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Density functional theory (DFT) and the coupled cluster method have been used to study the mechanism of the cycloaddition of acetylene to silene, H2Si=CH2, and germene, H2Ge=CH2, at the B3LYP/6-311++G(d,p) and CCSD/6-311++G(d,p) levels of theory. Diradical, zwitterionic, and concerted pathways were located for both metallenes. The computational results were compared to experimental data to propose the most likely reaction pathway for each metallene.

The energetics of all the possible isomers of the didehydronaphthalene (naphthalyne or naphthyne) diradical has been examined using the reduced multi-reference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that employs a modest size MR CISD wave function as an external source of the most important (primary) triples and quadruples to account for nondynamic correlation effects in the presence of quasi-degeneracy, as well as by its perturbatively corrected version for the secondary triples, the RMR CCSD(T) method. The resulting energies are compared with those obtained by the standard single-reference (SR) CCSD and CCSD(T) approaches. In all cases, we used both the cc-pVDZ basis set, as well as its restricted version cc-pVDZ* with deleted p-functions on hydrogens. Once the optimal geometry for each isomer was found, we computed the energy of the lowest-lying singlet and triplet states using the above mentioned CC methods, as well as the implied singlet–triplet splittings. These results enabled us to classify the isomers into three groups according to their stability and to determine, whenever possible, the spin multiplicity of the ground state. Finally, we point out the relationship between the extent of the diradical character of naphthyne isomers, the degree of their MR nature, the distance separating the radical centers, and, finally, the size of the largest doubly-excited cluster amplitude in their CC wave functions.

Reduction of [([Formula: see text])2][As6]2 with triphenylantimony and tetrabutylammonium chloride produced the diradical 5,5'-bis(1,3,2,4-dithiadiazolyl) [[Formula: see text]-[Formula: see text]] in high yield as a black solid with widely ranging magnetic susceptibilities (e.g., 0.6 to 2.6 μB), which on oxidation with AsF5 regenerated [([Formula: see text])2][AsF6]2. The identity of [[Formula: see text]-[Formula: see text]] was established from EPR, vibrational, and mass spectra. Ab initio molecular orbital [MPW1PW91/6-311G(2df)] calculations show the lowest energy structure to consist of two coplanar rings separated by a C—C single bond (1.444 Å), reflected in the comparison of the vibrational spectra of the diradical with that of [([Formula: see text])2][AsF6]2 and the calculated spectra. Confidence in the calculated [MPW1PW91/6-311G(2df)] structure of the diradical is supported by the excellent agreement between the calculated and X-ray single crystal structure geometries of [[Formula: see text]]2 and [([Formula: see text])2]2+ in [([Formula: see text])2][AsF6]2. The molecular orbitals indicate the diradical is essentially disjoint, confirmed by a very small (0.07 kJ mol–1) GVB-PP(TC-SCF)/6-311G* calculated singlet–triplet energy gap. Accordingly, the EPR spectrum of the diradical (in tetrahydrofuran, 293 K) shows a simple 3-line pattern (g = 2.0043, a(14N) = 1.11 mT) with no observable exchange coupling between the two radical centers. Mechanical grinding of the diradical results in a large increase in paramagnetism (e.g., from 1.03 to 2.55 μB) that is unprecedented in main group chemistry. The X-ray diffraction data of the ground and unground powder are consistent with a second order phase change on grinding. Attempts to obtain crystals of the diradical by sublimation led instead to numerous decomposition and rearrangement products, including 4-cyano-1,2,3,5-dithiadiazolyl [NC-[Formula: see text]], monoclinic space group C2/c, a = 9.46(2) Å, b = 7.625(5) Å, c = 13.17(2) Å, β = 107.94(4)°, Z = 8. The structure consists of axially symmetric, co-facial, cis dimers linked to form strands through two sets of C[Formula: see text]Nδ–···Sδ+ contacts. More efficient and larger scale preparations of [SNS][AsF6] and [([Formula: see text])2][AsF6]2 are reported. Key words: 5,5'-bis(1,3,2,4-dithiadiazolyl), diradical, paramagnetism, mechanical grinding, second order phase change, 4-cyano-1,2,3,5-dithiadiazolyl.

Ab initio GVB(6/12)/6-31G** calculations were performed on A2X==YB2 (A, B = H, F; X, Y = C, Si) to obtain the optimized geometries for planar and twisted singlet structures, and to also calculate pi bond energies (rotational barriers). The nature of C-C, Si-Si, and C-Si pi bonds has been investigated. The results show that the C-C pi bond energy (E pi (ethene) = 65.4 kcal/mol) decreases with increasing fluorine substitution. The pyramidalization at the carbon or silicon center for the twisted structures decreases the pi bond energies in the substituted ethenes and their silicon counterparts. The Si-Si (E pi (disilene) = 23.2 kcal/mol) and C-Si (E pi (silaethene) = 31.6 kcal/mol) pi bonds become much weaker. Fluorine substitution stabilizes both the diradical and the dipolar twisted singlet structures.Key words: pi bond energy, ab initio calculations, generalized valence bond, fluorine substitution, disilene, and silaethene.

Electron spin exchange of positronium (Ps = e+e−) with spin 1/2 paramagnetic species was investigated. First, the concept of spin flip (nonflip) collisions is examined in the light of the difference in phase shifts between electron spin singlet and triplet encounters. Then, the effects of spin exchange on positron spin dynamics and on Ps lifetimes are discussed and results are compared to muon (electron) spin dynamics in the muon spin rotation (electron spin resonance) techniques.

All Diels–Alder reactions between 1,3-butadiene and cyclopentadiene or 2H-phosphole have been examined at the MP4SDQ/6-31G*//HF/6-31G* level. There is remarkable similarity between the two systems. The thermodynamic product is the bicyclo[4.2.0]nonadiene while the kinetic product is the norbornene product. There is a slight kinetic preference for the endo addition and for the butadiene to be in the s-trans conformation. Except for the case where butadiene is the diene component and addition is endo, the reactions are concerted and synchronous. In these other two cases, the reaction is stepwise with a diradical intermediate.Key words: phosphole, Diels–Alder reaction, topological electron density analysis.

The properties of the atoms in a number of substituted carbenes and silylene provide an understanding of the relative stabilities of their lowest lying singlet and triplet states and of their differing chemical reactivities. The theory of atoms in molecules defines all of an atom's properties, including its energy. In this research one finds that the methylenic carbon or silicon atom is energetically most stable in the triplet state of each system, while the ligands are most stable in the corresponding singlet state. It is also found that the average electron populations of the carbon and silicon atoms are largest in the triplet states. In systems where the carbon or silicon atom bears a net positive charge, as found in CHNH2, CF2, and SiH2, the excess in the transfer of charge to the more electronegative ligands in the singlet states stabilizes the ligands more than it destabilizes the central atom. These systems have singlet ground states. The relative susceptibility of singlet and triplet carbenes to electrophilic and nucleophilic attack is determined by the properties of the Laplacian of the charge distribution. This quantity assimilates the model of localized electrons in terms of local concentrations of electronic charge. It also determines regions from which electronic charge is locally depleted. These regions are found to coincide with those where the lowest-lying vacant orbital is concentrated.

Photolysis of 5-hydroxy-5β-cholest-3-en-6-one (2) in hexanes gave 5-oxo-5,6-seco-cholest-3-en-6-al (13) and [(1R, 4R, 7aR)-1-(1S,5-dimethylhexyl)-7a-methyl-2,3,3a,4,7,7a-hexahydro-1H-inden-4-yl]acetaldehyde (14). Similar irradiation of 13 gave 14 and a small amount of 5-hydroxy-B-nor-5β-cholest-3-ene-6α-carbaldehyde (15). The structures of 13 and 15 were confirmed by independent synthesis: the former from cholesteryl acetate via 3β-acetoxy-6,6-dimethoxy-5,6-seco-cholestan-5-one and the latter from 13 by treatment with methanolic sodium methoxide. Photolysis of 5-deuteroxy-5β-cholest-3-en-6-one (26) in hexanes gave the -CDO analogues of the aldehydes 13, 14, and 15. These observations are interpreted in terms of α-cleavage of the C-5—C-6 bond of 2 to give an acyl allyl diradical, which undergoes O—H hydrogen abstraction to give 13. The occurrence of α-cleavage as the major reaction pathway is attributed to the increased stabilization of the alkyl radical center by the hydroxyl substituent. The preference for O—H rather than C—H abstraction in the diradical is attributed to the planar nature of the allyl radical center. The indenylacetaldehyde 14 is considered to arise via a Norrish type II reaction of 13, while the formation of B-nor aldehyde 15 is ascribed to a "dark" aldol reaction of 13.

Thermolyses of epimeric 3-alkyl-2-phenyloxetanes (1c, 1t, 2c, 2t, 3c, and 3t), 3,3-dimethyl-2-aryloxetanes (4, 5, 6, 7, and 8), 3,3,4,4-tetramethyl-2,2-diphenyloxetane (9), and 3,3-dimethyl-2,2-diphenyloxetane (10) were studied in degassed N,N,N′,N′-tetramethylethylenediamine at 270–350 °C. Although the fragmentation of 9 and 10 can be understandable on the basis of a diradical mechanism, there were several observations, in the reaction of certain other oxetanes, which could hardly be explained simply in terms of such a mechanism. Namely, (1) less strained 1t reacted faster than more strained 1c; (2) a major mode of the fragmentation for 1c, 2c, and 3c was "B" (forming an alkene and benzaldehyde), whereas that for 1t, 2t, and 3t was "A" (forming an alkenylbenzene and formaldehyde); (3) the apparent energy of activation for the "B" process seemed to be larger than that for the "A"; (4) a dramatic change of the major fragmentation mode from "B" to "A" was brought about by a substituent on the phenyl group, as was observed in 4–8. These results may be explained reasonably by assuming that the fragmentation proceeds, at least, in dual reaction courses. In competition with an anticipated diradical pathway, there will be another process, which is energetically little more favorable than the diradical fragmentation, rather specific to the "A" mode of fragmentation, and important particularly in the reaction of the trans isomers. Probable candidates for the second process are discussed.

Ab initio SCF potential energy surfaces for the cycloaddition reactions C2H4(1Alg) + O(1D,3P) are reported. The lowest energy singlet path corresponds to the symmetric, concerted addition reaction yielding ethylene oxide. The lowest energy triplet path corresponds to an asymmetric approach of the reactants to form an open ring adduct [Formula: see text] The spatial distribution of the spin density in this adduct conforms to that expected for a diradical. The barrier to internal rotation of the terminal methylene is calculated to be ∼5 kcal mol−1. The lowest intersystem crossing from the triplet to the singlet surface, as required for ring closure to yield ground state product, is found to occur ∼36 kcal mol−1 above the energy of the triplet reactants.The differences in singlet and triplet charge distributions and energies for similar approach geometries may be rationalized in terms of the additional restraints imposed on the course of the triplet reaction by the Pauli principle, restraints which are made evident by the spatial distribution of the spin density. Two principal mechanisms of spin polarization, spin unpairing and spin decoupling, are found to provide a rationale for the observed differences of singlet and triplet reactants in abstraction, insertion and addition reactions. The favoured course of a triplet reaction is found to be governed by the axiom that localized distributions of spin density are to be maximally separated in space.

The photolysis of bicyclo(3.2.1)octan-2-one (XVII), bicyclo(2.2.2)octan-2-one (XXV), and 1,8,8-trimethyl-bicyclo(3.2.1)octan-2-one (homocamphor) (IX) in nucleophilic solvents (methanol, water, cyclohexylamine) is reported. The structures of the products were established by synthesis and comparison with authentic materials, and, in the case of XVII, the two aldehydes (XVIII, XIX) were isolated and the position of the double bond established by nuclear magnetic resonance shift measurements and confirmed by degradation. The spectroscopic properties of the products were recorded. -- The photochemical products were predominantly derived from aldehydes and ketenes and the product ratios are tabulated. The results are rationalized in terms of conformational analysis of the diradical intermediates involved. A potential general method of altering the product ratio using cyclohexylamine is described, as are various synthetic approaches to these ketones.

The effect of wavelength and pressure on the distribution of products has been studied in the photolysis of cyclopentanone and cyclopentanone-2,2,5,5-d4. At low pressures, C2D4 and C2H4 are formed in addition to the expected high pressure product C2H2D2, confirming the intermediate formation of excited cyclobutane. The high pressure ratio of cyclobutane to ethylene is attributed to alternate reactions of the tetramethylene diradical which have equal energy sensitivity, and some evidence is presented for a further mechanism of ethylene formation at short wavelengths. Evidence is also presented for the formation of cyclobutane and ethylene following thermal re-excitation of the precursor of 4-pentenal.

Previous theories of acetylene pyrolysis are reviewed in the light of recent work by Minkoff, Newitt, and Rutledge. It is shown that the relatively large rates observed at the beginning of the induction period do not agree with mechanisms involving the intervention of comparatively stable dimers. The required kinetic form is obtained, however, if a triplet diradical is produced on the surface in a bimolecular process, followed by gas phase polymerization, with some chain ending on the surface. The detailed mechanism closely resembles Flory's scheme for liquid phase vinyl polymerization. The shape of the radical is discussed, and it is suggested that the trans-configuration leads to polymerization, while the cis facilitates dehydrogenation.

The photolysis of thiolane vapor [Formula: see text] was studied at room temperature with 2139 Å radiation. The major product was C2H4, whose quantum yield decreased as the pressure was increased. Next in importance were 1-C4H8, C3H6, and C2H6. Also produced were CH4, c-C3H6, 1,3-C4H6, CH2CHSH, [Formula: see text], 1-C4H9SH, H2, C3H8, n-C4H10, c-C4H8, polymer, and an unstable product tentatively identified as 1-butene-1-thiol. All products were initial products of the reaction. Experiments with added C3H6 showed the absence of sulfur atoms. The results are interpreted in terms of two excited states and an intermediate which might be the diradical •CH2CH2CH2CH2S•.

The triplet mercury photosensitization of spiropentane affords ethylene, allene, and methylenecyclobutane as major reaction products. This indicates an olefinic-type interaction with the quaternary C—C σ-bond of spiropentane, the cleavage of which results in a triplet state diradical. The initially formed diradical may undergo isomerization through ring cleavage or deactivation by collision. The open chain diradical then either decomposes unimolecularly into C2H4 + C3H4 or on collisional relaxation isomerizes to methylenecyclobutane.The paraffinic-type interaction between Hg* and spiropentane which would lead to C—H bond scission appears to have minor significance.

The e.p.r. spectra from the low temperature matrix photolysis of several mercaptans, disulfides, and cyclic sulfides have been recorded and compared to those obtained in the photolysis of these materials adsorbed on a porous Vycor glass. The spectra of the adsorbed radicals are characterized by an enhanced thermal stability and a shift of the highest sulfur g-value to higher fields. The only free radical product in the photolysis of mercaptans and disulfides is the thiyl radical. The cyclic sulfides [Formula: see text] appear to yield•CH2—(CH2)n—S•type diradicals, while the cyclic disulfide [Formula: see text][Formula: see text] gives the [Formula: see text] diradical.

2,2-Bis(ethylthio)-3,3-dimethyl-4-pentenal was found to quantitatively isomerize to 2,2-bis(ethylthio)-5-methyl-4-hexenal over a wide temperature range (130–170 °C). This rearrangement can formally be regarded as a [1,3] sigmatropic shift. The reaction, which could be conveniently monitored by 1Hmr spectroscopy, was found to obey first order kinetics. The substantial positive entropy of activation (+ 61.5 J deg−1 mol−1) for the reaction and the negligible solvent effect (decane vs. DMF) are both consistent with a proposed diradical pathway.