Campanelli Anna Rita, Arcadi Antonio, Domenicano Aldo, Ramondo Fabio, Hargittai István
Department of Chemistry, University of Rome La Sapienza, I-00185 Rome, Italy.
J Phys Chem A. 2006 Feb 9;110(5):2045-52. doi: 10.1021/jp058174w.
The molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C2v, D3h, and D2h symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of Cs, C3v, and C2v symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 A per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C[triple bond]C bond of the ethynyl group is unaffected by the pattern of substitution, the C(ipso)-C(ethynyl) bonds in p-diethynylbenzene are 0.001-0.002 A shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the C(ipso)-C(ethynyl) and C(ipso)-C(ortho) bonds in the three molecules, 0.023-0.027 A, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level.
通过气相电子衍射和从头算/密度泛函理论分子轨道计算，已精确测定了乙炔基苯和间三乙炔基苯的分子结构，并与先前研究中对二乙炔基苯的结构进行了比较[多梅尼卡诺，A.；阿尔卡迪，A.；拉蒙多，F.；坎帕内利，A. R.；波塔洛内，G.；舒尔茨，G.；哈吉塔伊，I.《物理化学杂志》1996年，第100卷，第14625页]。尽管这三种分子的平衡结构分别具有C2v、D3h和D2h对称性，但气相中相应的平均结构最好分别用Cs、C3v和C2v对称性的非平面模型来描述。对称性的降低是由于取代基在苯环平面外的大幅度运动。在电子衍射分析中使用非平面模型得到的环角与分子轨道计算结果一致。微波光谱研究报道的乙炔基苯的分子结构在苯环的本位区域被证明是不准确的。二乙炔基苯和间三乙炔基苯中环C-C键和C-C-C角的变化可以很好地解释为每个取代基独立作用的叠加。特别是，实验和计算一致表明，每增加一个乙炔基，环C-C键的平均长度增加约0.002埃。不同理论水平的分子轨道计算表明，尽管乙炔基中C≡C键的长度不受取代模式的影响，但二乙炔基苯中C(本位)-C(乙炔基)键比乙炔基苯和间三乙炔基苯中的相应键短0.
