Interpretation of substituent‐induced chemical shifts in ¹³C NMR spectra of 2‐substituted norbornadienes. Influence of homoconjugation

Electron-Withdrawing Substituents Decrease the Electrophilicity of the Carbonyl Carbon. An Investigation with the Aid of13C NMR Chemical Shifts, ν(CO) Frequency Values, Charge Densities, and Isodesmic Reactions To Interprete Substituent Effects on Reactivity

“…Although the NMR shielding is not determined only by electron density, linear correlations with positive slopes between the charge densities and the 13 C NMR chemical shifts for probe nucleus in closely similar surroundings have been observed in several systems when varying the substitution. [14][15][16][17][18][19] The carbon resonates on the higher field the higher the electron density at the carbon is. The Karplus-Pople approximation states that the paramagnetic term of the shielding constant, σ p , which dominates the structure sensitivity of the chemical shift, is determined by eq 1 where ∆E is the average excitation energy, r the mean radius of the 2p-orbital at the atom measured, and Σq the charge density-bond order matrix that contains the electron density.…”

Evidence of Substituent-Induced Electronic Interplay. Effect of the Remote Aromatic Ring Substituent of Phenyl Benzoates on the Sensitivity of the Carbonyl Unit to Electronic Effects of Phenyl or Benzoyl Ring Substituents

“…Although the NMR shielding is not determined only by the electron density, linear correlations with positive slopes between the atomic charges and the 13 C NMR chemical shifts for probe nucleus in closely similar surroundings have been observed in several systems when the substitution was varied. [4][5][6][7][8][9][10][11] A carbon nucleus resonates at the higher field the higher the electron density at the carbon. A decrease in electron density of the carbonyl carbon, on then other hand, can be considered to facilitate nucleophilic attack on the carbon.…”

Comprehensive Experimental and Theoretical Study into the Complete Valence Electronic Structure of Norbornadiene

“…Contrary to the situation described immediately above for the electronic structure of NBD, experimental and theoretical studies into its physicochemical properties have been more prevalent. They include molecular geometry experiments , and calculations, − infrared and Raman spectroscopy results and NMR results . More recently, Penning ionization electron spectroscopy (PIES) data has also become available.…”

“…Standard UniChem , features then allow us to utilize this “optimum” wave function to extract the chemically important molecular property information for the NBD system including, for example, its molecular geometry, vibrational spectra and NMR. A selection of these data, along with a comparison of them with previous work, − is given and discussed in section 5 of this paper. Finally, in section 6, conclusions from the results of the present study are drawn.…”

“…They include molecular geometry experiments 19,20 and calculations, [21][22][23] infrared and Raman spectroscopy results 24 and NMR results. 25 More recently, Penning ionization electron spectroscopy (PIES) data 26 has also become available.…”