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To reproduce the experimental splittings, we developed a numerical 1D model to solve the rovibrational problem. In this work, we apply the model using the dihedral angle between the planes defined by (i) the C(phen)—OH group and (ii) the phenyl ring as the leading coordinate. This represents a periodic intramolecular motion, whose periodicity is influenced by the identities and positions of the substituents on the phenyl ring.

The rotational spectroscopy of several sulfur bearing molecules and their 1:1 water complex, cysteamine, cysteamine monohydrate, 1-thioglycerol and 1-propanethiol were studied in the micro-wave and (or) millimeter-wave range. Precise laboratory spectra and conformational information were obtained. For cysteamine, the conformational space (at the B3LYP-GD3(BJ)/Def2-TZVP level) and the measurement and analysis of its rotational spectra in the 6 - 18 and 59.6 - 120 GHz are reported. The hyperfine structure of the rotational spectra was observed and analyzed for the first time. Based on the measured spectra, a search of the different conformers of cysteamine was performed toward the G+0.693-0.027 molecular cloud. We computed the upper limit of the ratio of ethanolamine to cysteamine, which is >0.8−5.3. For the cysteamine monohydrate, the conformational space was explored (at the B3LYP-GD3(BJ)/Def2-TZVP level). The rotational spectra of the cysteamine monohydrate complex have been assigned in the frequency range 6 – 18.5 GHz. The global minimum, Conf A1, was the only observed one. The 34S isotopologue of Conf A1 was observed in natural abundance, while 18O isotopologue was detected by introducing the H218O. In this conformer, the water molecule plays both proton donor and acceptor roles, forming a OHw···N interaction, a SH···Ow interaction and a CH···Ow interaction. The conformational space of 1-thioglycerol has been characterized by quantum mechanical calculation and its rotational spectrum has been recorded and analyzed in the frequency range 59.6 - 78.4 GHz. The global minimum of 1-thioglycerol is gTg’Gg’ and were detected together with gTg’Tg and gGgG’g, while the two detected conformers are g’G’gGg’ and tGgGg. The high-resolution rotational spectrum of 1-propanethiol in the frequency range 59.6 – 78.4 GHz was measured. Two conformers, Gg and Tg, were observed and their spectra were analyzed. Considering the overall conformational space calculated at the B3LYP-GD3(BJ

We recorded the rotational spectra of six isotopologues of the most stable conformer of 1,2-butanediol in the 59.6–74.4 GHz frequency region. The observed species are the four monosubstituted 13C, the OD···OH and the OD···OD, and the measurements were performed using a Free-Jet Absorption Millimeter Wave spectrometer. The analysis of the 18 experimental rotational constants determined for all the isotopologues together with those of the parent species, lead to the determination of the coordinates in the principal axis system for the substituted atoms. From the theoretical structure, calculated at the B3LYP-D3/aug-cc-pVTZ level of theory, a refinement of the structural parameters led to a partial r0 geometry, which reproduced all the rotational constants within 1.3 MHz.

My research interests during my doctoral years have been focused on high resolution rotational studies of molecules and weakly bound molecular complexes. Information on the molecular structure, internal motions and intermolecular interactions that can be obtained by applying suitable theoretical models to the analysis of these unusually complex spectra allows the determination and understanding of the driving forces involved in formation of the molecular complex. In this way, many types of non-covalent interactions have been characterized, from pure van der Waals interactions in complexes of rare gases to moderate-strength and weak hydrogen bonds (HBs) and to the most recent halogen bonds, pnicogen4 or chalcogen bonds. In this thesis, we first introduce the theory of rotational spectroscopy, including that of the asymmetrical rotor, the effects of centrifugal distortion, nuclear quadrupole coupling effects end those of internal motions In the second part, we introduce the experimental apparatuses that were used and related theoretical knowledge. In the third part, chloropentafluorobenzene (C6F5Cl) and bromopentafluorobenzene (C6F5Br) are chosen as case studies to investigate the effect of perfluorination on the molecular structure and electronic properties.In the fourth and fifth parts, we discuss the 1:1 complexes of acrolein-methanol and acrolein-ethanol. In chapter six to eight I report the results on the microwave detection and analysis of the 1:1 complexes of dimethyl sulfoxide (DMSO) with water, methanol and ethanol, respectively, in the gas phase.

The conformational space of methacrylamide was explored by quantum mechanical modeling and surveyed in the 59.6–104.0 GHz frequency range using a millimeter-wave Stark-modulated free-jet absorption spectrometer. According to the relative orientation of the two unsaturated bonds, two conformers were observed, namely s-trans (A=5234.360(1), B=3364.9717(8) and C=2173.099(1) MHz) and s-cis (A=5207.292(1), B=3470.930(1) and C=2113.496(1) MHz). The s-trans conformation is the global minimum, with relative energy 4(2) kJ mol−1 and calculated isomerization barrier 15 kJ mol−1. Except for the methyl hydrogen atoms, s-cis-methacrylamide is planar and its methyl internal rotation barrier is 10.2(1) kJ mol−1. In s-trans-methacrylamide the allyl and amino frames form a dihedral angle of about 30° and the methyl internal rotation barrier is 7.4 kJ mol−1. This different behaviour is explained in terms of attractive and repulsive intramolecular interactions between groups: CH2/CO and CH3/NH2 for s-cis, CH2/NH2 and CH3/CO for s-trans. The tunneling splitting related to the double-well potential describing the interconversion between the two equivalent s-trans forms is 837.97(2) MHz and was reproduced by a one-dimensional flexible model using a 3.6 kJ mol−1 interconversion barrier.

The interaction between carbon dioxide and planar carboxylic acids has been investigated through the analysis of the microwave spectrum of the acrylic acid·CO2 complex and quantum chemical modeling of the R-COOH·(CO2)1,16 clusters, where R = H, CH2CH. As regards the 1 : 1 compounds, two species, involving the s-cis and s-trans conformers of acrylic acid were observed. For both of them, a similar bidentate interaction arises between the carbonyl group of CO2 and the carboxylic group of the organic acid, leading to the formation of a planar six-membered ring. The binding energy is estimated to be De ≃ 21 kJ mol-1, 1/3 being the energy contributions of the tetrel to hydrogen bonds, respectively. In the 1 : 16 clusters, the ring arrangement is broken, allowing for the interaction of the acid with several CO2 molecules. The CO2 molecules completely surround formic acid, whereas, in the case of acrylic acid, they tend to avoid the allyl chain.

The hydrogen-bonded complex between acrylonitrile (CH2=CHCN) and methanol has been characterized spectroscopically in the millimeter wave range (59.6-74.4 GHz) using a free jet absorption millimeter wave spectrometer. Precise values of the rotational and centrifugal distortion constants were obtained from the measured frequencies of the complex of acrylonitrile with CH3OH and CD3OD. The analysis of the splittings of the rotational lines due to the hindered internal rotation of the methanol methyl group led to the determination of a V-3 value of 221.9(7) and 218(5) cm(-1) for the complexes of CH3OH and CD3OD, respectively, and these values are about 40% lower than that of free methanol. The structure of the observed conformation is in agreement with the global minimum determined at the MP2/aug-cc-pVTZ level of calculation, and the counterpoise corrected intermolecular binding energy, obtained at the same theoretical level, is D-e = 26.3 kJ mol(-1)

The hydrogen-bonded complex between acrylonitrile (CH2=CHCN) and methanol has been characterized spectroscopically in the millimeter wave range (59.6-74.4 GHz) using a free jet absorption millimeter wave spectrometer. Precise values of the rotational and centrifugal distortion constants were obtained from the measured frequencies of the complex of acrylonitrile with CH3OH and CD3OD. The analysis of the splittings of the rotational lines due to the hindered internal rotation of the methanol methyl group led to the determination of a V-3 value of 221.9(7) and 218(5) cm(-1) for the complexes of CH3OH and CD3OD, respectively, and these values are about 40% lower than that of free methanol. The structure of the observed conformation is in agreement with the global minimum determined at the MP2/aug-cc-pVTZ level of calculation, and the counterpoise corrected intermolecular binding energy, obtained at the same theoretical level, is D-e = 26.3 kJ mol(-1).

Direct spectroscopic evidence of a reaction occurring between acrolein and water and involving the exchange of an oxygen atom has been obtained by characterizing the non-covalently bound water complexes and their isotopic forms, via rotational spectroscopy. The experimental geometries of the binary and ternary water complexes have been determined, and other stationary points on the reaction path have been characterized using ab initio quantum chemical methods at the MP2/6-311++G(d,p) level. These results can enhance the understanding of the water-mediated atmospherically important reactions involving acrolein.

In this dissertation, the introduction of the basic knowledge of microwave spectroscopy including the fundamental theory of the rotational spectroscopy and the experimental techniques was reviewed firstly. Then the work that I have performed during my PhD career is discussed. Pulsed-jet Fourier transform microwave (PJ-FTMW) spectroscopy has been used to study the monomers, and the bimolecular clusters formed with hydrogen bond or weakly non-covalent bonds. Difluoromethane and water oligomers have been studied by Chirped-Pulse Fourier transform microwave (CP-FTMW) spectroscopy. The spectroscopic work in this dissertation includes three parts: 1) Isolated molecules with large amplitude motions. Three subjects are involved in this part, which are the rotational studies of indan, 1,2-dimethoxylethane, and 1-methylcyclohexanol. The transitions were split due to ring puckering motion, the internal rotations of methyl groups, and the rotation of the hydroxyl group, respectively. 2) Clusters formed by the cooperation of different classes of HBs. Four subjects are involved in this chapter. Firstly, formic acid clustering with dimethyl ether and cyclobutanone, respectively, indicate that both complexes are formed with one typical O-H···O HB and two C-H···O weak hydrogen bonds (WHBs). Secondly, the investigation of the complex of indan with trifluoromethane showed a cage structure based on the cooperative effects of C-H···π and C-H···F WHB interactions. Lastly, The study of oligomers of (CH2F2)m-Wn, (m,n = 1,2) has broaden our knowledge of the cooperation of different HBs in a cluster. 3) Another interesting result is the observation of the competition between reactivity and the pre-reactivity in the mixtures of carboxylic acids and alcohols, which is a basic phenomenon in organic synthesis. The experimental results on the primary and secondary alcohols mixed with carboxylic acids have shown that the esterification reaction occurs, whereas only the spectra of the adducts could