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 ~1A_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"'.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 ~(11)B_4 are calculated to be(splittings in parentheses):G(0)=2352(22)cm~(-1),nu_1(A_(1g))=1136(24)cm~(-1),nu_2(B_(1g))=209(144)cm~(-1),nu_3(B_(2g))=1198(19)cm~(-1),nu_4(B_(2u))=271(24)cm~(-1),and nu_5{E_u)=1030(166)cm~(-1)(D_(4h)notation).Their variations in all stable isotopomers 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 higher overtones and combination levels are difficult to assign unequivocally.
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