Nicolaos D. Epiotis

The π-donating ability of heteroatoms in α-substituted methyl cations

Abstract An ab initio SCF geometry optimization on the simple cations H 2 X, with X = F, Cl, NH 2 and PH 2 has been performed at the split valence shell (4-31G) level. The computed optimum conformations correspond in each case to a structure in which all atoms lie in the same plane. Comparison of the computed charge distributions reveals that the third period heteroatom (Cl and P) is a better π electron donor than the corresponding second period analogue (F and N). This result parallels that obtained recently [ Can. J. Chem. 53 , 1144 (1975)] for S and O in H 2 OH and H 2 SH, but contradicts current notions based on assumed values of the C π -X π overlap integrals. It is shown here by explicit calculations of overlap integrals that these assumptions are not always correct. Furthermore, it is shown that arguments based only on overlap are necessarily incomplete since they neglect terms like the energy difference between the interacting orbitals which can play a dominant role. The relative importance of such terms is discussed for these species.

The “Forbidden” World of Chemistry

Huckel’s rule1, the Woodward-Hoffmann rules2, and, in a broader sense, Huckel Molecular Orbital (HMO) theory3 have had profound impact on chemistry in a way which is now well recognized and admired. Perhaps, the most important and time-lasting contribution of HMO theory has been the revelation of what in Valence Bond (VB) terms we call parity control of stereoselection4, or, in more familiar language, the revelation of the fact that a ground state molecule, a transition state, or, any molecular system, in general, can be thought of as the product of a “forbidden” or “allowed” union of two component fragments with the latter mode of union being energetically more favorable than the former one.

The art of “arrow pushing”: Is the hyperconjugation model theoretically justified?

Abstract The language of the chemist is used in order to argue that intuitive “arrow pushing” is not theoretically justified, i.e. trends traditionally attributed to the contribution of certain resonance structures can actually be due to the contribution of entirely different resonance structures. The hyperconjugation recipe of “arrow pushing” is investigated and the approach used is illustrated by reference to reactions of the type 2 MAD → MA 2 + MD 2 and to conformational isomerism.

Beyond symmetry: From organic to main group to stereochemistry and back

Abstract The thesis of this paper is: molecular stereoelectronics cannot be understood on the basis of orbital symmetry considerations alone. Rather, it is symmetry coupled to color , a code word for the intrinsic ability of an atom to form bonds by overlap, that jointly dictate molecular shape. This can only be seen by a theory which explicitly and pictorially projects the interplay between atomic excitation necessary for bond formation and the energy gain due to bond-making itself. Such is MOVB theory, a theory tailored for the human mind rather than for the computer and based on a fusion of MO and VB concepts. According to MOVB, one views a molecule as a composite of two or more fragments and writes the total wavefunction as a linear combination of substates, called bond diagrams, each of which is nothing other than a distinct, symmetry-consistent way of connecting the fragments by bonds or antibonds. The method is first illustrated and then applied to specific stereochemical problems drawn from various subdisciplines of chemistry. The following conclusions emerge: (a) There is no such thing as an “Isoelectronic Principle” in molecular stereochemistry; this is, at best, a local concept often useful for “row comparisons”. (b) Organometallic fragments, such as metal carbonyls, resemble heavy main group atoms in the way they bind. (c) With respect to the entire Periodic Table, “organic” stereochemical principles are the exception and not the rule. (d) From knowledge of heavy main group structures, one may predict organometallic structures, and conversely, by reference to a dictionary of symmetry/color analogies. (e) The fact that valence isoelectronic species can have different shapes is the physical manifestation of the fact that the mechanism of electron delocalization is color-dependent, as shown in a separate paper.

Thiacyclobutadiene and thiabenzene: A comparative theoretical analysis

Abstract The two title compounds have been investigated theoretically using an ab initio SCF-MO treatment at the minimal STO-3G level. It has been found that in the most stable conformation thiacyclobutadiene has all atoms approximately in the same plane except for the H atom bonded to sulphur (i.e. a pyramidal sulphur). On the other hand, in the most stable conformation of thiabenzene, the sulphur atom not only is pyramidal, but also ~10° out of the plane containing the carbon system. The barriers to pyramidal inversion at the sulphur centres are ~48 and ~56 kcal mol for thiacyclobutadiene and thiabenzene, respectively. The properties of these molecules are rationalized by means of Perturbation Molecular Orbital (PMO) theory.

The manifestation of non-bonded attraction in the physical properties of cis and trans olefins

Abstract Non-bonded attraction is suggested to account for a host of differences in the physical properties of cis and trans olefins of the type XHCCHX. The main predictions are: (i) The cis isomer is more stable than the trans isomer; (ii) The CC bond is longer and the C-X bonds are shorter for the cis isomer; (iii) The π MOs orbital energies of the two isomers differ such that the trans isomer is a better electron donor and electron acceptor than the cis isomer. Ab initio calculations at the STO-3G and the 4-31G levels in support of the model are presented. The photoelectron spectra of cis and trans difluoro, dichloro and dibromoethylene are discussed, and found to be in accord with our qualitative model.

The electronic structure of methane and its derivatives: apparent versus effective symmetry control of sigma bonding

Abstract Resonance theory is one major conceptual basis of chemistry. However, it has been shown that resonance theory is “VB theory without symmetry control”. Hence, many qualitative concepts of chemistry must either be incorrect or accidentally correct. Molecular orbital valence bond (MOVB) theory is “VB theory over canonical fragment MOs” or “VB theory with symmetry control made clear” and it is ideally suited for explaining old facts and for designing new chemistry. In this paper, a systematic presentation is given of central problems which are now recognized to have no satisfactory solution. The MOVB solution is put forward, and the problems with current interpretations are explained. The electronic structures of methane and fluoromethane are discussed in order to illustrate the following general MOVB concept: the nature of the AOs of an atom creates a distinction between apparent and effective molecular symmetry. When the constituent atoms are all first-row non-metal atoms the former is different from the latter. For example, the apparent symmetry of fluoromethane is C3v, but the effective symmetry is Td, i.e., the σ bonds are made as if the true molecular symmetry were Td (methane-type bonds). By contrast, the apparent and effective symmetries of, for example, chlorostannane are the same, C3v. This difference between C and Sn is a result of the different absolute and relative radial extensions of the valence AOs of the central atom.

The echinos model of cluster electronic structure: What is angle strain?

Abstract According to Molecular Orbital Valence Bond (MOVB) theory (based on a fusion of MO and VB concepts) a molecule can be viewed as a composite of two or more fragments and the total wavefunction is a linear combination of substates, called bond diagrams, each of which is a pictorial representation of a distinct, symmetry-consistent way of connecting fragments by bonds or antibonds. In any cluster, the atomic orbitals (AOs) can be partitioned into radial and tangential AOs with the former interacting to produce the symmetry adapted MOs of the needle (N) fragment and the latter interacting to yield the symmetry adapted MOs of the surface (S) fragment. A cluster is then a species in which electrons are deposited in the needle and surface fragments with coordinate-type bonds connecting these two fragments and with ligands attaching themselves on the composite NS system via covalent bonds. This MOVB Echinos (Greek sea urchin) model is applied to the problem of “angle strain”. Contrary to common intuition, it is concluded that cyclopropane is strained relative to cyclohexane because, while overall bonding is stronger in cyclopropane, the amount of atomic excitation needed for bond making is disproportionately large. The reason behind this is an odd-even distinction of rings which originates from the fact that odd members have coupled Huckel and Mobius, whereas even members have coupled Huckel and Huckel needle and surface AO rings, respectively. The implication is that σ bonds are not system-invariant, i.e. the CC and CH bonds of cyclopropane and cyclohexane are completely different from one another. MOVB theory explains the interplay of atom excitation (investment) and bond making (return) which is actually what determines “stability”, and can be used to answer some vexing questions. Why is cubane so much more strained than cyclopropane? Why does replacement of C by Si sometimes increase (e.g. cyclopropane) and sometimes decrease (e.g. cubane) strain? Why is tetrahedral N4 so unstable but tetrahedral P4 a stable allotrope? Why do metal atoms form stable three-dimensional clusters, i.e. why does strain disappear in metallic systems? What is wrong with current approaches which have failed to provide us with a clear understanding of strain, as is now well recognized?

THERMAL CHEMISTRY AS AN EXERCISE IN PHOTOCHEMISTRY

Abstract The Linear Combination of Fragment Configurations approach is used to generate qualitative potential energy surfaces. The theory is applied to “forbidden” cycloadditions, solvolysis, and electrophilic aromatic substitution. It is also used to illuminate principles of chemical selectivity and the electronic structure of diradicals. It is argued that chemical intermediates in thermal reactions arise from excited surfaces under the influence of perturbations such as substituent effects and solvation.

The concept of interstitial bonding

Abstract Molecular orbital valence bond (MOVB) theory suggests that, in addition to the well-known covalent and ionic mechanisms, there exist two distinct types of bonding: overlap dispersion (discussed in a previous paper) and ionic overlap induction where overlap is assisted by an overlap-independent coulomb binding mechanism. Ionic overlap induction is exemplified by the RLi tetramer in which the Li 4 unit relinquishes, e.g. one electron to the R 4 unit setting up an electric field which glues the four Li species in Li + 4 by interstitializing the remaining three electrons. As a result, three R radicals can now make three interstitial covalent bonds with the Li + 4 fragment with one R − becoming bound to the same fragment by ionic forces. Our analysis implies that there exists a well-defined and separate bonding mode which can exist only when electropositive metals are combined with electronegative nonmetals. One of the additionally cited illustrative examples is a CuO dimer of biological significance. This sets the stage for the eventual presentation of a theory of high temperature superconductivity based on these ideas.