Theoretical investigation of the heterofullerenes C59N and C69N and their dimers

The heterofullerenes C₅₉N and C₆₉N and their dimers are examined by the intermediate neglect of differential overlap (INDO/1 and INDO/S) models. The results confirm the stability of the (C₅₉N)₂ and (C₆₉)₂ (C_2h) isomers with a 6–6 closure fusion link between the two monomers. The nearest neighbor C of nitrogen, at the hexagon–hexagon fusion in C₅₉N and in C₆₉N where the N replaces one C in the pentagon of the fullerene has the largest electronic spin density, and this is where the two radicals of C₅₉N or C₆₉N link to form dimers. Also the dimers readily dissociate to free radical monomers to react regioselectively with other reagents. The electronic spectra of the radicals are similar to those of the dimers, which suggest essentially identical electronic structures. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 422–436, 2000     

Hyperfine coupling constants of Mg-centered radicals and radical ions

The energies, geometric parameters, spin densities, and HFC constants of the Mg-centered radicals ^•MgH, ^•MgOH, ^•MgMe, and monovalent magnesium aqua complexes Mg(H₂O)_n ⁺ (n = 0–3) were calculated using the density functional theory. As n increases, the HFC constants decrease for Mg atoms and increase for O atoms. The theoretical HFC constants in the Mg-centered radicals are in good agreement with experimental data. Published In Russian In Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, Pp. 861–865, May, 2007.     

15N13C coupling constants in 15N labelled azaadamantane

The ¹⁵N¹³C coupling constants in ¹⁵N labelled azaadamantane have been measured and interpreted in terms of their conformational dependence as compared with ¹⁵N-quinuclidine.     

13C, 15N coupling constants in substituted 1,2,4-triazines-4-15N

The ¹³C,¹⁵N coupling constants in three 1,2,4-triazine-4-¹⁵N derivatives and in a substituted pyrimidine-1-¹⁵N have been determined and are compared with those in pyridine and quinoline. Of special interest are the data of the 5-(2-dimethylamino-1-propenyl)-3,6-dimethyl-1,2,4-triazine-4-¹⁵N, since one observes a characteristic alternation of the absolute value of J(¹³C,¹⁵N) along the sidechain and a hitherto unobserved very large ¹³C,¹⁵N coupling constant through four bonds of 3·9 Hz, the origin of which is attributed to the influence of the lone pair orbital on the ¹⁵N at position 4.     

C,H coupling constants of propene

The proton coupled ¹³C NMR spectrum of propene has been analysed at 100.6 and 25.2 MHz and a complete set of C,H coupling constants is reported. In particular, vicinal and one-bond coupling constants are discussed with respect to substituent effects of alkyl groups. An incremental method allows us to predict ¹J(CH) in more highly methyl-substituted alkenes.     

13C,13C coupling constants of conjugated dienes

One bond ¹³C,¹³C- and ¹³C,¹H-coupling constants have been measured for some 1,2-dimethylene-cycloalkanes, as well as for 2,3-dimethylbuta-1,3-diene and methylenecyclobutane. The results for 2,3-dimethylbuta-1,3-diene confirm the findings for buta-1,3-diene, i.e. that J(C-1, C-2) is smaller for the diene than for the correspondingly substituted monoene. No differences have been found between the ¹J(CC) exocyclic coupling constants of the dimethylene and monomethylene cycloalkanes.     

自由基C59N及双体(C59N)2的理论研究

用INDO系列方法对自由基C59N及双体(C59N)2进行了理论研究,结果表明: N的掺入使C60笼发生畸变, N向笼外突出, 碳氮6-6键上的C自旋密度较大, 两C59N自由基在这个碳上以C-C单键连接形成双体为C2h, C2v对称性。其中C2v构型更稳定, 且N与附近的三个碳均以单键连接。理论计算的电子光谱与实验吻合较好。(C59N)2易分解为单体C59N。     

13C, 1H spin coupling constants of dimethylacetylene

¹³C, ¹H spin coupling constants of dimethylacetylene have been determined by the complete analysis of the proton coupled ¹³C NMR spectrum. For the methyl carbon ¹J(CH) = + 130.6₄ Hz and ⁴J(CH) = + 1.5₈ Hz, and for the acetylenic carbon ²J(CH) = ? 10.34 Hz and ³J(CH) = +4.30 Hz. The ⁵J(HH) long-range coupling constant (+2.79 Hz) between the methyl protons was also determined.     

N. M. R. coupling constants

Approximate ab-initio calculations of NMR coupling constants are reported, for a number of small molecules. The agreement with experiment is erratic, and is shown to depend critically on the form of the excited state wavefunctions. This suggests that the use of an average energy approximation is far from valid.

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Zusammenfassung NMR Kopplungskonstanten werden für eine Anzahl von kleinen Molekülen mitgeteilt, wobei sich keine rechte Übereinstimmung mit dem Experiment ergibt. Es zeigt sich, daß die Werte kritisch von der Form der Wellenfunktion für die angeregten Zustände abhängen. Das läßt vermuten, daß die Verwendung eines mittleren Energienenners nicht zu empfehlen ist.

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Résumé Calculs ab-initio approchés des constantes de couplage NMR d'un certain nombre de petites molécules. L'accord avec l'expérience est erratique et dépend d'une façon critique de la forme des fonctions d'onde excitées. Ceci suggère que l'emploi d'une approximation d'«énergie moyenne» est loin d'être valable.

     

19F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

In the process of studying the chemistry of perfluoro[2.2]paracyclophanes (PFPCs), a novel class of compounds, it became necessary to identify some disubstituted products. To achieve this goal, we characterize in this work some monosubstituted PFPCs, identifying their (19)F-(19) F coupling patterns, and establishing a methodology for the assignment of their (19)F chemical shifts. The pattern of coupling constants indicates a skewed geometry in which the upper deck moves towards or away from the substituent, depending on the substituent electron-donor character and size. Quantum chemical calculations, performed at the HF/6-311 + G(d,p)//B3LYP/EPR-III level of theory, confirmed the conformations inferred from coupling constants and reproduced well the values of the couplings. Transmission mechanisms for the FC term of four- and five-bond (19)F-(19) F couplings are discussed in detail. Understanding the conformational preferences of PFPCs and how they are reflected by the coupling constants facilitates the assignment of (19)F chemical shifts in monosubstituted PFPCs and the identification of the disubstituted products.     

自由基C69N及双体(C69N)2结构和电子光谱的理论研究

用INDO系列方法对自由基C₆₉N(Cs)及双体(C₆₉N)₂(C₂h)进行了理论研究,结果表明:笼骨架上N的掺入使C₇₀笼发生畸变,N向笼外突出,与氮相连的碳(6-6环上的C)自旋密度较大,2个C₆₉N自由基在这个碳上以C-C单键连接,形成双体为C₂h对称性,N与附近的3个碳均以单键连接,并不断开。理论计算的电子光谱与实验吻合较好,(C₆₉N)₂易分解为单体C₆₉N.     

The structural dependence of 13C, 13C coupling constants in cyclopropanes

The smallest one-bond carbon—carbon coupling constants of cyclopropane derivatives have been measured for [1-¹³C]cyclopropane-1,1-dicarboxylic acid and cyclopropanecarboxylic acid. The ¹³C, ¹³C coupling constants of cyclopropylcarbinol have also been determined. The dependence of ¹J(CC) of the three-membered ring on different substituents and hybridization effects is evaluated. The carbon—carbon coupling constant of the unsubstituted cyclopropane is discussed on the basis of self-consistent perturbation theory (SCPT) and s character calculations (localized bond orbitals), and the results are compared with the experimental finding. It is shown that the ¹J (CC) can be interpreted by assuming a dual-pathway coupling mechanism.     

Interbenzylic 19F,19F spin-spin coupling constants in bis(fluoromethyl) arenes

Five- to eleven-bond F,F spin coupling is observed in bis(fluoromethyl)arenes. According to INDO MO calculations, these couplings are transmitted mainly via the aromatic π-system. The low preference of 1,4-(CH₂F)₂-benzene for the conformation in which the C? F bonds are perpendicular to the aromatic ring is shown by the strong increase of⁷J(FF) upon double ortho-substitution.     

Calculation of 14N hyperfine coupling constants by the INDO-MO method

The proportionality constant between the hyperfine splitting constant for ¹⁴N and the spin density calculated by the INDO-MO method has been redetermined using a larger sample of results. A revised value of 332 Gauss is suggested.     

Vicinal C,H spin coupling constants in determining alkene stereochemistry

Further examples of the diagnostic vicinal C,H coupling constants of trisubstituted alkenes are reported, and the application of the method confirms the structures of four dimethyl acetylenedicarboxylate adducts.     

13C13C spin coupling constants in phenanthrene derivatives

The synthesis of six 1-X- (X = OH, OCH₃, OCOCH₃, CH₃, CHO and CN) phenanthrene derivatives with a ¹³C label at C-1 is described. An analysis of the ¹³C¹³C spin coupling constants shows the importance of π-interaction for the coupling constant transmission. Small ¹³C¹³C spin coupling constants over 6 bonds are reported.     

Non empirical quantum mechanical calculations of the1H,13C,15N and17O magnetic shielding constants and of the spin-spin coupling constants in formamide,hydrated formamide and N-methylformamide

The magnetic shielding constants of the different atoms of formamide, hydrated formamide and N-methylformamide are calculated by anab initio method. For the protons of formamide the measured differences between their chemical shifts are correctly reproduced by theory, provided that the molecular geometry used as input is carefully chosen. The differences between the values of the magnetic shielding constants calculated for formamide and hydrated formamide show that intermolecular hydrogen bonding produces variations of chemical shifts for all the atoms of the molecule except the formyl proton. The calculated chemical shift variations between formamide and N-methylformamide are compared to the experimental values and discussed in relation with different hydrogen bonding possibilities of the two molecules. The calculation of the contact term of the spin-spin coupling constants of formamide and hydrated formamide shows that in most cases the measured trends are satisfactorily reproduced and that the variations of these terms upon hydration are less than 3%.