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Chemical Physics Letters 396 (2004) 150–154

Bending vibration of platinum monocarbonyl PtCO: observationof 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 cm�1, which resolved the reported discrepancy between the previous matrix-infrared and theoretical

estimates.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Carbon monoxide adsorbed on the surface of plati-

num metal is an important chemical system in several as-pects [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 suchsystems 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 espe-

cially been studied in great detail as a benchmark mole-

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.08.040

* Corresponding author.

E-mail address: sctokab@ipc.shizuoka.ac.jp (T. Okabayashi).

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 deter-

mined 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 tran-

sitions 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 spe-

cies 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 ofquantum-chemical calculations agreed basically with

306284 306290(MHz)

194PtCO ν2(e)

J=46–45

Fig. 1. Rotational transition of 194PtCO in the m2 vibrational state.

E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154 151

the experimental values reported by Evans and Gerry

[20]. The theoretical wavenumbers of the m1 (�2000

cm�1) and m3 (�600 cm�1) stretching vibrations also

agreed well with the experimental values. However, the

theoretical value of the m2 bending mode (�400 cm�1)

deviated significantly from the experimental value(917 cm�1) [16]. No absorption feature due to PtCO

was observed around 400 cm�1, 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 cm�1.

We have applied microwave spectroscopy in the pre-sent study to elucidate this discrepancy by determination

of the l-type doubling constant q in the m2 excited vibra-

tional state. Our estimate of the bending frequency has

confirmed that predicted by theoretical calculations.

2. Experimental

The present experiment was carried out using a

source-modulated microwave spectrometer [23]. Milli-

meter- and submillimeter-wave radiations were gener-

ated by frequency-multipliers driven by klystrons. The

radiation transmitted through a free space cell was de-

tected by an InSb bolometer cooled by liquid helium.

The cell was equipped with a pair of cylindrical elec-

trodes for a dc glow discharge and was covered by acooling 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 conditionwas determined by monitoring the line-intensity in the

ground state. Transition frequencies in the ground state

were predicted using the molecular constants deter-

mined 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 wasset 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 ob-

served on a cathode-ray oscilloscope without data accu-

mulation. Weak doublet lines in the excited vibrational

state (v2 = 1) were also detected by carrying out a careful

examination with data accumulation. The line-intensityin the m2 state was about 10 times weaker than that of

the ground state lines. Fig. 1 displays a typical spectrum

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.

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 for-

mula for a linear molecule,

Ev;J ¼ Bv½JðJ þ 1Þ � l2� � Dv½JðJ þ 1Þ � l2�2

� 12½qv þ qvJ JðJ þ 1Þ�JðJ þ 1Þ; 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, ac-

counts for the l-type doubling in the m2 state. The qv va-lue of a linear triatomic molecule usually has a positive

value, and the + and � signs in Eq. (1) correspond to thef and e levels, respectively. Analysis of our millimeter-

and submillimeter-wave data combined with the micro-

wave data from [20] led to the molecular constants listed

in Table 1. The observed rotational transition frequen-

cies 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 excitedvibrational state. The harmonic vibrational wavenum-ber of the lowest stretching vibration m3 (Pt–C str.) is

estimated by

Table 2

Observed transition frequencies of PtCO in MHza

J 0–J00 l 194PtCO 195PtCO 196PtCO 198PtCO

Ground

1–0 0 6649.7173(�7)b 6645.6647(�7)b 6641.6606(0)b 6633.7609(�11)b

2–1 0 13299.4251(�1)b 13291.3200(1)b 13283.3101(�2)b 13267.5141(9)b

3–2 0 19949.1101(�4)b 19936.9527(0)b 19924.9387(4)b 19901.2421(�6)b

29–28 0 192797.614(0) 192680.140(8) 192564.051(0)

30–29 0 199442.590(�6) 199200.966(�20)

34–33 0 226019.162(16) 225881.433(6) 225745.364(6)

35–34 0 232662.391(6) 232520.623(3) 232380.555(1) 232104.256(�27)

36–35 0 239305.242(�2) 239159.438(5) 239015.367(�4) 238731.224(7)

37–36 0 245947.722(12) 245797.851(�2) 245649.799(3) 245357.779(19)

38–37 0 252589.760(�13) 252435.866(�6) 252283.833(13) 251983.915(12)

43–42 0 285793.679(14) 285619.527(�17) 285447.530(3) 285108.213(�7)

44–43 0 292433.066(�16) 292254.912(�7) 292078.914(3) 291731.741(12)

45–44 0 299072.015(�5) 298889.805(�11) 298709.812(�5) 298354.760(�1)

46–45 0 305710.466(�3) 305524.236(14) 305340.224(�9) 304977.295(�10)

47–46 0 312348.420(4) 312158.140(12) 311970.154(4) 311599.348(�2)

m236–35 1e 239757.953(2) 239611.937(21) 239467.645(18)

37–36 1e 246412.886(�12) 246262.792(�21) 246114.500(�19)

38–37 1e 253067.449(14) 252913.303(4) 252761.005(4)

44–43 1e 292985.399(�13) 292806.970(�12) 292630.640(�26)

45–44 1e 299636.765(2) 299454.293(6) 299273.979(11)

46–45 1e 306287.623(7) 306101.080(�13) 305916.791(18)

47–46 1e 312938.015(57)c 312747.404(15) 312559.061(�7)

36–35 1f 239921.880(5) 239775.636(9) 239631.150(�6)

37–36 1f 246581.358(�7) 246431.050(�11) 246282.557(�23)

38–37 1f 253240.452(8) 253085.994(�90)c 252933.619(25)

44–43 1f 293185.652(�3) 293006.981(12) 292830.457(26)

45–44 1f 299841.537(�5) 299658.798(�7) 299478.251(�8)

46–45 1f 306496.910(�19) 306310.138(�2) 306125.573(�15)

47–46 1f 313151.826(22) 312961.032(68)c 312772.407(1)

a Values in parentheses represent the residuals (Obs. � Calc.) to the last digits.b Cited from [20]. Frequencies of 195PtCO are calculated values without hyperfine splitting.c Excluded from the fit.

Table 1

Molecular constants of PtCOa

194PtCO 195PtCO 196PtCO 198PtCO

This work

B0 (MHz) 3324.859918(95) 3322.833613(94) 3320.831201(76) 3316.88191(15)

D0 (kHz) 0.453696(29) 0.453251(28) 0.452652(23) 0.451608(44)

B2 (MHz) 3332.30874(24) 3330.27764(29) 3328.27149(35)

D2 (kHz) 0.462874(66) 0.462347(79) 0.462012(93)

q2 (MHz) 2.27923(48) 2.27616(59) 2.27361(69)

q2J (Hz) �0.97(13) �0.92(16) �0.92(19)

Previous workb

B0 (MHz) 3324.85989(43) 3322.83356(31) 3320.83107(43) 3316.88224(43)

D0 (kHz) 0.455(28) 0.450(20) 0.442(28) 0.474(28)

a Values in parentheses represent 1 SD.b [20].

152 E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154

x3 ’4B3

e

De

� �1=2

; 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 cm�1. This is in good agreement with the

previous estimates by FTMW (605 cm�1) [20], matrix-

isolation infrared spectroscopy (581 cm�1) [16], photo-

Table 3

Comparison of harmonic vibrational wavenumbers of PtCO in cm�1

x1 x2 x3 Ref.

Experimental

mmW 420 600 This work

FTMW 605 [20]

matrix IR 2052a 917a 581a [16]

PE 2040a 360a 550a [18]

Theoretical

B3LYP 2114 407 585 [18]

MP2/LanL2DZ 2047 441 636 [16]

MP2/Stoll 2042 429 618 [16]

QCISD/Stoll 2124 415 565 [16]

B3LYP/Stoll 2119 395 577 [16]

B3LYP 2121 405 590 [17]

GVB(6/12)-PP 1976 561 600 [10]

SCF 2157 550 527 [6]

a Effective values including anharmonic terms.

E. Yamazaki et al. / Chemical Physics Letters 396 (2004) 150–154 153

electron spectroscopy (550 cm�1) [18], and theoretical

calculations as summarized in Table 3. Since the dia-

tomic 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 bend-

ing vibration m2 is estimated from the l-type doublingconstant q2 through the following equation [24]:

x2 ’2:6B2

e

q2: ð3Þ

Using the molecular constants in Table 1, the vibra-tional wavenumber is calculated to be x2 � 420 cm�1.

This agrees qualitatively with the results of photoelec-

tron spectroscopy (360 cm�1) [18] and theoretical calcu-

lations at various levels shown in Table 3. However, it is

not consistent with the wavenumber of matrix-isolation

infrared spectroscopy (917 cm�1) [16]. In the light of

these experimental and theoretical results, the assign-

ment of the infrared band of 917 cm�1 to the bendingfundamental [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 Manc-

eron et al. observed no corresponding absorption in the

predicted region near 400 cm�1 as shown in Fig. 3 of

[16]. This finding means that the intensity of the m2 bandis much weaker than that of the m3 band. It is uncom-

mon that the 2m2 band was observed whereas the m2band 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 m2band and the Fermi resonance between the m3 and 2m2states. 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 cm�1 band to that of the m3 band, about 1:10

[16], is an acceptable value as a result of intensity-

borrowing 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 cm�1) is

slightly higher than twice the estimated m2 value (about

420 cm�1).

Acknowledgements

The research was supported by Japan Society for the

Promotion of Science through Grant-in-Aid for Scien-

tific Research (Nos. 12740316 and 15656184). E.Y.

thanks the Japan Science Society through the SasagawaScientific 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 Technol-

ogy Promotion.

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