THE MOLECULAR-STRUCTURE AND QUADRATIC FORCE-FIELD OF TETRAMETHYLBUTATRIENE (2,5-DIMETHYL-2,3,4-HEXATRIENE) STUDIED BY GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO MO CALCULATIONS, AND ADJUNCT AB-INITIO STUDIES OF BUTATRIENE

The molecular structure of (CH3)(2)C=C=C=C(CH3)(2) (tetramethylbutatriene) has been determined by gas-phase electron diffraction (GED) at 20 degrees C and by ab initio MO calculations at the HF/6-31G* and MP2/6-31G* levels. All results are consistent with D-2h molecular symmetry. A HF/6-31G*//HF/6 - 31G* scaled quantum mechanical force field has been established and used to compute vibrational amplitude quantities needed in the GED structural analysis. GED results with effects from correlation in the data and uncertainty in the s scale (0.1%) included in the standard deviations, are: r(a)(C-H) = 110.5(2) pm, r(a)(C-C) = 151.1(2) pm, r(a)(C(sp(2))=C(sp)) = 133.0(2) pm, r(a)(C(sp)=C(sp)) = 127.1(3) pm, measured angle(alpha)(C-C-H) = 111.4(3)degrees, and measured angle(alpha)(C=C-C) = 121.9(1)degrees. This corresponds to a weighted average for the C=C bond length of r(a) (C=C) = 131.0(1) pm and a difference value of Delta(a)(C=C) = 5.9(4) pm compared with MO results (r(e);Delta(e)) of 129.3 and 4.4 pm (KF) and 131.2 and 5.7 pm (MP2). These GED results were obtained with simultaneous adjustment of ten I value groups, including tied refinement of I values for the two types of C=C bond distances (3.86(10) and 3.64(10) pm). When they were fixed at computed values (4.15 and 3.93 pm), Delta(a)(C=C) was determined as 5.3(2) pm. Ab initio calculations at the HF/6-31G*, MP2/6-31G* and MP3/6-31G* levels of butatriene, C4H4, are consistent with D-2h molecular symmetry and gave for r(e)(C=C) and Delta(e)(C=C), 129.0 and 3.8 pm, 130.8 and 4.8 pm, and 130.5 and 4.4 pm for the HF, MP2 and MP3 calculations, respectively, compared with literature r(a), values of 130.7(2) pm and 3.5 pm, The computed C(sp(2))=C(sp) bond lengths (130.3, 132.4, 132.0 pm) are longer than the isolated counterparts of 129.6, 131.3 and 131.1 pm, determined in supplementary HF, MP2 and MP3 calculations on allene (C3H4; D-2d symmetry) which has a literature r(g) value of 131.3(1) pm. Literature assignments of vibrational spectroscopic data and ab initio frequencies for both title molecules are compared and some reassignments are suggested.