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Publisher’s version / Version de l'éditeur:

Canadian Journal of Physics, 79, 2-3, pp. 461-466, 2001

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Infrared spectrum of the CH3OH - CO complex in the C-O stretching

region

Xia, C.; McKellar, A

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Infrared spectrum of the CH

3

OH – CO

complex in the C–O stretching region

Changhong Xia and A.R.W. McKellar

Abstract: The infrared spectrum of the weakly-bound molecular complex methanol – carbon

monoxide has been observed, for the first time in the gas phase, by means of a parallel (1K = 0) band in the region of the carbon monoxide C–O stretching vibration. The band origin, 2154.5 cm−1, represents a blue shift of 11.2 cm−1relative to the free CO molecule; its proximity to that of the H2O – CO complex points to a similarity of the hydrogen bonding in the two systems. The observed structure of the band could be well reproduced by a simulation based on the methyl internal rotation component of A-symmetry. A component of E-symmetry must also be present, with roughly equal intensity, but the two components were not individually resolved. The observed transitions showed evidence of predissociation broadening, leading to an estimate of t & 0.5 ns for the lifetime of the upper (C–O stretching) state.

PACS Nos.: 3320E, 3425, 3520P, 3640

Résumé: Pour la première fois en phase gazeuse, nous avons observé le spectre infrarouge

du complexe méthanol – monoxyde de carbone par le biais d’une bande parallèle (1K = 0) dans la région de vibration par étirement du monoxyde de carbone C–O. L’origine de la bande à 2154,5 cm−1montre un déplacement vers le bleu de 11,2 cm−1par rapport à la molécule de CO. Sa proximité du complexe H2O – CO suggère une similarité de liaison hydrogène entre les deux systèmes. La structure du lien observé pourrait bien être reproduite par une simulation basée sur la composante de symétrie A de la rotation interne du radical méthyl. Une composante de symétrie E doit aussi être présente, avec à peu près la même intensité, mais nous n’avons pas réussi à séparer les deux composantes. Les transitions observées montrent un élargissement de ligne prédissociatif, menant à un estimé de t & 0,5 ns pour la demi-vie de l’état supérieur (étirement C–O).

1. Introduction

The spectroscopic study of weakly-bound molecular complexes in the gas phase provides a direct and precise method to probe intermolecular forces. The methanol – carbon monoxide complex was first studied in the microwave region by Lovas et al. [1], who noted the relevance of the forces in this system to the catalytic production of gasoline in the MTG (methanol to gasoline) process. Their microwave spectrum could be analyzed by regarding the complex as a near-prolate asymmetric rotor molecule, with two distinct sets of transitions corresponding to the A- and E-symmetry internal rotor components of the methyl top within the complex. The A-state transitions could be easily fitted in terms of a conventional

Received July 7, 2000. Accepted August 20, 2000. Published on the NRC Research Press Web site on Novem-ber 30, 2000.

C. Xia and A.R.W. McKellar.1Steacie Institute for Molecular Sciences, National Research Council of Canada,

Ottawa, ON K1A 0R6, Canada.

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462 Can. J. Physics Vol. 79, 2001

asymmetric rotor Hamiltonian, but the E-transitions required the use of a specialized internal rotation Hamiltonian. By analyzing six different isotopic forms of the complex, it was possible to derive an approximate molecular structure which involved a bent hydrogen bond to the CO carbon atom at a distance of about 2.41 Å from the hydroxyl hydrogen, and a near-planar configuration of all the heavy atoms. However, the large amplitude internal motions typical of weakly-bound complexes precluded the determination of a precise and unambiguous molecular structure.

In this paper, we report the first observation of an infrared spectrum for gas phase CH3OH – CO.

The band we have studied is that due to the C–O stretching vibration of carbon monoxide, which was

found to be centered at 2154.499 cm−1, representing a blue shift of 11.228 cm−1relative to the free

CO molecule. It exhibits a-type selection rules, with fully-resolved J-structure and partially resolved

K-structure. There is evidence for a small degree of predissociation broadening of the lines due to the

finite lifetime of the upper state. In many respects, this spectrum resembles that of the water – carbon monoxide complex as studied recently in the same region by Brookes and McKellar [2], except that its details are not as well resolved.

2. Experimental details

The spectra were obtained using a pulsed supersonic slit jet source and a tunable diode laser spec-trometer, as described previously [3–5]. Compared with earlier versions of this apparatus, the most significant recent modification involved replacement of the previous external mirror system, which gave 8–10 passes of the laser beam through the jet, with an internal astigmatic mirror system giving 182 passes. The new optical system, based on a New Focus Model 5612 Cell, increased the absorption path by a factor of about 20, and was also more stable and less prone to optical interference fringes.

The gas mixture for the jet expansion was made up of about 95% neon, 5% carbon monoxide, and a trace of methanol. The backing pressure was about 1.4 atm, and the slit opening was 0.1 × 20 mm in size. Methanol was introduced by passing the Ne + CO gas mixture over a reservoir containing liquid

CH3OH, which was held in a temperature controlled bath. We found that the optimum spectra were

obtained with a bath temperature of about –25◦C. At this temperature, the vapor pressure of methanol

is only about 0.007 atm, and, therefore, the fraction of methanol in the gas was approximately 0.5%. However, we had no direct measurement of this quantity. The reason for this low optimum methanol concentration is probably related to the balance in the jet expansion between formation of the desired

CH3OH – CO complex and the more strongly bound (CH3OH)2dimer.

3. Results and analysis

We anticpated that the nature of the hydrogen bonding in CH3OH – CO would be similar to that in

H2O – CO, and therefore began searching for the spectrum of the former in the region where the latter

was observed [2], namely, around 2154 cm−1. Indeed, the desired band was detected here with its center

at 2154.5 cm−1and an extent from about 2153 to 2156 cm−1. This spectrum is illustrated in the upper

trace of Fig. 1, where it should be noted that the strong feature which goes off scale near 2155 cm−1

is not due to CH3OH – CO, but rather is the R(2) transition of the CO monomer. The band in Fig. 1

has the appearance of an a-type (1Ka =0) parallel band of a near-prolate asymmetric rotor molecule,

with a mostly unresolved Q-branch at about 2154.5 cm−1flanked by resolved P- and R-branches. The

spacing of the P- and R-branch lines, about 0.12 cm−1, is just what is expected for CH

3OH – CO, for

which the known [1] ground-state value of (B + C) is 3639 MHz, or 0.1214 cm−1.

We first analyzed the spectrum considering only the A internal rotation component, for which an ordinary asymmetric rotor Hamiltonian sufficed in fitting the microwave spectrum [1], as noted above.

By using the known ground-state parameters for CH3OH – CO and varying the excited state parameters

ν0, A, and12(B +C), we could obtain a good simulation of the observed profile of the band.Also adjusted

in the fit were the effective rotational temperature and the line width of the individual transitions, which

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Fig. 1.Observed and simulated spectra of the weakly-bound complex CH3OH – CO in the region of the carbon monoxide C–O stretching vibration. The strong R(2) line of the CO monomer which goes off scale at 2154.596 cm−1 is not part of the spectrum of the complex. There are also a few weak and sharp observed lines whose identification is uncertain.

were assumed to have a Lorentzian shape. This simulated spectrum is shown in the lower trace of Fig. 1, the observed line positions and their assignments are listed in Table 1, and the parameters used in the simulation are listed in Table 2.

The simulation provides an excellent representation of the observed spectrum. The splitting into doublets of the P- and R-branch lines for J values above about five is revealed to be due to the combined effects of asymmetry doubling and a modest decrease in the A rotational constant between the ground and excited states. That is, the stronger component (at higher frequency) is due to the K = 0 transition and one of the K = 1 transitions, while the weaker one is due to the other K = 1 transition and the

K = 2 transitions (which are considerably weaker due to their unfavorable Boltzmann factor at the

low effective temperature of 3.1 K). It thus became evident that the two internal rotation components,

Aand E, were not resolved in the infrared spectrum. The most obvious discrepancies between observed

and calculated spectra occur in the Q-branch region, where the observed spectrum appears to have

rather stronger absorption than in the calculation, particularly for the peak at 2154.493 cm−1assigned

to K = 1. Unfortunately, some of this region is partly obscured by the strong R(2) CO monomer line

at 2154.596 cm−1.

While the success of the calculation is gratifying, it is also somewhat puzzling, since it includes only the A internal rotation component and neglects the E-component, which should make up approximately half the intensity of the spectrum. Lovas et al. [1] showed that the E-state microwave transitions of

CH3OH – CO occur quite close to the corresponding A-state transitions, and that they could not be fitted

in detail except by using a specialized internal rotor Hamiltonian. In our case, the fact that the simulated

A-state spectrum already accounts well for the observed spectrum strongly suggests that the rotational

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464 Can. J. Physics Vol. 79, 2001

Table 1. Observed features in the spectrum of the CH3OH – CO complex.

Observed Primary Obs. – Calc.

(cm−1) assignment (cm−1) Secondary assignments 2153.3772 817←918 –0.0003 82←92 2153.4029 808←909 0.0014 818←919 2153.5033 716←817 0.0014 72←82 2153.5273 707←808 0.0036 717←818 2153.6253 615←716 –0.0009 62←72 2153.6473 606←707 0.0013 616←717 2153.7513 514←615 0.0009 52←62 2153.7690 505←606 0.0008 515←616 2153.8761 413←514 0.0016 42←52 2153.8903 404←505 0.0000 414←515 2154.0008 312←413 0.0023 32←42 2154.0110 303←404 –0.0014 313←414 2154.1343 202←303 0.0000 212←313; 211←312; 22←32 2154.2561 101←202 0.0001 111←212; 110←211 2154.3784 000←101 0.0008 2154.4386 J,3←J,3 2154.4720 J,2←J,2 2154.4931 J,1←J,1 2154.7415 202←101 0.0005 212←111; 211←110 2154.8606 303←202 –0.0010 313←212; 312←211; 32←22 2154.9813 404←303 –0.0006 414←131; 413←312; 42←32 2155.0869 515←414 0.0002 52←42 2155.1022 505←404 0.0004 514←413 2155.2063 616←515 0.0017 62←52 2155.2225 606←505 0.0011 615←514 2155.3239 717←616 0.0017 72←62 2155.3417 707←606 0.0012 716←615 2155.4389 818←717 –0.0005 82←92 2155.4592 808←707 –0.0001 817←716 2155.5562 919←818 0.0000 92←82 2155.5786 909←808 0.0011 918←817

Notes:The quantum numbers given in the assignments are either (J′, K

a, Kc′) ← (J′′, Ka′′, Kc′′),

or else (J′, K

a) ← (J′′, Ka′′).

The primary assignments are those with the greatest calculated strength. The secondary assignments are other transitions which contribute to the observed feature.

The residuals (Obs. – Calc.) refer to the primary assignment, as calculated using the parameters of Table 2.

(C–O stretching) states. Unfortunately, since the spectrum is essentially accounted for already by the

A-component there is little additional information content relating to the E-component.

Part of the problem is due to the width of the observed lines, which was about 0.011 cm−1(FWHM).

This is considerably larger than the experimental resolution, which is about 0.003 cm−1or better. The

observed broadening of the CH3OH – CO lines must partly be due to the presence of the additional,

unresolved, E-state transitions not included in the simulation. But we conclude that the broadening is mostly due to predissociation — that is, to the finite lifetime of the excited (C–O stretching) state of the complex. Such lifetime broadening is possible in the infrared spectra of weakly-bound complexes

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Table 2.Parameters for CH3OH – CO as used in the simu-lation of Fig. 1.

Ground statea Excited stateb ν0(cm−1) 2154.4990(5) A(MHz) 28644.468 28453(7) 1 2(B + C) (MHz) 1819.509 1816(1) 1 4(B − C) (MHz) 27.678 27.678c T (K) 3.1(3)d 1σ(cm−1) 0.011(3)e aGround-state parameters from Table IV of ref. 1. These are

for the A internal rotation state of the complex. Centrifugal distortion parameters from the same source were also used in the simulation, with the excited state fixed to ground-state values.

bThe parameter uncertainties in parentheses are approximate

estimates.

cThis parameter was fixed to its ground-state value. dEffective rotational temperature in the simulated spectrum. eLorentzian line width (full width at half maximum) in the

simulated spectrum.

because the binding energy of the complex is usually much less than the vibrational energy of the state being excited. Often, however, the coupling between this vibration and the dissociative motion is very weak, leading to long lifetimes and sharp spectral features. In the present case, the relatively rich vibrational manifold of the methanol molecule itself provides more opportunities for coupling in

CH3OH – CO relative, for example, to H2O – CO, where no broadening was detected [2]. Our fitted

width of 0.011 cm−1corresponds to a time of about 0.5 ns, which should be considered as a lower limit

to the real dissociative lifetime.

4. Conclusions

A possible next step in the analysis of the present spectrum would be to simulate the E internal rotation transitions using the known [1] ground-state parameters and a specialized internal rotation fitting program [6,7]. Inclusion of the E-component might well improve the agreement of the observed and calculated spectra, especially in the K = 1 region of the Q-branch where the agreement is presently poorest. The effects of internal rotation on the E-component will be to modify the energy levels and selection rules so that they resemble more closely those of a symmetric rotor at lower J-values. This will tend to give a sharper and less widely split K = 1 Q-branch, just as observed. However, we consider that there is little point in carrying out further calculations since there is virtually no additional information to be gained from the present spectrum. The main problem is the fact that, due to lifetime broadening effects, we do not individually resolve the A- and E-transitions in the spectrum. However, what is clear is that the A- and E-states must have very similar band origins, and show a similar variation of rotational parameters between the ground and excited states (see Table 2).

In conclusion, an infrared spectrum of the weakly-bound complex CH3OH – CO has been observed

for the first time by means of a parallel (1K = 0) band in the region of the C–O stretch of carbon monoxide. The spectrum could be successfully modeled on the basis of its A internal rotation component alone, indicating that the additional effects of the E-component are very similar. A small decrease (3.5 ±

1 MHz) was noted in the rotational parameter 1

2(B + C)upon vibrational excitation, together with a

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466 Can. J. Physics Vol. 79, 2001

indicated that predissociation lifetime of the upper C–O stretching state is &0.5 ns. The close proximity

of the observed CH3OH – CO band origin to that of the H2O – CO complex indicates that the nature

and strength of the hydrogen bonding is similar for the two systems.

Acknowledgments

This paper is dedicated to the memory of Gerhard Herzberg. The apparatus used here was originally set up by Matthew Brookes, with refinements in the data acquisition by James Anstey and Bryan Fulsom. We are grateful to them, to Michael Barnett for continuing technical support, and to Frank Lovas and Rick Suenram for useful communications. We also thank a referee for interesting comments on the effects of internal rotation on the E-state energy levels and selection rules.

References

1. F.J. Lovas, S.P Belov, M.Y. Tretyakov, J. Ortigoso, and R.D. Suenram. J. Mol. Spectrosc. 167, 191 (1994). 2. M.D. Brookes and A.R.W. McKellar. J. Chem. Phys. 109, 5823 (1998).

3. M.D. Brookes and A.R.W. McKellar. J. Chem. Phys. 111, 7321 (1999).

4. J.A. Anstey, M.D. Brookes, and A.R.W. McKellar. J. Mol. Spectrosc. 194, 281 (1999). 5. C. Xia, A.R.W. McKellar, and Y. Xu. J. Chem. Phys. 113, 525 (2000).

6. I. Kleiner, M. Godefroid, M. Herman, and A.R.W. McKellar. J. Mol. Spectrosc. 142, 238 (1990). 7. I. Kleiner, J.T. Hougen, R.D. Suenram, F.J. Lovas, and M. Godefroid. J. Mol. Spectrosc. 148, 38 (1991);

153, 578 (1992).

Figure

Fig. 1. Observed and simulated spectra of the weakly-bound complex CH 3 OH – CO in the region of the carbon monoxide C–O stretching vibration
Table 1. Observed features in the spectrum of the CH 3 OH – CO complex.
Table 2. Parameters for CH 3 OH – CO as used in the simu- simu-lation of Fig. 1.

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