Spectroscopic identification of ethanol-water conformers by large-amplitude hydrogen bond librational modes

Abstract

The far-infrared absorption spectra have been recorded for hydrogen-bonded complexes of water with ethanol embedded in cryogenic neon matrices at 2.8 K. The partial isotopic H/D-substitution of the ethanol subunit enabled by a dual inlet deposition procedure enables the observation and unambiguous assignment of the intermolecular high-frequency out-of-plane and the low-frequency in-plane donor OH librational modes for two different conformations of the mixed binary ethanol/water complex. The resolved donor OH librational bands confirm directly previous experimental evidence that ethanol acts as the O · · · HO hydrogen bond acceptor in the two most stable conformations. In the most stable conformation, the water subunit forces the ethanol molecule into its less stable gauche configuration upon dimerization owing to a cooperative secondary weak O · · · HC hydrogen bond interaction evidenced by a significantly blue-shift of the low-frequency in-plane donor OH librational band origin. The strong correlation between the low-frequency in-plane donor OH librational motion and the secondary intermolecular O · · · HC hydrogen bond is demonstrated by electronic structure calculations. The experimental findings are further supported by CCSD(T)-F12/aug-cc-pVQZ calculations of the conformational energy differences together with second-order vibrational perturbation theory calculations of the large-amplitude donor OH librational band origins. C 2015 AIP Publishing LLC. [http://dx

Ethanol dimer: Observation of three new conformers by broadband rotational spectroscopy

“…The observed relative abundances of the ethanol dimer conformer, The switch from trans to gauche ethanol upon complexation has also been observed for the ethanol-water complex 32,33,34,35 . However, this is not a widespread occurrence.…”

Competition between weak OH⋯π and CH⋯O hydrogen bonds: THz spectroscopy of the C2H2—H2O and C2H4—H2O complexes

“…These direct spectroscopic observables detected in the challenging far-infrared spectral region between 300 and 600 cm −1 have been shown to enable an accurate characterization of the conformational potential energy landscape spanned by the hydrogen-bonded subunits. [12][13][14][15][16][17][18] In the present work, we demonstrate how the class of large-amplitude hydrogen bond vibrational modes arising from the hindered rotational motion of the H 2 O subunits in the THz region below 300 cm −1 provides direct excellent spectroscopic probes for the much weaker intermolecular CH••O and OH••π hydrogen bond motifs in these hydrocarbon-H 2 O complexes. The infrared activity for large-amplitude librational modes of complexes between H 2 O and nonpolar hydrocarbons originates substantially from the dipole moment change associated with the hindered rotational motion of the H 2 O subunits.…”

“…However, anharmonicity contributions in the same order of 15-20% for large-amplitude intermolecular vibrational modes have been observed for a variety of more strongly OH••O hydrogen-bonded molecular complexes. 14,17,18 The predicted (harmonic) value of the dissociation energy D 0 of 11.3 kJ•mol −1 for the most stable conformation of the 1:1 C 2 H 2 -H 2 O complex should thus be regarded as a lower limit relative to the true value including anharmonicity corrections.…”

High‐Resolution Infrared Synchrotron Investigation of (HCN)2 and a Semi‐Experimental Determination of the Dissociation Energy D0

“…The observed reduction of the electric dipole moment of 0.54(5) D in the excited state relative to the ground‐state value of 6.023(31) D demonstrated a highly anharmonic nature of this vibrational normal coordinate. The large‐amplitude vibrational motion involving intermolecular hydrogen bonds is in general found to be highly anharmonic in nature and challenging for ab initio methodologies . The second fundamental transition associated with the class of large‐amplitude anharmonic intermolecular vibrational modes, the hindered translational motion involving both HCN subunits or intermolecular stretching ν 5 , has been observed indirectly at ca.…”

“…The large-amplitude vibrational motion involving intermolecular hydrogen bonds is in general found to be highly anharmonic in nature and challenging for ab initio methodologies. [28][29][30][31][32] The second fundamental transition associated with the class of large-amplitude anharmonic intermolecular vibrational modes, the hindered translational motion involving both HCN subunits or intermolecular stretching ν 5 , has been observed indirectly at ca. 101 cm À 1 from vibrational satellites in the microwave region [8] .…”