Benzdiynes (1,2,4,5-tetradehydrobenzenes): direct observation by wavelength-selective photolyses of benzenetetracarboxylic dianhydrides in low-temperature nitrogen matrixes

MgC6H2 Isomers: Potential Candidates for Laboratory and Radioastronomical Studies

Eighty three stationary points of MgC₆H₂ isomers spanning from 0 to 215 kcal mol^-1 have been theoretically identified using density functional theory at the B3LYP/6-311++G(2d,2p) level of theory. Among them, four low-lying isomers lying within 23.06 kcal mol^-1 (1 eV) have been further characterized in detail using high-level coupled-cluster (CC) methods. The thermodynamically most stable isomer turns out to be 1-magnesacyclohepta-4-en-2,6-diyne (1). The other three isomers, 3-magnesahepta-1,4,6-triyne (2), 1-magnesacyclohepta-2,3,4-trien-6-yne (3), and 1-magnesahepta-2,4,6-triyne (4) lie 8.24, 19.76, and 21.36 kcal mol^-1, respectively, above 1 at the ae-CCSD(T)/cc-pCVTZ level of theory. All the four isomers are polar with a permanent electric dipole moment (μ ≠ 0). Hence, they are potential candidates for rotational spectroscopic studies. Considering the recent identification of magnesium-bearing hydrocarbons such as, MgC₂H and MgC₄H in IRC+10216, it is believed that the current theoretical data may be of relevance to laboratory molecular spectroscopic and radioastronomical studies on MgC₆H₂ isomers. The energetic and spectroscopic information gathered in this study would aid the detection of low-lying MgC₆H₂ isomers in the laboratory, which are indispensable for radioastronomical studies. It is also noted here that neither the National Institute of Standards and Technology Chemistry WebBook nor the Kinetic Database for Astrochemistry lists any isomer of MgC₆H₂ at the moment. Therefore, these isomers are studied here theoretically for the very first time.

2,6,10-Triphenylenotriyne: a star-shaped trisaryne

The synthesis and reactivity of an efficient precursor of a triphenylene-based trisaryne synthon is reported for the first time.

1,7-Naphthodiyne: a new platform for the synthesis of novel, sterically congested PAHs

The synthesis of an efficient precursor of the novel 1,7-naphthodiyne synthon is reported.

Matrix Isolation and Electronic Structure of Di- and Tridehydrobenzenes

Within the past four decades, matrix isolation spectroscopy has emerged as the method of choice for obtaining direct structural information on benzynes and related dehydroaromatics. In combination with quantum chemical computations, detailed insight into the structure and reactivity of di-, tri-, and tetradehydrobenzenes has been obtained. This Review focuses on rather recent developments in aryne chemistry with a special emphasis on the matrix isolation of tridehydrobenzenes and related systems.

Thermochemical Properties of the Benzynes

The thermochemical properties of the benzynes have been the subject of investigation for nearly 50 years. This work provides an overview and assessment of all the experimental thermochemical properties that have been reported for the benzynes, or can be derived from reported thermochemical data. These properties include enthalpies of formation and thermochemical values that correspond to formation and dissociation of the benzynes by neutral and ionic processes. Thermochemical values are provided for both the ground-state singlet and the excited-state triplet states of the benzynes. The starting point for all the thermochemical consideration of the benzynes are the enthalpies of formation, which, in this work, are recommend to be 107.3 ± 3.5, 121.9 ± 3.1, and 138.0 ± 1.0 kcal mol–1 for ortho-, meta-, and para-benzyne, respectively (1 kcal mol–1 = 4.184 kJ mol–1). Whereas the paper predominantly focuses on the experimentally determined values, it also provides a comparison with theoretical studies that have examined the absolute thermochemical properties of the benzynes.

Benzdiynes and Related Dehydroaromatics

Benzdiynes, which are more unsaturated than benzyne, were proposed as reactive intermediates in the pyrolyses of aromatic dianhydrides in 1966. The generation and direct observation of these highly unsaturated species has been an experimental challenge. More than 30 years later, the first direct observation of a benzdiyne, 1,4-bis(trifluoromethyl)-2,3,5,6-tetradehydrobenzene, was reported. We outline the progress in the generation and observation of benzdiynes as well as theoretical studies on the background of benzyne chemistry. Spectroscopic observation of the benzdiynes, in particular detection of an IR band due to the asymmetric stretching of the two triple bonds, in conjunction with quantum chemical computations reveals their multireference character. In connection with the studies for generating benzdiynes, the studies for generating bisarynes with extended π-systems and the most highly unsaturated cyclic C6 are briefly reviewed.

The Experimental and Calculational Thermochemistry of 1,2,4,5-Benzenetetracarboxylic Dianhydride: Is This 10 π Multiring Species Aromatic?

The standard (p degrees = 0.1 MPa) molar enthalpy of formation of 1,2,4,5-benzenetetracarboxylic dianhydride in the gaseous phase, -826.8 +/- 3.1 kJ mol-1, was derived from the standard molar enthalpy of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry and the standard molar enthalpy of sublimation, at T = 298.15 K, measured by Calvet microcalorimetry. In addition, density functional theory calculations have been performed with the B3LYP, MPW1B95, and B3PW91 density functionals and the cc-pVTZ basis set for 1,2,4,5-benzenetetracarboxylic dianhydride and 1,2,4,5-benzenetetracarboxylic diimide. Nucleus-independent chemical shifts calculations show that the aromaticity is restricted to the benzenic ring in both compounds even though they are formally 10 pi polynuclear species.

1,4-Difluoro-2,5-dimethoxybenzene as a Precursor for Iterative Double Benzyne−Furan Diels−Alder Reactions

[reaction: see text] The use of 1,4-difluoro-2,5-dimethoxybenzene as a novel precursor for iterative two-directional benzyne-furan Diels-Alder reactions, using a range of 2- and 3-substituted furans, is reported. Substituted oxabenzonorbornadienes were synthesized following the initial Diels-Alder reaction, which upon ring opening under acidic conditions gave substituted naphthol derivatives. Highly substituted anthracenols were generated in the second benzyne-furan Diels-Alder reaction following acid-catalyzed isomerization of the cycloadducts.

Substituent effects on benzdiyne: a density functional theory study

Density functional theory (DFT) studies were performed to investigate the effect of substituents on the properties of benzdiyne derivatives. Twelve substituted benzdiynes-C(6)X(2), where X = F, Cl, Br, Me, CF(3), CN, OH, NO(2), NH(2), OMe, NMe(2), and Ph-were considered along with the unsubstituted 1,4-benzdiyne. The structures, vibrational frequencies, and IR intensities of these benzdiynes were studied with a popular three-parameter hybrid density functional (B3LYP) combined with the split-valence 6-31G(d) basis set and Dunning's correlation-consistent polarized triple-zeta (cc-pVTZ) basis set. The relative stabilities of the substituted benzdiynes were studied with the help of reaction energies of isodesmic reactions, which showed that the electron-withdrawing groups destabilized the benzdiynes more than they did the corresponding benzenes, whereas the electron-donating groups stabilized the benzdiynes more than they did their benzene counterparts. Correlation analyses revealed that field/inductive effects played a more important role than did resonance effects. The changes in atomic charges and spin populations due to the substituents were also studied. The asymmetric nu(Ctbd1;C) stretching modes obtained were close to the 1500-cm(-)(1) mark. Reinvestigation of the experimental results supported these results; a weak IR band at 1486 cm(-)(1) was assigned to this asymmetric stretching mode in C(6)(CF(3))(2) F. Some other benzdiynes also had large IR intensity values for their asymmetric nu(Ctbd1;C) vibrational modes due to the coupling with other vibrational modes. Heats of formation for the substituted benzdiynes were obtained from the reaction energies calculated at the B3LYP/cc-pVTZ level of theory.