Are 1,2-dihydrodiazetes aromatic? An experimental and computational investigation

A thorough experimental and computational investigation of the aromaticity of the 1,2-dihydrodiazete ring system was carried out. The X-ray crystal structure of 1,2-dihydrodiazete 6 is reported, and the alkene-like reactivity of compound 6 is described. The compound's structure and reactivity suggest that 6 is not aromatic. This conclusion is corroborated by computational results on 6 and related compounds including homodesmotic reactions to test for aromatic stabilization, NICS calculations, and NBO calculations. Compound 6, and 1,2-dihydrodiazetes in general, are concluded to be strained heterocycles with no indication for aromatic stabilization.     

Are antiaromatic rings stacked face-to-face aromatic?

The stacking of 4n pi electron hydrocarbon rings into superphane structures can eliminate their antiaromaticity and result in through-space three-dimensional aromatic character. This is demonstrated by the bond length equalized geometries and diatropic NICS values of the methano-bridged superphane series with interacting three- to nine-membered 4n pi electron rings. Along with triplet and M?bius strategies, stacking is the third way to achieve aromatic ring systems with 4n pi electrons.     

Butalene and related compounds: aromatic or antiaromatic?

Density functional theory (DFT) has been used to study the first three members of the condensed cyclobutadienoid series, butalene (3), bicyclobutadienylene (12), and dicyclobutenobutalene (20). The first is planar and is judged "aromatic" by comparisons with suitable models using both energetic and magnetic criteria. The second is nonplanar, and not aromatic, but not so antiaromatic as cyclobutadiene (11). The third is slightly more antiaromatic and best viewed as a butalene fused to two cyclobutadiene rings; its properties are the sum of aromatic and antiaromatic components, like benzocyclobutadiene. Ring-opening transition states for both 3 and 12 have been located, and these are conrotatorily twisted. The ring-opening barrier for 12 is more than twice that for 3. Ring-opening of 20 involves ring inversion as the only barrier.     

Proton transfers in aromatic and antiaromatic systems. How aromatic or antiaromatic is the transition state? An ab initio study

An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6H7(+)/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5(-)), and the cyclobutenyl cation/cyclobutadiene (C4H5(+)/C4H4) systems and their respective noncyclic reference systems, that is, [structure: see text], [structure: see text] and [structure: see text]. For the aromatic C6H7(+)/C6H6 and C5H6/C5H5(-) systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4H5(+)/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving pi-acceptor groups.     

Are N,N-dihydrodiazatetracene derivatives antiaromatic?

The synthesis and X-ray characterization of two new dialkynylated diazatetracenes and the corresponding N, N-dihydrodiazatetracenes are reported. The dialkynylated heteroacenes are packed in a brick-wall motif that enforces significant overlap of their pi-faces. Cyclic voltammetry indicates that the dehydrogenated forms are easily reduced to their radical anions in solution. The planarity of these species validates the discussion of their aromaticity. Nucleus Independent Chemical Shift (NICS) computations demonstrate that both of these 20 pi and 24 pi electron systems are aromatic. Both experimental and computational results suggest that the aromaticity of the dihydroheteroacenes is reduced.     

What controls the magnetic interaction in bis-μ-alkoxo Mn(III) dimers? A combined experimental and theoretical exploration

The synthesis and magnetic characterisation of a series of bis-μ-alkoxide bridged Mn(III) dinuclear complexes of general formula [Mn(III)(2)(μ-OR)(2)(biphen)(2)(ROH)(x)(L)(y)] (where R = Me, Et; H(2) biphen = 2,2'-biphenol and L = terminally bonded N-donor ligand) is described, doubling the literature basis set for this type of complex. Building on these findings we have categorised all known μ-OR bridged Mn(III) dinuclear complexes into one of three classifications with respect to their molecular structures. We have then employed DFT and MO calculations to assess all potential magneto-structural correlations for this class of compound in order to identify the structural requirements for constructing ferromagnetic family members. Our analysis indicates that the most influential parameter which governs the exchange interaction in this class of compounds is the relative orientation of the JT axes of the Mn(III) atoms. A perpendicular orientation of the JT axes leads to a large ferromagnetic contribution to the exchange. These results also suggest that a large ferromagnetic interaction and a large anisotropy are unlikely to co-exist in such structural types.     

Two-photon absorption properties of two-dimensional π-conjugated chromophores: combined experimental and theoretical study

The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis.     

Are oxocarbon dianions aromatic?

Assessment of the cyclic electron delocalization of the oxocarbon dianions, C(n)()O(n)()(2)(-) and their neutral counterparts C(n)()O(n)() (n = 3-6), by means of structural, energetic, and magnetic criteria, shows that C(3)O(3)(2)(-) is doubly aromatic (both sigma and pi cyclic electron delocalization), C(4)O(4)(2)(-) is moderately aromatic, but C(5)O(5)(2)(-), as well as C(6)O(6)(2)(-), are less so. Localized orbital contributions, computed by the individual gauge for localized orbitals method (IGLO), to the nucleus-independent chemical shifts (NICS) allow pi effects to be disected from the sigma single bonds and other influences. The C-C(pi) contribution to (NICS(0,pi) (i.e., at the center of the ring) in oxocarbon dianions decreases with ring size but shows little ring size effect at points 1.0 A above the ring. On the basis of the same criteria, C(4)O(4) exhibits cyclic electron delocalization due to partial occupancy of the sigma CC bonds. However, the dissociation energies of all the neutral oxocarbons, C(n)()O(n)(), are highly exothermic.     

Chemical bonds without "chemical bonding"? A combined experimental and theoretical charge density study on an iron trimethylenemethane complex

High-resolution X-ray diffraction data, in conjunction with DFT(B3LYP) quantum calculations, have been used in a QTAIM analysis of the charge density in the trimethylenemethane (TMM) complex Fe(eta(4)-C[CH(2)](3))(CO)(3). The agreement between the theoretical and experimental topological properties is excellent. Only one bond path is observed between the TMM ligand and the Fe atom, from the central C(alpha) atom. However, much evidence, including from the delocalization indices and the source function, suggests that there is a strong chemical interaction between the Fe and C(beta) atoms, despite the formal lack of chemical bonding according to QTAIM.     

Quantum chemical properties investigation and molecular docking analysis with DNA topoisomerase II of β-carboline indole alkaloids from Simaba guianensis: a combined experimental and theoretical DFT study

Structural Chemistry - A theoretical and experimental DFT study of the vibrational, structural, and quantum properties of 9-methoxy-canthin-6-one (I) and...     

Lemniscular hexaphyrins as examples of aromatic and antiaromatic double-twist Möbius molecules

Two reported [26] and [28]hexaphyrins are analyzed via measured and computed geometries and NMR-shieldings as examples of respectively 4n + 2 pi-electron aromatic and 4n pi-electron antiaromatic double-twist M?bius ring systems, adopting a lemniscular/figure-eight topology with linking number LK = 2pi. Values of local twist (TW) and nonlocal writhe (WR) derived from the relation Lk = Tw + WR appear relatively insensitive to the aromatic/antiaromatic character. The [26]hexaphyrin may adopt differing solution and solid-state conformations.     

AsS2Cl-an Arsenic(V) compound? Formation, stability and structure of gaseous AsSCl and AsS2Cl--a combined experimental and theoretical study

By reaction of solid As(4)S(4) with gaseous Cl(2) at a temperature of 410 K gaseous AsSCl and AsS(2)Cl are formed. Unexpectedly in AsS(2)Cl the arsenic is not of formal oxidation state +V but +III: the molecular structure of AsS(2)Cl is arranged as a 1-chloro-dithia-arsirane and comprises an hitherto unknown AsS(2) three-membered ring. Thermodynamic data on AsSCl and AsS(2)Cl are obtained by mass spectrometry (MS). The experimental data are extended and confirmed by ab initio quantum chemical calculations (QC). The following values are given: Δ(f)H(0)(298)(AsSCl) = -5.2 kJ mol(-1) (MS), Δ(f)H(0)(298)(AsSCl) = 1.7 kJ mol(-1) (QC), S(0)(298)(AsSCl) = 296.9 J K(-1) mol(-1) (QC) and c(p)(0)(T)(AsSCl) = 55.77 + 3.97 × 10(-3)T- 4.38 × 10(5)T(-2)- 1.83 × 10(-6)T(2) and Δ(f)H(0)(298)(AsS(2)Cl) = -39.0 kJ mol(-1) (MS), Δ(f)H(0)(298)(AsS(2)Cl) = -20.2 kJ mol(-1) (QC), S(0)(298)(AsS(2)Cl) = 321.3 J K(-1) mol(-1) (QC) and c(p)(0)(T)(AsS(2)Cl) = 80.05 + 5.09 × 10(-3)T- 7.61 × 10(5)T(-2)- 2.35 × 10(-6)T(2) (298.15 K < T < 1000 K) (QC). The ionization energies are determined (IP(AsSCl) = 10.5, IP(AsS(2)Cl) = 10.2 eV). The IR spectrum of AsSCl is detected by means of matrix isolation spectroscopy. The estimated force constant f(As=S) = 4.47 mdyn·?(-1) gives rise to an As=S double bond.     

Effect on ring current of the Kekulé vibration in aromatic and antiaromatic rings

Derivative current-density maps are used to follow the changes in ring-current (and hence, on the magnetic criterion, the changes in aromaticity) with the Kekulé vibrations of the prototypical aromatic, antiaromatic, and nonaromatic systems of benzene, cyclooctatetraene (COT), and borazine. Maps are computed at the ipsocentric CHF/6-31G**//RHF/6-31G** level. The first-derivative map for benzene shows a growing-in of localized bond currents, and the second-derivative map shows a pure, paratropic "antiring-current", leading to the conclusion that vibrational motion along the Kekulé mode will reduce the net aromaticity of benzene, on average. For planar-constrained D(4h) COT, the Kekulé mode (positive for reduction of bond-length alternation) increases paratropicity at both first and second order, indicating an average increase in antiaromaticity with zero-point motion along this mode. On the ring-current criterion, breathing expansions of benzene and D(4h) COT reduce aromaticity and increase antiaromaticity, respectively.     

A theoretical investigation of Zn?Zn and Be?Be one electron bond

Attachment of one electron to 1,2-diBeX-benzene and 1,2-diZnX-benzene derivatives leads to the formation of stronger Be Be and Zn Zn interaction compared to the neutral one. This is reflected in the dramatic shortening of the Be Be and Zn Zn distance. The formation of these 2-center-1-electron bonds have also been confirmed by topological survey of electron density using quantum theory of atoms in molecules and electron localization function. The formation of these bonds is expected to render stability to these radical anions. These radical anions are stable toward electron detachment and computed bond dissociation energy values are also significant.     

A theoretical investigation of the interaction between substituted carbonyl derivatives and water: Open or cyclic complexes?

The structures and binding energies of complexes between substituted carbonyl bases and water are the B3LYP/6‐311++G(d,p) computational level. The calculations also include the proton affinity (PA) of the O of the C?O group, the deprotonation enthalpies (DPE) of the CH bonds along a natural bond orbital analysis. The calculations reveal that stable open C?O···H_wO_w as well as cyclic CH···O_wH_w···O?C complexes are formed. The binding energies for the open complexes are linearly related to the PAs, whereas the binding energies for the cyclic complexes depend on both the PA and DPE. Different indicators of hydrogen bonds strength such as electron charge density, intramolecular and intermolecular hyperconjugation energy, occupation of orbitals, and charge transfer show significant differences between open and cyclic complexes. The contraction of the CH bond of the formyl group and the corresponding blue shift of the ν(CH) vibration are explained by the classical trans lone pair effect. In contrast, the elongation or contraction of the CH₃ group involved in the interaction with water results from the variation of the orbital interaction energies from the σ(CH) bonding orbital to the σ* and π* antibonding orbitals of the C?O group. The resulting blue or red shifts of the ν(CH₃) vibrations are calculated in the partially deuterated isotopomers. © 2012 Wiley Periodicals, Inc.     

Sodium dialkyl-amidozincates: alkyl or amido bases? An experimental and theoretical case study

Alkali metal zincate reagents are attracting considerable attention at present in respect to their often special reactivity/selectivity in hydrogen-metal and halogen-metal interconversion reactions. Heteroleptic diorgano-amidozincates, typified by lithium di-tert-butyltetramethylpiperidinozincate, have proved to be especially useful reagents in such applications. In this paper the related sodium TMP-zincate, prepared as its TMEDA (N,N,N',N'-tetramethylethylenediamine) adduct, [TMEDA.Na(mu-tBu)(mu-TMP)Zn(tBu)], 1, is introduced. This new zincate was synthesized from a 1:1:1 mixture of tBu2Zn, NaTMP, and TMEDA in hexane solution, as a colorless crystalline solid in an isolated yield of 58%. It has been characterized in solution by 1H and 13C NMR spectroscopic studies. An X-ray crystallographic study reveals that 1 adopts a five-membered (NaNZnCC) ring system featuring a TMP bridge and an unusual, asymmetrical tBu bridge involving a Na...Me agostic contact. Probing the basicity of 1, reaction with benzene affords the new hetero(tri)leptic zincate [TMEDA.Na(mu-Ph)(mu-TMP)Zn(tBu)], 2, which has also been crystallographically characterized. Thus, in this hydrogen-metal exchange reaction 1 functions as an alkyl base, with the elimination of butane, as opposed to an amido base. Also reported are DFT calculations using B3LYP functionals and the 6-311G** basis set on model zincate systems, which intimate that the preference of 1 for tBu ligand transfer over TMP ligand transfer in the reaction toward benzene is due to favorable thermodynamic factors.     

Comparative theoretical and experimental analysis of hydrocarbon σ-radical cations

The structures of σ-radical cations formed by ionization of adamantane, twistane, noradamantane, cubane, 2,4-dehydroadamantane, and protoadamantane were optimized at the B3LYP, B3LYP-D, M06-2X, B3PW91, and MP2 levels of theory using 6-31G(d), 6-311+G(d,p), 6-311+G(3df,2p), cc-PVDZ, and cc-PVTZ basis sets. On the whole, single-configuration approximations consistently describe the structure and transformations of the examined σ-radical cations. The best correlations (r = 0.97–0.98) between the calculated adiabatic ionization potentials and experimental oxidation (anodic) potentials of hydrocarbons were obtained in terms of B3PW91 approximation.