1H,2D,3C NMR Investigation of the Potential Energy Surfaces and of the Proton Exchange Mechanisms Through Hydrogen Bond

Unraveling the Electronic and Vibrational Contributions to Deuterium Isotope Effects on 13C Chemical Shifts Using ab Initio Model Calculations. Analysis of the Observed Isotope Effects on Sterically Perturbed Intramolecular Hydrogen-Bonded o-Hydroxy Acyl Aromatics

“…The decrease in R O‚‚‚O is mainly coupled to a decrease in the C-1,C-2, O and C-2,C-1,CdO angles (see Scheme 1 and Figure 5a). This is seen from the following numbers: the angles A comparison of calculated bond length of 3 and the corresponding molecules in which one, two, or three of the acetyl groups are replaced by formyl groups (11)(12)(13), for data see Table 1S) revealed that the R O‚‚‚O distances for an intramolecular hydrogen bond of CH 3 CO‚‚‚HO type are similar for similar steric motifs (Figure 5b) irrespective of the number of acetyl or formyl groups on the benzene ring. The same is true for a HCO‚‚‚HO type of hydrogen bonds.…”

“…In addition to the compounds shown in Scheme 1, structures of compounds at the same level of calculations are given in Table 1S (Supporting Information): salicylaldehyde (7), 1-acetyl-2,4-dihydroxy-3-formylbenzene (8), 1-acetyl-2,6-dihydroxy-3-formylbenzene (9), 1,3diformyl-2,4-dihydroxybenzene (10), 1,3-diacetyl-5-formyl-2,4-dihydroxybenznene (11), 1-acetyl-3,5-diformyl-2,4,6-trihydroxybenene (12), and 1,3,5-triformyl-2,4,6-trihydroxybenzene (13).…”

“…NMR parameters often used to characterize hydrogen bonds are OH chemical shifts, primary isotope effects, [7][8][9] or secondary deuterium isotope effects on chemical shifts. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] For 13 C the latter is defined as n ∆C(OD) ) δC(OH) -δC (OD) on 13 C chemical shifts, n ∆C(OD), have been studied extensively in intramoleculary hydrogen-bonded systems 4, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] and are shown qualitatively to describe hydrogen-bond strength. The latter is described e.g.…”

“…16,22 With the present set of compounds this is now changed. Deuterium isotope effects on chemical shifts have likewise been shown to distinguish "localized" and tautomeric hydrogen-bonded systems, 10,11,15,26,27 and some interesting isotope effects in o-hydroxy acyl aromatic compounds in which the acceptor group (RCO) is sterically perturbed previously have been described. 28 In recent years ab initio methods to calculate nuclear shieldings [31][32][33][34][35][36][37] and deuterium isotope effects on nuclear shieldings 38,39 have been developed to a high degree of accuracy and have become generally accessible due to still faster computers and commercial software.…”

Fluorine-containing 2,4-dioxo acids in the synthesis of heterocyclic compounds

“…On the basis of the 1 H and 13 C NMR spectral data, it was concluded that there was no tautomerism between two possible enol forms. 28,29 The proton is delocalised between two potential wells due to tunnelling. This agrees with the results of quantum-chemical SCF MO LCAO calculations by the CNDO/2 method.…”