Structural and conformational analysis of the isoelectronic isovalent molecules: (ethynylmethyl)cyclopropane and ethynylcyclobutane. A combined electron diffraction, rotational constants and ab initio study

The structures and conformations of gaseous isoelectronic and isovalent molecules (ethynylmethyl)cyclopropane (EMCP) and ethynyl-cyclobutane (ECB) have been determined by combined analysis of electron diffraction and microwave spectroscopic data. The refinement of the experimental data provided the geometrically consistent r(av) structure. The vibrational harmonic corrections necessary for the conversion of the r(alpha)(0) structural parameters to the r(a) set appropriate for comparison with the diffraction data and the transformation of the observed rotational constants into B(Z) were derived from quadratic force fields MP2/6-31G** and HF/6-31G** for EMCP and ECB, respectively. The combined experimental data for EMCP are consistent with two stable conformers, the gauche and cis with the latter lower in energy than the former by about 0.92 kJ/mol. This result is in fairly good agreement with the energy differences (Delta E = E(cis) - E(gauche)) Obtained from ab initio calculations at DFT (b3pw91/6-31G*) (-0.68 kJ/mol) and MP2/6-311++G(2d,p) (-1.78 kJ/mol) levels of theory. HF/6-31G** calculations, however, predicted 1.5 kJ/mol. The major bond distances (r(av)) and bond angles for the cis conformer obtained from least-squares analysis are: r(C(1)-C(2)) = 1.509(9) Angstrom, r(C(2)-C(3)) = 1.517(14) Angstrom, r(C-C=) = 1.450(13) Angstrom, r(C=C) = 1.215(3) Angstrom, r([C-H](ring))= 1.107(6) Angstrom, angle(C(1)-C(4)-C=) = 114.0(8)degrees, angle(C(4)-C(1)(C(1)-C(2)-C(3))(ringplane) = 126.5(9)degrees, angle(C(4)-C(1)-C(2)-C(3)) = 110.4(19)degrees, angle(C(2)-C(1)-C(4)-C(5)) = 35.9(4)degrees. The combined study of ECB reveals that this molecule exists in axial and equatorial forms with the latter more stable in energy by about 4.0 kJ/mol (corresponds to eq : ax = 84 : 16) which is in excellent agreement with the values observed by means of microwave and vibrational spectroscopy. This result is also in good agreement with the ab initio calculations which were carried out using the methods: HF/6-31G**, MP2/6-31G** and DFT (b3pw91/6-31G*). These methods predicted the occurrence of the equatorial form to be 85, 81 and 88%. The most important structural parameters (r(av)) for the equatorial conformer obtained from the final refinement are: r(C(1)-C(2)) = 1.555(16) Angstrom, r(C-C=) = 1.452(8) Angstrom, r(C=C)= 1.211(2) Angstrom, ([C-H](ring)) = 1.095(2) Angstrom, angle(C(2)-C(1)-C(4))(ring) = 88.0(16)degrees, angle(C(5)-C(1)(C(2)-C(1)-C(4))(ring) = 132.3(17)degrees, puckering angle = 29.2(99)degrees, For the axial conformer: angle(C(5)-C(1)(C(2)-C(1)-C(4))(ring) = 124 6(50)degrees and the puckering angle = 28.5(42)degrees.