Rotationally Resolved Spectra of Isovalent NbCr and VCr

Resonant two-photon ionization spectroscopy has been used to study the isovalent metal molecules NbCr and VCr. The first experimental observation of 93Nb52Cr yielded a ground-state rotational constant of B‘ ‘0 = 0.141 04(2) cm-1, which corresponds to a bond length of r‘ ‘0 = 1.8940 ± 0.0003 Å. The excited state, which lies at 14 440 cm-1, has B‘0 = 0.144 39(2) cm-1 and r‘0 = 1.8720 ± 0.0001 Å. The bond energy of NbCr is measured as 24 409 ± 5 cm-1 (3.0263 ± 0.0006 eV) by the onset of predissociation in a congested vibronic spectrum. For 51V52Cr, the ground state is identified as 2Δ5/2 with B‘ ‘0 = 0.219 91(26) cm-1 and r‘ ‘0 = 1.7260 ± 0.0011 Å. The excited state, which lies at 14 371 cm-1, has B‘0 = 0.221 53(27) cm-1 and r‘0 = 1.7201 ± 0.0011 Å. On the basis of these results, a consistent set of multiple bonding radii are presented for the V, Cr, Nb, and Mo atoms, and the bond lengths (r 0) of VMo and NbMo are predicted to be 1.859 and 2.013 Å, respectively. A comparison of bond energies for the NbM molecules is also presented, along with an analysis of the trend in bond energies as the metal M is varied.