Naphthazarin Derivatives in the Light of Intra- and Intermolecular Forces

Abstract

Our long-term investigations have been devoted the characterization of intramolecular hydrogen bonds in cyclic compounds. Our previous work covers naphthazarin, the parent compound of two systems discussed in the current work: 2,3-dimethylnaphthazarin (1) and 2,3-dimethoxy-6-methylnaphthazarin (2). Intramolecular hydrogen bonds and substituent effects in these compounds were analyzed on the basis of Density Functional Theory (DFT), Møller-Plesset second-order perturbation theory (MP2), Coupled Clusters with Singles and Doubles (CCSD) and Car-Parrinello Molecular Dynamics (CPMD). The simulations were carried out in the gas and crystalline phases. The nuclear quantum effects were incorporated a posteriori using the snapshots taken from ab initio trajectories. Further, they were used to solve a vibrational Schrödinger equation. The proton reaction path was studied using B3LYP, ωB97XD and PBE functionals with a 6-311++G(2d,2p) basis set. Two energy minima (deep and shallow) were found, indicating that the proton transfer phenomena could occur in the electronic ground state. Next, the electronic structure and topology were examined in the molecular and proton transferred (PT) forms. The Atoms In Molecules (AIM) theory was employed for this purpose. It was found that the hydrogen bond is stronger in the proton transferred (PT) forms. In order to estimate the dimers' stabilization and forces responsible for it, the Symmetry-Adapted Perturbation Theory (SAPT) was applied. The energy decomposition revealed that dispersion is the primary factor stabilizing the dimeric forms and crystal structure of both compounds. The CPMD results showed that the proton transfer phenomena occurred in both studied compounds, as well as in both phases. In the case of compound 2, the proton transfer events are more frequent in the solid state, indicating an influence of the environmental effects on the bridged proton dynamics. Finally, the vibrational signatures were computed for both compounds using the CPMD trajectories. The Fourier transformation of the autocorrelation function of atomic velocity was applied to obtain the power spectra. The IR spectra show very broad absorption regions between 700 cm-1-1700 cm-1 and 2300 cm-1-3400 cm-1 in the gas phase and 600 cm-1-1800 cm-1 and 2200 cm-1-3400 cm-1 in the solid state for compound 1. The absorption regions for compound 2 were found as follows: 700 cm-1-1700 cm-1 and 2300 cm-1-3300 cm-1 for the gas phase and one broad absorption region in the solid state between 700 cm-1 and 3100 cm-1. The obtained spectroscopic features confirmed a strong mobility of the bridged protons. The inclusion of nuclear quantum effects showed a stronger delocalization of the bridged protons.

Keywords: AIM; CCSD; CPMD; DFT; MP2; SAPT; crystalline phase; gas phase; intramolecular hydrogen bonds; nuclear quantum effects.

Figure 1

Molecular structures of the studied 2,3-dimethylnaphthazarin (1) and 2,3-dimethoxy-6-methylnaphthazarin (2) with the atom numbering scheme applied in the current study.

Figure 2

The potential energy profiles for the proton motion in the hydrogen bridges of compounds 1 (a) and 2 (b), respectively. The hydrogen bridges denoted as O8-H$${}^{BP1}$$...O1 (Bridge 1) for compound 1 and O8-H$${}^{BP1}$$...O1 (Bridge 1) and O5-H$${}^{BP2}$$...O4 (Bridge 2) for compound 2 are presented. In the case of 2, the solid line denotes Bridge 1 while the dotted line denotes Bridge 2.

Figure 3

Topology maps of electron density obtained on the basis of AIM theory at the B3LYP/6-311++G(2d,2p) level of theory for compounds 1 (a) and 2 (b). The molecular (left) and proton transferred (right) forms are presented. The black solid and dashed lines indicate the intramolecular interaction paths. The green and red dots mark the presence of BCPs and RCPs, respectively.

Figure 4

Two distinct types of stacking in the crystal structures of 1, 2,3-dimethylnaphthazarin (anti-parallel stacking), and 2, 2,3-dimethoxy-6-methylnaphthazarin (parallel stacking). The interplanar distances are 3.614 Å and 3.442 Å, respectively.

Figure 5

Dimers extracted from the crystal structures of compounds 1 (upper part) and 2 (lower part), used in the SAPT study.

Figure 6

Hydrogen bridge structural parameters during the CPMD simulation of 2,3-dimethylnaphthazarin (1). The graphs show gas phase results for (a) Bridge 1 and (b) Bridge 2, and solid state results for (c) Bridge 1 and (d) Bridge 2. For atom numbering scheme, see Figure 1.

Figure 7

Hydrogen bridge structural parameters during the CPMD simulation of 2,3-dimethoxy-6-methylnaphthazarin (2). The graphs show gas phase results for (a) Bridge 1 and (b) Bridge 2, and solid state results for (c) Bridge 1 and (d) Bridge 2. For atom numbering scheme, see Figure 1.

Figure 8

Histograms for the donor-proton distances in the two hydrogen bridges of the studied compounds—results of the CPMD simulation for (a) 1 in the gas phase, (b) 1 in the solid state, (c) 2 in the gas phase, (d) 2 in the solid state. Color scale represents probability density in Å$${}^{-2}$$.

Figure 9

Impact of nuclear quantum effects for the H$${}^{BP1}$$ bridge proton on the O8-H$${}^{BP1}$$ distance. Green circles—classical value of the distance; red crosses—quantum expectation value of the O8-H$${}^{BP1}$$ distance operator. Results of a posteriori quantum treatment of CPMD trajectory for (a) 1 in the gas phase, (b) 1 in the solid state, (c) 2 in the gas phase, (d) 2 in the solid state.

Figure 10

Vibrational signatures (atomic velocity power spectra) of the bridge protons calculated from the CPMD simulation of 1 and 2. In the case of 2, signatures of the non-equivalent bridge protons are presented as separate curves placed back to back. For atom numbering scheme, see Figure 1.