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Bending vibration of platinum monocarbonyl PtCO: observation of the millimeter- and submillimeter-wave spectra in the ν2 excited vibrational state
Chemical Physics Letters 396 (2004) 150–154 www.elsevier.com/locate/cplett
Bending vibration of platinum monocarbonyl PtCO: observation of the millimeter- and submillimeter-wave spectra in the m2 excited vibrational state Emi Yamazaki, Toshiaki Okabayashi *, Mitsutoshi Tanimoto Department of Chemistry, Faculty of Science, Shizuoka University, Oya 836, Shizuoka 422-8529, Japan Received 19 March 2004; in final form 26 July 2004
Abstract The millimeter- and submillimeter-wave spectra of PtCO in the ground and m2 excited vibrational states were observed by employing a source-modulated microwave spectrometer. The PtCO molecule was generated in a free space cell by the sputtering reaction from a platinum sheet lining the inner surface of a stainless steel cathode using a dc glow plasma of CO and Ar. From the molecular constants determined for the m2 excited state, especially the l-type doubling constant, its harmonic wavenumber was determined to be 420 cm1, which resolved the reported discrepancy between the previous matrix-infrared and theoretical estimates. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide adsorbed on the surface of platinum metal is an important chemical system in several aspects [1]. It is considered to be the first step in the reaction of CO with O2 catalyzed by platinum metals. Clarification of the mechanism of CO chemisorption on the platinum surface and subsequent oxidation is a subject of active research by all available experimental and theoretical tools. A recent development in computer technology has enabled quantum-chemical calculations [2,3] for such systems as a complex of several platinum atoms, taken as a model of a catalyst surface. The behavior of a CO molecule on the surface has been analyzed theoretically. Platinum monocarbonyl, PtCO, serves as the simplest model for platinum–CO chemisorption [4] and has especially been studied in great detail as a benchmark mole*
Corresponding author. E-mail address: [email protected] (T. Okabayashi).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.040
cule for understanding the Pt–CO chemical bonding [5– 18]. In contrast, experimental evidence for PtCO had long been limited to matrix isolation infrared spectroscopy [16,17,19], by which the three fundamental bands, m1–m3, were observed and assigned by Manceron et al. [16]. The molecular structure in the gas phase was determined by Evans and Gerry [20] by Fourier-transform microwave (FTMW) spectroscopy. They generated PtCO by the reaction of laser-ablated platinum atoms with CO in a supersonic jet and observed its low-J transitions in the vibrational ground state. Quite recently, Chatterjee et al. [18] observed the anion photoelectron (PE) spectrum of PtCO and estimated the vibrational frequencies of the fundamental bands of the neutral species from the partially resolved shoulder structure of the anion spectrum. These experimental findings have been compared with those obtained by quantum-chemical calculations. The geometrical structure obtained by a number of quantum-chemical calculations agreed basically with
E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154
the experimental values reported by Evans and Gerry [20]. The theoretical wavenumbers of the m1 (2000 cm1) and m3 (600 cm1) stretching vibrations also agreed well with the experimental values. However, the theoretical value of the m2 bending mode (400 cm1) deviated significantly from the experimental value (917 cm1) [16]. No absorption feature due to PtCO was observed around 400 cm1, and the authors [16] suggested that the discrepancy was within the range of uncertainty in the quantum-chemical calculations. Nevertheless, a recent theoretical calculation at the MPWPW91 and several other levels [21] still yielded the harmonic vibrational wavenumber of 400 cm1. We have applied microwave spectroscopy in the present study to elucidate this discrepancy by determination of the l-type doubling constant q in the m2 excited vibrational state. Our estimate of the bending frequency has confirmed that predicted by theoretical calculations.
194
151
PtCO ν2(e)
J=46–45 306284
(MHz)
Fig. 1. Rotational transition of
306290
194
PtCO in the m2 vibrational state.
of PtCO in the m2 vibrational state. In total, 44 lines in the ground state and 42 lines in the m2 state of PtCO were observed between 192 and 313 GHz.
2. Experimental The present experiment was carried out using a source-modulated microwave spectrometer [23]. Millimeter- and submillimeter-wave radiations were generated by frequency-multipliers driven by 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 PtCO species were generated in the free space cell by a dc glow discharge in CO with Ar using a method similar to that used in our previous experiment on NiCO [22]. Atoms of Pt were supplied by sputtering from a small piece of a platinum sheet lining the inner surface of a stainless steel cathode. The generation condition was determined by monitoring the line-intensity in the ground state. Transition frequencies in the ground state were predicted using the molecular constants determined by FTMW spectroscopy [20]. The line-intensity of PtCO was sensitive to the discharge conditions, such as the cell temperature, the discharge current and the sample pressure. Optimum sample pressure was 1 mTorr of CO with 4 mTorr of Ar. The discharge current was set to about 200 mA. The cell temperature needed to be kept below 150 °C for efficient generation of PtCO. Under the above experimental conditions, the lines of PtCO in the ground state were strong enough to be observed on a cathode-ray oscilloscope without data accumulation. Weak doublet lines in the excited vibrational state (v2 = 1) were also detected by carrying out a careful examination with data accumulation. The line-intensity in the m2 state was about 10 times weaker than that of the ground state lines. Fig. 1 displays a typical spectrum
3. Analysis The observed spectrum showed a typical pattern of a linear molecule in the 1R state. Transition frequencies were analyzed using the standard rotational energy formula for a linear molecule, Ev;J ¼ Bv ½J ðJ þ 1Þ l2 Dv ½J ðJ þ 1Þ l2 12½qv þ qvJ J ðJ þ 1ÞJ ðJ þ 1Þ;
2
ð1Þ
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 one in the m2 excited state. The last term in Eq. (1), which should be neglected for the ground state, accounts for the l-type doubling in the m2 state. The qv value of a linear triatomic molecule usually has a positive value, and the + and signs in Eq. (1) correspond to the f and e levels, respectively. Analysis of our millimeterand submillimeter-wave data combined with the microwave data from [20] led to the molecular constants listed in Table 1. The observed rotational transition frequencies and residuals of the fit are summarized in Table 2. The standard deviations of the fits, 10–20 kHz for each vibrational state, are reasonable in view of the expected measurement error.
4. Results and discussion The present measurement has led to an improvement of the molecular constants of PtCO in the ground state and to the first determination of those in the m2 excited vibrational state. The harmonic vibrational wavenumber of the lowest stretching vibration m3 (Pt–C str.) is estimated by
152
E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154
Table 1 Molecular constants of PtCOa 194
195
196
198
This work B0 (MHz) D0 (kHz) B2 (MHz) D2 (kHz) q2 (MHz) q2J (Hz)
3324.859918(95) 0.453696(29) 3332.30874(24) 0.462874(66) 2.27923(48) 0.97(13)
3322.833613(94) 0.453251(28) 3330.27764(29) 0.462347(79) 2.27616(59) 0.92(16)
3320.831201(76) 0.452652(23) 3328.27149(35) 0.462012(93) 2.27361(69) 0.92(19)
3316.88191(15) 0.451608(44)
Previous workb B0 (MHz) D0 (kHz)
3324.85989(43) 0.455(28)
3322.83356(31) 0.450(20)
3320.83107(43) 0.442(28)
3316.88224(43) 0.474(28)
PtCO
a b
PtCO
PtCO
PtCO
Values in parentheses represent 1 SD. [20].
Table 2 Observed transition frequencies of PtCO in MHza J 0 –J00
l
194
195
196
198
Ground 1–0 2–1 3–2 29–28 30–29 34–33 35–34 36–35 37–36 38–37 43–42 44–43 45–44 46–45 47–46
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
6649.7173(7)b 13299.4251(1)b 19949.1101(4)b 192797.614(0) 199442.590(6) 226019.162(16) 232662.391(6) 239305.242(2) 245947.722(12) 252589.760(13) 285793.679(14) 292433.066(16) 299072.015(5) 305710.466(3) 312348.420(4)
6645.6647(7)b 13291.3200(1)b 19936.9527(0)b 192680.140(8)
6633.7609(11)b 13267.5141(9)b 19901.2421(6)b
225881.433(6) 232520.623(3) 239159.438(5) 245797.851(2) 252435.866(6) 285619.527(17) 292254.912(7) 298889.805(11) 305524.236(14) 312158.140(12)
6641.6606(0)b 13283.3101(2)b 19924.9387(4)b 192564.051(0) 199200.966(20) 225745.364(6) 232380.555(1) 239015.367(4) 245649.799(3) 252283.833(13) 285447.530(3) 292078.914(3) 298709.812(5) 305340.224(9) 311970.154(4)
m2 36–35 37–36 38–37 44–43 45–44 46–45 47–46 36–35 37–36 38–37 44–43 45–44 46–45 47–46
1e 1e 1e 1e 1e 1e 1e 1f 1f 1f 1f 1f 1f 1f
239757.953(2) 246412.886(12) 253067.449(14) 292985.399(13) 299636.765(2) 306287.623(7) 312938.015(57)c 239921.880(5) 246581.358(7) 253240.452(8) 293185.652(3) 299841.537(5) 306496.910(19) 313151.826(22)
239611.937(21) 246262.792(21) 252913.303(4) 292806.970(12) 299454.293(6) 306101.080(13) 312747.404(15) 239775.636(9) 246431.050(11) 253085.994(90)c 293006.981(12) 299658.798(7) 306310.138(2) 312961.032(68)c
239467.645(18) 246114.500(19) 252761.005(4) 292630.640(26) 299273.979(11) 305916.791(18) 312559.061(7) 239631.150(6) 246282.557(23) 252933.619(25) 292830.457(26) 299478.251(8) 306125.573(15) 312772.407(1)
a b c
PtCO
PtCO
PtCO
PtCO
232104.256(27) 238731.224(7) 245357.779(19) 251983.915(12) 285108.213(7) 291731.741(12) 298354.760(1) 304977.295(10) 311599.348(2)
Values in parentheses represent the residuals (Obs. Calc.) to the last digits. Cited from [20]. Frequencies of 195PtCO are calculated values without hyperfine splitting. Excluded from the fit.
3 1=2 4Be x3 ’ ; De
ð2Þ
in a diatomic approximation [20,24]. If the rotational and centrifugal distortion constants in the equilibrium
state are approximated by those in the ground state, the vibrational wavenumber is calculated to be x3 600 cm1. This is in good agreement with the previous estimates by FTMW (605 cm1) [20], matrixisolation infrared spectroscopy (581 cm1) [16], photo-
E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154
153
Table 3 Comparison of harmonic vibrational wavenumbers of PtCO in cm1 x1
x2
x3
Ref.
420
This work [20] [16] [18]
[18] [16] [16] [16] [16] [17] [10] [6]
Experimental mmW FTMW matrix IR PE
2052a 2040a
917a 360a
600 605 581a 550a
Theoretical B3LYP MP2/LanL2DZ MP2/Stoll QCISD/Stoll B3LYP/Stoll B3LYP GVB(6/12)-PP SCF
2114 2047 2042 2124 2119 2121 1976 2157
407 441 429 415 395 405 561 550
585 636 618 565 577 590 600 527
a
Effective values including anharmonic terms.
electron spectroscopy (550 cm1) [18], and theoretical calculations as summarized in Table 3. Since the diatomic approximation for the lowest stretching mode also results in good estimates for NiCO [22] and PdCO [25], this method of estimation seems to be a good model for this type of a metal complex. The harmonic vibrational wavenumber of the bending vibration m2 is estimated from the l-type doubling constant q2 through the following equation [24]: x2 ’
2:6B2e : q2
ð3Þ
Using the molecular constants in Table 1, the vibrational wavenumber is calculated to be x2 420 cm1. This agrees qualitatively with the results of photoelectron spectroscopy (360 cm1) [18] and theoretical calculations at various levels shown in Table 3. However, it is not consistent with the wavenumber of matrix-isolation infrared spectroscopy (917 cm1) [16]. In the light of these experimental and theoretical results, the assignment of the infrared band of 917 cm1 to the bending fundamental [16] seems to be inconsistent. This band might be assigned to the 2m2 band rather than to the m2 band (see below). A DFT calculation [17] predicted that the intensity of the m2 band is similar to that of the m3 band, but Manceron et al. observed no corresponding absorption in the predicted region near 400 cm1 as shown in Fig. 3 of [16]. This finding means that the intensity of the m2 band is much weaker than that of the m3 band. It is uncommon that the 2m2 band was observed whereas the m2 band was not, because the fundamental band should be much stronger than the overtone band. This anomaly can be explained by the low transition moment of the m2 band and the Fermi resonance between the m3 and 2m2 states. Strong Fermi interactions between the m3 and 2m2 states have often been reported for linear triatomic
molecules [24]. The ratio of the observed intensities of the 917 cm1 band to that of the m3 band, about 1:10 [16], is an acceptable value as a result of intensityborrowing due to the Fermi resonance. The lifting of the 2m2 state by this resonance is perhaps a part of the reason why the observed 2m2 band (917 cm1) is slightly higher than twice the estimated m2 value (about 420 cm1).
Acknowledgements The research was supported by Japan Society for the Promotion of Science through Grant-in-Aid for Scientific Research (Nos. 12740316 and 15656184). E.Y. thanks the Japan Science Society through the Sasagawa Scientific Research Grant and the Hayashi Memorial Foundation for Female Natural Scientists through the Hayashi Fellowship. T.O. thanks the Kawasaki Steel 21st Century Foundation for financial support. T.O. and E.Y. also acknowledge the financial support from the Hamamatsu Foundation for Science and Technology Promotion.
References [1] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [2] P.J. Feibelman, B. Hammer, J.K. Nørskov, F. Wagner, M. Scheffler, R. Stumpf, R. Watwe, J. Dumesic, J. Phys. Chem. B 105 (2001) 4018. [3] H. Orita, N. Itoh, Y. Inada, Chem. Phys. Lett. 384 (2004) 271. [4] S.P. Walch, W.A. Goddard III, J. Am. Chem. Soc. 98 (1976) 7908. [5] H. Basch, D. Cohen, J. Am. Chem. Soc. 105 (1983) 3856. [6] H. Basch, Chem. Phys. Lett. 116 (1985) 58. [7] C.M. Rohlfing, P.J. Hay, J. Chem. Phys. 83 (1985) 4641.
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[8] A. Gavezzotti, G.F. Tantardini, M. Simonetta, Chem. Phys. Lett. 129 (1986) 577. [9] A. Gavezzotti, G.F. Tantardini, H. Miessner, J. Phys. Chem. 92 (1988) 872. [10] G.W. Smith, E.A. Carter, J. Phys. Chem. 95 (1991) 2327. [11] M. Ohno, W. von Niessen, Phys. Rev. B 45 (1992) 9382. [12] M. Ohno, W. von Niessen, Surf. Sci. 269/270 (1992) 258. [13] S. Roszak, K. Balasubramanian, J. Phys. Chem. 97 (1993) 11238. [14] S.-C. Chung, S. Kru¨ger, G. Pacchioni, N. Ro¨sch, J. Chem. Phys. 102 (1995) 3695. [15] S.-C. Chung, S. Kru¨ger, S.P. Ruzankin, G. Pacchioni, N. Ro¨sch, Chem. Phys. Lett. 109 (1996) 248. [16] L. Manceron, B. Tremblay, M.E. Alikhani, J. Phys. Chem. A 104 (2000) 3750.
[17] B. Liang, M. Zhou, L. Andrews, J. Phys. Chem. A 104 (2000) 3905. [18] B. Chatterjee, F.A. Akin, C.C. Jarrold, K. Raghavachari, J. Chem. Phys. 119 (2003) 10591. [19] E.P. Ku¨ndig, D. McIntosh, M. Moskovits, G.A. Ozin, J. Am. Chem. Soc. 95 (1973) 7234. [20] C.J. Evans, M.C.L. Gerry, J. Phys. Chem. A 105 (2001) 9659. [21] Y. Ono, Private communication. [22] E. Yamazaki, T. Okabayashi, M. Tanimoto, J. Am. Chem. Soc. 126 (2004) 1028. [23] T. Okabayashi, M. Tanimoto, J. Chem. Phys. 99 (1993) 3268. [24] C.H. Townes, A.L. Schawlow, Microwave Spectroscopy, McGraw-Hill, New York, 1955. [25] N.R. Walker, J.K.-H. Hui, M.C.L. Gerry, J. Phys. Chem. A 106 (2002) 5803.
Bending vibration of platinum monocarbonyl PtCO: observation of the millimeter- and submillimeter-wave spectra in the m2 excited vibrational state Emi Yamazaki, Toshiaki Okabayashi *, Mitsutoshi Tanimoto Department of Chemistry, Faculty of Science, Shizuoka University, Oya 836, Shizuoka 422-8529, Japan Received 19 March 2004; in final form 26 July 2004
Abstract The millimeter- and submillimeter-wave spectra of PtCO in the ground and m2 excited vibrational states were observed by employing a source-modulated microwave spectrometer. The PtCO molecule was generated in a free space cell by the sputtering reaction from a platinum sheet lining the inner surface of a stainless steel cathode using a dc glow plasma of CO and Ar. From the molecular constants determined for the m2 excited state, especially the l-type doubling constant, its harmonic wavenumber was determined to be 420 cm1, which resolved the reported discrepancy between the previous matrix-infrared and theoretical estimates. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide adsorbed on the surface of platinum metal is an important chemical system in several aspects [1]. It is considered to be the first step in the reaction of CO with O2 catalyzed by platinum metals. Clarification of the mechanism of CO chemisorption on the platinum surface and subsequent oxidation is a subject of active research by all available experimental and theoretical tools. A recent development in computer technology has enabled quantum-chemical calculations [2,3] for such systems as a complex of several platinum atoms, taken as a model of a catalyst surface. The behavior of a CO molecule on the surface has been analyzed theoretically. Platinum monocarbonyl, PtCO, serves as the simplest model for platinum–CO chemisorption [4] and has especially been studied in great detail as a benchmark mole*
Corresponding author. E-mail address: [email protected] (T. Okabayashi).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.040
cule for understanding the Pt–CO chemical bonding [5– 18]. In contrast, experimental evidence for PtCO had long been limited to matrix isolation infrared spectroscopy [16,17,19], by which the three fundamental bands, m1–m3, were observed and assigned by Manceron et al. [16]. The molecular structure in the gas phase was determined by Evans and Gerry [20] by Fourier-transform microwave (FTMW) spectroscopy. They generated PtCO by the reaction of laser-ablated platinum atoms with CO in a supersonic jet and observed its low-J transitions in the vibrational ground state. Quite recently, Chatterjee et al. [18] observed the anion photoelectron (PE) spectrum of PtCO and estimated the vibrational frequencies of the fundamental bands of the neutral species from the partially resolved shoulder structure of the anion spectrum. These experimental findings have been compared with those obtained by quantum-chemical calculations. The geometrical structure obtained by a number of quantum-chemical calculations agreed basically with
E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154
the experimental values reported by Evans and Gerry [20]. The theoretical wavenumbers of the m1 (2000 cm1) and m3 (600 cm1) stretching vibrations also agreed well with the experimental values. However, the theoretical value of the m2 bending mode (400 cm1) deviated significantly from the experimental value (917 cm1) [16]. No absorption feature due to PtCO was observed around 400 cm1, and the authors [16] suggested that the discrepancy was within the range of uncertainty in the quantum-chemical calculations. Nevertheless, a recent theoretical calculation at the MPWPW91 and several other levels [21] still yielded the harmonic vibrational wavenumber of 400 cm1. We have applied microwave spectroscopy in the present study to elucidate this discrepancy by determination of the l-type doubling constant q in the m2 excited vibrational state. Our estimate of the bending frequency has confirmed that predicted by theoretical calculations.
194
151
PtCO ν2(e)
J=46–45 306284
(MHz)
Fig. 1. Rotational transition of
306290
194
PtCO in the m2 vibrational state.
of PtCO in the m2 vibrational state. In total, 44 lines in the ground state and 42 lines in the m2 state of PtCO were observed between 192 and 313 GHz.
2. Experimental The present experiment was carried out using a source-modulated microwave spectrometer [23]. Millimeter- and submillimeter-wave radiations were generated by frequency-multipliers driven by 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 PtCO species were generated in the free space cell by a dc glow discharge in CO with Ar using a method similar to that used in our previous experiment on NiCO [22]. Atoms of Pt were supplied by sputtering from a small piece of a platinum sheet lining the inner surface of a stainless steel cathode. The generation condition was determined by monitoring the line-intensity in the ground state. Transition frequencies in the ground state were predicted using the molecular constants determined by FTMW spectroscopy [20]. The line-intensity of PtCO was sensitive to the discharge conditions, such as the cell temperature, the discharge current and the sample pressure. Optimum sample pressure was 1 mTorr of CO with 4 mTorr of Ar. The discharge current was set to about 200 mA. The cell temperature needed to be kept below 150 °C for efficient generation of PtCO. Under the above experimental conditions, the lines of PtCO in the ground state were strong enough to be observed on a cathode-ray oscilloscope without data accumulation. Weak doublet lines in the excited vibrational state (v2 = 1) were also detected by carrying out a careful examination with data accumulation. The line-intensity in the m2 state was about 10 times weaker than that of the ground state lines. Fig. 1 displays a typical spectrum
3. Analysis The observed spectrum showed a typical pattern of a linear molecule in the 1R state. Transition frequencies were analyzed using the standard rotational energy formula for a linear molecule, Ev;J ¼ Bv ½J ðJ þ 1Þ l2 Dv ½J ðJ þ 1Þ l2 12½qv þ qvJ J ðJ þ 1ÞJ ðJ þ 1Þ;
2
ð1Þ
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 one in the m2 excited state. The last term in Eq. (1), which should be neglected for the ground state, accounts for the l-type doubling in the m2 state. The qv value of a linear triatomic molecule usually has a positive value, and the + and signs in Eq. (1) correspond to the f and e levels, respectively. Analysis of our millimeterand submillimeter-wave data combined with the microwave data from [20] led to the molecular constants listed in Table 1. The observed rotational transition frequencies and residuals of the fit are summarized in Table 2. The standard deviations of the fits, 10–20 kHz for each vibrational state, are reasonable in view of the expected measurement error.
4. Results and discussion The present measurement has led to an improvement of the molecular constants of PtCO in the ground state and to the first determination of those in the m2 excited vibrational state. The harmonic vibrational wavenumber of the lowest stretching vibration m3 (Pt–C str.) is estimated by
152
E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154
Table 1 Molecular constants of PtCOa 194
195
196
198
This work B0 (MHz) D0 (kHz) B2 (MHz) D2 (kHz) q2 (MHz) q2J (Hz)
3324.859918(95) 0.453696(29) 3332.30874(24) 0.462874(66) 2.27923(48) 0.97(13)
3322.833613(94) 0.453251(28) 3330.27764(29) 0.462347(79) 2.27616(59) 0.92(16)
3320.831201(76) 0.452652(23) 3328.27149(35) 0.462012(93) 2.27361(69) 0.92(19)
3316.88191(15) 0.451608(44)
Previous workb B0 (MHz) D0 (kHz)
3324.85989(43) 0.455(28)
3322.83356(31) 0.450(20)
3320.83107(43) 0.442(28)
3316.88224(43) 0.474(28)
PtCO
a b
PtCO
PtCO
PtCO
Values in parentheses represent 1 SD. [20].
Table 2 Observed transition frequencies of PtCO in MHza J 0 –J00
l
194
195
196
198
Ground 1–0 2–1 3–2 29–28 30–29 34–33 35–34 36–35 37–36 38–37 43–42 44–43 45–44 46–45 47–46
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
6649.7173(7)b 13299.4251(1)b 19949.1101(4)b 192797.614(0) 199442.590(6) 226019.162(16) 232662.391(6) 239305.242(2) 245947.722(12) 252589.760(13) 285793.679(14) 292433.066(16) 299072.015(5) 305710.466(3) 312348.420(4)
6645.6647(7)b 13291.3200(1)b 19936.9527(0)b 192680.140(8)
6633.7609(11)b 13267.5141(9)b 19901.2421(6)b
225881.433(6) 232520.623(3) 239159.438(5) 245797.851(2) 252435.866(6) 285619.527(17) 292254.912(7) 298889.805(11) 305524.236(14) 312158.140(12)
6641.6606(0)b 13283.3101(2)b 19924.9387(4)b 192564.051(0) 199200.966(20) 225745.364(6) 232380.555(1) 239015.367(4) 245649.799(3) 252283.833(13) 285447.530(3) 292078.914(3) 298709.812(5) 305340.224(9) 311970.154(4)
m2 36–35 37–36 38–37 44–43 45–44 46–45 47–46 36–35 37–36 38–37 44–43 45–44 46–45 47–46
1e 1e 1e 1e 1e 1e 1e 1f 1f 1f 1f 1f 1f 1f
239757.953(2) 246412.886(12) 253067.449(14) 292985.399(13) 299636.765(2) 306287.623(7) 312938.015(57)c 239921.880(5) 246581.358(7) 253240.452(8) 293185.652(3) 299841.537(5) 306496.910(19) 313151.826(22)
239611.937(21) 246262.792(21) 252913.303(4) 292806.970(12) 299454.293(6) 306101.080(13) 312747.404(15) 239775.636(9) 246431.050(11) 253085.994(90)c 293006.981(12) 299658.798(7) 306310.138(2) 312961.032(68)c
239467.645(18) 246114.500(19) 252761.005(4) 292630.640(26) 299273.979(11) 305916.791(18) 312559.061(7) 239631.150(6) 246282.557(23) 252933.619(25) 292830.457(26) 299478.251(8) 306125.573(15) 312772.407(1)
a b c
PtCO
PtCO
PtCO
PtCO
232104.256(27) 238731.224(7) 245357.779(19) 251983.915(12) 285108.213(7) 291731.741(12) 298354.760(1) 304977.295(10) 311599.348(2)
Values in parentheses represent the residuals (Obs. Calc.) to the last digits. Cited from [20]. Frequencies of 195PtCO are calculated values without hyperfine splitting. Excluded from the fit.
3 1=2 4Be x3 ’ ; De
ð2Þ
in a diatomic approximation [20,24]. If the rotational and centrifugal distortion constants in the equilibrium
state are approximated by those in the ground state, the vibrational wavenumber is calculated to be x3 600 cm1. This is in good agreement with the previous estimates by FTMW (605 cm1) [20], matrixisolation infrared spectroscopy (581 cm1) [16], photo-
E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154
153
Table 3 Comparison of harmonic vibrational wavenumbers of PtCO in cm1 x1
x2
x3
Ref.
420
This work [20] [16] [18]
[18] [16] [16] [16] [16] [17] [10] [6]
Experimental mmW FTMW matrix IR PE
2052a 2040a
917a 360a
600 605 581a 550a
Theoretical B3LYP MP2/LanL2DZ MP2/Stoll QCISD/Stoll B3LYP/Stoll B3LYP GVB(6/12)-PP SCF
2114 2047 2042 2124 2119 2121 1976 2157
407 441 429 415 395 405 561 550
585 636 618 565 577 590 600 527
a
Effective values including anharmonic terms.
electron spectroscopy (550 cm1) [18], and theoretical calculations as summarized in Table 3. Since the diatomic approximation for the lowest stretching mode also results in good estimates for NiCO [22] and PdCO [25], this method of estimation seems to be a good model for this type of a metal complex. The harmonic vibrational wavenumber of the bending vibration m2 is estimated from the l-type doubling constant q2 through the following equation [24]: x2 ’
2:6B2e : q2
ð3Þ
Using the molecular constants in Table 1, the vibrational wavenumber is calculated to be x2 420 cm1. This agrees qualitatively with the results of photoelectron spectroscopy (360 cm1) [18] and theoretical calculations at various levels shown in Table 3. However, it is not consistent with the wavenumber of matrix-isolation infrared spectroscopy (917 cm1) [16]. In the light of these experimental and theoretical results, the assignment of the infrared band of 917 cm1 to the bending fundamental [16] seems to be inconsistent. This band might be assigned to the 2m2 band rather than to the m2 band (see below). A DFT calculation [17] predicted that the intensity of the m2 band is similar to that of the m3 band, but Manceron et al. observed no corresponding absorption in the predicted region near 400 cm1 as shown in Fig. 3 of [16]. This finding means that the intensity of the m2 band is much weaker than that of the m3 band. It is uncommon that the 2m2 band was observed whereas the m2 band was not, because the fundamental band should be much stronger than the overtone band. This anomaly can be explained by the low transition moment of the m2 band and the Fermi resonance between the m3 and 2m2 states. Strong Fermi interactions between the m3 and 2m2 states have often been reported for linear triatomic
molecules [24]. The ratio of the observed intensities of the 917 cm1 band to that of the m3 band, about 1:10 [16], is an acceptable value as a result of intensityborrowing due to the Fermi resonance. The lifting of the 2m2 state by this resonance is perhaps a part of the reason why the observed 2m2 band (917 cm1) is slightly higher than twice the estimated m2 value (about 420 cm1).
Acknowledgements The research was supported by Japan Society for the Promotion of Science through Grant-in-Aid for Scientific Research (Nos. 12740316 and 15656184). E.Y. thanks the Japan Science Society through the Sasagawa Scientific Research Grant and the Hayashi Memorial Foundation for Female Natural Scientists through the Hayashi Fellowship. T.O. thanks the Kawasaki Steel 21st Century Foundation for financial support. T.O. and E.Y. also acknowledge the financial support from the Hamamatsu Foundation for Science and Technology Promotion.
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