Observation of the high-resolution spectrum of the N–H bending vibration of ketenimine CH2CNH
Introduction
Ketenimine, CH2
CNH, is an unstable molecule produced in pyrolytic reactions of various nitriles, and has attracted much attention as a reactive intermediate in organic chemistry [1] and astrochemistry [2], [3]. Its isomerization to acetonitrile CH3CN, i.e. most stable C2H3N species, has been experimentally [4] and theoretically investigated [5].
In spite of the interest in such wide areas, spectroscopic information on ketenimine is still limited. The first observation of its infrared spectrum has been done in an Ar matrix [6], followed by microwave spectroscopy by Rodler et al. [7], [8]. The inversion splitting associated with the N–H bending motion was measured to be 2.2 × 10−6 cm−1 (66 kHz) in vibrational ground state, thereby inversion potential function was determined with the use of semi-rigid-bender calculation [8]. Rodler et al. thus showed that ketenimine is the first imine molecule that exhibits inversion splitting observable experimentally. In 1990, we reported the high-resolution IR spectrum of the CCN stretching band and its rotational analysis [9]. The present study reports on the subsequent infrared study of this molecule. As listed in Table 1, ketenimine has 12 vibrational modes and the CCN stretching mode (previously denoted as ν4 and should be renewed as ν3) has the largest infrared intensity. Our next target was the second strongest band which involves the N–H in-plane bending. Hereafter we call this mode as ν6, in accordance with the Herzberg notation [10]. Since this mode is directly correlated to the N–H inversion motion, it would be interesting to see how the ν6 mode is affected by the large amplitude motion.
Section snippets
Experimental
The experimental procedure was already described [9]. In brief, the pyrolytic product of ethylenecyanohydrine (HOCH2CH2CN) in a quartz tube heated at ca. 1050 °C was flowed into a White-type long path cell (l = 10 m). The cell was evacuated by a roots pump (Edwards EH250) backed up by a rotary pump (Edwards E2M40), and the pressure inside the cell was maintained at ca. 80 mTorr to minimize the loss of the reactive species. The spectrum of the ν6 band was measured by using a BOMEM DA 3.36
Assignment and analysis
The shape of the ν6 band was, as shown in Fig. 1, not typical for a parallel- nor a perpendicular band of a near-prolate asymmetric-top molecule. A closer look at the spectrum, however, revealed sequences with intervals of ca. 0.64 cm−1 (=B + C), as shown in the inset. Moreover, the DFT calculation predicted |μb(ν6)/μa(ν6)| = 0.196, where μa(ν6) and μb(ν6) denote transition moment of the ν6 mode along a- and b-axis, respectively. It was therefore reasonable to conclude that the ν6 band is of a-type
Results and discussion
For these two effects, the following assessments can be made quantitatively.
Conclusion
We measured a high-resolution infrared spectrum of the N–H bending vibration of ketenimine, and analyzed its rotational structure. We found that the molecular constants in vibrational excited state show anomalies due to Coriolis couplings with two nearby states and large amplitude motion. For obtaining deperturbed constants, it would be necessary to utilize non-perturbational treatment of the three relevant vibrational levels in combination with theoretical model to describe large amplitude
Acknowledgments
The authors are grateful to Dr. Harutoshi Takeo and Prof. Yoshiaki Hamada (The Open University of Japan) for helpful discussion.
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