Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates

Abstract

Noncovalent interactions are among the main tools of molecular engineering. Rational molecular design requires knowledge about a result of interplay between given structural moieties within a given phase state. We herein report a study of intra- and intermolecular interactions of 3-nitrophthalic and 4-nitrophthalic acids in the gas, liquid, and solid phases. A combination of the Infrared, Raman, Nuclear Magnetic Resonance, and Incoherent Inelastic Neutron Scattering spectroscopies and the Car-Parrinello Molecular Dynamics and Density Functional Theory calculations was used. This integrated approach made it possible to assess the balance of repulsive and attractive intramolecular interactions between adjacent carboxyl groups as well as to study the dependence of this balance on steric confinement and the effect of this balance on intermolecular interactions of the carboxyl groups.

Keywords: CPMD; DFT; IINS; IR; NMR; Raman; carboxyl group; proton dynamics.

Figure 1

Chemical structures of 3-nitrophthalic (3) and 4-nitrophthalic (4) acids.

Figure 2

Conformers of monomeric 3 (upper row) and 4 (bottom row) and their relative energies (ΔE = E_min(conformer) – E_i(conformer), kcal/mol) obtained at the B3LYP/6-311+G(d,p) level of theory for the gas phase and in acetonitrile (CH₃CN). E_min(conformer) stands for the energy of 3(I) or 4(I). E_i(conformer) stands for the energy of the conformer under consideration.

Figure 3

Calculated potential energy curves for the carboxyl group rotation in 3 (solid line) and 4 (dashed line).

Figure 4

The potential energy profile for a gradual displacement of one proton within the H-bond in the 4(II) monomer (a) and the D4(I) dimer (b and c) calculated in the PCM approximation in acetonitrile. The curves a and c represent a case when all other structural parameters are optimized. The curve b represents a case when the position of the adjacent bridged proton is fixed.

Figure 5

Schemes of the prototropic equilibria for the carboxyl aryl derivatives and their intermolecular complexes.

Figure 6

Time evolution of the H-bridge metric parameters. The CPMD gas phase simulations of the monomeric 3 and 4. Red: Donor-proton distance, green: Proton-acceptor distance, blue: Donor-acceptor distance.

Figure 7

Possible scenarios of phthalic acid interaction with bases in nonpolar solution: (a) One intra- and one intermolecular H-bond, (b) single intermolecular H-bond, and (c) two intermolecular H-bonds. Molecular structures of the considered bases.

Figure 8

Characteristic ¹H NMR spectra of 3 and 4 in CDCl₃ at 300 K in the presence of Et₃N. The signals of OH-protons are marked by asterisks. The mole fractions are (a) water:4:Et₃N = 1:1.2:35, (b) water:4:Et₃N = 1:8.7:35, and (c) water:3:Et₃N = 1:8.7:35.

Figure 9

Characteristic ¹H NMR spectra of 4 in CDCl₃ at 300 K in the presence of DMAP. The mole fractions are (a) water:4:DMAP = 1:1.0:260, (b) water:4:DMAP = 1:3.9:260, and (c) water:4:DMAP = 1:0.24:0.30.

Figure 10

Normalized experimental IR and Raman spectra of 3 (A and B) and 4 (C and D) (black spectra) and their deuterated (OD) derivatives (red spectra).

Figure 11

Normalized IINS (A, D), Raman (B, E), and IR (C, F) spectra of compounds 3 (A—C, black spectra) and 4 (D–F, black spectra) and their deuterated derivatives (red spectra).

Figure 12

Calculated power spectra of atomic velocity–results of the CPMD runs for the monomers of 3 and 4 (3m and 4m) as well as for the dimers of 3 and 4 (3d and 4d). The CPMD power spectra are presented only for the bridged protons vibrational modes. The stretching vibration area is shown in red. The bending vibration areas are shown in blue and yellow.