Electron density of a benzoylated tetrafructopyranose

2.1 Structural properties

The molecular structure in the crystal is shown in Figure 1 in MERCURY representations [16]. In contrast to inulin, all fructose units are in the pyranose form. The atomic numbering scheme from Ref. [15] was maintained. As expected, the pyranosyl rings are in chair conformations ¹C₄, more precisely ²C₅ in the notation for a keto sugar, as indicated by the θ-values of the Cremer & Pople puckering parameters [17, 18], which are close to 180°. This holds also for free β-D-fructopyranose [19]. The 2 → 1 glycosidic linkages all exhibit a -gauche-trans conformation, see also Figure 1 (bottom).

Figure 1:

MERCURY representations [16] of the title compound. Hydrogen atoms are omitted for clarity. The contributing tetrabenzoylpyranose sugars are numbered (I)–(IV). The benzoyl phenyl rings and their centers (shown in yellow) are numbered P11, P12, … P43 in the representation (top). To make the pyranosyl rings and the glycosidic linkages more clearly visible, the benzoyl substituents are omitted in the representation (bottom).

The 12 phenyl rings that are part of the benzoyl groups are all planar. Most of them form insignificant interplanar angles. Exceptions are the ring pairs P12/P23 and P32/P43, which are coplanar with interplanar angles of 6.5(4) and 8.9(1)°, respectively. Their interplanar distances are 5.32 and 5.28 Å. A closer approach is seen between rings P21 and P33 (to 4.24 Å) with an interplanar angle of 35.9(3)°.

Due to the absence of all but one OH group, only one O–H⋯O hydrogen bond exists, namely O(2)–H(1)⋯O(34), the primed oxygen atom being related by a translation in x-direction. In addition, Platon [18] lists seven intermolecular C–H⋯O contacts with C⋯O distances of 3.04–3.55 Å.

2.2 Electron density results, bonding and atomic properties

An illustration of the non-spherical ED distribution in the form of deformation densities is shown in selected intramolecular planes in Figure 2, confirming the proper assignment of the library multipoles. Deformation densities in the O–H⋯O and a C–H⋯O hydrogen bond region are shown in Figure 3. The O–H and C–H bond vectors are directed towards the accepting oxygen-atom lone-pair region.

Figure 2:

Deformation densities in selected intramolecular planes: Above left: C(19)–O(26)–C(23), above right: C(19)–O(25)–C(18); below: phenyl ring through C(103)–C(107)–C(104).

Figure 3:

Deformation densities in the planes of the intermolecular hydrogen bonds O(2)–H(1)⋯O(34) and C(107)–H(88)⋯O(24).

For a quantitative analysis of the electron density, Bader’s QTAIM formalism [20] was applied to yield bond critical points (BCPs) and atomic properties. Averages of bond critical point properties (BCP’s, defined by the property that the gradient ∇ρ(r) vanishes at this point) on the covalent bonds are given in Table 1. For a summary of all BCP properties on the covalent bonds (and ring critical point properties) see Supplementary material, Table S1. There is a large number of 105 C–C bonds in the molecule which permits a reliable statistic. One can distinguish two carbon–carbon bond types: 33 single and 72 aromatic bonds. From the electron density ρ(r_BCP) at their bond critical points, the topological bond order n_B was calculated [21]. We obtain averages n_B = 1.23(6) for the single, and 1.71(6) for the aromatic bonds. In an earlier study on a rotaxane, where averages over 22 single and 46 aromatic bonds were calculated, bond orders of 1.03 and 1.61 were derived [22]. In the present case, electron delocalization for the C–C single bonds can be discussed. For the 54 C–O bonds, three types can be distinguished as specified in Table 1. They show the expected behavior and do not need further discussion. Ring critical points r_cp were identified for the 4 pyranosyl and the 12 phenyl rings, see also Table 1.

Topological properties of the intermolecular hydrogen bonds (HBs) are summarized in Table 2. The HB energy of O–H⋯O units is around 10 kcal mol⁻¹ [23], indicating a weak HB. The ρ(r_BCP) values on the BCPs of the C–H⋯O HBs are all very low, so that in total the intermolecular interactions in terms of HBs are marginal. As a result, the packing in the crystal is less dense than in carbohydrates, with their larger number of hydroxyl groups giving rise to more or less strong hydrogen bonds. We refer to the low density of the crystals of the title compound of 1.32 g cm⁻³ as determined by XRD. For comparison, β-D-fructopyranose, β-D-glucopyranose and sucrose have a much higher density of around 1.55–1.60 g cm⁻³. Non-substituted oligosaccharides, as for example the tetrasaccharides coded PEKHES or STACHY10, or the pentasaccharide trehalose (DEKYEE) [1, 2], have a substantial higher density, 1.50 g cm⁻³ or higher. In all those cases, the molecules are linked by a variety of hydrogen bonds.

Atomic properties [25] were calculated by integration over the atomic basins bound by zero flux surfaces of the gradient vector field ∇ρ(r), which subdivide a structure into transferable substructures. The algorithm introduced by Volkov et al. [26], as implemented in the Xdprop subprogram of XD [24], was used. To illustrate the atomic properties in different neighborhoods, a fragment consisting of a pyranosyl ring with one benzoyl substituent is plotted in Figure 4 together with the atomic properties in this region, which is representative for all other groups. The oxygen atoms carry a strong negative charge close to −1 e, which is in part compensated by the charge of the directly bonded carbon atoms. Carbon atoms C(1) and C(25) bonded to two oxygen atoms are more positively charged than the other carbon atoms in the pyranosyl ring.

Figure 4:

The pyranosyl ring (IV) with one benzoyl substituent (P41) is displayed together with atomic charges and volumes of carbon and oxygen atoms (in blue). Not shown: average charges/volumes at the pyranosyl hydrogen atoms H(2)⋯H(6) are 0.08(2) e/7.5(12) Å³, at the phenyl hydrogen atoms H(30)⋯H(34) 0.15(1) e/8.0(14) Å³.

All carbon atoms of the phenyl rings have atomic charges close to zero. Atomic volumes of the formal sp³ hybridized atoms at the pyranosyl ring are smaller than those of the sp² carbon atoms of the phenyl rings. Obviously, the number of neighboring atoms has a small but noticeable influence on the atomic volume within each of these two groups. The hydrogen atom of the only OH group, which is the donor of an O–H⋯O hydrogen bond, has an extremely small volume close to 1 Å³, and a rather high atomic charge. The phenyl hydrogen atoms have twice the charges of the pyranosyl ring hydrogen atoms, and contribute to the positive electrostatic potential belt of the molecule, see next section.

The volume of the entire molecule, as obtained from the summation over the atomic volumes, is 2436.81 Å³. It reproduces half the unit cell volume (V_cell/2 = 2452.0 Å³) within less than 1%, which is a good check whether the integration procedure has worked properly.

2.3 Electron density, Hirshfeld and electrostatic potential surfaces

Hirshfeld surfaces [28, 29] can provide information about intermolecular interactions based on local ED concentrations. Mapping the aspherical ED by a color code onto this surface, ED concentrations indicate sites and strengths of these interactions. The electrostatic potential (ESP) makes sites of molecular polarization visible, where positive and negative ESP regions can be identified. The joint information that ESP and Hirshfeld surfaces provide helps to understand interactions and reactivity of a structure of interest. The generation of both types of surfaces was carried out using the graphical software Moliso [27].

In Figure 5 the Hirshfeld surface of the title molecule is represented. Due to the large size of the molecule, this surface is difficult to display and to interpret. We note that there are only small and weak signals (see also the color bar on the left in Figure 5). The viewing direction was chosen to highlight ED concentration at the donor and the acceptor site of the only O–H⋯O hydrogen bond. Some additional signals occasionally appearing on this surface representation stand for a few C–H⋯O interactions. For comparison, we have generated the Hirshfeld surface for sucrose (Figure 6). On this surface, broad and strong signals (compare the color bar with the one for the title molecule shown in Figure 5) are easy to interpret. The ED concentrations indicated by white circles are caused by some of the strongest hydrogen bond interactions in sucrose.

Figure 5:

Electron density mapped onto the Hirshfeld surface of the benzoylated tetrasaccharide. Small ED concentrations are seen at the donor and acceptor sites of the O–H⋯O hydrogen bond, and occasionally at one of the C–H⋯O HB’s (drawn with the program Moliso [27]).

Figure 6:

Electron density mapped onto Hirshfeld surfaces of sucrose; above: transparent representation with the molecular skeleton visible, below: non-transparent representation; four of the strongest hydrogen bonds are indicated by local electron density concentrations (representations generated with Moliso [27]).

The ESP surface shown in Figure 7 was calculated by using the method of Volkov et al. [30] with the Xdprop subprogram of Xd2006 [24], and color coded onto the 0.067 e Å⁻³ (=0.01 a.u.) ED [27] isosurface. The terminal phenyl rings of the benzoyl groups cause a positive ESP on the outer shell of the ED isosurface, so that a rather strongly positive ESP belt exists. The oxygen atoms of the benzoyl groups cause an almost neutral ESP. A weakly negative ESP in the interior at the pyranosyl oxygen atoms is visible through a pale red coloring. In total, we see a polarization from the positive ESP belt in the outer regions of the molecule towards neutral and negative regions in the interior. The almost completely positive outer surface is expected to lead to repulsive forces with adjacent molecules in the crystal. This result explains the less dense packing mentioned before, as also suggested by the weak signals on the Hirshfeld surface.

Figure 7:

Electrostatic potential of the title compound. For guidance, the sites of the pyranosyl residues are labeled in roman letters, see also Figure 1 (representation with Moliso [27]).