High-resolution FTIR spectroscopy of HNSO—Analysis of the highly perturbed ν4, ν6 and 2ν5 bands

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Abstract

We report a rovibrational analysis of the ν4 and ν6 fundamentals and the 2ν5 overtone of HNSO from high-resolution Fourier transform infrared spectra. The ν6 band (out-of-plane bend) centred at 757.5 cm−1 is c-type. The ν4 band (HNS bend) centred at 905.9 cm−1 is predominantly a-type with a very weak b-type component (μa2/μb2=12.4). Numerous global perturbations and localized avoided crossings affecting the v4 = 1 rotational levels were successfully treated by inclusion of Fermi and c-axis Coriolis resonance terms between v4 = 1 and v5 = 2, and a b-axis Coriolis resonance term between v4 = 1 and v6 = 1. The latter term gives rise to an avoided crossing with an extraordinary ∣ΔKa = 5 selection rule. The Fermi resonance between v4 = 1 and v5 = 2 gives rise to strong mixing of their rotational wavefunctions in the vicinity of Ka = 18. The resultant borrowing of intensity made it possible for 2ν5 transitions in the range Ka = 16–19 to be assigned and included in a global rovibrational treatment of all three band systems.

Introduction

N-Sulfinylamines, compounds featuring the nonlinear functional group single bondNdouble bondSdouble bondO, have drawn considerable interest due to their geometry. Large number of compounds of the type R-NSO have been prepared and their structure and behavior studied, [1] with the conclusion being that a planar structure with syn configuration is the preferred form. The simplest sulfinylimine HNSO, also known as thionylimide, was first prepared by the spontaneous gas phase reaction of ammonia and thionyl chloride at room temperature [2]. Theoretical studies using ab initio molecular orbital and density functional calculations have identified cis-HNSO as the most stable structure on the potential energy surface (PES) and predicted an additional seven structural isomers [3], four of which had already been prepared photolytically from HNSO in a low-temperature matrix and probed by spectroscopic methods [4], [5], [6], [7]. Ultraviolet absorption spectroscopy of HNSO indicated electronic excitation centered at the S atom [8]. A planar, cis conformation for the HNSO molecule, in agreement with theory, was established from analysis of its microwave spectrum at room temperature [9] and quartic centrifugal distortion constants [10] and the hyperfine constants [11] were determined later. The frequencies of the six fundamental vibrations and a harmonic force field were first derived from low-resolution studies of the IR spectra of HNSO and DNSO in the gas phase [12], [13] and in matrices [14]. The structure and harmonic frequencies obtained from ab initio calculations [15] were found to be in good agreement with experimental data [16], especially when improved larger basis sets combined with experimental mass-dependent methods were used [17]. Carlotti et al. recorded the ν1 fundamental at medium resolution (0.04–0.06 cm−1) and reported partial rotational analyses for both HNSO [18] and DNSO [19]. The Ssingle bondO stretching fundamental ν2 of thionylimide, first analyzed at low resolution [16], was subsequently recorded and fully analyzed by high-resolution IR spectroscopy [20]. The upper state was found to be substantially perturbed by the v5 = 1, v6 = 1 combination level through both a- and b-axis Coriolis coupling. Even though no ν5 + ν6 band transitions were identified, treatment of the perturbations and avoided crossings in ν2 permitted Joo and Clouthier to obtain the band origin and a set of rotational constants for the dark v5 = 1, ν6 = 1 state [20]. An analysis of the Nsingle bondS stretching (ν3) and NSO bending (ν5) fundamentals was reported using high-resolution FTIR spectroscopy [21]. An improved set of ground state rotational constants was obtained along with excited vibrational state constants for both bands, neither of which exhibited any perturbations.

In this work, we report the full analysis of two fundamentals of HNSO, ν4 (HNS bending) and ν6 (out of plane bending), using high-resolution Fourier transform infrared (FTIR) spectroscopy. The v4 = 1 rotational levels are found to be seriously disturbed by global perturbations and local avoided crossings. We report how a global fit to transitions from ν4, ν6 and 2ν5 can successfully treat all of these perturbations.

Section snippets

Experimental details

The high-resolution (0.0030 cm−1 unapodized) Fourier transform IR spectra of HNSO were recorded on a Bruker IFS 120 HR interferometer coupled to a multi-pass sample cell. The spectrometer was equipped with a mid-infrared Globar light source, a KBr beamsplitter and a liquid nitrogen cooled mercury-cadmium-telluride (MCT) photodetector. A longwave-pass optical filter (1% transmission at 1550 cm−1) was inserted prior to the detector to restrict incident radiation while the 4P (4-point) apodization

Results

As first reported from the microwave study [9] HNSO has a cis planar near-prolate structure of Cs symmetry. Its six fundamental IR modes of vibration are characterized as either a/b-type bands of a′ symmetry (ν1ν5) or c-type bands of a″ symmetry (ν6) as indicated in Table 1. Also shown in the table are the band origins of all six fundamentals and two other bands. Apart from ν1 all of the other band centres have been obtained from high-resolution IR studies.

A high-resolution IR spectrum of the ν

Discussion and conclusions

The molecular parameters of Table 2 were used to calculate the energies of rotational levels plotted in Fig. 4. As the interacting v4 = 1 and v5 = 2 levels are only 9.694 cm−1 apart, the extent of local crossings seen between the states is not at all surprising. The crossing and re-crossing of their energy levels is encouraged by Ka splittings that appear at higher J values with increasing Ka as a result of asymmetry splitting. It can be seen that for each value of Ka in v4 = 1 there are rotational

Acknowledgments

We gratefully acknowledge financial support from the Australian Research Council and from Monash University.

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