Thomas L. Allen

Bio: Thomas L. Allen is an academic researcher from University of California, Davis. The author has contributed to research in topics: Molecular geometry & Ab initio. The author has an hindex of 16, co-authored 26 publications receiving 630 citations. Previous affiliations of Thomas L. Allen include Indiana University & University of Nottingham.

Synthesis and characterization of the monomeric imides Ar'MNAr' ' (M = Ga or In; Ar' or Ar' ' = terphenyl ligands) with two-coordinate gallium and indium.

Abstract: Reaction of Ar‘MMAr‘ (M = Ga or In) with N3Ar‘ ‘ (Ar‘ = C6H3-2,6-Dipp2, Dipp = C6H3-2,6-Pri2, Ar‘ ‘ = C6H3-2,6(Xyl-4-But)2) afforded the first monomeric imides of heavier group 13 elements with two-coordinate metals. Planar, trans-bent structures with short M−N bond distances were observed, which are consistent with lone pair character at both M and N and a bond order less than the formally expected triple one.

Theoretical studies of multiple bonds in gallium–gallium and germanium–germanium compounds

Abstract: A recent publication concerning the synthesis and structure of the compound Na2{GaC6H3-2,6-Trip2}2 (Trip = C6H2-2,4,6-iPr3), which has a trans-bent geometry, has generated considerable discussion owing to the description of its gallium–gallium bond as a triple one. To provide a theoretical perspective on this subject, we have studied a series of model compounds by the methods of molecular electronic structure theory. For the species trans-Li2MeGaGaMe we find a Ga–Ga bond order somewhat less than two, instead of a triple bond, owing to the antibonding character of one of the molecular orbitals. In the isoelectronic trans-MeGeGeMe we find an essentially GeGe double bonded structure. The neutral trans-MeGaGaMe molecule has a weak Ga–Ga single bond rather than a Ga–Ga double bond. Each of these molecules features a lone pair orbital of bu symmetry, with the main regions of electron density located on the gallium or germanium centers, formed by mixing a bonding π orbital and an antibonding σ* orbital in a second-order Jahn–Teller effect.

A possible role for triplet H2CN+ isomers in the formation of HCN and HNC in interstellar clouds

Abstract: The structures and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self‐consistent field and configuration interaction calculations. When both hydrogen atoms are attached to the nitrogen atom, H2NC+, the molecule has its lowest triplet state energy, which is 97.2 kcal mol−1 above the energy of the linear singlet ground state. The structure has C2v symmetry, with an HCH bond angle of 116.8°, and bond lengths of 1.009 A (H–N) and 1.268 A (N–C). Other isomers investigated include the H2CN+ isomer at 104.7, the cis‐HCNH+ isomer at 105.3, and the trans‐HCNH+ isomer at 113.6 kcal mol−1. The H2CN+ isomer has an unusual ’’carbonium nitrene’’ structure, with a C–N bond length of 1.398 A. It is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3. Specifically, a triplet path is postulated in which C+ and NH3 yield the triplet H2NC+ isomer, which then yields the singlet H2NC+ isomer by phospho...

A Combined Charge and Energy Decomposition Scheme for Bond Analysis

Abstract: In the present study we have introduced a new scheme for chemical bond analysis by combining the Extended Transition State (ETS) method [Theor. Chim. Acta 1977, 46, 1] with the Natural Orbitals for Chemical Valence (NOCV) theory [J. Phys. Chem. A 2008, 112, 1933; J. Mol. Model. 2007, 13, 347]. The ETS-NOCV charge and energy decomposition scheme based on the Kohn−Sham approach makes it not only possible to decompose the deformation density, Δρ, into the different components (such as σ, π, δ, etc.) of the chemical bond, but it also provides the corresponding energy contributions to the total bond energy. Thus, the ETS-NOCV scheme offers a compact, qualitative, and quantitative picture of the chemical bond formation within one common theoretical framework. Although, the ETS-NOCV approach contains a certain arbitrariness in the definition of the molecular subsystems that constitute the whole molecule, it can be widely used for the description of different types of chemical bonds. The applicability of the ETS-...

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

Abstract: This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

Calculation of Molecular Structure and Energy by Force-Field Methods

Abstract: Publisher Summary This chapter discusses the calculation approaches to determine molecular structure and energy by force-field methods. These include the ab initio method, the force-field method, mechanical model, and energy minimization schemes. Methods for determining the internal energy of an organic molecule traditionally depend on heats of combustion. Because calculations of the force-field variety have become so accurate and efficient, many studies using such calculations to attack problems in molecular structure have been and are currently being carried out. These studies give, as a minimum, the bond lengths, bond angles, and torsional angles of the molecules examined, in addition to information concerning energy. The force-field method offers a rapid, convenient and reliable method for the determination of molecular structures and energies. While there are limitations to the method, as there are with each of the experimental methods, the usefulness of this technique is generally appreciated.