## 1 Introduction  
### Background  
Bitumen, as one of the materials obtained from crude oil, is a viscoelastic material, which is remarkably soluble in carbon disulfide (CS\({}_{2}\)) and holds adhesive and waterproofing properties. From the chemical point of view, bitumen contains approximately 80% carbon, 15% hydrogen and the remaining part consists of two types of atoms: heteroatoms and metals [1]. Considering the chemical composition, it is highly dependent on the source (crude oil) and on the refinery process, which contribute to its unique chemical and physical properties [2]. Although in this context chemistry of bitumen has been considered as a key parameter in fundamental understanding of bitumen characterization, the knowledge about such a complex material is still limited. Bitumen is typically constituted of two main groups of compounds: (a) asphaltenes, defined as \(n\)-heptane-insoluble and toluene-soluble part,which are complex mixtures of high molecular weight hydrocarbons present in a percentage ranging between 5% and 25% by weight, consisting mainly of condensed aromatic compounds, as well as oxygen, nitrogen, sulfur and metals; and (b) maltenes, as soluble part in \(n\)-heptane, can in turn be subdivided into saturates, aromatics, and resins, which, together with the asphaltenes, are known as SARA fractions [3]. While asphaltenes have the highest polarity, saturates are the least polarized molecules following the aromatics and resins, which rank in between the two extremes. Several former studies have indicated that the relative amount of bitumen fractions considerably affect both the physical and mechanical properties. It has been found that, while the polar fraction directly affects the elastic phase, the presence of non-polar fraction contributes to the viscous behavior of bitumen [4]. Therefore, a better understanding of the structural characteristics and composition of the various fractions is of utmost importance in the industrial use of these complex mixtures to apply appropriate methodologies to adapt their chemical-physical properties in a controlled manner [5; 6]. Since the bitumen fractions obviously determine the bitumen mechanical properties and characteristics, several attempts have been made to analyze the bitumen composition and to fractionate the bitumen compounds. Beside Atomic Force Microscopy (AFM) images and Fourier Transform-Infrared Spectroscopy (FT-IR) analysis, recently Nuclear Magnetic Resonance (NMR) spectroscopy has been found to be a practically efficient and reliable technique for complex material characterization such as bitumen [7; 8]. It is worth underlining that the NMR has become one of the most powerful tools for probing in all states of matter. The technique in isotropic solution is commonly used for the characterization of synthetic or natural compounds, while structural and conformational information on even large-sized molecules can be obtained in anisotropic media through the anisotropic NMR observables, such as the residual dipolar couplings (RDCs) [9; 10; 11; 12]. Nowadays, the NMR technique is also well known as an analytical technique for qualitative and quantitative study, at molecular level, of complex mixtures. Compared to the conventional analysis, NMR spectroscopy does not need the pre-treatment of samples that could considerably reduce the time consuming process of their preparation. In addition, since the amount of solvent that is used and the waste generation is minimum, the technique is considered environmentally friendly [13]. The superiority of this technique is the ability to simultaneously investigate several components of the mixture with a single \({}^{1}\)H-NMR spectrum and to evaluate the relative amount of the aliphatic and aromatic hydrogens portion in the mixture [8; 14; 15]. On the other hand, the separation technique to prepare a sufficient sample for NMR analysis can be a challenge.