Abstract:Resonant two-photon ionization, IR−UV hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopies
have been employed along with density functional theory (DFT) calculations to assign and characterize the
hydrogen-bonded topologies and structures of eight benzene−(H2O)
n
(CH3OH)
m
, cluster isomers (hereafter
shortened to BW
n
M
m
) with n + m ≤ 4. The O−H stretch infrared fundamentals are used to determine the
H-bonding topology of the clusters. However, in several cases, the O−H stretch spectrum leaves… Show more
“…It corresponds to the band at 3508 cm À1 observed in size selected ternary benzene-methanol-water trimers. 10 Two dimers mw and wm are found at all levels of calculation (see Fig. 3).…”
Section: Methanol-watermentioning
confidence: 89%
“…One reason for the sparsity of experimental data is the absence of a suitable UV chromophore which would allow for sensitive double-resonance spectroscopy approaches. 10 VUV double resonance techniques might be feasible, but are still in their infancy 11 and do not always provide faithful infrared spectra. 12 Therefore, the direct absorption approach is currently more practical in the infrared.…”
The vibrational dynamics of vacuum-isolated hydrogen-bonded complexes between water and the two simplest alcohols is characterized at low temperatures by Raman and FTIR spectroscopy. Conformational preferences during adaptive aggregation, relative donor/acceptor strengths, weak secondary hydrogen bonding, tunneling processes in acceptor lone pair switching, and thermodynamic anomalies are elucidated. The ground state tunneling splitting of the methanol-water dimer is predicted to be larger than 2.5 cm(-1). Two types of alcohol-water trimers are identified from the spectra. It is shown that methanol and ethanol are better hydrogen bond donors than water, but even more so better hydrogen bond acceptors. As a consequence, hydrogen bond induced red shifts of OH modes behave non-linearly as a function of composition and the resulting cluster excess quantities correspond nicely to bulk excess enthalpies at room temperature. The effects of weak C-H···O hydrogen bonds are quantified in the case of mixed ethanol-water dimers.
“…It corresponds to the band at 3508 cm À1 observed in size selected ternary benzene-methanol-water trimers. 10 Two dimers mw and wm are found at all levels of calculation (see Fig. 3).…”
Section: Methanol-watermentioning
confidence: 89%
“…One reason for the sparsity of experimental data is the absence of a suitable UV chromophore which would allow for sensitive double-resonance spectroscopy approaches. 10 VUV double resonance techniques might be feasible, but are still in their infancy 11 and do not always provide faithful infrared spectra. 12 Therefore, the direct absorption approach is currently more practical in the infrared.…”
The vibrational dynamics of vacuum-isolated hydrogen-bonded complexes between water and the two simplest alcohols is characterized at low temperatures by Raman and FTIR spectroscopy. Conformational preferences during adaptive aggregation, relative donor/acceptor strengths, weak secondary hydrogen bonding, tunneling processes in acceptor lone pair switching, and thermodynamic anomalies are elucidated. The ground state tunneling splitting of the methanol-water dimer is predicted to be larger than 2.5 cm(-1). Two types of alcohol-water trimers are identified from the spectra. It is shown that methanol and ethanol are better hydrogen bond donors than water, but even more so better hydrogen bond acceptors. As a consequence, hydrogen bond induced red shifts of OH modes behave non-linearly as a function of composition and the resulting cluster excess quantities correspond nicely to bulk excess enthalpies at room temperature. The effects of weak C-H···O hydrogen bonds are quantified in the case of mixed ethanol-water dimers.
“…One explanation for this sparsity of spectroscopic data might be the absence of any suitable UV chromophores in these systems which would otherwise enable sensitive electronic double-resonance spectroscopy investigations. 9 A series of pure rotational spectroscopic studies of adiabatically cooled mixed complexes of water with methanol 10 and t-butanol 11 has unraveled the structure of the conformers where the water subunit acts as the hydrogen bond donor. The other conformations with the alcohol subunit acting as the hydrogen bond donor were not detected under the cold conditions of the seeded molecular beam studies.…”
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 OHO hydrogen bond formation. The experimental findings are supported by complementary electronic structure calculations at the CCSD(T)-F12/aug-cc-pVTZ level of theory.
“…In this paper, the pure methanol and BM m clusters will serve as the basis for presenting the C−H stretch modes as secondary probes of H-bonding. In the paper on BW n M m clusters which follows, the C−H stretch fundamentals are used in several cases to distinguish between possible isomers differing in the position of the methanol(s) in the mixed cluster. 1 H-bonding topologies deduced for the BM 1 - 4 clusters from the OH stretch RIDIR spectra.…”
Resonant ion-dip infrared spectroscopy has been used to record infrared spectra of a series of benzene-(methanol) m clusters with m ) 1-5 in the O-H and C-H stretch regions. Previous work has used the O-H stretch region as a probe of the H-bonding topologies of these clusters, from which it was deduced that benzene-(methanol) 1-3 contain H-bonded methanol chains and benzene-(methanol) 4-6 H-bonded methanol cycles. In the present work, the C-H stretch fundamentals of the methyl group of methanol and the aryl C-H groups of benzene are studied. While benzene's C-H stretch Fermi triad is virtually unchanged in frequency from one cluster to the next, the methyl C-H stretch vibrations undergo systematic wavenumber shifts characteristic of the H-bonding arrangement for each methanol in the cluster. Density functional theory calculations on the pure methanol and benzene-(methanol) m clusters faithfully reproduce the directions and approximate magnitudes of the observed shifts and provide a basis for assignment of the observed transitions to acceptor, donor, and acceptor-donor methanol subunits. The experimental results on the ν 2 fundamental of methanol in benzene-(methanol) 1-5 show characteristic frequency shifts due to (i) donor (D, -20 to -15 cm -1 ), (ii) acceptor-donor (AD) and π donor (π) (-6 to -9 cm -1 ), and (iii) OH ... O acceptor/π donor (Aπ, -4 to +2 cm -1 ). Calculations on (methanol) m and benzene-(methanol) m clusters extend the predictions to include characteristic shifts for (iv) double-acceptor/single-donor (AAD, +5 to +15 cm -1 ), (v) single-acceptor (A, +15 to +30 cm -1 ), and (vi) double-acceptor (AA) (+20 to +30 cm -1 ). FTIR spectra of liquid methanol and of binary solutions of methanol with acetone-d 6 , CDCl 3 , and D 2 O indicate that methanol's CH stretch frequency shifts reflect methanol's H-bonding environment in solution as well.
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