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Pressure-Generated Hydrogen Bonds and the Role of Subtle Molecular Features in Tetrahydrofuran ski, and Andrzej Katrusiak* Kamil F. Dziubek, Damian Je˛czmin Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Pozna n, Poland
ABSTRACT Pressure-promoted formation of intermolecular C-H 3 3 3 O contacts has been rationalized by the electrostatic potential distribution and molecular conformation in tetrahydrofuran, C4H8O. Its isochoric crystallization at 2.25, 3.26, and 3.80 GPa and isobaric freezing at ambient pressure lead to a monoclinic phase, space group C2/c. The sequence of C-H 3 3 3 O distances in these structures is consistent with the electrostatic potential distribution and favored acceptor directionality for hydrogen atoms. The pressure evolution of intermolecular interactions increases the number of C-H 3 3 3 O contacts short enough to be classified as hydrogen bonds in the compressed crystal. SECTION Molecular Structure, Quantum Chemistry, General Theory
T
he C-H 3 3 3 O interactions are considered as an archetype of weak hydrogen bonds.1 Although their energy is much lower than that of classical (e.g., O-H 3 3 3 O or N-H 3 3 3 O) hydrogen bonds, they play an important role in biological systems, supramolecular chemistry, and crystal engineering.2-4 Properties of C-H 3 3 3 O interactions were investigated by infrared absorption and Raman scattering,5 X-ray and neutron diffraction,6 and computational methods.7 It was postulated that pressure strongly favors the formation of C-H 3 3 3 O hydrogen bonds,5 although no structural information has been available. Moreover, in most of the structures investigated at ambient conditions, weak hydrogen bonds coexisted with stronger interactions, and the effects of CH 3 3 3 O contacts were obscured. The strength of the C(spn)H 3 3 3 O interaction decreases in the order C(sp)-H 3 3 3 O > C(sp2)-H 3 3 3 O > C(sp3)-H 3 3 3 O,7 and it increases with the number of adjacent electron-withdrawing groups. In tetrahydrofuran (THF, C4H8O), which is a saturated ether without other functional groups or π electrons in the molecule, only weak C-H 3 3 3 O hydrogen bonds and H 3 3 3 H interactions8 stabilize the crystal structure. These subtle cohesive forces can be analyzed in THF due to the absence of stronger interactions. Therefore, for this study of weak C-H 3 3 3 O hydrogen bonds, THF was chosen, and we have focused on the interplay of electrostatic potential, mutual orientation, and packing arrangement for the formation of the contacts in the crystal structure. We have applied high pressure to relate a contraction of intermolecular contacts with the changes in the molecular arrangement and to check if the crystal packing based mainly on the C-H 3 3 3 O contacts can be destabilized in the compressed structure. THF is a simple heterocyclic compound, and its fivemembered ring is a prototypical structural unit of furanose sugars, among others ribose and deoxyribose, being the components of RNA and DNA, respectively. Recent ab initio calculations and electron momentum spectroscopy experiments9,10
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suggest that a global energy minimum conformation of THF, most populated in the gas phase, is the envelope of Cs symmetry. Still, the energy differences between the conformers of C1, C2, and Cs symmetry are very small, and they interconvert rapidly in an internal motion known as pseudorotation.11 This process is quenched in the solid state as evidenced by single-crystal X-ray diffraction at 103 and 143 K12 and, for perdeuterated THF, by powder neutron diffraction at 5 and 120 K.13 THF crystallizes in space group C2/c, where the molecule adopts a twisted conformation coinciding with the crystal two-fold axis. We have studied THF in a diamond anvil cell just above the freezing pressure at 2.25 GPa and also at 3.26 and 3.80 GPa, where no phase transition occurs (crystal data and refinement details are summarized in Supporting Information, Table S1). Over the pressure range from 0.1 MPa to 3.80 GPa, the THF becomes slightly flatter, for example, the torsion angle C2-C3-C3i-C2i (symmetry code: i=-x, y, -z þ 0.5) varies from -35.2(4) at 148 K/0.1 MPa to -31.7(4) at 295 K/3.80 GPa, while q2, the Cremer and Pople puckering parameter,14 changes from 0.355(4) Å at 148 K/0.1 MPa to 0.309(4) Å at 295 K/3.80 GPa (for the full list of bond angles, torsion angles, and q2 parameters, see Supporting Information, Table S2). As shown in Figure 1 (see also Supporting Information, Figure S1), each oxygen atom is involved in six (three symmetry-related pairs) intermolecular C-H 3 3 3 O contacts. The next H 3 3 3 O distances are considerably longer, and this gap widens with increasing pressure (between 0.88 Å at 148 K/0.1 MPa and 1.03 Å at 295 K/3.80 GPa), which indicates enhanced stabilizing features of C-H 3 3 3 O interactions in dense THF. The six-fold hydrogen-acceptor capacity of the
Received Date: December 11, 2009 Accepted Date: January 21, 2010 Published on Web Date: February 12, 2010
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DOI: 10.1021/jz9003894 |J. Phys. Chem. Lett. 2010, 1, 844–849
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Table 1. Geometric Parameters of Short C-H 3 3 3 O Contacts in the THF Crystal Structure at Different Thermodynamic Conditionsa d(H 3 3 3 O) [Å]
T/p
d(C 3 3 3 O) [Å]
— (C-H 3 3 3 O) []
ii
5 K/0.1 MPab
C3-H31 3 3 3 O1 2.51 3.560(3)
103 K/0.1 MPa
2.51
3.550(6)
161
120 K/0.1 MPab
2.54
3.592(3)
163
148 K/0.1 MPa 295 K/2.25 GPa
2.55 2.35
3.594(5) 3.406(3)
161 166
295 K/3.26 GPa
2.29
3.351(3)
165
295 K/3.80 GPa
2.27
3.328(5)
165
5 K/0.1 MPa
C2-H22 3 3 3 O1iii 2.77 3.753(3)
151
103 K/0.1 MPa
2.80
3.786(7)
151
120 K/0.1 MPab
2.83
3.820(3)
152
148 K/0.1 MPa
2.83
3.820(5)
152
b
Figure 1. The crystal structure of THF at 3.80 GPa projected along [100]. The shortest C-H 3 3 3 O contacts have been indicated by the dashed lines. Displacement ellipsoids have been drawn at the 50% probability level for non-H atoms.
oxygen is highly unlikely for any strong hydrogen bonds. For example, the four-fold hydrogen-acceptor capacity of the carbonyl oxygen atom in urea is extremely rare.15 In THF, the C-H 3 3 3 O distances are longer, which extends the hydrogen-bond coordination sphere around the oxygen, and these contacts are less sensitive to the deviations from ideal geometry than stronger hydrogen bonds.16,17 There are several geometric descriptors used as the basic criteria for classifying genuine hydrogen bonds. While the shorter C-H 3 3 3 O contacts display H 3 3 3 O (d) distances of 2.1-2.5 Å and C 3 3 3 O (D) distances of 3.1-3.5 Å, more liberal cutoff thresholds (d < 2.8 Å or D < 4.0 Å) are often adopted.1 The C-H 3 3 3 O interaction is primarily electrostatic in nature but exhibits a preference to be linear even for very weak hydrogen-bond donors;18 the most populated C-H 3 3 3 O angles (θ) typically vary between 150 and 180. In THF (Table 1), for the three closest symmetry-independent H 3 3 3 O distances, the C-H 3 3 3 O (θ) angles fall in this range, and they lie in a narrow interval of a few degrees only, testifying to the significance of molecular arrangement for the strength and hierarchy of the C-H 3 3 3 O interactions. The H 3 3 3 O contacts, referred to the 148 K/0.1 MPa structure, are gradually compressed either upon decreasing the temperature or increasing the pressure (Figure 2, Table 1). The d(H31 3 3 3 O1ii) distance is the shortest one in any of the structures, while the sequence of similar d(H22 3 3 3 O1iii) and d(H32 3 3 3 O1iv) reverses due to a stronger compression of initially slightly longer contact H32 3 3 3 O1iv (cf. Table 1). Despite some reservations regarding the sum of van der Waals radii in structure analysis,21,22 together with the angular dimensions, they can be reliable criteria for identifying hydrogen bonds. In THF at ambient pressure, only the closest d(H31 3 3 3 O1ii) distance is shorter than the sum of H and O van der Waals radii, and at high pressure, also d(H22 3 3 3 O1iii) and d(H32 3 3 3 O1iv) cross this threshold value. Specific intermolecular interactions can be also discussed in the terms of electrostatic complementarity between adjacent molecules in the crystal structure. It occurs that of
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162
295 K/2.25 GPa
2.60
3.567(4)
149
295 K/3.26 GPa
2.53
3.494(4)
148
295 K/3.80 GPa
2.51
3.472(6)
147
5 K/0.1 MPa
C3-H32 3 3 3 O1iv 2.75 3.825(2)
175
103 K/0.1 MPa
2.80
3.880(8)
174
120 K/0.1 MPab
2.82
3.896(3)
176
148 K/0.1 MPa
2.85
3.927(4)
175
295 K/2.25 GPa 295 K/3.26 GPa
2.58 2.49
3.665(4) 3.572(4)
177 177
295 K/3.80 GPa
2.45
3.531(6)
178
b
Symmetry codes: ii = -0.5 þ x, 0.5 þ y, z; iii = -x, -2 - y, 1 - z; and iv=0.5 - x, -1.5 - y, 1 - z. b The structures of perdeuterated THF. a
four symmetry-independent H atoms in THF, the shortest H 3 3 3 O contact is formed by H31, which is the equatorial β-H atom (hereafter Hβ,eq), and two similar H 3 3 3 O distances are those of H32 (axial β, Hβ,ax) and H22 (equatorial R, HR,eq), the latter slightly longer at high pressure. This sequence, d(Hβ,eq 3 3 3 O) < d(Hβ,ax 3 3 3 O) < d(HR,eq 3 3 3 O), is strikingly consistent with the calculated magnitudes of the electrostatic potential at the sites of hydrogen nuclei (EPN, i.e., the molecular electrostatic potential at a particular nucleus, excluding the term at this nucleus from the summation), which efficiently indicates the H atoms' capability to become donors in hydrogen bonds;23,24 EPN(Hβ,eq)>EPN(Hβ,ax)>EPN(HR,eq) are -2926, -2943, and -2953 kJ mol-1, respectively. The smallest value of EPN(HR,ax), of -2968 kJ mol-1, coincides with no short intermolecular contacts between the H21 atom and O1 atom in the crystal. The same trend is observed in the molecular electrostatic potential mapped on the electron density isosurface (Figure 3a). This correlation corroborates the electrostatic nature of C-H 3 3 3 O interactions. Moreover, the angle between the H 3 3 3 O contact direction and the putative direction of the closer lone pair of the oxygen atom (Figure 3b) is smaller for the H31 3 3 3 O1ii contact than those for contacts H22 3 3 3 O1iii and H32 3 3 3 O1iv (Supporting Information Table S3). This is consistent with the tendency of
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DOI: 10.1021/jz9003894 |J. Phys. Chem. Lett. 2010, 1, 844–849
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Figure 2. The distance-distance plot19 drawn for three shortest C-H 3 3 3 O contacts in THF (marked by the green, orange, and blue symbols; the color code corresponds to that in Figure 1). The distances at 0.1 MPa/5 K (diamonds), 0.1 MPa/103 K (circles), 0.1 MPa/120 K (hexagons), 2.25 GPa/295 K (triangles up), 3.26 GPa/ 295 K (triangles down), and 3.80 GPa/295 K (squares) have been plotted against the distances at 0.1 MPa/148 K (abscissa axis; the empty symbols represent the neutron powder diffraction data for perdeuterated THF). The pink horizontal line indicates the sum of van der Waals radii of H and O atoms.20
hydrogen-donor groups to approach the acceptor atom from the side of individual lone pairs, as predicted by simple hybridization arguments. The potential-energy well associated with hydrogen bonds to ether oxygen atoms is very shallow,25,26 and the role of oxygen lone pairs was recently critically discussed.27 However, numerous electron density determinations28 as well as computational29 and microwave spectroscopy30 studies of hydrogen-bonded heterodimers of THF with strong hydrogen-bond donors show the electrostatic preference toward the lone pair positions. Moreover, the location of hydrogen atoms in a bisector plane of the C-O-C angle in crystals is clearly favored.25,26 The deviation of ∼30 between the H31 3 3 3 O1ii contact and this plane observed in the THF structure (Supporting Information Figure S1b) is likely to result from steric effects of six H atoms overcrowded around the acceptor O atom. In the open-ring analogue of THF, diethylether crystal, each oxygen atom of two symmetry-independent molecules is involved in three short (d
