Probing the Electronic Manifold of MgCl with Millimeter-Wave Spectroscopy and Theory: (3)(2)Σ(+) and (4)(2)Σ(+) States.

The millimeter/submillimeter spectrum of magnesium chloride (MgCl) has been observed in two new electronic excited states, (3)(2)Σ(+) and (4)(2)Σ(+), using direct absorption methods. The molecule was synthesized in a mixture of Cl(2), argon, and magnesium vapor. For the (3)(2)Σ(+) state, multiple rotational transitions were measured in the v = 0 level for all six isotopologues ((24)Mg(35)Cl, (24)Mg(37)Cl, (25)Mg(35)Cl, (25)Mg(37)Cl, (26)Mg(35)Cl, and (26)Mg(37)Cl), as well as up to v = 13 for (24)Mg(35)Cl. For the (4)(2)Σ(+) state, less intense spectra were recorded for (24)Mg(35)Cl (v = 0-2). Equilibrium rotational parameters were determined for both states for (24)Mg(35)Cl, as well as rotational constants and (25)Mg hyperfine parameters for the other isotopologues. A perturbation was observed between rotational levels of the two states due to an avoided crossing. Computations were also carried out at the CASPT2 and MRCISD+Q levels, and the resulting bond lengths for (3)(2)Σ(+) and (4)(2)Σ(+) states agree well with the experimental values of r(e) = 2.536 and 2.361 à . The computations show that the (3)(2)Σ(+) state has a double-well potential; however, the state behaves as a single well with unperturbed vibrational levels up to v = 13 due to nonadiabatic interactions with the (4)(2)Σ(+) state.