Refinement of the Townes-Dailey Theory. MNDO Calculation of 35CI NQR Frequencies

Nuclear Quadrupole Resonance Spectroscopy: Tautomerism and Structure of Functional Azoles

“…The formation heats (H, kcal/mol) of the conformers A and C of 2-trichloromethyl-5(6)nitrobenzimidazole and 35 Cl NQR frequencies (v, MHz), obtained from Townes-Dailey (TD) and modified Townes-Dailey (MTD) equations [38,46,68]. The analyses of these, others, and our data [84,[87][88][89][90][91] shows that usage of the modified Townes-Dailey equation is more preferable then Townes-Dailey equation for the elucidation of structure and assignment of signals in the 35 Cl NQR spectra of organochlorine compounds. The average frequency of the 35 Cl NQR signals of compound 14 (Table 2) is higher than compound 13, that can be rationalized by the electron-withdrawing influence of the nitro group, despite its being removed from the indicator atom.…”

“…As mentioned above, the numbering of azoles starts from the NH nitrogen atom (or N-organyl) to other heteroatoms. The π-electron density and N-1 bond population, calculated by the Townes-Dailey approach, described by Dr. Lucken [86] and Dr. Dolgushin [68,83,84,[87][88][89][90][91], enhances with lengthening of the substituent in the position 1 (N-1) [94,95]. The data of NMR-NQR study and quantum-chemical investigations of thermodynamic stability of the tautomeric forms of indazole show that the 1H form is more highly stable (21.4 kJ mol −1 ) than the 2H form [98].…”

“…The modification of the Townes-Dailey equation does not account for the effect of the different diffusivity of the p i -orbital on gradient of the electric field (Equation (1)) [68,84].…”

“…The modification of the Townes-Dailey equation does not account for the effect of the different diffusivity of the pi-orbital on gradient of the electric field (Equation (1)) [68,84]. (1) where ν is the calculated NQR frequency, k is the empirical constant, Pxx, Pyy and Pzz are the population of the corresponding р-orbital of the indicator atom, and ξz is the exponent index of the corresponding pi-orbital of the Slater type.…”

“…The data of the quantum-chemical calculations demonstrate a preference of conformer C, while H-bonding stabilizes the conformer A (Scheme 3) [83,85]. The enthalpy of the conformers A and C of 2-trichloromethyl-5(6)-nitrobenzimidazole 14 calculated by AM1 and PM3 methods and the 35 Cl NQR frequencies computed from the Townes-Dailey equation (TD) [86], and the modified Townes-Dailey Equation (1) [68,84], have been studied (Table 3) [83,85]. Table 3.…”