Main group elements excited states

Until the early 1980s, the predominant view was expressed by the so-called double bond rule , which stated that elements with a principal quantum number greater than 2 do not form mnltiple bonds with themselves or with other elements. Exciting events in 1981 saw this rule overturned, with the annonncement of the first sUene Si=C and disilene Si=Si donble bonds, followed not long after by a P=P donble bond in Mes P=PMes . In the 25 years since that time, a phenomenal advance in experimental results has taken place in this area, accompanied by a parallel advance, which has not been without controversy, in the understanding of the phenomenon of multiple bonding in the heavier main group elements. 

Relativistic and electron correlation effects play an important role in the electronic structure of molecules containing heavy elements (main group elements, transition metals, lanthanide and actinide complexes). It is therefore mandatory to account for them in quantum mechanical methods used in theoretical chemistry, when investigating for instance the properties of heavy atoms and molecules in their excited electronic states. In this chapter we introduce the present state-of-the-art ab initio spin-orbit configuration interaction methods for relativistic electronic structure calculations. These include the various types of relativistic effective core potentials in the scalar relativistic approximation, and several methods to treat electron correlation effects and spin-orbit coupling. We discuss a selection of recent applications on the spectroscopy of gas-phase molecules and on embedded molecules in a crystal enviromnent to outline the degree of maturity of quantum chemistry methods. This also illustrates the necessity for a strong interplay between theory and experiment. 

Excited states for molecules containing main group elements... 

A rule of thumb, based on observation, is that the need for electron correlation becomes more important as one descends to the heavier main group elements and toward the right in the first transition series.This observation can be rationalized in terms of weaker bonding for heavier MG elements as well as first row TMs, hence lower energy excited states and a greater electron correlation contribution (see Eq. [2]). One final point is that quantum mechanical methods for including correlation scale as the fifth to seventh power of N, where N is the number of basis functions. 

Zhao, Y Truhlar, D. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements two new functionals and systematic testing of four M06-class functionals and 12 other functionals, Theor. Chem. Acc. 2008,120, 215-241. 

The electron affinities of the main group homonuclear diatomic anions have been measured by PES. A few experimental values for the transition metal dimers are also available. The electron affinities of all the 3d homonuclear diatomic molecules have been calculated using density functional methods [1-4], Only the AEa of I2, 2.524 eV C2, 3.27 Si2, 2.2o S2, 1.67 F2, 3.0g Cl2, 2.4s Br2, 2.5, and 02, 1.07 have been measured by more than one method [1-3]. CURES-EC calculations confirm these to within 0.1 eV. Positive excited states Ea have been measured for 02, C2, and I2 and are inferred for other X2 [5-8]. Just as in the case of the atomic Ea, the trends in the Periodic Table can support the assignments of AEa for the other elements. 

Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 120,215-241 (2008). The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. 

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