A critical comparison of the ab initio geometry and zero-point-average structure of bicyclo(3.1.0)hexane (original) (raw)
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Journal of Molecular Structure, 2010
Variable temperature (−55 to −100 • C) studies of the infrared spectra (3500-400 cm −1 ) of fluorocyclobutane, c-C 4 H 7 F, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 496 ± 40 cm −1 (5.93 ± 0.48 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 8 ± 1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 732 ± 47 cm −1 (9.04 ± 0.44 kJ/mol) and the average value of 602 ± 20 cm −1 from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial [axial] conformer are: distances (Å) C-F = 1.383 [1.407(3)], C ␣ -C  = 1.543(3) [1.546(3)], C  -C ␥ = 1.554(3) [1.554(3)] and angles ( • ) ∠C ␣ C  C ␥ = 85.0(5) [89.2(5)], ∠C  C ␣ C  = 89.3(5) [89.2(5)], ∠F-(C  C ␣ C  ) = 117.4(5) [109.2(5)] and a puckering angle of 37.4(5) [20.7(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some other monosubstituted cyclobutanes with halogen and pseudohalogen substituents.
Tetrahedron, 1975
Abetract-Using an INDO approximation to Von Niessea's charge density localization method, the percent s character of the C atom hybrid orbitals engaged in C-H bonds is computed for a series of open-chain and cyclic saturated and unsaturated hydrocarbons. A semi-empirical linear relationship is found between these quantities and the experimental nuclear spin-spin coupling constants JCH (standard deviation 493 Hz). The experimental values of JCH were measured for quadricyclane (1) and tricyclo[3.1.0.6ulhexane (2) and were shown to be in fair agreement with the theoretical pred&ions. An analogous relationship is also established between the percent s charac-k of the involved C atom hybrid orbital and the C-H bond distance. the standard deviation beinn OM40 A. This relation is used in order to predict the diierent C-H bond lengths in the vibrational ground state of iand 2, for which equal CH distances were assumed in the analysis of electron diffraction experiments. The results indicate that molecular geometries can be retined with the use of semi-empirical correlations of the above mentioned type. lNrsloDlJmoN No accurate experimental molecular geometry has been reported hitherto for the highly strained molecules tetracyclo[3.2.0.a2'.O'~heptane (1) or quadricyclane and tricyclo[3.1 .O.@']hexane (2). 1 2 Kutchitsu et al.' described the results of a gas phase electron di&action experiment on quadricyclane. The experimental radial distribution curve was analysed with the help of several assumptions, one of them being that all C-H distances were equal. Assuming further either the r, (G-G) distance equal to that of cyclopropane (Method A), or r, (C&) equal to that of norbornane (Method B), the authors proposed two diierent sets of geometrical parameters. A similar procedure was followed by Geise et al.' in their electron diffraction investigations on tricyclo[3.l.O.~]hexane. Here too, several restrictions were imposed on the molecular geometry; once more, all C-H distances were set equal. In this paper, we will try to provide supplementary information on the nature of these C-H bonds with an experimental study of the JCH coupling constants and a theoretical investigation of these molecules using the *Author to whom inquiries should be addressed. tResearch candidates of the National Foundation for Scientific Research.
J Am Chem Soc, 1981
that for CI-CI so that our measurements with their large uncertainties provide no experimental evidence on the point. Some supporting evidence is available in the results of X-ray diffraction investigation of [4.4.2]propella-3,8-diene-l1,1 2-dione,I5 a molecule with a CBD subunit, and trans-3,4-di-tert-butylcyclobutane-1,2-dione" The first of these has distances of 1.551 (3) A, 1.534 (3) A, and 1.572 (3) A for the bonds corresponding to C&, C1-C4, and C -C4 in CBD; the second has distances of 1.560 A, 1.527 (2) A, and 1.560 (3) A. Thus the CI-C1 type distance is in each case longer than normal and the CI-C4 type smaller than its neighbors.
Journal of Molecular Structure, 1994
For floppy molecules, changes in the bond distances and bond angles as a function of the large amplitude motion have to be carefully accounted for to obtain the equilibrium structure. This effect is discussed in connection with transformation of the operational bond distances r, to the equilibrium distances r, for comparison with ab initio computed values, where CaXz (X=Cl, Br and I) molecules are used as examples. The root-mean-square amplitudes of vibration for the bond distances are affected such that the experimentally determined values from electron diffraction should always be larger than the values calculated from the vibrational force field if the bending potential is not explicitly included in the analysis. The shrinkage and the asymmetry constants are also affected by the large amplitude motion and can be either larger or smaller in magnitude compared to values calculated from the vibrational force field. The effective symmetry of the molecules and the symmetry at the equilib~um configuration is discussed when structural constraints have to be made. The use of rotational constants from microwave spectroscopy in a joint analysis is also discussed.
Chemosphere, 2008
The 1 H NMR spectrum and the crystal structure of HCDBCO [(1R,2R,5R,6R,9S,10S)-5,6-dibromo-1,10,11,12,13,13hexachlorotricyclo[8,2,1,0 2,9 ]-tridec-11-ene)] are reported. The measured dihedral angles from the X-ray structure correlate very well with those calculated from the proton-proton coupling constants indicating that the conformations in solution and in the solid state are probably very similar. Attempts at calculating the 3D model structure of HCDBCO only produced a very poor match between the measured dihedral angles between vicinal protons and the observed proton-proton coupling constants from the 1 H NMR spectrum.
Acta Crystallographica Section B Structural Science, 1984
The interring dihedral angles for the three independent molecules are 28.7 (2), 35.7 (2), and 32.2 (2) °. The structure was refined both in the usual way JR, wR = 0.038, 0.040 for 406 variables and 2848 reflections having I > 3o'(I)], and using a rigid-group, TLS model with o0e internal degree of freedom to describe the CI2H9 biphenyl moieties (R, wR = 0.061, 0.073 for 166 variables and 2848 observed reflections). Descriptions of the rigid-body thermal motion derived from the two types of refinement are essentially the same, although the agreement of the positional parameters is limited by the rather serious assumptions of the rigid-group model. Although rigid-group refinements incorporating anisotropic thermal parameters have been used previously, this appears to be the first direct comparison of the thermal-motion descriptions obtained from the two procedures. The rigid-group refinement procedure has also been tested for several other previously published structures of related molecules with similar results.
The Journal of Organic Chemistry, 1993
that for CI-CI so that our measurements with their large uncertainties provide no experimental evidence on the point. Some supporting evidence is available in the results of X-ray diffraction investigation of [4.4.2]propella-3,8-diene-l1,1 2-dione,I5 a molecule with a CBD subunit, and trans-3,4-di-tert-butylcyclobutane-1,2-dione" The first of these has distances of 1.551 (3) A, 1.534 (3) A, and 1.572 (3) A for the bonds corresponding to C&, C1-C4, and C -C4 in CBD; the second has distances of 1.560 A, 1.527 (2) A, and 1.560 (3) A. Thus the CI-C1 type distance is in each case longer than normal and the CI-C4 type smaller than its neighbors.
Journal of Molecular Structure, 2009
Variable temperature (À55 to À100°C) studies of the infrared spectra (3500-400 cm À1 ) of cyanocyclobutane, c-C 4 H 7 CN, dissolved in liquid xenon have been carried out. The infrared spectra of the gas and solid, as well as, the Raman spectrum of the liquid have also been recorded. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 254 ± 12 cm À1 (3.03 ± 0.15 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 23 ± 1%. The ab initio MP2(full) average predicted energy difference for a variety of basis sets is 238 ± 23 cm À1 in excellent agreement with the experimental DH value, but the average value of 331 ± 7 cm À1 from density functional theory predictions by the B3LYP method is significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for both the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C"N = 1.160 [1.160(3)], C a A(CN) = 1.461(3) [1.468(3)], C a AC b = 1.557(3) [1.562(3)], C b AC c = 1.549(3) [1.550(3)] and angles (°) \C a C b C c ¼ 87:8ð5Þ ½89:1ð5Þ; \C b C a C b ¼ 88:3ð5Þ ½87:9ð5Þ; \NCðC b C a C b Þ ¼ 130:9ð5Þ ½120:0ð5Þ and a puckering angle of 28.5(5) [24.1(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some similar molecules.
Journal of Molecular Structure, 2010
The FT-microwave spectrum of ethynylcyclohexane, cC 6 H 11 CCH has been investigated from 11,000 to 21,000 MHz and 32 transitions for the chair-equatorial conformer and 22 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C 1 AC 9,10 = 1.532(3)[1.537(3)], C 9,10 AC 15,16 = 1.544(3)[1.534(3)], C 4 AC 15,16 = 1.541(3)[1.545(3)], C 6 "C 7 = 1.210(3)[1.212(3)] and angles in degrees: \C 1 C 9,10 C 15,16 = 111.0(5) [111.2(5)], \C 6 C 4 C 15,16 = 110.7(5)[110.3(5)] and sC 1 C 9,10 C 15,16 C 4 = 56.4(10)[55.3(10)]. Variable temperature (À60 to À100°C) studies of a xenon solution were carried out and by using 10 conformer pairs an enthalpy difference of 124 ± 13 cm À1 (1.48 ± 0.16 kJ/mol) was obtained with the more stable form the chair-equatorial conformer. At ambient temperature, the abundance of the chair-equatorial conformer is 55 ± 3%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 822 (axial)/842 (equatorial) cm À1 , a standard enthalpy difference of 172 ± 32 cm À1 (2.06 ± 0.38 kJ/mol) was obtained again with the chair-equatorial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 135 cm À1 (1.62 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis set gives the chair-equatorial conformer more stable by 289 cm À1 (3.46 kJ/mol). A complete vibrational assignment is given for the chair-equatorial conformer and most of the fundamentals for the chair-axial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.