J. Brunvoll

Molecular vibrations of footballene

Abstract A normal coordinate analysis has been carried out for C 60 “footballene”. A five-parameter force field was employed in order to calculate the vibrational frequencies. Results for the 10 Raman-active and 4 infrared-active frequencies are reported.

The molecular structure of decamethylferrocene studied by gas phase electron diffraction. Determination of equilibrium conformation and barrier to internal rotation of the ligand rings

Abstract The molecular structure of decamethylferrocene, (η-C 5 Me 5 ) 2 Fe, has been determined by gas phase electron diffraction. The FeC and C(Cp)C(Cp) bond distances, 2.064(3) and 1.439(2) A, respectively, are indistinguishable from those in ferrocene, Cp 2 Fe. But, while the equilibrium conformation of gaseous Cp 2 Fe is eclipsed ( D 5h ), the equilibrium conformation of (C 5 Me 5 ) 2 Fe is staggered ( D 5d ) with a barrier to internal rotation of the ligand rings V 5 = 1.0(0.3) kcal mol −1 . And while the CH bonds in Cp 2 Fe are bent about 4° out of the plane of the C 5 ring towards the metal atom, the C(Cp)C(Me) bonds in (C 5 Me 5 ) 2 Fe are bent 3.4(0.5)° out of the plane in the opposite direction.

Dynamic Jahn–Teller effect and average structure of dicyclopentadienylcobalt, (C5H5)2Co, studied by gas phase electron diffraction

The molecular structure of (C5H5)2Co has been determined by gas phase electron diffraction. The best agreement between calculated and experimental intensity curves is obtained with a model with eclipsed C5H5 rings (symmetry D5h), but a model with staggered rings (symmetry D5d) cannot be ruled out. The mean CoC and CC bond distances are 2.119(3) A and 1.429(2) A respectively. The average angle between the CH bonds and the C5 ring is 2.1(0.8)°. The value obtained for the CC vibrational amplitude, l(CC) = 0.055(1) A, is significantly larger than the amplitude calculated from a molecular force field and the corresponding amplitudes in (C5H5)2Fe and (C5H5)2Ni determined by electron diffraction, and confirms the presence of a dynamic Jahn—Teller effect of the magnitude calculated from ESR data. The average structure is compared with those of the metallocenes of the other first row transition elements.

Symmetry Coordinates, Harmonic Force Field, and Mean Amplitudes for Light and Heavy Dibenzene Chromium

The molecular vibrations of dibenzene chromium are analyzed in terms of the eclipsed 25 atomic model of symmetry D6h with planar ligands. A complete set of symmetry coordinates is constructed and based almost entirely on conventional types of valence coordinates. A harmonic force field is given, and is used to calculate the vibrational frequencies, mean amplitudes of vibration, and perpendicular amplitude correction coefficients for (C6H6)2Cr and (C6D6)2Cr.

Tetrathiafulvalene: gas-phase molecular structure from electron diffraction

Abstract The gas-phase molecular structure of 2,2-bi-1,3-dithiole (tetrathiafulvalene) was determined by electron diffraction and compared with the crystal molecular structure. A nonplanar “boat” structure was found to give the best agreement with experimental electron diffraction results. The electron diffraction results were found to be consistent overall with those from X-ray crystallographic studies.

Normal coordinate analysis of the total ferrocene complex

The results of a total normal coordinate analysis of ferrocene, Fe(C5H5)2, are reported. As in a previously published total vibrational analysis of dibenzenechromium, interesting kinematic couplings between some ligand and some framework normal modes of the complex can be observed, and these account for some frequency shifts from free (ionic) to complexed cyclopentadienide without any charge in the harmonic force field of the ligand. Calculated mean amplitudes for ferrocene are compared with those determined by an electron diffraction study and with the corresponding values in the free cyclopentadienide anion.

The molecular structure of gaseous monobromobenzene

Abstract The molecular structure of gaseous monobromobenzene has been studied by the electron diffraction method. The molecular geometry was determined by a conjoint analysis based on electron diffraction intensities and microwave rotational constants, assuming C 2v molecular symmetry. The angular distortion of the benzene ring mainly affects the internal angle at the ipso carbon atom: this angle is determined to be ∠ α (C 2 C 1 C a ) = 121.5(4)° which, as expected for an electronegative substituent, is significantly larger than 120°. The other geometrical parameters are: r a (C 1 Br) = 1.898(1) A, r a (C 1 C 2 ) = 1.394(3) A, r n (C 2 C 1 ) = 1.396(5) A, r a (C 2 C 1 ) = 1.394(7) A r a (C 2 H 2 ) = 1.097(3) A, r a (C 2 H 2 ) = 1.086(3) A, r a (C 4 H 9 ) = 1.085(3) A, ∠ α C 3 = 119.0(7)°, ∠ α C 1 C 2 H 7 = 121.7(1.1)° and ∠ α C 4 C 3 H 3 = 120.5(1.1)°. The r o α (CH) bond lengths are assumed to be equal and are refined in one group. Parenthesized values are one standard deviation from the least-squares refinement.

Enumeration and classification of coronoid hydrocarbons: Part V. Primitive coronoids

Some topological properties of primitive coronoids are discussed. A method of generating such systems from the corona hole is described in detail. The results from computer-aided enumerations of primitive coronoids are given for h (the number of hexagons ) up to 25. The results account for 1 075 554 nonisomorphic systems. The distributions into symmetry groups are specified. The forms of all the primitive coronoids for h ⩽ 15 are depicted and supplied with K numbers ( Kekule structure counts ). Finally the average K values and related quantities for the systems under consideration are reported.

Benzenechromium tricarbonyl III. Mean amplitudes of vibration and related quantities for C6H6Cr(CO)3

Abstract The results of a normal coordinate analysis of the whole molecule of benzenechromium tricarbonyl are reported. The calculations have revealed similar kinematic coupling phenomena to those previously described for transition metal sandwich complexes. Mean amplitudes of vibration (u), perpendicular amplitude correction coefficients (K) and selected shrinkage effects (δ) for C6H6Cr(CO)3 are given which were calculated on the basis of this normal coordinate analysis. The u and δ values are compared with the corresponding quantities in related molecules, viz. free benzene, bis(benzene)chromium and chromium hexacarbonyl.

Symmetry coordinates, harmonic force field, and mean amplitudes for light and heavy ferrocene

Abstract The molecular vibrations of ferrocene were analyzed on the basis of an eclipsed structure ( D 5h ). Harmonic force constants are presented in terms of thoroughly specified symmetry coordinates, and were developed so as to fit exactly the observed vibrational frequencies for Fe(C 5 H 5 )2. Calculations for Fe(C 5 D 5 ) 2 suggest some reassignments of observed frequencies for this isotopic species. The force field was used to calculate the mean amplitudes of vibration for the two isotopic molecules; for Fe(C 5 H 5 ) 2 the values are compared with electron diffraction data. Calculated perpendicular amplitude correction coefficients are also given.