Thomas Bally

Calculating Accurate Proton Chemical Shifts of Organic Molecules with Density Functional Methods and Modest Basis Sets

The purpose of this paper is to convince practitioners of (1)H NMR spectroscopy to consider simple quantum chemical calculations as a viable option to aid them in the assignment of their spectra. To this end, it is demonstrated, on a test set of 80 conformationally stable molecules of various kinds carrying different functional groups, that, in contrast to what is claimed in the literature, large basis sets are not needed to obtain rather accurate predictions of (1)H NMR chemical shifts by quantum chemical calculations. On the other hand, modeling the solvent by an SCRF-type calculation may improve certain predictions significantly. The best accuracy/cost ratio is provided by GIAO calculations in chloroform as a solvent with the specially parametrized WP04 functional of Cramer et al. using the cc-pVDZ or 6-31G** basis set, closely followed by similar calculations with the ubiquitious B3LYP functional (both predict (1)H chemical shifts with an average deviation of ca. 0.12 ppm, if the results are scaled linearly). A slightly higher accuracy can be attained by adding diffuse functions to the basis set, but going to the triple-zeta basis sets which have invariably been used hitherto in calculations of chemical shifts does not lead to any improvement. The popular increment schemes such as those implemented in the ChemDraw or ACD programs do not do nearly as well and are often incapable of correctly distinguishing stereoisomers.

Quadrannulene: A Nonclassical Fullerene Fragment

The presence of nonhexagonal rings in an otherwise graphitic lattice induces curvature. Pentagons are common—twelve pentagons surrounded by hexagons make up C60. Larger rings are present in Stone–Wales defects, and the polyhedral formula of Euler mandates their existence in carbon nanotube Y-junctions. Except for a single, partially saturated example, four-membered rings in graphitic structures are, however, unknown. The smallest examples of these graphitic structures are the [n]circulenes, wherein a central n-sided polygon is surrounded by n-fused benzenoid rings. [7]Circulene, first prepared by Yamamoto, Nakazaki, and coworkers in 1983, is saddle shaped. [6]Circulene, or coronene, is the trivial, planar case, and it was first synthesized by Scholl andMeyer in 1932 but also occurs naturally. [5]Circulene, or corannulene, comprises 1/3 of the C60 skeleton and has been intensely studied, and it was first prepared by Lawton and Barth in 1971. Whereas a few pioneering attempts have been reported, [4]circulene has never been synthesized before. We report herein the preparation and characterization of a stable [4]circulene. By analogy with Lawton s naming of corannulene (Latin: cor, heart; annula, ring), we suggest the trivial name quadrannulene (Latin: quadra, square; annula, ring) for the [4]circulene parent. Hence, we name this derivative 1,8,9,16-tetrakis(trimethylsilyl)tetra-cata-tetrabenzoquadrannulene, abbreviated TMS4-TBQ. The IUPAC name and atom numbering are given in the Supporting Information. Our unoptimized five-step synthesis (Scheme1) provides TMS4-TBQ in very low yield. Hopf and co-workers recently summarized two synthetic strategies to the quadrannulene core: making the four-membered ring from [2,2]-paracyclo-

Matrix isolation, time-resolved IR, and computational study of the photochemistry of benzoyl azide

It was shown recently on the basis of DFT calculations (N. P. Gritsan and E. A. Pritchina, Mendeleev Commun., 2001, 11, 94) that the singlet states of aroylnitrenes undergo tremendous stabilization due to an extra N–O bonding interaction. To test experimentally the multiplicity and the structure of the lowest state of benzoylnitrenes we performed a study of their photochemistry in Ar matrices at 12 K. Formation of two species was observed on irradiation of benzoyl azide (1b) and its 4-acetyl derivative (1c). One of these species has an IR spectrum, which is consistent with that of isocyanate (2b,c). The IR and UV spectra of the second intermediate are in very good agreement with the calculated spectra of the singlet species (3b,c), whose structure is intermediate between that of a carbonylnitrene and an oxazirene. We further examined the photochemistry of benzoyl azide in solution at ambient temperatures by nanosecond time-resolved IR methods and obtained additional evidence for the singlet ground state of benzoylnitrene as well as insight into its reactivity in acetonitrile, cyclohexane, and dichloromethane. The above experiments were accompanied by quantum chemical calculations which included also a thorough investigation of the parent species, formylnitrene, at different levels of theory.

On the photoelectron spectrum of p-benzoquinone

A high-resolution photoelectron spectrum of p-benzoquinone in the low energy (9.5–11.5 eV) region is reported and analyzed with the aid of simulations based on high-level ab initio calculations. The results generally support the notion that the two prominent spectral features in this region are each due to a pair of final ion states. The lower energy feature beginning near 10 eV is due to oxygen lone-pair ionizations, while that beginning near 11 eV comes from π electron removal. Contrary to previous interpretations of the spectrum, however, the results of this study indicate that the two π states are nearly degenerate, with the strongest peak in the photoelectron spectrum representing a convolution of the corresponding pair of 0–0 ionizations.

The Pyrolysis of Isoxazole Revisited: A New Primary Product and the Pivotal Role of the Vinylnitrene. A Low-Temperature Matrix Isolation and Computational Study

This paper describes the pyrolysis of parent isoxazole and of its 5-methyl and 3,5-dimethyl derivatives by the high-pressure pulsed pyrolysis method, where activation of the precursor molecules occurs predominantly by collisions with the host gas (Ar in our case), rather than with the walls of the pyrolysis tube, where catalyzed processes may occur. The products were trapped at 15 K in Ar matrices and were characterized by vibrational spectroscopy. Thereby, hitherto unobserved primary products of pyrolysis of isoxazole and of its 5-methyl derivative, 3-hydroxypropenenitrile or 3-hydroxybutenenitrile, respectively, were observed. E-Z photoisomerization could be induced in the above hydroxynitriles. On pyrolysis of isoxazole, ketenimine and CO were observed as decomposition products, but this process did not occur when the 5-methyl derivative was pyrolyzed. Instead, the corresponding ketonitrile was formed. In the case of 3,5-dimethylisoxazole, 2-acetyl-3-methyl-2H-azirine was detected at moderate pyrolysis temperatures, whereas at higher temperatures, 2,5-dimethyloxazole was the only observed rearrangement product (next to products of dissociation). These findings are rationalized on the basis of quantum chemical calculations. Thereby it becomes evident that carbonyl-vinylnitrenes play a pivotal role in the observed rearrangements, a role that had not been recognized in previous theoretical studies because it had been assumed that vinylnitrenes are closed-shell singlet species, whereas they are in fact open-shell singlet biradicaloids. Thus, the primary processes had to be modeled by the multiconfigurational CASSCF method, followed by single-point MR-CISD calculations. The picture that emerges from these calculations is in excellent accord with the experimental findings; that is, they explain why some possible products are observed while others are not.

Isomerism: The same but different

Electromerism is an unfamiliar concept to many chemists and refers to molecules that are not conventional isomers but instead differ in how the electrons are distributed across their structure. A novel example of such electromers has now been demonstrated.

Spectroscopic evidence for a new type of bonding between a thioether radical cation and a phenyl group

The oxidation potential of thioethers constrained to be near aromatic rings is lowered, due to an antibonding interaction between the p-type sulfur lone pair with the neighboring phenyl π-system which on removal of an electron becomes a new kind of 3-electron S∴π bonding that reveals itself in the photoelectron spectrum and by an electronic transition involving the orbitals participating in the S∴π bond.

Excited states in polyene radical cations. An ab initio theoretical study

Abstract The lowest π- and σ-doublet excited states of butadiene, bauntriene and octatetraene radical cation are studied using multiconfigurational second-order perturbation theory. The calculated excitation energies are found to be within ⩽ 0.2 eV of the experimental energies. Our results allow us to interpret the different features observed in photoelectron spectra which are measured at the ground state geometry of the neutral molecules as compared to the electronic absorption spectra which are taken at the ground state geometry of the cations.

Light induced processes in organic molecular ions

Of the many different chemical processes where organic molecular ions (in particular cations Nt) are known to engage in, most are thermally activated ones. Recently, however, a growing body of evidence has begun to emerge which indicates that Mt also undergo a variety of photochemical transformations. They are quite distinct from those undergone by their parent neutrals (M) in that they can usually be induced with visible light and often follow different courses. This short review intends to summarize the present state of knowledge about Mt-photoprocesses and highlights some pertinent photophysical parameters of Mt used to rationalize these reactions. A general comparison between gas— and condensed phase results is made and the example of trans-stilbenet and cis_stilbenet interconversion which appears at present the best studied Mt-photoreaction is discussed in some detail.

Synthesis and structure of m-terphenyl thio-, seleno-, and telluroethers.

Several routes for the synthesis of m-terphenyl thio-, seleno-, and telluroethers were investigated. m-Terphenyl iodides react with diphenyl diselenides or ditellurides (CsOH·H(2)O, DMSO, 110 °C) to give the desired compounds in 19-84% yield which significantly extends the previously reported such reactions because o-benzyne cannot be an intermediate as previously suggested. However, the most general synthetic route was that involving reaction of 2,6-diaryl Grignard reagents with sulfur, selenium, or tellurium electrophiles. The m-terphenyl thio-, seleno-, and telluroethers were characterized spectroscopically and, in one case, by single-crystal X-ray analysis. Certain of these compounds showed atropisomerism and barriers for interconversion of isomers were determined by variable-temperature NMR spectroscopy. The barriers for interconverting the syn and anti atropisomers increase on going from the analogous S to Se to Te compounds. Calculations on this isomerization revealed that the barriers are due to rotation about the aryl-aryl bond and that the barriers for rotation about the aryl-chalcogen bond are much lower.