Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with theoretical results, determining their structures. Through chemical bonding analyses, we have identified unique aromatic characters in these metallaboron clusters.

Spatial overlap of two non-bonding orbitals of ionized and neutral molecules can result in formation of a hemibond (two-center three-electron (2c3e) bond). Though hemibond formation between water and rare gas atom has been theoretically predicted, no definite experimental evidence has been reported. In the present study, we perform infrared spectroscopy of [H₂O-(Kr)_n]⁺ (n=1-3) clusters in the gas phase. Comparison of the observed spectral features in the OH stretch region with the ab initio anharmonic vibrational simulations demonstrates the hemibond formation between water and Kr in all the observed cluster sizes.

The rotational spectrum of the 2-methylaminoethanol-water (2MAE-water) complex has been observed using a cavity based Fourier-transform microwave spectrometer in the range of 10-19 GHz. 2MAE exists in trans and gauche conformationsR. E. Penn and L. W. Buxton, J. Mol. Spectrosc. 56, 229-238 (1975).^,C. Calabrese, A. Maris, L. Evangelisti, A. Piras, V. Parravicini and S. Melandri, Front. Chem. 6:25 (2018).^,Y. Liu, C.A. Rice and M.A. Suhm, Can. J. Chem. 82, 1006-1012 (2004). We modeled 10 possible conformers of the 2MAE-water complex, five trans and five gauche conformers, using ab initio calculations (MP2/6-311++G(d,p)). 14 rotational transitions were fit to Watson's A-reduced Hamiltonian: A=3368.02 MHz, B=2282.60 MHz, and C=1538.00 MHz. ¹⁴N nuclear quadrupole hyperfine splittings were resolved, and the 38 hyperfine components were fit to χ_aa = 1.543(7) MHz and χ_bb = -1.083(25) MHz. The measured spectrum is assigned to the lowest energy model structure of the complex, which has two intermolecular hydrogen bonds: from hydroxyl group to water and from water to the methylamino group. The structure of 2MAE-water is compared with 2-aminoethanol-water (2AE-water) and 2-methoxyethylamine-water (2MEA-water) complexes.

---

Footnotes:

R. E. Penn and L. W. Buxton, J. Mol. Spectrosc. 56, 229-238 (1975).\end C. Calabrese, A. Maris, L. Evangelisti, A. Piras, V. Parravicini and S. Melandri, Front. Chem. 6:25 (2018). Y. Liu, C.A. Rice and M.A. Suhm, Can. J. Chem. 82, 1006-1012 (2004)..

r0.33 Figure We have recorded gas-phase Fourier transform Infrared spectra at room temperature of several hydrogen (H) and deuterium (D) bound bimolecular complexes. These complexes were formed using methanol and ethanol as OH donors, and methanol-d₁ and ethanol-d₁ as OD donors, to compare isotopic effects in H/D-bond stability. The stability of these complexes are governed by the Gibbs energy of complex formation. By combining experimental and calculated intensities, the pressure of the complex can be determined, and from that and monomer pressures the Gibbs energy of complex formation. At room temperature, we find similar Gibbs energies for corresponding H/D-bound complexes. For the pair of H/D-complexes, methanol·dimethylether (OH·O) and methanol-d₁·dimethylether (OD·O), Gibbs energies of 8.3 and 7.7 kJ/mol were determined, and for methanol·trimethylamine (OH·N) and methanol-d₁·trimethylamine (OD·N) were determined Gibbs energies of 3.2 and 2.7 kJ/mol. Our findings of similar Gibbs energies for H/D-bound complexes at room temperature, is in contrast to what has been observed in spectra recorded in cold conditions. The deuterium bound complex is heavily favored, at low temperatures due to a lower zero-point vibrational energy. From calculations, we find that the difference in stability of the H/D-bound complexes become smaller with increasing temperature. The change in stability with temperature, arise from changes in both the rotational and vibrational entropic contributions.[1]

[1] A. Kjaersgaard, E. Vogt, N. F. Christensen, H. G. Kjaergaard, J. Phys. Chem. A., 2020,
       DOI: 10.1021/acs.jpca.9b11762.

The large number of spectroscopic and theoretical studies of NH₃-Rg complexes reported for three decades clearly proves that they are good prototypes to investigate intermolecular dynamics of van der Waals interactions. The inversion coordinate for which the inversion-tunneling splitting in NH₃ is not quenched in the case of NH₃-Rg represents a valuable probe of asymmetry in the intermolecular potential energy surface (IPES).G. T. Fraser, D. D. Nelson Jr, A. Charo and W. Klemperer, J. Chem. Phys. 82, 2535 (1985). recent high resolution infrared laser jet- cooled study realized in the ν₂ umbrella mode region of NH₃-Ar enabled to detect five ortho and para bands and to provide accurate band centres and upper state rotational constants for the ortho ones.P. Asselin, Y. Belkhodja, A. Jabri, A. Potapov, J. Loreau and A. van der Avoird, Mol . Phys. 116, 3642 (2018)he puzzling para bands observed in the region of the lower and upper components of the inversion splitting doublets have been assigned by comparison with calculations of vibration-rotation-tunneling (VRT) levels based on a 4D PES that includes the umbrella inversion motion.J. Loreau and A. van der Avoird, J. Chem. Phys. 143,184303 (2015)he present study aims to investigate more in detail the intense ortho bands of the NH₃-Rg family (Rg= Ne, Ar, Kr, Xe) taking advantage of recent improvements in the pulsed slit expansion to reduce line widths and to increase gain in absorption. These advances enabled to derive accurate excited rotational and quartic parameters and l-type doubling constant q for the four binary NH₃-Rg complexes from rovibrational analyses of the ortho Π_s(j=1,k=0) ← Σ_a(j=1,k=0) transition. Using a pseudodiatomic model, structural parameters and force constants of the NH₃-Rg family could be estimated from the experimental set of spectroscopic constants obtained and then compared with those calculated using ab initio VRT levels, transition frequencies and line strengths from 4D PES of NH₃-Rg. The predictive character of the IPES is discussed on the grounds of band origins, rovibrational states and l-type doubling contribution.

---

Footnotes:

G. T. Fraser, D. D. Nelson Jr, A. Charo and W. Klemperer, J. Chem. Phys. 82, 2535 (1985).A P. Asselin, Y. Belkhodja, A. Jabri, A. Potapov, J. Loreau and A. van der Avoird, Mol . Phys. 116, 3642 (2018)T J. Loreau and A. van der Avoird, J. Chem. Phys. 143,184303 (2015)T

One of the largest unknowns in the understanding of aerosol particles is the structure and growth process that drives new particle formation, particularly from a molecular point of view. These particles typically form by the nucleation of water onto oxygenated byproducts of terpenes. Their structure and growth are dictated by the underlying interplay of hydrogen-bonding interactions of the organic molecules with water and water with itself. The presence of amine groups may further accelerate new particle formation. Here, we determine the structures of oxygenated byproducts of isoprene complexing with water and ammonia by employing infrared spectroscopy to reveal the important hydrogen-bonding interactions likely at play in larger clusters. We model the OH stretching region of the clusters' IR spectra through a suite of approaches that have favorable computational scaling and cost compared to standard anharmonic approaches. This acceleration allows us to consider the numerous candidate clusters and provide assignments. Finally, the wealth of spectral data (both theoretical and experimental) allows us to test the performance of data-driven spectroscopic models.

Anisole (C6H5OCH3) has two prominent hydrogen bonding sites, namely oxygen and the π-electrons of the phenyl ring. Earlier studies on anisole-water^1,2 and anisole-methanol³ complexes show that in both cases the interaction with the oxygen atom is preferred and complex formation takes place via OH-O hydrogen bonding, where water/methanol acts as hydrogen bond donor. We have studied anisole-methanol complexes in superfluid helium droplets, using high-resolution infrared spectroscopy. Several bands corresponding to (anisole)_m-(methanol)_n complexes (where m=1,2 and n=1) were observed. The size of the clusters corresponding to the observed bands was determined by recording the band intensity as a function of the partial pressures of the constituent molecules, resulting in so-called pickup curves.⁴ Based on a comparison of the observed spectra with the predicted spectra, at MP2/6-311++G (d,p) level of theory, we propose that, in helium droplets, for 1:1 anisole-methanol clusters, structures bound via OH-O and OH-π hydrogen bonding are almost equally likely. References: 1. B. Reimann, K. Buchhold, H.-D. Barth, B. Brutschy, P. T arakeshwar, and Kwang S. Kim, J. Chem. Phys., 2002, 117, 8805. 2. M. Becucci, G. Pietraperzia, M. Pasquini, G. Piani, A. Zoppi, R. Chelli, E. Castellucci and W. Demtroeder, J. Chem. Phys., 2004, 120, 5601. 3. M. Heger, J. Altno, A. Poblotzki and M. A. Suhm, Phys. Chem. Chem. Phys., 2015, 17, 13045. 4. M. Lewerenz, B. Schilling and J. P. Toennies, J. Chem. Phys., 1995, 102, 8191.

Superfluid Helium-4 shows a variety of unique effects. For instance, the effective rotational constants for molecules embedded in ⁴He droplets are usually quite different than their gas-phase values. The observed molecular rotational constants shrink with increasing number of He atoms until the "turnaround point," where part of the ⁴He density rotationally decouples from a surrounding ⁴He superfluid shell, leading to anomalous rotational constants as the number of ⁴He atoms increases. Such behavior can appear well below the droplet limit; high-resolution studies of linear molecules embedded in small ⁴He clusters containing as few as 6 ⁴He atoms reveal superfluidic behavior.[1] Tang, J.; Xu, Y.; McKellar, A. R. W.; Jäger, W. Science 2002, 297, 2030. [2] Dempster, S. P.; Sukhorukov, O.; Lei, Q. -Y.; Jäger, W., J. Chem. Phys. 2012, 137, 174303.lthough rovibrational spectra of asymmetric top molecules embedded in nanodroplets are routinely acquired using IR techniques, MW spectroscopy of small molecule-(⁴He)_n clusters has been limited to linear molecules. In fact, only a small handful of theoretical studies of asymmetric tops embedded in superfluids are currently available.[1] Zeng, T.; Li, H.; Roy, P. -N., J. Phys. Chem. Lett. 2013, 4, 18. [2] Zeng. T.; Roy, P. -N., Rep. Prog. Phys. 2014, 77, 046601.he potential experimental complications are foreboding, and are exacerbated by limitations of the Balle-Flygare cavity experiments used in many of the early MW studies. Thankfully, chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy circumvents some of these limitations. Here, we show exciting new results on ⁴He clusters doped with pyridine and benzonitrile. These systems show intriguing and unexplained spectroscopic phenomena and dynamics. In the case of pyridine, we can identify spectroscopic transitions for clusters containing as many as 20 ⁴He atoms. We will also describe experimental techniques for sensitivity enhancement to detect such large molecule-doped helium clusters. These techniques, coupled with new efforts to characterize these systems theoretically, should extend generally to studies of more complicated chromophores.

---

Footnotes:

[1] Tang, J.; Xu, Y.; McKellar, A. R. W.; Jäger, W. Science 2002, 297, 2030. [2] Dempster, S. P.; Sukhorukov, O.; Lei, Q. -Y.; Jäger, W., J. Chem. Phys. 2012, 137, 174303.A [1] Zeng, T.; Li, H.; Roy, P. -N., J. Phys. Chem. Lett. 2013, 4, 18. [2] Zeng. T.; Roy, P. -N., Rep. Prog. Phys. 2014, 77, 046601.T

Gas-phase hydrogen-bonded clusters are treated as microscopic models of hydrogen bond networks. We have been investigating the temperature effect on microscopic hydrogen bond structures. In our previous study, we measured ultraviolet photodissociation (UVPD) spectra of hydrated phenol cations [PhOH(H₂O)₅]⁺ trapped in our temperature-variable ion trap. We revealed temperature dependence of relative populations between two isomers having distinct hydrogen bond structures.H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017).t is known that water and methanol molecules construct different hydrogen bond networks. Thus, we measured UVPD spectra of phenol-methanol cluster cations [PhOH(MeOH)_n]⁺ (n=3, 4) in the present study. As in the case of [PhOH(H₂O)₅]⁺, isomers having ring type hydrogen bond structures are dominant in cold condition for both n = 3 and 4 cases. As the temperature elevates, populations of the chain type isomers become large in the case of the n = 3. Both bands for the ring and chain type isomers exhibit red shifts of the band positions and broadening of the band widths along the temperature elevation were observed. These changes were attributed mainly by hot bands of the intermolecular vibrational modes. In contrast, only the bands assigned as the ring isomers were observed with the temperature below 150 K. However, changes in the band profiles indicate structural changes within a ring type hydrogen bond motif.

---

Footnotes:

H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017).I

Since gas-phase hydrogen-bonded clusters are treated as microscopic models of hydrogen bond networks, numerous spectroscopic studies have been performed, so far. Structural fluctuations are one of features of hydrogen bond network. Such fluctuations correspond to isomerizations among isomers having distinct hydrogen bond structures in the cases of clusters. To investigate microscopic natures of structural fluctuations of hydrogen bond networks, we observed IR-induced isomerizations of hydrogen-bonded phenol cluster cations trapped in a cold ion trap. In a cold condition, all hydrogen-bonded phenol-methanol cluster cations [PhOH(MeOH)₃]⁺ have a ring type hydrogen bond structures. In contrast, isomers having a chain type hydrogen bond structure are dominant in a hot condition. In the present experiment, we excited the ring isomers in a cold condition by IR laser pulses. Since the energy of IR laser photon exceeded the height of isomerization barriers, isomerizations from the ring to chain isomers occurred. We successfully detected the chain type isomers in ultraviolet photodissociation spectra. Additionally, we also observed reverse isomerizations from the chain back to the ring isomers by the collisional cooling with the buffer gas. Details of the observations are presented in the paper.