Indirect carbon–carbon spin–spin couplings across one,two and three bonds in pyridine and diazine systems: experiment and theory

An excellent linear correlation is found between a large body of experimental spin–spin carbon–carbon couplings, J(CC), across one, two and three bonds in pyridine and diazine ring systems and the corresponding B3PW91/6‐311++G(d,p)//B3PW91/6‐311++G(d,p) computations. The correlation does not differ significantly from the simplest relationship possible, J(CC)_exp. = J(CC)_calcd., within a small and random spread of about 1 Hz. There are 276 experimental values considered, and 124 out of these are new and come from the present work. The aromatic carbon–carbon couplings vary from ?7.6 through +78.5 Hz. It is shown that the correlation provides a reliable tool for predictions of the signs of aromatic J(CC)'s even if the magnitudes of the latter are of the order of 1 Hz. It is demonstrated, for the first time, that the relatively weak ² J(CC) couplings, in the heteroaromatic systems studied, can bear either sign and span a considerable range of about 11 Hz. The character of the correlation indicates that rovibronic effects on aromatic J(CC)'s and those of nuclear motions on aromatic J(CC)'s are practically negligible. All of this is in a perfect agreement with our recent extensive studies on aromatic J(CC)'s in analogous benzene ring system. Substituent effects on the aromatic J(CC)'s turn out to be significant not only for ¹J(CC)'s but also for most of ³J(CC)'s and ² J(CC)'s, and the computation neatly reciprocates these trends. Copyright © 2008 John Wiley & Sons, Ltd.     

Substituent effects on nuclear spin–spin carbon–carbon coupling constants in derivatives of acetylene

¹J(¹³C?¹³C) nuclear spin–spin coupling constants in derivatives of acetylene have been measured from natural abundance ¹³C NMR spectra and in one case (triethylsilyllithiumacetylene) from the ¹³C NMR spectrum of a ¹³C-enriched sample. It has been found that the magnitude of J(C?C) depends on the electronegativity of the substituents at the triple bond. The equation ¹J(¹³C?¹³C) = 43.38 E_x + 17.33 has been derived for one particular series of the compounds Alk₃SiC?CX, where X denotes Li, R₃Sn, R₃Si, R₃C, I, Br or Cl. The ¹J(C?C) values found in this work cover a range from 56.8 Hz (in Et₃SiC?Li) to 216.0 Hz (in PhC?CCI). However, the ¹J(C?C) vs E_x equation combined with the Egli–von Philipsborn relationship allows the calculation of the coupling constants in Li₂C₂ (32 Hz) and in F₂C₂ (356 Hz). These are probably the lowest and the highest values, respectively, which can be attained for ¹J(CC) across a triple bond. The unusually large changes of the ¹J(C?C) values are explained in terms of substituent effects followed by a re-hybridization of the carbons involved in the triple bond. INDO FPT calculations performed for two series of acetylene derivatives, with substituents varied along the first row of the Periodic Table, corroborate the conclusions drawn from the experimental data.     

Substituent effects on indirect carbon–carbon couplings,J(CC), in substituted thiophenes,pyrroles, and furans studied by experiment and theory

An excellent linear correlation is found between a large set of experimental spin–spin carbon–carbon coupling constants, J(CC), in thiophene, pyrrole, and furan systems and the corresponding B3PW91/6‐311++G(2d,p)//B3PW91/6‐311++G(2d,p) calculated estimates. The correlation does not differ significantly from the simplest relationship possible, J(CC)_exp. = J(CC)_calcd., within a small and random spread of about 1 Hz. There are 285 experimental values considered, and 202 out of these are new and come from the present work. The character of the correlation indicates that rovibronic effects on aromatic J(CC)'s, and those of nuclear motions on aromatic J(CC)'s are practically negligible. All of this is in a perfect agreement with our recent extensive studies on aromatic J(CC)'s in pyridine and benzene ring systems. As has been shown by computations, not only large one‐bond couplings but also almost all long‐range ones occurring between the carbons of the heteroaromatic rings are, with a few exceptions, positive. Significant substituent effects experimentally observed in the one‐bond as well as long‐range couplings are very accurately reproduced by the computation. The experimental coupling magnitudes vary from ca. 1 to 98 Hz. The J(CC)'s computed for the model variously substituted trimethylsilyl and fluoro derivatives, which are not easily accessible experimentally, span a range of about 130 Hz, from ca. ?2 in up to ca. +125 Hz . Copyright © 2012 John Wiley & Sons, Ltd.     

Origin of significant solvent effects on 1J(CC) spin–spin coupling in some acetylenes: hydrogen bonding and solvent polarity

It is demonstrated that some acetylenes, those of the R? C?CH structure, display anomalously high sensitivity to solvent effects of their ¹J(C?C) coupling while R? C?CR acetylenes fail to show that. The solvent‐induced variation in the latter coupling does not exceed 3 Hz; this seems to be the upper limit of variation of any J(CC) and J(CH) coupling in the molecular system studied which included: acetylene (in 13 solvents), phenylacetylene (in 12 solvents), 1‐phenylpropyne, and 2‐hexyne (two solvents each), and the only exceptions are ¹J(C?C) in acetylene, which is shown to vary within about 13 Hz, and that in phenylacetylene where the range amounts to about 8 Hz. These apparent anomalies are explained in the present study in terms of two effects of prime importance, solvent polarity and the solute‐to‐solvent hydrogen bonds where the CH moiety in R? C?CH acetylenes acts as a donor of hydrogen bonds to acceptor sites in the solvent concerned. Copyright © 2009 John Wiley & Sons, Ltd.     

Nitrogen NMR Shieldings of Some Nitro Derivatives of 2-Amino-4-methylpyridine Systems

High precision ¹⁴N NMR shieldings (chemical shifts), bulk susceptibility corrected, are reported for dilute solutions in pure, dry acetone for a group of nitro derivatives of (N-substituted) 2-amino-4-methylpyridines, with nitro substituents in either positions 5 or 3 or both, where at least some of these should reveal serious steric hindrance between the substituents involved. The nitrogen shieldings as well as the corresponding PM3 optimized geometries show quite clearly that the 5-nitro derivatives are planar or nearly planar, without any significant hindrance to the -conjugation throughout the molecular system concerned, while the 3-nitro derivatives experience deviations from coplanarity of the pyridine ring and the nitro and amino substituents, with concomitant impairment of -conjugation between these moieties. The nitrogen shieldings of the pyridine nitrogen atoms in N-nitramino-2-alkyl-4-methyl 3 (or 5)-nitropyridines indicate that the N-nitramino group acts as a modest donor of -electrons, much weaker with respect to the donor strength of amino, alkylamino, or phenylamino substituents.     

Solvent polarity and hydrogen-bonding effects on the nitrogen NMR shieldings of N-nitrosamines and DFT calculations of the shieldings of C-, N-, and O-nitroso systems

High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for dimethyl-N-nitrosamine (I) and diethyl-N-nitrosamine (II) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-induced nitrogen shielding variations of (I) and (II) is significant for the amino-type nitrogens, up to about 16 ppm, and originates essentially from the deshielding effect of the increasing polarity of solvent. On the other side, the nitroso nitrogen shieldings reveal an even stronger response to solvent effects, within about 20 ppm, but in this case the increasing polarity and hydrogen bond donor strength of solvent produce enhanced shielding. DFT quantum-mechanical calculations using the GIAO/B3PW91/6-311++G** approach and geometry optimizations employing the same basis set and hybrid density functionals show an excellent correlation with the experimental data on C-, N-, and O-nitroso moieties and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the nitroso systems as a whole. A combination of the calculations involving the corresponding N and O-protonated species and the trends observed in the solvent-induced nitrogen shielding variations shows clearly that the prime acceptor site for hydrogen bonding is the nitroso oxygen atom.