A Benefit of Using the IDSCRF- over UFF-Radii Cavities and Why Joint Correlations of NMR Chemical Shifts Can Be Advantageous: Condensed Pyridines as an IEF-PCM/GIAO/DFT Case Study

Abstract

Herein, an advantage of the use of IDSCRF- over UFF-radii-based solute cavities in GIAO/DFT calculations is presented for the C and especiallyN NMR chemical shifts made for several bicyclic aromatic nitrogen heterocycles in CDCl solution treated within the classical IEF-PCM solvation scheme. Successful application of the IDSCRF-radii in the non 1:1 joint multinuclear H/C and particularly H/C/N correlations of the measured δ values to those obtained theoretically is also documented for a series of test systems (-268 ≤ δ ≤ -72 ppm). The experimentally yet unknown δ's were found in this way for the title compounds via a trinuclear eq 1 determined for an optimally chosen value of the multiplication factor of initial raw δ data (m = 10). Such a simultaneous analysis of the δ data is proposed as a novel method to study the solution structure of the other similar conformationally homogeneous (bio)organic compounds. The issue of small spurious imaginary vibrational frequencies computed for a few molecular systems using the Gaussian 09 default UFF-radii is briefly considered as well.

Combining Explicit Quantum Solvent with a Polarizable Continuum Model

“…A discrete-continuum solvation model is also recommended to accurately predict solvation free energies and p K a values, − interaction free energies, and rotational free energy barriers . However, the accuracy of some molecular properties, including solvatochromic shifts, ,, proton affinities, solvation free energies and p K a values, ,, NMR shifts, and metal ion–water distances, depends on the definition of the molecular cavity used to define the boundary between the discrete molecular system (i.e., QM region) and the continuum. Many PCM cavity definitions exist, but benchmarking of these cavities usually does not include explicit QM solvent and the reliability of PCM methods with explicit QM solvent is relatively unexplored.…”

On the Use of Deuterated Organic Solvents without TMS to Report 1H/13C NMR Spectral Data of Organic Compounds: Current State of the Method, Its Pitfalls and Benefits, and Related Issues

“…The use of the δ H and δ C data with higher precision than is currently suggested is particularly important for all species with high C/H ratios or with proton deficiency and many heteroatoms, or more generally, for organic systems that provide a small number of δ X data. A suitable method to use in such cases is the multinuclear method [124], which includes a non-1:1 correlation between theoretical predictions and experimental δ X data. This approach uses δ H values multiplied by an adjustable factor n, for example, n = 10.…”

Shortfall of B3LYP in Reproducing NMR JCH Couplings in Some Isomeric Epoxy Structures with Strong Stereoelectronic Effects: A Benchmark Study on DFT Functionals

“…The Gaussian 09 default parameters (including the UFF atomic radii) were applied when constructing solute cavities, whereby a cavity is understood as a free space inside a bulk solvent. A few calculations were also performed with nonstandard IDSCRF atomic radii, which were found beneficial in recently reported structure optimizations and calculations of NMR chemical shifts . The UA0 model and unscaled ( α =1) van der Waals surfaces were used in this case, and related input files were produced with the SCRF‐RADII program .…”

“…Af ew calculations were also performed with nonstandard IDSCRF atomic radii, [28] which were found beneficial in recently reported structure optimizations [12o] and calculations of NMR chemical shifts. [29] The UA0 model and unscaled (a = 1) van der Waals surfaces were used in this case, and related input files were produced with the SCRF-RADII program. [30] The Cartesian coordinates of all ten finally considered conformational structures of epoxides 1-6 are listed in Table S9 (Supporting Information), and their Chemcraft [31] molecular representations are shown in Figure S2.…”