deuteration. The corresponding Raman bands appear at 749 and 752 cm-l in the light and the d3 species, respectively. Presumably the intensity of the 749-cm-l Raman band is enhanced by overlap with one of the split components of the GaH3 antisymmetric deformational mode. In the Raman spectra of both species, the symmetric P-C3 stretch, v8(Al), occurs at approximately 675 cm-'. The symmetric stretch is also observed as a weak band at 673 cm-l in the infrared spectrum of only the light compound. The frequencies of the symmetric and antisymmetric P-C3 stretching modes are considerably 8 4 3 = 2 6 3 4 - €35 - €45 higher than those of the corresponding modes in the free 8 4 4 = E 3 s - €45 base.12 The situation is analogous to that observed in 8 4 5 = 2~ a 3 - ~ 2 -4 Y a 5 s46= 7 2 4 - Y2S (CH3)3P.BH3,and, as in the latter case, the increase in the s,, = 2 T 3 - T 4 - 75 P-C3 stretching frequencies may be attributed to a des48=74-Tj crease of the P-C bond length on complexation. S49b= 2 0 1 , + 20, + CY, + 20, + 20, + 2R8- 0 1 ~ - a l Q -a l l The Raman bands observed at 198 cm-', vlO(A1),and 258 01 - Pc,- 0 1 0 - 0 1 1 - a r 2 - 0 1 1 3 - O ( 1 4 - P i a - 0 1 3 - P i 4 cm-l, ~29(E), are assigned to the P--C3symmetric and anS j O b 1 = a 9 ~ a 1 0 + a l l f P 9 + B l O + ~ 1 1 - a 1 2 - a 1 3 - a 1 4 012 - 013 - 014 tisymmetric defornnational modes. It should be noted that although the frequency of the antisymmetric P-C3 dea Not normalized; refer to Figure 3. Redundant. formation of'the free base itself, which appears at 255 cm-l, does not change appreciably by attachment of the GaH3 and 1793 cm-l. Splitting of the antisymmetric Ga-D3 group, that of thle symmetric modes decreases by apstretch is also observed in the deuterated species, where proximately 100 cm-l. A much smaller decrease in the the two bands appear at 1306 and 1295 cm-l in the infrared frequency ( w 10 cni-l) of the symmetric P-C3 deformation spectrum and at 1306 and 1292 cm-' in the Raman specwas observed in the case of the (CH3),P.BH3 comp1ex.l trum. These assignments agree with the calculated isotopic GaH3 Modes. The Ga-H3 symmetric stretch, ~ 3 ( A 1 ) , shift factors which are 1.40 and 1.39 for the symmetric and appears as a strong band at 1827 cm-l in the Raman the antisymmetric stretching modes, respectively. spectrum and as a band of medium intensity at 1832 cm-l In the light compound, the strong infrared band at 690 in the infrared spectrum. Upon deuteration the Raman cm-l which corresponds to the Raman band of medium band shifts to 131%cm-l, and it is observed as a shoulder intensity at 684 cm-l is assigned to the Ga-H3 symmetric deformation,u,(A1). These bands shift to 498 and 490 cm-l, in the same region of the infrared spectrum. In the infrared spectrum of the light compound the antisymmetric respectively, on deuteration. In the infrared spectrum the Ga-H3 stretching mode, vlS(E),is observed at about 1808 antisymmetric Ga-H3 deformation, vz5(E),splits into two cm-l as a broad and intense band, but it appears as a components, a strong shoulder at 765 cm-l and a weak doublet in the Raman spectrum with peak maxima at 1817 band at 747 cm-l. Only one of the components is observed
1846
The Journal of Physical Chemistry, Vol. 84,No. 14, 1980
Odom et al.
TABLE 11: Fundamental Frequencies and Assignments for (CH,),P.GaH,a (CH,),P.GaH, v,
IR cm-'
2983 vw 2970 vw 2905 vw 1832 m 1808 vs
Raman A v , cm-'
V,
(CH, ),Pb IR Raman cm-' A U , cm-'
(CH, )3P. GaH, calcd, U , cm-' 2979 2978 2978 2905 1836 1809
1287 m 970 ms
2983 vs 2971 vs 2904 vs 1827 vs 1817 s 1793 s 1431 w 1422 w 1412 vw 1304 w 1286 vw 966 w
952 ms 765 sh
954 vw 767 m
940
948
973 94 2 764
747 wm 753 s
749 s
709
708
748
690 s 673 w
684 m 675 m
653
684 672
498 w 485 w
497 w 491 w 341 w
1422 w 1416 w
258 mw 198 s
2978 2900
1441 1430 1416
2969 2954 2894
1421
973
1425 1425 1426 1302 1286 975
496
assignments and approximate descriptions vI6(E),u,(A,) CH, antisym str 100% ,(E)CH, antisym str 100% v,(A,), v,,(E)sym,str 100% v,(A1) GaH, sym str 100% v,,(E) GaH, antisym str 100% v
u,,(E) CH, antisym def 92% + ( E ) CH, rock 8% v,,(E) CH, antisym def 92% t (E) CH, rock 8% v,(A,) CH, antisym def 92% t ( A , ) CH, rock 8% v,,(E) CH, sym def 100% u,(A,)CH, sym def 100% v,(A,) CH, rock 92% t (A,) CH, antisym def 8%, vz3(E)CH, rock 80% + (E) CH, antisym def 6% t (E) PC, antisym str 8% uz4(E)CH, rock 92% + 7% (E) CH, antisym def v,,(E) GaH, antisym def 89% + (E) GaH, rock 5% t (E) PC, str 6%
u,,(E) PC, antisym str 73% t (E) CH, sym def 22% t (E) CH, rock 5 .,(A,) GaH, sym def 100% v,(A,) PC, sym str 76% t (A,) CH, antisym def 20% + (A,) GaP stretch 4% v 2 , ( E ) GaH, rock 94% + (E) GaH, antisym def 5%
v,(A,) GaP str 76% + (A,) PC, sym str 18% + ( A , ) PC, sym def 1 2 258 v,,[E) PC, antisym def 100% 196 ul,(A,) PC, sym def 78% + (A,) PC, str 12% t (A,) GaP str 5% t (A,) GaH, sym def 5% 151 v , , ( E ) GaPC, rock 100% Data taken from ref 12, a and b. Infrared data are for vapor, 342
255 298
263 305
150 w
Abbreviations used: m, medium;^, strong; w, weak. and the Raman frequencies are for the liquid state,
a t 767 cm-l in the Raman spectrum; the other one, as stated earlier, overlaps with the P-C, antisymmetric stretch which occurs at 749 cm-l. In the deuterated complex the antisymmetric Ga-D, deformation is also split with doublets at 545 and 533 cm-' in the Raman spectrum. In both the infrared and the Raman spectra, the Ga-H3 also forms a pair of weak bands with rocking mode, vZ7(E), frequencies of 498 and 485 cm-I and 497 and 491 cm-l, respectively. In the d3 complex the infrared bands shift to 366 and 358 cm-', whereas the Raman bands shift to 363 and 358 cm-l, respectively, Isotopic shift factors for the three bending modes, using an average frequency when splitting of the E modes was observed, are 1.38 and 1.40 for the symmetric and antisymmetric deformational modes, respectively, and 1.37 for the Ga-H, rocking motion. As an aid in checking the assignments, the Teller-Redlich product rule was calculated for the A, and E symmetry species of the two isotopic molecules. The observed values for the Al and E symmetry species were 0.512 and 0.370, respectively, and the calculated values were 0.505 and 0.372, respectively. The agreement appears quite satisfactory and supports the proposed assignment. The removal of the degeneracies of the antisymmetric Ga-H3 stretching and bending modes as discussed above would suggest that the effective symmetry of the molecule in the crystalline state is actually lower than C3u. No information on the crystal structure of trimethylphosphine-gallane is available at this time. However, the most probable site symmetries which could explain the observed splittings of the Ga-H3 modes are C1 and C,. One important question which remains to be answered is why the A2 modes were not observed in either the infrared or
the Raman spectra of the solid if the site symmetry is C1 or C,. Normal Coordinate Calculations. Wilson's FG-matrix method1, and programs written by Scha~htschneiderl~ were used for the calculation of the normal coordinates of the fundamental vibrations. The structural parameters used in generating the G matrix were as follows: r(CP) = 1.819 A, LHCH = 109.3', LCPC = 105O, r(GaP) = 2.52 A, r(GaH) = 1.59 A, and LHGaP = 102O. The first three parameters were assumed to be equal to those determined by Bryan and Kuczkowski16for (CH3),P.BH3. The value of r(GaP) was taken from the results of an electron diffraction studylGof (CH3)3P.Ga(CH3).The values of r(GaJ3) and L H G ~ are P the same as those calculated for the Ga-H bond distance and the H-Ga-N bond angle from the study of the microwave spectra of trimethylamine-gallane by Durig et al.G The basis set was composed of 50 internal coordinates which were used to construct the 50 symmetry coordinates listed in Table I (also see Figure 3). Most of the initial values for the principal force constants and the interaction force constants for the trimethylphosphine moiety were the same as those used by Odom et al.l for (CH3),P-BH3,whereas the force constants associated with the stretching and bending of the GaH, group were assumed to be smaller than the analogous force constants for the BH, group. An initial force field of 26 force constants was selected to fit 45 frequencies. In the least-squares refinement the observed frequencies were weighted by 1/X, with the Jacobian matrix being constantly checked to guide the process of refinement. The calculated frequencies of (CH,),P-GaH3and (CH3),P-GaD3 are listed in Tables I1 and 111with average errors of 0.40
The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
Spectra and Structure of Gallium Compounds
1847
TABLE 111: Fundamental Frequencies and Assignments for (CH, ),P.GaD,a
IR v , cm-'
Raman Au,cm-'
2983 vw
2983 s
2970 vw 2905 vw
2972 s 2908 vw 1432 w 1423 w
1423 mr; 1417 m 1312 sh 1306 s 1295s 1287 s 969 s
1312 vs 1306 s 1292 m
953 s 753 m
952 g 752 m
IR v,cm-'
Raman Av,cm-l
2978
2969 29 54
2900 1441 1480 14116
2894
973
962 w
940 709
674 m 545 sh 539 s 498 366 w 358 w
255 298
148 w See corresponding footnotes t o Table 11.
2979 2978 2978 2905 1425 1425 1426 1300 1302 1293 1286 975
708
973 942 748
653
672
948
545 m 533 m 490 w 368 w 358 w 341 w 257 mw 197 s
a
1421
calcd, u,cm-'
263 305
assigament and approximate descriptions v , , ( E ) , u , ( A , )CH, antisym str 100% v , , ( E ) CH, antisym str 100% u,(A,),v , , ( E ) CH, sym str 100%
u , , ( E ) CH, antisym def 92% + (E) CH, rock 8% v , , ( E ) CH, antisym def 92% + ( E ) CH, rock 8% u,(A,)CH, antisym def 92% + ( A , )CH, rock 7% v , ( A , )GaD, sym str 100% v , , ( E ) CH, sym def + v , , ( E ) GaD, antisym str v , , ( E ) GaD, antisym str 100% v , ( A , )CH, sym def 100% v , ( A , )CH, rock 92% + ( A , )CH, antisym def 8%, v , , ( E ) CH, rock 80% t ( E ) CH, antisym def 6% ( E ) PC, antisym str 8%
+
v Z 4 ( E CH, ) rock 92% + (E) CH, antisym def 7% u Z 6 ( E )PC, antisym str 73% + ( E ) CH, antisym
543
def 22% + ( E ) CH, rock 5% v , ( A , )PC, sym str 76% + ( A , )CH, antisym def 20% + ( A , )GaD str 4 v Z S ( EGaD, ) antisym def 96% + ( E ) GaD, rock 4%
492 359
v , ( A , )GaD, sym def 100% u 2 , ( E ) GaD, rock 94% + ( E ) GaD, antisym def 5%
340
v , ( A , )GaP stretch 70% + ( A , )PC, sym str 18%+ ( A , )PC, sym def 1 u,,(E) PC, antisym def 100% v , , ( A , )PC, sym def 78% + ( A , )PC, str 12% + A + Gap str 5% + (A,) GaD, def 5% v , , ( E ) GaPC, rock 100%
257 196 150
.TABLE IV: Internal Force Constants for (CH,),P.GaH, and (CH,),P.GaD, force constants WUP mdyn/Aa
\
KR KQ K,
P-Ga stretch P-C stretch Ga-H stretch K, C-H stretch' HE LC-P-C bend H, LC-P-Ga bend H@ LH-Ga-H bend H6 LH-Ga-P bend H, LH-C-H bend Hp LP-C-H bend F, C-H stretch/C-H stretch F, Ga-P stretch/Ga-H stretch FR, P-Ga stretch/iC-P-Ga bend F R ~P-Ga stretch/LH-Ga-P bend F Q p P-C stretchlip-C-H bend F, LC-P-C bend/LC-P-C bend FQ, P-C stretchlLC-P-C bend FQ P-C stretchlic-P-Ga bend F,6 LC-P-Ga bend/LH-Ga--P bend F,p LC-P-Ga bendlLH-C-P bend Fe LH-Ga-H bend/LH-Ga--H bend F6 LH-Ga-P bend/LH-Ga-P bend a All bending coordinates weighted by
,
0 Figure 3. Internal coordinates for (CH,),P.GaH,.
and 0.45%, respectively. Only 22 force constants (Table IV) with magnitudes greater than 0.02 mdyn/A were used in the calculation of the final force field. Four interaction force constants-F ,F,, FRQ and FQg-which were included in the initial calcufations were ultimately dropped as they were found not to irnprove appreciablythe force field. The principal force constants during the later stages of re-
2.006 f 0.05 3.585 f 0.024 1.934 f 0.007 4.765 f 0.005 0.742 f 0.017 0.467 f 0.017 0.332 f 0.003 0.424 k 0.005 0.519 f 0,001 0.608 f 0.002 0.045 f 0.003 0.031 i- 0.005 0.189 I 0.07 0.123 i 0.05 0.632 f 0.006 -0.073 ?: 0.02 -0.276 f 0.03 -0.095 i 0.05 0.067 i 0.02 0.023 i: 0.018 0.031 5 0.003 0.039 f 0.004 1A .
finement were invariant to changes in the interaction force constants. Discussion In the methyl stretching and bending regions the fundamental frequencies may well be predicted by the normal
1848
modes of vibrations of trimethylphosphine; in other words, the force constants do not change appreciably. On the other hand, there is a large increase in the C-P stretching force constant, from 2.78 in the free base to 3.585 in the complex. Similar behavior was observed in the borane adduct of trimethylphosphine,*and, as in the latter case, the increase in C-P force constant may be due to the decrease in the C-P bond length accompanied by an opening of the C-P-C bond angle. The value of the force constant for the Ga-N stretching mode in trimethylamine.gal1anehas been found to be approximately 2.43 mdyn/A.6 From the smaller value of the Ga-P stretching force constant one would expect trimethylphosphineto be a weaker base than trimethylamine. + (CH3),N(g) Studies on the equilibrium (CH3)3P-GaH3(~) (CH3)3N.GaH3(~) + (CH,),P(g) by Greenwood et al.5 using infrared and NMR techniques showed that the equilibrium mixture contains approximatelyequimolecular quantities of (CH3)3Pand (CHJ3N, and it was concluded that the donor strength of (CH3)3Ntowards GaH3is either equal to or only slightly greater than that of (CH3)3P. It would appear from these observations that the adduct stability, for this class of compounds, cannot be predicted directly from the magnitudes of the stretching force constants of the dative bonds. Numerous s t u d i e ~ ’ ~on~ Jborane ~ complexes with group VA Lewis bases have shown that (a) the LHBH bending force constant is relatively invariant to the nature of the donor atom and (b) the external top-angle bending force constant LHBX (X = N,P, or As) decreases in value while the separation between the antisymmetric and symmetric BH3 deformation modes increases as one descends group VA of the periodic table. A similar pattern of behavior is also observed with the GaH3 bending modes in (CH3),N.GaH3 and (CH3)3P-GaH3.In both compounds the GaH3antisymmetric deformation appears around 765 cm-’, which shows that the force constant of the LHGaH bend is insensitive to the nature of the atom to which the GaH3 group is attached. Secondly, in the trimethylamine complex the GaH3 symmetric deformation and rocking modes occur at 712 and 528 cm-l, respectively, both of which are higher in frequency than those of the corresponding modes in the trimethylphosphine complex. Consistent with the foregoing, the separation between the GaH, antisymmetric and symmetric deformations increases as the donor atom is changed from N to P.
-
Odom et
The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
al.
The potential energy diagram shows that both symmetric and antisymmetric P-C3 stretching modes couple with other vibrational modes, the extent of coupling being approximately the same for the light and the deuterated compound. Maximum coupling of the Ga-P stretching mode is found to occur with the symmetric P-C3 stretch. Some coupling is also observed between the C3P symmetric deformation and the symmetric P-C3 stretching modes. Such coupling would tend to increase the energy of the C,P deformation. Evidently, the increase in energy is more than compensated by the relatively low values of the force constants of the C-P-C and Ga-P-C bending motions which together constitute the C3P symmetric deformation mode of vibration. Acknowledgment. The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-77-08310.
References and Notes (1) J. D. Odom, 8. A. Hudgens, and J. R. Durig, J . Phys. Chem., 77, 1972 (1973). (2) J. D. Odom, J. A. Barnes, and J. R. Durig, J. Phys. Chem., 78, 1503 (1974). (3) W. A. G. Graham and F. G. A. Stone, J. Inorg. Nucl. Chem., 3, 164 (1956). (4) N. N. Greenwood, A. Storr, and M. 0. H. Wallbridge, Proc. Chem. SOC.,London, 249 (1962). (5) N. N. Greenwood, E. J. F. Ross, and A. Storr, J . Chem. Soc., 1400 (1965). (6) J. R. Durig, K. K. Chatterjee, Y. S. Li, and J. D. Odom, J. Chem. Phys., in press. (7) D. F. Shriver, ”The Manipulation of Air-Sensitive Compounds”, McGraw-Hill, New York, 1969. (8) N. N. Greenwood, A. Storr, and M. G. H. Wallbridge, Inorg. Chem., 2, 1036 (1963). (9) IUPAC, “Tables of Wavenumbers for the Calibration of Infrared Spectrometers”, A. R. H. Cole, Ed., 2nd ed;Butterworth, Washington, D. C., 1977; R. T. Hall and J. M. Dowling, J. Chem. Phys., 47, 2452 (1967); 52, 1161 (1970). (10) R. C. Taylor and T. C. Bissot, J . Chem. Phys., 25, 780 (1956). (1 1) J. D. Odom, S. Riethmiller, J. D. Witt, and J. R. Durig, Inorg. Chem., 12, 1123 (1973). (12) (a) E. J. Rosenbaum, D. J. Rosenbaum, D. J. Rubin, and C. R. Sandberg, J. Chem. Phys., 8 , 366 (1940); (b) J. R. Durlg, S. M. Craven, and J. Bragin, J . Chem. Phys., 53, 38 (1970). (13) E. B. Wilson, J. C. Declus, and C. Cross, ”Molecular Vlbrations, The Theory of Infrared and Raman Vibrational Spectra”, McGraw-Hill, New York, 1955. (14) J. H. Schachtschneider, Technical Report No. 231-64 and 57-65, Shell Development Company. (15) P. S. Bryan and R. L. Kuczkowski, Inorg. Chem., 11, 553 (1972). (16) L. M. Golubinskaya, A. V. Golubinskii, V. S. Mastryukov, L. V. Vilkov, and V. I.Bregadze, J . Organomet. Chem., 117, C4 (1976). (17) R. C. Taylor, Adv. Chem. Ser., No. 42, 59 (1964).
