The structures of zinc carbene ZnCH2and zinc carbyne HZnCH, and the conversion transition states between them are optimized at B3LYP/aug‐cc‐pVTZ, MP2/aug‐cc‐pVTZ, and CCSD/aug‐cc‐pVTZ levels of theory. The thermodynamic energies with CCSD(T) method are further extrapolated to basis set limit through a series of basis sets of aug‐cc‐pVXZ (X=D, T, Q, 5). The Zn−C bonding characteristics are interpreted by molecular plots, Laplacian of density plots, the integrated delocalization indices, net atomic charges, and derived atomic hardness. On the one hand, the studies demonstrated the efficiency of DFT method in structure optimizations and the accuracy of CBS method in obtaining thermodynamic energies; On the other hand, the density analysis of CCSD/aug‐cc‐pVDZ density demonstrates that both the sharing interaction and ionic interaction are important in ZnCH2ad HZnCH. The3B1state of ZnCH2is the global minimum and formed in visible light, but its small bond dissociation energy (47.0 kcal/mol) cannot keep the complex intact under UV light (79.4–102.1 kcal/mol). However, the3Σ−state of HZnCH can survive the UV light due to the greater Zn−C dissociation energy (100.7 kcal/mol). The delocalization indices of Zn…C in both3B1of ZnCH2(0.777) and the3Σ−state of HZnCH (0.785) are close to the delocalization index of the single C−C bond of ethane (0.841), i. e. the nomenclature of Zinc carbene and Zinc carbyne is incorrect. The stronger Zn−C bond in the3Σ−state of HZnCH than in the3B1state of ZnCH2can be attributed to the larger charge separation in the former. It was found that the Zn−C bonds in related Zinc organic compounds were also single bonds no matter whether the organic groups are CR, CR2, or CR3. The ionic interactions were discussed in terms of the atomic hardness that were in turn related to ionization energy and electron affinity. The unique combination of covalent and ionic characteristics in the Zn−C bonds of organic Zinc compounds could be the origin of many interesting applications of organic Zinc reagents.
Ring Expansion of Alkylidenecarbenes Derived from Lactams, Lactones, and Thiolactones into Strained Heterocyclic Alkynes: A Theoretical Study
Strained cycloalkynes are of considerable interest to theoreticians and experimentalists, and possess much synthetic value as well. Herein, a series of cyclic alkylidenecarbenes—formally obtained by replacing the carbonyl oxygen of four-, five-, and six-membered lactams, lactones, and thiolactones with a divalent carbon—were modeled at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G** and CCSD(T)/cc-pVTZ//CCSD/6-311+G** levels of theory. The singlet carbenes were found to be more stable than the triplets. The strained heterocyclic alkynes formed by ring expansion of these singlet carbenes were also modeled. Interestingly, the C≡C bonds in the five-membered heterocycles, obtained from the rearrangement of β-lactam- and β-lactone-derived alkylidenecarbenes, displayed lengths intermediate between formal double and triple bonds. Furthermore, 2-(1-azacyclobutylidene)carbene was found to be nearly isoenergetic with its ring-expanded isomer, and 1-oxacyclopent-2-yne was notably higher in energy than its precursor carbene. In all other cases, the cycloalkynes were lower in energy than the corresponding carbenes. The transition states for ring-expansion were always lower for the 1,2-carbon shifts than for 1,2-nitrogen or oxygen shifts, but higher than for the 1,2-sulfur shifts. These predictions should be verifiable using carbenes bearing appropriate isotopic labels. Computed vibrational spectra for the carbenes, and their ring-expanded isomers, are presented and could be of value to matrix isolation experiments.
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- Award ID(s):
- 1665278
- NSF-PAR ID:
- 10100148
- Date Published:
- Journal Name:
- Molecules
- Volume:
- 24
- Issue:
- 3
- ISSN:
- 1420-3049
- Page Range / eLocation ID:
- 593
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)We investigated the collision-induced dissociation (CID) reactions of a protonated Hoogsteen 9-methylguanine–1-methylcytosine base pair (HG-[9MG·1MC + H] + ), which aims to address the mystery of the literature reported “anomaly” in product ion distributions and compare the kinetics of a Hoogsteen base pair with its Watson-Crick isomer WC-[9MG·1MC + H] + (reported recently by Sun et al. ; Phys. Chem. Chem. Phys. , 2020, 22 , 24986). Product ion cross sections and branching ratios were measured as a function of center-of-mass collision energy using guided-ion beam tandem mass spectrometry, from which base-pair dissociation energies were determined. Product structures and energetics were assessed using various theories, of which the composite DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97XD/6-311++G(d,p) was adopted as the best-performing method for constructing a reaction potential energy surface. The statistical Rice–Ramsperger–Kassel–Marcus theory was found to provide a useful framework for rationalizing the dominating abundance of [1MC + H] + over [9MG + H] + in the fragment ions of HG-[9MG·1MC + H] + . The kinetics analysis proved the necessity for incorporating into kinetics modeling not only the static properties of reaction minima and transition states but more importantly, the kinetics of individual base-pair conformers that have formed in collisional activation. The analysis also pinpointed the origin of the statistical kinetics of HG-[9MG·1MC + H] + vs. the non-statistical behavior of WC-[9MG·1MC + H] + in terms of their distinctively different intra-base-pair hydrogen-bonds and consequently the absence of proton transfer between the N1 position of 9MG and the N3′ of 1MC in the Hoogsteen base pair. Finally, the Hoogsteen base pair was examined in the presence of a water ligand, i.e. , HG-[9MG·1MC + H] + ·H 2 O. Besides the same type of base-pair dissociation as detected in dry HG-[9MG·1MC + H] + , secondary methanol elimination was observed via the S N 2 reaction of water with nucleobase methyl groups.more » « less
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Abstract Density functional theory and extrapolated CCSD(T) computations of several “anti‐Bredt” alkenes were carried to explore possible 1,2‐diyl “alkene” candidates with a triplet ground state. Ten candidates containing twisted double bonds at the bridgehead positions of bicyclic structures (
1‐6 ) or adamantene (7‐10 ) derivatives were studied. Based on a combination of ring strain, rigid scaffolding, and steric crowding, four species were identified to have surprisingly low singlet‐triplet energy gaps (lower than 4 kcal/mol). Atert‐ butyl substituted bicyclic structure (4 ) was identified to have a near‐zero singlet‐triplet energy gap, but no triplet ground‐state alkene was found. Ring strain energy (RSE) calculations, π‐orbital axis vector (POAV) analyses, and multiple linear regression models were performed to elucidate the geometric and energetic effects of double bond twisting in1‐10 . Based on our computational exploration, it appears unlikely that there is a ground‐state triplet olefin. -
The strained heterocyclic alkyne, 3-oxacyclohexyne, was generated photochemically for the first time using a cyclopropanated phenanthrene precursor, and trapped by cyclopentadienones as Diels–Alder adducts. The precursor initially produced the putative 3-oxacyclopentylidenecarbene that subsequently rearranged to the cycloalkyne. Computational studies indicate that the carbene favors a singlet state, and the barrier for its ring expansion by a 1,2-shift of the carbon proximal to oxygen is lower in energy than the corresponding shift of the distal carbon.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/molecules24030593
