Search

Abstract: Infrared (IR)–IR double resonance spectroscopy is used to probe the isomerization dynamics of helium solvated HF–N2O following the vibrational excitation of one of two stable isomers on the intermolecular potential energy landscape. An upstream pump laser excites the H–F stretching fundamental of either the linear ONN–HF or the bent NNO–HF isomer, while a downstream probe laser is scanned through the bands of both isomers to test for photo-induced isomerization. Vibrational excitation of the H–F stretch mode leads to the isomerization of a fraction of the upstream population, regardless of the identity of the pumped isomer. The measured isomer interconversion is consistent with vibrational predissociation followed by geminate recombination. A fraction of the upstream population is unaccounted for by the downstream probe. We propose that a fraction of the missing population is due to the mismatch in the timescales of complex recombination and pump, probe events. [Copyright &y& Elsevier]

The deuteroxyl radical (OD) has been isolated in superfluid helium nanodroplets and characterised by infrared depletion spectroscopy. Two resolvedQ(3/2) lines are observed, with a separation that is 4.88 (10) times larger than in the gas phase. This is similar to that previously reported for He-solvated OH (5.30 (2)), for which it was shown that the splitting could be reproduced by a model that assumes a small parity dependence of the rotor's effective moment of inertia [P.L. Raston, T. Liang, and G.E. Douberly, J. Phys. Chem. A (2013). DOI:10.1021/jp312335q]. With this model, the OD Λ-doublet splitting in liquid He is reproduced withBeandBfrotational constants that differ by ≈0.24%. [ABSTRACT FROM PUBLISHER]

• We present the IR spectra of the propyl radicals measured in para -hydrogen matrices. • Experimental results were compared to anharmonic theory calculations. • Propene hydrogenation reactions were employed in the matrices to study tunneling. • Experimental results were compared to WKB model calculations of tunneling rates. We report infrared spectra of the n - and i -propyl radicals isolated in solid para -hydrogen (p -H 2). Propyl radicals were produced via 248 nm irradiation of matrices formed by co-deposition of p -H 2 and either 1-iodopropane (forming n -propyl) or 2-iodopropane (forming i -propyl). Secondary photolysis was used to group spectral bands arising from the same species. In addition to those observed previously in either argon matrix or helium droplet spectra, we assign approximately 20 previously unreported bands. The assignment of features to propyl radicals is made via comparisons to VPT2+K anharmonic frequency computations. In separate measurements, propyl radicals were generated via the reaction between propene and hydrogen atoms produced within the p -H 2 matrix. Because the barriers are between 700 and 1500 cm−1, the H + propene reaction is necessarily under tunneling control when carried out in solid p -H 2 at 3.3 K. i -Propyl radicals were produced in ∼5–20 times greater abundance than n -propyl radicals, indicating that H atom addition to the terminal carbon is faster than addition to the central carbon. The Wentzl-Kramers-Brillouin approximation is applied to one-dimensional ab initio potentials to estimate the tunneling time constants for formation of n - and i -propyl. This model predicts the rate of addition to the terminal carbon (forming i -propyl) to be 17 times faster, in qualitative agreement with the experimental branching ratio. [ABSTRACT FROM AUTHOR]

We measured the rotationally resolved infrared spectra of helium solvated methyl fluoride at 3 μm and 10 μm, wherein lies C-H and C-F stretching bands, respectively. The linewidths (FWHM) were found to increase with increasing vibrational energy and range from 0.002 cm -1 in the v 3 band (C-F stretch) at ~1047 cm -1 , to 0.65 cm -1 in the v 4 band (asymmetric C-H stretch) at ~2997 cm -1 . In between these two bands we observed the lower and upper components of the Fermi triad bands (ν 1 /2ν 2 /2ν 5 ) at ~2859 and ~2961 cm -1 . We carried out Stark spectroscopy on the lower band on account of its narrower linewidths (0.04 vs. 0.14 cm -1 , respectively). The objective of performing Stark spectroscopy was to see if there is any evidence for a rotational linewidth dependence on the external field strength, due to a reduced difference in between methyl fluorides rotational energy gap and the roton-gap of superfluid helium. We did not find any evidence for such an effect, which we largely attribute to the rotational energy gap not increasing significantly enough by the external field. We point to another molecule (formaldehyde) whose energy levels are predicted to show a more promising response to application of an external field., (© 2024 Wiley-VCH GmbH.)

Thallium chemistry is experiencing unprecedented importance. Therefore, it is valuable to characterize some of the simplest thallium compounds. Stationary points along the singlet and triplet Tl   2 H   2 potential energy surface have been characterized. Stationary point geometries were optimized with the CCSD(T)/aug-cc-pwCVQZ-PP method. Harmonic vibrational frequencies were computed at the same level of theory while anharmonic vibrational frequencies were computed at the CCSD(T)/aug-cc-pwCVTZ-PP level of theory. Final energetics were obtained with the CCSDT(Q) method. Basis sets up to augmented quintuple-zeta cardinality (aug-cc-pwCV5Z-PP) were employed to obtain energetics in order to extrapolate to the complete basis set limits using the focal point approach. Zero-point vibrational energy corrections were appended to the extrapolated energies in order to determine relative energies at 0 K. It was found that the planar dibridged isomer lies lowest in energy while the linear structure lies highest in energy. The results were compared to other group 13 M   2 H   2 (M = B, Al, Ga, In, and Tl) theoretical studies and some interesting variations are found. With respect to experiment, incompatibilities exist., (© 2024 Wiley Periodicals LLC.)

Five structures of Ge 2 H 2 and Ge 2 H 2 + are investigated in this study. Optimized geometries at the CCSD(T)/cc-pwCVQZ-PP level of theory were obtained. Focal point analyses were performed on these optimized geometries to determine relative energies using the CCSD(T) method with polarized basis sets up to quintuple-zeta. Energy corrections include full T and pertubative (Q) coupled-cluster effects plus anharmonic corrections to the zero-point vibrational energy. Relative ordering in energy from lowest to highest of the five Ge 2 H 2 + structures is butterfly, germylidene, monobridged, trans , then linear. In neutral Ge 2 H 2 , the monobridged structure lies lower in energy than the germylidene structure. Fundamental vibrational frequencies and IR intensities were computed for the minima at the CCSD(T)/cc-pwCVTZ-PP level of theory to compare with experimental research. Partial atomic charges and natural bonding orbital analyses indicated that the positive charge of Ge 2 H 2 + is contained in the region of the Ge-Ge bond.

High-level potential energy surfaces for three reactions of hypobromous acid with atomic hydrogen were computed at the CCSDTQ/CBS//CCSDT(Q)/complete basis set level of theory. Focal point analysis was utilized to extrapolate energies and gradients for energetics and optimizations, respectively. The H attack at Br and subsequent Br-O cleavage were found to proceed barrierlessly. The slightly submerged transition state lies -0.2 kcal mol-1 lower in energy than the reactants and produces OH and HBr. The two other studied reaction paths are the radical substitution to produce H2O and Br with a 4.0 kcal mol-1 barrier and the abstraction at hydrogen to produce BrO and H2 with an 11.2 kcal mol-1 barrier. The final product energies lie -37.2, -67.9, and -7.3 kcal mol-1 lower in energy than reactants, HOBr + H, for the sets of products OH + HBr, H2O + Br, and H2 + BrO, respectively. Additive corrections computed for the final energetics, particularly the zero-point vibrational energies and spin-orbit corrections, significantly impacted the final stationary point energies, with corrections up to 6.2 kcal mol-1., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Criegee intermediates, formed from the ozonolysis of alkenes, are known to have a role in atmospheric chemistry, including the modulation of the oxidizing capacity of the troposphere. Although studies have been conducted since their discovery, the synthesis of these species in the laboratory has ushered in a new wave of investigations of these structures, both theoretically and experimentally. In some of these theoretical studies, high-order corrections for correlation energy are included to account for the mid multi-reference character found in these systems. Many of these studies include a focus on kinetics; therefore, the calculated energies should be accurate (<1 kcal/mol in error). In this research, we compute the enthalpies of formation for a small set of Criegee intermediates, including higher-order coupled cluster corrections for correlation energy up to coupled cluster with perturbative quintuple excitations. The enthalpies of formation for formaldehyde oxide, anti-acetaldehyde oxide, syn-acetaldehyde oxide, and acetone oxide are presented at 0 K as 26.5, 15.6, 12.2, and 0.1 kcal mol -1 , respectively. Additionally, we do not recommend the coupled cluster with perturbative quadruple excitations [CCSDT(Q)] energy correction, as it is approximately twice as large as that of the coupled cluster with full quadruple excitations (CCSDTQ). Half of the CCSDT(Q) energy correction may be included as a reliable, cost-effective estimation of CCSDTQ energies for Criegee intermediates.

Butyl radicals (n-, s-, i-, and tert-butyl) are formed from the pyrolysis of stable precursors (1-pentyl nitrite, 2-methyl-1-butyl nitrite, isopentyl nitrite, and azo-tert-butane, respectively). The radicals are doped into a beam of liquid helium droplets and probed with infrared action spectroscopy from 2700 to 3125 cm -1 , allowing for a low temperature measurement of the CH stretching region. The presence of anharmonic resonance polyads in the 2800-3000 cm -1 region complicates its interpretation. To facilitate spectral assignment, the anharmonic resonances are modeled with two model Hamiltonian approaches that explicitly couple CH stretch fundamentals to HCH bend overtones and combinations: a VPT2+K normal mode model based on coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] quartic force fields and a semi-empirical local mode model. Both of these computational methods provide generally good agreement with the experimental spectra.

Four different reaction pathways are initially located for the reaction of Cl atom plus water trimer Cl + (H 2 O) 3 → HCl + (H 2 O) 2 OH using a standard DFT method. As found for the analogous fluorine reaction, the geometrical and energetic results for the four chlorine pathways are closely related. However, the energetics for the Cl reaction are very different from those for fluorine. In the present paper, we investigate the lowest-energy chlorine pathway using the "gold standard" CCSD(T) method in conjunction with correlation-consistent basis sets up to cc-pVQZ. Structurally, the stationary points for the water trimer reaction Cl + (H 2 O) 3 may be compared to those for the water monomer reaction Cl + H 2 O and water dimer reaction Cl + (H 2 O) 2 . Based on the CCSD(T) energies, the title reaction is endothermic by 19.3 kcal mol -1 , with a classical barrier height of 16.7 kcal mol -1 between the reactants and the exit complex. There is no barrier for the reverse reaction. The Cl⋯(H 2 O) 3 entrance complex lies 5.3 kcal mol -1 below the separated reactants. The HCl⋯(H 2 O) 2 OH exit complex is bound by 8.6 kcal mol -1 relative to the separated products. The Cl + (H 2 O) 3 reaction is somewhat similar to the analogous Cl + (H 2 O) 2 reaction, but qualitatively different from the Cl + H 2 O reaction. It is reasonable to expect that the reactions between the chlorine atom and larger water clusters may be similar to the Cl + (H 2 O) 3 reaction. The potential energy profile for the Cl + (H 2 O) 3 reaction is radically different from that for the valence isoelectronic F + (H 2 O) 3 system, which may be related to the different bond energies between HCl and HF.

The utility of high energy density materials (HEDMs) comes from their thermodynamic properties which arise from specific structural features that contribute to energy storage. Studies of such structural features seek to increase our understanding of these energy storage mechanisms in order to further enhance their properties. High-nitrogen-containing HEDMs are of particular interest because they are less toxic than traditional HEDMs. Pentazole is the largest of the nitrogen rings which has been synthesized and considered for an HEDM; however, few experimental studies exist due to the difficulty involved in the synthesis, and most previous theoretical studies employed composite methods where lower level geometries were used with higher level methods. Here, the decomposition reaction of pentazole is studied. Geometries, fundamental frequencies, and energies for each of the stationary points of the decomposition pathway are computed using ab initio methods up to CCSDT(Q). Decomposition rates are calculated over a range of temperatures using canonical transition state theory in order to determine the kinetic stability of pentazole. Based on the present results, it would be difficult for pentazole to act as an HEDM, requiring temperatures close to 200 K to achieve a suitable level of stability.

The H + (CO) 2 and D + (CO) 2 molecular ions were investigated by infrared spectroscopy in the gas phase and in para-hydrogen matrices. In the gas phase, ions were generated in a supersonic molecular beam by a pulsed electrical discharge. After extraction into a time-of-flight mass spectrometer, the ions were mass selected and probed by infrared laser photodissociation spectroscopy in the 700 cm -1 -3500 cm -1 region. Spectra were measured using either argon or neon tagging, as well as tagging with an excess CO molecule. In solid para-hydrogen, ions were generated by electron bombardment of a mixture of CO and hydrogen, and absorption spectra were recorded in the 400 cm -1 -4000 cm -1 region with a Fourier-transform infrared spectrometer. A comparison of the measured spectra with the predictions of anharmonic theory at the CCSD(T)/ANO1 level suggests that the predominant isomers formed by either argon tagging or para-hydrogen isolation are higher lying (+7.8 kcal mol -1 ), less symmetric isomers, and not the global minimum proton-bound dimer. Changing the formation environment or tagging strategy produces other non-centrosymmetric structures, but there is no spectroscopic evidence for the centrosymmetric proton-bound dimer. The formation of higher energy isomers may be caused by a kinetic effect, such as the binding of X (=Ar, Ne, or H 2 ) to H + (CO) prior to the formation of X H + (CO) 2 . Regardless, there is a strong tendency to produce non-centrosymmetric structures in which HCO + remains an intact core ion.

Molecular cations of HO 4 + and DO 4 + are produced in a supersonic expansion. They are mass-selected, and infrared photodissociation spectra of these species are measured with the aid of argon-tagging. Although previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen, infrared spectra have free OH stretching bands, suggesting other isomeric structures. As a consequence, we undertook extensive computational studies. Our conformer search used a composite method based on an economical combination of single- and multi-reference theories. Several conformers were located on the quintet, triplet, and singlet surfaces, spanning in energy of only a few thousand wavenumbers. Most of the singlet and triplet conformers have pronounced multiconfigurational character. Previously unidentified covalent-like structures (H-O-O-O-O) on the singlet and triplet surfaces likely represent the global minima. In our experiments, HO 4 + is formed in a relatively hot environment, and similar experiments have been shown capable of producing multiple conformers in low-lying electronic states. None of the predicted HO 4 + isomers can be ruled out a priori based on energetic arguments. We interpret our argon-tagged spectra with Second-Order Vibrational Perturbation Theory with Resonances (VPT2+K). The presence of one or more covalent-like isomers is the only reasonable explanation for the spectral features observed.

Sulfurous acid (H 2 SO 3 ) is an infamously elusive molecule. Although some theoretical papers have supposed possible roles for it in more complicated systems, it has yet to be experimentally observed. To aid experiment in detecting this molecule, we have examined the H 2 O + SO 2 potential energy surface at the CCSDT(Q)/CBS//CCSD(T)-F12b/cc-pVTZ-F12b level of theory to resolve standing discrepancies in previous reports and predict the gas-phase vibrational spectrum for H 2 SO 3 . We find that sulfurous acid has two potentially detectable rotamers, separated by 1.1 kcal mol -1 ΔH 0K with a torsional barrier of 1.6 kcal mol -1 . The sulfonic acid isomer is only 6.9 kcal mol -1 above the lowest enthalpy sulfurous acid rotamer, but the barrier to form it is 57.2 kcal mol -1 . Error in previous reports can be attributed to misidentified stationary points, the use of density functionals that perform poorly for this system, and, most importantly, the basis set sensitivity of sulfur. Using VPT2+K, we determine that the intense S=O stretch fundamental of each species is separated from other intense peaks by at least 25 cm -1 , providing a target for identification by infrared spectroscopy.

Vinoxy radicals are involved in numerous atmospheric and combustion mechanisms. High-level theoretical methods have recently shed new light on the reaction of the unsubstituted vinoxy radical with O 2 . The reactions of 1-methylvinoxy radical and 2-methylvinoxy radical with molecular oxygen have experimental high pressure limiting rate constants, k ∞ , 5-7 times higher than that of the vinoxy plus O 2 reaction. In this work, high-level ab initio quantum chemical computations are applied to the 2-methylvinoxy radical plus O 2 system, namely, the formation and isomerization of the 1-oxo-2-propylperoxy radical, the immediate product of O 2 addition to the 2-methylvinoxy radical. Multireference methods were applied to the entrance channel. No barrier to O 2 addition could be located, and more sophisticated treatment of dynamic electron correlation shows that the principal difference between O 2 addition to the vinoxy and 2-methylvinoxy radicals is a larger steric factor for 2-methylvinoxy + O 2 . This is attributed to the favorable interaction between the incoming O 2 molecule and the methyl group of the 2-methylvinoxy radical. Via the focal point approach, energetics for this reaction were determined, in most cases, to chemical accuracy. The coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] correlation energy and Hartree-Fock energies were independently extrapolated to the complete basis set limit. A correction for the effect of higher excitations was computed at the CCSDT(Q)/6-31G level. Corrections for the frozen-core approximation, the Born-Oppenheimer approximation, the nonrelativistic approximation, and the zero-point vibrational energy were included. From the 1-oxo-2-propylperoxy radical, dissociation to reactants is competitive with the lowest energy isomerization pathway. The lowest energy isomerization pathway ultimately forms acetaldehyde, CO, and · OH as the final products.

Three constitutional isomers of both Ca 2 H 2 and Ca 2 H 4 have been characterized with molecular electronic structure theory. Correlation methods as complete as CCSDT(Q) and basis sets as large as cc-pwCV5Z have been used to converge the relative energies within chemical accuracy (≤1 kcal mol -1 ). Anharmonic vibrational frequencies were computed using second-order vibrational perturbation theory employing CCSD(T)/cc-pwCVTZ cubic and quartic force-fields and a CCSD(T)/cc-pwCVQZ quadratic force field. The monobridged [Ca(μ 2 -H)CaH] and dibridged [Ca(μ 2 -H) 2 Ca] isomers of Ca 2 H 2 were predicted to lie 6.5 and 12.9 kcal mol -1 below the energy of the classical HCaCaH linear isomer, respectively. Despite the energetic favorability of the bridged Ca 2 H 2 isomers, we conclude (surprisingly) that only the higher energy linear structure has been observed in the laboratory. At 0 K, the tribridged [Ca(μ 2 -H) 3 CaH] isomer of Ca 2 H 4 is predicted to be enthalpically favored by 0.9 kcal mol -1 in comparison to the enthalpy of the dibridged [HCa(μ 2 -H) 2 CaH] structure. Comparison of experiment with our computed frequencies suggests that the observed vibrational features arise from both the dibridged and the tribridged Ca 2 H 4 structures.

Helium-solvated ethylperoxy radicals (CH 3 CH 2 OO • ) are formed via the in situ reaction between 2 A' ethyl radical and 3 Σ g - dioxygen. The reactants are captured sequentially through the droplet pick-up technique. Helium droplets are doped with ethyl radical via pyrolysis of di- tert-amyl peroxide or n-propylnitrite in an effusive, low-pressure source. An infrared spectrum of ethylperoxy, in the CH stretching region, is recorded with species-selective droplet beam depletion spectroscopy. Spectral assignments are made via comparisons to second-order vibrational perturbation theory with resonances (VPT2 + K) based on coupled-cluster full quartic force fields. Cubic and quartic force constants, evaluated using a small basis set, are transformed into the normal coordinate system of the higher level quadratic force constants. This transformation procedure eliminates the mismatch between normal modes, which is a source of error whenever normal coordinate force constants from different levels of theory are combined. The spectrum shows signatures of both the C 1 gauche and C s trans rotamers in an approximate 2:1 ratio; this is despite the prediction that the gauche rotamer lies 44 cm -1 lower on the zero-Kelvin enthalpic potential surface for torsional interconversion. Helium droplets are 0.4 K at equilibrium; therefore, in situ ethylperoxy production is highly nonthermal.

Catalytic thermal cracking of O 2 is employed to dope helium droplets with O( 3 P) atoms. Mass spectrometry of the doped droplet beam reveals an O 2 dissociation efficiency larger than 60%; approximately 26% of the droplet ensemble is doped with single oxygen atoms. Sequential capture of O( 3 P) and HCN leads to the production of a hydrogen-bound O-HCN complex in a 3 Σ electronic state, as determined via comparisons of experimental and theoretical rovibrational Stark spectroscopy. Ab initio computations of the three lowest lying intermolecular potential energy surfaces reveal two isomers, the hydrogen-bound ( 3 Σ) O-HCN complex and a nitrogen-bound ( 3 Π) HCN-O complex, lying 323 cm -1 higher in energy. The HCN-O to O-HCN interconversion barrier is predicted to be 42 cm -1 . Consistent with this relatively small interconversion barrier, there is no experimental evidence for the production of the nitrogen-bound species upon sequential capture of O( 3 P) and HCN.

We report the ro-vibrational spectrum of the ν3(e') band of the methyl radical (CH3) solvated in superfluid (4)He nanodroplets. Five allowed transitions produce population in the N(K) = 0(0), 1(1), 1(0), 2(2) and 2(0) rotational levels. The observed transitions exhibit variable Lorentzian line shapes, consistent with state specific homogeneous broadening effects. Population relaxation of the 0(0) and 1(1) levels is only allowed through vibrationally inelastic decay channels, and the (P)P1(1) and (R)R0(0) transitions accessing these levels have 4.12(1) and 4.66(1) GHz full-width at half-maximum line widths, respectively. The line widths of the (P)R1(1) and (R)R1(1) transitions are comparatively broader (8.6(1) and 57.0(6) GHz, respectively), consistent with rotational relaxation of the 2(0) and 2(2) levels within the vibrationally excited manifold. The nuclear spin symmetry allowed rotational relaxation channel for the excited 1(0) level has an energy difference similar to those associated with the 2(0) and 2(2) levels. However, the (P)Q1(1) transition that accesses the 1(0) level is 2.3 and 15.1 times narrower than the (P)R1(1) and (R)R1(1) lines, respectively. The relative line widths of these transitions are rationalized in terms of the anisotropy in the He-CH3 potential energy surface, which couples the molecule rotation to the collective modes of the droplet.

HO3-(O2)n clusters are formed by the sequential addition of the hydroxyl radical and O2 molecules to superfluid helium nanodroplets. IR laser spectroscopy in the fundamental OH stretching region reveals the presence of several bands assigned to species as large as n=4. Detailed ab initio calculations are carried out for multiple isomers of cis- and trans-HO3-O2, corresponding to either hydrogen- or oxygen-bonded van der Waals complexes. Comparisons to theory suggest that the structure of the HO3-O2 complex formed in helium droplets is a hydrogen-bonded (4)A' species consisting of a trans-HO3 core. The computed binding energy of the complex is approximately 240 cm(-1). Despite the weak interaction between trans-HO3 and O2, nonadditive redshifts of the OH stretching frequency are observed upon successive solvation by O2 to form larger clusters with n>1., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

High-resolution infrared (IR) spectroscopy is used to investigate the Mg-HF and Mg-(HF)(2) van der Waals complexes. Both complexes are formed and probed within helium nanodroplets. Rotationally resolved zero-field and Stark spectra are assigned to a linear binary complex composed of a Mg atom bound to the hydrogen end of the HF molecule. Although high level ab initio calculations predict a fluorine bonded complex, none of the observed IR bands can be assigned to this complex. The collocation method is employed to determine the bound states on the two-dimensional intermolecular Mg-HF potential energy surface. The ground and first excited state wave functions for this potential surface have zero amplitude in the well corresponding to the fluorine bonded complex, consistent with experiment. The two HF stretching bands of the Mg-(HF)(2) complex are observed and assigned using a combination of the spectral symmetry, ab initio calculations, pick-up cell pressure dependencies, and dipole moment measurements. Comparisons with the helium solvated HF dimer show large changes to the HF stretching frequencies upon the addition of a single Mg atom to the hydrogen side of (HF)(2).

Infrared laser spectroscopy was used to probe the unique rotational dynamics of the HCN-M (M = Na, K, Rb, Cs) complexes formed on the surface of helium droplets. The nu1 CH stretch ro-vibrational spectra were measured revealing what appears to be the P and R contours of a nearly rigid linear rotor. To simulate the linear molecule spectra, given a rotational temperature of 0.37 K, effective moments of inertia, IB, were required to be 10(4)-10(5) amu.A2 larger than the ab initio predicted values. The large moments of inertia were found to be strongly dependent on both the mass of the complex and the size of the helium droplet, consistent with a model where the dopant is located in a dimple site on the surface of the droplet. In this model, the moment of inertia is representative of the rotational motion of the dopant on the surface about an inertial axis through the center of the droplet.

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.