Accurate Determination of the Deformation of the Benzene Ring upon Substitution: Equilibrium Structures of Benzonitrile and Phenylacetylene

Accurate equilibrium, r_e, structures of the monosubstituted benzene molecules benzonitrile, C₆H₅CN, and phenylacetylene, C₆H₅CCH, have been determined using two different, to some extent complementary techniques. The semiexperimental, r_e^SE, structural parameters are the result of a least-squares fit to equilibrium rotational constants derived from experimental effective ground-state rotational constants and rovibrational corrections based principally on an ab initio cubic force field. The composite ab initio Born–Oppenheimer, r_e^BO, structural parameters are obtained from frozen-core and all-electron MP2 and the CCSD­(T) geometry optimizations using Gaussian basis sets up to quintuple-zeta quality. The DFT­(B3LYP) method, with two different Gaussian basis sets, 6-31G* and 6-311+G­(3df,2pd), was used to calculate the cubic force field employed during the r_e^SE structure determination. With the 6-31G* basis set, the error of the rovibrational correction is to a large extent random, whereas with the 6-311+G­(3df,2pd) basis set it is mainly systematic. As shown here, systematic errors do not have a significant effect on the accuracy of the derived structure; the quality of the structural fit, however, is sensitive to the true accuracy of the ground-state rotational constants. An even more important general conclusion of this study is that the addition of extra rotational constants from multisubstituted species does not seem to improve the accuracy of the r_e^SE structures, quite in contrast to the highly desirable availability of data corresponding to all singly substituted species.