Elsevier

Chemical Physics Letters

Volume 301, Issues 1–2, 19 February 1999, Pages 200-204
Chemical Physics Letters

Laboratory microwave spectroscopy of aluminium cyanide

https://doi.org/10.1016/S0009-2614(99)00013-5Get rights and content

Abstract

The pure rotational spectrum of aluminium cyanide (AlCN) between 10 and 21 GHz has been investigated by Fourier transform microwave spectroscopy. Molecular samples were produced by reacting ablated Al metal with cyanogen present in an Ar supersonic jet. Rotational and centrifugal distortion constants and 27Al and 14N nuclear quadrupole coupling constants and nuclear spin–rotation constants have been determined for the main isotopomer, 27Al12C14N. The spectrum of AlCN was found to be much weaker than that of AlNC thus confirming that AlCN is the less stable of the two isomers.

Introduction

Over the past decade, transitions of several metal-containing species, including aluminium, sodium, and potassium monohalides 1, 2and sodium and magnesium monocyanides 3, 4, 5, 6, have been identified in the circumstellar envelope of IRC+10216. The metal monocyanides are particularly interesting because they have three possible structural isomers: they can take the linear cyanide, the linear isocyanide or the non-linear T-shaped configurations. Experimentally the most stable isomer of sodium monocyanide is T-shaped [7]but that of magnesium monocyanide is the linear isocyanide species [5]. The T-shaped NaCN isomer [3]has been detected in IRC+10216, as have both MgNC 4, 5and its metastable linear cyanide isomer MgCN [6]. The two linear isomers of aluminium monocyanide have also been suggested as possible candidates for detection in this object [8]. As was found for magnesium monocyanide, the linear isocyanide species is again the lowest energy isomer [9]: calculations on the formation of metal monocyanide species in this molecular cloud suggest that most of the aluminium monocyanide should be in the form AlNC not AlCN [10]. Therefore sensitive searches need to be made for both linear isomers to verify this prediction.

Until recently, only theoretical calculations of ground-state structural parameters were available for AlCN and AlNC 8, 11, 12. In the past year, several experimental studies of gas-phase AlNC have been published. Millimeter wave transitions, in both the ground vibrational state and excited states of the bending mode (ν2), have been measured by Robinson et al. [9]. The hyperfine structure in the pure rotational spectrum was investigated by Walker and Gerry [13]using Fourier transform microwave (FTMW) spectroscopy. Fukushima observed the vibronic structure of an electronic band system at 28754 cm−1 using laser-induced fluorescence (LIF) [14]. He assigned this system as the 1A′–1Σ+ transition of AlNC based on the vibrational analysis of the ground electronic state and the results of his own ab initio calculations. There has also been a matrix isolation infrared study of the monocyanides of all the group 13 metals, including AlNC and AlCN [15]. Very recently, Gerasimov et al. obtained rotationally resolved fluorescence excitation spectra of an electronic band system at 36389 cm−1 which they assigned as the Ã1ΠX̃1Σ+ transition of AlNC [16]. They also reassigned the band reported by Fukushima to the 1Π1Σ+ transition of AlCN. This spectrum is the only reported observation of gas phase AlCN so far.

This Letter is the first report of the microwave spectrum of AlCN. The two lowest frequency rotational transitions, J=1–0 and J=2–1, have been measured using a cavity Fourier transform microwave (FTMW) spectrometer. The nuclear hyperfine structure due to 27Al and 14N nuclei has been observed and nuclear quadrupole and nuclear spin–rotation constants have been calculated. The molecular constants determined from the FTMW spectrum of AlCN will aid in the search for this molecule in circumstellar regions.

Section snippets

Experimental

The transitions of AlCN were measured using a pulsed jet cavity FTMW spectrometer [17]which has been described in detail elsewhere [18]. The spectrometer cavity is formed by two spherical aluminium mirrors 28 cm in diameter and placed ∼30 cm apart. A pulsed nozzle laser ablation source is mounted near the centre of one of the mirrors so that the molecular jet travels parallel to the axis of microwave propagation. This configuration enhances the sensitivity of the spectrometer [19]. It also

Observed spectra and analyses

The initial search parameters were determined using the ab initio calculations by Ma et al. [8], since the experimental results of Gerasimov et al [16]. were not available when our study was undertaken. For AlNC, it was found that the calculations made at the TZ2P+fCISD level of theory produced rotational constants closest to the experimental values [13]. Accordingly, the predicted value at the same level of theory was used for AlCN. Nuclear quadrupole coupling constants eQq0(27Al) from AlNC

Discussion

The 27Al and 14N nuclear quadrupole coupling constants can be used to examine the bonding in AlCN. These constants can be interpreted in terms of valence p-electron densities using the Townes–Dailey model [23]. This relates the measured nuclear quadrupole coupling constant, eQq(mol), to the nuclear quadrupole coupling constant of one atomic np electroneQq(mol)=nznx+ny2eQqn10(atom),where nx, ny, and nz are the number of electrons in the npx, npy and npz orbitals, respectively, and z axis is

Conclusion

The pure rotational spectrum of AlCN has been studied by FTMW spectroscopy. The nuclear quadrupole coupling constants have been used to investigate the electronic structure of this molecule. The eQq(27Al) value is consistent with the Al–N bond in AlNC having more `double-bond character' than the Al–C bond in AlCN. The bonding in the CN groups in AlNC and HCN are found to be similar. The molecular constants determined in this study can be used to predict rest frequencies for astronomical

Acknowledgements

This work has been supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and by the Petroleum Research Fund administered by the American Chemical Society.

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    Present address: Molecular Spectroscopy Group, Steacie Institute for Molecular Sciences, 100 Sussex Drive, Ottawa, Ont., Canada K1A 0R6.

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