3.2.3. Ortho Phenols and Naphthols
In this subsection, both 5- and 6-member rings for forming intramolecular H-bonds will be considered in connection to the phenolic (naphtholic) OH. This author considers comparison of ortho phenols more important than strictly maintaining the categorization by the number of ring members. The only noteworthy difference being that the O–H…X bond angles could deviate.
Compilations for experimental O–H vibrational frequencies of ortho phenols measured in the gas phase or in dilute solutions of low-dielectric-constant solvents [181,182,183] suggest that the O–H…X intramolecular H-bond exists in these phases. Appearance of this bond in aqueous solution is a more complicated question.
2-OH phenol (catechol). By interpreting the gas-phase microwave spectrum, Caminati et al., [184] concluded that the structure forms an intramolecular H-bond. However, Mandado et al., [7] did not find a (3, –1) BCP and a related H-bond in their AIM analysis using the B3LYP/6-311++G** electron charge density. Reynolds [185] calculated the relative free energy of the catechol conformers and found that the O–H…O structure with a H-bond (HB) is preferred in comparison to the H–O…O–H disrupted H-bond (DHB) form in aqueous solution.
2-OH benzylalcohol. Kumar et al., [186] recorded the UV, IR and microwave absorption spectra in a supersonic jet. A single conformation was identified, where the phenolic OH is a donor in the intramolecular H-bond to the alcohol oxygen. The authors also assume the existence of a weak O–H…π interaction between the alcohol OH and the aromatic ring on the basis of a second minimum in the spectrum. The two types of intramolecular hydrogen bonds were assigned to absorption in the RIDIR spectrum at 3494 cm−1 (O–H…O) and at 3636 cm−1 (O–H…π). The minima were reproduced at the M05/cc-pvtz level. A quite different theoretical spectrum was predicted for the conformer where the alcohol OH is the proton donor to the phenolic O. This local-minimum-energy structure is higher in energy than the global minimum by 10.5 kJ/mol at the M05/aug-cc-pvtz level after zero-point energy correction. No solvent effect study was provided.
Simplerer et al., [181] compared the phenolic OH IR-frequencies for ortho substituted phenols in dilute CCl4 solutions. The experimentally observed red-shift of 202 cm−1 for the OH frequency in the 2-OH benzylalcohol methyl ether compared with the pure phenol supports the model that there must be a strong O–H…O intramolecular H-bond in 2-OH benzylalcohol and in their derivatives.
2-Halogen phenol. The gas-phase electron diffraction experiment predicts a mixture of HB and DHB structures for 2-F phenol [187], with preference for the HB structure. 2-Cl phenol forms an intramolecular H-bond in the gas phase and both ortho-halogen phenols maintain a HB structure in dilute solutions of low-dielectric-constant solvents [181,182,183]. Recent theoretical studies by Nagy [15,64] confirm this finding: the HB structure is almost exclusive either with a 2-F or a 2-Cl substituent in CCl4, and the DHB 2-F phenol fraction was estimated at less than 10% in chloroform. In aqueous solution, the hydration itself favors the H–O…X (X = F, Cl) DHB structure, but the total relative free energy is still favorable for the HB conformers by about 3–5 kJ/mol, corresponding to at least of 80% HB structure in the equilibrium composition [15].
2-NH2 phenol. The term “aminophenol” may be used in a more general sense as regarding the species when the –NH2 group is a substituent on the benzene ring for a phenol, or resides on an alkyl substituent connecting to a benzene ring bearing one, two, etc., OH substituent(s). This latter group will be discussed as β-substituted ethylamines in the next section.
Probably due to the high melting point of 174 °C for 2NH2-phenol, no experimental gas-phase study has been found in the literature. The neutral (non-zwitterionic) form of 2NH2-phenol was studied by Nagy [64] in the gas phase, chloroform and water solvents. In principle, there can exist two intramolecular H-bonds for this molecule, namely O–H…N and N–H…O. The so-called aniline-type-NH2 group, as a benzene-ring substituent, is much less basic than an amino group on a saturated chain. The calculated free energy difference is almost zero in the gas phase for the two types of the intramolecular H-bonds. In both solvents the N–H…O bond was found to prevail, although the calculated relative free energies strongly depend on the applied level of theory and the manner of calculating the solvent effects.
2-NO2 phenol. Despite the weak hydrogen-bond acceptor character of the –NO2 group, the authors of the gas-phase electron diffraction study [188] convincingly argue in favor of the O–H…O(NO) intramolecular H-bond for the isolated molecule. The six-member ring can be conveniently formed. The optimized H…O and O–H…O H-bond parameters calculated at the B97D/aug-cc-pvtz level [64] agree with the experimental values within their respective certainties.
Both in chloroform and water, the theoretical calculations predict a negligible fraction for the DHB conformation with a disrupted intermolecular H-bond. The calculated O–H stretching frequency for the H-bond donor group deviates only by 2 cm−1 from the experimental value. The good agreement was considered as an indication of the need for high-level, IEF-PCM/B97D/aug-cc-pvtz geometry optimizations for exploring the relative free energies between HB and DHB conformers in solutions.
2-COOH phenol. The intramolecular H-bond is formed within a six-member ring including the phenolic OH. The molecule may be considered as a β-hydroxy carboxylic acid, as well. Accordingly, it will be compared with the saturated β-hydroxy carboxylic acids in the next section.
The molecule can adopt several conformations, although only one of them is highly populated and was assigned in IR experiments. The spectrum was recorded by Fiedler et al., [189] in tetrachloride solution and indicated a strong intramolecular H-bond. The deviation of the OH stretching frequency from that in phenol was 395 cm−1. The theoretically calculated deviation is 359 cm−1 at the B3LYP/6-311+G(d,p) level. The lowest-energy conformer is planar, the phenolic OH is a H-bond donor to the carbonyl oxygen of the syn carboxylic group. The =O…H distance and the =O…H–O bond angle were calculated at 176 pm and 145°, respectively. The second-most-stable conformer is higher in energy by 14.3 kJ/mol, where the phenolic OH is the H-bond donor to the syn carboxylic OH. Similar conclusions were drawn by Yahagi et al., [190] by interpreting the gas-phase IR frequencies of the phenolic OH.
In a former calculation by Nagy et al. [45], the two conformers above were found also to be the most stable with MP2/6-31G*//HF/6-31G* energy separation of 13.7 kJ/mol and free energy difference of 12.1 kJ/mol at T = 298 K. All other conformers are much higher in free energy, supporting the estimate of Fiedler that the population of the lowest-energy form is 99.7%.
Nagy et al., also investigated if the intramolecular H-bond would be maintained in aqueous solution by performing NpT MC/FEP simulations using the OPLS pair-potential. Comparing the two stable conformers, the solvation itself would favor the conformation with an H–O(carboxyl)…H–O(phenol) bond by 6.1 kJ/mol, but the total relative free energy is still 6.0 kJ/mol in favor of the =O…H–O(phenol) form. Even much larger solvent effect, 30.0 kJ/mol, was calculated in favor of the conformer when the phenolic OH rotates by 180°, thus when the phenolic group is free for hydration. However, the total relative free energy still remains too high by 17.4 kJ/mol at this new geometry when is compared with the most stable one. In conclusion, the conformer most stable in the gas phase with =O…H–O intramolecular H-bond remains as the predominant species in solution, although 8% second-stable form is also expectable in comparison with its gas-phase population calculated at about 1%.
1-NO and 2-NO naphthols. Ivanova and Enchev [191] performed in-solution experimental and theoretical studies for these molecules. Using NMR spectroscopy in CHCl3 and DMSO solvents, they found that both structures exist only in the tautomeric =N–O–H oxime form at an observable fraction. This suggests that the relevant structures correspond to 1,2-naphthoquinone monooximes. The theoretical studies at the MP4(SDTQ)/6-31G*//6-31G* level augmented with PCM solvent calculations found an equilibrium between the syn and anti oximes, although the preferences are different by the two solvents. H-bonds are only possible in the syn oxime conformation with the neighboring quinone oxygen. This conformer is favored for the 1-syn-oxime-2-naphthoquinone (1-NO-2-naphthol) in both solvents, although the anti oxime was also found experimentally and predicted theoretically. For the 2-oxime-1-naphthoquinone (2-NO-1-naphthol), the authors found experimentally that only the anti oxime form exists in solution, in contrast to prior experimental results. The calculated barrier for the oxim to nitroso form tautomerization is too high along an intramolecuar proton transfer path, explaining the absence of the 1-NO form.