Breathing viability into cyclonona‐3,5,7‐trienylidenes via α‐dimethyl and ά‐moieties at DFT

Cyclonona‐3,5,7‐trienylidene ( 1 ) changes from being a transition state (TS) to minimum states when substituted by α‐methyl groups and ?‐X, where X = CMe₂, NMe, PMe, O, S, cyclopropyl, and SiMe₂ ( 2 , 3 , 4 , 5 , 6 , 7 , 8 , respectively) at density functional theory. Specifically, the parent carbene 1 exhibits a negative vibrational force constant and proves to be an unreachable electrophilic TS while shows C_s symmetry with an NBO atomic charge of +0.70 on its carbenic center. It has a triplet ground state with a rather small singlet‐triplet energy gap (ΔE_s–t = ?4.1 kcal/mol). In contrast, all of its seven scrutinized derivatives enjoy reachable global minima, with C₁ symmetry, desired nucleophilicity, and singlet closed shell (S_cs) ground states (for all but 8 which remains triplet). Stability is indicated by relative ΔE_s–t values: 2 > 3 > 4 > 5 > 6 > 7 > 1 > 8 . The highest ΔE_s–t as well as NBO carbenic atomic negative charge (?0.74) are displayed by 2 . Our carbenes ( 2 , 3 , 4 , 5 , 6 , 7 ) appear more nucleophilic than the synthesized N‐heterocyclic carbenes (imidazol‐2‐ylidenes). Copyright © 2013 John Wiley & Sons, Ltd.     

2,2,9,9‐Tetramethylcyclonona‐3,5,7‐trienylidene vs. its heterocyclic analogues: A quest for stable carbenes at DFT

Replacement of α‐methylenes with BH, AlH, CMe₂, SiH₂, NH, NMe, N^tButyl, NPh, PH, O, and S in non‐planar cyclonona‐3,5,7‐trienylidene (CH₂) alters its status from an unstable transition state to rather stable minima, at B3LYP/6‐311++G**//B3LYP/6‐31 + G* levels of theory. All species appear with singlet closed shell (S_cs) global minima, except for SiH₂ and CH₂ which exhibit triplet electronic ground states. The order of stability based on singlet–triplet energy gap (ΔE_s–t / kcalmol^?1) is: CMe₂ (45.8) > NH (35.8) > NMe (32.3) > O (31.5) > N^tButyl (27.7) ≥ NPh (27.5) ≥ BH (27.4) > S (21.9) > PH (17.0) > CH₂ (?4.4) > SiH₂ (?12.5). In contrast to many reports on N‐heterocyclic carbenes, here alkyl groups appear to exert a higher stabilizing effect than heteroatoms, making CMe₂ the most stable. In addition bulky NMe, N^tButyl, and NPh appear more nucleophilic than their synthesized imidazol‐2‐ylidene congeners. Excluding SiH₂, isodesmic reactions reveal that all substituents stabilize singlet state considerably more than the corresponding triplet. Finally, this work is hoped to pave the path for future matrix isolations and IR studies of these rather stable cyclic non‐planar carbenes. Copyright © 2013 John Wiley & Sons, Ltd.     

Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (ΔE_{HOMO ? LUMO}), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization, and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1_CH2) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ?‐SiMe₂, ?‐NMe, ?‐PMe, ?‐O, ?‐S, ?‐CH₂, ?‐cyclopropyl, and ?‐CMe₂ (2_SiMe2, 2_NMe, 2_PMe, 2_O, 2_S, 2_CH2, 2_cyclopropyl, and 2_CMe2, respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant p_π orbital of the carbene center (Walsh_cyclopropyl → p_{π carbene}). Among them, the singlet closed shell 2_NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (ΔE_{s ? t} = 27.1 kcal mol^?1). In contrast, the least stable derivative is triplet 2_SiMe2, which exhibits the lowest relative ΔE_{s ? t} of ?5.5 kcal mol^?1. The overall trend of ΔE_s‐t is 2_NMe > 2_PMe > 2_S > 2_O > 2_cyclopropyl > 2_CMe2 > 2_CH2 > 1_CH2 > 2_SiMe2. With one negative force constant, the unsubstituted 1_CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.     

A theoretical investigation into dimethylcarbene and its diamino and diphosphino analogs: effects of cyclization and unsaturation on the stability and multiplicity

High levels of ab initio and DFT calculations (B3LYP/6‐311++G**, B3LYP/AUG‐cc‐pVTZ, and CCSD(T)/6‐311++G** levels) coupled with isodesmic reactions are used to compare and contrast the multiplicities and relative stabilities of singlet (s) and triplet (t) acyclic carbenes, including: dimethylcarbene, diaminocarbene, and diphosphinocarbene along with their saturated and unsaturated cyclic ones. Cyclization is unfavorable for all acyclic carbenes while unsaturation of cyclic analogs appears favorable. The simultaneous cyclization and unsaturation of dimethylcarbene increases the singlet–triplet energy gap (ΔE_s–t), while for diphosphinocarbene the situation is reversed. For diaminocarbene the increase of ΔE_s–t is encountered only during cyclization. Copyright © 2009 John Wiley & Sons, Ltd.     

Ylide stabilized carbenes: a computational study

High‐level Density Functional Theory calculations, coupled with appropriate isodesmic reaction, are employed to investigate the effects of α‐carbon, ammonium, phosphorus, and sulfur ylides, cyclization, and unsaturation on the stability, multiplicity, and reactivity of novel singlet (S) and triplet (T) carbenes. Among them the highly π‐donating α‐ammonium ylide is found to exert the highest stabilizing effect on the carbenic center. α‐Ammonium ylides resist dimerization and hydrogenation. They show wider singlet–triplet energy gap (ΔΕ_S–T), broader band gap (ΔΕ_HOMO–LUMO), and higher nucleophilicity compared to the reported stable N‐heterocyclic carbenes. Aromatic cyclic unsaturated ammonium, phosphorus, and sulfur ylide carbenes appear more stable than their saturated cyclic analogs which are in turn more viable than their acyclic counterparts. Copyright © 2014 John Wiley & Sons, Ltd.     

From acyclic dialkylcarbene to the unsaturated cyclic heteroatom substituted ones: a survey of stability

In regard to the worldwide interest in synthesis and application of stable carbenes, ab initio and DFT calculations (CCSD(T)/6‐311G*//QCISD/6‐31G* and B3LYP/AUG‐cc‐pVTZ//B3LYP/6‐31 + G* levels) are employed to reach at a series of symmetrically substituted diheteroatom carbenes. In this way the multiplicity, stability, and reactivity of diethylcarbene and its saturated and unsaturated cyclic congeners are compared and contrasted to the corresponding disilyl‐, diamino‐, diphosphino‐, dioxy‐, and dithiocarbenes. The investigations reveal the enlargement of the singlet–triplet energy gaps (ΔE_S‐T) in the order of (amino ≈ oxy) > thio > phosphino > alkyl > silyl. The observed trend is thoroughly analyzed applying appropriate isodesmic reactions which differentiate the substituent effects on each of the singlet and triplet states of all the carbenes. In contrast to previous reports, it is found that π‐donor/σ‐acceptor amino substituents stabilize not only the singlet but also the triplet states. The effects of cyclization and unsaturation are also probed in each series of the symmetrically substituted carbenes. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity. Copyright © 2010 John Wiley & Sons, Ltd.     

P‐Heterocyclic silylenes: a survey of stability with density functional theory

The effects of phosphorous atom on the stability, multiplicity, and reactivity of six‐member cyclic silylenes are investigated at B3LYP/AUG‐cc‐pVTZ//B3LYP/6‐31+G* and MP2/6‐311++G**//B3LYP/6‐31+G* coupled with appropriate isodesmic reactions. From a thermodynamic point of view, 1H‐2‐silaphosphinine‐2‐ylidene ( 1_a ) and 1H‐4‐silaphosphinine‐4‐ylidene ( 2_a ) are relatively the most stable with singlet–triplet energy gaps (ΔE_S–T) of 37.0 and 28.1 kcal/mol, respectively. The calculated energy barrier for the 1,2‐H shift of 1_a to the corresponding 2‐silapyridine ( 1 ) is 26.5 kcal/mol, which is lower than the 28.8 kcal/mol required for the 1,4‐H shift of 2_a to the corresponding 4‐silapyridine ( 2 ). In contrast to the previous reports, isodesmic reactions indicate that π‐donor/σ‐donor phosphorous destabilizes the singlet while stabilizes the triplet state. Both 1_a and 2_a silylenes appear invulnerable to the head‐to‐head as well as the head‐to‐tail dimerization, inviting experimental explorations. Copyright © 2011 John Wiley & Sons, Ltd.     

Reaching for [6]n SiC‐cyclacenes and [6]n SiC‐acenes: A DFT approach

Density functional theory (DFT) calculations introduced triplet ground states for [6]_n SiC‐cyclacenes and ‐acenes with alternate silabenzene rings including silicon atoms in 2 opposite edges (n = 6, 8, 10, 12). The singlet‐triplet energy gap (ΔE_(S‐T)), binding energy per atom (BE/n), and NBO calculation with very small band gap (ΔE_LUMO‐HOMO) confirmed the triplet ground states. In contrast to polyacenes, the singlet [6]_n SiC‐cyclacenes displayed more stability improvement than triplets, through n increasing. This may open the way for synthesis of larger stable [6]_n SiC‐cyclacenes. The ΔE_(S‐T), BE/n, and the strain energy through homodesmic equations indicated more stability for larger [6]_n SiC‐cyclacenes, which was more noticeable in singlet states. Cyclacenes and acenes with high conductivity and full point charge were introduced as suitable candidates for hydrogen storage.     

Computational study of singlet and triplet sulfonylnitrenes insertion into 1,3‐butadienes: 1,2‐ or 1,4‐cycloaddition?

The formation of N‐trifluoromethylsulfonyl‐2‐vinylaziridine and N‐trifluoromethylsulfonyl‐3‐pyrroline by the reaction of the singlet and triplet trifluoromethanesulfonylnitrenes with s‐cis‐ and s‐trans‐1,3‐butadienes was studied theoretically at the B3LYP/6‐311++G(d,p) and M06‐2X/6‐311++G(d,p) levels of theory. The singlet trifluoromethanesulfonylnitrene adds to s‐cis‐ and s‐trans‐1,3‐butadiene exothermally in one step to give the product of 1,2‐cycloaddition, N‐trifluoromethylsulfonyl‐2‐vinylaziridine, the energy decreasing by 88.5 and 86.2 kcal/mol at the B3LYP level and by 105.2 and 103.0 kcal/mol at the M06‐2X level, respectively. The formed 2‐vinylaziridine can undergo rotation about the C(2)–C_sp2 bond with the barrier not exceeding 3.5 kcal/mol and to rearrange into N‐trifluoromethylsulfonyl‐3‐pyrroline. The triplet trifluoromethanesulfonylnitrene reacts with s‐cis‐ and s‐trans‐1,3‐butadiene in two steps. The first exothermic step is the formation of the triplet diradical adducts. The second step is the spin inversion with the energy raising by 5.8 and 17.8 kcal/mol at the B3LYP level and by 11.0 and 20.8 kcal/mol at the M06‐2X level for the adducts to s‐cis‐ and s‐trans‐1,3‐butadiene, respectively. Recombination of the radical centers occurs selectively to give N‐trifluoromethylsulfonyl‐2‐vinylaziridine that is exothermally rearranged into N‐trifluoromethylsulfonyl‐3‐pyrroline with the energy barrier of 40 kcal/mol at the B3LYP level and of 50 kcal/mol at the M06‐2X level. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrogen bond and internal rotations barrier: DFT study on heavier group‐14 analogues of formamide

A theoretical study on heavier group‐14 substituting effect on the essential property of formamide, strong hydrogen bond with water and internal rotational barrier was performed within the framework of natural bond orbital (NBO) analysis and based on the density functional theory calculation. For heavier group‐14 analogues of formamide (YHONH₂, Y = Si, Ge and Sn), the n_N–π_Y=O conjugation strength does not always reduce as Y becomes heavier, for example, silaformamide and germaformamide have similar strength of delocalization. Heavier formamides prefer being H‐bond donors to form FYO–H₂O complexes to being H‐bond acceptors to form FYH–H₂O complexes. The NEDA analysis indicates that H‐bond energies of FYO–H₂O complexes increase as moving down group 14 due to concurrently stronger charge transfer (CT) and electrostatic attraction and for the FYH–H₂O complexes H‐bond strengths are similar. The model of CTs from FYO to H₂O differs from that at FYH–H₂O complexes, which are contributed not only by aligning lone‐pair orbital of O but also by another lone‐pair orbital. At two lowest lying excited states (the triplet and S₁ excited states), formamide and its heavier analogues form double H‐bonds with H₂O molecule at the same time. The barrier heights of internal rotation become gradually low from C to Sn, formamide (15.73 kcal/mol) > silaformamide (11.73 kcal/mol) > germaformamide (9.45 kcal/mol) > stannaformamide (7.50 kcal/mol) at the CCSD(T)/aug‐cc‐pVTZ//B3LYP/cc‐pVTZ level. NBO analysis indicates that the barrier does not only come from the n_N→π*_YO conjugation, and for heavier analogues of formamide, the n_N→σ*_YO hyperconjugation effect and steric effect considerably contribute to the overall rotational barrier. Copyright © 2013 John Wiley & Sons, Ltd.     

Computational study of singlet and triplet sulfonylnitrenes insertion into the C―C or C―H bonds of ethylene

Formation of N‐sulfonylaziridines, N‐ethylidenesulfonamides, N‐vinylsulfonamides and 4,5‐dihydro‐1,2,3‐oxathiazole 2‐oxides by the reaction of singlet and triplet trifluoromethyl‐, methyl‐ and tosylnitrenes with ethylene is studied computationally at the B3LYP/6‐311++G(d,p) level of theory in both gas phase and in solution. Singlet sulfonylnitrenes react with ethylene via [1 + 2]‐cycloaddition exothermically to give N‐sulfonylaziridines. Triplet sulfonylnitrenes are formed from the singlet ones by the intersystem crossing with the energy barrier not exceeding 2.5 kcal/mol and react in a stepwise fashion by C‐addition or H‐abstraction. The C‐addition gives rise to the formation of N‐sulfonylaziridines or N‐ethylidenesulfonamides depending on the S―N―C_sp3―C_sp2 dihedral angle, with the barrier to rotation about the N―C_sp3 bond not exceeding 2.5 kcal/mol. The H‐abstraction results in N‐vinylsulfonamides. Transformation of N‐sulfonylaziridines to N‐ethylidenesulfonamides requires to overcome the barrier of 57–60 kcal/mol, N‐ethylidenesulfonamides to 4,5‐dihydro‐1,2,3‐oxathiazole 2‐oxides—74–80 kcal/mol and N‐vinylsulfonamides to N‐ethylidenesulfonamides—about 64 kcal/mol. The use of the polarizable continuum model does not lead to a change of the course of the reaction of trifluoromethanesulfonylnitrene with ethylene and only slightly affects the relative energies of the products, intermediates and transition states. Copyright © 2014 John Wiley & Sons, Ltd.     

Spectroscopic and stability studies on unsymmetrical 1,3‐dialkyl‐1,3‐diphosphacyclobutane‐2,4‐diyls

Air‐tolerant 2,4‐bis(2,4,6‐tri‐t‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl singlet biradicals can be prepared by utilizing the unique reactivity of a kinetically stabilized P≡C triple bond compound. In this procedure, we studied the spectroscopic properties of a fundamental unsymmetrical P‐heterocyclic biradical containing both PEt and PMe moieties, and the effects of the PCH₂OMe group in relation to the stability of the P‐heterocyclic biradical skeleton. The experimentally observed nuclear magnetic resonance and photo‐absorption parameters of 1‐ethyl‐3‐methyl‐2,4‐bis(2,4,6‐tri‐t‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl were discussed based on our previous findings and density functional theory calculations, suggesting particular structural characteristics of the P‐heterocyclic biradical skeleton and aromatic substituent effects on the sp²‐C atoms in the 4‐membered ring. Introduction of the methoxymethyl group in the P₂C₂ biradical moiety gave more stabilized 1,3‐diphosphacyclobutane‐2,4‐diyl derivatives. In comparison with considerably unstable biradicals bearing propargyl substituents, relatively higher lowest unoccupied molecular orbital energies suggest reluctant oxidation of the P‐heterocyclic skeleton. Copyright © 2011 John Wiley & Sons, Ltd.     

Conformational analysis of 3,3‐dimethyl‐3‐silathiane, 2,3,3‐trimethyl‐3‐silathiane and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane—preferred conformers,barriers to ring inversion and substituent effects

The first conformational analysis of 3‐silathiane and its C‐substituted derivatives, namely, 3,3‐dimethyl‐3‐silathiane 1 , 2,3,3‐trimethyl‐3‐silathiane 2 , and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6‐311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers ΔG^≠ for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1‐dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (?ΔG° = 0.35 kcal/mol) derived from low‐temperature ¹³C NMR data is fairly consistent with the calculated value. For compound 2 , theoretical calculations give ΔE value close to zero for the equilibrium between the 2 ‐Me_ax and 2 ‐Me_eq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (?ΔG° = 1.8 kcal/mol) and the predominance of the 3 ‐SiMe_{3 eq} conformer at room temperature was confirmed by the simulated ¹H NMR and 2D NOESY spectra. The effect of the 2‐substituent on the structural parameters of 2 and 3 is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

The synthesis and photolysis mechanisms of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid

Photolabile protecting groups have been extensively studied and applied for protection of small biological molecules, which make it convenient to detect the biological processes of the caged compounds. In this study, a series of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid were synthesized with improved photolysis efficiency. Among them, 6‐bromo‐8‐nitro‐1, 2‐dihydroquinolinyl chromophore was proven the best derivative on account of its longest absorption wavelength (345 nm), highest caging ability, and quantum yield (Φ = 0.003). Moreover, density functional theory calculations were performed in order to study the photolysis mechanisms. Theoretical calculations revealed that the reaction was kinetically inert under general mild condition with the high barrier height of 34.3 kcal/mol at carbonyl migration step, while under the photolysis condition, because of the large energy gap (64.5 kcal/mol) between S₀ and S₁ states, the reaction should be accessible in the triplet ground state (T₁) through successive excitation of S₀ → S₁ states, subsequent intersystem crossing of S₁ → T₁ states, and finally returned to the stable S₀ state for product via potential energy surface crossing between T₁ and S₀ states. Copyright © 2014 John Wiley & Sons, Ltd.     

1,5‐electrocyclization versus 1,5‐proton shift in imidazolium allylides and 2‐phospha‐allylides: a DFT investigation

The competitive 1,5‐electrocyclization versus intramolecular 1,5‐proton shift in imidazolium allylides and imidazolium 2‐phosphaallylides has been investigated theoretically at the DFT (B3LYP/6‐311 + +G**//B3LYP/6‐31G**) level. 1,5‐Electrocyclization follows pericyclic mechanism and its activation barrier is lower than that for the pseudopericyclic mechanism by ～5–6 kcal mol^?1. The activation barriers for 1,5‐electrocyclization of imidazolium 2‐phosphaallylides are found to be smaller than those for their nonphosphorus analogues by ～3–5 kcal mol^?1. There appears to be a good correlation between the activation barrier for intramolecular 1,5‐proton shift and the density of the negative charge at C8, except for the ylides having fluorine substituent at this position ( 7b and 8b ). The presence of fluorine atom reduces the density of the negative charge at C8 (in 7b it becomes positively charged) and thus raises the activation barrier. The ylides 7f and 8f having CF₃ group at C8, in preference to the 1,5‐proton shift, follow an alternative route leading to different carbenes which is accompanied by the loss of HF. The carbenes Pr _{7 , 8b – e} resulting from intramolecular 1,5‐proton shift have a strong tendency to undergo intramolecular S_N2 type reaction, the activation barrier being 7–28 kcal mol^?1. Copyright © 2011 John Wiley & Sons, Ltd.     

On the nature of some biradicaloid and antiaromatic Vis/NIR‐chromophores

Dyes derived from the biradicalic oxyallyl and cyclopentadienylium were calculated by time‐dependent density functional theory (TDDFT) and the characteristics of the prominent low‐energy transitions revealed by graphical means. According to theoretical and experimental studies, 4‐aminophenyl‐substituted dyes absorb intensely at long wavelengths up to the near infrared. If the amino groups are removed the absorption wavelengths are changed little. As found in the previous studies on the squaraine and croconaine oxyallyl dyes, the substituents play only a minor role in the spectral excitation. Charge‐transfer‐type excitations do not occur between the donor aryl substituents and the central oxyallyl or cyclopentadienylium acceptor group. This behaviour is exceptional since donor–acceptor compounds tend to produce charge‐transfer‐ or polymethine‐type electronic transitions. The hitherto rarely used electron density difference (EDD) maps clearly unveiled the spectral excitation features. The spectral excitation of the title compounds is predominantly localized at the oxyallyl and cyclopentadienylium groups, respectively. Characteristics of simple chromophoric compounds and of conventional CT‐ and polymethine dyes are given for comparison. The biradicaloid character of these dyes is supported by the calculated low singlet–triplet splitting energies. Spin properties were characterized in terms of expectation values of the S²‐operator and antiaromatic properties in terms of nucleus‐independent chemical shifts (NICS). According to the ΔE_S/T and <S²> criteria compounds with the acyclic oxyallyl fragment are more biradicaloid. The parent compounds oxyallyl, thioxyallyl and cyclopentadienylium, display extremely large <S²> values. These compounds are triplets in the ground state. The absorption wavelengths of selected biradicaloid species were also calculated by the multi‐reference SORCI method. Copyright © 2010 John Wiley & Sons, Ltd.     

The DFT study on non‐conjugated polymer host materials based on styrene derivatives for phosphorescent polymer light‐emitting diodes

A series of poly(4,4‐vinyltriphenylamine) based non‐conjugated polymer as host molecules are designed and studied by density functional theory. The results show that the substituent has a great influence on the properties of polymer. The parent molecule directly linked para‐carbazole, β‐pyrrole and triphenylamine are favorable to hole injection, and para‐carbazole could significantly increase E_T of the host molecules. The large changes of structural parameters between the lowest triplet state and ground state can cause the decrease of E_T. Moreover, parent molecule directly linked carbazole and triphenylamine units possess strong intramolecular charge transfer and low singlet and triplet energy difference (?E_ST). The calculated results also show that all designed host molecules are suitable for green emitter by comparing with the E_T. S₁ → S₁ and T₁ → T₁ energy transfer mechanism between host and guest is thermodynamically feasible. In addition, host–guest model is built to study the charge transfer nature, and the results indicate that a good intermolecular charge transfer can be achieved between host and guest materials. In the designed host molecules, the N atom of parent molecule linked para‐carbazole substituent shows a great potential for the green phosphorescent polymer light‐emitting diodes. Copyright © 2015 John Wiley & Sons, Ltd.     

Intramolecular Diels–Alder reaction in enyne–allenes: a computational investigation and comparison with the Myers–Saito and Schmittel reactions

The reaction mechanisms as well as substituted effect and solvent effect of the enyne–allenes are investigated by Density Functional Theory (DFT) method and compared with the Myers–Saito and Schmittel reactions. The Myers–Saito reaction of non‐substituted enyne–allenes is kinetically and thermodynamically favored as compared to the Schmittel reaction; while the concerted [4 + 2] cycloaddition is only 1.32 kcal/mol higher than the C₂? C₇ cyclization and more exothermic (Δ_RE = ?69.38 kcal/mol). For R₁ = CH₃ and t‐Bu, the increasing barrier of the C₂? C₇ cyclization is higher than that for the C₂? C₆ cyclization because of the steric effect, so the increased barrier of the [4 + 2] cycloaddition is affected by such substituted electron‐releasing group. Moreover, the strong steric effect of R₁ = t‐Bu would shift the C₂? C₇ cyclization to the [4 + 2] cycloaddition. On the other hand, for R₁ = Ph, NH₂, O^?, NO₂, and CN substituents, the barrier of the C₂? C₆ cyclization would be more diminished than the C₂? C₇ cyclization due to strong mesomeric effect; the reaction path of C₂? C₇ cyclization would also shift to the [4 + 2] cycloaddition. The solvation does not lead to significant changes in the potential‐energy surface of the reaction except for the more polar surrounding solvent such as dimethyl sulfoxide (DMSO), or water. Copyright © 2009 John Wiley & Sons, Ltd.     

A theoretical study on the stability and detonation performance of 2,2,3,3‐tetranitroaziridine (TNAD)

In recent years, there has been a considerable interest in developing high oxygen compounds as oxidizers for, for example, composite explosives. 2,2,3,3‐Tetranitroaziridine (TNAD) is a new designed compound with high oxygen balance (25.11%) and is environmentally friendly. A synthesis route of TNAD was suggested in this study, and the thermodynamic possibilities of reactions were evaluated by the changes in the free energy obtained with the density functional theory (DFT). The strong strain energy (E_s = 292.28 kJ/mol) of TNAD leads the C–C bond in the ring more fragile than the C–NO₂ bond, and the activation energy (E_a) of pyrolysis of the C–C bond (119.14 kJ/mol at the B3LYP/6‐31G* level of DFT) is higher than that of 2,4,6‐trinitrotoluene (TNT) (113.00 kJ/mol). The topological analysis with the contour maps of electron density was used to show the changes of the electron density at the critical points (BCP) in the process of homolysis of the C–C bond. In addition, the energy gap between the frontier orbitals of TNAD (ΔE_g = 5.22 eV) is slightly higher than that of 1,3,3‐trinitroazetidine (TNAZ, 5.06 eV). And the HOMO → LUMO transition plays important roles in the UV spectrum. The noncovalent interactions in the TNAD/RDX composite were estimated to be stronger than that in TNAZ/RDX, that is, the former may have better compatibility than the latter. TNAD/RDX with the weight ratio of w_TNAD/w_RDX = 0.46/0.54 has the wonderful performance (D = 9.14 km/s, P = 37.30 GPa, and I_s = 285.47 s) which is better than that (D = 8.85 km/s, P = 35.09 GPa, and I_s = 272.33 s) of TNAZ/RDX with the same weight ratio. Copyright © 2014 John Wiley & Sons, Ltd.     

Sulfonyl nitrenes from different sources: computational study of formation and transformations

The formation of sulfonyl nitrenes RSO₂N (R = Me, p‐Tol, CF₃) from different sources, such as sulfonyl azides RSO₂N₃, N‐halosulfonamides RSO₂NHHal, salts RSO₂NNaCl, N‐hydroxysulfonamides RSO₂NHOH and sulfonylimino‐λ³‐iodanes ArI = NSO₂R or ‐bromanes, by elimination of neutral molecules (N₂, HCl, NaCl, H₂O and ArHal, respectively) was studied theoretically at the density functional theory and second‐order Møller‐Plesset perturbation theory using the 6‐311++G(d,p) and cc‐pVTZ basis sets. The originally formed singlet nitrenes suffer nonradiative intersystem crossing to the triplet state (S₁ ? T₁) or, sometimes, undergo spontaneous barrierless pseudo‐Curtius rearrangement into the corresponding sulfonylamines RN = SO₂. The activation barriers decrease in the order (kcal/mol): RSO₂NHHal ~ RSO₂NHOH (60–70) ≥ RSO₂N(Na)Hal (40–50) > RSO₂N₃ (35) > RSO₂N = IPh (9–13) ≥ RSO₂N = BrC₆H₄CF₃ (4–8). Copyright © 2013 John Wiley & Sons, Ltd.