The equilibrium structure of the more stable conformer of H2SO4, of C2 symmetry, has been calculated ab initio using the CCSD(T) method and taking into account the core correlation correction. The accuracy of this structure has been checked by comparing it to that of similar molecules and by estimating the effects of basis set enlargement and of diffuse functions. Furthermore, the quadratic, cubic, and quartic force fields have been calculated at the MP2 level of theory using a basis set of triple-zeta quality. The spectroscopic constants derived from the force field are in satisfactory agreement with the experimental ones. The resulting band origins are compared to literature infrared values, including those for overtone and combination bands. Normal modes of vibration are pictured. Using this force field, semiexperimental equilibrium rotational constants are determined which allows us to check the accuracy of the ab initio structure and to refine it using a mixed regression method.
The microwave rotational spectrum of toluene has been investigated in the region from 8 to 23 GHz. The three types of absorption lines to be expected in the case of a molecule of this symmetry with a very low sixfold barrier hindering internal rotation of the methyl top have been found: to=0; | to | 0, 3 n; |m| = 3n. From the lines to=0 the rotational constants and the dipole moment (0.375 ± 0.01 D) have been determined; from the lines |TO| = 3 the internal rotation barrier could be calculated as V6= (13.94 ± 0.1) cal/mole. This value is discussed and compared with the very much larger value derived from solid-phase heat capacity data.
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