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Matrix isolation and theoretical study of the photochemical reaction of CH3CN with CrO2Cl2 and OVCl3
Journal of Molecular Structure 740 (2005) 125–131 www.elsevier.com/locate/molstruc
Matrix isolation and theoretical study of the photochemical reaction of CH3CN with CrO2Cl2 and OVCl3 Nicola Goldberg, Sarah R. Lubell, Bruce S. Ault* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA Received 30 November 2004; revised 16 January 2005; accepted 18 January 2005
Abstract The matrix isolation technique has been combined with theoretical calculations to identify and characterize the photoproducts in the reactions of CH3CN with CrCl2O2 and OVCl3. Twin jet co-deposition of these reagents led to the formation of a 1:1 molecular complex which was observed using UV/visible spectroscopy. Irradiation of these matrices with light of lO300 nm led to the observation of new bands in the infrared spectra, the most intense of which was seen at 1942 cmK1 for the CrCl2O2/CH3CN system. The product bands are assigned to the 2h complexes of acetonitrile n-oxide with CrCl2O and VCl3, respectively. Identification of these species was supported by extensive isotopic labeling (2H and 15N), as well as by B3LYP/6-311CCG(d,2p) density functional calculations. q 2005 Elsevier B.V. All rights reserved. Keywords: Matrix isolation; Complex; Infrared spectra; Visible/UV spectra; Theoretical calculations
1. Introduction High valent transition metal oxo compounds such as CrO2Cl2 and OVCl3 are very strong oxidizing agents, a property that has been utilized in a variety of applications ranging from organic synthesis [1–3] to chemical vapor deposition [4] to catalysis [5]. Despite the importance of these compounds, knowledge of the mechanisms of these reactions is incomplete. Recent matrix isolation studies from this laboratory have investigated the thermal and photochemical reactions of high valent transition metal compounds with a number of small organic and inorganic substrates [6–12]. Two types of reactions have been observed to date, the first of which is formation of an initial complex, followed by HCl elimination from the complex and addition of the organic (or inorganic) fragment to the metal center to retain a 4-coordinate complex [6–10]. The second is oxygen atom transfer from the metal center to the substrate, leading to an oxidized substrate species [11–13]. When this reaction
* Corresponding author. Tel.: C1 51 3556 9238; fax: C1 51 3556 9239. E-mail address: [email protected] (B.S. Ault). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.032
occurred as a result of irradiation after deposition, the two species (e.g. CrCl2O and oxidized substrate) then formed a relatively strongly bound complex. The matrix isolation technique [14–16] has been used on numerous occasions to isolate and characterize reactive intermediates, including radicals, ions and complexes. While solution reactions between these compounds and acetonitrile (CH3CN) have not been reported, Mielke et al. [17] studied the reactions of O atoms with CH3CN in a matrix and observed two major product species, hydroxyacetonitrile (HOCH 2 CN) and acetonitrile N-oxide (CH3CNO), as well as a complex of the two molecules. Acetonitrile has also been used as a reagent in the study of partially bonded molecules, for example with BF3 [18,19]. In addition, reactions of CH3CN with ZrCl4 and HfCl4 have been reported by Hase and Alves [20]. In all of these cases the position of the CbN stretching mode was important in confirming the formation of a complex. The present study was undertaken to investigate oxidation reactions of the nitrile group, through photochemical reactions of acetonitrile with CrO2Cl2 and OVCl3 in argon matrices. High level theoretical calculations were employed to complement the experimental data.
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2. Experimental
3. Results
All of the experiments in this study were carried out on conventional matrix isolation apparatus that has been described [21]. Chromyl chloride, CrO2Cl2, and OVCl3 (both Aldrich), were introduced into the vacuum system as the vapor above the room-temperature liquid, after purification by freeze-pump-thaw cycles at 77 K. CH3CN (Pharmco), CD3CN (99% D), CH3C15N (99% 15N) (both Acros Organics) were introduced in a similar manner into a separate vacuum manifold and were purified by repeated freeze-pump-thaw cycles at 77 K. Argon (Wright Brothers) was used as a matrix gas without further purification. All matrix samples were deposited in the twin-jet mode where the two gas samples were sprayed from separate nozzles onto the 14 K cold window, allowing for a very brief mixing time prior to matrix deposition. The matrices were then irradiated for 1.0 or more hours with the H2O/Pyrex filtered output of a 200 W medium pressure Hg arc lamp, after which additional spectra were recorded on a Perkin–Elmer Spectrum One Fourier transform infrared spectrometer at 1 cmK1 resolution. UV–visible spectra were recorded on a Varian Cary 4000 UV/visible spectrophotometer between 800 and 200 nm using a 0.5 nm bandwidth. Theoretical calculations were carried out using the GAUSSIAN 03W suite of programs [22]. Density functional calculations using the B3LYP functional were used to locate energy minima, determine structures and calculate vibrational spectra of potential intermediate species. Final calculations with full geometry optimization employed the 6-311CCG(d,2p) basis set, after initial calculations with smaller basis sets were run to approximately locate energy minima.
Prior to any co-deposition experiments, blank experiments were run on each of the reagents used in this study. In each case the blanks were in good agreement with literature spectra [23–25] and with blanks run previously in this laboratory. Each blank experiment was then irradiated by the H2O/Pyrex filtered output of a 200 W Hg arc lamp for 2 h. No changes were observed in any of these spectra as a result of irradiation. 3.1. Infrared studies 3.1.1. CrO2Cl2CCH3CN In an initial experiment, a sample of Ar/CH3CNZ 250 was co-deposited with a sample of Ar/CrO2Cl2Z 250. No distinct new infrared absorptions were apparent upon initial matrix deposition. When this matrix was annealed to 25 K and re-cooled, the resultant spectrum again showed no distinct absorptions. On irradiation with the H2O/Pyrex filtered output of a mediumpressure Hg arc (lO300 nm), a moderately intense new doublet was observed at 1942 cmK1, along with several very weak new absorptions at 2750, 1506, 1490 and 679 cmK1. In addition, bands identified as arising from the photochemical reaction of CrCl2O2 with impurity H2O at 3438, 3428 and 755 cmK1 were seen throughout [7]. Since these bands are well known, they will not be discussed further. Fig. 1 shows the region of the spectrum containing the 1942 cmK1 band (and its isotopic equivalents) from a representative experiment with this pair of reagents. In addition, the CrO2Cl2 peaks at 466, 500, 983 and 997 cmK1 were also
Fig. 1. Infrared spectra, in the range 2080–1860 cmK1 of matrices prepared by the twin-jet co-deposition of a sample of Ar/CrO2Cl2Z125 with samples of Ar/CH3CNZ125 (lower trace), Ar/CH3C15NZ125 (middle trace) and Ar/CD3CNZ125 (upper trace) after 3 h of irradiation with light of lO300 nm. Other features in the spectra are due to the parent species, and were present before irradiation.
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noticeably broader suggesting that new bands have formed but are being overlapped by strong reactant bands in these regions. This experiment was repeated several times, using a range of sample concentrations for each reagent, along with different irradiation times. Similar results were obtained throughout, with product band intensities that varied as the concentration of the reactants was varied. The optimum conditions (for maximum product yield) were found to be Ar/CH 3CNZ125 and Ar/CrO 2 Cl 2Z125, and 3 h of irradiation. 3.1.2. CrO2Cl2CCD3CN, CH3C15N When a sample of Ar/CD3CNZ125 was co-deposited with a sample of Ar/CrO2Cl2Z125, and irradiated with light of lO300 nm, new bands were observed at 1943, 1013, and 471 cmK1. The latter two bands appeared as distinct shoulders on the nearby intense parent bands of CrCl2O2. This experiment was repeated with similar results. Experiments were also conducted with 15N labeled acetonitrile. After irradiation, a moderately intense doublet was observed at 1919 cmK1, along with possible very weak bands at 2752, 1508, 1490, and 466 cmK1. Broadening of the CrO2Cl2 bands were also seen in both sets of experiments. Table 1 summarizes product band positions for the photochemical products of CrO2Cl2 with CH3CN and its isotopomers in argon matrices. 3.1.3. OVCl3CCH3CN, CH3C15N Similar experiments were conducted in which samples of Ar/OVCl 3Z125 were co-deposited with samples of Ar/CH3CN and CH3C15NZ125. No distinct new infrared absorptions were apparent upon initial matrix deposition. On irradiation a weak, broad product band was observed at 1930 cmK1 for CH3CN, along with some broadening of parent bands. When CH3C15N was employed, a weak broad product band was seen at 1921 cmK1.
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3.2. UV–visible studies Prior to co-deposition, blank experiments were run on each of the reagents. No bands were observed between 800 and 200 nm for acetonitrile. Blank spectra for CrO2Cl2 and OVCl3 were in good agreement with the literature spectra of these compounds [26,27] and with blanks run previously in this laboratory [28]. Twin-jet co-deposition experiments were then conducted employing samples of Ar/CrO2Cl2Z1400 and Ar/CH3 CNZ500. As the spectra in Fig. 2 show three of the CrO2Cl2 bands have blue shifted, from 303/278 nm, 281/235 nm and 420/387 nm. Upon irradiation, all of the bands were destroyed. Co-deposition experiments using Ar/OVCl3Z1400 and Ar/CH3CNZ500 were also undertaken. No band shifts or new product bands were observed between 800 and 200 nm before or after irradiation.
4. Results of calculations As will be discussed below, there are two likely products for each of these two systems: the complexes of CrCl2O (or VCl3) with ONCCH3 (acetonitrile n-oxide) and with NCCH2OH (hydroxyacetonitrile), based on the observations of Mielke et al. [17]. In addition, several different possible geometries could be envisioned for each complex. Consequently, DFT calculations were undertaken on all of these species using the B3LYP hybrid functional and basis sets as high as 6-311CCG(d,2p). Initial intermediate 1:1 complexes between the two reactants were also calculated. Fig. 3 shows the relative energies and calculated structures for the photochemical reaction of CrCl2O2 with CH3CN, while Fig. 4 shows the analogous diagram for the reaction CH3CN with OVCl3. All of these species optimized to energy minima on the potential energy surface, with all positive vibrational frequencies. Vibrational frequencies were calculated for the normal isotope, the 2H and 15N
Table 1 Product band positionsa from the photochemical reactions of CH3CN with CrCl2O2 and OVCl3 CH3CNCCrCl2O2
Calculated bandb
CH3C15NCCrCl2O2
Calculated shiftc
2750 1942 1930e 1506 1490 1012 (sh) 679 466 (sh)
3035 1972 1749e 1482 1461 1116 642 461
2752 1919 1921e 1508 1490 1012 (sh)
0 K35 K31e 0 0 0 K8 0
a b c d e
466 (sh)
Band positions in cm . Calculated bands, unscaled, at B3LYP/6-311CCG(d,2p) level. Calculated 15N shift, unscaled (15N calculated band–14N calculated band). Calculated deuterium shift, unscaled (2H calculated band–1H calculated band). Reaction with OVCl3. K1
CD3CNCCrCl2O2 1943
1012 (sh) 466 (sh)
Calculated shiftd
Assignment
K835 K1 0e K419 K410 0 K37 0
CH3 stretch C–N stretch C–N stretche CH3 bend CH3 bend CrCl2O C–C–N bend CrCl2O
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Fig. 2. UV/visible spectrum of a matrix after co-deposition of Ar/CrO2Cl2Z1400 with Ar/CH3CNZ500 (Ar/CrO2Cl2Z1400 is shown for comparison).
labeled isotopomers for comparison with experimental spectra.
5. Discussion Co-deposition of CH3CN with either CrO2Cl2 or OVCl3 into argon matrices did not lead initially to any distinct product bands in the infrared. However, irradiation of these matrices resulted in the growth of new product bands. For both systems, the single most identifiable product band was
located near 1940 cmK1, although the band produced in the CrO2Cl2CCH3CN experiments was significantly greater in intensity than the band produced in the OVCl3CCH3CN experiments. This suggests that the products in these two systems are quite similar, if not identical. For a photochemical reaction to occur under the experimental conditions, the two reagents must be trapped within the same matrix cage or site and that the stoichiometry is 1:1, i.e. one molecule of CH3CN and one molecule of CrO2Cl2 (or OVCl3). Whether the cage pair is a distinct molecular complex, or simply trapped in the same site, cannot be
˚ and angles in Fig. 3. DFT B3LYP/6-311CCG(d,2p) calculated structures of the reaction products of CrO2Cl2 with CH3CN. Selected distances are given in A degrees.
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˚ and angles in Fig. 4. DFT B3LYP/6-311CCG(d,2p) calculated structures of the reaction products of OVCl3 with CH3CN. Selected distances are given in A degrees.
determined from the infrared spectra. However, formation of a distinct complex is supported by the visible/UV spectral data for the CrCl2O2/CH3CN system (see below). Two mechanisms for such photoreactions have been seen to date, one involving HCl elimination, and the other involving oxygen atom transfer. The former can be ruled out, since no production of HCl was noted in these experiments (other than a very small amount produced by the reaction of the metal oxo compound with residual H2O). Thus, it is very likely that oxidation of acetonitrile by the transition metal oxo compound occurs, leaving a CrOCl2 or VCl3 fragment. This view is supported by the observation of shoulders near 1012 and 470 cmK1 in the CrCl2O2 experiments, after irradiation. These match the known band positions of the CrCl2O fragment [11,13]. The oxygen atom could either transfer to the terminal carbon or to the nitrogen atom, leading to NCCH2OH or ONCCH3. As the two fragments are trapped within the same matrix cage it seems reasonable that the two are bound together in a complex, a result supported by the DFT calculations. Three of the possible complexes that might be formed were calculated to be stable at the highest levels of theory: an 2h complex between the CrOCl2 (VCl3) fragment and ONCCH3, an 2h complex to the nitrogen atom of NCCH2OH and an 1h complex to the oxygen atom of NCCH2OH, as shown in Fig. 3. The scarcity of product bands makes the identification of the reaction product from among these three complexes difficult, and theoretical calculations must guide the considerations (all of the calculated frequencies below are unscaled). The most intense and distinct band, and the band
for which isotopic data is available, was observed at 1942 cmK1 when CrCl2O2 was employed and at 1930 cmK1 with OVCl3. The third listed complex, an 1h complex to the oxygen atom of NCCH2OH, can most readily be eliminated in that (a) it has no calculated infrared absorptions within 300 cmK1 of this position, (b) the nearest, calculated at 2366 cmK1, is computed to have a relatively low intensity, and (c) the most intense band calculated for the complex is the O–H stretching mode above 3000 cmK1. No product bands were found in the 3000 cmK1 region. On the other hand, the first two options both have a band calculated in the general region of 1940 cmK1, namely 2067 cmK1 for the 2h complex to the CbN bond of NCCH2OH and at 1972 cmK1 for the 2h to the CbN bond of ONCCH3. The latter is a better fit overall, but the former is within accepted error. Both of these correspond to C–N stretching modes of 2h complexes to the respective CbN bonds. In both cases, the bond order of the C–N bond is reduced from 3 (that of a nitrile) to a lower value through p electron donation to the metal center of the CrCl2O species. This reduces the stretching force constant, lowering the vibrational frequency significantly, as is observed. The observed isotopic shifts (approximately 0 for deuterium substitution and K23 cmK1 for 15N substitution) also support a C–N stretching motion (or C–N stretch coupled to a N–O stretch for the complex of CrCl2O with ONCCH3). The calculated isotopic shifts are similar as well, with the fit for the 2h complex to the CbN bond of NCCH2OH being somewhat better. These shifts, then, do not provide a definitive means for making an assignment to one of the two possible complexes.
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While calculated intensities at this level of theory are not perfect, they do provide some guidance. The calculated intensity of the 1972 cmK1 band of the 2h to the CbN bond of ONCCH3 was 239 km/mol, one of two most intense bands of this complex (by far). The calculated intensity of the 2067 band of the 2h complex to the CbN bond of NCCH2OH was calculated to be much lower, 28 km/mol, while 12 other bands of this complex were calculated to be more intense, yet no other product bands were observed. This argument supports assignment to the 2h complex of ONCCH3 with Cl2CrO (VCl3). The 1972 cmK1 band calculated for the 2h complex of ONCCH3 with Cl2CrO is one of three rather intense bands for the complex, with all of the remainder being quite weak. The second of the more intense bands is the CrbO stretch of CrCl2O. This has been observed [11,13] in several studies at 1012 cmK1. A distinct broadening was observed on the shoulder of the very intense mode of parent CrCl2O2 at 997 cmK1 which satisfactorily accounts for this calculated band. The third is calculated near 1300 cmK1, corresponding to the NbO stretch of the complex. No product bands were observed in this region. It is possible that the calculations predict too high a frequency, and that this mode lies underneath the intense parent bands of CrCl2O2, or that the calculated intensity is too high. Nonetheless, failure to observe this band is a weakness in the identification of the 2h complex of ONCCH3 with Cl2CrO as the photoproduct in this system. It should be noted that if this identification is made, then the quite weak product bands that were also observed fit quite well to less intense calculated absorptions of this complex, as shown in Table 1. Reaction energetics and the relative energies of the different complexes can be determined from the calculations as well. As shown in Figs. 3 and 4, all of the separated reaction products as well as all of the above complexes are higher in energy than the reactants. Thus, these complexes represent local minima on the global potential energy surface. Despite the endothermicity of these reactions, the energy gained by the absorption of a visible or near-UV photon is more than enough to form any of the complexes from the reactants. Comparing the relative energies of the three complexes, it is clear that the two complexes of CrCl2O with hydroxyacetonitrile are lower in energy than the complex with acetonitrile n-oxide. This result is consistent with the relative energies of the uncomplexed products. If thermodynamic considerations controlled the photochemical reaction, then one would expect the hydroxy species, not the n-oxide to form (however, strict thermodynamic control would lead to the initial molecular complex). However, kinetic, not thermodynamic control is often observed in cryogenic matrices, due to the low temperature and the presence of the rigid matrix cage around the initial molecular complex. The matrix cage, then, limits the rearrangements that can occur before excess energy is dissipated to the phonon modes of the matrix.
Although an initial complex was not observed upon initial deposition in the infrared spectra, evidence for formation of a complex was shown by the observation of new bands in the UV/visible spectrum. The spectra provide direct evidence for a change in electronic structure as a result of the formation of an initial complex. This result is consistent with DFT calculations, that show a small binding energy (around 1 kcal/mol) for the initial complex of CH3CN with CrCl2O2 and with OVCl3. From the optimized structures, it can be clearly seen that the interaction is taking place through electron donation from the lone pair on the nitrogen atom to the metal center. Since this geometry (or something similar, considering possible matrix perturbations) is trapped in the matrix cage, the photoproduct that is formed must pass through a transition state originating with this structure. Comparison of this structure with those of the acetonitrile n-oxide and hydroxyacetonitrile complexes with CrCl2O suggests that it is relatively easy to form the n-oxide complex from the initial 1:1 complex. On the other hand, is appears to be rather difficult for the CH3CN molecule to rotate sufficiently in the matrix to enable bond formation between the terminal carbon (opposite the site of initial complexation) and one of the oxygen atoms. This comparison supports formation of the noxide as the photoproduct in this system. Finally, the activation barrier to formation of the hydroxy species, which involves rupture of a C–H bond prior to formation of the C–O and O–H bonds, may also be significantly higher than the barrier to addition of an oxygen atom to the nitrogen atom to form the n-oxide.
6. Conclusions The photochemical reaction between the transition metal compounds CrO2Cl2 and OVCl3 with CH3CN leads to oxygen atom transfer, and formation of CrOCl2- or VCl3 acetonitrile N-oxide complexes in argon matrices. This takes place via the formation of an initial complex in which the metal atom is bound to the lone pair on the nitrogen, evidence for which is provided by the UV/visible spectra and theoretical calculations.
Acknowledgements The National Science Foundation is gratefully acknowledged for support of this research through grant CHE 02-43731.
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Matrix isolation and theoretical study of the photochemical reaction of CH3CN with CrO2Cl2 and OVCl3 Nicola Goldberg, Sarah R. Lubell, Bruce S. Ault* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA Received 30 November 2004; revised 16 January 2005; accepted 18 January 2005
Abstract The matrix isolation technique has been combined with theoretical calculations to identify and characterize the photoproducts in the reactions of CH3CN with CrCl2O2 and OVCl3. Twin jet co-deposition of these reagents led to the formation of a 1:1 molecular complex which was observed using UV/visible spectroscopy. Irradiation of these matrices with light of lO300 nm led to the observation of new bands in the infrared spectra, the most intense of which was seen at 1942 cmK1 for the CrCl2O2/CH3CN system. The product bands are assigned to the 2h complexes of acetonitrile n-oxide with CrCl2O and VCl3, respectively. Identification of these species was supported by extensive isotopic labeling (2H and 15N), as well as by B3LYP/6-311CCG(d,2p) density functional calculations. q 2005 Elsevier B.V. All rights reserved. Keywords: Matrix isolation; Complex; Infrared spectra; Visible/UV spectra; Theoretical calculations
1. Introduction High valent transition metal oxo compounds such as CrO2Cl2 and OVCl3 are very strong oxidizing agents, a property that has been utilized in a variety of applications ranging from organic synthesis [1–3] to chemical vapor deposition [4] to catalysis [5]. Despite the importance of these compounds, knowledge of the mechanisms of these reactions is incomplete. Recent matrix isolation studies from this laboratory have investigated the thermal and photochemical reactions of high valent transition metal compounds with a number of small organic and inorganic substrates [6–12]. Two types of reactions have been observed to date, the first of which is formation of an initial complex, followed by HCl elimination from the complex and addition of the organic (or inorganic) fragment to the metal center to retain a 4-coordinate complex [6–10]. The second is oxygen atom transfer from the metal center to the substrate, leading to an oxidized substrate species [11–13]. When this reaction
* Corresponding author. Tel.: C1 51 3556 9238; fax: C1 51 3556 9239. E-mail address: [email protected] (B.S. Ault). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.032
occurred as a result of irradiation after deposition, the two species (e.g. CrCl2O and oxidized substrate) then formed a relatively strongly bound complex. The matrix isolation technique [14–16] has been used on numerous occasions to isolate and characterize reactive intermediates, including radicals, ions and complexes. While solution reactions between these compounds and acetonitrile (CH3CN) have not been reported, Mielke et al. [17] studied the reactions of O atoms with CH3CN in a matrix and observed two major product species, hydroxyacetonitrile (HOCH 2 CN) and acetonitrile N-oxide (CH3CNO), as well as a complex of the two molecules. Acetonitrile has also been used as a reagent in the study of partially bonded molecules, for example with BF3 [18,19]. In addition, reactions of CH3CN with ZrCl4 and HfCl4 have been reported by Hase and Alves [20]. In all of these cases the position of the CbN stretching mode was important in confirming the formation of a complex. The present study was undertaken to investigate oxidation reactions of the nitrile group, through photochemical reactions of acetonitrile with CrO2Cl2 and OVCl3 in argon matrices. High level theoretical calculations were employed to complement the experimental data.
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2. Experimental
3. Results
All of the experiments in this study were carried out on conventional matrix isolation apparatus that has been described [21]. Chromyl chloride, CrO2Cl2, and OVCl3 (both Aldrich), were introduced into the vacuum system as the vapor above the room-temperature liquid, after purification by freeze-pump-thaw cycles at 77 K. CH3CN (Pharmco), CD3CN (99% D), CH3C15N (99% 15N) (both Acros Organics) were introduced in a similar manner into a separate vacuum manifold and were purified by repeated freeze-pump-thaw cycles at 77 K. Argon (Wright Brothers) was used as a matrix gas without further purification. All matrix samples were deposited in the twin-jet mode where the two gas samples were sprayed from separate nozzles onto the 14 K cold window, allowing for a very brief mixing time prior to matrix deposition. The matrices were then irradiated for 1.0 or more hours with the H2O/Pyrex filtered output of a 200 W medium pressure Hg arc lamp, after which additional spectra were recorded on a Perkin–Elmer Spectrum One Fourier transform infrared spectrometer at 1 cmK1 resolution. UV–visible spectra were recorded on a Varian Cary 4000 UV/visible spectrophotometer between 800 and 200 nm using a 0.5 nm bandwidth. Theoretical calculations were carried out using the GAUSSIAN 03W suite of programs [22]. Density functional calculations using the B3LYP functional were used to locate energy minima, determine structures and calculate vibrational spectra of potential intermediate species. Final calculations with full geometry optimization employed the 6-311CCG(d,2p) basis set, after initial calculations with smaller basis sets were run to approximately locate energy minima.
Prior to any co-deposition experiments, blank experiments were run on each of the reagents used in this study. In each case the blanks were in good agreement with literature spectra [23–25] and with blanks run previously in this laboratory. Each blank experiment was then irradiated by the H2O/Pyrex filtered output of a 200 W Hg arc lamp for 2 h. No changes were observed in any of these spectra as a result of irradiation. 3.1. Infrared studies 3.1.1. CrO2Cl2CCH3CN In an initial experiment, a sample of Ar/CH3CNZ 250 was co-deposited with a sample of Ar/CrO2Cl2Z 250. No distinct new infrared absorptions were apparent upon initial matrix deposition. When this matrix was annealed to 25 K and re-cooled, the resultant spectrum again showed no distinct absorptions. On irradiation with the H2O/Pyrex filtered output of a mediumpressure Hg arc (lO300 nm), a moderately intense new doublet was observed at 1942 cmK1, along with several very weak new absorptions at 2750, 1506, 1490 and 679 cmK1. In addition, bands identified as arising from the photochemical reaction of CrCl2O2 with impurity H2O at 3438, 3428 and 755 cmK1 were seen throughout [7]. Since these bands are well known, they will not be discussed further. Fig. 1 shows the region of the spectrum containing the 1942 cmK1 band (and its isotopic equivalents) from a representative experiment with this pair of reagents. In addition, the CrO2Cl2 peaks at 466, 500, 983 and 997 cmK1 were also
Fig. 1. Infrared spectra, in the range 2080–1860 cmK1 of matrices prepared by the twin-jet co-deposition of a sample of Ar/CrO2Cl2Z125 with samples of Ar/CH3CNZ125 (lower trace), Ar/CH3C15NZ125 (middle trace) and Ar/CD3CNZ125 (upper trace) after 3 h of irradiation with light of lO300 nm. Other features in the spectra are due to the parent species, and were present before irradiation.
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noticeably broader suggesting that new bands have formed but are being overlapped by strong reactant bands in these regions. This experiment was repeated several times, using a range of sample concentrations for each reagent, along with different irradiation times. Similar results were obtained throughout, with product band intensities that varied as the concentration of the reactants was varied. The optimum conditions (for maximum product yield) were found to be Ar/CH 3CNZ125 and Ar/CrO 2 Cl 2Z125, and 3 h of irradiation. 3.1.2. CrO2Cl2CCD3CN, CH3C15N When a sample of Ar/CD3CNZ125 was co-deposited with a sample of Ar/CrO2Cl2Z125, and irradiated with light of lO300 nm, new bands were observed at 1943, 1013, and 471 cmK1. The latter two bands appeared as distinct shoulders on the nearby intense parent bands of CrCl2O2. This experiment was repeated with similar results. Experiments were also conducted with 15N labeled acetonitrile. After irradiation, a moderately intense doublet was observed at 1919 cmK1, along with possible very weak bands at 2752, 1508, 1490, and 466 cmK1. Broadening of the CrO2Cl2 bands were also seen in both sets of experiments. Table 1 summarizes product band positions for the photochemical products of CrO2Cl2 with CH3CN and its isotopomers in argon matrices. 3.1.3. OVCl3CCH3CN, CH3C15N Similar experiments were conducted in which samples of Ar/OVCl 3Z125 were co-deposited with samples of Ar/CH3CN and CH3C15NZ125. No distinct new infrared absorptions were apparent upon initial matrix deposition. On irradiation a weak, broad product band was observed at 1930 cmK1 for CH3CN, along with some broadening of parent bands. When CH3C15N was employed, a weak broad product band was seen at 1921 cmK1.
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3.2. UV–visible studies Prior to co-deposition, blank experiments were run on each of the reagents. No bands were observed between 800 and 200 nm for acetonitrile. Blank spectra for CrO2Cl2 and OVCl3 were in good agreement with the literature spectra of these compounds [26,27] and with blanks run previously in this laboratory [28]. Twin-jet co-deposition experiments were then conducted employing samples of Ar/CrO2Cl2Z1400 and Ar/CH3 CNZ500. As the spectra in Fig. 2 show three of the CrO2Cl2 bands have blue shifted, from 303/278 nm, 281/235 nm and 420/387 nm. Upon irradiation, all of the bands were destroyed. Co-deposition experiments using Ar/OVCl3Z1400 and Ar/CH3CNZ500 were also undertaken. No band shifts or new product bands were observed between 800 and 200 nm before or after irradiation.
4. Results of calculations As will be discussed below, there are two likely products for each of these two systems: the complexes of CrCl2O (or VCl3) with ONCCH3 (acetonitrile n-oxide) and with NCCH2OH (hydroxyacetonitrile), based on the observations of Mielke et al. [17]. In addition, several different possible geometries could be envisioned for each complex. Consequently, DFT calculations were undertaken on all of these species using the B3LYP hybrid functional and basis sets as high as 6-311CCG(d,2p). Initial intermediate 1:1 complexes between the two reactants were also calculated. Fig. 3 shows the relative energies and calculated structures for the photochemical reaction of CrCl2O2 with CH3CN, while Fig. 4 shows the analogous diagram for the reaction CH3CN with OVCl3. All of these species optimized to energy minima on the potential energy surface, with all positive vibrational frequencies. Vibrational frequencies were calculated for the normal isotope, the 2H and 15N
Table 1 Product band positionsa from the photochemical reactions of CH3CN with CrCl2O2 and OVCl3 CH3CNCCrCl2O2
Calculated bandb
CH3C15NCCrCl2O2
Calculated shiftc
2750 1942 1930e 1506 1490 1012 (sh) 679 466 (sh)
3035 1972 1749e 1482 1461 1116 642 461
2752 1919 1921e 1508 1490 1012 (sh)
0 K35 K31e 0 0 0 K8 0
a b c d e
466 (sh)
Band positions in cm . Calculated bands, unscaled, at B3LYP/6-311CCG(d,2p) level. Calculated 15N shift, unscaled (15N calculated band–14N calculated band). Calculated deuterium shift, unscaled (2H calculated band–1H calculated band). Reaction with OVCl3. K1
CD3CNCCrCl2O2 1943
1012 (sh) 466 (sh)
Calculated shiftd
Assignment
K835 K1 0e K419 K410 0 K37 0
CH3 stretch C–N stretch C–N stretche CH3 bend CH3 bend CrCl2O C–C–N bend CrCl2O
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Fig. 2. UV/visible spectrum of a matrix after co-deposition of Ar/CrO2Cl2Z1400 with Ar/CH3CNZ500 (Ar/CrO2Cl2Z1400 is shown for comparison).
labeled isotopomers for comparison with experimental spectra.
5. Discussion Co-deposition of CH3CN with either CrO2Cl2 or OVCl3 into argon matrices did not lead initially to any distinct product bands in the infrared. However, irradiation of these matrices resulted in the growth of new product bands. For both systems, the single most identifiable product band was
located near 1940 cmK1, although the band produced in the CrO2Cl2CCH3CN experiments was significantly greater in intensity than the band produced in the OVCl3CCH3CN experiments. This suggests that the products in these two systems are quite similar, if not identical. For a photochemical reaction to occur under the experimental conditions, the two reagents must be trapped within the same matrix cage or site and that the stoichiometry is 1:1, i.e. one molecule of CH3CN and one molecule of CrO2Cl2 (or OVCl3). Whether the cage pair is a distinct molecular complex, or simply trapped in the same site, cannot be
˚ and angles in Fig. 3. DFT B3LYP/6-311CCG(d,2p) calculated structures of the reaction products of CrO2Cl2 with CH3CN. Selected distances are given in A degrees.
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˚ and angles in Fig. 4. DFT B3LYP/6-311CCG(d,2p) calculated structures of the reaction products of OVCl3 with CH3CN. Selected distances are given in A degrees.
determined from the infrared spectra. However, formation of a distinct complex is supported by the visible/UV spectral data for the CrCl2O2/CH3CN system (see below). Two mechanisms for such photoreactions have been seen to date, one involving HCl elimination, and the other involving oxygen atom transfer. The former can be ruled out, since no production of HCl was noted in these experiments (other than a very small amount produced by the reaction of the metal oxo compound with residual H2O). Thus, it is very likely that oxidation of acetonitrile by the transition metal oxo compound occurs, leaving a CrOCl2 or VCl3 fragment. This view is supported by the observation of shoulders near 1012 and 470 cmK1 in the CrCl2O2 experiments, after irradiation. These match the known band positions of the CrCl2O fragment [11,13]. The oxygen atom could either transfer to the terminal carbon or to the nitrogen atom, leading to NCCH2OH or ONCCH3. As the two fragments are trapped within the same matrix cage it seems reasonable that the two are bound together in a complex, a result supported by the DFT calculations. Three of the possible complexes that might be formed were calculated to be stable at the highest levels of theory: an 2h complex between the CrOCl2 (VCl3) fragment and ONCCH3, an 2h complex to the nitrogen atom of NCCH2OH and an 1h complex to the oxygen atom of NCCH2OH, as shown in Fig. 3. The scarcity of product bands makes the identification of the reaction product from among these three complexes difficult, and theoretical calculations must guide the considerations (all of the calculated frequencies below are unscaled). The most intense and distinct band, and the band
for which isotopic data is available, was observed at 1942 cmK1 when CrCl2O2 was employed and at 1930 cmK1 with OVCl3. The third listed complex, an 1h complex to the oxygen atom of NCCH2OH, can most readily be eliminated in that (a) it has no calculated infrared absorptions within 300 cmK1 of this position, (b) the nearest, calculated at 2366 cmK1, is computed to have a relatively low intensity, and (c) the most intense band calculated for the complex is the O–H stretching mode above 3000 cmK1. No product bands were found in the 3000 cmK1 region. On the other hand, the first two options both have a band calculated in the general region of 1940 cmK1, namely 2067 cmK1 for the 2h complex to the CbN bond of NCCH2OH and at 1972 cmK1 for the 2h to the CbN bond of ONCCH3. The latter is a better fit overall, but the former is within accepted error. Both of these correspond to C–N stretching modes of 2h complexes to the respective CbN bonds. In both cases, the bond order of the C–N bond is reduced from 3 (that of a nitrile) to a lower value through p electron donation to the metal center of the CrCl2O species. This reduces the stretching force constant, lowering the vibrational frequency significantly, as is observed. The observed isotopic shifts (approximately 0 for deuterium substitution and K23 cmK1 for 15N substitution) also support a C–N stretching motion (or C–N stretch coupled to a N–O stretch for the complex of CrCl2O with ONCCH3). The calculated isotopic shifts are similar as well, with the fit for the 2h complex to the CbN bond of NCCH2OH being somewhat better. These shifts, then, do not provide a definitive means for making an assignment to one of the two possible complexes.
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While calculated intensities at this level of theory are not perfect, they do provide some guidance. The calculated intensity of the 1972 cmK1 band of the 2h to the CbN bond of ONCCH3 was 239 km/mol, one of two most intense bands of this complex (by far). The calculated intensity of the 2067 band of the 2h complex to the CbN bond of NCCH2OH was calculated to be much lower, 28 km/mol, while 12 other bands of this complex were calculated to be more intense, yet no other product bands were observed. This argument supports assignment to the 2h complex of ONCCH3 with Cl2CrO (VCl3). The 1972 cmK1 band calculated for the 2h complex of ONCCH3 with Cl2CrO is one of three rather intense bands for the complex, with all of the remainder being quite weak. The second of the more intense bands is the CrbO stretch of CrCl2O. This has been observed [11,13] in several studies at 1012 cmK1. A distinct broadening was observed on the shoulder of the very intense mode of parent CrCl2O2 at 997 cmK1 which satisfactorily accounts for this calculated band. The third is calculated near 1300 cmK1, corresponding to the NbO stretch of the complex. No product bands were observed in this region. It is possible that the calculations predict too high a frequency, and that this mode lies underneath the intense parent bands of CrCl2O2, or that the calculated intensity is too high. Nonetheless, failure to observe this band is a weakness in the identification of the 2h complex of ONCCH3 with Cl2CrO as the photoproduct in this system. It should be noted that if this identification is made, then the quite weak product bands that were also observed fit quite well to less intense calculated absorptions of this complex, as shown in Table 1. Reaction energetics and the relative energies of the different complexes can be determined from the calculations as well. As shown in Figs. 3 and 4, all of the separated reaction products as well as all of the above complexes are higher in energy than the reactants. Thus, these complexes represent local minima on the global potential energy surface. Despite the endothermicity of these reactions, the energy gained by the absorption of a visible or near-UV photon is more than enough to form any of the complexes from the reactants. Comparing the relative energies of the three complexes, it is clear that the two complexes of CrCl2O with hydroxyacetonitrile are lower in energy than the complex with acetonitrile n-oxide. This result is consistent with the relative energies of the uncomplexed products. If thermodynamic considerations controlled the photochemical reaction, then one would expect the hydroxy species, not the n-oxide to form (however, strict thermodynamic control would lead to the initial molecular complex). However, kinetic, not thermodynamic control is often observed in cryogenic matrices, due to the low temperature and the presence of the rigid matrix cage around the initial molecular complex. The matrix cage, then, limits the rearrangements that can occur before excess energy is dissipated to the phonon modes of the matrix.
Although an initial complex was not observed upon initial deposition in the infrared spectra, evidence for formation of a complex was shown by the observation of new bands in the UV/visible spectrum. The spectra provide direct evidence for a change in electronic structure as a result of the formation of an initial complex. This result is consistent with DFT calculations, that show a small binding energy (around 1 kcal/mol) for the initial complex of CH3CN with CrCl2O2 and with OVCl3. From the optimized structures, it can be clearly seen that the interaction is taking place through electron donation from the lone pair on the nitrogen atom to the metal center. Since this geometry (or something similar, considering possible matrix perturbations) is trapped in the matrix cage, the photoproduct that is formed must pass through a transition state originating with this structure. Comparison of this structure with those of the acetonitrile n-oxide and hydroxyacetonitrile complexes with CrCl2O suggests that it is relatively easy to form the n-oxide complex from the initial 1:1 complex. On the other hand, is appears to be rather difficult for the CH3CN molecule to rotate sufficiently in the matrix to enable bond formation between the terminal carbon (opposite the site of initial complexation) and one of the oxygen atoms. This comparison supports formation of the noxide as the photoproduct in this system. Finally, the activation barrier to formation of the hydroxy species, which involves rupture of a C–H bond prior to formation of the C–O and O–H bonds, may also be significantly higher than the barrier to addition of an oxygen atom to the nitrogen atom to form the n-oxide.
6. Conclusions The photochemical reaction between the transition metal compounds CrO2Cl2 and OVCl3 with CH3CN leads to oxygen atom transfer, and formation of CrOCl2- or VCl3 acetonitrile N-oxide complexes in argon matrices. This takes place via the formation of an initial complex in which the metal atom is bound to the lone pair on the nitrogen, evidence for which is provided by the UV/visible spectra and theoretical calculations.
Acknowledgements The National Science Foundation is gratefully acknowledged for support of this research through grant CHE 02-43731.
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