Periodicity and Peculiarity in 120 First- and Second-Row Diatomic Molecules

Alexander I. Boldyrev, Nick Gonzales, Jack Simons
1994 The Journal of Physical Chemistry  
The ground and very low-lying excited states of all 120 first-and second-row diatomic molecules are surveyed. Three quarters of these molecules have had their ground state term symbols reliably experimentally determined. However, one quarter remain predicted only theoretically. For all 120 species, the best available experimental (where known) and theoretical values for the dissociation energies to ground state atoms are also presented. The Aufbau principle, combined with standard energy
more » ... g for the valence molecular orbitals, is able to properly account for the ground state term symbols of all but 20 of the diatomics studies. The 20 exceptions produce higher than expected ground state spin multiplicity and arise when there are 4-5 or 7-8 valence electrons and group 3, 4, or 5 (but not group 6 or 7) atoms are involved. @ Abstract published in Advance ACS Abstracts, September 1, 1994. 0022-365419412098-993 1$04.5010 the corresponding hydrides but excluding rare-gas-containing species. We emphasize (i) species that have yet to be studied experimentally, (ii) species whose ground states do not involve maximal double orbital occupancy, (iii) trends and exceptions to trends in the spin multiplicity of ground states. Sophisticated ab initio techniques were applied to many of the 33 experimentally uncharacterized diatoms shown in Figure 1 in burgundy. In particular, the following 23 have been studied in earlier theoretical works: LiB,l0 LiC loc,ll LiN loC,l2 LiMg,13 LiA1,l°C LiSi,locJ1 LiP,and SiP.14 In the present work, we present our new results on the remaining 10 diatomic molecules: BeB, NaB, NaC, NaN, MgB, MgC, MgN, AlB, NaAl and MgAl. We repeat high-level calculations on several of the other 23 molecules for which the ground state has not yet been identified with certainty. In addition, we attempt to examine patterns in ground state spin multiplicity for the species in Figure 1 , in particular noting circumstances where ground states with higher than expected spin multiplicities occur. Computational Details The bond lengths and harmonic vibrational frequencies of the 10 diatomics (BeB, NaB, NaC, NaN, MgB, MgC, MgN, AlB, NaAl, and MgAl) for which new data are presented here were optimized using analytical gradients18 and polarized split-
doi:10.1021/j100091a001 fatcat:z4vmqkctdfd3zjnk5qmhycnzne