Elsevier

Chemical Physics Letters

Volume 731, 16 September 2019, 136610
Chemical Physics Letters

Research paper
The most stable isomer of H2C4-(OCS)2 van der Waals complex: Theory and experiment agree on a structure with C2 symmetry

https://doi.org/10.1016/j.cplett.2019.136610Get rights and content

Highlights

  • The infrared spectrum of H2C4-(OCS)2 trimer around 4 μm is studied.

  • The trimer has C2 point group symmetry.

  • Theoretical calculations confirm that the observed structure is the most stable form.

Abstract

We report the infrared spectrum of H2C4-(OCS)2 trimer in the region of the ν1 fundamental vibration of the OCS monomer. The van der Waals complexes are generated in a supersonic slit-jet apparatus and probed using a rapid-scan tunable diode laser. Both H2C4-(OCS)2 and D2C4-(OCS)2 are studied. Analysis of their spectra establishes that the trimer has C2 point group symmetry. Theoretical calculations performed to find stationary points on the potential energy surface confirm that the observed structure is the most stable form. The experimental rotational parameters are in very good agreement with those computed using double hybrid functionals.

Introduction

Mixed dimers and trimers formed from OCS and the first member of the polyyne series, acetylene, have been studied extensively by high resolution spectroscopy. Two isomers of the mixed dimer H2C2-OCS were first studied by Peebles and Kuczkowski using microwave spectroscopy [1], [2]. One of them [1] was observed to be planar and near-parallel analogous to the observed H2C2-N2O and H2C2-CO2 dimers. The other one [2] was also planar but T-shaped similar to acetylene dimer. The T-shaped isomer has C2v symmetry, with OCS forming the stem of the T, and the S atom in the inner position. Infrared spectra of both dimers were later studied in the OCS ν1 region [3] and they showed rather different vibrational shifts, a small red shift (−0.3 cm−1) for the near-parallel form and a larger red-shift (−5.7 cm−1) for the T-shaped form.

The first identification of the mixed trimer H2C2-(OCS)2 was also made by Peebles and Kuczkowski [4]. This trimer (isomer b) was found to possess a twisted barrel shaped structure similar to (OCS)3 [5] and (N2O)3 [6]. It has a polar OCS dimer unit, hence its rotational spectrum was studied by means of microwave spectroscopy. However, the pure rotational spectrum of the ground state isomer of H2C2-(OCS)2 with a non-polar OCS dimer unit could not be detected due to a very small permanent dipole moment which could arise only from induced effects. This lowest energy isomer (isomer a) was later observed in the infrared region [7]. It has a C2 symmetry axis perpendicular to and passing through the center of mass of C2H2, in contrast to isomer b which has no symmetry. Two fundamental bands for isomer a were observed in the OCS ν1 region. One of the bands is a relatively strong c-type band associated with the out-of-phase vibration of the OCS monomers. The other band, with b-type selection rules, arises from their in-phase vibration and would have zero intensity in the planar limit. Observation of a b-type band thus establishes the nonplanarity of the OCS dimer unit within the trimer.

A related trimer is (C2H2)2-OCS. Two distinct isomers of this trimer have been observed. First is a barrel-shaped isomer whose precise structure is still unclear [8]. This uncertainty is probably due to the presence of large amplitude intermolecular motions. The second isomer is a planar form with the C2H2 monomers in a nearly T-shaped orientation, like C2H2 dimer, and OCS approximately parallel to the ‘stem’ of the T [9]. The energy ordering of the two isomers has not been well established.

Much less is known on complexes containing OCS and the second member of the polyyne series, diacetylene. The only study is that of the mixed dimer of H2C4-OCS in the infrared region reported recently by our group [10]. Like H2C2-OCS, H2C4-OCS was found to have a planar structure with nearly parallel monomer units, and with the help of theoretical calculations, it was established that the observed structure is the lowest energy form on the potential energy surface.

In the present letter, we report observation of a fundamental band for H2C4-(OCS)2 in the region of the ν1 fundamental vibrations of the OCS monomer. This is a hybrid band with a very strong c-type component accompanied by a much weaker a-type. The trimer observed here has C2 point group symmetry with the C2 symmetry axis perpendicular to and passing through the center of mass of HCCCCH. We also carried out various levels of theoretical calculations in support of our experimental findings. The theoretical calculations confirm that the observed trimer is the most stable form and has an OCS dimer unit which is planar and non-polar. This is in contrast to H2C2-(OCS)2 where the OCS dimer has a substantial dihedral angle giving rise to a b-type band due to the in-phase vibration of the OCS monomers. In addition to the normal isotopologue, we also study a band for D2C4-(OCS)2. The results were found to be consistent with those of the normal species. The experimental rotational parameters are in very good agreement with those computed using double hybrid functionals.

Section snippets

Computational details

The experimental investigation was supported by theoretical calculations to characterize the different minima on the potential energy surface (PES) of H2C4-(OCS)2, and for determining their relative binding energies. Employing different basis sets, several computational approaches were used, from density functional theory (DFT) to wavefunction-based methods, to the coupled cluster (CC) level of theory. When modelling molecular complexes, the dispersion forces must be included in the DFT

Results and analysis

The spectra were obtained using a pulsed supersonic slit-jet expansion of a dilute mixture of C4H2 (0.3%) and OCS (0.1%) in helium carrier gas, with a backing pressure of about 8 atmospheres. All spectra were recorded by direct absorption using a rapid-scan tunable diode laser spectrometer, as described previously [37]. The diacetylene was synthesized by the procedure described in Ref. [38]. DCCCCD was obtained by mixing HCCCCH with a 1 M solution of NaOD in D2O as described by Etoh et al. [39]

Discussion and conclusion

Comparison of the theoretical equilibrium rotational constants for H2C4-(OCS)2, columns 2 in Table 1 and columns 3 in Table 2, with the experimental rotational constants, column 2 in Table 2, clearly shows that the observed trimer corresponds to the lowest energy isomer. This is not surprising considering that the jet conditions employed in this work favor the formation of the ground state structure. The comparison with the computed rotational constants augmented by vibrational corrections

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. One of the author (A.C.P.) gratefully acknowledges financial support by University Ca’ Foscari Venezia (ADiR funds), and the computational facilities provided by CINECA (grant no. HP10C1RAPR) and by SCSCF (”Sistema per il Calcolo Scientifico di Ca’ Foscari”, a multiprocessor cluster system owned by Universita’ Ca’ Foscari Venezia).

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