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The N2O-CS2 dimer is cross-shaped

The infrared spectrum of the cross-shaped van der Waals complex N2O-CS2 is observed in the region of the N2O nu1 fundamental band (~2220 cm-1) using a tuneable diode laser to probe a pulsed supersonic slit jet expansion. Both 14N- and 15N-substituted species are studied. Analysis of their spectra establishes that this dimer has a cross-shaped structure, similar to its isoelectronic cousin CO2-CS2. This is the first spectroscopic observation of N2O-CS2, and the molecular parameters determined here should be useful for detection of its pure rotational microwave spectrum.


Introduction
It has been found that most weakly-bound dimers containing small linear polyatomic monomers have planar structures [1], following from the prototypical case of the CO2 dimer [2,3]. Thus it was a bit of a surprise when, in 1998, Dutton et al. [4] showed that the CO2 -CS2 dimer was non-planar and cross-shaped, as viewed along the intermolecular axis. More recently, similar cross-shaped structures have been reported as well for the CS2 dimer [5] and for one isomer of OCS -CS2 [6]. In the present paper, we show that N2O -CS2 is also cross-shaped (see Fig. 1) by analysing its infrared spectrum in the region of the N2O ν1 fundamental band.

Observed spectra
A pulsed supersonic slit jet apparatus was used with a lead salt tuneable diode laser as probe [1]. The expansion gas was a very dilute mixture of about 0.15% N2O plus 0.3% CS2 in helium. The backing pressure of about 10 atmospheres resulted in an effective rotational temperature around 2.5 K. Wavenumber calibration was made using a fixed etalon and a room temperature N2O reference gas cell. The PGOPHER computer package was used for spectral simulation and fitting [7].
The N2O and CO2 molecules are rather similar in terms of mass and intermolecular forces. We thus expected N2O -CS2 to have a similar cross-shaped structure, similar rotational parameters, and a similar c-type band as compared to CO2 -CS2 [4,8]. This was indeed the case, but there is an important difference between the Part of the observed N2O -CS2 spectrum is shown in Fig. 2. A strong central Q-branch at 2223.23 cm -1 is flanked by weaker P-and R-branches. The band is similar to that of CO2 -CS2 (see Fig. 2 of [8]), but denser and more symmetric due to the lack of missing levels. We had relatively little trouble assigning the spectrum since the rotational parameters were indeed similar to those of CO2 -CS2. However, there were some discrepancies in the analysis which we attributed to upper state perturbations. For this reason, we decided to analyse the ground state first using combination differences from the spectrum, so that it would not be contaminated by  Table 1. It should be noted that the 1 uncertainties given here for the upper state are probably unrealistically small since the lower state parameters were fixed in the infrared fit.
We were also able to observe the analogous band for 15 N2O -CS2 using an enriched sample of 15 N2O, and the central part of this spectrum is illustrated in Fig. 3.
Details of the analysis and fit were very similar to those of the normal isotopologue, and results are given in Table 1    are roughly analogous.

Discussion and conclusions
As shown in Table 2, the observed band origins of N2O -CS2 are shifted by only a relatively small amount (-0.5 cm -1 ) from those of the free N2O monomer, showing that the presence of the nearby CS2 molecule has little effect on the frequency of the N2O 1 mode. Interestingly, a considerably larger red shift of -2.6 cm -1 is observed for CO2 -CS2 [4,8] in the somewhat analogous CO2 3 mode.
We can also compare the rotational parameters of N2O -CS2 and CO2 -CS2, as shown in Table 1. Looking at 14 N2O -CS2, we find that the rotational constants A, B, and C are increased by 1.36, 2.21, and 1.02 %, respectively, relative to 12 C 16 O2 -CS2 (which has the same reduced mass). The increases are partly due to the fact that the B-value of N2O itself is larger than that of CO2 by 7.4%. But, in addition, it turns out that the intermolecular bond is slightly shorter for N2O -CS2. Determining this bond length as described in [4], and averaging the slightly different values obtained using B" or C", we obtain values of 3.368 and 3.367 Å for 14 N2O -CS2 and 15 N2O -CS2, respectively, as compared to 3.392 Å for CO2 -CS2 [4].
But there is an additional structural parameter to determine in the case of N2O -CS2, namely the angle between the intermolecular axis (connecting the monomer centers of mass) and the N2O (or CO2) axis. For CO2 -CS2, we know by symmetry that this angle has an equilibrium value of 90º, but for N2O -CS2 this is not necessarily the case. If CO2 -CS2 had a rigid cross-shaped structure, then its A-value It should be possible in the future to observe the pure rotational microwave spectrum of N2O -CS2 and thus obtain more precise rotational parameters and possibly improved structural data. We would expect c-type transitions arising from the small permanent dipole moment of N2O, as well as possible a-type transitions arising from a small induced dipole. The expected transition frequencies can be easily calculated from our ground state parameters in Table 1. One problem is that the intensity of each transition is likely to be spread out among many components due to 9 resolved hyperfine splitting in 14 N2O -CS2. In this respect, 15 N2O -CS2 would be more promising, if an enriched sample is available.
In conclusion, the weakly bound van der Waals complex N2O -CS2 has been observed spectroscopically for the first time by means of its infrared spectrum in the region of the N2O 1 stretch. Both 14 N2O-and 15 N2O-containing dimers were studied, and the structure was shown to be cross-shaped and very similar to that of the isoelectronic species CO2-CS2. The ground state parameters determined here should be useful for future observation of the microwave spectrum of N2O -CS2.