The Power of SPecTroScoPy CHIMIA 2017, 71, No. 1/2 7 doi:10.2533/chimia.2017.7 Chimia 71 (2017) 7–12 © Swiss Chemical Society Measuring Intermolecular Binding Energies by Laser Spectroscopy Richard Knochenmussa, Surajit Maityb, Géraldine Féraudc, and Samuel Leutwyler*a Abstract: The ground-state dissociation energy, D (S ), of isolated intermolecular complexes in the gas phase is a 0 0 fundamental measure of the interaction strength between the molecules. We have developed a three-laser, triply resonant pump-dump-probe technique to measure dissociation energies of jet-cooled M•S complexes, where M is an aromatic chromophore and S is a closed-shell ‘solvent’ molecule. Stimulated emission pumping (SEP) via the S ↔S electronic transition is used to precisely ‘warm’ the complex by populating high vibrational levels v" of 0 1 the S state. If the deposited energy E(v") is less than D (S ), the complex remains intact, and is then mass- and 0 0 0 isomer-selectively detected by resonant two-photon ionization (R2PI) with a third (probe) laser. If the pumped level is above D (S ), the hot complex dissociates and the probe signal disappears. Combining the fluorescence or SEP 0 0 spectrum of the cold complex with the SEP breakoff of the hot complex brackets D (S ). The UV chromophores 0 0 1-naphthol and carbazole were employed; these bind either dispersively via the aromatic rings, or form a hydrogen bond via the -OH or -NH group. Dissociation energies have been measured for dispersively bound complexes with noble gases (Ne, Kr, Ar, Xe), diatomics (N , CO), alkanes (methane to n-butane), cycloalkanes (cyclopropane 2 to cycloheptane), and unsaturated compounds (ethene, benzene). Hydrogen-bond dissociation energies have been measured for H O, D O, methanol, ethanol, ethers (oxirane, oxetane), NH and ND . 2 2 3 3 Keywords: Dispersive interactions · Hydrogen bonding · Intermolecular interactions · Stimulated emission pumping 1. Introduction For larger molecules, sequential up- intermolecular modes of the complex (thin pumping is impractical due to the com- gray lines), on a timescale of nanoseconds Spectroscopic methods for determining plexity of assigning high vibrational- to microseconds. If the energy deposited dissociation energies, D , of diatomic rotational levels of multidimensional in the now ‘hot’ M≠•S complex is larger 0 and triatomic molecules have a long potential energy surfaces. Furthermore, than D (S ), it undergoes vibrational 0 0 and illustrious history. Early techniques intramolecularvibrationalrelaxation(IVR) predissociation to M and S. Otherwise such as the Birge-Sponer extrapolation[1] from higher vibrational levels becomes it remains intact and can be detected by determine the D of a diatomic molecule faster than the achievable up-pumping resonant 2-photon ionization (R2PI, a third 0 by assuming a specific vibrational poten- rates. To bypass such difficulties, we laser) and mass spectrometry.[7.13] tial function (e.g. Morse[2] or R–6). A take advantage of properties of our target The dissociation energy is bracketed by series of spectroscopically measured and species, M•S intermolecular complexes, comparing two measurements. The R2PI assigned vibrational transitions can then where M is a UV chromophore and S is a probe laser can be set to specifically detect be extrapolated to the dissociation limit closed-shell ‘solvent’ molecule. hot M≠•S by exciting on so-called ‘hot for AB→ A• + B•.[3,4] More recently, the In this scheme,[7–12] high M•S inter­ bands’. When the dump laser is scanned, dissociation energy of the HO–H bond in molecular vibrational levels are indirectly peaks appear at the v" energies of M≠•S, the water molecule has been measured by a excited via intramolecular vibrational but break off when v" is above D . This 0 triply-resonant scheme employing a series levels of the chromophore, M. These could, corresponds to the gold spectrum on of vibrational overtone transitions. This in principle, be directly accessed in the the right of Fig. 1. The last peak in this gave direct spectroscopic access to the infrared region, but the relatively low IR spectrumisalowerlimitforthedissociation onset of the H O→H• + OH• dissociation absorption cross-sections and the restricted energy. 2 continuum.[5,6] tuning range of IR lasers make it more To find the upper limit for D , two 0 effective to use a pump-dump strategy, methods are used. First, the probe laser can also called stimulated-emission pumping be set to monitor cold M•S while the dump (SEP). As seen in Fig. 1, a first UV laser is scanned. Since the dump process heats transfers (pumps) ground-state population the complex, it depletes the cold signal, to the vibrationless (v'=0) first electronic regardless of whether the complexes excited state of M•S. Shortly thereafter, a dissociate or not. This is called a dump *Correspondence: Prof. Dr. S. Leutwylera E-Mail: [email protected] second UV laser transfers population down spectrum. Alternatively the fluorescence aUniversität Bern (dump, violet-green downward arrows) emission spectrum from the first singlet Departement für Chemie und Biochemie to vibrational levels (v") of M•S in the excited state (S ) can be measured, since it Freiestrasse 3 1 CH-3012 Bern electronic ground state (thick black lines). also does not depend on whether the lower bCurrent address: Dept. of Chemistry, IIT Hyderabad The pump and dump transitions are in the state dissociates. This is shown as the blue Kandi, Sangareddy-502285, Telangana, India UV and have high oscillator strengths, spectrum on the left of Fig. 1. The first cCurrent address: LERMA, Sorbonne Universités UPMC University Paris 06, Observatorie de Paris so the process is relatively efficient. peak above the previously identified lower The v" level of M undergoes IVR to limit is the upper limit for D . PSL Research University, CNRS, F-75252 Paris 0 8 CHIMIA 2017, 71, No. 1/2 The Power of SPecTroScoPy Fig.2showsthepump-dump-IVReffect in the 1-naphthol•cycloheptane complex. The black spectrum is the normal 1-color R2PI spectrum in the S →S origin region. 0 1 If the pump laser is then fixed on the isomer B origin, many ground-state complexes are up-pumped and lost. The blue spectrum shows this depletion of both the isomer B origin and vibronic bands. When the dump is added, also at the isomer B origin, not only is the depletion more extensive, but also a broad absorption appears below the origins (red spectrum and shaded area). These are the hot-band transitions of the vibrationally excited complexes. Fig. 3 is an example of a bracketing experiment on 1-naphthol•cyclohexane. The probe laser is first fixed on hot bands, as indicated in Fig. 2, while the dump is scanned. The signal of the R2PI ionized complex is observed with the mass spec- trometer. This produces a spectrum like the upper, gold, trace in Fig. 3. The arrow indicates the last peak for which the complex remains intact. In this case, Fig. 1. Stimulated-emission pumping part of the SEP-R2PI experiment of an M•S complex: Following the pump step at the S ←S origin, the excited a dump spectrum of cold complexes was 1 0 M•S complex either fluoresces back to many different S state vibrational used for the comparative measurement, 0 as shown by the blue trace. The dump levels (fluorescence spectrum on the left) or is dumped to a specific v'' level in the S state (violet to green vertical arrows). The hot M•S complex is spectrum is not inverted, the peaks are 0 negative, indicating depletion. The two detected by a third ‘probe’ laser (not shown), this is the gold SEP spectrum on the right. Note that the SEP spectrum breaks off as soon as the hot v" arrows show the bracket of D for this 0 level of M•S lies above D , leading to vibrational predissociation: M•S → –1 0 cluster, between 1697 and 1703 cm . M + S. The intermolecular dissociation energy D (S ) is bracketed by the 0 0 highest v'' level observed in the SEP spectrum and the next vibrational level that is observed in the fluorescence spectrum on the left. 2. Noble Gas Complexes WeaklyboundvanderWaalscomplexes between aromatics and noble-gas (NG) atoms are prototypical for the investigation of non-polar solvation and the buildup of solvent shells. The main attractive interaction of an aromatic with a noble-gas atom is quantum mechanical dispersion. In the limit of two interacting atoms, and expanded in terms of polarizability moments, the dominant dispersion term is from the dipole polarizabilities, which falls off as R–6, where R is the interatomic distance. The next term is the dipole- quadrupole polarizability contribution, with an R–8 dependence. The complexes of carbazole with NG = Ne, Ar, Kr and Xe offer a systematic test of this and other theories of dispersion. Ground-state dissociation energies of carbazole•NG were determined by the SEP-R2PI method[10] as shown in Fig. 4. Except for Ne, the relative uncertainty was <0.4%. For the Ne complex only an upper limit could be determined, the lower limit was estimated. The carbazole-NG distances have not been measured, except for carbazole•Ar, where it is 3.48 Å.[14] However, Neusser et al.[15] determined the benzene-NG distances by rotationally Fig. 2. Resonant two-photon ionization ‘probe’ spectra of the resolved R2PI spectroscopy. The 1-naphthol•cycloheptane complex. Two isomers exist in the supersonic jet, benzene•Ar distance is slightly longer, and are denoted A and B. See the text for details. The Power of SPecTroScoPy CHIMIA 2017, 71, No. 1/2 9 3.58 Å, reflecting the greater dispersion attraction of the larger carbazole.