The accuracy in the calculation of 31 P NMR chemical shifts in the series of the simplest phosphines, phosphine oxides, and phosphine sulfides was estimated in terms of the Hartree-Fock self-consistent final perturbation theory and density functional theory with different basis sets. The best agreement between the calculated and experimental data was achieved at the DFT/B3LYP/IGLO-III level of theory.NMR spectroscopy is one of the most important experimental tools for studying steric and electronic structures of molecules. In this connection, development of theoretical methods for the calculation of NMR parameters, in particular chemical shifts, becomes significant. Such methods could provide invaluable help both in assignment of signals in the spectra of complex organic molecules and determination of their structure and in conformational analysis, configurational assignment, studying intra-and intermolecular interactions, and prediction of the reactivity of organic compounds [1,2]. The present communication opens a new series of our theoretical works on the calculation of NMR shielding constants (chemical shifts) of nuclei in organic molecules at a high level of modern quantum chemistry.At present, most studies concerned with calculations of magnetic shielding constants make use of approaches based on the density functional theory (DFT) which ensures fairly good results for 1 H, 13 C, and 15 N chemical shifts of natural compounds [3, 4] and organometallic complexes [5,6]. These results are very useful for studying tautomeric equilibria and proving formation of intramolecular hydrogen bonds [7,8]. On the other hand, calculations on shielding constants of nuclei belonging to the third and higher Periods of the Periodic Table, e.g., 31 P, were reported in a few publications. Nevertheless, the available data attract considerable interest, taking into account exceptionally broad range of variation of phosphorus chemical shifts (up to 500 ppm) and important structural information that could be obtained therefrom. Analysis of various factors affecting the accuracy of calculation of δ P values for organophosphorus compounds was performed only in some recent studies [9][10][11]; in particular, comparison of different electron density functionals, basis sets, and methods for consideration of gauge invariance.In the present work we estimated the accuracy in the calculation of 31 P shielding constants (chemical shifts), depending on the calculation method (level of theory) and basis set (primarily its size and flexibility) for effective and appropriate use of these parameters in stereochemical studies on organophosphorus compounds. As subjects for study we selected nine simplest organophosphorus compounds of different natures: phosphines R 3 P (I-III), phosphine oxides R 3 P=O (IV-VI), and phosphine sulfides R 3 P=S (VII-IX) containing methyl, ethyl, and methoxy groups on the phosphorus (I, IV, VII, R = Me; II, V, VIII, R = Et; III, VI, IX, R = MeO).All calculations of 31 P chemical shifts were performed for the most favora...

Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ 6 -pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δ P 170 ppm) and that the contribution of the spin-orbital contribution to the 31 P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of 31 P NMR chemical shifts. * For communication II, see [1].The range of 31 P chemical shifts is known to exceed 500 ppm, which ensures high informative value of 31 P NMR spectroscopy in structural studies on organophosphorus compounds [2, 3]. One of the most interesting aspects of 31 P NMR spectroscopy is related to effects arising from intra-and intermolecular coordination of phosphorus to heteroatoms possessing an unshared electron pair (UEP). The effect of coordination may attain a considerable value comparable with the overall range of variation of phosphorus chemical shifts. For example, phosphorus pentachloride, which is a known electrophilic phosphorylating agent, both in crystal and in polar solvents undergoes dissociation into ions, whereas in nonpolar solvents it exists as five-coordinate phosphorus species, and the δ P value changes from -300 to 80 ppm, depending on the degree of coordination [4] (Scheme 1).In addition to available experimental data, up-todate quantum-chemical methods make it possible to get a deeper insight into the relation between δ P and the structure of organophosphorus derivatives with different coordination numbers of the phosphorus atom. The present communication continues the series of studies involving ab initio calculations on shielding constants [1,5]; in this work we examined effects of intramolecular coordination of the phosphorus atom in trichloro[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) (I) and 1,1,1,1-tetrachloro-1H-1λ 6 -pyrazolo[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide (II). Phosphorylation of 1-vinyl-1H-pyrazole with PCl 5 gave Z isomer of salt I, which was stable at room temperature; it underwent Z-E isomerization at elevated temperature (Scheme 2). This process is facilitated by the presence of hydrogen chloride in the reaction mixture (via reversible hydrochlorination of the double bond. Phosphorylation of 1-vinyl-1H-pyrazole with an equimolar amount of PCl 5 favors formation of structure Z-II with six-coordinate phosphorus atom, which may be regarded as dipolar ion with intramolecular coordination.

The effects of intramolecular and intermolecular coordination on (31)P nuclear shielding have been investigated in the series of tetracoordinated, pentacoordinated and hexacoordinated N-vinylpyrazoles and intermolecular complexes of N-vinylimidazole and 1-allyl-3,5-dimethylpyrazole with phosphorous pentachloride both experimentally and theoretically. It was shown that either intramolecular or intermolecular coordination involving phosphorous results in a dramatic (31)P nuclear shielding amounting to approximately 150 ppm on changing the phosphorous coordination number by one. A major importance of solvent effects on (31)P nuclear shielding of intramolecular and intermolecular complexes involving N → P coordination bond has been demonstrated. It was found that the zeroth-order regular approximation-gauge-including atomic orbital-B1PW91/DZP method was sufficiently accurate for the calculation of (31)P NMR chemical shifts, provided relativistic corrections are taken into account, the latter being of crucial importance in the description of (31)P nuclear shielding.

The influence of solvent nature, relativistic effects, and vibrational corrections on the accuracy of calculation of 31 P chemical shifts of the simplest phosphines, phosphine oxides, phosphine sulfides, and phosphine selenides was studied. Consideration of the above factors at the stage of both geometry optimization and calculation of magnetic shielding constants was found to appreciably improve the accuracy of calculation of 31 P NMR chemical shifts in the series of phosphines and phosphine chalcogenides. * For communication I, see [1].Interest in the chemistry of phosphines and phosphine chalcogenides has increased considerably since late 1980s when direct reaction of red phosphorus with electrophiles in the presence of superbasic catalysts (Trofimov-Gusarova reaction) has been discovered [2]. This reaction made it possible to synthesize previously unknown or difficultly accessible phosphines and phosphine oxides which could be converted into phosphine sulfides and phosphine selenides [3]. The compounds thus obtained exhibited many practically important properties, and some of them are now used in the design of catalysts [4] and for the synthesis of semiconducting nanomaterials [5] and extractants for noble, rare-earth, and transuranium elements [6].In addition, phosphines and phosphine chalcogenides are classical model compounds for stereochemical studies by various physicochemical methods and up-to-date computational methods of quantum chemistry [7]. In this connection, of exceptional interest is development of procedures for theoretical calculations of spin-spin coupling constants and magnetic shielding constants (NMR chemical shifts) for phosphorus nuclei. It is known [8] that these parameters provide unique information on stereochemical structure of organophosphorus compounds.The first communication in the given series [1] was concerned mainly with calculations of 31 P NMR chemical shifts of organophosphorus compounds in the gas phase at different levels of quantum-chemical theory with the use of different basis sets. In the present work we performed a detailed study on the influence of such factors as solvation, relativistic effects, and vibrational corrections on the accuracy of calculation of phosphorus magnetic shielding constants in the series of simplest phosphines, phosphine oxides, phosphine sulfides, and phosphine selenides.All calculations of 31 P NMR chemical shifts were performed for the most favorable conformations of phosphines and phosphine chalcogenides in terms of the well known density functional theory using B3LYP functional [9, 10], following the gauge-invariant atomic orbital approach (GIAO) [11] with the Kutzelnigg IGLO-III basis set [12] (GIAO-B3LYP/IGLO-III). According to our recent data [1], this procedure ensures most accurate calculation of phosphorus chemical shifts. Search for stable conformers (including harmonic vibration analysis of localized stationary points on the potential energy surface) and optimiza-

A configurational assignment of the isomeric methylglyoxal bisdimethylhydrazones derived from the 2-ethoxypropenal precursor has been performed based on experimental measurements and high-level ab initio calculations of 1J(C,C) and 1J(C,H) couplings. The results reveal the marked stereochemical dependence upon the orientation of the lone pairs of both nitrogen atoms in different isomers. Methylglyoxal bisdimethylhydrazone is shown to exist in a mixture of the EE and ZE isomers (ca. 75:25), both of which adopt predominant s-trans conformations with minor (up to 8°) out-of-plane deviations.