Anisotropic Reorientation of 9-Methylpurine and 7-Methylpurine Molecules in Methanol Solution Studied by Combining ¹³C and ¹⁴N Nuclear Spin Relaxation Data and Quantum Chemical Calculations

Abstract

Reorientation of 9-(trideuteromethyl)purine and 7-(trideuteromethyl)purine molecules in methanol-d4 solutions has been investigated on the basis of the interpretation of the nuclear spin relaxation rates of their 14N (or 1H) and 13C nuclei. The transverse quadrupole relaxation rates of 14N nuclei have been obtained from the line shape analysis of their 14N NMR spectra. Alternatively, the information on the longitudinal 14N relaxation rates has been obtained via the scalar relaxation of the second kind of protons coupled to 14N. The longitudinal dipolar relaxation rates of the protonated 13C nuclei in the investigated molecules have been determined by measuring their overall relaxation rates and NOE enhancement factors. The molecular geometries, scalar coupling constants, and EFG tensors needed for quantitative interpretation of the above data have been calculated theoretically [DFT B3LYP/6-311++G(2d,p) or B3PW91/6-311+G(df,pd)] including the impact of the solvent by using discrete solvation and the polarizable continuum model. The reorientation of the investigated purines has been described as rotational diffusion of an asymmetrical top. It has been found that to get a fully consistent interpretation of the relaxation data, effective C-H bond lengths being 3% longer than the calculated ones had to be used in analysis to compensate for the ground-state vibrations. The obtained rotational diffusion coefficients and orientations of the principal diffusion axes show that the investigated molecules reorient anisotropically and that the mode of their solvation is remarkably different, in spite of their structural similarity.

Interpretation of the Longitudinal 13C Nuclear Spin Relaxation and Chemical Shift Data for Five Bromoazaheterocycles Supported by Nonrelativistic and Relativistic DFT Calculations

“…These T 1,DD parameters could be established from the measured longitudinal relaxation times, T 1 , and NOE enhancement factors using the well-known relationship , T 1 , DD = ( η max / η ) T 1 In the course of the analysis, for a given carbon, the dipolar interactions with all of the protons in the molecule were taken into account. The necessary interatomic distances were adopted from the optimized molecular geometries, except in the case of the directly bonded C–H atoms, where the distance was assumed to be equal to 1.12 Å to compensate for the effects of vibrations. ,− Unfortunately, the data concerning the 13 C··· 1 H dipolar relaxation alone did not allow separation of the diffusion parameters describing reorientations about the axis perpendicular to the ring plane and about the other two axes, as all of the C–H relaxation vectors lay in the ring planes. To overcome this difficulty, we included in the analysis the longitudinal relaxation rates due to the shielding anisotropy mechanism, T 1,SA , estimated for the protonated carbons as T 1 , SA = [ η max / false( η max − η false) ] T 1 If necessary, the so-obtained values were corrected for dipolar relaxation due to 14 N nuclei of directly bonded nitrogens.…”

“…10,11,25 The computer program developed to retrieve rotational diffusion parameters from DD, SA, and QQ relaxation data was described elsewhere. 26 Other programs for data fitting were based on the Newton−Raphson algorithm of iterative nonlinear least-squares sum minimization.…”

Scalar Relaxation of the Second Kind. A Potential Source of Information on the Dynamics of Molecular Movements. 4. Molecules with Collinear C–H and C–Br Bonds

“…Owing to the symmetry, the molecules of 1 reorient in isotropic solutions as symmetrical tops. , The values of the appropriate rotational diffusion parameters: D ∥ = 3.0(3) × 10 9 s –1 and D ⊥ = 8.6(8) × 10 9 s –1 have been calculated using the measured T 1 and η data for protonated carbons, the DFT optimized molecular geometry, and the computer program described elsewhere . During the analysis of the relaxation data the theoretically calculated equilibrium C–H bond lengths have been multiplied by a factor of 1.03 to compensate for the effect of molecular vibrations. − The determined rotational diffusion coefficients and molecular geometry allowed us to calculate the contributions to the overall relaxation of C-9 carbon coming from the dipolar interactions , with protons, T 1,DD ( 13 C···H) = 165 s, from the dipolar interactions with bromine isotopes, T 1,DD ( 13 C– 79 Br) = 271 s and T 1,DD ( 13 C– 81 Br) = 234 s, and from the contribution from the relaxation mechanism caused by the magnetic shielding anisotropy, ,, T 1,SA ( 13 C-9) = 408 s. The last contribution has been estimated using the theoretically calculated Δσ( 13 C-9) = 39.2 ppm value. These contributions are hardly measurable and may seem unimportant, but it is not the case (see below).…”

Shielding and Indirect Spin–Spin Coupling Tensors in the Presence of a Heavy Atom: An Experimental and Theoretical Study of Bis(phenylethynyl)mercury

“…39 The diffusion constants, D ∥ and D ⊥ , were determined from the relaxation data for protonated carbons, using a computer program based on Canet's formulation of magnetic relaxation equations 40 and described elsewhere. 41 Next, the shielding anisotropy parameters for acetylene carbons were calculated, taking into account the corrections from the dipolar interactions of these carbons with protons. A similar method was used to determine the magnetic shielding anisotropy parameter of the mercury nucleus.…”