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Barrelene, H–C(CH=CH)3C–H, is an unsaturated polycyclic hydrocarbon containing three isolated double bonds in a non-planar arrangement. We have studied the transmission of field effects through the barrelene framework by analyzing the small structural changes occurring in the phenyl group of many Ph–C(CH=CH)3C–X molecules, where X is a variable substituent. Molecular geometries have been determined by quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Comparison with the results obtained for the corresponding saturated molecules, the bicyclo[2.2.2]octane derivatives Ph–C(CH2–CH2)3C–X, reveals a small, but significant, field-induced π-polarization of the barrelene cage, especially when the remote substituent is a charged group. Additional evidence of π-polarization is obtained by comparing the electric dipole moments of the two sets of uncharged molecules. The structural variation of the barrelene cage caused by the variable substituent in Ph–C(CH=CH)3C–X molecules has also been investigated. It is much larger than that of the phenyl group and depends primarily on the electronegativity of the substituent. Particularly pronounced is the concerted variation of the non-bonded distance between the bridgehead carbons of the cage, r(C···C) 1 BARR , and the average of the three C–C–C angles at the cage carbon bonded to the variable substituent, α 1 BARR . A scattergram of r(C···C) 1 BARR versus the corresponding parameter for bicyclo[2.2.2]octane derivatives, r(C···C) 1 BCO , shows that the variation of r(C···C) 1 BARR becomes gradually less pronounced than that of r(C···C) 1 BCO as the electronegativity of the substituent increases.

Electronic substituent effects in 4-substituted 1-phenylcubane derivatives, Ph–C8H6–X, have been investigated from the structural changes caused by the substituent X. The molecular structures of 34 derivatives with charged or dipolar substituents have been determined from quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Geometrical variations caused by substitution appear both in the cubane framework and in the benzene ring, but the two kinds of changes show no correlation. The rather small changes in the benzene ring geometry are caused by long-range polar effects (field effects), while the larger changes in the cubane cage are controlled primarily by electronegativity effects. A structural parameter measuring the long-range polar effect of the substituent, S F CUB , has been derived from the geometry of the phenyl group acting as a probe. This parameter correlates well with the calculated gas-phase acidities of 4-substituted cubane-1-carboxylic acids, HOOC–C8H6–X, and with other indicators of long-range polar effects obtained from bicyclo[2.2.2]octane derivatives. The correlations can further be improved by introducing a resonance parameter as an additional explanatory variable. This indicates that the electron delocalization resulting from hyperconjugative interactions between substituent and cage modifies the long-range polar effect of the substituent. Strong hyperconjugative interactions between some charged substituents and the cubane cage result in remarkable variations in the cage geometry, superimposed onto those ascribed to electronegativity effects.

The molecular structure of triphenylsilane has been investigated by gas-phase electron diffraction and theoretical calculations. The electron diffraction intensities from a previous study (Rozsondai B, Hargittai I, J Organomet Chem 334:269, 1987) have been reanalyzed using geometrical constraints and initial values of vibrational amplitudes from calculations. The free molecule has a chiral, propeller-like equilibrium conformation of C 3 symmetry, with a twist angle of the phenyl groups τ = 39° ± 3°; the two enantiomeric conformers easily interconvert via three possible pathways. The low-frequency vibrational modes indicate that the three phenyl groups undergo large-amplitude torsional and out-of-plane bending vibrations about their respective Si–C bonds. Least-squares refinement of a model accounting for the bending vibrations gives the following bond distances and angles with estimated total errors: r g(Si–C) = 1.874 ± 0.004 A, 〈r g(C–C)〉 = 1.402 ± 0.003 A, 〈r g(C–H)〉 = 1.102 ± 0.003 A, and ∠aC–Si–H = 108.6° ± 0.4°. Electron diffraction studies and MO calculations show that the lengths of the Si–C bonds in H4−n SiPh n molecules (n = 1–4) increase gradually with n, due to π → σ*(Si–C) delocalization. They also show that the mean lengths of the ring C–C bonds are about 0.003 A larger than in unsubstituted benzene, due to a one hundredth angstrom lengthening of the Cipso–Cortho bonds caused by silicon substitution. A small increase of r(Si–H) and decrease of the ipso angle with increasing number of phenyl groups is also revealed by the calculations.

The present electron diffraction study of the molecular structure of tetramethylsilane, augmented with theoretical calculations, answers the need for accurate and detailed information on the most fundamental molecules containing silicon. The Si—C bond length is r g = 1.877 ± 0.004 A, in perfect agreement with a previous study (Beagley, B.; Monaghan, J. J.; Hewitt, T. G. J. Mol. Struct. 1971 8 401). The C—H bond length is r g= 1.110 ± 0.003 A and the Si—C—H angle is 111.0 ± 0.2°. The experimental data are consistent with a model of T d symmetry and staggered methyl conformation. The barrier to methyl rotation is estimated to be 5.7 ± 2.0 kJ mol−1 on the basis of the experimentally observed average torsion of the methyl groups.

The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol−1. The calculated lengths of the $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ and $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Ph}}$$ bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Me}}$$ or $${\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{Ph}}$$ bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are $$\langle r_g {\text{(C}}{\kern 1pt} - {\kern 1pt} {\text{C)}}\rangle {\text{ }} = {\text{ 1}}{\text{.407 }} \pm {\text{ 0}}{\text{.003 }}{\AA}{\text{, }}\langle r_g ({\text{Si}}{\kern 1pt} - {\kern 1pt} {\text{C}})\rangle = {\text{ 1}}{\text{.881 }} \pm {\text{ 0}}{\text{.004 }}{\AA}{\text{, }}\langle r_g ({\text{C}}{\kern 1pt} - {\kern 1pt} {\text{H}})\rangle = 1.108{\text{ }} \pm {\text{ 0}}{\text{.003 }}{\AA}{\text{ }}$$ , and $$\angle {\text{C}}_{{\text{ortho}}} {\kern 1pt} - {\kern 1pt} {\text{C}}_{{\text{ipso}}} {\kern 1pt} - {\kern 1pt} {\text{C}}_{{\text{ortho}}} = 117.2{\text{ }} \pm {\text{ 0}}{\text{.3}}^\circ $$ . While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°].

The effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol has been studied by SCF ab initio MO calculations at the HF/6-31G * level. The systems investigated are eight phenol-water complexes and the dimer and trimer of phenol. Optimized geometries show that hydrogen bond formation causes a consistent pattern of changes in the structure of the molecule. When phenol acts as a proton donor, the expected increase ofr (O-H) is accompanied by a slight decrease ofr(C-O) and of the internal ring angles at theipso andpara positions, and by an increase ofr(Cipso©ortho). These changes suggest that the relative contribution of polar canonical forms to the electronic structure of the molecule increases upon hydrogen bond formation, since this enhances the strength of the interaction. The opposite changes occur when phenol acts as a proton acceptor, except forr(O-H), which is the same as in the free molecule. If phenol acts as a proton donorand as a proton acceptor, the two hydrogen bonds become stronger due to a synergic effect. In this case, however, the structural deformation of the molecule is less pronounced than in the previous cases, due to the opposite effect of the two hydrogen bonds. The available experimental evidence on gas-crystal structural differences for phenol is critically reviewed, also in the light of the present results on gas-phase complexes.

The structural properties and intramolecular hydrogen bonding of a series of structures of naphthazarin molecule were investigated by ab initio HF-SCF methods. The geometries of theC 2v ,C 2h ,D 2h , andC s symmetry structures were optimized using split-valence basis sets. MP2/6-31G*// HF/6-31G single-point energy calculations indicate that theC 2v isomer (5,8-dihydroxy-1,4-naphthoquinone) is the lowest energy structure of the molecule and that theC 2h symmetry one (4,8-dihydroxy-1,5-naphthoquinone), lying 37 kJ/mol above theC 2v form, is the other stable isomer of naphthazarin. At the HF/6-31G level, the intramolecular proton exchange between two equivalentC 2v structures is a two-step process where each proton can be independently transferred through an unsymmetrical potential having a 1,5-quinone intermediate, theC 2h symmetry structure, and two equivalent transition states ofC s symmetry, with a barrier height equal to 38 kJ/ mol (MP2/6-31G*//HF/6-31G). The study of naphthazarin molecule is flanked by a theoretical investigation on theC 2v andC 2h isomers of the parent naphthoquinone and dihydroxynaphthalene molecules. The SCF vibrational spectrum of the ground state of naphthazarin, harmonic frequencies, and infrared and Raman band intensities were computed at the HF/6-31G level. The results of the calculations are compared with the matrix isolation FT-IR spectroscopy measurements and with the infrared and Raman spectra of the crystal molecule.