following geometric parameters of the methylphosphine (MPN) molecule were used: ... force constants of MPN were recalculated in the present work, since the ...
FORCE CONSTANTS AND VIBRATIONAL SPECTRA OF METHYLPHOSPHINE DIMETHYLPHOSPHINE,
AND TRIMETHYLPHOSPHINE
S. A. Katsyuba, I. S. Pominov, and B. P. Khalepp
UDC 539.19+541.57
The intense development of calculational methods of vibrational spectroscopy [i] in recent years has led to the creation of a library of hydrocarbon-molecule force constants [2]. However, to date, no systematic correct investigations of the force constants of a series of phosphororganic compounds have appeared in the literature. The present work constitutes the first attempt of this kind. The equilibrium configuration of the given molecules and the system of natural coordinates adopted are shown in Fig. i. The method of solving the inverse vibrational problem (IVP) is analogous to [3]. The masses of hydrogen and deuterium atoms were taken to be equal to their "spectroscopic" values: 1.088 and 2.126 a.m.u., resDectively. In the calculations, theO following geometric parameters of the methylphosphine (MPN) molecule were used: PC = 1.863 A, PH = 1 . 4 1 4 ~ , CH = 1 . 0 9 3 A, ~ H P H = 9 3 ~ ZCPH=97~ ', L P C H ( 1 ) = l l l ~ 9 ZPCH(2,3)=108~ ' [4], as well as ~he following parameters for dimethylphosphine (DMPN): PC = 1.848 ~, PH = 1.419 ~, CH = 1.093 A, Z-CPC=99~ ', ZCPH=96~ LHCH= I08~ [5]; and trimethylphosphine (TMPN): PC = 1.846 ~, CH = 1,091 ~, L C P C = 9 8 ~ ', L H C H = I 0 8 ~ ' [ 6 ] . The values of the kinematic coefficients corresponding to the torsional coordinate were calculated from the formula of [7]. The force field of MPN in independent symmetry coordinates was determined in [8]. The force constants of MPN were recalculated in the present work, since the geometry of the molecules assumed in [8] does not correspond with that observed experimentally [4], and the model of the potential field differs from that adopted here for the whole series of phosphines investigated. In formulating the force-constant matrix of MPN in the zero approximation~ the dynamic coefficients of phosphine PH3 [9], the methyl group [i], and the PC bond [8] were used. No torsional coordinate was adopted in the calculation, since the corresponding frequency was not observed experimentally. In the course of solving the IVP, 19 independent parameters of the force field were varied, while using 56 vibrational frequencies [8] of the four isotopic samples of the MPN molecule H2PCHa, D2PCH3, H=PCD3, D2PCD3. The final values of the dynamic coefficients are shown in Table i. The force constants found for MPN were then used for a preliminary analysis of the vibrational spectra of DMPN and DMPN-d. The latter was differently interpreted in [I0, ii]. The vibration frequencies of HP(CH3)2 and DP(CH3)2 calculated in the zero approximation are in satisfactory agreement with those given in [ii], which were adopted as the basis in our solution of the IVP. The same comparison for the molecule HP(CD3)2 shows that, in this case, the interpretation of the experimental spectrum in [i0] is incorrect. Thus, in [i0], the frequency 1117 cm -I is attributed to deformational vibrations of the PH bond, However, since ~as (CPH~ = 1012 cm -I in the HP(CH3)2 molecules [ii], it may be assumed that the corresponding absorption band of HP(CDa)2 lies in the region of lower frequencies. Taking into account that ~as (PCH) = 932 cm -I for the H2PCD3 molecule [8]~ the absorption band at 935 cm -I should be assigned to the given vibration in the case of HP(CD3)=. The band at 1117 cm -I can hardly be assigned to deformational antisymmetric vibrations of the methyl groups CDa (~6), since these frequencies lie in the range 1000-1050 cm-: for H2PCD3 and P(CD3)3 [6, 8]. Evidently, the given band is due to the composite frequency v7 + ~ 2 (895 cm -z + 229 cm -I = 1124 cm-:), and the band at 1035 cm -~ should be assigned to ~:7. The weak absorption band at 806 cm -z [I0] is most probably due to hhe composite frequency ~23 + v2~ (665 cm - I + 1 3 7 c m -I =802cm-~), while v9 corresponds to the band at 768 cm -I. This set of assignments agrees with the experimental interpretation of the Raman spectra of the molecule P(CD3)3 [6]. It is assumed that ~zo corresponds to the band at 620 cm -z and v:~ to 594 cm -~ while v22 and v~z are randomly degenerate. 1982.
Translated from Zhurnal Prikladnoi Spektroskopii, Original article submitted June 15, 1981.
0021-9037/82/3605-0553507.50
Vol. 36, No. 5, pp, 783-787, May,
9 1982 Plenum Publishing Corporation
553
TABLE i. Force Constants of Phophines (i0 ~ cm-~). Coefficients Not Appearing in the Table Were Set Equal to Zero
Notation Kr
KQ
H=PCHz
5,144~0,086 4,474~0,058
Kq 1 Kq~
8,338To2o~8
Kt9
0,936~0,080 1,113~0,109
Kv
K~ x
]
8,212~0,064
0,706~-0,083
%
K{z 2
0,706" 0,753-T0,092 0,753 ?
1(• hqq HQQ Hrr HrQ AQ~, AQI~. AQ? AQr162 Art9 AQ~6t AQ.6._ Ar~/t Ar,w_ Aq{z Aqd~t Aqd5~
0,0505 0,006~0,055 --0,014 5 0,270~-0,102 0,242~0,093 O;037:1:
HP(CHa )
5,022-T0,037 4,522-T0 ,091 8,100T-0,027 8,18OTO,O16
I ,181~-0,056 0,709~-0 ,O67 1,755To,o57 0,709 * 0,680~0,083 0,714~0,070 0,084~-0,041 0,050 tt _0,01451:
--0,014 $ 0,250 $ 0,250 $ 0, O37:1:
P(CH3)~
4,582~0,050
7 ,919-T0,021 8,103-~0,013
0,693~0,030 1,744T-0,051 0,708~0,029 0,691~0,034 0,702T0,032 0,118T0,037 0,050 $ --0;014 5
5 0,250 o, 950 ~: 0,045T0,051
0, O37$ 0,037 5 --0,015 5 0,0375 --0,015:1: 0,350 $ 0,69s 0,748~0,109 --0,016~-0.087
0,350 5
0,350 :1:
o, 720 :]: 0,7205
O,720 5 0,720 :l:
o,139~o,Iol o ,o97~o ,o81
155 --0,029~0,086
0,060~0,040
lr162 lvfh l~fh
--0,093~0,079
l.~fh lwlL.
--0,086~0,079
Ivd~
0,160~0,081
--o,o9oTo,o77 --o, 13o~o,o95
--0,149-T-0,04l
--o ,o47~o, 060 --0,060~0,082
' -=0,050~0,033
0,241T0,095 Iccct l~ la13~ l~=lh
--0,010-T0,071 --0,041-T0,060 --0,025T0,089 --0,025"*
*Ks: = Kq2. tKBI = K~= = K~s.
554
0, 194~-0,085 --o ,024T0,067 --o, 050~0,050 --0,018~0,070 --0,044~0,071
O, 183~0,040
- - 0,020~0,027 --0 ,060-T0, 029 --0,020~0,022 --0,052~0,0 23
SNot varied. **Z~IB2 = Z~xBs =
TABLE 2. Experimental and Ca2culated Values of the Vibrational Frequencies (cm-I) of the HP(CD3)2 Molecules Vibrations o f type
No. 4 1
2 3 5 6 7 8 9 !0 I1 12 13
v exp 2268 2229 2223
Vibrations o f t y p e Vealc 2280 2225
2117 !048 1034 10!2 904 768 615 594
2215 2130 1047 1036 1019 923 78~ 597 573
229
224
137
131
----No. 14 I5 !6 i7 18 19 20 23 2i 22 24
Vexp
Vcalc
2229 2223
2221 2213
2!22 1043 1028 1012 938 659 647 594 !37
2130 1047 1037 1022 94i 672 618 579 !3l
Note. The numbering of the frequencies corresponds to that adopted in [i0].
b
Fig. I. Equilibrium configurations and the system of natural vibrational coordinates used for methylphosphine (a), dimethylphosphine (b), and trimethylphosphine (c), The values of the experimental frequencies of HP~CD3)= used in solving the IVP are shown in Table 2, together with the corresponding calculated values. Altogether, in refining the 22 force-field parameters of the DMPN fields, use was made of 70 vibrational frequencies of three isotopic samples of the molecules: HP(CH3)2, DP(CH~)= [ii] and HP(CD3)2 [i0] (Table 2). The final values of the force constants are shown in Table i. The force field of DMPS, as is evident from Table i, does not contain any new types of interaction at all in comparison with MPN, The identity of the force fields of the primary, secondary, and tertiary phosphines is confirmed by calculating the vibrations of trimethylphosphine (TMPN). The frequencies calculated from the zero-approximation potential-energy matrix formulated on the basis of the DMPN force field are so close to the experimental values [6] that one iteration was sufficient for the solution of the IVP. Altogether, in refining the 17 parameters of the TMPN force field, 38 values of the experimental frequencies of P(CH~)3 and P(CD~)3 were used [6]. The results are shown in Table i. As is evident from the tables, the force constants of the CH bond decrease regularly in the series (CH2)nPH3_n with n = l, 2, 3, For MPN, the force constant of the CH bond in the trans-position with respect to the unshared electron pair in the phosphorus atom (Kql) is larger than the corresponding coefficients for the two other CH bonds (Kq2 = Kqs), In the ease of TMPN, the opposite picture is observed. This pattern corresponds to the dependence of the CH bond strength on the number of methyl groups in phosphines observed experimentally [12].
555
Comparing the results of a normal-coordinate analysis of the given molecules, it may be concluded that valence, deformational, and torsional vibrations of methyl groups and valence vibrations of PH bonds are completely characteristic for all the given compounds. The absence of a shift in the torsional vibrations of phosphines with other low-frequency vibrations indicates that, if they are disregarded in solving the IVP for MPN, no marked distortion in the values of the strength constants will appear. LITERATURE CITED i. 2.
3. 4. 5. 6.
7. 8. 9. i0.
II. 12.
556
V.M. Vol'kenshtein, L. A. Gribov, M. A. El'yashevich, and B. I. Stepanov, Vibrations of Molecules [in Russian], 2nd edn., revised Nauka, Moscow (1972). L . A . Gribov and V. A. Dement'ev, Tables of Parameters for Calculating the Vibrational Spectra of Multiatomic Molecules [in Russian], Akad. Nauk SSSR, Nauchn. Sov. Spektrosk,, Moscow (1979), No. i. Abstracts from the Fourth All-Union Conference on the Use of Computers in Molecular Spectroscopy, Novosibirsk, September 19-21, 1977 [in Russian], NIOKh, Novosibirsk (1977). T. Kojima, E. L. Breig, and C, C. Linn, "Microwave spectrum and internal barrier of methylphosphine," J. Chem. Phys., 35, 2139-2144 (1961). R. Nelson, "Microwave spectrum, molecular structure, and dipole moment of methylphosphine," J. Chem. Phys., 32, 2382-2383 (1963). H. Rojhantalab, J. W. Nihler, and C. J. Wilkins, "Raman spectra and torsional barriers for multitop molecules: trimethylphosphine oxide, trimethylphosphine sulfide, and trimethylphosphine selenide," Spectrochim. Acta, 32A, 519-533 (1976). A . S . Kozlov and V. G. Dashevskii, "Torsional oscillations of multiatomic molecules," Zh. Prikl. Spektrosk., 31, 302-307 (1979). J . A . Lannon and E. R. Nixon, "Vibrational spectra and force constants of methylphosphine," Spectrochim. Acta, 23A, 2713-2732 (1967). G . I . Rybakova, D. S. Koval'chuk, and V. P. Morozov, "Force constants and influence coefficients of pyramidal hydrides," Opt, Spektrosk., ~, 34-39 (1960). J . R . Durig and J. E. Saunders, "Spectra and structure of organophosphorus compounds, XII. Infrared and Raman spectra of (CH3)=PH and ~CD3)2 PH ," J, Raman Spectrosc,, 4 121-130 (1975). A . J . F . Clark and J. E. Drake, "The vibrational spectrum of dimethylphosphine~ a reassessment," Spectrochim. Acta, 34A, 307-310 (1978). D . C . McKean and G. P. McQuillan, "Isolated CH stretching frequencies, bond strengths, and lengths in some MeP and MeS compounds," J. Mol. Struct., 49, 275-282 (1978).
