J. Phys. Chem. 1996, 100, 18357-18362
18357
Effects of Excitation Wavelength on the Raman Spectra of Vanadium(V) Oxo Compounds Jan-Christoph Panitz* and Alexander Wokaun General Energy Research Department, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland ReceiVed: June 27, 1996; In Final Form: September 16, 1996X
Raman spectra of the compounds vanadia, vanadium(V) oxytrichloride, and vanadium(V) triisopropoxide oxide were recorded using several laser wavelengths in the range 482.5-1064 nm for excitation. Data recorded were analyzed by comparing band intensities with those of reference compounds free from resonance Raman effects in the laser wavelength range used. Results obtained show that in the case of vanadia and vanadium(V) oxytrichloride the intensity ratio IVdO/Iref is largest for an excitation wavelength of 676.5 nm, whereas in case of vanadium(V) triisopropoxide oxide, IVdO/Iref is largest for excitation with the 482.5 nm laser line. The study provides guidelines for which laser wavelength is most useful for the characterization by Raman spectroscopy of vanadium oxide layers on catalysts and of compounds useful for the preparation of such catalysts.
Introduction Supported vanadia catalysts are used for a variety of industrial processes, for example the selective catalytic reduction (SCR) of nitric oxide by ammonia,1 partial oxidation reactions,2 and ammoxidation reactions3 of aromatic hydrocarbons. Characterization of supported vanadia catalysts by Raman spectroscopy has been reported by several authors.4-8 In the course of an investigation of the structure of vanadia layers supported on titania it was observed that unreduced intensities of Raman bands of vanadia were considerably higher when excited with the 647.1 nm line of a Kr+ ion laser, as compared to excitation with the 488 nm line of an Ar+ ion laser.6 In this article, the effects of excitation wavelength on the Raman spectra of vanadia are addressed in detail. To find out whether effects of excitation laser wavelength are also observable in other vanadium(V) oxo compounds, we extended the investigation to the liquid compounds vanadium(V) oxytrichloride, VOCl3, and vanadium(V) triisopropoxide oxide, abbreviated in the following as VOTIP. Both compounds are important starting materials used to obtain either complex vanadium compounds9 or catalysts.10 A further motivation to perform this study was to find out which excitation laser wavelength is the most suitable for Raman spectroscopy of vanadium(V) oxo compounds. In particular, Raman investigations of supported catalysts under in situ conditions, when only a few wt % vanadia is supported on a carrier, will benefit from an optimization of this parameter. A central point of this investigation is the spectroscopic characteristics of the structural element common to all compounds investigated, the vanadyl (VdO) group. Our interpretation of data recorded aims primarily at classifying the effects observed; possible influences of either resonant excitation of self-absorption effects of Raman intensities will be discussed on a qualitative basis. After a description of the experimental procedures, the mentioned compounds will be addressed in sequence. Experimental Section General. Vanadium(V) oxide (Fluka, purity 99+%), vanadium(V) oxytrichloride (Aldrich, 99%), and vanadium(V) * Author to whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01923-5 CCC: $12.00
triisopropoxide oxide (Alfa, 99%) were used as received. As reference compounds, naphthalene (Merck, z. synth.) and toluene (Merck, p.a.) were used. Liquid samples were placed in cylindrical tubes with a flat bottom that were preheated at 140 °C for 2 days. In the case of the vanadium compounds sensitive to moisture, syringe techniques under an argon atmosphere were used for the transfer of the samples. For the solid samples, a flat disc was made from polycrystalline vanadium(V) oxide and naphthalene, respectively. To characterize the vanadium(V) oxide used in this study, a diffuse reflectance spectrum was recorded in the wavelength range 400-750 nm. A flat disc containing 10 wt % V2O5, pressed from a blend of vanadium(V) oxide and anhydrous sodium sulfate (Fluka), was used in the measurement. A doublebeam UV/vis instrument (Lambda 19, Perkin-Elmer) equipped with an integrating sphere was used. The spectrum was recorded against a background of anhydrous sodium sulfate. Raman Spectroscopy. Raman spectra were acquired on a home-built high-resolution Raman spectrometer. Briefly, the spectrometer consists of a SPEX 1418 double monochromator (f ) 0.85m) and a backthinned CCD detector (Tektronix, TK512CB) mounted in a liquid nitrogen-cooled camera head (Photometrics, CH260). A more detailed description of the setup and control software is given in ref 11. Excitation was performed with a Ti:sapphire CW laser (Coherent, Model 890) operated at 780.4 or 818.2 nm; additionally, a Kr+ ion laser (Coherent, Innova 301) was used for excitation in the range 482.5-676.5 nm. Laser power and spectral resolution at each laser wavelength are included in Table 1. The slit width of the spectrometer was adjusted such that instrument resolution equals 3 times the pixel resolution of the multichannel detector. In the case of vanadia, it was possible to acquire a FT-Raman spectrum as well (Bruker IFS55 and FRA106 accessory), with details of the experimental setup and parameters given in ref 12. Whereas spectra of liquid samples were collected in a 90° scattering geometry, the backscattering (180°) geometry was used for solid samples. Care was taken that Raman spectra of sample and reference materials were acquired under identical conditions. Especially in the case of vanadium(V) oxytrichloride, it proved to be important to start from the red side of the excitation wavelength spectrum in order to avoid premature photodecomposition of the sample. © 1996 American Chemical Society
18358 J. Phys. Chem., Vol. 100, No. 47, 1996
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TABLE 1: Intensity Ratios of Selected Bands Observed in Vanadia, Which Have Been Assigned to Different O-Vn Coordination Polyhedra15 a
λ/nm 482.5 530.9 568.2 647.1 676.4 780.4 818.2 1064
spectral laser power/ resolution/ cm-1 mW 15 10 25 50 40 100 70 15
1.2 0.9 0.8 0.6 0.5 0.4 0.4 1.0
I996/ I1021
I703/ I762
I483/ I513
I996/ I703
I996/ I483
0.013 0.123 0.743 1.321 1.694 0.756 0.529 0.083
0.015 0.176 1.065 1.903 2.359 1.052 0.699 0.130
0.013 0.201 0.854 1.470 1.787 0.635 0.423 0.057
0.642 0.507 0.415 0.338 0.342 0.351 0.441 0.420
0.887 0.991 0.852 0.694 0.884 0.787 1.020 1.404
a Laser power and spectral resolution at each excitation wavelength used are given as well. In the case of the first three intensity ratios, intensities were compared with intensities of naphthalene bands observed in adjacent spectral regions. The two columns on the right show intensity ratios within a spectrum recorded at the given wavelength, after correction using eq 1.
Intensities of Raman bands were obtained by fitting Voigt profiles to the experimental line shapes using nonlinear least squares minimization (NLLSQ) routines. The Gaussian contribution to the Voigt profiles corresponds to the spectral slit width of the spectrometer at the given laser wavelength. Prior to running the NLLSQ routines, a baseline correction was performed. When comparing intensities of bands within one Raman spectrum, intensity ratios Iy/Ix were corrected for the V4-dependence of inelastic scattering by dividing by the factor F,
F ) (ν˜ i - y cm-1)4/(ν˜ i - x cm-1)4, y > x
(1)
νi ) energy of excitation line, in cm-1 if the difference in band positions was larger than 75 cm-1. Iy and Ix are the intensity of a signal recorded at a Raman shift of y cm-1 and x cm-1, respectively. Otherwise, results would be biased by laser wavelength, especially with red or near IR laser excitation. For a laser wavelength of 1064 nm (9398 cm-1) and bands with Raman shifts of 1000 and 1075 cm-1, eq 1 yields the result that a difference of 75 cm-1 corresponds to a value of F ) 0.96. This value is within the limit of reproducibility of the experimental data. Effects of a variation in excitation laser wavelength on recorded intensities are monitored by calculating an intensity ratio Isample/Ireference. Isample is the intensity of a Raman band of the compound investigated, and Ireference is the intensity of a Raman signal of the reference compound, occurring in close spectral vicinity of the sample signal. It is supposed that any resonance Raman effects in the reference compounds can be ruled out, an assumption that is valid in excellent approximation for the cases of naphthalene and toluene in the range of laser wavelengths used. Results and Discussion Vanadium(V) Oxide. Vanadium(V) oxide (orthorhombic, space group Pmnm - D2h13) is an example of a transition metal oxide with a layered structure.13,14 The assignments of its Raman bands according to Beattie and Gilson15 will be listed for future reference. Associated with the vanadyl group are the bands at 996 cm-1, symmetric stretch of character ag, and 406 cm-1, symmetric deformation (ag), and a deformation vibration at 285 cm-1 (b2g). The band at 483 cm-1 is assigned to a deformation vibration of oxygen atoms coordinated to two vanadium centers. Vibrations of oxygen atoms connected to
Figure 1. Raman spectra of vanadia excited with several different laser wavelengths.
three vanadium centers result in the modes at 703 cm-1 (stretching vibration, b2g) and 530 cm-1, symmetrical stretch, ag, and a deformational mode of character ag at 305 cm-1. For reasons of brevity, the discussion of our results will be restricted to the modes at 996, 483, and 703 cm-1. Following the interpretation given in ref 15, each type of local vanadiumoxygen coordination sphere (see above) is thereby accounted for. The vanadyl stretch at 996 cm-1 is referenced to a vibrational mode of naphthalene, namely, the band at 1021 cm-1.16 Similarly, bands at 703 and 483 cm-1 are compared to the modes at 762 and 513 cm-1. Figure 1 shows the sequence of Raman spectra recorded with excitation wavelength increasing from bottom to top. Results of an analysis of recorded data are given in Table 1. Out of the five intensity ratios displayed in Table 1, the first three columns show intensity ratios Isample/Ireference and the other two list results of intensity comparisons within one spectrum at the given wavelength. Intensity ratios I996/I1021, I703/I762, and I483/I513 follow the same course (cf. Table 1). With increasing excitation wavelength, an increase in intensity ratio is observed, followed by a drop of similar magnitude. At an excitation wavelength of 1064 nm, the ratio I996/I1021 is of the same order as with excitation at 530.9 nm. Figure 2 shows the dependence of the intensity ratio I996/ I1021 for the range of wavelengths investigated against the electronic spectrum of the investigated vanadia sample (10 wt % in anhydrous sodium sulfate). From this figure and Table 1 it follows that the Raman spectrum of vanadia can be best excited using red laser wavelengths. The intensity ratios obtained by comparing bands within one spectrum at a given wavelength give no clear picture whether there is a difference between V-O vibrations assigned to different coordination polyhedra when exciting with different laser wavelengths. Finally, an explanation is proposed for the behavior observed. The efficiency of spontaneous Raman scattering is related to possible parallel processes such as absorption, fluorescence, and self-absorption of inelastically scattered photons. For this reason, the electronic and optical properties of vanadia will be briefly discussed now. Optical properties of vanadia have been already investigated by several researchers.17-20 In addition, a variety of calculations concerning the band structure of vanadia
Raman Spectra of Vanadium(V) Oxo Compounds
J. Phys. Chem., Vol. 100, No. 47, 1996 18359 TABLE 2: Raman Spectra of VOCl3, Excited with a Laser Wavelength of 780 nm vibration
symmetry
cm-1
ν1 ν2 ν3 ν4 ν5 ν6
A1 A1 A1 E E E
1035 411 163 504 247 130
intensitya w vs m w m s
2.6 42.7 4.8 1.0 4.4 14.8
assignment VdO stretch V-Cl sym. stretch V-Cl sym. bend V-Cl asym. stretch Cl3-V-O def. V-Cl asym. bend
a Relative to the intensity of the V-Cl asymmetric stretching vibration.
Figure 2. Intensity ratio I996/I1021 (right scale) at the excitation wavelengths used, and reflection spectrum of the vanadia sample (left scale). The reflection spectrum is displayed in Kubelka-Munk units (KMU), representing absorption. Note that the maximum of the intensity ratio I996/I1021 does not coincide with the minimum of the absorption spectrum for the vanadia sample.
have been reported.21-23 Absorption measurements yielded the result that the edge of absorption is located at ∼520 nm, followed by a broad maximum around 400 nm.17,18 This result was confirmed by reflectance spectroscopy and ellipsometry.18,20. Kenny et al.18 have assigned the band edge to a forbidden direct transition. According to the mentioned band structure calculations, this transition can be characterized by a charge transfer O2p w V3d. Of further notice is the result that the diffuse reflectance spectrum of the sample investigated in this study shows an increase of absorption (expressed in Kubelka-Munk units) for wavelengths > 620 nm. Unfortunately, the equipment employed did not allow us to extend the measurement to wavelengths > 750 nm. According to the aforementioned investigation of Kenny et al.,18 an optical transmission study of a single crystal of vanadia yielded the result that absorption maxima at 830 and 1000 nm are observed if the single crystal is oriented such that the Poynting vector of the light wave is normal to the plane defined by crystallographic axes a and c, and the electrical field vector, E, of the incoming light is parallel to the crystallographic axis a. Since these transitions are observed at energies lower than the absorption edge of the O2p w V3d transition, it has been concluded that nonstoichiometric oxygen defects are the cause of these bands. This conclusion is supported by the observation that band intensities at 830 and 1000 nm are inversely proportional to the oxygen partial pressure during sample preparation.18 The rise in the ratio I996/I1021 may therefore be explained with the electronic properties of the material investigated, as documented by the reflectance spectrum displayed in Figure 2. This figure shows that the minimum of the reflectance spectrum is not coincident with the maximum of the ratio I996/I1021. In the excitation wavelength range 482.5-568.2 nm self-absorption certainly decreases the intensity of the Raman bands of vanadia. But with the results of the reflectance spectrum alone, the rise in the ratio I996/I1021 by changing excitation wavelength from 568.1 to 676.4 nm cannot be explained with self-absorption effects. Therefore, we believe that the increase of the ratio I996/I1021 observed when increasing excitation energy from 1064 to 676.4 nm might be explained by an enhancement of Raman signals based on coupling to the electronic transitions of the oxygen vacancies in the lattice. We are well aware that further experiments are needed to prove this assumption. Vanadium(V) Oxytrichloride. Characterization of vanadium(V) oxytrichloride with Raman spectroscopy has been reported by several authors.24-27 These investigations cover
Figure 3. Raman spectra of VOCl3 recorded with several wavelengths. With progressively longer excitation wavelengths, the spectra become more or less dominated by the V-Cl symmetric stretching vibration at 411 cm-1.
solid, liquid, and vapor phases of the substance. Apart from the observation of Eichhoff and Weigel24 that irradiation with light of wavelength < 500 nm results in considerable photodecomposition of the sample, no effects of excitation wavelength on the Raman spectrum of vanadium(V) oxytrichloride are reported in the literature. Vanadium(V) oxytrichloride is characterized by bond distances of 156 pm (V-O) and 212 pm (V-Cl), having symmetry C3V.28 In accordance with this geometry, six fundamental Raman bands are reported by all authors (see Table 2 for spectral positions and assignments). A sequence of Raman spectra acquired with various wavelengths between 482.5 and 780 nm are depicted in Figure 3. To allow direct visual comparison of the spectra, the intensity of the band at 411 cm-1 (V-Cl sym. stretch) is drawn to the same height in all traces of Figure 3. Evidently, shifting the excitation wavelength to the red leads to the effect that the spectra are progressively dominated by the band at 411 cm-1. The intensity of the vanadyl stretching vibration, in this case V1 at 1035 cm-1, is of main concern to this investigation. Therefore, the intensity of this mode was compared to the intensity of a ring deformational mode, ν11 (1004 cm-1), of toluene.16 Underlying is the assumption that toluene does not show any resonance Raman effects in the excitation wavelength range employed, as mentioned in the Experimental Section. Obtained intensity ratios I1035/I1004 as a function of laser excitation wavelength are given in Table 3. Shifting the excitation wavelength to the red leads to an increase of this parameter by a factor of 85, when comparing data for 482.5
18360 J. Phys. Chem., Vol. 100, No. 47, 1996
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TABLE 3: Changes in the Intensity Ratio I1035/I1004 of the ν1 Band in Vanadium(V) Oxytrichloride upon Variation of the Excitation Wavelength λ/nm
I1035/I1004
482.5 530.9 568.2 647.1 676.4 780.4
0.008 0.101 0.573 0.582 0.678 0.438
and 676.4 nm laser lines. On the average, it can be stated that after an increase the intensity ratio I1035/I1004 is fairly constant for wavelengths g 568.2 nm. The observed effects may be explained with the characteristics of the electronic spectrum of this compound. UV/vis spectra of vanadium(V) oxytrichloride have been published by several authors29-31 and are characterized by an absorption band with λmax ) 345 nm.29,30 Depending on the amount of water or other impurities present in the sample, this band may extend well down to 500 nm.31 Therefore, the possibility that the low values of the intensity ratio I1035/I1004 observed with excitation wavelengths of 482.5 and 530.9 nm are due to partial self-absorption of Raman scattered light cannot be ruled out. With red laser wavelengths, this self-absorption is absent and therefore a rise in the intensity ratio I1035/I1004 is observed. Therefore it is concluded that no resonance Raman effect is observed for this particular vibrational mode of vanadium(V) oxytrichloride. Vanadium(V) Triisopropoxide Oxide. A Raman spectrum of the title compound was reported by Witke et al.,32 who assigned measured band positions using depolarization ratios in conjunction with group frequency concepts. Bands in the spectral regions 980-1030 cm-1 and 595-675 cm-1 were assigned to vibrations of the OdV(-O-)n framework. It was found that the compound features more Raman bands in these regions than expected on symmetry arguments. Ideally, one would expect that the point group for VOTIP is C3V. In this case, only one VdO stretching vibration should be observed. Since investigations of the aggregation behavior of VOTIP using VOTIP/cyclohexane mixtures showed no signs of association,33 it was claimed that the compound existed in several rotational isomers.32 Contrary to this result, an investigation with NMR spectroscopy by Paulsen et al.34 led to the conclusion that neat VOTIP exists as a dimer or oligomer of low stability. By correlating results of EXAFS and 51V-NMR studies, Sanchez et al.35 found that a relationship exists between the line width of the 51V signal and the degree of association. In the case of monomeric VO(O-t-Am)3, Am ) Amyl, the line width amounts to 30 Hz, whereas the dimer [VO(O-n-Pr)3]2 has a line width of 50 Hz. Therefore, an investigation of VOTIP is motivated not only by our interest in the wavelength dependency of Raman excitation but also by the contradicting results reported concerning the structure of the compound. NMR spectra, UV/vis transmission spectra, and Raman spectra were acquired in order to contribute to a solution. Characterization of the investigated sample of VOTIP (VOTIP/ C6D6 ) 2:1 volume ratio) by NMR spectroscopy gave the following results. 1H: 1.24, d, J ) 6.5 Hz; 5.00, s, very broad. 13C: 25.72; 82.13, line width 42 Hz. 51V: -630.85, line width 75 Hz, recorded on a Bruker ARX spectrometer with a proton frequency of 250 MHz. An interpretation of the line width obtained with 51V-NMR has to be regarded with caution, since experimental conditions may differ considerably. Nevertheless, the line width is a very useful indicator of aggregation behavior and, according to ref 35, suggests here the existence of associated VOTIP molecules.
Figure 4. Raman spectra of VOTIP, recorded with six different laser wavelengths in the range 482.5-780.4 nm. Arrows indicate the position of the vanadyl stretching vibrations where changes are easily observed. For the raw data, the broad components of the vanadyl stretching bands dominate with excitation in the blue, whereas the sharp component is of greater intensity when the excitation laser wavelength is shifted to the red.
UV/vis transmission spectra of VOTIP and VOTIP/n-hexane mixtures were recorded at several temperatures in the range 230-323 K. At all temperatures, absorption sets in at a wavelength λ ≈ 420 nm. With increasing temperature, the absorption band broadens, but no indication of a temperature dependent equilibrium between differently associated VOTIP species was observed by this technique. Raman spectra recorded with six different laser wavelengths are given in Figures 4 and 5. Whereas Figure 4 shows spectra information in the range 200-1600 cm-1, Figure 5 gives a detailed view of the CH region (left, 2700-3100 cm-1) and the VdO stretching vibrations (right, 800-1200 cm-1). Due to the very low quantum yield of CCD detectors at wavelengths > 1000 nm, no trace for the CH region recorded with an excitation laser wavelength of 780.2 nm is shown on the left side. In general, measured vibrational band positions agree well with the values reported by Witke et al.32 Focusing attention on Figure 5, we note several changes in the Raman spectra when the excitation wavelength is shifted. Concerning the CH stretching bands, we note that the intensity of the band at 2984 cm-1 decreases, when compared to the band profile in the range 2870-2940 cm-1. By NLLSQ analysis, we find that the band contour of the CH stretching vibrations is best fitted by eight Voigt profiles. Best fitted means in this case that the sum of squares describing the deviation between fitted and experimental data is at a minimum with eight bands, as compared to runs with six, seven, and nine bands. Two of the fitted bands are assigned to antisymmetric methine CH stretching vibrations with an intensity ratio of ∼2:1. The remaining six bands, assigned to symmetric CH stretching vibrations, are composed of two subgroups: two bands at higher Raman shift and four bands at the lower wavenumber edge of the band profile. Again, the intensity ratio amounts roughly to 2:1, when comparing the two subgroups. Returning to the spectral traces on the right side of Figure 5, we note similar effects within the spectral region assigned to the VdO stretch. The band profile around 1000 cm-1 is
Raman Spectra of Vanadium(V) Oxo Compounds
J. Phys. Chem., Vol. 100, No. 47, 1996 18361
Figure 7. Intensity ratios of the three components of the vanadyl stretching vibration band profile (1006, 990, and 977 cm-1) plotted against the excitation laser wavelength. Figure 5. Raman spectra of VOTIP. On the left, the CH stretching vibrations are shown, whereas the panel on the right side allows detailed inspection of the vanadyl stretching vibrations.
Figure 6. Components of the band profile of the vanadyl stretching vibrations in VOTIP obtained by the NLLSQ routines used.
characterized by an increase in intensity of the sharp peak at 1006 cm-1, when compared to the broad band at around 990 cm-1. Again, analysis was done using nonlinear least squares fitting procedures. It emerged that the band profile is best fitted with three components with spectral positions 1006, 990, and 977 cm-1. All these vibrations are assigned to VdO stretching vibrations. A typical fit is given in Figure 6, showing the three Raman bands the band profile is composed of. Both spectral regions mentioned exhibit the most drastic changes under a change of the excitation wavelength. Whereas under blue and green excitation, the CH stretching region is dominated by the antisymmetric stretching vibrations, the situation is reversed at excitation with longer laser wavelengths. Here, the symmetric CH stretches are of prominent intensity. We note that the intensity ratios of the groups of fitted bands (2:1) are rather constant and are not prone to excitation laser wavelength induced intensity shifts. In the region at around 1000 cm-1, we note an increase of the sharp band at 1006 cm-1 with increasing excitation
wavelength, when compared to the two other bands. To analyze this observation, we compared the intensity of the components of the band profile of the VdO stretch with the intensity of the ν11 band of toluene, as already detailed for the case of vanadium(V) oxytrichloride. Although the intensity of the sharp band at 1006 cm-1 relative to the remaining two VdO stretching bands increases with an increase in excitation wavelength, a comparison with the signal of the reference compound presents the data from a different view. As shown in Figure 7, it is found that compared to the ν11 band, the intensity of the signal at 1006 cm-1 is fairly constant. Changes evident in the spectra are due to a decrease in intensity of the broad components when the excitation wavelength is shifted to the red. Since the absorption band of VOTIP has its edge around 420 nm, it is justified to classify the observed intensity shifts as a preresonant enhancement of the Raman signal. The question is now whether the results presented are of relevance to the actual structure of liquid VOTIP. The fact that the broad components of the VdO stretching band are observed at a lower Raman shift when compared to the sharp component might be a first indicator that VOTIP exists as an aggregate in the neat liquid. Two arguments support this assignment. First, coordination of VOTIP molecules via electronegative oxygen atom ligands should lead to an increase of the ionicity of the VdO chemical bond. This will result in a decrease of the vibrational frequency of the VdO bond.36 Second, coordination with oxygen-containing ligands will lead to a drop of the charge transfer transition energy O2p w V3d, shifting this transition to lower energy, thereby making a preresonance enhancement of the Raman signals of the associated VOTIP molecules possible. This resonant enhancement manifests itself in Figure 7 for the bands at 990 and 977 cm-1 attributed to the aggregated species. The line intensity ratios observed in the CH stretching region, which are close to 2:1 for the methine proton signals, give additional support to the conclusion that VOTIP exists in an associated form in the neat liquid. These intensity ratios support the structure proposed by Sanchez et al. for associated vanadium alkoxides.35 Figure 8 shows a proposal for the structure of associated VOTIP molecules.
18362 J. Phys. Chem., Vol. 100, No. 47, 1996
Figure 8. Proposal for the structure of VOTIP in the liquid state. The scheme was drawn according to ref 35.
Conclusions Raman spectroscopy of compounds investigated using several excitation laser wavelengths revealed that excitation with red laser wavelengths (647.1 and 676.5 nm) gave the best results in the case of the samples vanadia and vanadium(V) oxytrichloride, when compared to the signals of reference compounds that are free of resonance Raman effects. In the case of VOTIP, excitation with the blue laser line at 482.5 nm gave the highest Raman intensities, when compared to the reference compound. In the case of vanadyl oxytrichloride, this result may be explained with self-absorption of emitted Raman photons upon excitation in the blue. For vanadia, our results indicate that in addition to the influence of the self-absorption process another mechanism operates that is assumed to be a resonance enhancement based on coupling of the Raman scattering process to transitions caused by oxygen vacancies in the lattice. A preresonance effect is observed for the vibrational modes assigned to vanadyl stretches in the case of the VOTIP, where an increase of recorded intensity is observed when shifting the exciting wavelength to the blue. Further, our observations support the proposal that in the neat liquid VOTIP exists in the form of associates. Acknowledgment. Part of this work was conducted at the University of Bayreuth, Germany. Thanks are due to Robert Schenk for technical assistance with sample preparation and to Bernd Schwarze for recording the NMR spectra. We are indebted to Wolfgang Ha¨fner and Folker Zimmermann for making the nonlinear least squares fit program and data analysis routes available. Financial support by the Verband der Chemischen Industrie is gratefully acknowledged. CAS Registry Numbers. The following registry numbers were supplied by the author: Naphthalene, 91-20-3; toluene, 108-88-3; vanadium(V) oxide, 1314-62-1; vanadium(V) oxytrichloride, 7727 18-6; vanadium(V) triisopropoxide oxide 558884-1. References and Notes (1) Baiker, A.; Dollenmeier, P.; Glinski, M.; Reller, A. Appl. Catal. 1987, 35, 351. Baiker, A.; Dollenmeier, P.; Glinski, M.; Reller, A. Appl. Catal. 1987, 35, 365.
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