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Infrared−infrared double resonance spectroscopy is used to probe the vibrational dynamics of molecular complexes solvated in helium nanodroplets. We report results for the acetylene−HCN and cyanoacetylene−HCN binary complexes, each having two stable isomers. We find that vibrational excitation of an acetylene−HCN complex results in a population transfer to the other isomer. Photoinduced isomerization is found to be dependent on both the initially excited vibrational mode and the identity of the acetylene−HCN isomer. However, population transfer is not observed for the cyanoacetylene−HCN complexes. The results are rationalized in terms of the ab initio intermolecular potential energy surfaces for the two systems with particular emphasis on the long-range barriers to rearrangement. [ABSTRACT FROM AUTHOR]

Electronic absorption spectra of bare and methanol-solvated radical anions of benzophenone ((C6H5)2CO) and acetophenone ((C6H5)CH3CO) were measured by monitoring the photodetachment efficiency in the gas phase. Strong absorption bands due to autodetachment after transitions to bound excited states were observed. Stepwise spectral shifts approaching the limit of the condensed phase spectra were found to occur as the cluster size increases. In the case of benzophenone radical anion, the solvation of two methanol molecules exhibits the near convergence to the limit, representing the full coordination with the solvent molecules around the carbonyl group. For the acetophenone case, the coordination number was not apparently determined because of their relatively small shifts. Relationships between hydrogen bonding and electronic structure are analyzed for the spectral shifts with the aid of calculations based on density functional theory. The calculational results show that the coordination angle of the solvent molecule is affected mostly by steric hindrance around the carbonyl group, and that there is no evidence for reorientation due to specific hydrogen bonding interaction with the singly occupied orbital, which has been formerly persisted for an interpretation of the transient absorption following pulse radiolysis in alcoholic solutions. An alternative possibility involving deformation with respect to intramolecular coordinates is discussed. [ABSTRACT FROM AUTHOR]

The rotational excitation spectrum, including the vibrational shift of the rotational band, of several CO isotopomers solvated in He clusters has been calculated. Reptation quantum Monte Carlo simulations are used in conjunction with an accurate He−CO potential energy surface, which quantitatively describes the rovibrational spectrum of the binary complex. Our simulations, when compared with number-selective infrared spectra taken for different isotopomers, help discriminate among the alternative assignments proposed for cluster sizes around 15 He atoms. The origin of the vibrational band has a red shift that is nearly linear with the cluster size within the first solvation shell and is almost constant up to the largest cluster studied, well beyond completion of the second solvation shell. A blue upturn at even larger sizes would be needed to attain the nanodroplet limit, as recently estimated from the isotopic dependence of the measured R(0) transitions. [ABSTRACT FROM AUTHOR]

We have recorded the S1← S0electronic spectra of Biphenylene and its Ar and O2van der Waals complexes inside helium nanodroplets using beam depletion detection. In general, the spectrum is similar to the previously reported high-resolution REMPI spectrum. The zero phonon lines, however, are split similar to the previously reported tetracene case. The calculated potential energy surface predicts that helium atoms can simultaneously occupy all equivalent global minima positions. Therefore, it appears that the splitting cannot be explained either by different isomers or by tunneling. Furthermore, surprisingly the splitting is retained for the Ar van der Waals complexes (and possibly for the O2complex as well). This case suggests that the current models of the origin of zero phonon line splitting and the helium solvation are incomplete. [ABSTRACT FROM AUTHOR]

Rovibrational spectra are measured for the HCCCN·HCN and HCN·HCCCN binary complexes in helium droplets at low temperature. Though no Q-branch is observed in the infrared spectrum of the linear HCN·HCCCN dimer, which is consistent with previous experimental results obtained for other linear molecules, a prominent Q-branch is found in the corresponding infrared spectrum of the HCCCN·HCN complex. This Q-branch, which is reminiscent of the spectrum of a parallel band of a prolate symmetric top, implies that some component of the total angular momentum is parallel to the molecular axis. The appearance of this particular spectroscopic feature is analyzed here in terms of a nonsuperfluid helium density induced by the molecular interactions. Finite temperature path integral Monte Carlo simulations are performed using potential energy surfaces calculated with second-order Möller−Plesset perturbation theory, to investigate the structural and superfluid properties of both HCCCN·HCN(4He)Nand HCN·HCCCN(4He)Nclusters with N≤ 200. Explicit calculation of local and global nonsuperfluid densities demonstrates that this difference in the rovibrational spectra of the HCCCN·HCN and HCN·HCCCN binary complexes in helium can be accounted for by local differences in the superfluid response to rotations about the molecular axis, i.e., different parallel nonsuperfluid densities. The parallel and perpendicular nonsuperfluid densities are found to be correlated with the locations and strengths of extrema in the dimer interaction potentials with helium, differences between which derive from the variable extent of polarization of the CN bond in cyanoacetylene and the hydrogen-bonded CH unit in the two isomers. Calculation of the corresponding helium moments of inertia and effective rotational constants of the binary complexes yields overall good agreement with the experimental values. [ABSTRACT FROM AUTHOR]

The results of anharmonic frequency calculations on neutral imidazole (C3N2H4, Im), protonated imidazole (ImH), and its complexes with water (ImH)(H2O)n, are presented and compared to gas phase infrared photodissociation spectroscopy (IRPD) data. Anharmonic frequencies are obtained via ab initio vibrational self-consistent field (VSCF) calculations taking into account pairwise interactions between the normal modes. The key results are:  (1) Prediction of anharmonic vibrational frequencies on an MP2 ab initio potential energy surface show excellent agreement with experiment and outstanding improvement over the harmonic frequencies. For example, the ab initio calculated anharmonic frequency for (ImH)(H2O)N2exhibits an overall average percentage error of 0.6% from experiment. (2) Anharmonic vibrational frequencies calculated on a semiempirical potential energy surface fitted to ab initio harmonic data represents spectroscopy well, particularly for water complexes. As an example, anharmonic frequencies for (ImH)H2O and (ImH)(H2O)2show an overall average deviation of 1.02% and 1.05% from experiment, respectively. This agreement between theory and experiment also supports the validity and use of the pairwise approximation used in the calculations. (3) Anharmonic coupling due to hydration effects is found to significantly reduce the vibrational frequencies for the NH stretch modes. The frequency of the NH stretch is observed to increase with the removal of a water molecule or replacement of water with N2. This result also indicates the ability of the VSCF method to predict accurate frequencies in a matrix environment. The calculation provides insights into the nature of anharmonic effects in the potential surface. Analysis of percentage anharmoncity in neutral Im and ImHshows a higher percentage anharmonicity in the NH and CH stretch modes of neutral Im. Also, we observe that anharmonicity in the NH stretch modes of ImHhave some contribution from coupling effects, while that of neutral Im has no contribution whatsoever from mode−mode coupling. It is concluded that the incorporation of anharmonic effects in the calculation brings theory and experiment into much closer agreement for these systems. [ABSTRACT FROM AUTHOR]

In this article, a new technique we call Beam Action Spectroscopy via Inelastic Scattering (BASIS) is demonstrated. BASIS takes advantage of the sensitivity of rotational state distributions in a supersonic molecular beam to inelastic scattering within the beam. We exploit BASIS to achieve increased sensitivity in two very different types of experiments. In the first, the UV photodissociation spectrum of OClO is recovered by monitoring intensity changes in the pure rotational transition of a spectator molecule (OCS) downstream from the nozzle, revealing a new vibrational structure in the region between 30 000 and 36 000 cm-1. In the second, the mid-IR vibrational spectrum of acetylene is recorded simply by monitoring a single pure rotational transition of OCS co-expanded with acetylene. The technique may prove particularly fruitful when an excitation process produces product dark states that are not easily probed by conventional spectroscopy. [ABSTRACT FROM AUTHOR]