11228
J. Phys. Chem. 1996, 100, 11228-11234
Infrared Spectrum of Nickel-Monoethylene. An Infrared Argon Matrix and Normal Coordinate Analysis Study Y. K. Lee,† Y. Hannachi,‡ C. Xu,† L. Andrews,§ and L. Manceron*,† Laboratoire de Spectrochimie Mole´ culaire, CNRS URA 508, UniVersite´ Pierre et Marie Curie, boıˆte 49, 4 place Jussieu, 75252 Paris Cedex 05, France, Laboratoire de Photochimie et Photophysique Mole´ culaire, CNRS URA 348, UniVersite´ de Bordeaux, 351 Cours de la Libe´ ration, 33405 Talence Cedex, France, and Department of Chemistry, UniVersity of Virginia, McCormick Road, CharlottesVille, Virginia 22901 ReceiVed: January 30, 1996; In Final Form: April 25, 1996
The vibrational spectrum of ground state nickel-monoethylene isolated at low temperature in solid argon has been reinvestigated. NiC2H4 and its various isotopic forms NiC2D4, Ni13C2H4, and NiCH2CD2, have been synthetized by direct Ni vapor and ethylene co-condensation in the rare gas matrices. Nine to ten fundamentals are observed for each species, and additional data can be derived from the observation of four combination bands. A normal coordinate analysis has been carried out at the harmonic level, based on the experimental isotopic frequencies, and C-C, Ni-C, and C-H bond force constants can be estimated at 5.61, 1.91, and 4.68 mdyn Å-1, respectively. These results are compared to the most recent theoretical prediction by Papai et al. (J. Phys. Chem. 1993, 97, 9986).
The need to understand the nature of the bonding between nickel and alkene molecules has motivated a number of fundamental studies on the simplest system of all, the NiC2H4 complex. Owing to the difficulty of isolating the nickelmonoethylene species, in spite of a relatively large binding energy (35 ( 5 kcal/mol),1 experimental studies have been limited to indirect determination in the gas phase through flash photokinetic studies of metal atom removal via complexation or to vibrational studies of Ni (C2H4)n, n ) 1, 2, 3, complexes of the condensation products of nickel atoms and ethylene molecules in rare gas matrices2,3. On the other hand, in pace with the steady improvement of theoretical methods, several studies have been devoted to modeling and predicting the properties of this benchmark species (refs 4-6 and references therein). The recent study by Papai et al.,5 performed using density functional theory quantum chemical techniques, seems to provide the most reliable results on such complex, manyelectron systems. This study included a full prediction of the vibrational spectrum and infrared intensities at the harmonic level and underlined the need for more complete experimental data. Up to now, only 4 out of 12 IR-active fundamentals had been reported,3 concerning two isotopic species only, and no relative intensity measurement was available. This prompted us to reinvestigate the infrared spectrum of nickel-monoethylene isolated in an argon matrix. We report here the positions of 10 fundamentals and four combination bands relative to four different isotopic species (NiC2H4, NiC2D4, Ni13C2H4, NiCH2CD2) as well as normal coordinate and infrared intensity analysis.
capacity liquid nitrogen trap. The base pressure before starting an experiment, after outgassing but with the nickel furnace hot, was always less than 5 × 10-7 mbar. About 5.5 mmol of a mixture of ethylene and argon (1/501/1000 molar ratio) were deposited in 2 h onto one of four sides of a flat, highly polished, chromium-plated copper mirror cube maintained at ∼10 K by an APD displex system. The samples were probed in the transmission-reflection mode by the infrared beam of a Bruker 120 FTIR spectrometer with 0.5 cm-1 resolution in the 4500-500 cm-1 range and with 2 cm-1 resolution in the 500-150 cm-1 region. Each sample spectrum was processed against a bare mirror background recorded at 10 K just before sample deposition and was baseline-corrected to compensate for infrared light scattering. High-purity nickel (Johnson Matthey, 99.9970%) was evaporated from a wetted 20 mm long and 0.75 mm wide tungsten filament in the 1300-1500 °C range. The metal effusion rate was continuously monitored using a quartz microbalance and varied between 9 and 60 × 10-8 g/min. High-purity argon (Prodair, 99.995%), C2H4 (Air Liquide, 99.50%), C2D4 (MSD, 99.20% D), and 13C2H4 (MSD >90.0%) were used without further purification but after several freezethaw cycles to minimize possible atmospheric contamination. Another isotopic species of ethylene (1,1-D2) was prepared by the Wittig reaction of deuterated formaldehyde with triphenylmethylphosphonium according to the procedure described by Atkinson et al.7 The purity of this product was checked using IR spectroscopy. Its main impurities (CHD ) CD2 and CH2 ) CHD) were in quantities weak enough (∼1.7% and 2%, respectively) to be neglected.
Experimental Section
Results
The experiments were performed in a high-vacuum all stainless steel cryostatic vessel and a nickel evaporation furnace fitted directly in a 150 mm diameter vacuum chamber evacuated by a Leybold 1000 L/s diffusion pump equipped with a large-
Comparisons between the spectra of samples deposited with and without Ni have been made in order to find the absorptions associated with the Ni(C2H4)n (n ) 1, 2, 3) complexes. The aim of this work being mainly the complex with stoichiometry n ) 1, we focused on the observation of the largest number of its absorptions. Fourteen absorptions have been observed for NiC2H4 (Table 1, Figure 1). Among these, three (2839.2, 1803, 1798 cm-1) appeared with very weak intensity, and four (1468.2,
Introduction
†
Universite´ Pierre et Marie Curie. Universite´ de Bordeaux. § University of Virginia. X Abstract published in AdVance ACS Abstracts, June 1, 1996. ‡
S0022-3654(96)00292-4 CCC: $12.00
© 1996 American Chemical Society
IR Spectrum of Ni-Monoethylene
J. Phys. Chem., Vol. 100, No. 27, 1996 11229
Figure 1. IR spectra of the region 1500-700 cm-1 for different isotopic species: (a) Ni/C2H4/Ar = 0.8/1/500. (b) Ni/13C2H4/Ar = 0.4/2.5/500. (c) Ni/C2D4/Ar = 0.4/2.5/500. The arrows designate peaks due to NiC2H4 or Ni13C2H4 or NiC2D4.
TABLE 1: Frequencies and Relative IR Intensities Observed for Various Isotopic Species of the Nickel-Ethylene Complex Ni13C2H4
NiC2H4
NiC2D4
NiCH2CD2
frequencies (cm-1)
relative intensity
frequencies (cm-1)
relative intensity
frequencies (cm-1)
relative intensity
frequencies (cm-1)
relative intensity
3031.7 2965.2 2914.1 2839.2 1803 1798 1468.2 1386.4 1165.9 902.0 601.8 517.4 502.2
0.61 0.88 0.11 0.07 0.075 0.075 0.04 0.10 0.89 1.00 0.25 0.07 0.15
3020.4 2957.6 2886.9 2817.9 1788.3 1780.1 1452.7 1377.9 1135.4 891.9 599.9 505.5 488.4
0.40 0.73 0.35 0.04 0.06 0.037 0.014 0.046 0.70 1.00 0.145 0.045 0.094
2282.8 2157.2 2066.9
0.44 0.72 0.08
1489.3 1440.2 1276.3 1032.6 932.1 746.9
0.06 0.044 0.97 0.06 0.56 1.00
2970.8 2853.2 2271.1 2167.1 1802.2 1441.7 1437.4 1221.0 987.2 904.1 724.6
0.97 0.10 0.11 0.24 0.11 0.055 0.12 0.68 0.29 1.00 0.51
450.7
0.85
485.5
0.166
1165.9, 902.0, 502.0 cm-1) are observed close to the positions reported by Merle-Mejean et al. (1465, 1156, 901, 498 cm-1).3 The small shift in comparing our observation with previous studies can be explained by the different dilution of the matrix (typically C2H4/Ar ) 1/10 for Merle-Mejean et al., 1/200 in our experiments) and the observation of different sites of same species owing to better resolution. In the CH stretching region, many absorptions can be observed in a relatively narrow range (3050-2940 cm-1); only four of them can be attributed with certainty to NiC2H4 or Ni13C2H4. The same experimental procedure has been performed for deuterated species (Figures 1-3). All the bands of NiCH2CD2 and Ni13C2H4 that we have observed are collected in Table 1; they have not been reported before. In Figure 2 are presented 12C/13C and H/D isotopic effects for the low-frequency vibrations. The 12C/13C shift at ∼600 cm-1 is conspicuously smaller than those of the two absorptions at ∼500 cm-1. The CH and CD absorptions of NiCH2CD2, unlike NiC2H4, were distributed over two regions (Figure 3). For the NiC2D4 sample, we could likewise identify 10 peaks. Three of these are observed at about the same position as in the study by Merle-Mejean et al., but we have neVer observed any signal at or around 615 cm-1 in our spectra. Even with
samples in which the strongest band of NiC2D4 was observed with 0.16 absorbance unit, nothing was observable around 615 cm-1. On the other hand, a new band at 746.9 cm-1 is observed in our spectra with the strongest relative intensity, just like the 902.0 cm-1 band in NiC2H4. Note that it appears 30 cm-1 above the strong ν7 band of C2D4. This is the opposite situation to that for NiC2H4, a fact that could have misled other researchers. The 10 new peaks of NiC2H4 were characterized by altering the relative concentration of nickel and ethylene in argon and performing warmup experiments which favored larger aggregates, as previously reported. The relative intensities of these new bands vary always correlatively with the four bands previously assigned to NiC2H4, and in agreement with the conclusions of Merle-Mejean et al.3 and Ozin et al.,2 NiC2H4 is best observed when the matrix had lower ethylene concentration. In Figure 4, it can be seen that variations of the ethylene and nickel concentrations modified the relative intensities of the peaks due to other species. The intensity of NiC2H4 peaks (1468.2, 1165.9 cm-1) is proportional to the Ni/C2H4 ratio, which was reduced in Figure 4 from trace a to trace d (Ni/ C2H4 decreasing from 0.8 to ∼0.3/20). On the other hand, Ni(C2H4)3 appeared and Ni(C2H4)2 obviously increased in spectra c and d, which had a larger ethylene concentration than spectra
11230 J. Phys. Chem., Vol. 100, No. 27, 1996
Lee et al.
TABLE 2: Comparison of Experimental and Calculated Harmonic Frequencies for Various Isotopic Species of Nickel-Ethylenea NiC2H4 assignment νCH νCH νCH νCH δCH2 + νCC δCH2 νCC + δCH2 δCCH δNiCH δNiCH δNiCH + δCCH δNiCH δCCH + δNiCH νNiC νNiC
B1 A1 A2 B2 A1 B2 A1 A2 B2 A1 B1 A2 B1 A1 B2
ν9 ν1 ν6 ν12 ν2 ν13 ν3 ν7 ν14 ν4 ν10 ν8 ν11 ν5 ν15
Ni13C2H4
NiC2D4
NiCH2CD2
exp
calc
exp
calc
exp
calc
exp
calc
3031.7 2965.2
3035.4 2949.5 2923.3 2798.4 1472.0 1385.9 1170.1 1110.7 907.1 901.9 890.8 723.0 601.0 517.5 503.1
3020.4 2957.6
3022.2 2943.1 2909.2 2793.9 1455.6 1379.6 1140.7 1098.6 901.5 893.8 890.6 721.5 599.6 506.7 487.0
2282.8
2265.1 2199.2(A2) 2159.0(A1) 2022.3 1267.7 1033.9 929.3 876.9 717.2 706.8 636.8 519.7 470.8(A1) 466.0(B2) 437.0(B1)
2970.8 2853.2 2199.5b n.o. 1437.4 1221.0 n.o. 987.2 904.1 n.o. 724.6 n.o. n.o. 485.5 n.o.
2982.1 2879.9 2230.1 2085.4 1436.7 1214.0 1032.0 982.8 900.4 777.8 712.1 606.6 507.9 482.7 470.6
n.o.c 1468.2 1386.4 1165.9 907d 902.0 n.o. 601.8 517.4 502.2
n.o. 1452.7 1377.9 1135.4 900d 891.9 n.o. 599.9 505.5 488.4
2157.2 n.o. 1276.3 1032.6 932.1 746.9 700d n.o. n.o. 450.7e
attribution νCH νCH νCD νCD δCH2 νCC δCCH δCD2 δNiCH δNiCH δNiCD δCCH δNiCD δCCD δNiCD
A′′ A′ A′′ A′ A′ A′ A′ ′ A′ A′ A′′ A′ A′′ A′ A′′ A′
ν10 ν1 ν11 ν2 ν3 ν4 ν12 ν5 ν6 ν13 ν7 ν14 ν8 ν15 ν9
a Using the following harmonic force constants: F CH ) 4.68, FCC ) 5.61, FNi-C ) 1.91, FC-H,CC ) 0.022, FCC,NiC ) 0.03, FNiC,NiC ) -0.022, FCH1,CH2 ) 0.015, FCH1,CH3 ) 0.212, FCH1,CH4 ) 0.02, FCH,NiC ) 0.08 mdyn/Å; FHCH ) 0.404, FCCH ) 0.572, FNiCH ) 0.55 mdyn‚Å; FCCH1,CCH2 ) 0.011, FCCH1,CCH3 ) -0.083, FCCH1,CCH4 ) 0.002; FNiCH1,NiCH2 ) 0.038, FNiCH1NiCH3 ) 0.005, FNiCH1,NiCH4 ) -0.0035, FCCH1,NiCH1 ) -0.32, FCH1H2,CH3H4 ) 0.077 mdyn‚Å; FCH,HCH ) 0.01, FCC,HCH ) -0.23, FCC,NiCH ) 0.0365, FNiC,NiCH ) 0.135 mdyn. b Estimated from 2167.1 after Fermi resonance correction. c n.o., not obtained. d Estimated value from ν4 + ν14 combination band. e Two possible assignments.
in previous studies, enhancing Ni(C2H4)2 or Ni(C2H4)3 formation at the expense of NiC2H4. Discussion Ozin et al.2 and also Merle-Mejean et al.3 have already reached the conclusion that NiC2H4 is a π-type complex with C2V symmetry. For the symmetrical species NiC2H4, Ni13C2H4, or NiC2D4, the 15 normal vibrations factor as 5A1 + 3A2 + 3B1 + 4B2 symmetry motions, among which the A2 symmetry motions are IR-inactive. Thus there should be 12 fundamental vibrations appearing in the IR spectra. For the partially deuterated isotopic species NiCH2CD2, the symmetry is reduced to Cs (9A′ + 6A′′) and all 15 vibrations are IR-active. To assist in the attributions of the new bands of nickelethylene, we will first define a set of internal and symmetry coordinates as follows (Chart 1):
for the A1block S1 ) (1/2)(r1 + r2 + r3 + r4) S2 ) (1/x6)(R8 + R9 - β10 - β11 - β12 - β13) Figure 2. Low-frequency absorption region (650 = 400 cm ) for different isotopic species: (a) Ni/C2H4/Ar = 20.4/.5/500. (b) Ni/13C2H4/ Ar = 0.4/2.5/500. (c) Ni/CH2CD2/Ar = 0.2/2.5/500. Arrows designate peaks of NiC2H4 or Ni13C2H4 or NiCH2CD2. The band marked “?” is unassigned. -1
a and b. The band designated with “X” cannot be attributed to NiC2H4. It has not been identified in our study. The absorption at ∼1482 cm-1 was attributed to NiX since it could be observed even without C2H4 and appeared more clearly in samples containing more Ni. A few wavenumbers below the sharp absorptions assigned to NiC2H4, a group of weaker satellite signals was observed for each fundamental (for examples below ν2 and ν3 in Figure 4). These additional signals appear after deposition with constant relative intensities to the main signals, whatever the nickel and ethylene concentrations, but quickly decrease and eventually disappear upon a slight temperature increase. This process began to take place before significant molecular diffusion, and therefore, the weaker satellite signals are attributed to slightly different, unstable, argon matrix trapping sites of the NiC2H4 complex. Annealing effects gave results similar to those
S 3 ) R5 S4 ) (1/2)(ψ14 + ψ15 + ψ16 + ψ17) S5 ) (1/x2)(D6 + D7) for the A2block S6 ) (1/2)(r1 - r2 - r3 + r4) S7 ) (1/2)(β10 - β11 - β12 + β13) S8 ) (1/2)(ψ14 - ψ15 - ψ16 + ψ17) for the B1block S9 ) (1/2)(r1 - r2 + r3 - r4) S10 ) (1/2)(ψ14 - ψ15 + ψ16 - ψ17) S11 ) (1/2)(β10 - β11 + β12 - β13)
IR Spectrum of Ni-Monoethylene
J. Phys. Chem., Vol. 100, No. 27, 1996 11231
Figure 3. Infrared spectra for the stretching region of C-H (3180 = 2800 cm-1) and C-D (2400 = 2180 cm-1) for the NiC2H2D2 species: (a) CH2CD2/Ar = 2.5/500. (b) Ni/CH2HCD2/Ar = 0.4/2.5/500.
Figure 4. IR spectra of the region 1530-1450 cm-1 and 1260-1140 cm-1 regions displaying the evolution of the Ni (C2H4)n complexes as a function of Ni and C2H4 concentrations: (a) Ni/C2H4/Ar = 0.8/1/500. (b) Ni/C2H4/Ar = 0.8/2.5/500. (c) Ni/C2H4/Ar = 0.4/2.5/500. (d) Ni/C2H4/Ar = 0.3/20/500. “X” designates absorptions attributable to a species not yet identified. Nix, nickel aggregates. The absorbance scale in the 12601140 cm-1 region is reduced by a factor 4.
for the B2block S12 ) (1/2)(r1 + r2 - r3 - r4) S13 ) (1/x6)(R8 - R9- β10 - β11 + β12 + β13) S14 ) (1/2)(ψ14 + ψ15 - ψ16 - ψ17) S15 ) (1/x2)(D6 - D7) We then developed a normal coordinate analysis based on a harmonic force field model and the well-known Wilson method,8 using the ground state geometry calculated in the recent theoretical DFT study, and preliminary attributions based on simple logical criteria that we will now briefly justify. The ν2 and ν3 modes have already been assigned by MerleMejean et al.3 to the coupled δsCH2 + νCdC stretching motions (combinations of S2 and S3 coordinates) usually found in the
1500-1200 cm-1 region for π-complexes of ethylene. The new data obtained here with Ni13C2H4 confirm this attribution, because the ν2 and ν3 bands are shifted downward about -16 and -30 cm-1 for the 13C species and -192 and -230 cm-1, respectively, for the NiC2D4 species. The shifts are due not only to the mass effect but also to the change in the coupling between CdC and methylene scissoring coordinates. In fact, as we will show later in the force field calculation, the S2 and S3 coordinates are greatly mixed in ν2 and ν3. Since the 1468.2 cm-1 absorption in our work has been assigned as the ν2 mode (A1 symmetry C-C stretching and CH2 scissoring motion), we can easily attribute the 1386.4 cm-1 band to the ν13 mode (the B2 symmetry CH2 scissoring motion). The complexation by the nickel atom has relatively less effect on the CH2 scissoring vibration. Thus the CH2 symmetric and asymmetric scissoring motion (ν13) is not very far below the same vibration band in free ethylene. As expected, deuteration provided a much greater shift from NiC2H4 than the 13C substitution. In NiCH2CD2,
11232 J. Phys. Chem., Vol. 100, No. 27, 1996 CHART 1
similar bands for CH2 and CD2 were observed at 1437.4 and 1221.0 cm-1, respectively. Logically, given the coupling effect, the band at 1437.4 cm-1 was observed between the 1468.2 (ν2) and 1386.4 cm-1 (ν13) fundamentals of NiC2H4. In the CH stretching region of NiC2H4 or Ni13C2H4, at least four bands were found. The two higher frequency bands around 3000 cm-1 are much stronger than the two lower ones below 2920 cm-1. The upper band at 3031.7 cm-1 in Ni12C2H4 shifts ∼11.3 cm-1 to 3020.4 cm-1 in Ni13C2H4, while the second stretching vibration at 2965.2 cm-1 experiences only 7.6 cm-1 12C/13C shift. An isolated C-H oscillator should present, in the harmonic approximation, a 12C/13C shift of ∼9 cm-1 . However, in a CH2 group with a 120° bond angle, the same model now predicts about 5 and 12 cm-1 12C/13C shifts for the in-phase and out-of-phase motions, respectively. For molecules such as NiC2H4 or Ni13C2H4, these correspond to the A1 or B2 vibrations for the former, and A2 or B1 vibrations for the latter. The 3031.7 cm-1 band is thus assigned to the B1 symmetry stretching, while the 2965.2 cm-1 band could be either the A1 or the B2 vibration. The lower two bands in the CH stretching region (2914.1 and 2839.2 cm-1 in NiC2H4, 2886.9 and 2817.9 cm-1 in Ni13C2H4) have much weaker intensities. Moreover, their 12C/ 13C shifts are 27.2 and 21.3 cm-1, respectively. This is far greater than predicted shifts discussed above for CH stretching fundamentals. On the other hand, these weak lines could be attributed to the 2ν2 overtone and the combination of ν2 + ν13. The calculated positions and, more important, the predicted 12C/ 13C shifts would match acceptably. For NiC H one would have 2 4 1468.2 × 2 ) 2936.4 cm-1 (observed at 2914.1 cm-1) and 1468.2 + 1386.4 ) 2854.6 cm-1 (observed at 2839.2 cm-1), while for Ni13C2H4 one obtains and 1452.7 × 2 ) 2905.4 cm-1 (observed at 2886.9 cm-1) and 1452.7 + 1377.9 ) 2830.6 cm-1 (observed at 2817.9 cm-1). It is noteworthy that 2ν2 at 2914.1 or 2886.9 cm-1 (with 13C) is more intense than the ν2 band itself. This can be explained if a Fermi resonance occurs between the strong ν1 fundamental absorption and the overtone absorption, both of A1 symmetry. The bands at 902.0, 891.9, 746.9, and 904.1 cm-1 have the strongest relative intensities in their respective isotopic species and are likely to correspond to the same normal mode, ν4. In fact, the absorption at 902.0 cm-1 has been observed before and was attributed to a wagging vibration.3 It is thus only ∼50 cm-1 downshifted in comparison with the free ethylene value. In NiCH2CD2, two strong peaks are observed, at 904.1 and 724.6 cm-1, attributable to CdCH2 and CdCD2 wagging motions,
Lee et al. respectively. In the symmetrical isotopic species, there should be two wagging motions: in-phase (A1) and out-of-phase (B2). Only one band is observed for these three species. Another wagging band can be thus predicted, for example in NiC2H4, at a position above 902.0 cm-1 by comparing the relative position between 902.0 and 904.1 cm-1 (NiCH2CD2). The coupling between the motions of the CH2 groups in NiC2H4 no longer exists in NiCH2CD2. This implies the existence of an another band in NiC2H4, unseen and higher in energy than 904.1 cm-1. This needs to be discussed together with the weak bands around 1800 cm-1, which can thus be explained logically. If we make the supposition that the band at 1803 cm-1 is the combination of two wagging modes and 1798 cm-1 is the overtone of the 902 fundamental for NiC2H4, this gives us a rough estimate of the anharmonicity correction to be expected (2 × 902) - 1798.6 cm-1. If the same anharmonicity is assumed for the combination at 1803 cm-1, the calculation 1803 + 6 - 902 would provide 907 cm-1 as the position of the other wagging band. The same supposition and calculation for Ni13C2H4 gives a 900 cm-1 value. We undertook a similar analysis for NiC2D4. Since 746.9 cm-1 was higher than 724.6 cm-1 (NiCH2CD2), there should be another, unobserved wagging absorption at lower frequency around 700 cm-1. This thus explains well the overtone band at 1489.3 cm-1 (2 × 746.9 ) 1493.8 cm-1) and the combination at 1440.2 cm-1 (746.9 + 700 ) 1446.9 cm-1). In NiCH2CD2, the intensity ratio between the 904.1 and 724.6 cm-1 absorptions is about the same as that between 1802.2 and 1441.7 cm-1. Following the same scheme, it can be deduced that the 1802.2 and 1441.7 cm-1 peaks are the overtone bands of 904.1 and 724.6 cm-1, respectively. This assumption was checked with force field calculations in order to establish the proper attributions. A first attempt was made by attributing the higher frequency band to ν4 (A1 symmetry) in NiC2H4 and the lower frequency absorption at 902.0 cm-1 to ν14 (B2). This attribution produced an unsolvable contradiction between the positions of ν4 and ν14 and the magnitude of the predicted isotopic shifts. A much better result was obtained by inverting the attributions such that the lower frequency absorption at 902.0 cm-1 is now assigned to ν4 (A1 symmetry). The H4/D4 shifts are, however, different for ν4 and ν14 and the relative positions are reversed in NiC2D4. The isotopic shifts and relative positions are now correctly reproduced (see Table 2). There are three absorptions left in the low-frequency region in NiC2H4 and Ni13C2H4 complexes, and two of them should be Ni-C A1 and B2 stretching vibrations. The absorption at 601.8 cm-1 (12C complex) cannot be one of them due to the small 12C/13C shift (only 1.9 cm-1). More quantitatively, if we take a 45° value for the CNiC bond angle in a single 3 × 3 harmonic model, the 12C/13C isotopic shift of Ni-C stretching can be calculated as 15 cm-1 A1 and 19 cm-1 B2. The bands at 517.4 and 502.2 cm-1 can then be assigned to A1 and B2, respectively, by comparing their 12C/13C isotopic shifts (12 cm-1 for A1 and 14 cm-1 for B2) to this simple calculation. Since the two Ni-C motions are already assigned, the unique possible assignment for the 601.8 cm-1 band is ν11 (B1). The force field calculations developed next will quantitatively justify this assignment, with an acceptable reproduction of the 12C/ 13C isotopic shift. To facilitate quantitative comparisons with uncomplexed ethylene, we also calculated the force field of ethylene based on the matrix frequencies and the same approximation (with the true gas phase planar geometry). The result of this calculation is very similar to that of the ref 6 and fit our experimental observations in frequency positions isotopic shifts. As for the other complexes, the CdC bond is clearly the most
IR Spectrum of Ni-Monoethylene
J. Phys. Chem., Vol. 100, No. 27, 1996 11233
TABLE 3: Comparison of Experimental and Theoretical Harmonic Frequencies for Ground State NiC2H4a attribution DNiC dCC dCH RHCH ENiCH νCH νCH νCH νCH δCH2 + νCC δCH2 νCC + δCH2 δCCH δNiCH δNiCH δNiCH δNiCH δCCH + δNiCH νNiC νNiC
(B1) (A1) (A2) (B2) (A1) (B2) (A1) (A2) (B2) (A1) (B1) (A2) (B1) (A1) (B2)
exp
ν9 ν1 ν6 ν12 ν2 ν13 ν3 ν7 ν14 ν4 ν10 ν8 ν11 ν5 ν15
3031.7(0.61) 2965.2(0.88) n.o.b 1468.2(0.04) 1386.4(0.10) 1165.9(0.89) 907 902.0(1.0) n.o.b 601.8(0.25) 517.4(0.07) 502.2(0.15)
VWN TZP-A
B-P TZP-A
1.840 1.428 1.111 114.5 112.2 3066(7.6) 2981(14.6) 3049(0.0) 2978(14.3) 1443(0.0) 1354(5.4) 1174(0.8) 1136(0.8) 925(38.1) 901(0.6) 742(0.0) 808(0.0) 575(20.3) 582(5.7) 527(5.7)
1.878 1.439 1.108 114.1 112.1 3077(16.0) 2995(21.3) 3059(0.0) 2991(22.8) 1461(0.3) 1388(2.2) 1158(2.3) 1155(0.0) 923(35.3) 910(2.4) 757(0.0) 822(0.0) 595(21.4) 539(5.9) 510(4.4)
a Relative infrared experimental intensities and absolute theoretical intensities (km/mol) are indicated in parentheses. Calculated harmonic frequencies are from ref 5 with DFT method and two different functionals (VWN and B-P). b n.o., not obtained.
affected by the nickel complexation. Relative to free ethylene the FCC force constant is lowered from 9.09 to 5.61 mdyn/Å (∼38.3% decrease). In this model, the perturbation is only a little larger than other metal complexes such as AlC2H4 (-33%),9 GaC2H4 (-30.6%), InC2H4 (-29.5%)10 and LiC2H4 (-30%).11 If we follow Badger’s empirical relationship between force constant and bond length, by reference to other data concerning well-known hydrocarbons or coordination compounds, we can interpolate approximately a C-C lengthening of 0.11 ( 0.02 Å with respect to ethylene. This value compares indeed well with the bond lengthening calculated by Papai et al. in their DFT theoretical study (1.440 - 1.331 ) 0.109 Å)5 and by Pierloot et al. (1.444 - 1.341 ) 0.102 Å).6 The CC bond length of Ni(C2H4)(PPh3)2 has been measured using X-ray diffraction, and the estimated value of 1.43 Å indicates a similar perturbation of the ethylene ligand.12 The Ni(CO)4 complex has also been well characterized and studied before using diffraction techniques and vibrational spectroscopy.13,14 These studies gave estimates of 1.84 Å for the Ni-C bond length and 2.02 mdyn/Å for the Ni-C bond force constant. Since the force constant FNi-C for the NiC2H4 complex is estimated here at ∼1.9 mdyn/Å, the Ni-C length should be a little longer than 1.84 Å. Indeed, the theoretical studies by Papai et al. and Pierloot et al. provide estimates of 1.88 and 1.876 Å.5,6 As a reference point, X-ray diffraction data indicated that the NiC bond length in Ni(C2H4) (PPh3)2 is 1.99 Å.12 When the theoretically predicted infrared spectrum5 is compared with the experimentally observed one, the contrast between the excellent agreement for the vibrational frequencies and the poor reproduction of the infrared intensities is striking. For example, the A1 and B2 symmetry CH2 wagging motions (ν4 and ν14, respectively) were predicted to have about 2.2 ( 2 and 34 ( 6 km/mol infrared intensities (depending upon the basis set and functional used in the calculations), while the former is indeed the strongest band observed and the latter could not be detected in our experiments (see Table 3). More troubling is the systematic underestimation, in the calculations, of the intensity of the ν3 mode, that having the most C-C stretching character. The ν3 vibration in NiC2H4 and ν2 in NiC2D4 are the second strongest absorption bands but are
calculated among the weakest infrared absorbers. Interestingly, the relative intensities of the ν9, ν13, or ν10 bands, of B1 and B2 symmetries, are more acceptably reproduced. The relative intensities of the infrared absorptions of the different isotopic species can be analyzed in the simple framework described in refs 1-3. Supposing mechanical harmonicity the normal modes, Qi can be expressed as linear combinations of the internal coordinates rj, previously defined, with rj ) ΣjLijQi, where Lij are the eigenvectors of vibrations. Supposing electrical harmonicity,10 the infrared intensity of the ith vibrational mode is proportional to the square of the dipole moment (P) derivative with respect to the Qi normal coordinate, (∂P/∂Qi)2. It is thus possible to expand the expression ∂P/∂Qi as a linear combination of the electrooptical parameters (EOP),15 the dipole moment derivatives ∂P/rj with respect the internal coordinates: ∂P/∂Qi ) ΣjLij ∂P/∂rj. Our goal is therefore to discuss, even qualitatively, which oscillators give the most significant contribution to the net dipole moment change during the vibrations. It is possible to evaluate the different EOP by assuming different combination of signs for the ∂P/∂Qi. In our case, these are not known, but the system is small enough that the different combinations of signs can be successively tried to determine the set of ∂P/rj giving the most consistent results over all the isotopic species. If we first consider the A1 block vibrations, which seem the most poorly reproduced in the DFT calculation, five vibrations are observed. The relative intensities of NiC2H4 and NiC2D4 have also been estimated in an experiment run with equal amounts of C2H4 and C2D4. The ratio of the strongest band of NiC2H4 (ν4) to that of NiC2D4 is measured 1.7 ( 0.2. Accordingly, from all the possible sign combinations for the ∂P/∂Q of NiC2H4, 16 sets of the five electrooptical parameters, ∂Pz/∂RCC, ∂Pz/∂rC-H, ∂Pz/∂DNiC, ∂Pz/ ∂RHCH, ∂Pz/∂ψNiCH, can be derived. Only two sets, can, however, reproduce the relative intensities for NiC2D4 within 30%. The best set represents (in arbitrary units) values of -0.35, +1.96, and 0.66 for the three dipole moment derivative with respect to C-H, C-C, and Ni-C bond length variation. The largest contribution to dipole moment change along the molecular axis thus comes from C-C bond length fluctuation. Since free ethylene has no permanent dipole moment, the net charge transfer occurring during the complexation must create a permanent dipole and induce a change in the carbon-carbon equilibrium distance. The dipole moment is sharply modulated during the vibration involving the C-C stretching coordinate, an effect more pronounced than during the Ni-C bond stretching itself. It therefore seems that the theoretical model does not yet fully account for these properties. Modeling of these electrical properties (dipole moment, electrooptical parameters) might be a sensitive test of the validity of the various theoretical models, useful for a better understanding of the nickel-alkene bonding. Acknowledgment. We gratefully acknowledge financial support from NSF and CNRS (A.I. 91N92/0072) and are indebted to J. Mascetti for helpful discussion and the gift of some 13C-labeled sample. References and Notes (1) Brown, C. E.; Mitchell, S. A.; Haxkett. P. A. Chem. Phys. Lett. 1992, 191, 175. (2) Ozin, G. A.; Huber, H.; Power, W. J. J. Am. Chem. Soc. 1976, 98, 6508. (3) Merle-Mejean, T.; Cosse-Merzens, C.; Bouachered, S.; Galan, F.; Mascetti, J.; Tranquille, M. J. Phys. Chem. 1992, 96, 9148. (4) Widmark, P. O.; Roos, B.; Siegbahn, P. E. J. Phys. Chem. 1985, 89, 2180.
11234 J. Phys. Chem., Vol. 100, No. 27, 1996 (5) Papai, I.; Mink, J.; Fournier, R., Salahub, D. R. J. Phys. Chem. 1993, 97, 9986. (6) Pierloot, K.; Person, B. J.; Roos, B. J. Phys. Chem. 1995, 99, 3465. (7) Atkinson, J. G.; Fisher, M. H.; Horley, D.; Morse, A. T.; Stuart, R. S.; Symes, E. Can J. Chem. 1965, 43, 1614. (8) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955. (9) Manceron, L.; Andrews, L. J. Phys. Chem. 1989, 93, 2964. (10) Manceron, L.; Andrews, L. J. Phys. Chem. 1990, 94, 3513. (11) Manceron, L.; Andrews, L. J. Phys. Chem. 1986, 90, 4514.
Lee et al. (12) Cheng, P. T.; Cook, C. D.; Koo, C. H.; Nybring, S. C.; Shiomo, M. T. Acta Crystallogr. 1971, 27B, 1904. (13) Hedberg, L.; Ijima, T.; Hedberg, K. J. Chem. Phys. 1979, 70, 32246. (14) Jones, L. H.; McDowell, R. S.; Goldblatt, M. J. Chem. Phys. 1968, 48, 2663. (15) Person, W.; Zerbi, G. Vibrational Intensities in Infrared and Raman Spectroscopy; Elsevier: Amsterdam, 1982.
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