Search

It is commonly believed that accumulation of organic molecules of increasing complexity is an important step in the series of events that led from elementary particles to the emergence of life. For this reason a lot of resources have been and are currently devoted to the discovery and quantification of molecular species in extra-terrestrial environments. Through the recognition of the rotational spectral lines, radioastronomy represents one of the most important techniques for the unequivocal detection of complex organic molecules (COMs) in circumstellar envelopes and interstellar medium (ISM): of the among 130 compounds that have been detected about 90 species are neutral molecules, that typically are characterized by functional groups common in organic chemistry, such as amines, alcohols, aldehydes, ketones, etc. COMs of greater size are expected to be detected exploiting the increasing sensitivity and resolution of new radiotelescopes, therefore the challenge is nowadays twice: (i) on the one hand the spectral analysis of the huge amount of data contained in the ongoing astronomical observations, (ii) on the other hand the laboratory observation and assignment of the rotational spectra of COMs, which are often complicated due to flexibility typical of these molecules. Considering these tasks, we will here present first the rotational spectra of some COMs recorded in laboratory, highlighting the relationships between the spectral features and the conformational landscapes, then an example of spectral survey achieved by data reduction of the freely available Atacama Large Millimeter/submillimeter interferometer Array (ALMA) data on Class 0 protostellar binary IRAS 16293-2422, evidencing the issues related to the molecular recognition in dense spectra.

Conventional absorption spectroscopy is still the workhorse in high-resolution rotational laboratory spectroscopy.1 The data obtained from these kind of instruments are relevant for astronomical searches of complex molecules that represent excellent probes of the physical and chemical environments and history of the sources where they are detected.2 Nowadays, observations performed by the Atacama Large Millimeter Array (ALMA) open up new opportunities to reveal the chemical complexity of solar systems analogues. At the same time the huge amount of data collected and the extremely rich surveys represent a challenge for the astrochemistry community. The chance to detect molecules with an increasing large number of atoms, goes hand in hand with the complexity of their conformational equilibria, often associated with large amplitude motions, that need to be analysed in laboratory before taking on an astronomical search. For this reason a strong interplay between the laboratory spectroscopists and the observational astronomers is increasingly required to be able to unravel the spectra, which are rather difficult to predict theoretically, mainly in the sub-mm wave region. In this talk laboratory results on diols and thiols of potential astronomical interest, obtained using the only Free Jet Absorption Millimeter Wave (FJAMMW) spectrometer working at the University of Bologna,3 will be presented. The rotational spectra (59.6 - 74.4 GHz, corresponding to 5.03 - 4.03 mm) reveal the presence of six conformers for 1,2-butanediol (C4H10O2) and four conformers for 1,3-propanedithiol (C3H8S2), proving the complexity of the conformational landscapes of these kind of compounds. Moreover, taking advantage of the existing public ALMA data, some considerations on the rich molecular line spectrum of the Class 0 protostellar binary IRAS 16293-2422 will be discussed. References [1] S. Brünken, S. Schlemmer, arXiv:1605.07456, 2016 [2] E. Herbst, E. F. van Dishoeck, Annu. Rev. Astron. Astrophys. 47, 427, 2009 [3] C. Calabrese, A. Maris, L. Evangelisti, L. B. Favero, S. Melandri, W. Caminati, J. Phys. Chem. A. 117, 13712, 2013

The conformational space of non-covalently bound complexes of medium size organic molecules is shaped by competing interactions occurring within the molecules or with the partners. It usually presents a high number of low energy conformations very close in energy with shallow potential energy barriers through which the molecular system can tunnel. The conformational preferences of non-covalently bound complexes can be studied to a very high degree of accuracy by free jet rotational spectroscopy1 and from the detailed structural and dynamical data that can be obtained, the site and geometry of the interaction and information on the binding energy can be inferred without ambiguity. The questions usually addressed are: which is the preferred binding site, which type of interactions are established, and whether any conformational change takes place in the monomers upon complexation. Answers to these questions allow insight into the molecular interaction process at the molecular level, bridging the gap between gas-phase and bulk properties. Chosen examples of published and unpublished results of complexes of medium-size organic molecules with different partners formed in a supersonic expansion and characterized by rotational spectroscopy will be discussed. The partner molecules are held together by hydrogen bonds, weak hydrogen bonds and lone-pair--hole interactions. It will be shown how non-bonding interactions compete to shape the conformational space of the complexes, the structural changes brought to the conformers of the monomers by complexation and how these interactions can be drastically changed through atomic or functional group substitution.

Conventional absorption spectroscopy is still the workhorse in high-resolution rotational laboratory spectroscopy.1 The data obtained from these kind of instruments are relevant for astronomical searches of complex molecules that represent excellent probes of the physical and chemical environments and history of the sources where they are detected.2 Nowadays, observations performed by the Atacama Large Millimeter Array (ALMA) open up new opportunities to reveal the chemical complexity of solar systems analogues. At the same time the huge amount of data collected and the extremely rich surveys represent a challenge for the astrochemistry community. To reach this goal, the spectroscopic know-how is fundamental in recognizing typical pattern lines due to multiple internal interactions and motions that cause complicated energy level schemes, since the resulting spectra will be rather difficult to predict theoretically, mainly in the sub-mm wave region. For this reason a strong interplay between laboratory spectroscopists and observational astronomers is increasingly required to be able to unravel the experimental data. In this talk the features and the potential of the high resolution rotational spectroscopy technique will be pointed out, providing some results on different kind of molecules characterized by complex conformational landscapes. In particular, molecular spectra of potential astronomical interest will be presented, obtained using the free jet absorption millimeter wave and the free space cell absorption sub-mm wave spectrometers working at University of Bologna.3 References [1] S. Brünken, S. Schlemmer, arXiv:1605.07456, 2016 [2] E. Herbst, E. F. van Dishoeck, Annu. Rev. Astron. Astrophys. 47, 427, 2009 [3] C. Calabrese, A. Maris, L. Evangelisti, L. B. Favero, S. Melandri, W. Caminati, J. Phys. Chem. A. 117, 13712, 2013

The importance of organic fluorinated compounds in science and in everyday life is growing year after year.1 The effects of fluorination are related to the high electronegativity of this atom and its polarizability. In general the introduction of a fluorine atom is used in order to modulate physicochemical properties, like hydrophilicity and lipophilicity, of bio-organic molecules and functional materials.1,2 Additionally, structural changes can also be related to the electronic effect of the fluorine atom or to the molecule capability of creating new hydrogen bonds or non-covalent bonds, having fluorine as acceptor. For this reason we investigate clusters where a molecule of water is used as a probe to reveal the changes on the electrostatic potential on the fluorinated compounds. The experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy technique (MBFTMW).3 The high resolution and sensitivity of rotational spectroscopy give direct access to the structural arrangement of the systems, allowing the measurement of bond lengths and angles. Moreover this gas phase technique allows unveiling subtle structural and dynamical effects usually related to changes in non-covalent interactions. The series of clusters studied between different fluorinated pyridines and a molecule of water are: 2-fluoropyridine-water, 3-fluoropyridine-water and penta-fluoropyridine-water (see Figure 1) complexes. The results clearly show that the introduction of a single fluorine atom into a molecule already induces significant effects, but as the number of fluorine atoms increases, such as in the case of penta-fluoropyridine, the system starts to behave as a completely novel species.4 The perfluorination effect is clearly observable in the penta-fluoropyridine-water adduct where the water oxygen lone pairs point towards the aromatic ring. Figure 1: the observed adducts of 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine with a molecule of water.

Halogen substitution in molecules is a common practice to modulate physicochemical properties of bio-organic molecules and functional materials [1,2]. The effects of halogenation are related to the high electronegativity of the atoms and their different polarizabilities. The presence of a halogen atom greatly alters lipophilicity, dipole moment, reactivity, chemical stability and interactions with other functional groups present in the molecule. Above all, the introduction of a fluorine atom becomes particularly interesting because it can activate or increase the molecule capability of creating hydrogen bonds or non- covalent bonds. For this reason we also investigate clusters where a molecule of water is used as a probe to reveal the changes on the electrostatic potential on the halogenated compounds. The experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy (MBFTMW) [3] or Free Jet Absorption Microwave Spectroscopy (FJAMW) [4] techniques. The high resolution and sensitivity of rotational spectroscopy give direct access to the structural arrangement of the compounds, allowing the measurement of bond length and angles. Moreoverthis gas phase technique can help to unveil subtle structural and dynamical effects usually related to changesb in non-covalent interactions. Regarding chlorination we present results on model compounds such as 5-and 6- clorohydroxypyridine in which we investigate the effects of halogenations on the tautomeric equilibrium. Regarding fluorination we first present a study on the changes in the conformational potential energy surface of 2-fluorobenzylamine with respect to the un-substituted molecule. The conformational space of such molecules is shaped by non bonding interactions which can be changed drastically through substitution of even a single atom. We have also studied a series of clusters between different fluorinated pyridines and a molecule of water: 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine. The results clearly show that the introduction ofa single fluorine atom into a molecule already induces significant effects,but as the number of fluorine atoms increases, such as in the case of penta-fluoropyridine, the material starts to behave as a completely novel species [5]. The perfluorination effect is clearly observable: and in the penta-fluoropyridine-water adduct the water oxygen lone pairs point towards the aromatic ring. [1] K. Müller, C. Faeh, F. Diederich, Science 317, 1881 (2007). [2] H. Matter, M. Nazar, S. Gussregen, D. Will, H. Schreuder, A.,Angew. Chem. 121, 2955(2009). [3] Caminati W.; Millemaggi A.; Alonso J.L.; Lesarri A.; Lopez J.C.; Mata S., Chem. Phys. Lett. 392, 1 (2004). [4] C. Calabrese, A. Maris, L. Evangelisti, L.B. Favero, S. Melandri and W. Caminati, J. Phys. Chem. A, 117, 13712 (2013). [5] Cametti M.,Crousse B., Metrangolo P., Milanicd R., Resnati G., Chem. Soc. Rev. 41, 31 (2012).

2-Oxazolidinone (systematic name 1,3-oxazolidin-2-one) is a five-member heterocyclic ring exhibiting potential medicinal roperties. It is the pharmacophore of a class of antimicrobial agents which have a unique structure and good activity gainst Gram-positive pathogenic bacteria (e.g. linezolid, the first in clinical use 2-oxazolidinone antibiotic). Often the functionality of drugs is related to their conformational flexibility and ability to form non covalent interactions with ts urroundings. In order to understand how substitution affect these properties, a series of oxazolidone compounds has been studied by means of millimeter wave free jet Stark modulated absorption spectroscopy [1] and quantum mechanical calculations. The achieved data will be presented. [1] C. Calabrese, A. Maris, L. Evangelisti, L.B. Favero, S. Melandri and W. Caminati, J. Phys. Chem. A, 117, 13712 (2013).

We present the rotational spectra of different organic molecules, Benzylamine (BA) and 2-Fluorobenzylamine (2FBA), and we report the effects of water complexation for BA. These flexible molecules are characterized by high number of low energy conformations and the presence of large amplitude motions. Non-bonding interactions compete to shape their conformational space and potential energy surfaces and these interactions can be changed drastically with water complexation and halogenation. Halogen substitution is in fact a common practice in chemistry in order to modulate physicochemical properties of bio-organic molecules and functional materials.[1,2] In particular, the role of organic fluorine atoms, has been extensively investigated at the molecular level and its effects are correlated to the high electronegativity of the atoms and their different polarizabilities. We recorded the spectra with Free Jet Absorption Microwave Spectroscopy (FJ-AMMW) or Fourier Transform Microwave Spectroscopy (FTMW). The high resolution and sensitivity of these techniques can help unveil subtle structural and dynamical effects usually related to changes in non-covalent interactions. In the study of the two monomers BA and 2FBA we want to evaluate the changes that substitution of H atoms by fluorine causes to Benzylamine. The rotational spectra of 2-FBA, recorded with FTMW spectrometer, show the presence of two of the four stable conformers predicted with quantum chemical calculations: the global minimum is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group, whereas the other conformer is characterized by a complex tunnelling motion of the aminic hydrogen atom. On the basis of the BA monomer, we predicted a fourth conformer and we found a greater splitting of the same tunneling motion due to the new geometry generated by the presence of the fluorine. The motion was analysed with a one-dimensional flexible model which allowed to estimate the energy barrier for the transition state. As to the rotational spectrum of the complex of BA with one molecule of water, recorded with FJ-AMMW spectrometer, it shows only one conformation with the water molecule’s hydrogen bound to the nitrogen atom of BA. We can note that the introduction of the water molecule does not disrupt the initial conformation of the monomer BA, but inverts the stability order of the conformations and blocks the tunneling motion. REFERENCES [1] K. Müller, C. Faeh, F. Diederich, Science, 2007, 317, 1881; H. Matter, M. Nazar, S. Gussregen, D.W. Will, H. Schreuder, A., Angew. Chem., 2009, 121, 2955. [2] R. Berger, G. Resnati, P. Metrangolo, E. Weberd and J. Hulliger, Chem. Soc. Rev., 2011, 40, 3496.

Hydrogen bonds play a crucial role in chemistry and biology. Numerous experimental and theoretical studies have been devoted to the elucidation of the structures and the energetics of hydrogen bonds, establishing key features of both intra-molecular and inter-molecular hydrogen bonds by means of different spectroscopic measurements and quantum chemical calculations. In particular, the interaction between acids and water occurs via formation of a hydrogen bond with the acid as the hydrogen donor and the water oxygen as the acceptor. If an electronegative center exists in the acid, another weaker hydrogen bond may be formed with this center as the hydrogen acceptor and the water as the hydrogen donor. Formation of two hydrogen bonds therefore constructs a ring-like structure in the 1:1 acid-water complex. Microwave spectroscopy studies of the dimers of nitric acid [1], sulfuric acid [2], and several carboxylic acids (formic [3], acetic [4], propanonic [5], trifluoroacetic acid [6], trans- and gauche-difluoroacetic [7]) with water have confirmed this structural feature. In many of such spectra, large amplitude motions of the water moiety produce a hyperfine structure of the transition lines. With the aim of getting insight on the behaviour of this kind of dimers, we recorded and assigned the mua- and mub-type rotational spectra of the water complex with both the s-cis and s-trans forms of acrylic acid, the simplest unsaturated carboxylic acid. Besides the parent species, also the spectra of eight isotopomers (COOH-H218O, COOH-D2O, COOD-H2O, COOD-D2O, COOH-DOH, COOH-HOD, COOD-DOH, COOD-HOD) for each conformer were analysed. From the comparison of the rotational constants of all species, precise structural information is obtained. The mub-type transition lines of the parent species and 18O isotopomers are split in two components and a picture of the potential energy surface that leads to this motion has been attempted by quantum-mechanical calculations.

The detection of molecules in space, is based on their spectroscopic features and high resolution spectral data is needed to allow an unambiguous identification of them. Many of the molecules detected in space are complex organic molecules containing chains of carbon atoms and which therefore show a high degree of molecular flexibility. The high number of low energy conformations and the presence of large amplitude motions on shallow potential energy surfaces are peculiar to this kind of systems. The presence of a high number of stable conformers - often interconverting through small energy barriers - usually gives rise to very complex spectra, which represent a challenge for spectroscopic and computational methods. We report the rotational study of methylaminoethanol (MAE) performed by Free Jet Absorption Microwave Spectrocopy (FJAMW). For this species it has proved essential to compute the complete potential energy surfaces related to the low amplitude modes. This has been calculated at the B3LYP/6-311++G** level of theory while the stable geometries have been characterized MP2/6-311++G**. The interest in the conformational properties of MAE is twofold: in the first place, aminoethanol and thus also MAE can be considered precursors of aminoacids in the interstellar medium \footnote{ S.~Charnley, in Proceedings of the workshop: The bridge between the Big Bang and Biology, CNR, Italy 1999.} and secondly, the MAE side chain is present in important biological molecules such as adrenaline. The conformational preferences of MAE are dominated by the intramolecular hydrogen bond between the OH and NH2 groups and its flexibility and asymmetry generate a very high number of conformers. 24 stable conformations have been predicted and two conformers were observed by FJAMW spectroscopy with our 60-72 GHz spectrometer. With respect to a previous study\footnote{ R.~E.~Penn and L.~W.~Buxton \textit{J.~ Mol.~Spectrosc.} \underline{\textbf{56}} 229 1975.} we have extended the observed frequency range, partly reassigned the rotational spectrum of one of the conformers and determined the nuclear quadrupole constants. The search for higher energy conformers has also been undertaken.

The investigation of phenomena related to the chemistry of the Cosmos is strongly based on the identification of molecules by spectroscopic methods and a knowledge of their spectroscopic parameters or their transition frequencies is needed. Rotational spectroscopy, with its high resolution, is the most powerful tool for the unambigous identification of molecules and laboratory work is needed to provide the community with the spectral features to analyze the cosmological surveys. Many of the molecules detected in space are complex organic molecules containing chains of carbon atoms and which therefore show a high degree of molecular flexibility. The high number of low energy conformations and the presence of large amplitude motions on shallow potential energy surfaces are peculiar to this kind of systems. The presence of a high number of stable conformers - often interconverting through small energy barriers - usually gives rise to very complex rotational spectra, which represent a challenge for spectroscopic and computational methods. Spectroscopic strategies for the rotational study of flexible molecules include the use of the cold and isolated conditions of a free jet expansion and laser ablation sources for the non volatile systems while the computational methods must deal with complex conformational surfaces and large amplitude motions which can be coupled to the overall rotation causing a hyperfine splitting of the rotational transitions. We will give examples of rotational spectroscopy of highly flexible organic molecules of astrophysical interest studied by the Free Jet Spectroscopy Group in Bologna with instruments going from 6 to 78 GHz. They include methylacetate [1], methylaminoethanol and biological building blocks like C4 sugars [2] The interpretation of their rotational spectra with appropriate models is essential to correctly predict their spectral features at higher frequencies. REFERENCES [1] M. Tudorie, I. Kleiner, J. T. Hougen, S. Melandri, L. W. Sutikdja, W. Stahl, J. Mol. Spectr., 269, 211 (2011) [2] B. M. Giuliano, S. Blanco, S. Melandri, and W. Caminati, Astrophy. J. Suppl,.179, 355 (2008); B. M. Giuliano, S. Blanco, S. Melandri, W. Caminati, Chem. Phys. Lett., 467, 74 (2008)

Flexible molecules show a complex conformational space, generally shaped by non bonding interactions occurring within the molecule or with the surroundings and represent therefore a challenge for computational methods and rotational spectroscopy because of the presence of high number of stable conformations and large amplitude motions. We report the rotational study of 2(N)-methylaminoethanol (MAE) performed by Free Jet Absorption Microwave Spectrocopy (FJAMW). This choice of study the conformational properties of MAE is mainly due to the fact that aminoethanol and thus also MAE can be considered a precursors of aminoacids in the interstellar medium (1) and secondly, the methyl-aminoethanol side chain is present in important biological molecules such as adrenaline, so we can consider it like a model for the chain’s interactions. The high vacuum conditions that we obtained with this technique reproduce those of the interstellar medium and this makes these kind of experiments very useful to guide astrophysical studies that aim at finding life precursors. Infact, because of the unique spectral signatures provided by rotational spectroscopy, much of the chemical inventory of the interstellar medium has been detected using radio telescopes operating in the centimeter and millimeter wavelenghts regions (2). Moreover the unusual chemical environments in this spatial region can produce novel chemical reaction processes, that involve less stable conformational species. The conformational preferences of MAE are dominated by the intramolecular hydrogen bond between the OH and NH2 groups. The high flexibility of this molecule involves a complex energy potential surface with a large number of possible stable conformations. Infact for MAE 24 stable conformations have been predicted and two conformers were observed by FJAMW spectroscopy with our 60-72 GHz spectrometer. With respect to a previous study (3) we have extended the frequency range to 72 GHz and partly reassigned the rotational spectrum of one of the conformers. (1) S. Charnley , in Proceedings of the workshop: The bridge between the Big Bang and Biology, CNR, Italy 1999. (2) J. L. Neill et all., J. Phys. Chem. A, 2011, 115 (24), pp 6472–6480. (3) R. E. Penn and L.W. Buxton, J. Mol. Spectrosc. 56, 229, 1975.

Rotational spectroscopy is an important tool to evaluate the conformational attitude of molecules, to define their structure and to characterize the large amplitude vibrational modes that can be present. Conformationals studies are interesting for example to understand the biological activity of small organic molecules, such as drugs and pollutantes and to verify “lock and key” principle effect. Moreover the experimental results may be used as benchmarks for computational data since they are obtained in the same conditions: the isolated molecule. Rotational spectroscopy is performed in gas-phase in high vacuum conditions and so are the quanto-mechanic ab initio calculations, which are used to describe the PES which governs the geometrical arrangements of conformational isomers and to obtain the spectroscopic constants which allow the research and the assignment of rotational transitions. Finally it should be noted that high vacuum conditions reproduce those of the interstellar medium and this makes these kind of experiments very useful to guide astrophysical studies that aim at findig life precursors. In our laboratory the experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy (MBFTMW) [1] or Free Jet Absorption Microwave Spectrocopy (FJAMW) [2] [3] techiniques. To support all these considerations we show some examples of different kind of molecules studied with this technique. At first a study which compares the conformers of Benzylamine (BA) and 2-Fluorobenzylamine (2-FBA). Because of the importance of the organic fluoride compounds, we want to evaluate the changes that substitution of H atoms by fluorine causes to these molecules. The rotational spectra of 2-FBA show the presence of two of the four stable conformers predicted with quantum chemical calculations: the global minimum is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group, whereas the other conformer is characterized by a complex tunnelling motion of the aminic hydrogen atom. Compare of to BA we predicted a new conformer and we found a different splitting of the same tunneling motion, due to the new geometry caused by the presence of the fluorine. The second example is 2-Methylaminoethanol (MAE) that is relevant both for astrophysical research and also in biological chemistry. Because of the high flexibility of its chain, MAE presents an elaborate conformational surface, which is explored by B3LYP and MP2 quanto-mechanic methods. The conformational preferences of MAE are dominated by the intramolecular hydrogen bond between the OH and NH2 groups, that gives rise to 24 different conformers and two of them were observed. The search for higher energy conformers has been undertaken. References [1]Caminati W.; Millemaggi A.; Alonso J.L.; Lesarri A.; Lopez J.C.; Mata S., «CHEMICAL PHYSICS LETTERS», 2004, 392, pp. 1 - 6 [2] S. Melandri, W. Caminati, L. B. Favero, A. Millemaggi, P. G. Favero, J. Mol. Struct. (THEOCHEM) 1995, 352/353, 253 ± 258. [3] S. Melandri, G. Maccaferri, A. Maris, A. Millemaggi, W. Caminati, P. G. Favero, Chem. Phys. Lett. 1996, 261, 267 ± 271

Fluorine substitution in molecules is a common practice in bio-organic chemistry in order to modulate physicochemical properties and biological activity of molecules and in the case of adrenergic neurotransmitters cases the selectivity of the molecules is dramatically altered depending on the site of fluorination.(1) 4-fluoro and 2-fluoro-2-phenylethylamine (analogs of the prototype 2-phenylethylamine) were studied by rotational spectroscopy and ab initio calculations with the scope of obtaining information on the effects of fluorination on the conformational energy landscape. The conformational space is dominated by low energy gauche conformations stabilized by weak hydrogen bonds and higher energy anti conformations. The order of stability of the conformers is altered with respect to the prototype molecule, and these observations are rationalized in terms of the effect of fluorine on the charge distribution of the  cloud. (1) C. Pooput, E. Rosemond, J. Karpiak, F. Deflorian, S. Vilar, S. Costanzi, J. Wess and K. L. Kirk, Bioorg. & Med Chem., 2009, 17, 7987.