Probing the Electronic\nManifold of MgCl with Millimeter-Wave\nSpectroscopy and Theory: (3)<sup>2</sup>Σ<sup>+</sup> and (4)<sup>2</sup>Σ<sup>+</sup> States

The millimeter/submillimeter spectrum of magnesium chloride\n(MgCl)\nhas been observed in two new electronic excited states, (3)<sup>2</sup>Σ<sup>+</sup> and (4)<sup>2</sup>Σ<sup>+</sup>, using\ndirect absorption methods. The molecule was synthesized in a mixture\nof Cl<sub>2</sub>, argon, and magnesium vapor. For the (3)<sup>2</sup>Σ<sup>+</sup> state, multiple rotational transitions were measured\nin the <i>v</i> = 0 level for all six isotopologues (<sup>24</sup>Mg<sup>35</sup>Cl, <sup>24</sup>Mg<sup>37</sup>Cl, <sup>25</sup>Mg<sup>35</sup>Cl, <sup>25</sup>Mg<sup>37</sup>Cl, <sup>26</sup>Mg<sup>35</sup>Cl, and <sup>26</sup>Mg<sup>37</sup>Cl), as well as up to <i>v</i> = 13 for <sup>24</sup>Mg<sup>35</sup>Cl. For the (4)<sup>2</sup>Σ<sup>+</sup> state, less intense spectra were recorded\nfor <sup>24</sup>Mg<sup>35</sup>Cl (<i>v</i> = 0–2).\nEquilibrium rotational parameters were determined for both states\nfor <sup>24</sup>Mg<sup>35</sup>Cl, as well as rotational constants\nand <sup>25</sup>Mg hyperfine parameters for the other isotopologues.\nA perturbation was observed between rotational levels of the two states\ndue to an avoided crossing. Computations were also carried out at\nthe CASPT2 and MRCISD+Q levels, and the resulting bond lengths for\n(3)<sup>2</sup>Σ<sup>+</sup> and (4)<sup>2</sup>Σ<sup>+</sup> states agree well with the experimental values of <i>r</i><sub>e</sub> = 2.536 and 2.361 Å. The computations\nshow that the (3)<sup>2</sup>Σ<sup>+</sup> state has a double-well\npotential; however, the state behaves as a single well with unperturbed\nvibrational levels up to <i>v</i> = 13 due to nonadiabatic\n interactions with the (4)<sup>2</sup>Σ<sup>+</sup> state.