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

Herein, an advantage of the use of IDSCRF- over UFF-radii-based solute cavities in GIAO/DFT calculations is presented for the ¹³C and especially ¹⁵N NMR chemical shifts made for several bicyclic aromatic nitrogen heterocycles in CDCl₃ solution treated within the classical IEF-PCM solvation scheme. Successful application of the IDSCRF-radii in the non 1:1 joint multinuclear ¹H/¹³C and particularly ¹H/¹³C/¹⁵N correlations of the measured δ_H,C(,N) values to those obtained theoretically is also documented for a series of test systems (−268 ≤ δ_N ≤ −72 ppm). The experimentally yet unknown δ_N’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_H data (<i>m</i>_H = 10). Such a simultaneous analysis of the δ_H,C(,N) 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

Flash vacuum thermolysis of N-(3- and 4-pyridylmethylidene)-tert-butylamines: mechanisms of formation of pyrrolopyridines and naphthyridines

Pyrrolopyridines and naphthyridines are formed by flash vacuum thermolysis (FVT) of 3-and 4-pyridylmethylidene-tert-butylimines 8 and 15. Elimination of a methyl radical generates resonance stablized 2-azaallyl radicals a1 and b1. The formation of pyrrolopyridines 9, 16 and 17 is rationalized in terms of cyclization of 1-aziridinyl radicals a2 and b2. Formation of naphthyridine 10 from imine 8, and of 11 and 18 from imine 15, are in accord with cyclization of 1-azaallyl radicals a6 and b9. Formation of naphthyridine 11 from 8, and of 10 and 19 from 15, indicate the operation of the spiro-cyclization pathways forming intermediates a9 and b14. Formation of the 1,8-naphthyridine 20 (3%) indicates a rearrangement through aziridine b22 and biradical b23. DFT calculations at the CAM-B3LYP/6-311G(d,p) level support the proposed reaction mechanisms

Carbon-13 longitudinal relaxation time measurements and DFT-GIAO NMR computations for two ammonium ions of a tetraazamacrocyclic scorpiand system

Spin-lattice relaxation times, T1s, for 13C nuclei in two cations Hn1n+ (n = 1, 5)of N-(2-amino-ethyl)-cyclam (1, scorpiand) were determined by means of 13C{1H} NMR experiments in aqueous solution at pH 11.5 and 0.2. The theoretical study [modeling with OPLS-AA, B3LYP/6-31G(d) geometry optimizations, dispersion-corrected energies (DFT-D3), and DFT-GIAO predictions of the NMR chemical shifts (including an IEF-PCM simulation of hydration)] was also done for several conformers of the tautomer iso-H414+ not investigated before. The binding directions in protonated polyamino receptors necessary for efficient complexation of the nitrate anion(s) were briefly outlined, as well. All these results were discussed in terms of 'abnormal' 13C chemical shift changes found previously for the side-chain carbons of amine 1 in strongly acidic solution (HNO3). In conclusion, an earlier proposal of its association with NO3- at pH=1 was rejected. Instead, the participation of small amounts of a microspecies iso-H414+Dhydr under such conditions can be proposed.Publikacja w ramach programu Springer Open Choice/Open Access finansowanego przez Ministerstwo Nauki i Szkolnictwa Wyższego i realizowanego w ramach umowy na narodową licencję akademicką na czasopisma Springer w latach 2010-2013