Acidity of ortho-substituted benzoic acids: an infrared and theoretical study of the intramolecular hydrogen bonds

Abstract

The structures of ortho-substituted benzoic acids with substituents bearing hydrogen atoms (OH, NH2, COOH and SO2NH2) were investigated by means of IR spectroscopy and of density functional theory at the B3LYP/6-311 + G(d,p) level. All possible conformations, hydrogen bonds, tautomeric forms and zwitterions were taken into consideration and particular attention was given to intramolecular H-bonds and their effect on acidity. Strong H-bonds in the anions of all four acids, were revealed by calculations. In three cases they were confirmed by the IR spectra of the tetrabutylammonium salts in tetrachloromethane solution, while the salt of 1,2-benzenedicarboxylic acid was not sufficiently soluble. The H-bonds are of different strengths but in all cases they are the main cause of the strengthened acidity of these acids in the gas phase and also in solution, although their effect is opposed by weaker H-bonds present in the undissociated acid molecules. The substituent effect on the acidity was evaluated in terms of isodesmic reactions, separately in the acid molecules and in the anions. While the acidity of the 2-OH and 2-NH2 acids is determined essentially by the H-bonds, that of the 2-COOH and 2-SO2NH2 acids is strengthened by the polar effect operating in the undissociated molecule in addition to the H-bond in the anion. The steric inhibition of resonance (SIR), estimated from model conformations with fixed torsional angles, is of little importance. This analysis goes significantly beyond the classical explanation obtained from the acidities in solution but essentially conforms with it.

Electropolymerization molecularly imprinted polymer (E-MIP) SPR sensing of drug molecules: Pre-polymerization complexed terthiophene and carbazole electroactive monomers

“…The bifunctional NH 2 of both carbazole and terthiophene-based monomers exhibited lower template rebinding. This might be a result of the weaker H-bonding ability of the NH 2 than -COOH and -OH (Fiedler et al, 2006;Sibrian-Vazquez and Spivak, 2004;Jeffrey, 1997) when complexing with the analyte drug, and thus less number of templates were imprinted. However, the E-MIP films generated by the bifunctional G1 monomers required more time (>30 min) to reach the saturation point.…”

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

“…Differences in the calculated bond lengths are not larger than 0.0002 Å. Since the DFT(B3LYP)/6-311+G(d,p) level has been very often used in the literature to study the geometry of π-electron systems [10,11,[36][37][38][39], we chose this level of theory for the HOMED estimation of all heteroatomic π-electron systems considered in this paper. Table 7 summarizes the HOMED indices for the DFT structures of the simplest triad molecules.…”

“…hyperconjugation, conjugation, aromaticity, tautomerism, proton-transfer, and/or hydrogen bonding, may take place [10,11,[36][37][38][39][40]. We analyzed π-electron delocalization for simple σ-π hyperconjugated, n-π conjugated, π -π conjugated, and aromatic compounds including those displaying prototropic tautomerism.…”

Noncovalent complexation of glucosylthioureidocalix[4]arenes with carboxylates and their gas‐phase characteristics: an ESI‐FTICR mass spectrometric study

“…[21] Relevant, too, is the more pronounced preference of hosts 2 and 3 for isophthalate (5) and terephthalate (6) over phthalate (4) as compared with the preferences of host 1. It must be emphasized, however, that in dicarboxylic acids (and especially in their monoanions) where H-bonding donor and acceptor groups are in close proximity, the existence of strong intramolecular H-bonds [22] presumably hampers the intermolecular association between host and guest. To investigate this, we evaluated the possible intramolecular H-bonding of relevant guests with ab initio calculations.…”