ARTICLE pubs.acs.org/JPCA
Low-Energy Vibrations of the Group 10 Metal Monocarbonyl MCO (M = Ni, Pd, and Pt): Rotational Spectroscopy and Force Field Analysis Toshiaki Okabayashi,*,† Takuya Yamamoto,‡ Emi Y. Okabayashi,‡ and Mitsutoshi Tanimoto§ †
Graduate School of Science and Technology, and ‡Department of Chemistry, Faculty of Science, Shizuoka University, 836 Oya, Suruga-ku, Shizuoka 422-8529, Japan § Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan
bS Supporting Information ABSTRACT: The rotational spectra of NiCO and PdCO in the ground and ν2 excited vibrational states were observed by employing a source-modulated microwave spectrometer. The NiCO and PdCO molecules were generated in a free space cell by the sputtering reaction of nickel and palladium sheets, respectively, lining the inner surface of a stainless steel cathode with a dc glow plasma of CO and Ar. The molecular constants of NiCO and PdCO were determined by least-squares analysis. By force field analysis for the molecular constants of not only NiCO and PdCO but also of PtCO as previously reported, the harmonic force constants were determined for these three group 10 metal monocarbonyls. The vibrational wavenumbers derived for the lower M-C stretching vibrations were in good agreement with those obtained from the IR spectra in noble gas matrices and those predicted by several quantum chemical calculations published in the past. The bending vibrational wavenumbers derived by the force field analysis were also consistent with most quantum chemical calculations previously reported, but showed systematic discrepancies from the matrix IR values by about 40 cm-1, even after reassignment (ν2 band f 2ν2 band) of the matrix IR spectra of PdCO and PtCO.
’ INTRODUCTION Unsaturated transition metal carbonyls are important in processes such as organometallic synthesis, homogeneous catalysis, and photochemical decomposition of organometallics.1 In particular, a metal monocarbonyl offers a zeroth-order model for interpreting the chemisorption of a CO molecule on a metal surface in catalytic activation processes.2 Thus, a large number of theoretical papers have appeared, which predict the spectroscopic and structural properties of transition metal carbonyls. In general, the strong bonding between a transition metal and CO is interpreted as arising from the synergistic combination of σ donation from the filled CO π orbital to the metal orbital and πback-donation from the metal dπ to the CO π* orbital. A recent review article by Zhou et al.3 summarizes the experimental and theoretical studies of the unsaturated metal carbonyls. Historically, the group 10 metal monocarbonyls, MCO (M = Ni, Pd, and Pt), have been extensively studied by theoretical calculations, because such metals belong to an especially well exploited group of metals in heterogeneous and homogeneous catalytic systems. Lots of ab initio calculations2,4-84 P predicted that the group 10 metal monocarbonyls have 1 þ electronic ground states. In contrast, experimental evidence for the group 10 metal monocarbonyls is sparse, and matrix-isolation infrared spectroscopy has been the only method for studying MCO experimentally.64,67,68,85-91 This method has been mainly applied to the study of the ν1 C-O stretching band (around 2000 cm-1) to investigate the π-back-donation contribution to the M-C bond. Several recent studies also discussed the low-energy ν2 bending and the ν3 M-C stretching bands.67,89-91 However, r 2011 American Chemical Society
on the basis of the vibrational wavenumbers, especially the bending wavenumbers, of such species obtained by recent high-level theoretical calculations, Taketsugu et al. suggested that the carriers of infrared spectra observed in noble gas matrices, which had been believed to be an MCO species, were actually noble gas (Ng) complexes Ng-MCO.74,84 Experimental spectroscopists offered a counterargument to this conclusion from the observation of the infrared bands, including the bending fundamental and its overtone bands, of NiCO in solid argon, neon, and mixed Ar/Ne matrices.91 In response to this counterargument, theoretical chemists claimed that experimental findings in Ar/Ne matrices were consistent with the theoretical behavior of Ng-NiCO.82 To settle this controversy, spectroscopic evidence in the gas phase has strongly been desired. However, until recently, it has been difficult to detect these reactive species in the gas phase. The experimental evidence of the group 10 metal monocarbonyls in the gas phase was obtained only by anion photoelectron (PE) spectroscopy,72,92,93 which roughly estimated the vibrational wavenumbers of the fundamental bands of the neutral species from the partially resolved structure of the anion spectrum. In the 2000s, Walker et al.94 and Evans et al.95 determined the molecular structure of PdCO and PtCO in the gas phase using Fourier-transform microwave (FTMW) spectroscopy. They generated these species by the reaction of laser-ablated metal atoms with CO in a supersonic jet Received: October 28, 2010 Revised: January 19, 2011 Published: February 20, 2011 1869
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The Journal of Physical Chemistry A and observed its low-J transitions in the ground vibrational state. Subsequently, we reported the millimeter-wave spectra of NiCO in the ground vibrational state96 and PtCO in the ground and ν2 vibrationally excited states.97 These species were generated by the sputtering reaction in the CO/Ar discharge plasma. For the latter species, the l-type doubling constants suggested that the 916.8 cm-1 band in the Ar matrix,67 which had been assigned to ν2, should be reassigned to the overtone band. Moreover, onehalf of the matrix IR value, 458 cm-1, was still too large as compared to the vibrational energy in the gas phase, ν2 = 420 cm-1. Recently, infrared diode laser spectroscopy of NiCO98 has been used to determine the vibrational wavenumber of the CO stretching band in the gas phase. As an extension of our interest in metal monocarbonyls, we have studied the millimeter-wave spectra of NiCO and PdCO in the ν2 excited bending state in the present work. The measured data of rotational transition frequencies yielded the molecular constants of NiCO and PdCO in the ν2 excited state. To discuss the bending vibration of MCO more precisely, force field analysis was carried out for the molecular constants of not only NiCO and PdCO but also of PtCO previously reported.97 The vibrational energies obtained from the present analysis were compared with the values predicted by theoretical calculations as well as those determined by matrix-isolation infrared spectroscopy.
’ EXPERIMENTAL SECTION The present experiment was performed using a source-modulated microwave spectrometer.99 Millimeter- and submillimeterwave radiations were generated by frequency-multipliers driven by a series of klystrons. The radiation transmitted through a free space cell was detected by an InSb bolometer cooled by liquid helium. The cell was equipped with a pair of cylindrical electrodes for a dc glow discharge and was covered by a cooling jacket made of copper, through which liquid nitrogen was circulated. The NiCO and PdCO species were generated in the free space cell by the dc glow discharge in CO with Ar. The atoms of Ni and Pd were generated by sputtering from a nickel and palladium sheet, respectively, lining the inner surface of a stainless steel cathode. The generation condition was determined by monitoring the line intensity in the ground state. The transition frequencies of NiCO in the ground state were taken from our millimeter-wave result.96 In contrast, for PdCO, the transition frequencies in the ground state were predicted using the molecular constants determined by the FTMW study.94 The line intensity was sensitive to the discharge conditions, such as the cell temperature, discharge current, and sample pressure. The optimum sample mixture was a trace amount of CO with 4 mTorr of Ar. The discharge current was set to about 300 mA. The cell temperature was kept below -150 C for efficient MCO generation. Under the above experimental conditions, the lines of NiCO and PdCO in the ground state were strong enough to be found on a cathode-ray oscilloscope without data accumulation. The weak doublet lines of abundant isotopomers, 58NiCO, 105PdCO, 106 PdCO, and 108PdCO, in the excited vibrational state (v2 = 1) were also detected by a careful examination with data accumulation. The intensities of the lines in the ν2 state of PdCO and NiCO were about 5 and 10 times weaker than those of the ground-state lines, respectively. Other rare isotopomers, 60 NiCO, 104PdCO, and 110PdCO, were not observed in the ν2 excited state because of their poor S/N ratio. Figures 1 and 2
ARTICLE
Figure 1. A schematic spectral pattern of NiCO and the observed transition of 58NiCO in the ν2 vibrational state in 10 s of data accumulation.
Figure 2. A schematic spectral pattern of PdCO and the observed transition of 106PdCO in the ν2 vibrational state in 10 s of data accumulation.
display the schematic spectral patterns and typical spectra of NiCO and PdCO in the ν2 vibrational state, respectively. In total, 27 lines of NiCO in the ν2 state in the range of J = 15-14 to 3433 and 207 lines of PdCO in the ground and ν2 states in the range of J = 19-18 to 45-44 were observed between 130 and 312 GHz. Accurate transition frequency of every rotational line was measured by computer software on a PC.
’ ANALYSIS The observed P spectrum showed a typical pattern of a linear molecule in the 1 state. The rotational transition frequencies were analyzed using the standard rotational energy formula for a linear molecule:100 Ev, J ¼ Bv ½JðJ þ 1Þ - l2 - Dv ½JðJ þ 1Þ - l2 2 1 þ Hv ½JðJ þ 1Þ - l2 3 ( ½qv þ qvJ JðJ þ 1ÞJðJ þ 1Þ ð1Þ 2 where v and l are the quantum numbers of the bending vibration. The value of l was fixed to zero in the ground state and to one in the ν2 excited state. The last term in eq 1, which should be neglected for the ground state, accounts for the l-type doubling in the ν2 state. The qv value of a linear triatomic molecule usually has a positive value, and the þ and - signs in eq 1 correspond to the f and e levels, respectively. Analysis of our millimeter- and submillimeter-wave data combined with the microwave data94 led to the molecular constants listed in Table 1. The observed rotational transition frequencies and residuals of the fit are summarized in Table S1 for NiCO in the ν2 state, Table S2 for PdCO in the ground state, and Table S3 for PdCO in the ν2 state. 1870
dx.doi.org/10.1021/jp110325q |J. Phys. Chem. A 2011, 115, 1869–1877
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ARTICLE
Table 1. Molecular Constants of NiCO and PdCOa 58
NiCO
104
PdCO
105
106
PdCO
108
PdCO
110
PdCO
PdCO
This Study B0/MHz
4529.97820(17)b
3459.01952(14)
3452.44955(17)
3446.01791(12)
3433.48610(16)
D0/kHz
1.13419(10)b
0.80979(14)
0.80716(16)
0.80397(11)
0.79863(15)
H0/mHz
0.0b,c
-0.201(39)
-0.132(46)
-0.204(31)
4543.96733(25)
3465.552625(84)
3459.095357(90)
D2/kHz
1.16554(16)
0.836363(30)
0.833061(33)
H2/mHz
0.0c
q2/MHz
4.71077(51)
-0.204c
3.43005(17)
-5.89(32)
q2J/Hz
-0.194(38)
3446.513888(99) 0.827550(36) -0.129c
3.41746(18)
-5.744(61)
0.79305(14)
-0.129(42)
B2/MHz
-0.132c
3421.39955(14)
3.39389(20)
-5.633(66)
-5.702(72)
FTMWd
a
B0/MHz
3459.01878(36)
3452.44864(21)
3446.01811(36)
3433.48621(36)
3421.39982(36)
D0/kHz
0.758(24)
0.726(19)
0.810(24)
0.803(24)
0.820(24)
Values in the parentheses represent one standard deviation. b From ref 96. c Fixed in the analysis. d From ref 94.
Table 2. Harmonic Force Field Constants of NiCO, PdCO, and PtCOa NiCO
fM-C/aJ Å-2 fMC,CO/aJ Å-2 fC-O/aJ Å-2 -2
fMCO/aJ rad a
PdCO
PtCO
this study
previous studyb
this study
previous studyc
this study
previous studyc
4.09 0.65d
4.07 0.65
2.95 0.60d
2.98 0.60
5.40 0.85d
5.146 0.85
15.44
16.57d
16.57
16.22d
15.70 0.334
0.4915
0.195
1.0212
16.22
0.450
2.249
Subscript M of the force constant represents Ni, Pd, or Pt. b From ref 89. c From ref 67. d Fixed in the analysis.
The standard deviations of the fits, 10-20 kHz for each vibrational state of each isotopic species, are reasonable in view of the expected measurement error.
’ RESULTS AND DISCUSSION Force Constants of MCO. In the previous microwave
studies,95-97 the harmonic vibrational wavenumbers of the bending ν2 and the lower stretching ν3 vibrations were, respectively, estimated by the following approximate equations:101 2:6Be 2 ω2 = ð2Þ q2 and 4Be 3 ω3 = De
!1=2 ð3Þ
These approximations are well-known to give good estimates for most of the triatomic linear molecules, but the obtained values possibly have uncertainties as much as 10%. Thus, we decided to carry out the force field analysis to obtain more reliable vibrational wavenumbers of MCO. By theoretical analysis of the vibrational-rotational interaction,102-104 the l-type doubling constant q2 and the centrifugal distortion constant De of a triatomic linear XYZ molecule have been given by ! 2 X B2e ω 1þ4 ξ22i 2 2 2 q2 ¼ 2 ð4Þ ω2 ωi - ω2 i
and De ¼
4B3e
ξ221 ξ223 þ ω23 ω21
! ð5Þ
respectively, where ω1 and ω3 are the bond-stretching wavenumbers and ξ21 and ξ23 are the Coriolis coupling constants, which depend on the masses, dimensions, and harmonic force constants of the molecule.100 On the basis of the force constants previously reported for NiCO,89 PdCO,67 and PtCO,67 the diagonal force constants of the — MCO bending and M-C stretching vibrations were fitted to reproduce the molecular constants of NiCO and PdCO listed in Table 1, as well as those of PtCO in our previous study.97 For NiCO, the recent infrared diode laser result of the ν1 band98 was also used in the analysis. The force constants determined in this study are summarized in Table 2. Table 3 lists centrifugal distortion constants, l-type doubling constants, and vibrational wavenumbers derived from the force constants in Table 2. The stretching force constants are not significantly different from those of previous studies, but the bending ones, especially for PdCO and PtCO, are changed quite significantly. Figure 3 presents the evolution of the harmonic force constants determined in the present study for the group 10 metal monocarbonyls. As predicted from the C-O bond lengths,96 NiCO and PtCO have larger M-C force constants than does PdCO because of the M dπ f CO pπ*back-donation. It is consistent with the finding that PdCO has the smallest force constants for not only the M-C stretching vibration but also for the bending vibration. The consistency between the relative magnitudes of the stretching and bending 1871
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Table 3. Observed and Calculated Centrifugal Distortion Constants, l-type Doubling Constants, and Vibrational Wavenumbers of 58NiCO, 106PdCO, and 195PtCO 58
NiCO
obs. 1.134
a,b
q2/MHz
4.711
c
ω1/cm-1
2010.69e
D/kHz
calc. 1.134 4.713
106
PdCO
obs. 0.804
a,c
3.417
c
calc. 0.802 3.417
Table 4. Comparison of Harmonic Vibrational Wavenumbers of NiCO in cm-1 method
ω1
ω2
ω3
ref
195
PtCO
obs.
Experimentala
calc.
mmW
0.453
a,d
0.454
2.276
d
2.277
IR in Ne matrix IR in Ar matrix
2006.7 1994.5
363
593
this study
398.9 409.1
592.2 591.1
91 89
2013
2044
2058
ω2/cm-1
363
270
412
PW91PW91
2026
364
620
82
ω3/cm-1
593
470
593
CCSD
2091
332
585
78
CCSD
2099
352
579
78
CCSD(T)
2012
369
596
78
CCSD(T)
2028
372
600
78
MPWPW91
2011.3
362.3
609.8
74
CCSD(T) B3LYP
2017.3 2079.9
401.7 362.5
633.3 595.5
74 74
BPW91
2008
351
599
70
MP2
1980
483
691
67
B3LYP
2083.6
345.0
585.9
68
CASSCF(6,6)
2008
358
611
65
CASSCF(8,7)
2011
360
617
65
BP86
2006.2
348.1
602.9
64
B3PW91 B3LYP
2086.5 2073.2
355.4 353.9
668.8 652.6
63 63
PW91
2003.2
352.5
661.3
63
BLYP
1981.1
349.7
645.5
63
BP86
1999.9
352.8
661.3
63
PW91
1995.9
342.2
617.6
63
BLYP
1962.4
345.4
593.5
63
BP86
1996.5
343.4
620.1
63
LDA/BP B3LYP
2005 2073
342 365
617 620
61 58
B3LYP
2099
355
604
56
VWN
2076
324, 295
646
49
BP
2019
311, 293
614
49
SCF
2294
315
458
48
CCSD(T)
2002
369
592
48
CCSD(T)
2016
374
596
48
SCF SCF
2240 2376
350 336
535 483
26 26
SCF
2379
290
413
26
SCF
2209
345
513
26
MP2
1931
548
678
26
MP2
2062
496
679
26
MP2
1991
460
607
26
Theoretical
a
D0 value. b From ref 96. c This study. d From ref 97. e From ref 98. Effective value including anharmonic terms.
Figure 3. Comparison of force constants of MCO (M = Ni, Pd, and Pt).
vibrations for all three molecules supports the validity of our force field analysis. Vibrational Frequencies of MCO in the Gas Phase and the Noble Gas Matrices. Tables 4, 5, and 6 list experimental and theoretical vibrational wavenumbers of NiCO, PdCO, and PtCO, respectively. As suggested in the recent two theoretical studies71,84 and our previous microwave study,97 the bending vibrational wavenumbers for PdCO (270 cm-1) and PtCO (412 cm-1) are now almost one-half the values of those reported by the matrix IR studies.90,67 After reassignment (ν2 band f 2ν2 band) in the matrix IR spectra, the — MCO bending wavenumbers in the noble gas matrices are estimated to be 308 and 458 cm-1 for PdCO and PtCO, respectively. Judging from the reassignment for PdCO, a weak tentative band observed at 317 cm-1 in the Kr matrix87 should be assigned to the bending band, although it was thought to be a band of Pd(CO)2.90 For PtCO, the fundamental bending band was not observed in the infrared region because of its small transition moment.67,97 To explain this relative intensity anomaly between ν2 and 2ν2 bands, we proposed intensity borrowing from the Pt-C stretching band through the Fermi resonance.97 In contrast, the fundamental bending band for NiCO with an enough amount of transition moment was much more strongly observed than its overtone band.91 We will return to this problem later. The vibrational wavenumbers for the M-C stretching vibrations (M = Ni, Pd, and Pt) derived by the present force field analysis are in good agreement with those from the IR spectra
a
Effective values including anharmonic terms.
observed in the noble gas matrices and several quantum chemical calculations. On the other hand, the bending vibrational wavenumbers derived by the present force field analysis (363 cm-1 for NiCO, 270 cm-1 for PdCO, and 412 cm-1 for PtCO) are consistent with most quantum chemical calculations, but they show systematic discrepancies from the matrix IR values (409 cm-1 for NiCO, 308 cm-1 for PdCO, and 458 cm-1 for PtCO) by about 40 cm-1, even after reassignment (ν2 band f 2ν2 band) in the matrix IR spectra of PdCO and PtCO. 1872
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Table 5. Comparison of Harmonic Vibrational Wavenumbers of PdCO in cm-1 method
ω1
ω2
ω3
Table 6. Comparison of Harmonic Vibrational Wavenumbers of PtCO in cm-1
ref
method
ω1
Experimentala mmW
ω3
ref
Experimentala
270
470
this study
mmW
IR in Ar matrix IR in Kr matrix
2044.2 2045.0
615.7b 316.8c
472.0
90 87
IR in Ar matrix PE
PE
2114
210
449
72
2051.9 2040
412
593
this study
916.8b 360
580.8 550
67 72
Theoretical
Theoretical
a
ω2
QCISD(T)
2081
419
605
84
QCISD(T)
2071
273
479
84
B3LYP
2118.1
392.4
565.0
77
B3LYP
2111.5
242.3
443.5
77
BLYP
2018.9
385.9
565.8
77
BLYP
2006.1
245.5
456.3
77
B3P86
2141.4
400.5
586.1
77
B3P86
2129.0
251.4
468.7
77
B3PW91
2137.1
400.0
584.0
77
B3PW91
2123.8
249.9
464.7
77
BHLYP
2230.1
396.4
545.6
77
BHLYP BP86
2237.9 2023.6
234.3 256.6
407.6 481.9
77 77
BP86 PBE1PBE
2042.7 2157.7
394.2 400.8
586.9 586.2
77 77
PBE1PBE
2145.9
247.8
465.1
77
B3LYP
2114
407
585
72
B3LYP
2114
249
449
72
MP2
2047
441
636
67
MP4(SDQ)(Rel)
2086
298
466
71
MP2
2042
429
618
67
MP4(SDQ)(NRel)
2090
218
383
71
QCISD
2124
415
565
67
MP2
2026
294
503
67
B3LYP
2119
395
577
67
B3LYP
2112.9
242.4
445.3
68
B3LYP
2121.4
404.7
590.3
68
LDA/LDA MP2
2099 1969.2
261 343.7
541 462.0
46 47
GVB(6/12)-PP SCF
1976 2157
561 550
600 527
41 20
GVB(6/12)-PP
2253
561
428
41
b
Effective values including anharmonic terms. Misassignment of the overtone band of the bending vibration. The fundamental band was observed near 310 cm-1, but it was misinterpreted as a band of Pd(CO)2 (see text). c Assignment was tentatively given as “a second fundamental vibration”.
However, because our harmonic force field calculation ignored anharmonic terms, it was necessary to assess the accuracy of our we applied the very same ω2 values for MCO species. Thus, P procedure to several well-known 1 þ triatomic molecules. Table 7 shows the result for halogeno cyanides (ClCN, BrCN, and ICN) and carbonyl chalcogenides (OCS and OCSe). The ω2 values derived by the present force field analysis are slightly underestimated by 5-9 and 2-5 cm-1 to the experimental ω2 and ν2 values, respectively, but there are not so large differences as seen in the MCO systems. Table 7 also shows so-called matrix shifts observed for ClCN and BrCN. The ν2 vibrational wavenumbers of ClCN and BrCN in the Ar matrix are about 10 cm-1 “blue-shifted” from the gasphase value, but these shifts are much smaller than those observed in the MCO systems. Judging from the bending force constants in Table 7, the stiffness of the group 10 metal monocarbonyls is close to that of halogeno monocyanides, and the bending motion of the group 10 metal monocarbonyls shows neither quasilinearity nor floppiness. This means that the energy hypersurface for — MCO bending should not be so perturbed through the interaction with the noble gas matrix, even though this interaction is well-known to cause a remarkable “blue-shift” for the large amplitude bending band of a quasilinear molecule like HCNO in the noble gas matrix.119 Therefore, the group 10 metal atom in MCO should possess another kind of a special interaction with the noble gas matrix. Taketsugu et al. proposed that the apparent bond-formation between the metal and noble
a
Effective values including anharmonic terms. b Misassignment of the overtone band of the bending vibration. No trace of the fundamental band existed in the 350-600 cm-1 region in Figure 3 in ref 67, which suggested that its transition moment should be almost zero (see text).
gas atoms is the origin of the large “blue-shift”.74,82,84 Their theoretical calculations indicated that the carriers of infrared spectra in the noble gas matrices are not free MCO species but rather their noble gas complexes Ng-MCO. The theoretical — MCO bending wavenumbers of Ar-MCO (409 cm-1 for Ar-NiCO,82 315 cm-1 for Ar-PdCO,84 and 461 cm-1 for Ar-PtCO84) are in good agreement with the matrix IR values. Taketsugu et al.84 also predicted that the intensity of the ν2 band of PtCO is about one-half of that of the ν3 band, but Manceron et al. observed no corresponding absorption in the predicted region near 460 cm-1, as shown in Figure 3 of ref 67. This finding means that the intensity of the bending band is much weaker than that of the Pt-C stretching band. Although it is uncommon that the overtone band was observed and that the fundamental band was not, this anomaly can be explained by the small transition moment of the fundamental band and the intensity borrowing through the Fermi resonance between the Pt-C stretching vibrational state and the bending overtone states.97 Taketsugu et al.84 explained the lack of the fundamental band by the theoretical calculation, according to which the intensity of the — PtCO bending of the Ar-PtCO complex is almost zero. On the other hand, Taketsugu et al.84 suggested that the intensity of the — PdCO bending band of the Ar-PdCO complex was not zero but about 1/10 of that of the Pd-C stretching band. This suggestion is consistent with the fact that weak bands were observed at 317 cm-1 in the Kr matrix87 and near 310 cm-1 in the Ar matrix,90 although the latter being misassigned to a band of Pd(CO)2. These two experimental 1873
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Table 7. Bending Vibrational Parameters Observed and Derived from the Force Field Analysis for Several 1 Molecules
molecule
ν2/cm-1
Triatomic
force field analysis
experimental in Ar matrix
Pþ
in gas
adjusted
ν2/cm-1
ω2/cm-1
q2/MHz
fXYZ/aJ rad-2
derived q2/MHz
ω2/cm-1
35
387a
379b
382c
7.459d
0.341
7.460
377
79
a
351
e
342
347
f
3.914e
0.298
3.914
338
(307)g
304h 520k
307i 524k
2.660j 6.361l
0.254 0.628
2.658 6.361
301 515
463m
466n
ClCN BrCN
127
ICN OC32S
3.172m
0.550
3.171
460
o
409
4.711
0.334
4.713
363
106
(308)p
3.417
0.195
3.417
270
195
(458)q
2.277r
0.450
2.277
412
OC80Se 58
NiCO PdCO PtCO
a Reference 105. b Reference 106. c Reference 107. d Reference 108. e Reference 109. f Reference 110. g Reference 111. Estimated value from the combination band. h Reference 112. i Reference 113. j Reference 114. k Reference 115. l Reference 116. m Reference 117. n Reference 118. o Reference 89. p Estimated value from the overtone band in ref 90. See text. q Estimated value from the overtone band in ref 67. See text. r Reference 97.
bending wavenumbers agree with the theoretical — PdCO bending wavenumber of Ar-PdCO, 315 cm-1 (ref 84). The higher experimental vibrational wavenumber seen in Kr-PdCO than that in Ar-PdCO probably indicates that the Kr-Pd bond is stronger than the Ar-Pd one, as suggested in the case of Ng-NiCO.74 Vibrational Frequencies of MCO (M = Ni, Pd, and Pt) and Isoelectronic M0 CN (M0 = Cu, Ag, and Au). It is beneficial to compare the molecular properties like vibrational wavenumbers, potential functions, and bond lengths among the isoelectronic molecules. Such molecules have the same number of valence electrons in similar molecular orbitals of different shapes. One of the best-known examples is hydrogen cyanide, HCN, and its isoelectronic species, HBFþ, HBO, HCOþ, HNNþ, HNC, and HOCþ.120,121 The group 11 metal cyanides M0 CN (M0 = Cu, Ag, and Au) are isoelectronic with the group 10 metal monocarbonyls MCO (M = Ni, Pd, and Pt). The vibrational wavenumbers of monocarbonyls MCO are compared here with those of their isoelectronic metal cyanides M0 CN.122,123 Figure 4 shows the changes in the ω2 (bend) and ω3 (M-C str.) values in the two groups of species. For ω2 and ω3, the group 10 metal monocarbonyls have higher vibrational wavenumbers than do the group 11 metal monocyanides. In the group 10 metal monocarbonyls, chemical bonding between the metal and carbon atoms is enhanced due to the M dπ f CO pπ*-back-donation. Because this back-donation is strongly affected by the nature of the metal, the vibrational wavenumbers are staggered, with a pronounced dip at Pd, in the group 10 metal monocarbonyls. The same pattern is observed in the group 11 metal monocyanides, with a roughly similar shift of each fundamental vibrational wavenumber, although the staggering is less accentuated than in the group 10 metal monocarbonyls, because the chemical bond between a group 11 metal and a cyano group is mainly constructed by a σ bond that is not as drastically affected by the nature of the metal atom.123 Rigid Bender Fit. If a linear molecule is undergoing a large amplitude bending vibration, an “average” bond length tends to be shortened, because the experimental bond length seems to be the projection of the bending vibration to the molecular axis. Because molecules including transition metals often have ionic and lowly directed bonds between metal and nonmetal atoms, they possibly possess large amplitude bending vibrations.
Figure 4. Comparison of the vibrational wavenumbers in the group 10 metal monocarbonyls MCO (M = Ni, Pd, and Pt) and the isoelectronic group 11 metal monocyanides M0 CN (M0 = Cu,122 Ag,123 and Au123).
According to Walker et al.,124 in the perspective of a rigid rotor, the rotational constant in the ground state, B0, can be expressed for a linear molecule XYZ with its molecular mass M as p2 mX mZ dYX dYZ mY dYX dYZ 1þ 1þ ð6Þ B0 ¼ ÆR2 æ 2hIe MIe 2Ie where mX, mY, and mZ are atomic masses, and dYX and dYZ are nonprojected bond lengths averaged over the ground state, which are not projections to the molecular axis, but “actual” lengths of oblique bonds. Under the assumption of a harmonic bending vibration, the bending displacement R = 180 — (XYZ) is related to the bending energy: p2 ð7Þ ÆR2 æ ¼ LR2 2 ÆQ2 2 æ ¼ GRR hcω2 and GRR ¼
MIe mX mY mZ dYX 2 dYZ 2
ð8Þ
where LR2 and GRR are standard Wilson matrices.125 The simultaneous fit of the B0 values of all isotopomers yielded the 1874
dx.doi.org/10.1021/jp110325q |J. Phys. Chem. A 2011, 115, 1869–1877
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Table 8. Geometries and Angle Bendinga dMC or rMC/Å
dCO or rCO/Å
NiCO
1.6715(19)
1.1596(25)
rigid bender model
this study
1.67230(41)
1.15124(54)
r0
96
1.670125(17)
1.152546(13)
rs
96
1.6689002(90)
1.1526057(80)
r(2) m
96
1.8439(10)
1.1484(15)
1.8447(1)
PdCO
PtCO
a
ÆR2æ1/2/deg
molecule
method
8.32
9.35
ref
rigid bender model
this study
1.1374(2)
r0
94
1.84127(4)
1.14034(4)
r(2) m
94
1.8401(2) 1.7615(15)
1.1360(2) 1.1545(21)
re rigid bender model
94 this study
7.64
1.76249(42)
1.14662(59)
r0
95
1.76039(13)
1.14627(16)
r(2) m
95
1.76046
1.14354
re(est.)
95
Values in the parentheses represent one standard deviation.
bond lengths dYX and dYZ, as listed in Table 8. However, in the present analysis, the mean-square angular displacement ÆR2æ did not converge in the iterative process. Hence, ÆR2æ values were estimated by a trial-and-error method so as to reproduce the bending vibrational energies in Table 3 and were fixed in the subsequent least-squares analysis to obtain the bond lengths. As reported for the MCN molecules,123 the nonprojected MC bond lengths of MCO averaged over the ground vibrational state were rather close to those obtained by other methods, such as r0 and r(2) m listed in Table 8. In contrast, the nonprojected CO distances were about 0.01 Å longer than those obtained by methods such as r0 and r(2) m as seen in the case of metal monocyanides.123 The averaged bending displacements for MCO, 7.6-9.4, are slightly smaller than those for isoelectronic monocyanides, 8.6-9.7, because of the bond enhancement due to the π-back-donation seen in MCO.
’ CONCLUSIONS The low-energy vibrational properties of the group 10 metal monocarbonyls, NiCO, PdCO, and PtCO, have been determined in detail by the measurement of their rotational spectra and the force field analysis. The derived vibrational wavenumbers for the lower M-C stretching vibrations are in good agreement with the IR spectra observed in the noble gas matrices and the prediction by several quantum chemical calculations. The wavenumbers derived for the bending vibration are consistent with most of the quantum chemical calculations, but show a systematic discrepancy by about 40 cm-1 from the matrix IR values, even after reassignment (ν2 band f 2ν2 band) of the matrix IR spectra of PdCO and P PtCO. The very same force field analysis for several well-known 1 þ triatomic molecules suggests that the present large discrepancies for MCO between in the Ar matrix and in the gas phase are significant even if the so-called matrix shift and the anharmonic vibrational effects are considered. This finding indicates that the group 10 metal atom in MCO possesses a kind of a special interaction with the noble gas matrix, for example, as the bond-formation between the metal and noble gas atoms predicted theoretically. ’ ASSOCIATED CONTENT
bS
Supporting Information. Observed transition frequencies. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This study was supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (nos. 15656184 and 20550010). E.Y.O. thanks the Hayashi Memorial Foundation for Female Natural Scientists for the Hayashi Fellowship. ’ REFERENCES (1) Tumas, W.; Gitlin, B.; Rosan, A. M.; Yardley, J. T. J. Am. Chem. Soc. 1982, 104, 55. (2) Walch, S. P.; Goddard, W. A., III. J. Am. Chem. Soc. 1976 98, 7908. (3) Zhou, M.; Andrews, L.; Bauschlicher, C. W., Jr. Chem. Rev. 2001, 101, 1931. (4) Clark, D. T.; Cromarty, B. J.; Sgamellotti, A. Chem. Phys. Lett. 1978, 55, 482. (5) Itoh, H.; Kunz, A. B. Z. Naturforsch., A: Phys. Sci. 1979, 34, 114. (6) Rives, A. B.; Weinhold, F. Int. J. Quantum Chem. 1980, 14, 201. (7) Howard, I. A.; Pratt, G. W.; Johnson, K. H.; Dresselhaus, G. J. Chem. Phys. 1981, 74, 3415. (8) Rives, A. B.; Fenske, R. F. J. Chem. Phys. 1981, 75, 1293. (9) Bagus, P. S.; Roos, B. O. J. Chem. Phys. 1981, 75, 5961. (10) Saddei, D.; Freund, H.-J.; Hohlneicher, G. Chem. Phys. 1981, 55, 339. (11) Pacchioni, G.; Koutecky , J.; Fantucci, P. Chem. Phys. Lett. 1982, 92, 486. (12) Dunlap, B. I.; Yu, H. L.; Antoniewicz, P. R. Phys. Rev. A 1982, 25, 7. (13) Basch, H.; Cohen, D. J. Am. Chem. Soc. 1983, 105, 3856. (14) Ha, T.-K.; Nguyen, M. T. J. Mol. Struct. (THEOCHEM) 1984, 109, 331. (15) Huzinaga, S.; Klobukowski, M.; Sakai, Y. J. Phys. Chem. 1984, 88, 4880. (16) Blomberg, M. R. A.; Brandemark, U. B.; Siegbahn, P. E. M.; Mathisen, K. B.; Karlstr€om, G. J. Phys. Chem. 1985, 89, 2171. (17) Rohlfing, C. M.; Hay, P. J. J. Chem. Phys. 1985, 83, 4641. (18) Kao, C. M.; Messmer, R. P. Phys. Rev. B 1985, 31, 4835. (19) Bauschlicher, C. W., Jr. Chem. Phys. Lett. 1985, 115, 387. (20) Basch, H. Chem. Phys. Lett. 1985, 116, 58. (21) Koutecky, J.; Pacchioni, G.; Fantucci, P. Chem. Phys. 1985 99, 87. 1875
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