High resolution infrared spectroscopy of propargyl alcohol-water complex embedded in helium nanodroplets

Propargyl alcohol (hereafter abbreviated as PA) is a molecule of astrophysical interest and has been probed extensively using microwave spectroscopy.$^{1,2}$ It is a multifunctional molecule and offers multiple sites for hydrogen bonding interactions. Therefore, it has also attracted the attention of groups interested in weak intermolecular interactions. Recently, the Ar…PA complex$^{3}$ and PA-dimer$^{4}$ have been studied using microwave spectroscopy. More recently, there have been matrix-isolation infrared spectroscopic studies on PA-water$^{5}$ and PA-acetylene$^{6}$ complexes. In the present work, clusters of PA and water were formed in the helium nanodroplets and probed using a combination of infrared spectroscopy and mass spectrometry. Using ab-initio quantum mechanical calculations, PA-water clusters were optimised and five minimum structures were found on the potential energy hypersurface, which were used as a guidance to the experiments. We used D$_{2}$O for the experiments since our laser sources at Bochum do not cover the IR spectral region of H$_{2}$O. IR spectra of PA-D$_{2}$O complex were recorded in the region of symmetric and antisymmetric stretches of the bound D$_{2}$O. Multiple signals were found in these regions which were dependent on the concentration of PA as well as D$_{2}$O. Using pickup curves most of these signals could be assigned to 1:1 PA:D$_{2}$O clusters. The ab-initio calculations helped in a definitive assignment of the spectra to the different conformers of PA-D$_{2}$O complex. The details will be presented in the talk. References: 1.E. Hirota, J. Mol. Spec. 26, 335 (1968). 2.J.C. Pearson and B.J. Drouin, J. Mol. Spectrosc. 234, 149 (2005). 3.D. Mani and E. Arunan, ChemPhysChem 14, 754 (2013). 4.D. Mani and E. Arunan, J. Chem. Phys. 141, 164311 (2014). 5.J. Saini, K.S. Vishwanathan, J. Mol. Struct. 1118, 147 (2016). 6.K. Sundararajan et al., J. Mol. Struct. 1121, 26 (2016)

IR spectroscopic studies on microsolvation of HCL by water

Acid dissociation reactions are at the heart of chemistry. These reactions are well understood at the macroscopic level. However, a microscopic level understanding is still in the early stages of development. Questions such as \textit{‘how many \chem{H_2O} molecules are needed to dissociate one HCl molecule?’} have been posed and explored both theoretically and experimentally.$^{1-5}$ Most of the theoretical calculations predict that four \chem{H_2O} molecules are sufficient to dissociate one HCl molecule, resulting in the formation of a solvent separated \chem{H_3O}$^{+}$(\chem{H_2O})$_{3}$Cl$^{-}$ cluster.$^{1-3}$ IR spectroscopy in helium nanodroplets has earlier been used to study this dissociation process.$^{3-5}$ However, these studies were carried out in the region of O-H and H-Cl stretch, which is dominated by the spectral features of undissociated (HCl)$_{m}$-(\chem{H_2O})$_{n}$ clusters. This contributed to the ambiguity in assigning the spectral features arising from the dissociated cluster.$^{4,5}$ Recent predictions from Bowman’s group, suggest the presence of a broad spectral feature (1300-1360 \wn) for the \chem{H_3O}$^{+}$(\chem{H_2O})$_{3}$Cl$^{-}$ cluster, corresponding to the umbrella motion of \chem{H_3O}$^{+}$ moiety.$^{6}$ This region is expected to be free from the spectral features due to the undissociated clusters. In conjunction with the FELIX laboratory, we have performed experiments on the (HCl)$_{m}$(\chem{H_2O})$_{n}$ (m=1-2, n$\geq$4) clusters, aggregated in helium nanodroplets, in the 900-1700 \wn region. Mass selective measurements on these clusters revealed the presence of a weak-broad feature which spans between 1000-1450 \wn and depends on both HCl as well as \chem{H_2O} concentration. Measurements are in progress for the different deuterated species. The details will be presented in the talk. \\\textbf{References}: \textbf{1)} C.T. Lee et al., \textit{J. Chem. Phys.}, \textbf{104}, 7081 (1996). \textbf{2)} H. Forbert et al., \textit{J. Am. Chem. Soc.}, \textbf{133}, 4062 (2011). \textbf{3)} A. Gutberlet et al., \textit{Science}, \textbf{324}, 1545 (2009). \textbf{4)} S. D. Flynn et al., \textit{J. Phys. Chem. Lett.}, \textbf{1}, 2233 (2010). \textbf{5)} M. Letzner et al., \textit{J. Chem. Phys.}, \textbf{139}, 154304 (2013). \textbf{6)} J. M. Bowman et al., \textit{Phys. Chem. Chem. Phys.}, \textbf{17}, 6222 (2015)

HIGH-RESOLUTION INFRARED SPECTROSCOPY OF IMIDAZOLE CLUSTERS IN HELIUM DROPLETS USING QUANTUM CASCADE LASERS

Imidazole ring is a part of many biologically important molecules and drugs. Imidazole monomer, dimer and its complexes with water have earlier been studied using infrared spectroscopy in helium droplets$^{1,2}$ and molecular beams$^{3}$. These studies were focussed on the N-H and O-H stretch regions, covering the spectral region of 3200-3800 wn._x000d_ _x000d_ We have extended the studies on imidazole clusters into the ring vibration region. The imidazole clusters were isolated in helium droplets and were probed using a combination of infrared spectroscopy and mass spectrometry. The spectra in the region of 1000-1100 wn and 1300-1460 wn were recorded using quantum cascade lasers. Some of the observed bands could be assigned to imidazole monomer and higher order imidazole clusters, using pickup curve analysis and ab initio calculations. Work is still in progress. The results will be discussed in detail in the talk. _x000d_ \textbf{References}:_x000d_ textbf{1)} M.Y. Choi and R.E. Miller, textit{J. Phys. Chem. A}, textbf{110}, 9344 (2006)._x000d_ textbf{2)} M.Y. Choi and R.E. Miller, textit{Chem. Phys. Lett.}, textbf{477}, 276 (2009)._x000d_ textbf{3)} J. Zischang, J. J. Lee and M. Suhm, textit{J. Chem. Phys.}, textbf{135}, 061102 (2011)._x000d_ _x000d_ textbf{Note:} This work was supported by the Cluster of Excellence RESOLV (Ruhr-Universitat EXC1069) funded by the Deutsche Forschungsgemeinschaft

Observation of the Low-Frequency Spectrum of the Water Trimer as a Sensitive Test of the Water-Trimer Potential and the Dipole-Moment Surface

© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. Intermolecular interactions in bulk water are dominated by pairwise and non-pairwise cooperative interactions. While accurate descriptions of the pairwise interactions are available and can be tested by precise low-frequency spectra of the water dimer up to 550 cm−1, the same does not hold for the three-body interactions. Here, we report the first comprehensive spectrum of the water trimer in the frequency region from 70 to 620 cm−1 using helium-nanodroplet isolation and free-electron lasers. By comparison to accompanying high-level quantum calculations, the experimentally observed intermolecular bands can be assigned. The transition frequencies of the degenerate translation, the degenerate in-plane and the non-degenerate out-of-plane libration, as well as additional bands of the out-of-plane librational mode are reported for the first time. These provide a benchmark for state-of-the-art water potentials and dipole-moment surfaces, especially with respect to three-body interactions

Rotational spectra of propargyl alcohol dimer: A dimer bound with three different types of hydrogen bonds

Pure rotational spectra of the propargyl alcohol dimer and its three deuterium isotopologues have been observed in the 4 to 13 GHz range using a pulsed-nozzle Fourier transform microwave spectrometer. For the parent dimer, a total of 51 transitions could be observed and fitted within experimental uncertainty. For two mono-substituted and one bi-substituted deuterium isotopologues, a total of 14, 17, and 19 transitions were observed, respectively. The observed rotational constants for the parent dimer A = 2321.8335(4) MHz, B = 1150.4774(2) MHz, and C = 1124.8898(2) MHz] are close to those of the most stable structure predicted by ab initio calculations. Spectra of the three deuterated isotopologues and Kraitchman analysis positively confirm this structure. Geometrical parameters and ``Atoms in Molecules'' analysis on the observed structure reveal that the two propargyl alcohol units in the dimer are bound by three different types of hydrogen bonds: O-H center dot center dot center dot O, O-H center dot center dot center dot pi, and C-H center dot center dot center dot pi. To the best of our knowledge, propargyl alcohol seems to be the smallest molecule forming a homodimer with three different points of contact. (C) 2014 AIP Publishing LLC

The X-C center dot center dot center dot pi (X = F, Cl, Br, CN) Carbon Bond

High-level ab initio calculations have been used to study the interactions between the CH3 group of CH3X (X = F, Cl, Br, CN) molecules and pi-electrons. These interactions are important because of the abundance of both the CH3 groups and pi-electrons in biological systems. Complexes between C2H4/C2H2 and CH3X molecules have been used as model systems. Various theoretical methods such as atoms in molecules theory, reduced density gradient analysis, and natural bond orbital analysis have been used to discern these interactions. These analyses show that the interaction of the p-electrons with the CH3X molecules leads to the formation of X-C...p carbon bonds. Similar complexes with other tetrel molecules, SiH3X and GeH3X, have also been considered

Dynamics of the chemical bond: inter- and intra-molecular hydrogen bond

In this discussion, we show that a static definition of a `bond' is not viable by looking at a few examples for both inter-and intra-molecular hydrogen bonding. This follows from our earlier work (Goswami and Arunan, Phys. Chem. Chem. Phys. 2009, 11, 8974) which showed a practical way to differentiate `hydrogen bonding' from `van der Waals interaction'. We report results from ab initio and atoms in molecules theoretical calculations for a series of Rg center dot center dot center dot HX complexes (Rg = He/Ne/Ar and X = F/Cl/Br) and ethane-1,2-diol. Results for the Rg center dot center dot center dot HX/DX complexes show that Rg center dot center dot center dot DX could have a `deuterium bond' even when Rg center dot center dot center dot HX is not `hydrogen bonded', according to the practical criterion given by Goswami and Arunan. Results for ethane-1,2-diol show that an `intra-molecular hydrogen bond' can appear during a normal mode vibration which is dominated by the O center dot center dot center dot O stretching, though a `bond' is not found in the equilibrium structure. This dynamical `bond' formation may nevertheless be important in ensuring the continuity of electron density across a molecule. In the former case, a vibration `breaks' an existing bond and in the later case, a vibration leads to `bond' formation. In both cases, the molecule/complex stays bound irrespective of what happens to this `hydrogen bond'. Both these cases push the borders on the recent IUPAC recommendation on hydrogen bonding (Arunan et al. Pure. Appl. Chem. 2011, 83 1637) and justify the inclusive nature of the definition

The X-C center dot center dot center dot Y (X = O/F, Y = O/S/F/Cl/Br/N/P) `carbon bond' and hydrophobic interactions

While the tetrahedral face of methane has an electron rich centre and can act as a hydrogen bond acceptor, substitution of one of its hydrogens with some electron withdrawing group (such as -F/OH) can make the opposite face electron deficient. Electrostatic potential calculations confirm this and high level quantum calculations show interactions between the positive face of methanol/methyl fluoride and electron rich centers of other molecules such as H2O. Analysis of the wave functions of atoms in molecules shows the presence of an unusual C center dot center dot center dot Y interaction, which could be called `carbon bonding'. NBO analysis and vibrational frequency shifts confirm the presence of this interaction. Given the properties of alkyl groups bonded to electronegative elements in biological molecules, such interactions could play a significant role, which is yet to be recognized. This and similar interactions could give an enthalpic contribution to what is called the `hydrophobic interactions'

MICROWAVE SPECTRUM OF HEXAFLUOROISOPROPANOL

Author Institution: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, IndiaHexafluoroisopropanol (HFIP) is an important organic solvent and probably the only solvent which can dissolve polythene. IR studies, on this molecule confirm the existence of antiperiplanar ({\bf{ap}}) and synclinical ({\bf{sc}}) conformers. We have observed pure rotational spectrum of this molecule and the fitted rotational constants (A= 2105.1208(11) MHz, B= 1053.9942(3) MHz, C= 932.3398(3) MHz) confirm the presence of {\bf{ap}} conformer. There are many other observed lines which most probably corresponds to {\bf{sc}} structure and due to the large amplitude motion of H-atom, some of these transitions show tunneling splitting. Work is in progress for the deuterated (OD) and C-13 isotopologues of the monomer. HFIP is expected to exhibit interesting hydrogen bonding properties and we are planning to investigate them by studying its complex with water. The results will be presented in this talk