Quantum-chemical calculations of NMR chemical shifts of organic molecules: IV. Effect of intermolecular coordination on 31P NMR shielding constants and chemical shifts of molecular complexes of phosphorus pentachloride with azoles

Abstract

Intermolecular coordination effects on the 31 P NMR spectra of molecular complexes of N-vinylimidazole and 1-allyl-3,5-dimethylpyrazole with phosphorus pentachloride were studied by theoretical and experimental methods. The formation of intermolecular dative N→P bond was shown to be accompanied by upfield shift of the phosphorus resonance signal by more than 200 ppm. Appreciable contribution of relativistic effects to 31 P NMR chemical shifts was revealed; the spin-orbital contribution to 31 P shielding constant was estimated at >210 ppm. Consideration of solvent effect was found to be crucial while studying steric structure of molecular complexes of azoles with phosphorus pentachloride and intermolecular coordination effects on 31 P NMR chemical shifts. * For communication III, see [1].We previously studied by theoretical methods the effect of intramolecular coordination on 31 P NMR shielding constants (chemical shifts) of [2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ 5 -pyrazolo[1,2-a][1,2,3]-diazaphosphol-8-ium-1-ide [1]. While developing our studies on theoretical calculations of 31 P chemical shifts [1][2][3], in the present work we examined molecular complexes of phosphorus pentachloride with some azoles, N-vinylimidazole (I) and 1-allyl-3,5-dimethylpyrazole (II). In keeping with the results of our experimental [4] and theoretical studies [1], the phosphorus atom in chlorophosphonium group is capable of forming a dative bond with pyridine-type nitrogen atom (i.e., sp 2 -nitrogen atom possessing an unshared electron pair). As a result, the coordination number of phosphorus increases to 5 or 6, and its resonance signal in the 31 P NMR spectrum is displaced upfield; therefore, 31 P NMR spectroscopy can be used to analyze structure of such intramolecular complexes.On the other hand, the presence in the heterorings of I and II of pyridine-type nitrogen atoms possessing enhanced nucleophilicity suggests that attack by phosphorus pentachloride can be directed at the heteroring with formation of the corresponding donor-acceptor molecular complexes III and IV [5, 6] (Scheme 1) rather than at the side-chain double bond. The 31 P NMR spectra of III and IV contained a singlet in the region δ P -258 to -296 ppm, which is typical of six-coordinate phosphorus atom. With a view to estimate the effect of intermolecular coordination on the 31 P shielding constants (chemical shifts) and prove the formation of dative bond in molecular complexes III and IV, we performed quantum-chemical calculations of 31 P NMR chemical shifts for the gas phase and with account taken of solvent effect (see table).The phosphorus shielding constants were calculated at the GIAO-B3LYP/DZP level of theory with account

Quantum chemical calculations of31P NMR chemical shifts: scopes and limitations

“…Namely, if molecules in solution are prone to association or selfassociation, the isolated molecular model will not be correct. 53,55 Intra-or intermolecular coordination involving phosphorus results in a dramatic 31 P nuclear shielding amounting to approximately 150 ppm upon changing the phosphorus coordination number by one. 55 Thus, the impact of associated forms in solution may dramatically change the observed 31 P CSs.…”

“…In fact, there are relatively few reports on 31 P NMR CS calculations using quantum chemical methods. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] In several systematic studies it was shown [48][49][50][51][52][53][54][55] that a satisfactory agreement between calculations and experiments is observed when quite a ''heavy'' basis set or high level of theory ‡ was used although deviations and exceptions were also found. It is necessary to stress that all these studies were focused only on the restricted types of small size compounds.…”

Organophosphorus Azoles Incorporating a Tetra-, Penta-, and Hexacoordinated Phosphorus Atom: NMR Spectroscopy and Quantum Chemistry

“…Theoretical examination of molecular complexes of phosphorus pentachloride with 1-vinylimidazole ( 28) and 1-allyl-3,5-dimethylpyrazole (32) have been carried out [41,56]. The phosphorus atom of the chlorophosphonium group can form a donor-acceptor bond with a pyridine nitrogen atom containing the lone electron pair (i.e., an sp2-hybridized nitrogen atom).…”

Recent advances in computational 31P NMR: Part 1. Chemical shifts

“…In a continuation of the earlier publications by Chernyshev and coauthors, in the paper, the molecule of phosphine, PH 3 , was taken as a reference with a value of phosphorus shielding constant of 606.11 ppm, as evaluated by Lantto, et al at the CCSD(T)/cc‐pwCV5Z level without taking into account relativistic, rovibrational, and temperature corrections. This nonrelativistic GIAO‐CCSD(T)/cc‐pwCV5Z shielding constant was then corrected by evaluating relativistic contribution at the four‐component Dirac–Coulomb level to give 624.309 ppm and finally rovibrationally and temperature corrected to give the final value of 614.758 ppm.…”