Abstract/Summary

A double minimum six-dimensional Potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D-2h) B-4 isomer in its (1)A(g) electronic ground state by fitting to energies calculated ab initio. The PES exhibits a barrier to the D-4h square structure of 255 cm(-1). The vibrational levels (J=0) are calculated variationally using an approach which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm-1 for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B-4 it is the B-1g (D-4h mode which distorts the square molecule to its planar rhombic form. The anharmonic fundamental vibrational transitions of B-11(4) are calculated to be (splittings in parentheses): G(O) = 2352(22) cm(-1), v(1)(A(1g)) - 1136(24) cm(-1,) v(2)(B-1g)=209(144) cm(-1) v(3)(B-2g)=1198(19)cm(-1), v(4)(B-2u) = 271(24) cm(-1), and v(5) (E-u) = 1030( 166) cm(-1) (D-4h notation). Their variations in all stable isotoporners were investigated. Due to the presence of strong anharmonic resonances between the B-1g in-plane distortion and the B-2u, out-of-plane bending modes. the hiaher overtones and combination levels are difficult to assign unequivocally. (C) 2005 American Institute of Physics.