Rotational Spectrum of 1,3-Dioxolane–Argon

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

184, 145–149 (1997)

MS977311

Rotational Spectrum of 1,3-Dioxolane–Argon Assimo Maris, Adolfo C. Fantoni,* Walther Caminati, 2 and Paolo G. Favero Dipartimento di Chimica ‘‘G. Ciamician,’’ Universita` di Bologna, Via Selmi 2, I-40126 Bologna, Italy Received January 13, 1997; in revised form March 28, 1997

The free jet millimeter wave spectrum of 1,3-dioxolane–argon has been investigated in the 60–78 GHz frequency range. The argon atom is located over the center of mass of the monomer, roughly perpendicular to the 1,3-dioxolane ring. Very high values of quartic centrifugal distortion constants, compared to those of similar adducts, suggest a relatively weak linkage of Ar to the ring. q 1997 Academic Press INTRODUCTION

Although several van der Waals complexes of a rare gas atom with a ring molecule have been investigated by free jet techniques, only recently a relatively simple technique as the supersonic jet millimeter-wave absorption spectroscopy has been applied to the study of the rotational spectra of this kind of adducts (1–5). Molecular beam Fourier transform microwave (MBFTMW) spectrometers had previously been used (6–23). In most of these cases the ring molecule linked to the rare gas atom was an aromatic molecule. Let us now consider the interaction and trend to form complexes of Ar with five membered ring molecules containing one oxygen atom. The furan–Ar adduct was observed rather early by Kukolich and Campbell (6, 8), and later some isotopic species were investigated by Spycher et al. (17). The argon atom was located perpendicular to the ˚ , almost over its center of ring at a distance of 3.480 A ˚ towards the oxygen. A similar mass, but shifted about 0.3 A situation has been found for 2,5- and 2,3-dihydrofuran–Ar complexes (3, 4). The configuration of tetrahydrofuran–Ar was quite different, with the rare gas atom no longer over the plane of the ring, but in proximity to the oxygen atom. For this last molecule each rotational transition was split into two component lines, probably due to the residual effects of the ring pseudorotation in the complex (5). All these investigations suggest that argon is attracted to sites with high electronic density, such as double bonds and oxygen atoms. At this point we considered interesting to study the complex 1,3-dioxolane–Ar in order to investigate (i) the effect of replacing a second CH2 group with an oxygen; (ii) if, similarly to tetrahydrofuran–Ar, 1,3-dioxolane–Ar would exhibit doublings due to the pseudorotation of the ring. 1 Permanent address: PROFIMO, Departamento de Fisica, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina. 2 To whom correspondence should be addressed.

Here we report the results of this investigation. Figure 1 shows the monomer (DIOX) and the adduct (DIOX–Ar) and the axis inversion that takes place upon formation of the adduct. EXPERIMENTAL PART

The Stark and pulse modulated free jet absorption millimeter-wave spectrometer has been described elsewhere ( 1, 24 ) . A sample of DIOX was purchased from Aldrich and used without further purifications. The sample seeded in argon (approximately 1% concentration) at 07C and at a stagnation pressure of approximately 1 bar was expanded to about 5 1 10 03 mbar. ANALYSIS OF THE ROTATIONAL SPECTRUM

Trial calculations of the rotational constants of the complex have been based on the molecular model proposed in Fig. 1, that is, the Ar atom being roughly perpendicular to the quasi-plane of DIOX and over its center of mass. The r0 geometry of DIOX, obtained from the rotational constants of Ref. (25), has been assumed to be unaltered in the complex. In going from the molecule to the adduct, an inversion of the principal axis of inertia with respect to the ring skeleton takes place (see Fig. 1), so that the rotational spectrum of the adduct results to be predominantly mc-type. The existence of a small ma component, in turn, favors the Stark modulation of the rotational transitions. Five groups of evenly spaced rotational lines have been identified. They have been found to correspond to the K *01 – K 901 Å 9–8, 8–7, 7–6, 6–5, and 5–4 R-type families, with J from 8 to 18. The K/1 value is not specified since each line is due to the overlapping of two transitions connecting doubly asymmetry degenerate levels. The regular spac-

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FIG. 1. (a) Principal axes system in DIOX; (b) Principal axes system in DIOX–Ar.

ing, corresponding to (B / C), was in agreement with the proposed model Besides the mentioned transition progressions, several weaker lines with lower K01’s, resolved in asymmetry doublets, have been measured, up to a total of 41 transitions, listed in Table 1. They have been fitted with the Watson reduced axis Hamiltonian (26) giving the spectroscopic constants reported in Table 2. Two sextic terms were required to improve the quality of the fit. Since the complex is an almost prolate symmetric top, the I r representation and the S-reduction have been chosen. VIBRATIONS AND rs LOCATION OF THE ARGON ATOM

the determination of the potential energy parameters of the van der Waals motions. Nevertheless if we compare the three main quartic centrifugal distortion parameters, DJ , DJK , and DK , to those of the five-membered ring molecules containing one oxygen, we can see that these values suggest that DIOX–Ar is much floppier than the other complexes (see Table 3). The force constant of the stretching is directly related to DJ for a planar monomer with symmetry D2h (28). This would also be valid for DIOX–Ar when DIOX was assumed to be planar and the argon to be over the center of mass, and since furan–Ar and DIOX–Ar have similar values for DJ and the rotational constants, they should have similar values also for the stretching force constants. The equation kr Å 8p 2m/ DJ[B 3DIOX – Ar (1 0 b) / C 3DIOX – Ar (1 0 c)]

The rotational spectra of this kind of complex supply information on the potential energy surfaces concerning the three vibrations which arise from the three translational degrees of freedom of isolated argon upon its attachment to the monomer. For adducts with Cs or higher symmetry several procedures have been considered for this purpose. The rotational frequencies can be directly fitted to determine the potential energy parameters (3, 4, 21, 27), or centrifugal distortion constants can be interpreted in terms of these parameters (22, 23). Alternatively the changes of second moments of inertia upon attachment of the Ar can be used (2). In all cases the geometries of the monomers have been assumed unaltered in the complex, and the centrifugal distortion effects due to motions in the monomer sub-unit have been neglected in the complex. In the case of DIOX–Ar the monomer unit does not have any symmetry and undergoes large amplitude motions, so that none of the above-mentioned methods is suitable for

is adapted for DIOX–Ar, and can be used provided that b is defined as b Å (BDIOX – Ar /BDIO X ), instead as in Eq. (22) of Ref. (28). An analogous expression holds for c. There m Å (MDIOX MAr )/(MDIOX / MAr ), and DJ is the quartic centrifugal distortion parameter, numerically coinciding with DJ . Applying Eq. (1) to DIOX– Ar we implicitly assume that the Ar atom vibrates on the z axis. The obtained approximated value of 2.26 N/m is quite smaller than that of furan– Ar (3.07 N/m, Ref. 23) The two remaining motions can be seen as bendings of the Ar atom considered as attached to the center of mass of the monomer, or, alternatively, as Ar motions in a plane parallel to xy: the two ‘‘pictures’’ do not differ much from each other. Information on these motions is obtained, in a first approximation, from the DJK and DK parameters. Their values for DIOX–Ar are one order of magnitude larger than those for furan–Ar, suggesting that in the former case Ar is much more free to move parallel to the ring.

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1,3-DIOXOLANE–ARGON

TABLE 1 Frequencies of all Measured Transitions of DIOX–Ar (MHz)

as shown in Table 3, where the values for the related molecules are reported also. This effect could be due to a displacement of the argon atom from the z axis at equilibrium or to vibrational effects of the two argon bending modes. Provided that the force constants of the two bendings are very similar to each other, the Coriolis coupling effects between the two vibrations can be neglected (2), and then DMxx and DMyy are due to the mass distribution associated to the vibrations. A comparison of DMxx to the values of furan- and 2,5dihydrofuran–Ar indicates that the mass distribution is more broadened for DIOX–Ar. The same comparison between the DMyy values suggests that the displacements from the center of mass of the monomer along the y direction is smaller for DIOX–Ar. This is plausible since the two oxygen atoms are very close to the x axis in DIOX. Comparisons with 2,3-dihydrofuran–Ar and tetrahydrofuran–Ar are more difficult since 2,3-dihydrofuran does not have any symmetry and tetrahydrofuran–Ar behaves in a completely different way. The DMzz values are also given in Table 3. They mainly depend on the Ar–ring distance. From the DMgg values, following Jochims et al. (12), it is possible to obtain the rs coordinates (29) of the Ar atom assuming that a hypothetical atom of zero mass is replaced by an argon atom in going from DIOX to DIOX–Ar. Due to the large amplitude Ar vibrations only crude values can be obtained. In the case of DIOX–Ar É xÉ Å 0.468, É yÉ Å 0.456, ÉzÉ Å 3.457, and ÉrÉ (distance from the center of

TABLE 2 Spectroscopic Constants of DIOX–Ar

This is confirmed by the changes of the planar moments of inertia along the x, y, and z directions in going from the monomer to the adduct (see Fig. 1). The planar moments of inertia, defined as Maa Å (h/16p 2 )( 01/A / 1/B / 1/C), etc.,

[2]

satisfactorily account for the mass extension along a given axis. Considering the rotation of axis which occurs in going from the monomer to the complex (see Fig. 1), we should have, in the limit of a rigid molecule with the argon atom on the perpendicular to the xy plane through the center of mass, Maa (monomer) Å Mbb (complex) (from now on Å Mxx ), and Mbb (monomer) Å Mcc (complex) (from now on Å Myy ). These values are actually consistently smaller for the complex; that is, DMgg Å Mgg (DIOX–Ar) 0 Mgg (DIOX) õ 0

[3] (g Å x, y),

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TABLE 3 Comparison of the Main Quartic Centrifugal Distortion Parameters of DIOX–Ar, Shifts of Planar Moments of Inertia in Going from DIOX to DIOX–Ar, and rs Coordinates of Ar in DIOX–Ar to Those of Similar Complexes

˚ , respectively. Comparing mass of the monomer) Å 3.519 A the values of DIOX–Ar once again to those of furan- and 2,5-dihydrofuran–Ar, we can see that É xÉ, indicative of the amplitude of the corresponding bending, is much larger for DIOX–Ar, while É yÉ, which accounts for the shift from the perpendicular to the center of mass of the monomer, is smaller. All this is in agreement with a larger amplitude of the motions in the x and y directions and with the equilibrium value of y close to zero. Since isolated DIOX adopts a puckered configuration (25, 31) two conformers would be expected for DIOX–Ar. Probably because of relative stability reasons we observed only one of them. We think that the Ar atom is located in the concave side of DIOX because, in spite of the weaker linkage, the coordinate ÉzÉ of Ar in DIOX–Ar is smaller than those of the related complexes furan–Ar and 2,5- and 2,3dihydrofuran (see Table 3). CONCLUSIONS

We showed that free jet millimeterwave absorption spectroscopy is a suitable tool for the observation and investigation of van der Waals complexes, even when weakly bounded, as it is the case for DIOX–Ar. In the specific case of DIOX–Ar we found that, rather surprisingly, the introduction of a second oxygen into the ring decreases the stability of the complex. Furthermore, we did not observe for DIOX–Ar the doubling of the lines already observed for tetrahydrofuran–Ar. Since the ground state splittings due to the pseudorotation are 21 307.71 and 64 840.23 MHz for tetrahydrofuran (30) and DIOX (31), respectively, we would have expected the doubling in the complex to be more pronounced for DIOX–Ar. Probably the different position of Ar in the two complexes may have

an important effect on the coupling between the pseudorotation and overall rotation angular momenta. ACKNOWLEDGMENTS We thank Mr. A. Millemaggi for technical help and the Ministero dell’ Universita` e della Ricerca Scientifica e Tecnologica and the C.N.R. for financial support.

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