The polarized Raman spectra of N-methylpyrrolidone-an effective method for determination of intermolecular interaction and H-bond formation

Highlights

• Two distinctive clusters constrained by H-bond were determined by linear extension method.

• The transformation from dimer to H-bond cluster till monomer was concentration-determined.

• Different model were proposed for different volume fraction of the mixture.

• Vibrational coupling induced splitting theory can reproduce the noncoincidence effects.

Abstract

The isotropic and anisotropic component Raman spectra and H NMR of N-methylpyrrolidone (NMP)/carbon tetrachloride and NMP/methanol binary mixture at different volume fractions have been collected. The polarization Raman frequencies and frequency differences of CO stretching vibration for NMP/methanol mixture show unique concentration-dependence and abrupt jump feature. It is found that the H-bond between solute and solvent does not destroy the noncoincidence (NCE) phenomenon, but has a significant synergistic effect on the NCE. Two distinctive clusters constrained by H-bond and intermolecular interactions were easily determined by means of linear extension method from abrupt jump curve. The experimental phenomena can be well explained by aggregation-induced splitting theory with the proposed dimer structure and H-bond cluster model. Applying the same methodology the conformation of NMP in water has been determined successfully. The establishment of this method will play an important role in the determination of biomolecule aggregation behavior and supramolecular self-assembly structure.

Introduction

In recent years, the research on the self-assembly and ordered aggregation behavior of the solution phase gets more and more popular[1], [2], [3], [4], [5], [6], [7]. The dynamic behavior and equilibrium structure of self-assembly depend on intermolecular interactions, including solute–solute interaction, solute–solvent interaction and solvent–solvent interaction[8], [9], [10]. Although the chemical bond determines the molecular structure of self-assembly, the final morphology of self-assembly in solution is the equilibrium competition result of the above-mentioned intermolecular interactions[9], [11].

Dipole-dipole interactions play an important role[12]. Polar bonds, such as CO, CS, CO, SO, etc. their stretching vibration frequencies of isotropic component and anisotropic component Raman spectra are inconsistent, their Raman frequency and IR frequency are also inconsistent, scientists name these phenomena as noncoincidence effect (NCE)[13], [14]. NCE value (Δν_NCE) was defined as the vibration frequency difference between the anisotropic and the isotropic component of these polar bonds[15], [16], [17]. The behavior of NCE value vs concentration is related to the dielectric constant of solute to solvent[17], [18], [19]. Giorgini et al. systematically studied the NCE effect of a series of chemicals containing CO group, including acetone[20], 4-hexylbenzoic acid-4-cyanophenyl ester[21], benzophenone[21], phenyl benzoate, and methyl acetate[21]. It was found that the steric hindrance of phenyl ring substituents also affected the NCE value. Giorgini, Musso, and Torii, et al. [22], [23] studied the NCE effect of acetone / DMSO mixture by Raman spectroscopy and MD/TDC theoretical simulation method. It was found that the change law of Δυ_NCE vs volume fraction in non-polar solvent presented an upward convex curve, while in a polar solvent it showed a downward concave curve. Our group studied the Δυ_NCE vs concentration behavior of the CO stretching of the Dimethyl Carbonate(DMC)[19], it exhibits a convex (upward) curvature in the DMC/cyclohexane mixture while a concave (downward) curvature in the DMC/acetone mixture. We proposed an aggregation induced split (AIS) model for the explanations of the NCE phenomenon and concentration-effect for these polar bonds[17]. The vibrational coupling of polar bonds makes the molecules orderly arranged along the polar bonds, forming stable aggregate structures, dimers, etc. The vibrational coupling of the adjacent polar bond makes the stretching vibration of the polar bond split into two kinds of stretching vibration frequencies, that is, in-phase stretching vibration and out of phase stretching vibration. Due to the different symmetry of these two stretching vibrations, they also have different depolarization ratios. When the frequency difference of these two frequencies is less than the resolution of Raman spectroscopy, it shows a broadening single peak. However, if the depolarization analyzer is applied, the vertical direction can preferentially obtain the peak of the vertical component with a high depolarization ratio, while the parallel direction will preferentially collect the peak of the parallel component with a low depolarization ratio, so as to obtain the peaks that the two frequencies do not coincide, that is, the so-called noncoincidence effect. However equipped with high-resolution techniques such as Raman spectra of matrix isolated dimers sometimes they can be well separated with no help from a depolarization analyzer [16], [24].

According to our model, the NCE value (Δν_NCE) represents the formation and coupling degree of the aggregation state. Therefore, we want to use Δν_NCE as a probe to study the effect of hydrogen bonds on the aggregation state of molecules.

N-Methyl-2-pyrrolidinone (NMP) is a good solvent containing carbonyl group, which can dissolve most organic and inorganic compounds, gases, and polymers [25], [26]. The study of the interaction between NMP and CH₃OH help us to deepen our understanding of hydrogen bonds. It is well known that methanol has strong polarity and forms abundant hydrogen bonds[27], [28]. In NMP/CH₃OH binary mixture, hydrogen bonds can be formed between methanol molecules, or between methanol and carbonyl group of NMP, which will inevitably affect the coupling of the carbonyl group and NCE value. In this paper, the NCE and H NMR information of NMP in different volume fractions of binary mixture were systematically investigated, and the influence of hydrogen bond on carbonyl coupling was analyzed and discussed.