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The infrared spectra of the dimethyl complexes of zinc, cadmium and mercury
Spectrochimica Acta. Vol. 48A, No. 8. pp. 1173-1178, 1992
0584-8539/92 $5.00+0.00 (~ 1992 Pergamon Press Ltd
Printed in Great Bnlain
The infrared spectra of the dimethyl complexes of zinc, cadmium and mercury M. BOCHMANN,*M. A. CHESTERS,* A. P. COLEMAN,R. GRINTER* and D. R. LINDER School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
(Received 16 March 1991; in final form 6 January 1992; accepted 6 February 1992) Abstract--The infrared spectra of the dimethyl complexes of zinc, cadmium and mercury, isolated in argon matrices, have been measured. Earlier assignments of the spectrum of the zinc compound appear to have included a band due to a methane impurity. A study of the P A I R spectrum of a thin film of ZnMe2 on a copper surface has revealed that the molecules in the film are strongly orientated and the additional information thus obtained has made it possible to suggest some reassignment of the spectra of all three compounds.
INTRODUCTION T H E VIBRATIONAL s p e c t r a
of the group
liB
metaldimethyls
have
been
extensively
investigated. The most recent study appears to be that of BUTLERand NEWBURY[1], who reported the Raman and IR spectra of zinc-, cadmium- and mercurydimethyl in their gaseous, liquid and solid phases and discussed their spectra using G36 [2] symmetry. The earlier work has been assessed by SmMANOUCm [3], who compiled tables, using data from four sources [4-7], to assign the IR spectra of the gaseous phase and the Raman spectra of the liquids using G~ and D'ah symmetries. In view of the value of SHIMANOUCHI'Scompilation, to which we frequently refer, we have discussed our results in terms of the G3~ formalism, although the molecules are not freely rotating in the matrix. We have also used the same symbols for the irreducible representations of the group as SHIMANOUCHI[3], unlike BUTLER and NEWBURY [1], who adopted a somewhat different notation. Our interest in the spectroscopy, surface chemistry and reactivity of these compounds, and also those of the higher dialkyl species, led us to re-examine the spectra, the primary objective of the present work being to form the basis for our studies of further molecules in the series. We consider here the IR spectra of the matrix-isolated, group IIB metaldimethyls plus the spectra of the zinc species as a film condensed on a copper surface, as a liquid film and as a polycrystalline layer. Following SHIMANOUCm [3], we assign our spectra in the group D;h, since our data do not permit a more sophisticated analysis. EXPERIMENTAL
Preparation of the metaldimethyl compounds All preparations were carried out under an argon atmosphere using standard Sehlenk techniques and dried solvents distilled under nitrogen. Zincdimethyl was prepared as described by GALYER and WmKINSON[8]. Cadmiumdimethyl was prepared using a scaled-down version of the method described by ANDERSONand TAYLOR[9]. The sample was purified by successive microdistillations. Mercurydimethyl was prepared by a method based upon the procedures described by GILMANand BRowr~ [10, 11] and by MARVELand GOULD[12]. The purity of the products was determined by 1H NMR spectroscopy.
Matrix isolation The conditions used for the preparation of the matrices are recorded in Table 1. The materials were deposited with argon onto a CsI window cooled to approximately 15 K by an Air Products * Authors to whom correspondence should be addressed. 1173
M. BOCHMANNet al.
1174
Table 1. The conditions for the preparation of the matrices Sample
ZnMe~
Reservoir pressure (Tort) Reservoir temperature (°C) Isolation time (min) Matrix appearance
0.15 -85
CdMe2 0.43 -75
25 clear but crazed
90 clear
HgMe2 0.53 -70 15 clear
"Displex" CSA202 closed-cycle refrigerator. The desired guest/host ratio was obtained by holding the flow of argon constant and varying the temperature of the metaldimethyl reservoir. IR spectra were recorded with a Perkin-Elmer 577 spectrometer over the range 200-4000 cm -j. In all cases a baseline of the clean window was obtained at room temperature and 15 K and of the window covered with a layer of pure argon at 15 K.
Thin film spectra ( R A I R S ) The reflection-absorption infrared spectroscopy (RAIRS) experiments were carried out in an oil diffusion pumped, ultra-high vacuum chamber interfaced to a Mattson Sirius 100 FTIR spectrometer, the optical configuration of which has been described previously [13]. A narrowband mercury-cadmium telluride (MCT) IR detector was used which allowed spectra in the range 4000-650 era-' to be recorded. The spectra were measured at 8 cm- ~resolution by the co-addition of 3000 interferograms for each of the sample and reference (clean surface) single beam spectra. The Ru(0001) single crystal substrate was cleaned by cycles of argon ion bombardment followed by annealing at 1250 K. A clean copper film was deposited on the room temperature substrate using high-temperature evaporation source. The transmission experiments on polycrystalline layers were carried out in an evacuated glass cell positioned in the same spectrometer. Zincdimethyl was condensed on a KBr substrate cooled to ca 100 K. A wide-band MCT detector was used to extend the spectral range down to 500 cm -1. Spectra were recorded at 8 cm-1 resolution by the co-addition of 1000 interferograms. RESULTS AND DISCUSSION Matrix isolation I R spectra are s h o w n in Fig. 1 and thin film spectra of ZnMe2 in Fig. 2. T h e n u m e r i c a l data and assignments are given in Tables 2 a n d 3 w h e r e they are c o m p a r e d with literature values. W e consider zincdimethyl first.
408o
-¢a~~
~
"~
c"
ag~jegated sample
40 8°
,
i 30
25
I 20
15 Wavenumber x 10-2
10
5
Fig. 1. The IR spectra of zinc-, cadmium- and mercurydimethyls isolated m argon matrices. • indicates bands due to methane in the zincdimethyl spectrum.
IR spectra of group IIB metaldimethyls
35
30
25
20 _
15
1175
10
Wavenumber x 10-z Fig. 2. IR spectra of zincdimethyl. (a) The R A I R spectrum of a thin film on a copper surface. (b) The transmission IR spectrum of a polycrystaUine layer on a KBr plate.
Zincdimethyl T h e matrix s p e c t r u m of this m o l e c u l e (Fig. 1) shows a strong b a n d at 3015 c m -~, which has n o c o u n t e r p a r t in the data a s s e m b l e d by SHIMANOUCHI [3]. T h e b e h a v i o u r o f this b a n d a r o u s e d o u r suspicions in that, u p o n annealing the matrix, it d e c r e a s e d in intensity m u c h m o r e than the o t h e r b a n d s in the spectrum. Its position c o r r e s p o n d s very closely with that o f the 1'3 b a n d o f m e t h a n e in a nitrogen matrix r e p o r t e d by NELANDER [14] and b y BURCZVK a n d D o w N s [15], a n d we t h e r e f o r e assign it to m e t h a n e . T h e p r e s e n c e o f m e t h a n e in the matrix is not surprising, in view o f the reactivity o f z i n c d i m e t h y l , a n d the assignment o f the 3015 c m -~ b a n d leads i m m e d i a t e l y to the conclusion that the strong p e a k at 1303 c m - i should also be assigned to m e t h a n e , a n d in particular to v4 which is f o u n d in just this position in a nitrogen matrix [14, 15]. T h e assignment o f the 1303 cm -~ b a n d to m e t h a n e raises p r o b l e m s , h o w e v e r , b e c a u s e b o t h SHIMANOUCHI [3] a n d BUTLER and NEWBURY [1] have assigned it to an e' CH3 d e f o r m a t i o n m o d e of zincdimethyl. In o r d e r to clarify this p r o b l e m , we have r e c o r d e d the R A I R s p e c t r u m o f a film o f z i n c d i m e t h y l d e p o s i t e d on a clean c o p p e r surface (Fig. 2a). T h e fact that this s p e c t r u m shows just o n e intense b a n d in the C - H stretching region strongly suggests that we are Table 2. Observed bands and assignments for Z n M e 2 and a comparison with earlier assignments This work Mode no.
e' e' e~
e'
5 6 7 8 9 10
CH3 stretch CH3 deform. C-Zn stretch CH3 stretch CH3 deform. CH3 rock combination
*
t
*
§
II
2915 s 1183m 613 2966 s 1301 m 704 s --
2899 s 1164s 604 vs 2948 s 1302 w 695 vs 2850 w
2920 m 1185m 613 vs 2970 m 1440 vw 695 vs 2850w
2898 vw l158vw -- ¶ 2940 s 1451vvw 698 vs 2840 vw
2897 s I155m 598 vs 2940 s 1470vw 718 vs 2841 m
* SHIMANOUCHI [3], 1" BUTLER a n d NEWBURY [1] (liquid).
Spectrum of the matrix-isolated zincdimethyl. § RAIR spectrum of a thin film on a Cu surface. IITransmission infrared spectrum of a polycrystallin¢ layer on a KBr plate. ¶ Band not observed since measuring range was 4000-650 cm- t. The numbering of the modes follows that of SHZMANOUCHZ"[3].
1176
M. BOCHMANNet al.
Table 3. Observed bands and assignments for the spectra of matrix-isolated ZnMe2, CdMe2and HgMe2and a comparison with earlier assignments ZnMe2 Mode no. e' a~' e' e'
a~' e' a~'
*
t
8 2966 s 2948 s 5 2915 s 2899 s combtn. 2850 9 1301 m 1302 w 6 1183m 1164s 10 704 s 695 s 7 613 604 vs
CdMe2 This work 2970 m 2920 m 2850 w 1440 vw 1185m 695 vs 613 vs
* 2980 vs 2923 s
t
2958s 2902 vs 2858 1315 1314 w 1136m 1124m 700 s 692 vs 535 s 526 vs
HgMe2 This work
*
2990 m 2962s 2930m 2956b 2869 w 1445 w 1397 1 3 2 5 w l191m 693 vs 780 vs 538 vs 540 vs
t
This work
2964 s 2896 s
2960 m 2895 m 2920 w 1395 m 1480vw 1 1 8 5 m 1400vw 774 vs 770 m 538 vs 538 vs
* SHIMANOUCHI [3]. BUTLER a n d NEWBURY [1].
The numbering of the modes follows that of SHIMANOUCHI[3] (see Table 2). dealing here with a case of a high degree of orientation of the zincdimethyl molecule on the copper surface. I R spectra of thin films on metal surfaces measured by the R A I R S technique are subject to a restriction which has become known as the metal surface selection rule [13, 16]. This arises because the amplitude of the I R standing wave at the metal surface is necessarily zero in a direction parallel to the surface. The metal surface selection rule states that only vibrational modes polarized perpendicular to the metal surface may be excited and this will apply to molecules in a film of small thickness compared to the wavelength of I R radiation, i.e. ,~1 p. The film deposited on the copper surface in this study was estimated to be of thickness <100 nm. We assign the strong band at 2940cm -I in the R A I R spectrum to the e' methyl antisymmetric stretching mode (v8) of zincdimethyl. The e' modes of this molecule are polarized perpendicular to the C - Z n - C axis while the a~ modes are polarized parallel to this axis. The absence of a strong band in the range 2900-2920 cm -~, which could be assigned to the a2' methyl symmetric stretching fundamental, suggests that the molecules in the film are orientated with the C - Z n - C axis parallel to the metal surface. The fact that the only other strong band in the R A I R spectrum, at 698 cm -1, may be assigned to the e' methyl rocking m o d e is consistent with the suggested film structure. However, the original assignment [1, 3] of a medium intensity band at 1301 cm -~ to the e' antisymmetric methyl deformation mode means that this should also appear in the R A I R spectrum. This apparent anomaly prompted us to measure the IR spectrum of a thin film of solid dimethyl zinc under conditions where the metal surface selection rule does not operate. The transmission I R spectrum of a thin film of ZnMe2 on a KBr plate is compared to the R A I R spectrum in Fig. 2. The transmission I R spectrum does not show any significant band at 1301 cm -1 but in all other respects it is consistent with earlier literature. In particular, we now see the strong bands due to a~ modes which were absent from the R A I R spectrum. We therefore conclude that the 1301 cm-1 band observed in earlier work, and in our matrix isolation spectra, should be assigned to methane produced by partial decomposition of zincdimethyl prior to deposition. Methane does not contaminate the thin films on copper and KBr because the substrate temperature (ca 100 K) is too high for methane to condense. The assignment of our zincdimethyl spectra (Table 2) therefore differs from the previous literature in that we assign the weak bands at 1440 (matrix), 1451 (Cu-surface) and 1470 cm -1 (polycrystalline layer) to Vg, the e' CH3 deformation, and suggest that the 1434 cm -1 band in SHXMANOUCHI'Stable should be assigned to this deformation rather than to v13, the e" CH3 deformation. The change of the assignment of v9 implies that the prominent band seen in most spectra between 2840 and 2850 cm -1 and assigned by BUTLER and NEWBURY [1] to the e' combination 2Vl0+Vl3 should be reassigned to 2v10 + Vg.
IR spectra of group IIB metaldimethyls
1177
Apart from the reassignment of v9, the positions of the bands in all the spectra are in good agreement with earlier data and the observed shifts in position lie within the expected ranges for changes of phase,
Comparison of the matrix spectra of the dimethyl compounds of Zn, Cd and Hg Concerning the matrix spectra of the three metaldimethyls, we first note some further differences between our results and those of BUTLER and NEWBURY [1]. In all their metaldimethyl spectra and for all phases, BUTLER and NEWBURY observe IR activity, which they attribute to overtones, in the 2400-1600 cm -1 region. We do not observe such bands in any of our spectra; liquid film (recorded between KBr discs for comparison), polycrystalline layer, layer on copper or matrix-isolated. As far as the spectra of the last three types of sample are concerned, this may well be due to the low temperatures at which they were measured. We have no explanation for this difference in the case of our spectrum of liquid ZnMe2. The assignment of the C - H stretching modes has raised no problems in the past, and the same can be said of the methyl rocking and metal-carbon stretch. However, as we have already seen, the assignment of the methyl deformations in the 1100-1500cm -1 region has proved more difficult. Our assignments for this region for the cadmium- and mercurydimethyls (Table 3) require some comment. Firstly, we note three weak features observed in our matrix spectra which have not been assigned since there are good reasons to doubt that they belong to the molecules under investigation. Specifically, comparison with the spectra of our liquid ZnMe2 films leads us to the conclusion that the 1030 cm -~ band in the ZnMe2 matrix spectrum is the C-O stretching band of the decomposition product, Zn(OMe)2. A band seen at 1130 cm-t in the CdMe2 spectrum is very characteristic of diethyl ether, which was used as solvent in the preparation of this compound. Although no trace of ether could be seen in the NMR spectrum, we believe, on balance, that this band should be assigned to that impurity. Finally, the band at 1090 cm -1 in the spectrum of HgMe2 was noticeable in the baseline of the window measured before isolation. For CdMe2 our assignment of the e' (v9) and a~' (1'6) CH3 deformation modes places these at rather higher frequencies than hitherto. If we were to accept the 1130 cm- ~band as belonging to CdMe2, rather than to an impurity, an assignment very similar to that of SHIMANOUCHI [3] and of BUTLERand NEWBURY[1] could be made, and this must remain a possibility. However, following the RAIR results, the reassignment of v9 appears to be very well founded. In the case of HgMe2 a problem also arises with the assignment of the two CH3 deformations since the spectrum shows only extremely weak bands in the region of interest. To our knowledge, no interpretation of this remarkable decrease in the intensity of at least one of the CH3 deformation vibrations in the sequence Zn--* Cd--~ Hg has been given in the literature. The frequencies chosen in Table 3 are taken from a spectrum of a matrix which had been annealed to the extent that there was aggregation of the HgMe2 giving stronger, though rather broad, bands at 1480 and 1400 c m -1. The relevant portion of this spectrum is reproduced in Fig. 1. CONCLUSIONS
The IR-active band at 1301 cm -~ in the spectrum of gaseous ZnMe2 is due to methane which requires that previous assignments [1, 3], which attributed the band to the e' CHa deformation (v9), be revised. The RAIR spectrum of an orientated film of the compound strongly suggests that a band seen at 1451 cm -~ in the R A I R and at 1440cm -1 in the matrix should be assigned to v9. This band clearly corresponds to the band found at 1434 cm -1 in the gaseous phase and previously assigned [1, 3] to the e" CH3 deformation (v13). Thus, the reassignment raises a question concerning the experimental position of v13. Since via is predicted to be IR-inactive, this is a question upon which our data can shed no light. SA(A) 4a,a-i
1178
M. BOCHMANNet al.
The reassignment of v9 in the ZnMe2 spectrum suggests that a similar change should be made in the assignment of the spectra of the cadmium and mercury compounds. A tentative reassignment of v6 is also proposed for these two molecules. Acknowledgements--We are grateful to Professor N. Sheppard for many useful discussions and we thank the United Kingdom Science and Engineering Research Council for support of this work.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
I. S. Butler and M. L. Newbury, Spectrochim. Acta 33A, 669 (1977). P. R. Bunker, J. Chem. Phys. 47,718 (1967). T. Shimanouchi, J. Phys. Chem. Ref. Data 6, 993 (1977). H. S. Gutowsky, J. Chem. Phys. 17, 128 (1949). D. R. J. Boyd, R. L. Williams and H. W. Thompson, Nature 167, 766 (1951). A. M. W. Bakke, J. Molec. Spectrosc. 41, 1 (1972). J. R. Durig and S. C. Brown, J. Molec. Spectrosc. 45, 338 (1973). A. L. Galyer and G. Wilkinson, Inorg. Synth. 19, 253 (1979). R. D. Anderson and H. A. Taylor, J. Phys. Chem. 56, 161 (1952). H. Gilman and R. E. Brown, J. Am. Chem. Soc. Sl, 928 (1929). H. Gilman and R. E. Brown, J. Am. Chem. Soc. 52, 3314 (1930). C. S. Marvel and V. I. Gould, J. Am. Chem. Soc. 44, 153 (1922). M. A. Chesters, J. Electron Spectrosc. Relat. Phenom. 38, 123 (1986). B. Nelander, J. Chem. Phys. 82, 5340 (1985). K. Burczyk and A. J. Downs, J. Chem. Soc. Dalton Trans. 2351 (1990). M. A. Pearce and N. Sheppard, Surf. Sci. 59, 205 (1976).
0584-8539/92 $5.00+0.00 (~ 1992 Pergamon Press Ltd
Printed in Great Bnlain
The infrared spectra of the dimethyl complexes of zinc, cadmium and mercury M. BOCHMANN,*M. A. CHESTERS,* A. P. COLEMAN,R. GRINTER* and D. R. LINDER School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
(Received 16 March 1991; in final form 6 January 1992; accepted 6 February 1992) Abstract--The infrared spectra of the dimethyl complexes of zinc, cadmium and mercury, isolated in argon matrices, have been measured. Earlier assignments of the spectrum of the zinc compound appear to have included a band due to a methane impurity. A study of the P A I R spectrum of a thin film of ZnMe2 on a copper surface has revealed that the molecules in the film are strongly orientated and the additional information thus obtained has made it possible to suggest some reassignment of the spectra of all three compounds.
INTRODUCTION T H E VIBRATIONAL s p e c t r a
of the group
liB
metaldimethyls
have
been
extensively
investigated. The most recent study appears to be that of BUTLERand NEWBURY[1], who reported the Raman and IR spectra of zinc-, cadmium- and mercurydimethyl in their gaseous, liquid and solid phases and discussed their spectra using G36 [2] symmetry. The earlier work has been assessed by SmMANOUCm [3], who compiled tables, using data from four sources [4-7], to assign the IR spectra of the gaseous phase and the Raman spectra of the liquids using G~ and D'ah symmetries. In view of the value of SHIMANOUCHI'Scompilation, to which we frequently refer, we have discussed our results in terms of the G3~ formalism, although the molecules are not freely rotating in the matrix. We have also used the same symbols for the irreducible representations of the group as SHIMANOUCHI[3], unlike BUTLER and NEWBURY [1], who adopted a somewhat different notation. Our interest in the spectroscopy, surface chemistry and reactivity of these compounds, and also those of the higher dialkyl species, led us to re-examine the spectra, the primary objective of the present work being to form the basis for our studies of further molecules in the series. We consider here the IR spectra of the matrix-isolated, group IIB metaldimethyls plus the spectra of the zinc species as a film condensed on a copper surface, as a liquid film and as a polycrystalline layer. Following SHIMANOUCm [3], we assign our spectra in the group D;h, since our data do not permit a more sophisticated analysis. EXPERIMENTAL
Preparation of the metaldimethyl compounds All preparations were carried out under an argon atmosphere using standard Sehlenk techniques and dried solvents distilled under nitrogen. Zincdimethyl was prepared as described by GALYER and WmKINSON[8]. Cadmiumdimethyl was prepared using a scaled-down version of the method described by ANDERSONand TAYLOR[9]. The sample was purified by successive microdistillations. Mercurydimethyl was prepared by a method based upon the procedures described by GILMANand BRowr~ [10, 11] and by MARVELand GOULD[12]. The purity of the products was determined by 1H NMR spectroscopy.
Matrix isolation The conditions used for the preparation of the matrices are recorded in Table 1. The materials were deposited with argon onto a CsI window cooled to approximately 15 K by an Air Products * Authors to whom correspondence should be addressed. 1173
M. BOCHMANNet al.
1174
Table 1. The conditions for the preparation of the matrices Sample
ZnMe~
Reservoir pressure (Tort) Reservoir temperature (°C) Isolation time (min) Matrix appearance
0.15 -85
CdMe2 0.43 -75
25 clear but crazed
90 clear
HgMe2 0.53 -70 15 clear
"Displex" CSA202 closed-cycle refrigerator. The desired guest/host ratio was obtained by holding the flow of argon constant and varying the temperature of the metaldimethyl reservoir. IR spectra were recorded with a Perkin-Elmer 577 spectrometer over the range 200-4000 cm -j. In all cases a baseline of the clean window was obtained at room temperature and 15 K and of the window covered with a layer of pure argon at 15 K.
Thin film spectra ( R A I R S ) The reflection-absorption infrared spectroscopy (RAIRS) experiments were carried out in an oil diffusion pumped, ultra-high vacuum chamber interfaced to a Mattson Sirius 100 FTIR spectrometer, the optical configuration of which has been described previously [13]. A narrowband mercury-cadmium telluride (MCT) IR detector was used which allowed spectra in the range 4000-650 era-' to be recorded. The spectra were measured at 8 cm- ~resolution by the co-addition of 3000 interferograms for each of the sample and reference (clean surface) single beam spectra. The Ru(0001) single crystal substrate was cleaned by cycles of argon ion bombardment followed by annealing at 1250 K. A clean copper film was deposited on the room temperature substrate using high-temperature evaporation source. The transmission experiments on polycrystalline layers were carried out in an evacuated glass cell positioned in the same spectrometer. Zincdimethyl was condensed on a KBr substrate cooled to ca 100 K. A wide-band MCT detector was used to extend the spectral range down to 500 cm -1. Spectra were recorded at 8 cm-1 resolution by the co-addition of 1000 interferograms. RESULTS AND DISCUSSION Matrix isolation I R spectra are s h o w n in Fig. 1 and thin film spectra of ZnMe2 in Fig. 2. T h e n u m e r i c a l data and assignments are given in Tables 2 a n d 3 w h e r e they are c o m p a r e d with literature values. W e consider zincdimethyl first.
408o
-¢a~~
~
"~
c"
ag~jegated sample
40 8°
,
i 30
25
I 20
15 Wavenumber x 10-2
10
5
Fig. 1. The IR spectra of zinc-, cadmium- and mercurydimethyls isolated m argon matrices. • indicates bands due to methane in the zincdimethyl spectrum.
IR spectra of group IIB metaldimethyls
35
30
25
20 _
15
1175
10
Wavenumber x 10-z Fig. 2. IR spectra of zincdimethyl. (a) The R A I R spectrum of a thin film on a copper surface. (b) The transmission IR spectrum of a polycrystaUine layer on a KBr plate.
Zincdimethyl T h e matrix s p e c t r u m of this m o l e c u l e (Fig. 1) shows a strong b a n d at 3015 c m -~, which has n o c o u n t e r p a r t in the data a s s e m b l e d by SHIMANOUCHI [3]. T h e b e h a v i o u r o f this b a n d a r o u s e d o u r suspicions in that, u p o n annealing the matrix, it d e c r e a s e d in intensity m u c h m o r e than the o t h e r b a n d s in the spectrum. Its position c o r r e s p o n d s very closely with that o f the 1'3 b a n d o f m e t h a n e in a nitrogen matrix r e p o r t e d by NELANDER [14] and b y BURCZVK a n d D o w N s [15], a n d we t h e r e f o r e assign it to m e t h a n e . T h e p r e s e n c e o f m e t h a n e in the matrix is not surprising, in view o f the reactivity o f z i n c d i m e t h y l , a n d the assignment o f the 3015 c m -~ b a n d leads i m m e d i a t e l y to the conclusion that the strong p e a k at 1303 c m - i should also be assigned to m e t h a n e , a n d in particular to v4 which is f o u n d in just this position in a nitrogen matrix [14, 15]. T h e assignment o f the 1303 cm -~ b a n d to m e t h a n e raises p r o b l e m s , h o w e v e r , b e c a u s e b o t h SHIMANOUCHI [3] a n d BUTLER and NEWBURY [1] have assigned it to an e' CH3 d e f o r m a t i o n m o d e of zincdimethyl. In o r d e r to clarify this p r o b l e m , we have r e c o r d e d the R A I R s p e c t r u m o f a film o f z i n c d i m e t h y l d e p o s i t e d on a clean c o p p e r surface (Fig. 2a). T h e fact that this s p e c t r u m shows just o n e intense b a n d in the C - H stretching region strongly suggests that we are Table 2. Observed bands and assignments for Z n M e 2 and a comparison with earlier assignments This work Mode no.
e' e' e~
e'
5 6 7 8 9 10
CH3 stretch CH3 deform. C-Zn stretch CH3 stretch CH3 deform. CH3 rock combination
*
t
*
§
II
2915 s 1183m 613 2966 s 1301 m 704 s --
2899 s 1164s 604 vs 2948 s 1302 w 695 vs 2850 w
2920 m 1185m 613 vs 2970 m 1440 vw 695 vs 2850w
2898 vw l158vw -- ¶ 2940 s 1451vvw 698 vs 2840 vw
2897 s I155m 598 vs 2940 s 1470vw 718 vs 2841 m
* SHIMANOUCHI [3], 1" BUTLER a n d NEWBURY [1] (liquid).
Spectrum of the matrix-isolated zincdimethyl. § RAIR spectrum of a thin film on a Cu surface. IITransmission infrared spectrum of a polycrystallin¢ layer on a KBr plate. ¶ Band not observed since measuring range was 4000-650 cm- t. The numbering of the modes follows that of SHZMANOUCHZ"[3].
1176
M. BOCHMANNet al.
Table 3. Observed bands and assignments for the spectra of matrix-isolated ZnMe2, CdMe2and HgMe2and a comparison with earlier assignments ZnMe2 Mode no. e' a~' e' e'
a~' e' a~'
*
t
8 2966 s 2948 s 5 2915 s 2899 s combtn. 2850 9 1301 m 1302 w 6 1183m 1164s 10 704 s 695 s 7 613 604 vs
CdMe2 This work 2970 m 2920 m 2850 w 1440 vw 1185m 695 vs 613 vs
* 2980 vs 2923 s
t
2958s 2902 vs 2858 1315 1314 w 1136m 1124m 700 s 692 vs 535 s 526 vs
HgMe2 This work
*
2990 m 2962s 2930m 2956b 2869 w 1445 w 1397 1 3 2 5 w l191m 693 vs 780 vs 538 vs 540 vs
t
This work
2964 s 2896 s
2960 m 2895 m 2920 w 1395 m 1480vw 1 1 8 5 m 1400vw 774 vs 770 m 538 vs 538 vs
* SHIMANOUCHI [3]. BUTLER a n d NEWBURY [1].
The numbering of the modes follows that of SHIMANOUCHI[3] (see Table 2). dealing here with a case of a high degree of orientation of the zincdimethyl molecule on the copper surface. I R spectra of thin films on metal surfaces measured by the R A I R S technique are subject to a restriction which has become known as the metal surface selection rule [13, 16]. This arises because the amplitude of the I R standing wave at the metal surface is necessarily zero in a direction parallel to the surface. The metal surface selection rule states that only vibrational modes polarized perpendicular to the metal surface may be excited and this will apply to molecules in a film of small thickness compared to the wavelength of I R radiation, i.e. ,~1 p. The film deposited on the copper surface in this study was estimated to be of thickness <100 nm. We assign the strong band at 2940cm -I in the R A I R spectrum to the e' methyl antisymmetric stretching mode (v8) of zincdimethyl. The e' modes of this molecule are polarized perpendicular to the C - Z n - C axis while the a~ modes are polarized parallel to this axis. The absence of a strong band in the range 2900-2920 cm -~, which could be assigned to the a2' methyl symmetric stretching fundamental, suggests that the molecules in the film are orientated with the C - Z n - C axis parallel to the metal surface. The fact that the only other strong band in the R A I R spectrum, at 698 cm -1, may be assigned to the e' methyl rocking m o d e is consistent with the suggested film structure. However, the original assignment [1, 3] of a medium intensity band at 1301 cm -~ to the e' antisymmetric methyl deformation mode means that this should also appear in the R A I R spectrum. This apparent anomaly prompted us to measure the IR spectrum of a thin film of solid dimethyl zinc under conditions where the metal surface selection rule does not operate. The transmission I R spectrum of a thin film of ZnMe2 on a KBr plate is compared to the R A I R spectrum in Fig. 2. The transmission I R spectrum does not show any significant band at 1301 cm -1 but in all other respects it is consistent with earlier literature. In particular, we now see the strong bands due to a~ modes which were absent from the R A I R spectrum. We therefore conclude that the 1301 cm-1 band observed in earlier work, and in our matrix isolation spectra, should be assigned to methane produced by partial decomposition of zincdimethyl prior to deposition. Methane does not contaminate the thin films on copper and KBr because the substrate temperature (ca 100 K) is too high for methane to condense. The assignment of our zincdimethyl spectra (Table 2) therefore differs from the previous literature in that we assign the weak bands at 1440 (matrix), 1451 (Cu-surface) and 1470 cm -1 (polycrystalline layer) to Vg, the e' CH3 deformation, and suggest that the 1434 cm -1 band in SHXMANOUCHI'Stable should be assigned to this deformation rather than to v13, the e" CH3 deformation. The change of the assignment of v9 implies that the prominent band seen in most spectra between 2840 and 2850 cm -1 and assigned by BUTLER and NEWBURY [1] to the e' combination 2Vl0+Vl3 should be reassigned to 2v10 + Vg.
IR spectra of group IIB metaldimethyls
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Apart from the reassignment of v9, the positions of the bands in all the spectra are in good agreement with earlier data and the observed shifts in position lie within the expected ranges for changes of phase,
Comparison of the matrix spectra of the dimethyl compounds of Zn, Cd and Hg Concerning the matrix spectra of the three metaldimethyls, we first note some further differences between our results and those of BUTLER and NEWBURY [1]. In all their metaldimethyl spectra and for all phases, BUTLER and NEWBURY observe IR activity, which they attribute to overtones, in the 2400-1600 cm -1 region. We do not observe such bands in any of our spectra; liquid film (recorded between KBr discs for comparison), polycrystalline layer, layer on copper or matrix-isolated. As far as the spectra of the last three types of sample are concerned, this may well be due to the low temperatures at which they were measured. We have no explanation for this difference in the case of our spectrum of liquid ZnMe2. The assignment of the C - H stretching modes has raised no problems in the past, and the same can be said of the methyl rocking and metal-carbon stretch. However, as we have already seen, the assignment of the methyl deformations in the 1100-1500cm -1 region has proved more difficult. Our assignments for this region for the cadmium- and mercurydimethyls (Table 3) require some comment. Firstly, we note three weak features observed in our matrix spectra which have not been assigned since there are good reasons to doubt that they belong to the molecules under investigation. Specifically, comparison with the spectra of our liquid ZnMe2 films leads us to the conclusion that the 1030 cm -~ band in the ZnMe2 matrix spectrum is the C-O stretching band of the decomposition product, Zn(OMe)2. A band seen at 1130 cm-t in the CdMe2 spectrum is very characteristic of diethyl ether, which was used as solvent in the preparation of this compound. Although no trace of ether could be seen in the NMR spectrum, we believe, on balance, that this band should be assigned to that impurity. Finally, the band at 1090 cm -1 in the spectrum of HgMe2 was noticeable in the baseline of the window measured before isolation. For CdMe2 our assignment of the e' (v9) and a~' (1'6) CH3 deformation modes places these at rather higher frequencies than hitherto. If we were to accept the 1130 cm- ~band as belonging to CdMe2, rather than to an impurity, an assignment very similar to that of SHIMANOUCHI [3] and of BUTLERand NEWBURY[1] could be made, and this must remain a possibility. However, following the RAIR results, the reassignment of v9 appears to be very well founded. In the case of HgMe2 a problem also arises with the assignment of the two CH3 deformations since the spectrum shows only extremely weak bands in the region of interest. To our knowledge, no interpretation of this remarkable decrease in the intensity of at least one of the CH3 deformation vibrations in the sequence Zn--* Cd--~ Hg has been given in the literature. The frequencies chosen in Table 3 are taken from a spectrum of a matrix which had been annealed to the extent that there was aggregation of the HgMe2 giving stronger, though rather broad, bands at 1480 and 1400 c m -1. The relevant portion of this spectrum is reproduced in Fig. 1. CONCLUSIONS
The IR-active band at 1301 cm -~ in the spectrum of gaseous ZnMe2 is due to methane which requires that previous assignments [1, 3], which attributed the band to the e' CHa deformation (v9), be revised. The RAIR spectrum of an orientated film of the compound strongly suggests that a band seen at 1451 cm -~ in the R A I R and at 1440cm -1 in the matrix should be assigned to v9. This band clearly corresponds to the band found at 1434 cm -1 in the gaseous phase and previously assigned [1, 3] to the e" CH3 deformation (v13). Thus, the reassignment raises a question concerning the experimental position of v13. Since via is predicted to be IR-inactive, this is a question upon which our data can shed no light. SA(A) 4a,a-i
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M. BOCHMANNet al.
The reassignment of v9 in the ZnMe2 spectrum suggests that a similar change should be made in the assignment of the spectra of the cadmium and mercury compounds. A tentative reassignment of v6 is also proposed for these two molecules. Acknowledgements--We are grateful to Professor N. Sheppard for many useful discussions and we thank the United Kingdom Science and Engineering Research Council for support of this work.
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