The influence of large-amplitude librational motion on the hydrogen bond energy for alcohol–water complexes

The far-infrared absorption spectra have been recorded for hydrogen-bonded complexes of water with methanol and t-butanol embedded in cryogenic neon matrices at 2.8 K. The partial isotopic substitution of individual subunits enabled by a dual inlet deposition procedure provides for the first time unambiguous assignments of the intermolecular high-frequency out-of-plane and low-frequency in-plane donor OH librational modes for mixed alcohol-water complexes. The vibrational assignments confirm directly that water acts as the hydrogen bond donor in the most stable mixed complexes and the tertiary alcohol is a superior hydrogen bond acceptor. The class of large-amplitude donor OH librational motion is shown to account for up to 5.1 kJ mol(-1) of the destabilizing change of vibrational zero-point energy upon intermolecular OH center dot center dot center dot O hydrogen bond formation. The experimental findings are supported by complementary electronic structure calculations at the CCSD(T)-F12/aug-cc-pVTZ level of theory

The donor OH stretching–libration dynamics of hydrogen-bonded methanol dimers in cryogenic matrices

FTIR spectra of the methanol dimer trapped in neon matrices are presented.</p

The effect of hydrogen bonding on torsional dynamics: A combined far-infrared jet and matrix isolation study of methanol dimer.

The effect of strong intermolecular hydrogen bonding on torsional degrees of freedom is investigated by far-infrared absorption spectroscopy for different methanol dimer isotopologues isolated in supersonic jet expansions or embedded in inert neon matrices at low temperatures. For the vacuum-isolated and Ne-embedded methanol dimer, the hydrogen bond OH librational mode of the donor subunit is finally observed at ∼560 cm(-1), blue-shifted by more than 300 cm(-1) relative to the OH torsional fundamental of the free methanol monomer. The OH torsional mode of the acceptor embedded in neon is observed at ∼286 cm(-1). The experimental findings are held against harmonic predictions from local coupled-cluster methods with single and double excitations and a perturbative treatment of triple excitations [LCCSD(T)] and anharmonic. VPT2 corrections at canonical MP2 and density functional theory (DFT) levels in order to quantify the contribution of vibrational anharmonicity for this important class of intermolecular hydrogen bond vibrational motion

An Infrared Study of Intermolecular Interactions in Gaseous Hydrogen Bonded Molecular Complexes

Intermolecular interactions are responsible for the deviations of gases from ideality, determine the three- dimensional structure and folding dynamics of biological molecules, and the binding of molecules to surfaces as in heterogeneous catalysis. The hydrogen bond is the most important intermolecular interaction. The most simple intermolecular interaction free from solvent and lattice effects arises when two chemical species are allowed to combine into a weakly bound gaseous molecular complex. In the present doctoral thesis the intermolecular interactions in a number of simple binary prototype molecular complexes held together by hydrogen bonds are investigated. The molecular complexes are studied by means of static gas-phase high-resolution absorption spectroscopy in the far- and near-infrared spectral regions. The high-brightness source of synchrotron radiation from the electron storage ring MAX-I at MAX-lab in Lund is used as a far-infrared radiation source. The far-infrared absorption spectra of the hydrogen-bonded molecular complexes OC-HCl, HCN-HCl and ammonia-HCN are recorded and provide information about the intermolecular hydrogen bond vibrations of the molecular complexes (Papers III-VI) introduced upon the complex formation. These floppy intermolecular hydrogen bond vibrations are directly correlated to the relative translation (stretching of the hydrogen bond) and the internal rotations of the subunits (bendings of the hydrogen bond). The spectral analyses allow the characterization of the intermolecular potential energy surface for these molecular complexes over a wide range of geometries away from the equilibrium structure. The near-infrared absorption spectra of the hydrogen-bonded molecular complexes HCN-HCN and HCN-HCl are recorded and provide information about the modification of the intramolecular force fields within the donor subunits by the incorporation into the bimolecular complex (Papers I and II)

Observation and rovibrational analysis of the intermolecular HCl libration band nu(1)(6) of HCN-HCl, DCN-HCl and (HCN)-C-13-HCl

The high-resolution far-infrared absorption spectrum of the intermolecular HCl libration band nu(6)(1) (nu(B)) of the gaseous molecular complex (HCN)-C-12-HCl and the two isotopically substituted species (HCN)-C-13-HCl and (DCN)-C-12-HCl is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 205 K using an electron storage ring source. The rotational structure of the nu(6)(1) band has the typical appearance of a perpendicular type band of a linear polyatomic molecule. The structure is analyzed using a standard semi-rigid linear molecule model including l-type doubling to yield the band origin nu(0), together with values for the upper state rotational constant B', the upper state quartic centrifugal distortion constant D'(J) and the value for the l-type doubling constant q(6). The values for the ground-state spectroscopic constants B '' and D ''(J) for (DCN)-C-12-(HCl)-Cl-35 and (HCN)-C-13-(HCl)-Cl-35 are determined for the first time by ground state combination difference analyses. A number of nu(6)(1) + nu(7)(1) - nu(7)(1) and nu(6)(1) + 2 nu(2)(7) - 2 nu(2)(7) hot bands are observed in the spectra and the sum of the anharmonicity constants X-6,X-7 + g(6,7) is estimated. The observed decrease of the rotational constant B together with the simultaneous increase of the quartic centrifugal distortion constant D-J upon excitation of the HCl libration mode indicate that the hydrogen bond in the molecular complex is significantly destabilized upon intermolecular vibrational excitation. The calculated harmonic force constants for the intermolecular hydrogen bond stretching vibration nu(sigma) for the ground state and the excited HCl libration state indicate that the excitation of the HCl libration mode destabilizes the intermolecular interaction between HCN and HCl by almost 20%. The hydrogen bond is elongated by 0.030 angstrom upon excitation of the nu(6)(1) mode

Observation and rovibrational analysis of the v(2) band of HCN-(HCl)-Cl-35

The high-resolution infrared absorption spectrum of an equilibrium mixture of HCN and HCl in a static gas long-path absorption cell is recorded in the 2500 2900 cm(-1) spectral region at 205 K. The spectrum shows rovibrational structure which has the typical appearance of a parallel band of a linear molecule and is assigned to the intramolecular H-Cl stretching vibration band upsilon(2) of the linear HCN-(HCl)-Cl-35 heterodimer. The rovibrational analysis of the band yield a band origin upsilon(0) of 2779.0968(12) cm(-1) together with a value for the upper-state rotational constant B' of 0.067722(2) cm(-1). The observed red shift of 107 cm(-1) for the upsilon(2) band of HCN- (HCl)-Cl-35 relative to the H-Cl stretching vibration band of monomer (HCl)-Cl-35 is in excellent agreement with results from the MP2/ 6-311++G** level of theory. The value of the upper-state rotational constant shows that the intermolecular hydrogen bond shortens by 0.022 Angstrom upon intramolecular vibrational excitation of the upsilon(2) mode. (C) 2004 Elsevier B.V. All rights reserved