Raymond L. Disch

On symmetrical clusters of carbon atoms: C60

Preliminary results of theoretical calculations on C60 (“Buckminsterfullerene” or “footballene”) and other symmetrical clusters of carbon atoms are reported.

Symmetrical clusters of carbon atoms: The C24 and C60 molecules

Abstract We report ab initio and AM1 calculations on C 24 in octahedral symmetry, including geometry optimization and vibrational analysis. The beats of formation and first ionization potentials of C 24 and C 60 are computed.

Recognition of cholinergic agonists by the muscarinic receptor. 1. Acetylcholine and other agonists with the NCCOCC backbone.

A theoretical model is used to deduce the pharmacologically active conformation of acetylcholine and other agonists interacting with the muscarinic receptor of the parasympathetic and central nervous systems. This is accomplished by replacing the usual dihedral angles tau 1 and tau 2, which define the conformations of cholinergic drugs, with two new geometric parameters more suitable for describing the muscarinic pharmacophore: a characteristic distance, [PQ], and a dihedral angle, PNOQ. Values for these parameters are determined by conformational analysis on semirigid muscarinic agonists using molecular mechanics and ab initio molecular orbital methods. In addition to deducing the active conformation of acetylcholine and other agonists, the model also rationalizes the pattern of stereoselectivity in agonists related to 3-acetoxyquinuclidine (aceclidine) and furnishes a geometric criterion for partial agonism and antagonism.

A simple formula for the zero-point energies of hydrocarbons

Abstract A simple empirical formula exists for the calculation of accurate zero-point energies of hydrocarbons. The formula is based upon the numbers of C and H atoms present, and not at all upon any structural details. The rms deviation from the predictions of this formula for fifty-five molecules is 1.22 kcal/mol.

Borepin and its valence isomers

Ab initio calculations show borepin to be planar and more stable than its valence isomers boranorbornadiene and boranorcaradiene by a considerable amount; the latter have Cs symmetry.

BOWL-SHAPED HYDROCARBONS RELATED TO C60

Ab initio studies at the HF/6‐31G* and B3LYP/6‐31G* levels are reported for two bowl‐shaped hydrocarbons related to C60: C30H12 and C36H12, of C3 and C3v symmetry, respectively. The former has an approximate heat of formation of 211 kcal/mol. Bowl‐to‐bowl interconversion may occur through a planar (C3h) form of ca. 64 kcal/mol greater energy having one imaginary vibrational frequency. The larger C36H12 bowl has a calculated ΔH°f of 265 kcal/mol. Its HF/6‐31G*, B3LYP/6‐31G*, and MM3 bond lengths are in good agreement with a recent X‐ray structure. Chemical shifts for both compounds calculated by the GIAO method are in good agreement with the measured NMR spectra. The observed 13C chemical shifts increase with the extent of pyramidalization. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 189–194, 1998

A theoretical study of pagodane and related systems

Abstract The structure and thermochemistry of pagodane are studied by ab initio molecular orbital theory and molecular mechanics. Satisfactory agreement with the crystal geometry determined by X-ray diffraction and with the experimental heat of formation is obtained only at the highest level of theory used, HF/6-31G ∗ . The predicted ΔH o f of the 4,9-bis(methoxycarbonyl) derivative of pagodane is also in reasonable agreement with experiment, −114.1 vs −107.3 kcal mol −1 . In both cases, ΔH o f is derived from the ab initio energy by means of group equivalents. This method is also used to obtain the heat of formation of the bipentaprismane 2 .

Planar and non-planar borepins

Theoretical studies of borepin and its B-substituted derivatives show planar structures with weak out-of-plane bending modes, often leading to boat-shaped borepins. The lowest vibrational frequency of borepin is 167 cm-1 at both HF/6-31G* and MP2/6-31G* levels, compared to 248 cm-1 calculated for the isoelectronic tropylium ion. Chloro, methyl and amino substitution on boron leads to even lower frequencies. The MP2/6-31G* geometry of 1-chloroborepin is in good agreement with a recent X-ray structure. The low frequencies of out-of-plane bending of the ring atoms imply that suitable replacement of borepin hydrogens should lead to borepins having boat conformations. Several such cases are discussed.

Theoretical studies of triquinacene and diademane

Abstract Ab initio calculations on triquinacene and diademane am reported at the STO-3G and 6–31G* SCF levels. For triquinacene, the geometry, photoionization energies and heat of formation are in good agreement with experiment. The cyclopentene rings of triquinacene appear to be very similar in structure to cyclopentene itself.

Ab-initio heats of formation of medium-sized hydrocarbons. 4. Perhydrotriquinacene

Abstract The ΔH f of perhydrotriquinacene is obtained from that of adamantane and their enthalpy of isomerization computed by ab initio SCF methods, with corrections for zero-point and thermal effects.