Physical Chemistry The Journal of 0 zyxwvutsrq Copyright 1994 by the American Chemical Society VOLUME 98, NUMBER 40, OCTOBER 6,1994 FEATURE ARTICLE Periodicity and Peculiarity in 120 First- and Second-Row Diatomic Molecules Alexander I. Boldyrev, Nick Gonzales, and Jack Simons* Department of Chemistry, The University of Utah, Salt Lake City, Utah 84112 Received: April 12, 1994; zyxwvutsrq In Final Form: June 20, 1994@ 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 ordering 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. I. Introduction One might expect that essentially all of the 15 zyxwvuts x 1612 = 120 diatomic molecules comprised of first (H, Li, ..., F) and second (Na, ..., C1) row atoms have been thoroughly studied to the extent that their ground electronic states and the corresponding bond lengths (Re) and dissociation energies (De) are well established. However, such is not the case; in Figure 1 those diatomics for which even the ground electronic states have not been so characterized are displayed in burgundy. In purple are shown the diatomics whose ground electronic states are reason- ably well characterized. Most of the experimental data used to create Figure 1 were taken from the monograph of Huber and Herzberg,' although several species' properties were obtained from more recent ~ o u r c e s . ~ - ~ It probably surprises most students of chemistry to learn that more than one quarter of all the diatomic molecules formed by combining pairs of first- or second-row atoms have yet to be experimentally characterized. Many of the uncharacterized diatomic molecules are very reactive intermediates with unpaired electrons or unsaturated valences of one or both atoms, which therefore can exist and be studied only under special conditions. In this article, we consider the electronic structures of the ground and low-lying excited states of diatomic molecules composed of atoms from the first and second rows, including @ Abstract published in Advance zyxwvutsrqp 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,14LiS,15 BeN,16BeC,17 BeNa,13BeA1,13 BeSi,lIb BeP,14 BP,14 NaMg,13 NaSi,locNaP,14 NaS,15 MgSi,loC MgP,I4 AlP,14 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. 11. 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- 0 1994 American Chemical Society