Nucleus-independent Chemical Shift Values Research Articles

Induction of an Aromatic Six-Membered Nitrogen Ring via Cation−π Interaction

Nitrogen clusters have been intensively studied for their potential application as high-energy density materials, but a six-membered nitrogen ring (N6) was not found to be stable and aromatic. To explore the possibility of inducing an aromatic N6 ring via cation-pi interaction, quantum chemistry calculations were performed on the systems of Ca2N6, CaN6, CaN6(2-), N6, and N6(4-) at the B3LYP/6-311+G level. The optimized geometries reveal that the planar structure of the N6 ring is stable only in the Ca2N6 complex. The computed NBO and CHelpG charges demonstrate that the planar N6 moiety in the Ca2N6 complex is almost a 10pi-electron system. The predicted nucleus-independent chemical shift (NICS) values demonstrate that the N6 moiety is aromatic in comparison with the NICS values of benzene. The estimated enthalpy of formation for the Ca2N6 complex is 100.4 kcal/mol for the reaction of 2Ca and 3N2. The binding energy between the Ca2+ cation and the N6(4-) moiety is -1928.8 kcal/mol, with electrostatic interaction serving as the predominant component. When all the calculated results are taken into account, including the planar structure, 10pi-electron system, identical bond length, and negative NICS value of the N6(4-) moiety in the Ca2N6 complex, it is deduced that the alkaline earth metal Ca is capable of inducing an aromatic N6 ring through the cation-pi interaction formed by electron transfer from the Ca atom to the N6 ring.

Nonlinear Optical Properties and Excited-State Dynamics of Highly Symmetric Expanded Porphyrins

A strong correlation among calculated Nucleus-Independent Chemical Shift (NICS) values, molecular planarity, and the observed two-photon absorption (TPA) values was found for a series of closely matched expanded porphyrins. The expanded porphyrins in question consisted of [26]hexaphyrin, [28]hexaphyrin, rubyrin, amethyrin, cyclo[6]pyrrole, cyclo[7]pyrrole, and cyclo[8]pyrrole containing 22, 24, 26, 28, and 30 pi-electrons. Two of the systems, [28]hexaphyrin and amethyrin, were considered to be antiaromatic as judged from a simple application of Hückel's [4n + 2] rule. These systems displayed positive NICS(0) values (+43.5 and +17.1 ppm, respectively) and gave rise to TPA values of 2600 and 3100 GM, respectively. By contrast, a set of congeners containing 22, 26, and 30 pi-electrons (cyclo[n]pyrrole, n = 6, 7, and 8, respectively) were characterized by a linear correlation between the NICS and TPA values. In the case of the oligopyrrolic macrocycles containing 26 pi-electron systems, a further correlation between the molecular structure and various markers associated with aromaticity was seen. In particular, a decrease in the excited state lifetimes and an increase in the TPA values were seen as the flexibility of the systems increased. Based on the findings presented here, it is proposed that various readily measurable optical properties, including the two-photon absorption cross-section, can provide a quantitative experimental measure of aromaticity in macrocyclic pi-conjugated systems.

Structure and stability of Al13Hn (n=1–13) clusters: Exceptional stability of Al13H13

We have performed density functional calculations for the structure and stability of Al(13)H(n) (n=1-13) clusters. Population analysis has shown significant charge transfer occurring from the Al cluster to the H atoms. The population for Al(13) varies from 0.24 (Al(13)H) to 2.83 (Al(13)H(13)). The shape of Al(13) moieties in the Al(13)H(n) (n>/=8) clusters is significantly distorted from the icosahedral structure of Al(13) and is a "cagelike" form. Al(13)H(13) has a capped icosahedron as the ground-state structure, similar to B(13)H(13), while the shape of B(13) (planar) is different from Al(13) (icosahedral). The Al(13)H(13) cluster is predicted to be exceptionally stable on the basis of the high stabilization energy and the negative nucleus independent chemical shift value.

Structure and stability of Be6, Be, and Be clusters

AbstractThe structure and stability of Be6, Be, and Be clusters have been investigated at the B3LYP, B3PW91, and second‐order Møller–Plesset (MP2) levels of theory, along with the 6‐311G* basis set for neutral and cationic clusters and the 6‐311+G* basis set for anion clusters. CCSD(T)/6‐311+G* has also been used to calculate some neutral structure to find the most stable structure. Twelve Be6, six Be, and eight Be isomers are identified. The distortion octahedron structure, pentagonal pyramids structure, and trapezoidal bipyramid structure are found to be the most stable structure on the neutral, cationic, and anionic surface, respectively. They are in agreement with the results reported previously. Natural bond orbital (NBO) analysis, molecular orbital (MO) pictures, and the nucleus independent chemical shift (NICS) values suggest aromatic of the neutral and cationic clusters and antiaromatic of the anionic cluster. The multi‐center σ bonds and delocalized π bonds play important role in the bonding of the beryllium clusters. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Density functional theory study of tautomerization of 2‐aminothiazole in the gas phase and in solution

AbstractThe amino/imino tautomeric equilibrium in the isolated, mono‐, di‐, and trihydrate forms and dimer of 2‐aminothiazole, and the effects of hydration or self‐assistance on the transition state structures corresponding to proton transfer from the amino to imino form, have been investigated by the B3LYP method in conjunction with 6‐31+G(d,p) and 6‐311+G(3df,2p) basis sets in the gas phase and in solution. The amino form has been found to be the predominant tautomer. The tautomeric barrier heights for water‐ and self‐assisted tautomerization reactions are significantly lower than that from the amino to imino form by the intramolecular proton transfer, showing the catalytic effect of water molecule(s) and the important role of 2‐aminothiazole itself for intermolecular proton transfer. Comparison between the tautomeric barriers demonstrates that the self‐association tautomerization through the dimerization is the most favorable pathway. Bulk solvent effects have been taken into account using the polarizable continuum model (PCM) of water and CCl4. The polar medium is favorable for the population of the imino form. The amino/imino equilibrium is also analyzed using the aromaticity index nucleus‐independent chemical shift (NICS); the NICS values for the amino form (about −10 ppm) are more negative than the imino species (about −8 ppm), showing that the amino form is more stable. The time‐dependent density functional theory (TDDFT) calculations of electronic absorption spectra suggest that the λmax of dimer is 255 nm. The oscillator strength of the imino forms is less than the amino form, and increases with the polarity of the solvents. All calculations for the tautomerization of 2‐aminothiazole are in reasonable line with the available experiments. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

The Relationship Between Aromaticity and Charge‐Transfer Transitions: A Combined Study of Square‐Planar Metal Complexes Based on DFT, NMR and Nucleus‐Independent Chemical Shifts

AbstractThe aromaticity of square‐planar metal complexes, in this case mixed diiminodithiolate complexes of group VIII metals, has been investigated by 1H and 1H‐15N NMR spectroscopy, both experimentally and theoretically, as well as NICS indices. The electron‐donating/‐attracting ability of the substituent group and the metal ion interact with each other and control the aromaticity of the chelated rings and, as a consequence, the extent of the delocalization in the molecular plane. Evolution of the nucleus‐independent chemical shift (NICS) of the ligands upon complexation is consistent with the change in their electron distribution. At the same time, and in analogy with the EMF in closed circuits, a positive difference of the NICS value (ΔNICS) between two points (where HOMO and LUMO are located) can be envisioned as the driving force for charge‐transfer transitions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Novel Superalkali Superhalogen Compounds (Li3)+(SH)−(SH=LiF2, BeF3, and BF4) with Aromaticity: New Electrides and Alkalides

Optimized structures, with all real frequencies, of superalkali superhalides (Li(3))(+)(SH)(-) (SH=LiF(2), BeF(3), and BF(4)), are obtained, for the first time, at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. These superalkali superhalides possess three characteristics that are significantly different from normal alkali halides. 1) They have a variety of structures, which come from five bonding mode types: edge-face, edge-edge, face-face, face-edge, and staggered face-edge. We find that the bonding mode type closely correlates with the Li(3)-SH bond energy. 2) The valence electrons on the Li(3) ring are pushed out by the (SH)(-) anion, and become excess electrons, conferring alkalide or electride characteristics on these Li(3)-SH species, depending on the bonding mode type. 3) The highest occupied molecular orbital of each Li(3)-SH species is a doubly occupied delocalized sigma bonding orbital on the Li(3) ring, which indicates its aromaticity. It is noticeable that the maximum negative nucleus-independent chemical shift value (about -10 ppm) moves out from the center of the Li(3) ring, owing to repulsion by the SH(-) anion. We find that these superalkali superhalides are not only complicated "supermolecules", but are also a new type of alkalide or electride, with aromaticity.

Can Substituted Cyclopentadiene Become Aromatic or Antiaromatic?

Cyclopentadiene derivatives with electronegative (F, Cl) or electropositive (H(3)Si, Me(3)Si) bis-5,5-substituents were studied at the B3LYP/6-311G* level of theory. It was found that there is no special stabilization or destabilization for any of the derivatives; the energetic effects that were previously attributed to aromatic stabilization or antiaromatic destabilization are the result of interactions in the reference systems. A nucleus-independent chemical shift (NICS) scan study at the HF-GIAO/6-311+G* theoretical level of these and similar derivatives suggest that they all show different magnitudes of diamagnetic ring current. None of the derivatives shows a paramagnetic ring current. Thus, cyclopentadienes are neither aromatic nor antiaromatic. It is also concluded that a diamagnetic ring current is perhaps necessary but certainly not a sufficient condition for aromaticity. The NICS scan procedure describes the type of ring current in the system, whereas a single isotropic NICS value (i.e., NICS(1)) may wrongly assign the type of ring current. It is shown that neither NICS(1) nor the NICS scan procedure can be used as a single aromaticity criterion.

On the Aromaticity of the Planar Hydrogen-Bonded (HF)3 Trimer.

The nuclear magnetic shieldings and magnetically induced ring currents have been calculated for the planar ring-shaped hydrogen fluoride trimer (HF)3 at correlated ab initio and density functional theory levels. Calculations of the magnetically induced current densities using the gauge-including magnetically induced current (GIMIC) method show that, contrary to a recent suggestion, (HF)3 has, at the MP2/TZVPP level, a very small ring-current susceptibility of 0.37 nA/T. Thus, only a weak net current is passing across the H···F hydrogen bond. An external magnetic field perpendicular to the ring plane induces strong edge currents circling around each HF molecule giving rise to a nonvanishing magnetic shielding at the center of the ring. The GIMIC results are supported by calculations of the long-range magnetic shielding function; the long-range magnetic shielding is very small, indicating that the magnetically induced ring-current is very weak. The surprisingly large nucleus-independent chemical shift (NICS) value for (HF)3 was recently taken as an indication of "H-bonded aromaticity". The NICS value calculated at the CCSD/QZ2P level is 2.77 ppm. The present GIMIC and aromatic ring-current shielding study shows that some care has to be taken when using NICS values as aromaticity indices.

Role of Electron Density and Magnetic Couplings on the Nucleus-Independent Chemical Shift (NICS) Profiles of [2.2]Paracyclophane and Related Species

The nucleus-independent chemical shifts (NICS) and electron density profiles along the inner and outer regions defined by the two stacked aromatic rings of [2.2]paracyclophane have been analyzed and compared to those of free benzene and p-xylene and benzene dimers taken as reference models. It is found that stacked aromatic rings show a reduction of the NICS indicator of aromaticity as compared to the same free aromatic systems. This decrease of the NICS values upon stacking is not due to an increase of the electron density in the inner region between the rings (as claimed in a previous work) but is related to the magnetic couplings between superimposed rings that affect this measure of local aromaticity. The increase of local aromaticity in superimposed aromatic rings indicated by NICS is not real but the result of the coupling between the magnetic fields generated by the two stacked rings. This result warns about the use of NICS as a descriptor of aromaticity for species having superimposed aromatic rings.

Diels–Alder reactions of 5,6-dihydrothiazolo[3,2-d][1,4,2]diazaphospholes: A DFT investigation

The density functional theory level (B3LYP/6-311G**) computations of the Diels–Alder (DA) reactions of 5,6-dihydrothiazolo[3,2-d][1,4,2]-diazaphospholes with 1,3-butadiene and with isoprene confirm pericyclic mechanism via asynchronous transition states. The aromatic character of the transition states is established by negative nucleus independent chemical shift (NICS) values falling in the range from −14 to −16. Integration of the dienophilic >CP functionality in the 6π aromatic azaphosphole ring raises the activation energy barrier (B3LYP/6-311++G**//B3LYP/6-311G**) compared to that for the DA reaction of the acyclic phosphaethene, but it is lower than the activation energy barrier for the DA reaction of the corresponding 10π aromatic system, thiazolo[3,2-d][1,4,2]diazaphospholes. The experimentally observed stereo- and regioselectivities in the reactions can be accounted on the basis of secondary molecular orbital (SMO) interactions detected in the respective transition structures. The attachment of an electron-withdrawing group to the dienophilic moiety enhances both stereo- and regio- selectivities which agree well with the experimental values. Solvent (toluene) effect studied with polarizable continuum model (PCM) indicates that the stereo- and regioselectivities are not affected by the solvent. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:402–410, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20196

CHARACTERIZATIONS OF NOVEL BINUCLEAR ALKALINE-EARTH METAL COMPOUNDS: M2(ηn-N5)2 (M=Be AND Mg, n = 1, 2; M=Ca, n = 2, 5)

The first structural characterization for the twelve binuclear alkaline-earth metal compounds M 2(ηn- N 5)2 ( M=Be , Mg ; n = 1, 2) and Ca 2(ηn- N 5)2(n = 2, 5) have been optimized with local energy minimum by density functional theory (DFT). The most energetically favored structures in M 2(ηn- N 5)2( M=Be , Mg , Ca ) are of D2d symmetry Be 2(η1- N 5)2, Mg 2(η2- N 5)2 and Ca 2(η2- N 5)2 and the metal–metal distances are 2.03 Å for Be-Be , 2.77 Å for Mg-Mg and 3.72 Å for Ca-Ca , which are significantly shorter than the experiment values of weakly bound bare diatomic Be 2, Mg 2 and Ca 2.1,2 Ca 2(η5- N 5)2 (D5d or D5h) is the only stable specie with sandwiched structure, bearing an even shorter Ca-Ca distance of 3.66 Å, and lying 24 kcal/mol higher in energy than the D2d structure. The dissociation enthalpies of the twelve M 2(ηn- N 5)2( M=Be , Mg , Ca ) to two M (ηn- N 5) fragments are predicted to be 72.6–73.1, 41.2–43.8, and 27.4–29.7 kcal/mol, respectively, implying a substantial metal–metal bonding. Natural bond orbital (NBO) analysis suggests that metal–metal bonds are of σ-bond. The natural charge of the alkali earth metal atom in the twelve M 2(ηn- N 5)2 species is larger than +0.88, which is consistent with the +1 oxidation state of the metal atoms. Nucleus-independent chemical shift (NICS) values confirm that the planar [Formula: see text] exhibits characteristics of aromaticity for these M 2(ηn- N 5)2 species.

Nucleus-Independent Chemical Shifts (NICS): Distance Dependence and Revised Criteria for Aromaticity and Antiaromaticity

Nucleus-independent chemical shifts (NICS) have been used extensively for the identification of aromaticity properties of molecules, ions, intermediates, and transition states since their introduction in 1996 by Schleyer et al. Initially, probes (bq's) were placed at the centers of systems (NICS(0)) and later, 1A above the molecular planes (NICS(1)). However, contradicting assignments of aromaticity by NICS and other methods were found for some systems. In this article, an alternative NICS-based method is introduced. The method is based on scanning NICS values over a distance and separating them into in-plane and out-of plane contributions. The shapes of the plots of the chemical shifts and their components as a function of the distance of the NICS probe (bq) from the molecular plane give a clear indication of diamagnetic and paramagnetic ring currents. This method is applied to several (4n + 2)- and 4n pi-electron systems (molecules and ions) in the singlet and triplet electronic states, including some of the problematic systems mentioned above. It is also shown that relative aromaticities of rings in polycyclic systems (local aromaticities) cannot be estimated by comparing NICS or NICS-scan values.

The Trigonal Bipyramidal MN3M Species: A New Kind of Aromatic Complex Containing a Multiple‐Fold Aromatic N3 Subunit

A new kind of aromatic trigonal bipyramidal MN3M (M=Be, B, Mg, Al, and Ca) species, with all real frequencies, is obtained at the MP2/6-311+G(3d) level. The nucleus-independent chemical shift values are -102.16 ppm for the N3 (3-) ring, and -74.09, -79.39, -65.06, -74.44, and -62.33 ppm (at the geometrical center of the trigonal bipyramid) for BeN3Be, BN3B, MgN3Mg, AlN3Al, and CaN3Ca, respectively. Molecular orbital analysis indicates that the regular triangular N3 (3-) ring and each MN3M species have three aromatic six-electron systems (pi, sigma(p), and sigma(s)) and exhibit threefold aromaticity. The CaN3Ca species has a very low vertical ionization energy of 3.64 eV at the CCSD(T)/6-311+G(3d) level, which is even lower than the ionization energy (3.9 eV) of the Cs atom. Therefore, CaN3Ca can be considered as a new superalkali species. A further study on the CaN3CaCl molecule confirms the superalkali characteristics of CaN3Ca. Two interesting phenomena are explored in the MN3M species: the delocalized electron cloud of the N3 subunit is elongated by two M cations, and the electron clouds of two M cations are distended by the N3 (3-) ring.

M 4 @ Si 28 (M=Al,Ga): Metal-encapsulated tetrahedral silicon fullerene

It is known that silicon fullerenes cannot maintain perfect cage structures like carbon fullerenes. Previous density-functional theory calculations have shown that even with encapsulated species, nearly all endohedral silicon fullerenes exhibit highly puckered cage structures in comparison with their carbon counterparts. In this work, we present theoretical evidences that the tetrahedral fullerene cage Si(28) can be fully stabilized by encapsulating a tetrahedral metallic cluster (Al(4) or Ga(4)). To our knowledge, this is the first predicted endohedral silicon fullerene that can retain perfectly the same cage structure (without puckering) as the carbon fullerene counterpart (T(d)-C(28) fullerene). Density-functional theory calculations also suggest that the two endohedral metallosilicon fullerenes T(d)-M(4)@Si(28) (M=Al and Ga) can be chemically stable because both clusters have a large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap ( approximately 0.9 eV), strong spherical aromaticity (nucleus-independent chemical shift value of -36 and -44), and large binding and embedding energies.

Dications of Fluorenylidenes. The Relationship between Redox Potentials and Antiaromaticity for Meta- and Para-Substituted Diphenylmethylidenefluorenes

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.

Li 3 – O – Li 3 molecule: A metal-nonmetal-metal sandwichlike compound with a distending electron cloud

The D3d and D2d isomers of the Li3-O-Li3 molecule are metal-nonmetal-metal sandwichlike structures that contain two Li3 superalkali atoms. Their geometries and the real frequencies are obtained at the CCSD(T)/aug-cc-pVDZ level. They are different from the traditional types of the nonmetal-metal-nonmetal sandwich compounds. The natural bond orbital calculation and the topological property nabla2rho(r) calculation indicate that they are typical ionic compounds. In two isomers, the O2- anion is sandwiched in between two Li3+ cation rings. However, the different orientations of two Li3+ planes give the D3d isomer its own special characteristics. Under the action of the O2- anion in the center, the valence electrons of the D3d isomer are pushed out from two Li3+ triangle rings. This special interaction causes three phenomena. First, the valence electron clouds are distended. Second, the vertical ionization energy of the D3d isomer is considerably low, 4.39 eV, so that it may also be viewed as a superalkali atom. Third, we find that the D3d isomer owns the out-of-plane aromaticity and the largest negative nucleus-independent chemical shift value (-10.8 ppm) exists at 2.5 A above the center of the Li3+ ring, not at the center of the Li3+ ring like the isolated aromatic Li3+ cation.

Calculations on the Group 15 Intraring Substituted Benzenes (Pyridine to Bismin Series) and Their Datively Bonded Oxides and Sulfides

The series of heterocyclic compounds pyridine, phosphorin, arsenin, antimonin, bismin, and the related series of pyridine oxide, etc., and pyridine sulfide, etc., were investigated using density functional theory. In addition to molecular geometries and energies, the nucleus independent chemical shift (NICS) values were calculated for each ring, and the CHELPG atomic charges were calculated. Calculated heats of formation are: pyridine +133, phosphorin +181, arsenin +231, antimonin +329, bismin +468, pyridine oxide +143, phosphorin oxide +1, arsenin oxide +183, antimonin oxide +344, bismin oxide +570, pyridine sulfide +252, phosphorin sulfide +205, arsenin sulfide +328, antimonin sulfide +435, and bismin sulfide + 642 kJ/mol. The question of aromaticity for these heterocycles was discussed. On the basis of hydrogenation energies, NICS values, and lack of bond alternation, the entire series from pyridine to bismin appears to be aromatic and to a roughly equal degree. The situation for the oxides and sulfides of pyridine, phosphorin, and arsenin is much the same, but bismin oxide and bismin sulfide are nonplanar and according to NICS nonaromatic. Antimonin oxide is aromatic by some of the criteria and nonaromatic by others.

The NICS (Nucleus-Independent Chemical Shift) as a probe of the relative stability of β-chalcogenovinylaldehydes stabilized through intramolecular chalcogen–chalcogen interactions

Density functional calculations, at the B3LYP/6-311+G(3df,2p) level, have been carried out for the complete series of β-chalcogenovinylaldehydes, CH(X)–CH CH–YH (X, Y=O, S, Se, Te), to estimate the strength of H–X⋯Y or X⋯Y–H intramolecular chalcogen–chalcogen interactions, through the use of appropriate homodesmotic reactions. For the same set of compounds the value of the nucleus-independent chemical shift (NICS), on points 1 Å above the corresponding ring critical point, has been obtained at the B3LYP/6-311+G(3df,2p) level. For non-stabilizing chalcogen–chalcogen interactions the NICS value is positive, while the opposite is found when the interaction is stabilizing. In general, there is a good linear correlation between both magnitudes and therefore, we can conclude that NICS value is a reliable probe of the strength of intramolecular chalcogen–chalcogen interactions in this set of compounds.

A Comparison of the Cope Rearrangements of cis-1,2-Divinylcyclopropane, cis-2,3-Divinylaziridine, cis-2,3-Divinyloxirane, cis-2,3-Divinylphosphirane, and cis-2,3-Divinylthiirane: A DFT Study

Transition structures, energetics, and nucleus-independent chemical shifts (NICS) for Cope rearrangements of cis-2,3-divinylaziridine (1N), cis-2,3-divinyloxirane (1O), cis-2,3-divinylphosphirane (1P), and cis-2,3-divinylthiirane (1S), leading to 4,5-dihydro-1H-azepine (3N), 4,5-dihydrooxepine (3O), 4,5-dihydro-1H-phosphepine (3P), and 4,5-dihydrothiepine (3S), respectively, are reported at the (U)B3LYP/6-31G level and compared to those of cis-1,2-divinylcyclopropane (1C). The minimum energy path for all rearrangements proceeds through an endo-boatlike, aromatic transition structure. The predicted activation barriers increase in the order of 1C < 1N < 1O < 1P < 1S, which agrees qualitatively with the decreasing ring strain order of reference compounds (cyclopropane > aziridine > oxirane > phosphirane > thiirane). The exothermicities for these rearrangements decrease in the order of 1N > 1O > 1C > 1P > 1S. If the place of 1C in this sequence is ignored, the decreasing reaction exothermicity order correlates well with the increasing activation barrier order and with decreasing strain order of reference compounds. NICS values calculated for transition structures are typical of highly aromatic transition structures of thermally allowed pericyclic reactions.