Through-bond Orbital Interactions Research Articles

Spin-spin interaction and relaxation in two trityl-nitroxide diradicals

Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear polarization. To optimize performance it is important to understand the impact of spin–spin interactions on relaxation times of the diradicals. CW spectra and electron spin relaxation were measured for two trityl-nitroxides that differ in the substituents on the amide linker and have different strengths of the exchange interaction J. Analysis of the EPR spectra in terms of overlapping AB spin–spin splitting patterns explains the impact of J on various regions of the spectra. Even modest values of J are large relative to the separation between trityl and nitroxide resonances for some nitrogen nuclear spin state. Two conformations for each diradical were observed in CW spectra in fluid solution at X-band and Q-band. For one diradical J = 15 G (83%) and 5 G (17%) at 293 K, and J = 27 G (67%) and 3 G (33%) with interspin distances of 16 Å and 12 Å, respectively, at 80 K. For the second diradical the exchange interaction is stronger: the two conformations in fluid solution at 293 K had J = 113 G (67%) and 59 G (33%) and at 80 K the value of J was 43 G and there were two conformations with interspin distances of 13 and 11.5 Å. The observation of two conformations for each diradical, with different values of J, demonstrates the dependence of their exchange interactions on through-bond orbital interactions. X-band values of spin relaxation rates 1/T1 and 1/Tm at 80 to 120 K for the trityl-nitroxides are similar to values for nitroxide mono-radicals, and faster than for trityl radicals. These observations show that even for a relatively small value of J, the nitroxide is very effective in enhancing the relaxation of the more slowly relaxing trityl.

ChemInform Abstract: Structure of the Cyclohexa-1,4-diene Radical Cation: An ab initio Molecular Orbital and Electron Paramagnetic Resonance Study.

The structure of the radical cation of cyclohexa-1,4-diene, (1+), was explored using ab initio MO calculations and EPR spectroscopy. Geometry optimizations were carried out on neutral 1 and 1+ at the (U)HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory. The ground states of both 1 and 1+ were found to possess D2h structures, in agreement with experimental data. The ground electronic state of 1+ was found to be 2B1u and not 2B3g, and this is the result of through-bond orbital interactions. The computed MP2/6-31G(d, p) proton isotropic hfs values for 1+ were found to be in good agreement with the experimental values. The EPR spectra and proton hfs values for 1+ are similar to those obtained earlier by Shida et al.

Orbital interactions expressed in resonance structures: An approach to compute stabilisation of cyclobutanediyl diradicals

Ab initio valence bond (VB) calculations give direct evidence for the stabilisation and closed-shell character formation of formal diradicals by neighbouring group interactions. By allowing the singly occupied p-orbitals to delocalise during the orbital optimisation, neighbouring group effects are implicitly included in the wavefunction. By constraining the orbitals in the wavefunction to remain strictly atomic, through-bond effects are excluded from the wavefunction. These effects can then be included in the wavefunction by expanding the wavefunction using more valence bond structures. The choice of these structures used to built the wavefunction determines which interactions will be accounted for. For the compounds considered here, 2,4-dioxo-1,3-cyclobutanediyl ( 1), 2,4-bis(methylene)-1,3-cyclobutanediyl ( 2), 1,3-cyclobutanediyl ( 3), and 1,1,3,3-tetrahydro-1,3,2,4-diphosphadiboretane-1,3-diyl ( 4), the calculations show that the most important VB structures for mediating interactions between the opposing radical centres are those that withdraw electrons from the ring, thereby creating an aromatic, 2π-electron four-membered ring. For the 1,3-cyclobutanediyl analogues ( 3 and 4), the PH 2 group functions similar to the C O group, and is a better electron-accepting substituent than the CH 2 group. The energy differences between the strictly atomic and delocal orbital optimisation models show that the molecules with less diradical character are stabilised more by through-bond orbital interactions.

The occurrence of through-bond orbital interactions in an α,ω donor–acceptor substituted bi(cyclohexylidene) and bi(cyclohexyl). X-Ray diffraction, UV–Vis absorption and photoelectron spectroscopy, ab initio SCF-MO and natural bond orbital analyses†

The ground state σ–π and π–π* interactions in 2-[4-(1-phenylpiperidin-4-ylidene)cyclohexylidene]malononitrile (1) and 2-[4-(1-phenylpiperidin-4-yl)cyclohexylidene]malononitrile (2) have been studied. Both the ionization potentials of 1 and 2, and ab initio RHF/6-31G calculations in combination with a natural bond orbital analysis show that the ground state through-bond interaction (TBI) between the 1-phenylpiperidine electron donor and the dicyanoethylene electron acceptor in 1 and 2 is distinct but small. The olefinic bond in 1 enhances the interaction between the 1-phenylpiperidine electron donor and the dicyanoethylene electron acceptor as compared to 2. The TBI between the N,N-dialkylanilino donor and the olefinic bond in 1 can be modulated by rotation of the phenyl group around the C–N bond. The solid state structures of 1 and 2 have been determined by single crystal X-ray diffraction. In the crystal intermolecular electron-donor–acceptor complexes are formed, which give rise to an intermolecular charge transfer absorption in the solid state.

Long-Range Effects of Through-Bond Orbital Interactions on the Desilylation Rate of Silyl Ethers

Long-Range Effects of Through-Bond Orbital Interactions on the Desilylation Rate of Silyl Ethers

Analysis of the distance dependence and magnitude of the .pi.+,.pi.- and .pi.+*,.pi.-* splittings in a series of diethynyl[n]staffanes: an ab initio molecular orbital study

Through-bond orbital interactions between the π orbitals and between the π * orbitals of the first five members of the series of α,ω-diethynyloligo[1.1.1]propellanes, or α,ω-diethynyl[n]staffanes, 4(1)-4(5), have been investigated using ab initio MO theory. The π + ,π - and π + * ,π - * splittings of 4(1)-4(5) were obtained in the Koopmans'theorem approximation, using the STO-3G, 3-21G, and 6-31G * basis sets

Structure of the cyclohexa-1,4-diene radical cation: an ab initio molecular orbital and electron paramagnetic resonance study

The structure of the radical cation of cyclohexa-1,4-diene, (1+), was explored using ab initio MO calculations and EPR spectroscopy. Geometry optimizations were carried out on neutral 1 and 1+ at the (U)HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory. The ground states of both 1 and 1+ were found to possess D2h structures, in agreement with experimental data. The ground electronic state of 1+ was found to be 2B1u and not 2B3g, and this is the result of through-bond orbital interactions. The computed MP2/6-31G(d, p) proton isotropic hfs values for 1+ were found to be in good agreement with the experimental values. The EPR spectra and proton hfs values for 1+ are similar to those obtained earlier by Shida et al.

A cautionary comment on the use of orthogonal localized molecular orbitals for the quantitative analysis of through-space and through-bond orbital interactions

The quantitative dissection of orbital interactions into through-space (TS) and through–bond (TB) contributions using orthogonal localized MOs (LMOs), as originally proposed by Heilbronner and Schmelzer, is critically discussed, using as a test case, cyclohexa-1,4-diene. It was found that the (HF/STO-3G) TS interaction energies for this molecule, obtained using three different orbital localization procedures (Foster–Boys, the Weinhold natural bond orbitals and the Weinhold natural localized orbitals) are unsatisfactory in the sense that the calculations give a relatively small splitting between the π+ and π– orbitals and an inverted sequence of π* orbitals resulting from supposedly pure TS interactions. This result contradicts the Hoffmann conceptual model of orbital interactions. The source of the problem is traced to the presence of the ‘orthogonalization’ tails associated with the LMOs. An alternative strategy for dissecting orbital interactions, termed the ‘cluster’ approach is discussed using an ethene dimer model as an example.

ChemInform Abstract: Orbital Interactions. Part 13. The Observation of Through‐bond Orbital Interactions between Benzene and Double Bonds in Some Dimethanoanthracenes

Chemischer InformationsdienstVolume 17, Issue 38 Physical Organic Chemistry ChemInform Abstract: Orbital Interactions. Part 13. The Observation of Through-bond Orbital Interactions between Benzene and Double Bonds in Some Dimethanoanthracenes N. S. HUSH, N. S. HUSHSearch for more papers by this authorG. D. WILLETT, G. D. WILLETTSearch for more papers by this authorM. N. PADDON-ROW, M. N. PADDON-ROWSearch for more papers by this authorH. K. PATNEY, H. K. PATNEYSearch for more papers by this authorJ. B. PEEL, J. B. PEELSearch for more papers by this author N. S. HUSH, N. S. HUSHSearch for more papers by this authorG. D. WILLETT, G. D. WILLETTSearch for more papers by this authorM. N. PADDON-ROW, M. N. PADDON-ROWSearch for more papers by this authorH. K. PATNEY, H. K. PATNEYSearch for more papers by this authorJ. B. PEEL, J. B. PEELSearch for more papers by this author First published: September 23, 1986 https://doi.org/10.1002/chin.198638054AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume17, Issue38September 23, 1986 RelatedInformation

Orbital interactions. Part 13. The observation of through-bond orbital interactions between benzene and double bonds in some dimethanoanthracenes

The He1 photoelectron spectra of the structurally rigid dimethanoanthracenes (5)–(8) have been measured and their spectra have been assigned on the basis of band intensities, ionization energy correlations with related molecules, and ab initio SCF molecular orbital calculations. The results reveal the presence of large through-bond orbital interactions in (7) and (8) which destabilize the symmetric aromatic π-based orbital to a greater extent in the all-trans configuration (7) and the ethylenic π-based orbital more in the trans–cis configuration (8). This last result reveals yet another example which demonstrates the existence of a novel laticyclic through-bond orbital interaction which is relayed through methylene groups. The antisymmetric aromatic π-based orbitals in (7) and (8) appear not to be affected by any orbital interactions.

ChemInform Abstract: LONE PAIR INTERACTIONS IN DIMETHOXYMETHANE AND ANOMERIC EFFECT

Perturbation molecular orbital analysis has been used for a computation of the through-bond and through-space orbital interactions of oxygen lone pairs in dimethoxymethane (DMM). The analysis predicts the symmetrical combination of p-type lone pairs as a highest occupied orbital in an antiperiplanar conformation. The conformational dependence of through-bond orbital interactions has the character of the V2 term in the Fourier expansion of the rotation potential function about the C—O bond. Contrary to the recent theoretical interpretation that the anomeric effect (preference of gauche conformation) is caused by superjacent orbital control, the orbital interactions in DMM are not dominant terms with respect to the anomeric or exoanomeric effect. The dipole–dipole interactions of the C—O bonds stabilizing the gauche conformation should thus be considered as the primary cause of the anomeric effect in DMM. The frontier orbital energies and geometric parameters in DMM are strongly influenced by a variation of...

Lone pair interactions in dimethoxymethane and anomeric effect

Perturbation molecular orbital analysis has been used for a computation of the through-bond and through-space orbital interactions of oxygen lone pairs in dimethoxymethane (DMM). The analysis predicts the symmetrical combination of p-type lone pairs as a highest occupied orbital in an antiperiplanar conformation. The conformational dependence of through-bond orbital interactions has the character of the V2 term in the Fourier expansion of the rotation potential function about the C—O bond. Contrary to the recent theoretical interpretation that the anomeric effect (preference of gauche conformation) is caused by superjacent orbital control, the orbital interactions in DMM are not dominant terms with respect to the anomeric or exoanomeric effect. The dipole–dipole interactions of the C—O bonds stabilizing the gauche conformation should thus be considered as the primary cause of the anomeric effect in DMM. The frontier orbital energies and geometric parameters in DMM are strongly influenced by a variation of orbital interaction with rotation. Results obtained for DMM are used to explain the conformational behaviour of other molecules containing the acetal moiety, such as pyrane heterocycles, sugars, and polyoxymethylene.