The Microwave Spectrum of Imidazole; Complete Structure and the Electron Distribution from Nuclear Quadrupole Coupling Tensors and Dipole Moment Orientation

Abstract

Spectra have been measured for eleven isotopic forms of imidazole, including single substitutions at each nucleus in turn. A complete rN(1)-C(2) = 1.364 Å, C(2)-N(3) = 1.314 Å, N(3)-C(4) = 1.382 Å, C(4)-C(5) = 1.364 Å, C(5)-N(1) = 1.377 Å, ≮N(1)C(2)N(3) = 112.0°, ≮C(2)N(3)C(4) = 104.9°, ≮N(3)C(4)C(5) = 110.7°, ≮C(4)C(5)N(1) = 105.5° and ≮C(5)N(1)C(2) = 106.9°.The N(1)-H(1) distance is 0.998 Å, while the C-H distances are all very close to 1.078 Å. The bonds N(1)-H(1) and C(2)-H(2) lie close to the external bisectors of the respective ring angles, but C(4)-H(4) and C(5)-H(5) are each displaced by several degrees from these bisectors towards N(3) and N(1) respectively. The electric dipole moment is established as 3.67 (5) D from Stark effects, directed almost parallel with the line joining the nitrogen nuclei. The properties and orientations of the two

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

“…In Table 3, the structural parameters of caffeine monohydrate in the crystal [8] and the gas-phase structures of the related molecules, uracil [26] and imidazole [27,28] (see Fig. 1), are also listed.…”

“…Many of the experimental bond lengths in imidazole [27,28] are in disagreement with the corresponding values in caffeine. It should also be noted that caffeine has a longer C=C length than both of uracil and imidazole.…”

Structural Studies of the Bisimidazole 5,5-Dimethyl-1,3,2-Dioxaphosphorinane-2-Thioxo-2-Hydroxy Complex

“…In consequence, we observe distinct values of δ 22 in a series of calculated structures. The third relationship, between the N‚‚‚H + distance and κ (Figure 5c), is obvious given that κ ) 3(δ 22 -δ iso )Ω 34 . Table 5 shows the results of GIAO calculations for imidazole‚‚‚H + ‚‚‚NH 3 complexes for different distances.…”

Ab initio study of the EFG at the N sites in imidazole

“…The EFG at the N sites in the isolated imidazole molecule was calculated using the atomic positions measured at T = 293 K [2] and theoretical atomic positions optimized in this work. The theoretical and experimental geometrical parameters (bond distances and bond angles) are in very good agreement, differing by less than 1%.…”

“…The EFGs at the two N sites (imino and amino) in molecular imidazole have been measured by Christen et al [2] and by Blackman et al [3] through microwave spectroscopy. Using nuclear quadrupole resonance (NQR), Garcia et al [4] (at 291 K) and Edmond [5] (at 103 K) have measured the EFGs at the N sites in the crystalline phase of imidazole.…”