Bert Lutz

Spectroscopic evidence for CH⋯ interaction in crystalline steroids and reference compounds

Abstract The attractive interaction between CH donors and oxygen acceptors is demonstrated for a number of compounds measured as liquid, solid and in non-polar solution. Reference values for the “free” (isolated) molecules are derived from gas-phase or solution measurements. Infrared spectral data for the CH stretching mode of a series of compounds with geminal-substituted hydroxyl and ethynyl groups, are reported. Both the Δν SC ( ν CCl 4 − ν solid ) and Δν T ( ν 300 K − ν 100 K ) values are shown to be indicative of the strength of the CH⋯O interaction. For a limited number of steroids an attempt is made to correlate the H⋯O distance with the frequency shift, Δν SC . The steric effect of neighbouring alkyl groups is shown for some cyclohexanol and adamantanol derivatives.

Conformational isomerism and self-association of calixarene building blocks in non-polar solution studied by Fourier transform infrared spectrometry

In order to establish the conformational equilibrium of calix[6]arene derivatives, a series of phenolic building blocks differing in chain length (oligomers) have been studied in solution by Fourier transform infrared spectrometry (FT-IR). Different intramolecular OH ? X interactions have been established in the calixarene building blocks, i.e. OH ? ?, OH ? OH, OH ? OCH3 and a bifurcated OH ? OH/OCH3 or OH ? OH/OH one. Concentration-dependent measurements show that the ?biphenol? and ?triphenol? oligomers form cyclic dimers even at very low concentration ( 10?2 M). Preliminary MM3 calculations were performed to support conformational assignment.

Weak hydrogen bonding. Part 5. Experimental evidence for the long-range nature of CC–H ⋯π interactions: crystallographic and spectroscopic studies of three terminal alkynes

In the crystal structures of three terminal alkynes, long CC–H ⋯ CC contacts with H ⋯ C separations in the range 2.7–3.1 A are observed. Despite the long distances, these contacts possess the characteristic infrared spectroscopic features of weak hydrogen bonds. This provides direct evidence that the range of C–H ⋯π interactions donated by sufficiently acidic C–H groups extends beyond van der Waals separation. In two of the crystal structures, the C–H ⋯π interactions form interconnected systems CC–H ⋯ CC–H ⋯ CC–H.

Structural and spectroscopic evidence of hydrogen bonding in 3-bromo-5-hydroxy-2,6-dimethylpyridine and 3-chloro-5-hydroxy-2,6-dimethylpyridine

Abstract The crystal structures of 3-chloro-5-hydroxy-2,6-dimethylpyridine (ClHDMP) and 3-bromo-5-hydroxy-2,6-dimethyl-pyridine (BrHDMP) were determined at room temperature. Although ClHDMP is monoclinic, space group P2 1 /c, and BrHDMP is orthorhombic, space group Pna2 1 , the molecular conformation is essentially the same in both compounds. The structure consists of two parallel infinite chains of formula units linked through hydrogen bonds of the type OHtctdot;N. There is no interconnection between the chains. Fourier transform IR and Raman spectra of ClHDMP and BrHDMP were measured in the 50–3500 cm −1 frequency range at room temperature. The vibrational assignment of the observed bands is proposed. The appearance of the characteristic vibrational features in the spectra of these compounds is discussed in terms of strong asymmetric hydrogen bonding. The bands observed at approximately 2800–2850, 2500–2550 and 1800–1850 cm −1 are assigned to the OH stretching modes and those at 1400–1410 and 900–950 cm −1 to δ(OH) and γ(OH) modes, respectively. The ν(OH) vibrations of the hydrogen bonds appear at 130–140 cm −1 . The calculation of γ(OH) frequencies and other parameters describing the strong hydrogen bonding, such as the Fermi coupling constant |W|, the separation of the unperturbed vibrational levels (Δ 0 ), etc., has been performed. The positions of γ(OH) bands have been found to agree with those predicted from the perturbed ν(OH) patterns. Comparison of the structural and vibrational characteristics of hydrogen bonds in ClHDMP and BrHDMP has indicated that a stronger hydrogen bonding exists in the bromo-derivative.

Observation of attractive intermolecular C–H⋯O interaction in the crystal packing of 3-chloro- and 3-bromo-2,6-dimethyl-4-nitropyridine N-oxide

Abstract The X-ray crystal structure determination of 3-chloro-2,6-dimethyl-4-nitropyridine N-oxide (ClDMNPNO) and 3-bromo-2,6-dimethyl-4-nitropyridine N-oxide (BrDMNPNO) shows that the two pyridine derivatives are isomorphic with monoclinic space group P21/c, and four formula units in a cell with the following dimensions: a=7.933(1), b=9.721(1), c=11.419(1) A, β=107.70(1)° and a=7.981(1), b=9.817(2), c=11.515(1) A, β=106.54(1)° for ClDMNPNO and BrDMNPNO, respectively. The shortest intermolecular contacts form medium strong hydrogen bonds of the type C–H⋯O. Another relatively short intermolecular contact is established for CH3⋯Cl/Br. The planar pyridine rings do not show the C–C bonds alternation typical for resonance forms in aromatic rings. The IR and Raman spectra, measured in the 50–3500 cm−1 region at ambient temperature, are correlated with X-ray structural data. The assignment of IR and Raman bands is given. Comparison of the spectra of dissolved samples with the ones obtained from polycrystalline samples shows the attractive character of the intermolecular C–H⋯O contact for these molecules. No strong spectroscopic support is found for the existence of a significant CH3⋯Cl/Br intermolecular interaction.

The sensorial potentials of the OH stretching mode

Abstract The OH stretching band of 57 selected saturated alcohols has been investigated in two different apolar solvents, CCl4 and CS2, as an initial step to recognise and understand weak hydrogen bonding phenomena. Based on rotamer classification and mathematical curve fitting, complex OH bandshapes have been unraveled successfully. A detailed analysis shows that ten different classes of rotamers, AK, suffice to correlate molecular structure and band parameters. The sensing properties for variation in the local molecular structure are shown to be reflected in the band maximum, v OH , the half band with, HBWCCl4OH and the solvent shift value, v d . A scheme is presented to classify saturated mono-hydroxy alcohols. For bicyclic alcohols and adamantanol-2 derivatives a sterical lever effect is established. No experimental evidence is found to support the involvement of an oxygen lone pair·CαH of primary and secondary alcohols in the band maximum nor in the relative intensity of the rotamers.

Vibrational spectroscopic characteristics of CH ⋯ O and NH ⋯ π interaction in crystalline N-(2,6-dimethylphenyl)-5-methylisoxazole-3-carboxamide

Abstract Infrared and Raman spectra have been scrutinised for features reflecting the presence of a very short CH ⋯ O contact in crystalline N-(2,6-dimethylphenyl)-5-methylisoxazole-3-carboxamide (D2624). A tentative assignment of infrared and Raman bands is presented for the pure and partly deuterated compound measured in the solid state and in the non-polar solution. The intensity increase of the C4H stretching vibration upon changing the matrix from a solution to the crystalline state, is the most prominent feature particularly in infrared. The temperature sensitivity was determined for the C4H and NH stretching vibration; the wavenumber-temperature coefficients (WTc) are significant but small, 0.0137 and 0.0143 cm−1 K−1, respectively. The presence of an NH ⋯ π interaction in the crystalline phase is established by mimicking this type of interaction in solution. Infrared measurements at temperatures above the melting point show that an intermolecular NH ⋯ OC amide bond is present in the liquid phase. Upon cooling (crystallisation) this typical energetically favourable amide bond is disrupted as a result of several competing attractive forces (intermolecular CH ⋯ O and NH ⋯ π interaction, intramolecular N8-H ⋯ N2) and the effect of steric shielding of the amide functionality by the 2,6-dimethylphenyl group.

First example of cooperative O–H⋯CC–H⋯Ph hydrogen bonding: crystalline 7-ethynyl-6,8-diphenyl-7H-benzocyclohepten-7-ol

In the crystalline title compound, molecular dimers are joined by a system of interconnected O–H⋯CC–H and CC–H⋯Ph hydrogen bonds for which IR spectra indicate considerable strength; this hydrogen-bond configuration has not been observed previously.

Spectroscopic and structural evidence for the hydrogen bond nature of CCH…CC contacts in ethynyl steroids

Abstract Structural and IR-spectroscopic evidence is given that directed contacts from terminal alkynes to CC double bonds possess the essential characteristics of weak hydrogen bonds. The contacts are directed at the center of the π-bond rather than at one of the individual C-atoms. The contact distances from H to the center of the CC bond are typically 2.8 A, with the shortest distances being ∼2.5 A . The interaction is of pronounced long-range nature and can be detected in the infrared spectrum even with a long H…π distance of almost 3.0 A. The sample used is mainly composed of ethynyl steroids belonging to the progestine family.

Complexes of the proton sponge 1,8-bis(dimethylamino)naphthalene (DMAN). Part VI: The structure of [DMANH]+ [pentafluorophenolate]- [pentafluorophenol]2

Abstract The structure of the complex of 1,8-bis(dimethylamino)naphthalene (DMAN) with pentafluorophenol fluorophenol (PFP) with composition [DMANH] + [PFP] − [PFP] 2 has been determined at 100 K by X-ray diffraction. The crystals are orthorhombic, space group P 2 1 2 1 2 1 , with a = 13.5726(9), b = 14.1967(8), c = 15.8787(10) A and Z = 4. The structure was solved by direct methods and refined to R = 0.032 for 3735 reflections. The electron density in the strong intramolecular [NH⋯N] + hydrogen bond is diffuse and extends over the whole N⋯N region and this is interpreted as a disordering of the H atom. Refinement and difference Fourier syntheses indicate populations of 0.54 and 0.46 for the disordered H atom at distances of 1.07(6) and 0.86(7) A from the N atoms. The PFP residues each donate a hydrogen bond to the [PFP] − oxygen atom. Two weak intramolecular OH⋯F interactions result in two planar bifurcated hydrogen bonds. The IR protonic absorption of the PFP and [DMANH] + residues at ambient temperature and 100 K could be located. The characteristic broad signals associated with the [NH⋯N] + bond show frequency shifts and band narrowing on cooling.