Investigation of the effect of carbon monoxide on the oxidative carbonylation of methanol to dimethyl carbonate over CuX and CuZSM-5 zeolites

Direct synthesis of dimethyl carbonate offers prospects for a “green chemistry” replacement to eliminate use of phosgene for polymer production and other processes. The carbonylation of methanol to produce dimethyl carbonate over Cu⁺X and Cu⁺ZSM-5 zeolites prepared by solid-state ion exchange has been investigated, focusing on the interaction of carbon monoxide with the Cu⁺ zeolites. The methanol carbonylation mechanism reported previously has been extended to account for carbon monoxide adsorption at high pressure. The comparison of the results obtained from Cu⁺X and Cu⁺ZSM-5 show that strong CO adsorption on the catalyst is not related to increased rate of dimethyl carbonate production. The rate limiting step for DMC production is best described as the Eley-Rideal reaction of gas-phase CO with surface methoxide.     

Internal dynamics and optical rotations predicted for O(h)- and O-symmetric cubanes

B3LYP/6-31G(d) calculations find that cubanes, persubstituted with NO2 or BF2 groups, are predicted to undergo near-barrierless, internal disrotations. However, as a consequence of the intrinsically higher energies of eclipsed conformations for threefold than for twofold rotors, the threshold mechanisms for octamethyl-, octakis(trifluoromethyl)-, octakis(trichloromethyl)-, octakis(tribromomethyl)-, octasilylcubane, and octakis(trichlorosilyl)cubane are calculated to be mono- or conrotation. The cubanes with the larger substituents are predicted to be O-symmetric, resolvable, and thus optically active.     

How important is bishomoaromatic stabilization in determining the relative barrier heights for the degenerate Cope rearrangements of semibullvalene, barbaralane, bullvalene, and dihydrobullvalene?

[structure: see text] B3LYP/6-31G* calculations have been used to investigate the origins of the relative barrier heights for the degenerate Cope rearrangements of semibullvalene (1), barbaralane (2), bullvalene (3), and dihydrobullvalene (4). We conclude from our calculations that, of the four transition structures (TSs), that for rearrangement of 1 has the smallest amount of interallylic bonding. Nevertheless, relief of strain in the reactant confers on 1 the lowest barrier to Cope rearrangement. Conjugation between the cyclopropane ring and the pi bond of the etheno bridge in 3 makes the barrier for its Cope rearrangement higher than that for 4 and also contributes to making the barrier for 3 higher than that for 2. However, the relatively low barrier to the Cope rearrangement of 2 is largely due to the TS for this reaction having the largest amount of interallylic bonding of all four TSs.     

Reactions of sterically congested 1,5-hexadienes: Ab initio and DFT calculations on the competition between cope rearrangements and disrotatory cyclobutene ring-opening reactions of bridged syn-tricyclo[4.2.0.0(2,5)]octa-3,7-dienes

The thermolytic behavior of four syn-tricyclo[4.2.0.0(2,5)]octa-3,7-dienes, each spanned by four propano bridges (13, 14, 21, and 26), has been investigated by means of calculations at the UB3LYP/ 6-31G* and CASPT2/6-31G levels. These calculations predict that 13 should undergo a degenerate Cope rearrangement (E(A) = 19.6 kcal/mol), whereas the other three C(20)H(24) isomers should prefer a necessarily disrotatory cyclobutene ring-opening reaction. In the case of 14, the ring-opening reaction (E(A) = 27.2 kcal/mol) is concerted and leads directly to 15, a 4-fold bridged cyclooctatetraene. In the ring opening of 21, the 1,6-bridge in the 4-fold bridged bicyclo[4.2.0]octa-2,4,7-triene 31 prevents formation of the corresponding cyclooctatetraene. In the ring opening of 26, the 4-fold bridged bicyclo[4.2.0]octa-2,4,7-triene derivative 36 is predicted to form the corresponding bridged cyclooctatetraene 38, which should undergo bond shift. The results of these calculations are found to be in very good agreement with the results of experiments on these hydrocarbons.     

β‐Carboline (norharman)

The structure of β‐carboline, also called norharman (systematic name: 9H‐pyrido[3,4‐b]indole), C₁₁H₈N₂, has been determined at 110 K. Norharman is prevalent in the environment and the human body and is of wide biological interest. The structure exhibits intermolecular N—H...N hydrogen bonding, which results in a one‐dimensional herringbone motif. The three rings of the norharman molecule collectively result in a C‐shaped curvature of 3.19 (13)° parallel to the long axis. The diffraction data show shorter pyridyl C—C bonds than those reported at the STO‐3G level of theory.     

Molecular orbitals of the oxocarbons (CO)n, n = 2-6. Why does (CO)4 have a triplet ground state?

Cyclobutane-1,2,3,4-tetrone has been both predicted and found to have a triplet ground state, in which a b(2g) σ MO and an a(2u) π MO are each singly occupied. The nearly identical energies of these two orbitals of (CO)(4) can be attributed to the fact that both of these MOs are formed from a bonding combination of C-O π* orbitals in four CO molecules. The intrinsically stronger bonding between neighboring carbons in the b(2g) σ MO compared to the a(2u) π MO is balanced by the fact that the non-nearest-neighbor, C-C interactions in (CO)(4) are antibonding in b(2g), but bonding in a(2u). Crossing between an antibonding, b(1g) combination of carbon lone-pair orbitals in four CO molecules and the b(2g) and a(2u) bonding combinations of π* MOs is responsible for the occupation of the b(2g) and a(2u) MOs in (CO)(4). A similar orbital crossing occurs on going from two CO molecules to (CO)(2), and this crossing is responsible for the triplet ground state that is predicted for (CO)(2). However, such an orbital crossing does not occur on formation of (CO)(2n+1) from 2n + 1 CO molecules, which is why (CO)(3) and (CO)(5) are both calculated to have singlet ground states. Orbital crossings, involving an antibonding, b(1), combination of lone-pair MOs, occur in forming all (CO)(2n) molecules from 2n CO molecules. Nevertheless, (CO)(6) is predicted to have a singlet ground state, in which the b(2u) σ MO is doubly occupied and the a(2u) π MO is left empty. The main reason for the difference between the ground states of (CO)(4) and (CO)(6) is that interactions between 2p AOs on non-nearest-neighbor carbons, which stabilize the a(2u) π MO in (CO)(4), are much weaker in (CO)(6), due to the much larger distances between non-nearest-neighbor carbons in (CO)(6) than in (CO)(4).     

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

This perspective describes research, carried out in the authors' labs over the past forty years, aimed at understanding, predicting, and measuring the singlet-triplet energy differences (ΔE(ST)) in diradicals. A theory for qualitatively predicting the ground states of diradicals and the use of Negative Ion Photoelectron Spectroscopy (NIPES) for measuring ΔE(ST) are described. The application of this theory, ab initio calculations, and NIPES to the prediction and measurement of ΔE(ST) in a wide variety of organic diradicals is detailed. Among the diradicals that are discussed in this perspective are HN, CH(3)N, PhN, CH(2), trimethylenemethane (TMM), oxyallyl (OXA), meta-benzoquinodimethane (MBQDM), meta-benzoquinone (MBQ), tetramethyleneethane (TME), 1,2,4,5-tetramethylenebenzene (TMB), and D(8 h) cyclooctatetraene (COT). All of these diradicals have been studied in one and, in most cases, in both of the authors' laboratories. The studies of OXA and D(8h) COT were, in fact, collaborations between the research groups of the authors. These two projects both took advantage of the ability of NIPES to provide information about transition states. Transition-state spectroscopy was used to measure the carbonyl stretching frequency in the singlet state of OXA and to establish that D(8h) COT violates the strictest version of Hund's rule.     

What accounts for the difference between singlet phenylphosphinidene and singlet phenylnitrene in reactivity toward ring expansion?

(8/8)CASSCF and (8/8)CASPT2 calculations have been performed in order to investigate the potential surface for the ring expansion of the (1)A(2) state of phenylphosphinidene (1c) to 1-phospha-1,2,4,6-cycloheptatetraene (3c). Unlike the comparable ring expansion of the (1)A(2) state of phenylnitrene (1b) to 1-aza-1,2,4,6-cycloheptatetraene (3b), ring expansion of 1c to 3c is computed to be quite endothermic. Nevertheless, cyclization of 1c, to form the bicyclic intermediate 2c in the ring expansion reaction, is computed to be only slightly more endothermic than the comparable cyclization reaction of 1b to 2b. The origins of these differences between the ring expansion reactions of 1b and 1c have been elucidated through the calculation of the energies of relevant isodesmic reactions.