Single and double resonance spectroscopy of methanol embedded in superfluid helium nanodroplets

Abstract

Methanol is one of the simplest molecules that undergo torsional oscillations, and so it has been extensively studied in the gas phase by various spectroscopic techniques. At 300 K, a large number of rotational, torsional, and vibrational energy levels is populated, and this makes for a rather complicated spectrum, which is still not fully understood. It is expected that in going from 300 K to 0.4 K (the temperature of helium nanodroplets) the population distribution of methanol will mainly collapse into two states; the JK = 00 state for the A1 nuclear spin symmetry species (with ICH3 = 3/2), and the JK = 1-1 state for the E species (ICH3 = 1/2). This results in a simplified spectrum that consists of narrow a-type (ΔK = 0) lines and broader b- and c-type (ΔK = ±1) lines. We have recorded the rotovibrational spectrum of CH3OH in the OH stretching, CH3 stretching and bending, CH3 rocking, and CO stretching regions, and have firmly assigned five bands (v1, v2, v3, v7, and v8), and tentatively assigned five others (v9, 2v4, v4 + v10, 2v10, and v4 + v5). To our knowledge, the transitions we have assigned within the v4 + v10, 2v10, and v4 + v5 bands have not yet been assigned in the gas phase, and we hope that considering the very small "matrix" shift in helium nanodroplets (<1 cm(-1) for most subband origins of CH3OH), those made here can aid in their gas phase identification. Microwave-infrared double resonance spectroscopy was used to confirm the initially tentative a-type infrared assignments in the OH stretching (v1) band of A1 species methanol, in addition to revealing "warm" b-type lines. From a rotovibrational analysis, the B rotational constant is found to be reduced quite significantly (56%) with respect to the gas phase, and the torsional tunneling splittings are relatively unaffected and are at most reduced by 16%. While most rovibrational peaks are Lorentzian shaped, and those which are significantly perturbed by vibrational coupling in the gas phase are additionally broadened, the narrowest ΔJ = +1 peaks are asymmetric, and a skew-type analysis suggests that the response time of the helium solvent upon excitation is of the order of 1 ns.

Infrared Spectroscopy of NaCl(CH3OH)n Complexes in Helium Nanodroplets

“…If only this isomer is present in helium nanodroplets, then only a single OH stretching band is expected. The band assigned to NaCl(CH 3 OH) in the IR spectrum is red-shifted by ∼430 cm −1 from the OH stretching band of the methanol monomer in superfluid helium, 18 implying a very significant weakening of the O−H bond when methanol forms a complex with NaCl. This is consistent with the formation of an IHB between the OH and the chloride ion.…”

“…This is a relatively narrow range, and the frequencies of these absorption bands are well to the red of the OH stretching band of the isolated methanol molecule. 18 This red shift is characteristic of the formation of IHBs, which weaken the OH bonds and thereby lower the OH stretching frequencies. This effect has been seen previously in IR spectra of the anionic Cl − (CH 3 OH) n complexes, which have been studied in the gas phase by Lisy and co-workers.…”

To π or not to π – how does methanol dock onto anisole?

“…To the best of our knowledge, no microwave, infrared or IR/UV double resonance spectra of this binary complex have been reported so far, whereas its components are very well investigated. 13,14 Also, OH-O and OH-p isomers of methanol complexing the anthracene analog of anisole have been identified long ago by UV hole burning spectroscopy 15 and in that work, the study of methanol-anisole was actually suggested. In the context of calibrating quantum-chemical methods for the description of polar vs. dispersive interactions of alcohols with ethers, 16 we found that the most reliable among the routinely applicable methods predict the two docking sites to be essentially isoenergetic for methanolanisole, with at best a slight preference below 1 kJ mol À1 for the oxygen binding site.…”

A simple model for high rotational excitations of molecules in a superfluid

“…Figure 4 shows the effective centrifugal constant D * as a function of B * in comparison with the experimental data listed in Ref. 4,5,18,30,32,34,38,41,48,49,51,52 . In agreement with the established experimental and theoretical result, D * measured in helium droplets is found to be 10 2 − 10 4 times larger than the corresponding gas-phase value.…”